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Gaseous and particle emissions in low-temperature combustion diesel–HCNG dual-fuel operation with double pilot injection
Applied Energy 253 (2019) 113602
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Gaseous and particle emissions in low-temperature combustion diesel–HCNG dual-fuel operation with double pilot injection
T
Luigi De Simio , Sabato Iannaccone ⁎
Istituto Motori, National Research Council, Via Marconi 4, 80125 Napoli, Italy
HIGHLIGHTS
GRAPHICAL ABSTRACT
test of low-temperature • Experimental combustion and dual-fuel operation. engine fueled using NG • Reciprocating as primary fuel and H up to 25% by 2
• •
volume. Ignition triggered by double pilot injection of diesel fuel to reduce pressure rise. Benefit on NOx and equivalent CO2, without negative effects on particles number.
ARTICLE INFO
ABSTRACT
Keywords: Double-pilot injection Dual fuel Natural gas Hydrogen Particle emission
Alternative fuels and energy vectors are becoming increasingly important in terms of technical, geopolitical, economic, and environmental aspects. In particular, gaseous fuels and vectors, such as fossil or synthetic natural gas (NG) blended with hydrogen, commonly help provide optimal strategies to reduce global and toxic emissions of internal combustion engines, owing to their adaptability, anti-knock capacity, lower toxicity of pollutants, reduced CO2 emissions, and costeffectiveness. However, diesel engines still represent the reference category among internal combustion engines in terms of maximum thermodynamic efficiency. The possibility offered by dual-fuel (DF) systems to combine the efficiency and performance of diesel engines with the environmental advantages of gaseous fuels has been the subject of extensive investigations. However, the simple replacement of diesel fuel with gaseous fuel does not allow for optimising the engine performance, owing to the high percentage of unburned gaseous fuel, which compromises the potential reduction of CO2; therefore, more complex combustion strategies should be realised. In this study, with the aim of improving the DF combustion process, an experimental investigation was performed to analyse low-temperature combustion (LTC), using NG and two enriched hydrogen-compressed NG blends as primary fuels. The LTC mode was activated by means of a very early advanced pilot injection and carried out in two close steps. The double pilot injection was used to control the energy release rate in the first combustion stage, thereby minimizing the increase of the rate of pressure and allowing the extension of the operation range under LTC. The experimental activity was also focused on analysing the particle emissions, as it is well known that these emissions, together with those of nitrogen oxide, constitute the main pollutants resulting from diesel fuel combustion. The results demonstrated the potential to reduce the unburned fuel, NOx, and particle emissions simultaneously, while maintaining equivalent CO2 emissions to a diesel-only engine. Both the timing and pressure of the pilot injection proved to be critical parameters for optimising the emissions and performance.
⁎
Corresponding author. E-mail address: [email protected] (L. De Simio).
https://doi.org/10.1016/j.apenergy.2019.113602 Received 21 March 2019; Received in revised form 18 July 2019; Accepted 20 July 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.
Applied Energy 253 (2019) 113602
L. De Simio and S. Iannaccone
1. Introduction
three-way catalyst after-treatment rather than by means of high EGR. Abdelaal et al. analysed the effects of EGR on the oxygen excess at the exhaust on a DF diesel engine, and they identified thermal efficiency that was comparable to the conventional diesel mode [15]. In [16], the different effects of EGR (thermal, chemical and radical effects) were analyzed, finding that the thermal effect was the main reason for reducing NOx. In [17], the effects of cooled EGR were investigated and it was found that it could improve the brake thermal efficiency if the EGR was less than 5%. The EGR effect on abnormal combustion was analysed in [18], also considering the regulation of the incoming air by means of a throttle valve, and in [19], also considering the cooling of the EGR. In both cases, a substantial reduction in the pressure gradient was determined. At present, investigations have mainly focused on fuel injection strategies and the MF composition. Mann et al. proposed a system with direct gas injection to optimise the engine, achieving regulated emissions below the EPA 2010 levels over the 13-mode steady-state cycle [20]. Zhang et al. used an early PF injection in a heavy-duty DF engine with a 14:1 compression ratio to achieve LTC. The engine was operated up to 11.6 bar of brake mean effective pressure, not exceeding a pressure gradient of 15 bar/deg [21]. An LTC model was created by Krishnan et al. to predict the onset of ignition and the number of ignition centres [22]. Furthermore, the use of hydrogen or hydrogen/NG blends as primary fuel provides great opportunities for optimising DF combustion. In the initial studies on DF engines, the use of pure hydrogen was investigated, with particular reference to the knock because of its intensity, as reported in [23]. More recently, the use of such fuels has gained renewed interest in terms of smoke emissions and CO2 reduction. Karagoz et al. introduced hydrogen into the engine with 25% and 50% of the total charge, with the energy achieving a significant decrease in smoke emissions, even with a dramatic increase in the nitrogen oxides and maximum peak pressure [24]. However, the use of pure hydrogen as the primary fuel, which provides increased efficiency with respect to the FD mode, exhibits certain limitations on the input energy fraction, owing to the problem of pre-ignition occurring before the pilot diesel fuel injection [25]. Finally, the diesel fuel injection mode is a crucial factor. This aspect is related not only to the injection phasing and quantity, but also the pilot diesel injection profile, which is a topic of this study. In particular, the effect of a pilot split to realise a double pilot injection (DPI) is investigated. The DPI has proven to be a useful strategy for diesel engine optimisation, particularly for reducing the combustion noise, because the fuel burns with a more regular combustion rate [26]. This final aspect, which is strictly related to the heterogeneous combustion of the main diesel fuel injection in a diesel engine, would be significantly modified in a DF engine, where the main fuel is in a gaseous state, premixed with air, and involved in combustion though the propagation of flame fronts. Splitter et al. used a DPI such that 55% of the directinjected fuel was in the first pulse and 45% of the direct-injected fuel was in the second pulse on a single-cylinder 2.44 L engine [27]. Engine experiments were performed using port-fuel-injection of isooctane, and direct-injections of n-heptane with premixed isooctane air index approximately 3.3 (indicating a low load) in LTC regime. They found that at later injection timings that avoid liner impingement, double injections proved to be advantageous, thanks to the more distributed mixture of the direct-injected fuel. Bae et al. implemented a DPI strategy on a single-cylinder 0.98 L diesel engine under the low load condition of premixed charge compression ignition combustion [28]. The experimental results, indicated that a combination of an NG substitution of 40%, the DPI strategy of diesel, and a moderate EGR rate effectively reduced the HC and CO emissions and improved thermal efficiency. The DPI was used in the activity of this study to determine the effects of the formation of ignition sources on the DF combustion both at low and medium load of a multi-cylinder engine with the maximum possible substitution of diesel fuel with gaseous one, which represents the most severe conditions due to the considerable pressure gradients. Therefore,
Although internal combustion engines are expected to remain the major power sources for propulsion systems in the near future, strong and ever-increasing electrification of the powertrain has been occurring for several years. The aim of this electrification, in the form of hybrid systems or full electric systems, is to reduce toxic pollutants, particularly in urban areas, and reduce global CO2 emissions. In the future, an increasing number of hybrid thermal-electric systems is expected to emerge, owing to their higher flexibility compared with full electric systems; for example, in the autonomy of vehicles or the availability of the electric grid and materials for batteries production. Hybrid systems can significantly reduce the emissions and fuel consumption of the thermal unit by means of integrated electric and mechanic management, which can simplify the internal combustion engine operation. In fact, a thermal engine can operate with high efficiency, avoiding idling and part load operations when it operates as a battery charger, or it can be assisted by electric motor–generators, with the aim of reducing fuel consumption. Moreover, the increasing number of fully electric vehicles could lead to the development of small and medium-sized generators for electricity production from renewable sources or energy vectors. In this scenario, DF combustion strategy may attract significant interest. DF technology offers the simplest method for using a gaseous fuel in a diesel engine with a high compression ratio or a highly efficient diesel engine, in which a liquid fuel with a high cetane number is directly injected into the combustion chamber as a pilot source to ignite fuel with a high octane number fuel, which is the main fuel (MF). Either liquid or gaseous fuels may be used as the MF; however, gaseous fuels are generally favoured [1,2], such as natural gas (NG) or bio-methane, owing to the possibility of reducing CO2 emissions. The combustion process is a combination of the diffusive type, in which the liquid pilot fuel (PF) is the most involved, and a homogenous propagation type, which through numerous flame fronts extend the combustion from the ignition centres close to the PF to the entire combustion chamber [3,4]. This technology is very useful for emission reduction and fuel consumption; however, only in medium load areas of the engine operation map. At lower and higher loads, problems of incomplete or knocking combustion may occur, forcing the engine to be switched to full diesel (FD) mode. Such problems have been thoroughly investigated in several studies. Karim et al. analysed the effect of the total equivalence ratio (the stoichiometric mass air flow rate for the PF and MF divided by the actual mass air flow rate) [5] to propose a method for predicting the lean operational limits for DF combustion. The effect of the gaseous energy shares on the thermal efficiency enables the DF to exhibit efficiency similar to that of the diesel case, but only for high loads, as reported in [6–8]. The effect of the fast combustion achieved by DF on the risk of knock owing to end gas autoignition was analysed in [9,10], and it was found that increasing the mass of the gaseous fuel used increased the combustion noise. The most suitable applications of this technology could be in generators or the propulsion of hybrid vehicles, in which low and high loads can be performed with electric motors, while a DF engine of an appropriate size could output the mean power. The goal is to extend the load range in which DF combustion could be applied. Adb et al. [11] used a pilot injection with an increasing advance to achieve improved penetration of the PF jet inside the intake charge, with the aim of incorporating a greater amount of the air/gas mixture into the spray prior to the ignition process, and increasing the MF share involved in the combustion. In [12], the pilot injection timing was optimised to reduce the NOx, THC, and PM emissions. To reduce the minimum load at which DF could be used in a multi-cylinder engine substantially, the strategies of a skip-fire [13], which consists of increasing the load for certain cylinders and deactivating the others, could be followed. At high loads, exhaust gas recirculation (EGR) has been proven to be useful for DF mode extension. In [14], the authors used EGR instead of throttling to control the load in a stoichiometric diesel and gasoline DF engine, controlling the NOx emissions with a 2
Applied Energy 253 (2019) 113602
L. De Simio and S. Iannaccone
the potential of the DPI for realising LTC, by overcoming the problems of the NOx increasing and abnormal combustion occurring, was tested in a light-duty four-cylinder DF engine. The engine was fuelled with NG and two NG/hydrogen blends to explore the consequences of the fuel composition on the DPI strategy. The activity demonstrated the possibility of extending the LTC operation range with a DPI by retaining the maximum gradient of the pressure increase under 15 bar/deg. Effects of LTC on PM were also investigated by means of analysing the particle emissions at the exhaust, which are part of the regulated emissions for the majority of modern engines.
modules, which were used to set the engine load by adapting the fuel flows. The engine speed was set to 2000 rpm, while two different loads, namely 50 and 100 Nm, were considered for the FD and DF modes, respectively. The DF operation was performed with the highest possible gaseous fuel percentage, by decreasing the diesel fuel flow rate at the minimum quantity required for a stable combustion start. Therefore, the percentage of energy introduced into the gaseous fuel, compared to the total introduced also considering the diesel fuel, was found to be 83% and 75% at 100 and 50 Nm, respectively, and was almost the same for all of the tested mixtures. Owing to the dependence of the lower heating value (LHV) on the H2 content, as reported in Table 2, several differences can be noted if the mass percentage of the gaseous fuel compared to the total is considered. In fact, the gas fuel percentages were 82%, 80%, and 78% at 100 Nm, and 74%, 72%, and 70% at 50 Nm for CNG, HCNG15, and HCNG25, respectively. The pilot control strategy was modified to switch from normal combustion to LTC by setting a large advance and dividing the injection into two separate pilot injections. In this manner, it was possible to test the engine under reliable conditions, as the pressure gradient in the combustion chamber did not exceed 15 bar/CAD, which is generally considered as a limit value to avoid excessive mechanical and thermal stresses. The LTC was only activated for the DF mode, as this was the specific purpose of the study, while the injection advance was increased in the FD by means of simple translation of the injector control signal, until the maximum limit for the pressure gradient was reached. Therefore, a completely different injection law, aimed at obtaining the LTC in the FD case, was not investigated, as it was not an objective of this study, and data in the FD mode can be considered as a reference for normal combustion with diesel fuel only. A complete scheme of the experimental setup is presented in Fig. 1. The data collected during the test were processed to derive several parameters that are particularly useful for describing the DF mode:
2. Experimental setup and procedures 2.1. Reference engine, fuels, and instrumentation To assess the potential of the DPI for DF combustion, a multi-cylinder diesel engine was used, which was converted to operate in the DF mode. The engine was a light-duty four-cylinder diesel type (Table 1), with the diesel injection split into a pilot and main step. The DF mode was achieved by adding a timed gas injection system with four injectors, which introduced the fuel close to the intake valve of each cylinder during the corresponding intake stroke to prevent fuel bypassing of the combustion chamber owing to the exhaust-intake valve overlapping, which is generally significant in diesel engines. There was a glow plug in each combustion chamber to heat the air and aid in cold starting. The housing of one of these was used for the in-cylinder pressure cycle measurement through a piezoelectric transducer (Kistler 6058) and Kistler amplifier (Charge Amplifier, Type 5064) with a resolution of 0.1 crank angle degrees (CAD). The engine was fuelled with commercial diesel fuel, NG, and NG-H2 blends, with the compositions and properties listed in Table 2. Regarding the gaseous fuels, the remaining part of the composition consisted of hydrocarbons that are heavier than methane, such as the propane and ethane contained in NG. By increasing the H2 content, additional air was required to burn a fuel unit stoichiometrically, while less fuel was necessary to enter a unit of energy. Emission characterisation is essential for assessing the benefits of the DF mode with respect to the FD mode, as well as strengthening the LTC mechanism. In fact, the scope of LTC involves reducing both the unburned fuel and NOx, without increasing the particle emissions. During the tests, the instrumentation reported on in Table 3 was used. For the particle characterisation, the sample gas was analysed using a Cambustion DMS500 fast particulate spectrometer. This instrument uses a sorting plant column with 22 rings, which processes the current signals acquired by annular electrodes to provide a discrete numerical spectrum as a function of the sample flow diameter of the particles per cubic centimetre, expressed in dN/dlogDp/cm3, where N is the number and Dp is the diameter (in nm) of the particles. According to the spectral measurement, it is possible to obtain the distribution of the particles discerning the nucleation phase, in which the diameter of particles ranges from 5 to 100 nm, and the accumulation phase, with particles between 100 and 1000 nm, where the total number of particles and geometric mean diameter (GMD) of the particles is assumed to be spherical. The particle mass (M), in μg, was evaluated using Eq. (1), which assumes a rapid reduction in density of particles with diameters of 5–200 nm, and a slow reduction for larger particles, in agreement with [29].
M = 2.20·10
15· D 2.65 p
- the average air index, defined by Eq. (2), which is a parameter that allows for calculating the mean fuel in the combustion chamber; - the percentage of gaseous fuel (% gas), calculated according to the Eq. (3); and - the average LHV, according to Eq. (4), which is necessary for evaluating the overall efficiency, according to Eq. (5). Moreover, to provide a more in-depth understanding of the effect of LTC on the combustion completeness in both the CNG and HCNG cases, a new parameter was defined and analysed. This parameter was known as the unburned gas percentage (UGP), and was calculated according to Eq. (6). This determination was necessary, as the concentration of unburned H2 in the engine exhaust cannot be measured with a flame ionisation detector, which is widely used for accurate unburned hydrocarbon measurements, while a reduction in the THC, if part of the CNG is substituted with H2, is a direct consequence. The UGP compares the mass flow rate of the THC, which arises from the unburned species contained in the NG used to realise NG/H2 blends, with the fuel consumption of NG only, which is obtained from the mass flow rate of the Table 1 Main characteristics of engine.
(1)
2.2. Testing and data analysis procedures The test bench, on which the engine was tested under stationary conditions, featured an eddy current dynamometer, which was controlled to maintain a constant engine speed constant, and two electronic 3
Type
Four-stroke
Number of cylinders Displacement (cm3) Valves/cylinder Stroke (mm) Bore (mm) Compression ratio Torque @ Nm/rpm Power @ kW/rpm
4 1910 2 90.4 82 18:1 280/2000 77/5500
Applied Energy 253 (2019) 113602
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DPI, with the SOFPI reported in the figure, where the duration of the first pilot injection was equal to that of the second one (0.4 ms, approximately 5 CAD at 2000 rpm) and the start of the second pilot injection was delayed by approximately 1.7 ms compared to the first one. The diesel injection pressure was approximately 800 bar. It can be observed from the reported curves that the combustion switched from a conventional mechanism. By advancing the start of combustion (by increasing the SOFPI by 20 CAD, from 15 to 35 CAD), the heat release results advanced more rapidly to a different operating mode, in which further increases in the combustion advance (by increasing the SOFPI by 20 CAD, from 35 to 55 CAD) resulted in no corresponding increases in the maximum speed of the heat released. This final mechanism, which is representative of the LTC triggering; that is, of those reactions involving a chemical transformation of the fuel even before the actual flame fronts arise, causes more gradual combustion with respect to the conventional one. Therefore, further increasing the SOFPI can reduce the in-cylinder peak pressure, and consequently, the pressure gradient. Fig. 3b represents the range of all investigated pilot injections for all the tested fuels, together with the single pilot injection strategy for the CNG case at 100 Nm. In the following, SOPI indicates the start of the pilot injection for the diesel and CNG single pilot injection cases, while it corresponds to the SOFPI for the DPI cases. Starting from the SOPI closer to the TDC and moving to a higher injection advance, the combustion became increasingly rapid, which caused an increase in the maximum in-cylinder pressure gradient. At an SOPI of roughly 40 CAD before top dead centre (BTDC), the maximum pressure gradient progressed beyond the maximum allowable limit for the FD and DF with the single pilot injection cases, while for the DF with a DPI, it began to decrease and then settle on a lower value, as LTC was achieved. In Fig. 3a, the test results at 50 Nm are reported. The diesel injection was the same as for the 100 Nm DF cases, because the minimum diesel fuel flow rate for a stable combustion start was the same, while a slower main phase was obviously set for the FD case. Despite the reduced load compared to the 100 Nm operation, the 50 Nm FD cases exhibited a similar trend with the maximum pressure gradient. This is owing to the premixed combustion phase, which was not strongly influenced by the load. In fact, in the FD mode, the greatest difference between 50 and 100 Nm was observed in the diffusion combustion phase, where the pilot injection and injection pressure were approximately the same, while the main injection was reduced. Instead, for the DF cases, the maximum pressure gradient was reduced at 50 Nm with respect to 100 Nm, for both the conventional combustion and LTC mode, because of the lower concentration of gaseous fuel in the combustion chamber, and consequently, the reduction in the gas fuel portion burning close to the ignition. Even if the single pilot injection at a lower load could be used for the LTC strategy, without reaching the limit for the maximum pressure gradient, it is evident that the DPI ensured smoother combustion. Therefore, the splitting of the diesel pilot injection into two parts provides more favourable conditions for LTC, shifting the risk of a higher pressure gradient to greater engine loads, and thereby increasing
Table 2 Composition and main characteristics of CNG/H2. Name
CNG
HCNG15
HCNG25
Stoichiometric AFR Density (kg/sm3) LHV (MJ/kg) CH4 (% vol.) H2 (% vol.) H2 (% mass) CO2 & N2 (% vol.) H/C
15.9 0.83 46.3 85.4 0 0.00 3.4 3.7
16.5 0.68 48.2 77.3 15 1.98 2.5 4.2
17.4 0.58 51.4 72.1 25 3.88 1.0 4.6
gaseous fuels, weighed according to the NG mass concentration. In Eq. (6), yH2 is the mass fraction of H2 in the NG/H2 blend, as obtainable from Table 2. The UGP can range from 0% to 100% and provides an effective index of the combustion completeness for any NG/H2 blends, based on the flow rate measurement of the THC and gaseous fuels. Avg
=
%gas =
mair mdiesel · AFRdiesel + mgas · AFRgas
mgas mgas + mdiesel
LHVAvg = 1
g
=
·100
(3)
%gas %gas · LHVdiesel + ·LHVgas 100 100
Pbrake·3.6 ·100 LHVAvg ·mtot
UGP =
(2)
(4) (5)
mTHC mTHC = × 100 mNG mgas × (1 yH 2 )
(6)
Each measurement was repeated three times and the mean data values were stored. The accuracy of the testing method was established by performing the calculation of the percentage coefficient of variation (COV) of the acquired signals of three measurements. The results demonstrated less than 2% for the torque and mass flow measurement and almost 10% for the emission concentrations, while value of almost 25% was reached only for the particle number, as this parameter is significantly influenced by the combustion cyclic variation. 3. Results and discussion 3.1. Combustion development The rate of heat release (RHR) curves were calculated, starting from the in-cylinder pressure cycle, to analyse the combustion progress with the crank angle. In particular, the combustion rate was computed at different diesel injection laws, which are characterised in the following by the start of injection (SOI) for the FD cases and the start of the first pilot injection (SOFPI) for the DF cases. Fig. 2 presents a comparison among the RHR values at 100 Nm for the DF cases. The data refer to a Table 3 Instrumentation specifications for flow and emission measurement. Unit
Type
Range
Accuracy
PF MASS FLOW MF MASS FLOW AIR FLOW THC CH4 CO NOx CO2 Particles
AVL MOD 730 Coriolis MICRO MOTION ELITE Laminar flow meter CUSSONS BECKMANN HFID MOD 404 FID NGA2000 ABB URAS NDIR 14 EGA ABB LIMAS11 UV ABB URAS NDIR 14 EGA Cambustion DMS 500 electrical mobility detection
0–50 kg/h 0–50 kg/h 0–350 l/s 0–10,000 ppm C3 0–10,000 ppm C1 0–10% 0–5000 ppm 0–20% 5–1000 nm
± 0.15% of measurement < 1% of measurement ± 1% of measurement 0.5% of measurement range 0.5% of measurement range < 1% of measurement < 1% of measurement 1% of measurement Number: 10% Size: 5% (5–300 nm), 10% (> 300 nm) of measurement
4
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Fig. 1. Scheme of experimental setup.
efficiency, particularly regarding the maximum values achievable with conventional combustion and advanced pilot injection timing. In Fig. 5a, the maximum efficiency for the DF cases corresponds to approximately 32 SOFPI for both 50 and 100 Nm, which is an injection advance that is not sufficient for LTC. Further increasing the SOFPI to achieve LTC generally allows for maintaining the same efficiency levels. At 100 Nm, in the DF and LTC modes, the engine efficiency was close to that of the FD mode. This could indicate an effective result, because such values could be reached only in a heavy-duty spark ignition engine fuelled with gaseous fuels. Conventional light-duty spark ignition engines with the same displacement as that of the tested engine exhibit a lower maximum efficiency. At a lower load (50 Nm), the efficiency was significantly reduced with respect to the FD mode. This aspect is characteristic of DF combustion, and it is strictly related to the average air index, as reported in Fig. 5b, which reached a value close to the flammable limit at a lower load. Despite the lean mixture, the combustion development was quite stable, as identified by the value of the IMEP COV reported in Fig. 6, at less than 6% for almost all the tests. For most anticipated pilot injections, the combustion rapidly becomes unstable, and for a small further increase in the advance, the engine stops, even if the combustion remains effectively phased, resulting in the first at the beginning of the main combustion (HR10), which is still sufficiently close to the TDC. Therefore, this behaviour could be attributed to the ignition phase. This could be a result of the first pilot injection, which does not take part in the combustion process, and therefore, the second pilot injection is not sufficient to start the combustion. In fact, in a diesel engine, where the combustion chamber is formed in the piston crown, a strong SOFPI could cause part of the injected fuel to reach the cylinder wall or piston squish area directly, without proper mixing with the air and gaseous fuel.
Fig. 2. Comparison of RHR curves for DF mode at 100 Nm when varying SOFPI and MF.
the engine map range within which LTC is a practicable solution. In fact, the pilot diesel fuel resulted in greater mixing within the air/ gaseous fuel, reducing the local air index but increasing the number of areas in which the low-temperature chemical kinetic reactions could occur, with beneficial effects on the combustion development. The smoother combustion process allows for moving the crank angle at which the thermal flame reactions start away from the diesel pilot injection, as indicated by HR10 in Fig. 4, which represents the angle at which 10% of the fuel was burned. The main gaseous fuel composition appeared not to affect the HR10 position, and therefore on the chemical kinetics-controlled phase of the combustion, with the values being practically coincident. Therefore, the small differences observable in the maximum pressure gradient of Fig. 3, particularly for the HCNG25 mixture, which exhibited higher values, could be related to the slightly faster propagation during the rapid second stage, once the pre-mixed combustion had spread to a consistent fraction of the charge. Activating LTC does not involve a reduction in the engine global
3.2. Toxic and global gaseous emissions One of the most important goals to be achieved with improved combustion with respect to conventional combustion is the reduction in exhaust emissions at the same engine performance. The measurements and comparisons of engine pollutants are reported in the following as 5
Applied Energy 253 (2019) 113602
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Fig. 3. Pressure gradient for different SOPI values in FD and DF modes at (a) 50 Nm and (b) 100 Nm, varying MF and using DPI compared to single pilot injection.
Fig. 4. Start of combustion in FD and DF modes at 100 Nm when varying MF and using DPI compared to single pilot injection.
Fig. 6. Effects of SOPI on IMEP COV at 100 and 50 Nm in FD and DF modes with different MFs.
volume concentrations. This was possible because the flow rate of the intake air was maintained approximately the same in all the operation modes: FD, DF with NG, and DF with NG-H2 blends. Therefore, the
exhaust volumetric flow rate was almost constant, making it possible to use the emission data measured by the instrumentation directly, and avoiding the calculation on a mass basis.
Fig. 5. Effects of SOPI on (a) brake thermal efficiency and (b) average air index at 100 and 50 Nm in FD mode and DF with different MFs. 6
Applied Energy 253 (2019) 113602
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The CO emissions are presented in Fig. 7. In general, both the incomplete combustion and low reaction temperatures are the causes of high CO emissions in internal combustion engines. The CO emissions were higher for the DF cases than those of FD, but a consistent reduction was obtained when advancing the diesel pilot injections. Increasing the injection advance generates higher combustion temperatures when the combustion process is dominated by flame front propagation, which is typical of DF combustion with a higher gaseous fuel share, such as a spark ignition engine, and improves the oxidation process. When LTC was achieved at an earlier SOFPI, the CO trend stabilised close to the minimum value reached with conventional combustion and advanced injection timing. If the pilot injection is so advanced as to cause nonignition of the first injection, the combustion will be incomplete and the CO will increase, which is what occurred in the DF-CNG points at 50 Nm with an earlier SOFPI. The THC emissions are reported in Fig. 8a, and exhibit a similar trend to those of CO. The very lean air/gaseous fuel mixture, particularly at a low load, caused incomplete combustion in the areas far from the pilot diesel fuel jets and in the squish zone, and resulted in flame quenching close to the combustion chamber wall, which increased both the THC and CO despite excess oxygen. With an increasing injection advance, the greater combustion temperature allows for more efficient oxidation, until the LTC mode predominates and the emissions are stabilised. The hydrogen content had a positive effect on the THC emissions, at both the medium and low loads tested. With reference to the UGP, as reported in Fig. 8b, the positive effect (a THC reduction higher than the CNG substitution with H2) could be attributed to more complete combustion, owing to the lower ignition energy of H2, which could promote combustion in the leaner zone, as well as the faster combustion process, which contributed to reducing the quench layer thickness. The CO emissions could be drastically reduced with a relatively simple oxidation catalyst, while the THC, mainly constituted by methane, requires strong concentrations of noble metals to be oxidised, particularly at lower temperatures, which is a limitation for DF technology because of the excess air. However, methane emissions represent a real problem only for the global warming effect and not for human health, as methane is a non-toxic gas. Therefore, the CO2 and equivalent CO2, obtained by considering 1 g of methane in the exhaust gas, equivalent to 28 g of CO2 (IPCC Fifth Assessment Report), and 1 g of non-methane HC, equivalent to 7 g of CO2, are reported in Fig. 9. Firstly, it is evident that, despite the CO2 for the DF cases always being lower than the CO2 in the FD mode, the equivalent CO2 could be significantly higher. However, the lowest values were achieved under the LTC combustion regime, from both the medium and low loads. Moreover, at 100 Nm, the equivalent CO2 reached values very close to the FD case, particularly for the HCNG25, even without any post-treatment. Evidently, to obtain an equivalent effective CO2 reduction, the manner in which H2 is obtained as an energy vector is crucial. Finally, the NOx emissions are presented in Fig. 10. The NOx production is strictly dependent on the maximum in-cylinder temperature and oxygen concentration. At a low load, a sensible reduction in NOx in the DF with respect to the FD mode was achieved for all three compositions. In contrast, at the medium load, even if the activation of the LTC led to a reduction trend of the NOx, it still remained at a high level, particularly for gaseous mixtures with a higher H2 content. In the FD mode, the earlier combustion owing to the advanced injection caused increasing temperature peaks, and consequently, the NOx increased. This also occurred for the DF case, but the possibility of using a higher injection advance to achieve LTC resulted in a reduction trend of the NOx emissions, which reached a level close to zero in the case of the lower load. Therefore, LTC could be useful in DF engines for reducing the trade-off between NOx and THC emissions, which occurs when the injection advance is increased, while remaining in the conventional combustion regime, but it is essential to permit a higher injection advance. In the tests, the maximum SOFPI was limited from the higher IMEP COV values, probably owing to the first pilot injection
impingement on the piston squish area or cylinder wall. To increase the maximum SOFPI, different strategies could be investigated, such as a higher pilot injection pressure, which implies strong atomisation of the diesel pilot, and/or a different combustion chamber shape. In Fig. 11, several results of the tests carried out to evaluate the feasibility of a higher injection pressure to extend the LTC regime are presented. The pressure injection ranged from 380 to 1150 bar for the HCNG25 case at 50 Nm, with a SOFPI at approximately 55 CAD BTDC. The combustion development illustrated in Fig. 11a was not strongly influenced when the pressure injection increased from 800 to 1150 bar, or decreased to 380 bar, but in this final case with a lower pressure peak. In the range of 450 to 620 bar, the pressure peaks were similar to those of the case with 380 bar; however, they were reached slightly earlier. However, several effects of the injection pressure on the chemical reactions can be observed from the emission data in Fig. 11b. Although the highest injection pressure permitted by the experimental setup was limited, it can be noted that improvements in both the NOx and THC emissions were achievable with pressure injections higher than 800 bar. In contrast, lower injection pressures led to inferior combustion conditions in terms of the trade-off between NOx and THC. 3.3. Particle characterisation Particle emissions were compared as a concentration, because the exhaust dilution flow rates were almost the same for all of the test cases at 100 Nm, as illustrated previously. The number and mass spectra for the DF and FD operations with a SOPI at 25 CAD BTDC are illustrated in Fig. 12a. The weighted spectral curves reached the maximum concentration for particles with larger diameters compared to those of the numerical spectrum. This is a consequence of the fact that, although the small-diameter particles (10–100 nm) were more abundant, the major contribution in terms of mass was provided by particles with larger diameters (100–1000 nm). A significant difference is noticeable between the FD and DF spectra. The area underlying the spectra of the continuous lines, which represents the total particle number emission per volume unit, is plotted in Fig. 12b for all the tested cases. When the diesel fuel injection was not advanced, the main injection in the FD mode, which implies roughly the 95% of the fuel mass, underwent a relatively high share of the diffusion combustion mode, which was not present in the DF mode, where the primary fuel was in the gaseous state. At earlier injections, the pre-mixed combustion phase of the diesel fuel entering the chamber during the main injection became the most relevant, leading to a strong reduction in the particles, and reaching the levels of the DF cases. Thus, both the number and mass of particles emitted in the FD operation were significantly higher than those of the DF modes, when the SOPI was close to the TDC. In the DF operation, the
Fig. 7. Effects of SOPI on CO emissions at 100 and 50 Nm in FD and DF modes with different MFs. 7
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Fig. 8. Effects of SOPI on (a) THC emissions and (b) UGP at 100 and 50 Nm in FD and DF modes with different MFs.
nanoparticles, with a diameter of less than 50 nm, for both the FD and DF cases. Nanoparticles, as ultrafine particles, are considered very harmful to human health. A more in-depth investigation should be carried out to investigate the characteristics of the nanoparticles emitted in the FD and DF modes. In fact, in addition to the size, other characteristics of nanoparticles that are relevant to health effects are the chemical composition, surface characteristics, and shape [30]. The particles emitted in the DF mode could have less chemical substances adsorbed on their surfaces, owing to the low reactivity of methane and hydrogen compared to the long chain hydrocarbons of the FD mode. However, it is crucial to reduce the number, in both the combustion chamber and with a particulate filter at the exhaust, with the aim of reducing the exposition risk. The LTC mode was proven to be a viable solution, because it did not exhibit negative effects on the particle number production. Fig. 9. Effects of SOPI on CO2 and equivalent CO2 emissions at 100 and 50 Nm in FD and DF modes with different MFs.
4. Conclusions With the aim of improving the DF combustion process, an in-depth experimental campaign was carried out to investigate the feasibility of LTC, and its effects on the performance and emissions, on a compression ignition light-duty engine operated in the DF mode, using NG as primary fuel, adding H2 up to 25% by volume, and triggering combustion through a DPI. The DF tests were performed at the maximum percentage of diesel oil replacement, close to 80% of gaseous fuel on the energy basis, corresponding to the minimum amount of pilot fuel required for stable ignition. It was proven that it is possible to activate a stable LTC regime by splitting the pilot injection into two close steps, and increasing the injection advance by approximately 40–60 CAD BTDC. The DPI allowed for controlling the maximum pressure gradient, maintaining it at lower than 15 bar/deg up to 100 Nm of the engine torque output. The NOx emissions initially increased with the increasing pilot injection advance, and then, once reaching the conditions for LTC, began to decrease until a higher injection advance rapidly led to combustion instability. This was probably owing to the first pilot injection impingement on the cylinder wall or piston squish area, which compromised the ignition phase. To improve the ignition at an earlier pilot injection advance, a higher injection pressure was used. Although certain improvements were achieved, the investigation was not exhaustive owing to the limitations of the experimental setup (the maximum injection pressure and combustion chamber geometry). Therefore, a dedicated combustion chamber with a different volume and/or shape could be investigated to enhance the limit on the maximum injection advance that is achievable
Fig. 10. Effects of SOPI on NOx emission at 100 and 50 Nm in FD and DF modes with different MFs.
total particle concentration was less affected by the diesel fuel injection timing, probably because of the small amount of diesel fuel injected, which required a negligible diffusion flame phase, as with the SOFPI closer to the TDC. The GMD, as reported in Fig. 13, characterises the mean particles as 8
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Fig. 11. Effects of pilot injection pressure on (a) combustion development and (b) trade-off between NOx and THC in LTC regime.
prior to ignition problems occurring. This could be crucial for reaching an NOx of almost zero, together with high efficiency and CO2 equivalent emissions close to the FD mode, which was proven to be possible by adding 25% of H2 to the primary fuel. The activation of the LTC had no negative effects on the particle emissions, which resulted in close to the minimum value being reachable using an FD operation with higher injection advance, maximising the burning of the diesel fuel during the pre-mixed phase. In conclusion, the activity demonstrated that the LTC and DF technologies can achieve remarkable results in terms of high efficiency and low emissions, with engines set for a lower maximum power than that allowed by the available displacement. The achievable performance would not meet the requirements of engines used for transport, which take full advantage of supercharging, with a significant increase in the torque and power delivered per volume unit. However, the technology investigated in this work appears to be promising for vehicle propulsion with such hybrid systems, in which the engine can supply the average power level in an almost stationary manner, or for the generation of small and medium-scale electricity with engines powered by fuels of various types and compositions (biological or synthetic, with a low heat content, or energy carriers in general). In this scenario, specifically designed engines should be developed to expand the conditions for LTC activation beyond those obtained in the
Fig. 13. Effect of SOPI on GMD at 100 and 50 Nm in FD and DF modes with different MFs.
experimental activity, which was limited by the maximum injection pressure of the pilot fuel and combustion chamber shape, and optimised for diesel fuel.
Fig. 12. (a) Particle spectrum at 25 SOPI and (b) effect of SOPI on total particle number at 100 Nm in FD and DF modes with different MFs.
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Acknowledgements [20]
The authors would like to thank Vicenzo Bonanno for his valuable support in experimental activities.
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Glossary AFR: Air fuel ratio BTDC: Before top dead centre CAD: Crank angle degree COV: Coefficient of variation DF: Dual fuel Dp: Particle diameter FD: Full diesel GMD: Geometric mean diameter HCNG: Hydrogen-enriched compressed natural gas IMEP: Indicated mean effective pressure LHV: Lower heating value LTC: Low-temperature combustion MF: Main fuel N: Number of particles NG: Natural gas PF: Pilot fuel PM: Particulate matter RHR: Rate of heat release SOFPI: Start of first pilot injection SOPI: Start of pilot injection TDC: Top dead centre THC: Total hydrocarbons
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Gaseous and particle emissions in low-temperature combustion diesel–HCNG dual-fuel operation with double pilot injection
T
Luigi De Simio , Sabato Iannaccone ⁎
Istituto Motori, National Research Council, Via Marconi 4, 80125 Napoli, Italy
HIGHLIGHTS
GRAPHICAL ABSTRACT
test of low-temperature • Experimental combustion and dual-fuel operation. engine fueled using NG • Reciprocating as primary fuel and H up to 25% by 2
• •
volume. Ignition triggered by double pilot injection of diesel fuel to reduce pressure rise. Benefit on NOx and equivalent CO2, without negative effects on particles number.
ARTICLE INFO
ABSTRACT
Keywords: Double-pilot injection Dual fuel Natural gas Hydrogen Particle emission
Alternative fuels and energy vectors are becoming increasingly important in terms of technical, geopolitical, economic, and environmental aspects. In particular, gaseous fuels and vectors, such as fossil or synthetic natural gas (NG) blended with hydrogen, commonly help provide optimal strategies to reduce global and toxic emissions of internal combustion engines, owing to their adaptability, anti-knock capacity, lower toxicity of pollutants, reduced CO2 emissions, and costeffectiveness. However, diesel engines still represent the reference category among internal combustion engines in terms of maximum thermodynamic efficiency. The possibility offered by dual-fuel (DF) systems to combine the efficiency and performance of diesel engines with the environmental advantages of gaseous fuels has been the subject of extensive investigations. However, the simple replacement of diesel fuel with gaseous fuel does not allow for optimising the engine performance, owing to the high percentage of unburned gaseous fuel, which compromises the potential reduction of CO2; therefore, more complex combustion strategies should be realised. In this study, with the aim of improving the DF combustion process, an experimental investigation was performed to analyse low-temperature combustion (LTC), using NG and two enriched hydrogen-compressed NG blends as primary fuels. The LTC mode was activated by means of a very early advanced pilot injection and carried out in two close steps. The double pilot injection was used to control the energy release rate in the first combustion stage, thereby minimizing the increase of the rate of pressure and allowing the extension of the operation range under LTC. The experimental activity was also focused on analysing the particle emissions, as it is well known that these emissions, together with those of nitrogen oxide, constitute the main pollutants resulting from diesel fuel combustion. The results demonstrated the potential to reduce the unburned fuel, NOx, and particle emissions simultaneously, while maintaining equivalent CO2 emissions to a diesel-only engine. Both the timing and pressure of the pilot injection proved to be critical parameters for optimising the emissions and performance.
⁎
Corresponding author. E-mail address: [email protected] (L. De Simio).
https://doi.org/10.1016/j.apenergy.2019.113602 Received 21 March 2019; Received in revised form 18 July 2019; Accepted 20 July 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.
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1. Introduction
three-way catalyst after-treatment rather than by means of high EGR. Abdelaal et al. analysed the effects of EGR on the oxygen excess at the exhaust on a DF diesel engine, and they identified thermal efficiency that was comparable to the conventional diesel mode [15]. In [16], the different effects of EGR (thermal, chemical and radical effects) were analyzed, finding that the thermal effect was the main reason for reducing NOx. In [17], the effects of cooled EGR were investigated and it was found that it could improve the brake thermal efficiency if the EGR was less than 5%. The EGR effect on abnormal combustion was analysed in [18], also considering the regulation of the incoming air by means of a throttle valve, and in [19], also considering the cooling of the EGR. In both cases, a substantial reduction in the pressure gradient was determined. At present, investigations have mainly focused on fuel injection strategies and the MF composition. Mann et al. proposed a system with direct gas injection to optimise the engine, achieving regulated emissions below the EPA 2010 levels over the 13-mode steady-state cycle [20]. Zhang et al. used an early PF injection in a heavy-duty DF engine with a 14:1 compression ratio to achieve LTC. The engine was operated up to 11.6 bar of brake mean effective pressure, not exceeding a pressure gradient of 15 bar/deg [21]. An LTC model was created by Krishnan et al. to predict the onset of ignition and the number of ignition centres [22]. Furthermore, the use of hydrogen or hydrogen/NG blends as primary fuel provides great opportunities for optimising DF combustion. In the initial studies on DF engines, the use of pure hydrogen was investigated, with particular reference to the knock because of its intensity, as reported in [23]. More recently, the use of such fuels has gained renewed interest in terms of smoke emissions and CO2 reduction. Karagoz et al. introduced hydrogen into the engine with 25% and 50% of the total charge, with the energy achieving a significant decrease in smoke emissions, even with a dramatic increase in the nitrogen oxides and maximum peak pressure [24]. However, the use of pure hydrogen as the primary fuel, which provides increased efficiency with respect to the FD mode, exhibits certain limitations on the input energy fraction, owing to the problem of pre-ignition occurring before the pilot diesel fuel injection [25]. Finally, the diesel fuel injection mode is a crucial factor. This aspect is related not only to the injection phasing and quantity, but also the pilot diesel injection profile, which is a topic of this study. In particular, the effect of a pilot split to realise a double pilot injection (DPI) is investigated. The DPI has proven to be a useful strategy for diesel engine optimisation, particularly for reducing the combustion noise, because the fuel burns with a more regular combustion rate [26]. This final aspect, which is strictly related to the heterogeneous combustion of the main diesel fuel injection in a diesel engine, would be significantly modified in a DF engine, where the main fuel is in a gaseous state, premixed with air, and involved in combustion though the propagation of flame fronts. Splitter et al. used a DPI such that 55% of the directinjected fuel was in the first pulse and 45% of the direct-injected fuel was in the second pulse on a single-cylinder 2.44 L engine [27]. Engine experiments were performed using port-fuel-injection of isooctane, and direct-injections of n-heptane with premixed isooctane air index approximately 3.3 (indicating a low load) in LTC regime. They found that at later injection timings that avoid liner impingement, double injections proved to be advantageous, thanks to the more distributed mixture of the direct-injected fuel. Bae et al. implemented a DPI strategy on a single-cylinder 0.98 L diesel engine under the low load condition of premixed charge compression ignition combustion [28]. The experimental results, indicated that a combination of an NG substitution of 40%, the DPI strategy of diesel, and a moderate EGR rate effectively reduced the HC and CO emissions and improved thermal efficiency. The DPI was used in the activity of this study to determine the effects of the formation of ignition sources on the DF combustion both at low and medium load of a multi-cylinder engine with the maximum possible substitution of diesel fuel with gaseous one, which represents the most severe conditions due to the considerable pressure gradients. Therefore,
Although internal combustion engines are expected to remain the major power sources for propulsion systems in the near future, strong and ever-increasing electrification of the powertrain has been occurring for several years. The aim of this electrification, in the form of hybrid systems or full electric systems, is to reduce toxic pollutants, particularly in urban areas, and reduce global CO2 emissions. In the future, an increasing number of hybrid thermal-electric systems is expected to emerge, owing to their higher flexibility compared with full electric systems; for example, in the autonomy of vehicles or the availability of the electric grid and materials for batteries production. Hybrid systems can significantly reduce the emissions and fuel consumption of the thermal unit by means of integrated electric and mechanic management, which can simplify the internal combustion engine operation. In fact, a thermal engine can operate with high efficiency, avoiding idling and part load operations when it operates as a battery charger, or it can be assisted by electric motor–generators, with the aim of reducing fuel consumption. Moreover, the increasing number of fully electric vehicles could lead to the development of small and medium-sized generators for electricity production from renewable sources or energy vectors. In this scenario, DF combustion strategy may attract significant interest. DF technology offers the simplest method for using a gaseous fuel in a diesel engine with a high compression ratio or a highly efficient diesel engine, in which a liquid fuel with a high cetane number is directly injected into the combustion chamber as a pilot source to ignite fuel with a high octane number fuel, which is the main fuel (MF). Either liquid or gaseous fuels may be used as the MF; however, gaseous fuels are generally favoured [1,2], such as natural gas (NG) or bio-methane, owing to the possibility of reducing CO2 emissions. The combustion process is a combination of the diffusive type, in which the liquid pilot fuel (PF) is the most involved, and a homogenous propagation type, which through numerous flame fronts extend the combustion from the ignition centres close to the PF to the entire combustion chamber [3,4]. This technology is very useful for emission reduction and fuel consumption; however, only in medium load areas of the engine operation map. At lower and higher loads, problems of incomplete or knocking combustion may occur, forcing the engine to be switched to full diesel (FD) mode. Such problems have been thoroughly investigated in several studies. Karim et al. analysed the effect of the total equivalence ratio (the stoichiometric mass air flow rate for the PF and MF divided by the actual mass air flow rate) [5] to propose a method for predicting the lean operational limits for DF combustion. The effect of the gaseous energy shares on the thermal efficiency enables the DF to exhibit efficiency similar to that of the diesel case, but only for high loads, as reported in [6–8]. The effect of the fast combustion achieved by DF on the risk of knock owing to end gas autoignition was analysed in [9,10], and it was found that increasing the mass of the gaseous fuel used increased the combustion noise. The most suitable applications of this technology could be in generators or the propulsion of hybrid vehicles, in which low and high loads can be performed with electric motors, while a DF engine of an appropriate size could output the mean power. The goal is to extend the load range in which DF combustion could be applied. Adb et al. [11] used a pilot injection with an increasing advance to achieve improved penetration of the PF jet inside the intake charge, with the aim of incorporating a greater amount of the air/gas mixture into the spray prior to the ignition process, and increasing the MF share involved in the combustion. In [12], the pilot injection timing was optimised to reduce the NOx, THC, and PM emissions. To reduce the minimum load at which DF could be used in a multi-cylinder engine substantially, the strategies of a skip-fire [13], which consists of increasing the load for certain cylinders and deactivating the others, could be followed. At high loads, exhaust gas recirculation (EGR) has been proven to be useful for DF mode extension. In [14], the authors used EGR instead of throttling to control the load in a stoichiometric diesel and gasoline DF engine, controlling the NOx emissions with a 2
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the potential of the DPI for realising LTC, by overcoming the problems of the NOx increasing and abnormal combustion occurring, was tested in a light-duty four-cylinder DF engine. The engine was fuelled with NG and two NG/hydrogen blends to explore the consequences of the fuel composition on the DPI strategy. The activity demonstrated the possibility of extending the LTC operation range with a DPI by retaining the maximum gradient of the pressure increase under 15 bar/deg. Effects of LTC on PM were also investigated by means of analysing the particle emissions at the exhaust, which are part of the regulated emissions for the majority of modern engines.
modules, which were used to set the engine load by adapting the fuel flows. The engine speed was set to 2000 rpm, while two different loads, namely 50 and 100 Nm, were considered for the FD and DF modes, respectively. The DF operation was performed with the highest possible gaseous fuel percentage, by decreasing the diesel fuel flow rate at the minimum quantity required for a stable combustion start. Therefore, the percentage of energy introduced into the gaseous fuel, compared to the total introduced also considering the diesel fuel, was found to be 83% and 75% at 100 and 50 Nm, respectively, and was almost the same for all of the tested mixtures. Owing to the dependence of the lower heating value (LHV) on the H2 content, as reported in Table 2, several differences can be noted if the mass percentage of the gaseous fuel compared to the total is considered. In fact, the gas fuel percentages were 82%, 80%, and 78% at 100 Nm, and 74%, 72%, and 70% at 50 Nm for CNG, HCNG15, and HCNG25, respectively. The pilot control strategy was modified to switch from normal combustion to LTC by setting a large advance and dividing the injection into two separate pilot injections. In this manner, it was possible to test the engine under reliable conditions, as the pressure gradient in the combustion chamber did not exceed 15 bar/CAD, which is generally considered as a limit value to avoid excessive mechanical and thermal stresses. The LTC was only activated for the DF mode, as this was the specific purpose of the study, while the injection advance was increased in the FD by means of simple translation of the injector control signal, until the maximum limit for the pressure gradient was reached. Therefore, a completely different injection law, aimed at obtaining the LTC in the FD case, was not investigated, as it was not an objective of this study, and data in the FD mode can be considered as a reference for normal combustion with diesel fuel only. A complete scheme of the experimental setup is presented in Fig. 1. The data collected during the test were processed to derive several parameters that are particularly useful for describing the DF mode:
2. Experimental setup and procedures 2.1. Reference engine, fuels, and instrumentation To assess the potential of the DPI for DF combustion, a multi-cylinder diesel engine was used, which was converted to operate in the DF mode. The engine was a light-duty four-cylinder diesel type (Table 1), with the diesel injection split into a pilot and main step. The DF mode was achieved by adding a timed gas injection system with four injectors, which introduced the fuel close to the intake valve of each cylinder during the corresponding intake stroke to prevent fuel bypassing of the combustion chamber owing to the exhaust-intake valve overlapping, which is generally significant in diesel engines. There was a glow plug in each combustion chamber to heat the air and aid in cold starting. The housing of one of these was used for the in-cylinder pressure cycle measurement through a piezoelectric transducer (Kistler 6058) and Kistler amplifier (Charge Amplifier, Type 5064) with a resolution of 0.1 crank angle degrees (CAD). The engine was fuelled with commercial diesel fuel, NG, and NG-H2 blends, with the compositions and properties listed in Table 2. Regarding the gaseous fuels, the remaining part of the composition consisted of hydrocarbons that are heavier than methane, such as the propane and ethane contained in NG. By increasing the H2 content, additional air was required to burn a fuel unit stoichiometrically, while less fuel was necessary to enter a unit of energy. Emission characterisation is essential for assessing the benefits of the DF mode with respect to the FD mode, as well as strengthening the LTC mechanism. In fact, the scope of LTC involves reducing both the unburned fuel and NOx, without increasing the particle emissions. During the tests, the instrumentation reported on in Table 3 was used. For the particle characterisation, the sample gas was analysed using a Cambustion DMS500 fast particulate spectrometer. This instrument uses a sorting plant column with 22 rings, which processes the current signals acquired by annular electrodes to provide a discrete numerical spectrum as a function of the sample flow diameter of the particles per cubic centimetre, expressed in dN/dlogDp/cm3, where N is the number and Dp is the diameter (in nm) of the particles. According to the spectral measurement, it is possible to obtain the distribution of the particles discerning the nucleation phase, in which the diameter of particles ranges from 5 to 100 nm, and the accumulation phase, with particles between 100 and 1000 nm, where the total number of particles and geometric mean diameter (GMD) of the particles is assumed to be spherical. The particle mass (M), in μg, was evaluated using Eq. (1), which assumes a rapid reduction in density of particles with diameters of 5–200 nm, and a slow reduction for larger particles, in agreement with [29].
M = 2.20·10
15· D 2.65 p
- the average air index, defined by Eq. (2), which is a parameter that allows for calculating the mean fuel in the combustion chamber; - the percentage of gaseous fuel (% gas), calculated according to the Eq. (3); and - the average LHV, according to Eq. (4), which is necessary for evaluating the overall efficiency, according to Eq. (5). Moreover, to provide a more in-depth understanding of the effect of LTC on the combustion completeness in both the CNG and HCNG cases, a new parameter was defined and analysed. This parameter was known as the unburned gas percentage (UGP), and was calculated according to Eq. (6). This determination was necessary, as the concentration of unburned H2 in the engine exhaust cannot be measured with a flame ionisation detector, which is widely used for accurate unburned hydrocarbon measurements, while a reduction in the THC, if part of the CNG is substituted with H2, is a direct consequence. The UGP compares the mass flow rate of the THC, which arises from the unburned species contained in the NG used to realise NG/H2 blends, with the fuel consumption of NG only, which is obtained from the mass flow rate of the Table 1 Main characteristics of engine.
(1)
2.2. Testing and data analysis procedures The test bench, on which the engine was tested under stationary conditions, featured an eddy current dynamometer, which was controlled to maintain a constant engine speed constant, and two electronic 3
Type
Four-stroke
Number of cylinders Displacement (cm3) Valves/cylinder Stroke (mm) Bore (mm) Compression ratio Torque @ Nm/rpm Power @ kW/rpm
4 1910 2 90.4 82 18:1 280/2000 77/5500
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DPI, with the SOFPI reported in the figure, where the duration of the first pilot injection was equal to that of the second one (0.4 ms, approximately 5 CAD at 2000 rpm) and the start of the second pilot injection was delayed by approximately 1.7 ms compared to the first one. The diesel injection pressure was approximately 800 bar. It can be observed from the reported curves that the combustion switched from a conventional mechanism. By advancing the start of combustion (by increasing the SOFPI by 20 CAD, from 15 to 35 CAD), the heat release results advanced more rapidly to a different operating mode, in which further increases in the combustion advance (by increasing the SOFPI by 20 CAD, from 35 to 55 CAD) resulted in no corresponding increases in the maximum speed of the heat released. This final mechanism, which is representative of the LTC triggering; that is, of those reactions involving a chemical transformation of the fuel even before the actual flame fronts arise, causes more gradual combustion with respect to the conventional one. Therefore, further increasing the SOFPI can reduce the in-cylinder peak pressure, and consequently, the pressure gradient. Fig. 3b represents the range of all investigated pilot injections for all the tested fuels, together with the single pilot injection strategy for the CNG case at 100 Nm. In the following, SOPI indicates the start of the pilot injection for the diesel and CNG single pilot injection cases, while it corresponds to the SOFPI for the DPI cases. Starting from the SOPI closer to the TDC and moving to a higher injection advance, the combustion became increasingly rapid, which caused an increase in the maximum in-cylinder pressure gradient. At an SOPI of roughly 40 CAD before top dead centre (BTDC), the maximum pressure gradient progressed beyond the maximum allowable limit for the FD and DF with the single pilot injection cases, while for the DF with a DPI, it began to decrease and then settle on a lower value, as LTC was achieved. In Fig. 3a, the test results at 50 Nm are reported. The diesel injection was the same as for the 100 Nm DF cases, because the minimum diesel fuel flow rate for a stable combustion start was the same, while a slower main phase was obviously set for the FD case. Despite the reduced load compared to the 100 Nm operation, the 50 Nm FD cases exhibited a similar trend with the maximum pressure gradient. This is owing to the premixed combustion phase, which was not strongly influenced by the load. In fact, in the FD mode, the greatest difference between 50 and 100 Nm was observed in the diffusion combustion phase, where the pilot injection and injection pressure were approximately the same, while the main injection was reduced. Instead, for the DF cases, the maximum pressure gradient was reduced at 50 Nm with respect to 100 Nm, for both the conventional combustion and LTC mode, because of the lower concentration of gaseous fuel in the combustion chamber, and consequently, the reduction in the gas fuel portion burning close to the ignition. Even if the single pilot injection at a lower load could be used for the LTC strategy, without reaching the limit for the maximum pressure gradient, it is evident that the DPI ensured smoother combustion. Therefore, the splitting of the diesel pilot injection into two parts provides more favourable conditions for LTC, shifting the risk of a higher pressure gradient to greater engine loads, and thereby increasing
Table 2 Composition and main characteristics of CNG/H2. Name
CNG
HCNG15
HCNG25
Stoichiometric AFR Density (kg/sm3) LHV (MJ/kg) CH4 (% vol.) H2 (% vol.) H2 (% mass) CO2 & N2 (% vol.) H/C
15.9 0.83 46.3 85.4 0 0.00 3.4 3.7
16.5 0.68 48.2 77.3 15 1.98 2.5 4.2
17.4 0.58 51.4 72.1 25 3.88 1.0 4.6
gaseous fuels, weighed according to the NG mass concentration. In Eq. (6), yH2 is the mass fraction of H2 in the NG/H2 blend, as obtainable from Table 2. The UGP can range from 0% to 100% and provides an effective index of the combustion completeness for any NG/H2 blends, based on the flow rate measurement of the THC and gaseous fuels. Avg
=
%gas =
mair mdiesel · AFRdiesel + mgas · AFRgas
mgas mgas + mdiesel
LHVAvg = 1
g
=
·100
(3)
%gas %gas · LHVdiesel + ·LHVgas 100 100
Pbrake·3.6 ·100 LHVAvg ·mtot
UGP =
(2)
(4) (5)
mTHC mTHC = × 100 mNG mgas × (1 yH 2 )
(6)
Each measurement was repeated three times and the mean data values were stored. The accuracy of the testing method was established by performing the calculation of the percentage coefficient of variation (COV) of the acquired signals of three measurements. The results demonstrated less than 2% for the torque and mass flow measurement and almost 10% for the emission concentrations, while value of almost 25% was reached only for the particle number, as this parameter is significantly influenced by the combustion cyclic variation. 3. Results and discussion 3.1. Combustion development The rate of heat release (RHR) curves were calculated, starting from the in-cylinder pressure cycle, to analyse the combustion progress with the crank angle. In particular, the combustion rate was computed at different diesel injection laws, which are characterised in the following by the start of injection (SOI) for the FD cases and the start of the first pilot injection (SOFPI) for the DF cases. Fig. 2 presents a comparison among the RHR values at 100 Nm for the DF cases. The data refer to a Table 3 Instrumentation specifications for flow and emission measurement. Unit
Type
Range
Accuracy
PF MASS FLOW MF MASS FLOW AIR FLOW THC CH4 CO NOx CO2 Particles
AVL MOD 730 Coriolis MICRO MOTION ELITE Laminar flow meter CUSSONS BECKMANN HFID MOD 404 FID NGA2000 ABB URAS NDIR 14 EGA ABB LIMAS11 UV ABB URAS NDIR 14 EGA Cambustion DMS 500 electrical mobility detection
0–50 kg/h 0–50 kg/h 0–350 l/s 0–10,000 ppm C3 0–10,000 ppm C1 0–10% 0–5000 ppm 0–20% 5–1000 nm
± 0.15% of measurement < 1% of measurement ± 1% of measurement 0.5% of measurement range 0.5% of measurement range < 1% of measurement < 1% of measurement 1% of measurement Number: 10% Size: 5% (5–300 nm), 10% (> 300 nm) of measurement
4
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Fig. 1. Scheme of experimental setup.
efficiency, particularly regarding the maximum values achievable with conventional combustion and advanced pilot injection timing. In Fig. 5a, the maximum efficiency for the DF cases corresponds to approximately 32 SOFPI for both 50 and 100 Nm, which is an injection advance that is not sufficient for LTC. Further increasing the SOFPI to achieve LTC generally allows for maintaining the same efficiency levels. At 100 Nm, in the DF and LTC modes, the engine efficiency was close to that of the FD mode. This could indicate an effective result, because such values could be reached only in a heavy-duty spark ignition engine fuelled with gaseous fuels. Conventional light-duty spark ignition engines with the same displacement as that of the tested engine exhibit a lower maximum efficiency. At a lower load (50 Nm), the efficiency was significantly reduced with respect to the FD mode. This aspect is characteristic of DF combustion, and it is strictly related to the average air index, as reported in Fig. 5b, which reached a value close to the flammable limit at a lower load. Despite the lean mixture, the combustion development was quite stable, as identified by the value of the IMEP COV reported in Fig. 6, at less than 6% for almost all the tests. For most anticipated pilot injections, the combustion rapidly becomes unstable, and for a small further increase in the advance, the engine stops, even if the combustion remains effectively phased, resulting in the first at the beginning of the main combustion (HR10), which is still sufficiently close to the TDC. Therefore, this behaviour could be attributed to the ignition phase. This could be a result of the first pilot injection, which does not take part in the combustion process, and therefore, the second pilot injection is not sufficient to start the combustion. In fact, in a diesel engine, where the combustion chamber is formed in the piston crown, a strong SOFPI could cause part of the injected fuel to reach the cylinder wall or piston squish area directly, without proper mixing with the air and gaseous fuel.
Fig. 2. Comparison of RHR curves for DF mode at 100 Nm when varying SOFPI and MF.
the engine map range within which LTC is a practicable solution. In fact, the pilot diesel fuel resulted in greater mixing within the air/ gaseous fuel, reducing the local air index but increasing the number of areas in which the low-temperature chemical kinetic reactions could occur, with beneficial effects on the combustion development. The smoother combustion process allows for moving the crank angle at which the thermal flame reactions start away from the diesel pilot injection, as indicated by HR10 in Fig. 4, which represents the angle at which 10% of the fuel was burned. The main gaseous fuel composition appeared not to affect the HR10 position, and therefore on the chemical kinetics-controlled phase of the combustion, with the values being practically coincident. Therefore, the small differences observable in the maximum pressure gradient of Fig. 3, particularly for the HCNG25 mixture, which exhibited higher values, could be related to the slightly faster propagation during the rapid second stage, once the pre-mixed combustion had spread to a consistent fraction of the charge. Activating LTC does not involve a reduction in the engine global
3.2. Toxic and global gaseous emissions One of the most important goals to be achieved with improved combustion with respect to conventional combustion is the reduction in exhaust emissions at the same engine performance. The measurements and comparisons of engine pollutants are reported in the following as 5
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Fig. 3. Pressure gradient for different SOPI values in FD and DF modes at (a) 50 Nm and (b) 100 Nm, varying MF and using DPI compared to single pilot injection.
Fig. 4. Start of combustion in FD and DF modes at 100 Nm when varying MF and using DPI compared to single pilot injection.
Fig. 6. Effects of SOPI on IMEP COV at 100 and 50 Nm in FD and DF modes with different MFs.
volume concentrations. This was possible because the flow rate of the intake air was maintained approximately the same in all the operation modes: FD, DF with NG, and DF with NG-H2 blends. Therefore, the
exhaust volumetric flow rate was almost constant, making it possible to use the emission data measured by the instrumentation directly, and avoiding the calculation on a mass basis.
Fig. 5. Effects of SOPI on (a) brake thermal efficiency and (b) average air index at 100 and 50 Nm in FD mode and DF with different MFs. 6
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The CO emissions are presented in Fig. 7. In general, both the incomplete combustion and low reaction temperatures are the causes of high CO emissions in internal combustion engines. The CO emissions were higher for the DF cases than those of FD, but a consistent reduction was obtained when advancing the diesel pilot injections. Increasing the injection advance generates higher combustion temperatures when the combustion process is dominated by flame front propagation, which is typical of DF combustion with a higher gaseous fuel share, such as a spark ignition engine, and improves the oxidation process. When LTC was achieved at an earlier SOFPI, the CO trend stabilised close to the minimum value reached with conventional combustion and advanced injection timing. If the pilot injection is so advanced as to cause nonignition of the first injection, the combustion will be incomplete and the CO will increase, which is what occurred in the DF-CNG points at 50 Nm with an earlier SOFPI. The THC emissions are reported in Fig. 8a, and exhibit a similar trend to those of CO. The very lean air/gaseous fuel mixture, particularly at a low load, caused incomplete combustion in the areas far from the pilot diesel fuel jets and in the squish zone, and resulted in flame quenching close to the combustion chamber wall, which increased both the THC and CO despite excess oxygen. With an increasing injection advance, the greater combustion temperature allows for more efficient oxidation, until the LTC mode predominates and the emissions are stabilised. The hydrogen content had a positive effect on the THC emissions, at both the medium and low loads tested. With reference to the UGP, as reported in Fig. 8b, the positive effect (a THC reduction higher than the CNG substitution with H2) could be attributed to more complete combustion, owing to the lower ignition energy of H2, which could promote combustion in the leaner zone, as well as the faster combustion process, which contributed to reducing the quench layer thickness. The CO emissions could be drastically reduced with a relatively simple oxidation catalyst, while the THC, mainly constituted by methane, requires strong concentrations of noble metals to be oxidised, particularly at lower temperatures, which is a limitation for DF technology because of the excess air. However, methane emissions represent a real problem only for the global warming effect and not for human health, as methane is a non-toxic gas. Therefore, the CO2 and equivalent CO2, obtained by considering 1 g of methane in the exhaust gas, equivalent to 28 g of CO2 (IPCC Fifth Assessment Report), and 1 g of non-methane HC, equivalent to 7 g of CO2, are reported in Fig. 9. Firstly, it is evident that, despite the CO2 for the DF cases always being lower than the CO2 in the FD mode, the equivalent CO2 could be significantly higher. However, the lowest values were achieved under the LTC combustion regime, from both the medium and low loads. Moreover, at 100 Nm, the equivalent CO2 reached values very close to the FD case, particularly for the HCNG25, even without any post-treatment. Evidently, to obtain an equivalent effective CO2 reduction, the manner in which H2 is obtained as an energy vector is crucial. Finally, the NOx emissions are presented in Fig. 10. The NOx production is strictly dependent on the maximum in-cylinder temperature and oxygen concentration. At a low load, a sensible reduction in NOx in the DF with respect to the FD mode was achieved for all three compositions. In contrast, at the medium load, even if the activation of the LTC led to a reduction trend of the NOx, it still remained at a high level, particularly for gaseous mixtures with a higher H2 content. In the FD mode, the earlier combustion owing to the advanced injection caused increasing temperature peaks, and consequently, the NOx increased. This also occurred for the DF case, but the possibility of using a higher injection advance to achieve LTC resulted in a reduction trend of the NOx emissions, which reached a level close to zero in the case of the lower load. Therefore, LTC could be useful in DF engines for reducing the trade-off between NOx and THC emissions, which occurs when the injection advance is increased, while remaining in the conventional combustion regime, but it is essential to permit a higher injection advance. In the tests, the maximum SOFPI was limited from the higher IMEP COV values, probably owing to the first pilot injection
impingement on the piston squish area or cylinder wall. To increase the maximum SOFPI, different strategies could be investigated, such as a higher pilot injection pressure, which implies strong atomisation of the diesel pilot, and/or a different combustion chamber shape. In Fig. 11, several results of the tests carried out to evaluate the feasibility of a higher injection pressure to extend the LTC regime are presented. The pressure injection ranged from 380 to 1150 bar for the HCNG25 case at 50 Nm, with a SOFPI at approximately 55 CAD BTDC. The combustion development illustrated in Fig. 11a was not strongly influenced when the pressure injection increased from 800 to 1150 bar, or decreased to 380 bar, but in this final case with a lower pressure peak. In the range of 450 to 620 bar, the pressure peaks were similar to those of the case with 380 bar; however, they were reached slightly earlier. However, several effects of the injection pressure on the chemical reactions can be observed from the emission data in Fig. 11b. Although the highest injection pressure permitted by the experimental setup was limited, it can be noted that improvements in both the NOx and THC emissions were achievable with pressure injections higher than 800 bar. In contrast, lower injection pressures led to inferior combustion conditions in terms of the trade-off between NOx and THC. 3.3. Particle characterisation Particle emissions were compared as a concentration, because the exhaust dilution flow rates were almost the same for all of the test cases at 100 Nm, as illustrated previously. The number and mass spectra for the DF and FD operations with a SOPI at 25 CAD BTDC are illustrated in Fig. 12a. The weighted spectral curves reached the maximum concentration for particles with larger diameters compared to those of the numerical spectrum. This is a consequence of the fact that, although the small-diameter particles (10–100 nm) were more abundant, the major contribution in terms of mass was provided by particles with larger diameters (100–1000 nm). A significant difference is noticeable between the FD and DF spectra. The area underlying the spectra of the continuous lines, which represents the total particle number emission per volume unit, is plotted in Fig. 12b for all the tested cases. When the diesel fuel injection was not advanced, the main injection in the FD mode, which implies roughly the 95% of the fuel mass, underwent a relatively high share of the diffusion combustion mode, which was not present in the DF mode, where the primary fuel was in the gaseous state. At earlier injections, the pre-mixed combustion phase of the diesel fuel entering the chamber during the main injection became the most relevant, leading to a strong reduction in the particles, and reaching the levels of the DF cases. Thus, both the number and mass of particles emitted in the FD operation were significantly higher than those of the DF modes, when the SOPI was close to the TDC. In the DF operation, the
Fig. 7. Effects of SOPI on CO emissions at 100 and 50 Nm in FD and DF modes with different MFs. 7
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Fig. 8. Effects of SOPI on (a) THC emissions and (b) UGP at 100 and 50 Nm in FD and DF modes with different MFs.
nanoparticles, with a diameter of less than 50 nm, for both the FD and DF cases. Nanoparticles, as ultrafine particles, are considered very harmful to human health. A more in-depth investigation should be carried out to investigate the characteristics of the nanoparticles emitted in the FD and DF modes. In fact, in addition to the size, other characteristics of nanoparticles that are relevant to health effects are the chemical composition, surface characteristics, and shape [30]. The particles emitted in the DF mode could have less chemical substances adsorbed on their surfaces, owing to the low reactivity of methane and hydrogen compared to the long chain hydrocarbons of the FD mode. However, it is crucial to reduce the number, in both the combustion chamber and with a particulate filter at the exhaust, with the aim of reducing the exposition risk. The LTC mode was proven to be a viable solution, because it did not exhibit negative effects on the particle number production. Fig. 9. Effects of SOPI on CO2 and equivalent CO2 emissions at 100 and 50 Nm in FD and DF modes with different MFs.
4. Conclusions With the aim of improving the DF combustion process, an in-depth experimental campaign was carried out to investigate the feasibility of LTC, and its effects on the performance and emissions, on a compression ignition light-duty engine operated in the DF mode, using NG as primary fuel, adding H2 up to 25% by volume, and triggering combustion through a DPI. The DF tests were performed at the maximum percentage of diesel oil replacement, close to 80% of gaseous fuel on the energy basis, corresponding to the minimum amount of pilot fuel required for stable ignition. It was proven that it is possible to activate a stable LTC regime by splitting the pilot injection into two close steps, and increasing the injection advance by approximately 40–60 CAD BTDC. The DPI allowed for controlling the maximum pressure gradient, maintaining it at lower than 15 bar/deg up to 100 Nm of the engine torque output. The NOx emissions initially increased with the increasing pilot injection advance, and then, once reaching the conditions for LTC, began to decrease until a higher injection advance rapidly led to combustion instability. This was probably owing to the first pilot injection impingement on the cylinder wall or piston squish area, which compromised the ignition phase. To improve the ignition at an earlier pilot injection advance, a higher injection pressure was used. Although certain improvements were achieved, the investigation was not exhaustive owing to the limitations of the experimental setup (the maximum injection pressure and combustion chamber geometry). Therefore, a dedicated combustion chamber with a different volume and/or shape could be investigated to enhance the limit on the maximum injection advance that is achievable
Fig. 10. Effects of SOPI on NOx emission at 100 and 50 Nm in FD and DF modes with different MFs.
total particle concentration was less affected by the diesel fuel injection timing, probably because of the small amount of diesel fuel injected, which required a negligible diffusion flame phase, as with the SOFPI closer to the TDC. The GMD, as reported in Fig. 13, characterises the mean particles as 8
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Fig. 11. Effects of pilot injection pressure on (a) combustion development and (b) trade-off between NOx and THC in LTC regime.
prior to ignition problems occurring. This could be crucial for reaching an NOx of almost zero, together with high efficiency and CO2 equivalent emissions close to the FD mode, which was proven to be possible by adding 25% of H2 to the primary fuel. The activation of the LTC had no negative effects on the particle emissions, which resulted in close to the minimum value being reachable using an FD operation with higher injection advance, maximising the burning of the diesel fuel during the pre-mixed phase. In conclusion, the activity demonstrated that the LTC and DF technologies can achieve remarkable results in terms of high efficiency and low emissions, with engines set for a lower maximum power than that allowed by the available displacement. The achievable performance would not meet the requirements of engines used for transport, which take full advantage of supercharging, with a significant increase in the torque and power delivered per volume unit. However, the technology investigated in this work appears to be promising for vehicle propulsion with such hybrid systems, in which the engine can supply the average power level in an almost stationary manner, or for the generation of small and medium-scale electricity with engines powered by fuels of various types and compositions (biological or synthetic, with a low heat content, or energy carriers in general). In this scenario, specifically designed engines should be developed to expand the conditions for LTC activation beyond those obtained in the
Fig. 13. Effect of SOPI on GMD at 100 and 50 Nm in FD and DF modes with different MFs.
experimental activity, which was limited by the maximum injection pressure of the pilot fuel and combustion chamber shape, and optimised for diesel fuel.
Fig. 12. (a) Particle spectrum at 25 SOPI and (b) effect of SOPI on total particle number at 100 Nm in FD and DF modes with different MFs.
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Acknowledgements [20]
The authors would like to thank Vicenzo Bonanno for his valuable support in experimental activities.
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Glossary AFR: Air fuel ratio BTDC: Before top dead centre CAD: Crank angle degree COV: Coefficient of variation DF: Dual fuel Dp: Particle diameter FD: Full diesel GMD: Geometric mean diameter HCNG: Hydrogen-enriched compressed natural gas IMEP: Indicated mean effective pressure LHV: Lower heating value LTC: Low-temperature combustion MF: Main fuel N: Number of particles NG: Natural gas PF: Pilot fuel PM: Particulate matter RHR: Rate of heat release SOFPI: Start of first pilot injection SOPI: Start of pilot injection TDC: Top dead centre THC: Total hydrocarbons
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