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An experimental investigation of the combustion process of a heavy-duty diesel engine enriched with H2
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An experimental investigation of the combustion process of a heavy-duty diesel engine enriched with H2 C. Liew, H. Li*, J. Nuszkowski, S. Liu, T. Gatts, R. Atkinson, N. Clark Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506-6106, USA
article info
abstract
Article history:
This paper investigated the effect of hydrogen (H2) addition on the combustion process of
Received 6 April 2010
a heavy-duty diesel engine. The addition of a small amount of H2 was shown to have a mild
Received in revised form
effect on the cylinder pressure and combustion process. When operated at high load, the
10 June 2010
addition of a relatively large amount of H2 substantially increased the peak cylinder
Accepted 11 June 2010
pressure and the peak heat release rate. Compared to the two-stage combustion process of
Available online 7 August 2010
diesel engines, a featured three-stage combustion process of the H2ediesel dual fuel engine was observed. The extremely high peak heat release rate represented a combination of
Keywords:
diesel diffusion combustion and the premixed combustion of H2 consumed by multiple
Hydrogen
turbulent flames, which substantially enhanced the combustion process of H2ediesel dual
Diesel
fuel engine. However, the addition of a relatively large amount of H2 at low load did not
Dual fuel engine
change the two-stage heat release process pattern. The premixed combustion was
Combustion process
dramatically inhibited while the diffusion combustion was slightly enhanced and elongated. The substantially reduced peak cylinder pressure at low load was due to the deteriorated premixed combustion. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen has long been recognized as a fuel having excellent combustion and desirable emissions characteristics when burned in internal combustion (IC) engines [1]. The addition of H2 to spark ignition (SI) engines, known as H2-enrichment, was demonstrated to accelerate the flame propagation, extend the lean operational region, enhance the combustion stability at lean operation, improve the brake thermal efficiency, and reduce the exhaust emissions of carbon monoxide (CO), unburned hydrocarbons (HC) and possibly nitrogen oxide (NOx) if operated at an extremely lean mixture [1e5]. The mixture of H2enatural gas was used as fuel to power transit buses [6]. The lean burn SI H2 engines were developed to power shuttle van and passenger cars [7,8]. The detailed discussions of the positive features of H2 and H2-rich mixture
as clean alternative fuel of SI engines have been reported in the literature [9e11]. The use of H2 in diesel engines was initiated by its perceived potential to substantially reduce the emissions of particulate matter (PM) and the desired thermal efficiency of diesel engines. Also, the reduction in carbon dioxide (CO2) would clearly occur because the burning of H2 supplies energy without bringing carbon into the engine. The reduced CO2 emissions may also occur due to the improved thermal efficiency benefiting from the improved combustion. When burned in diesel engines, H2 is usually added into the intake mixture and burned with external ignition assistance, such as pilot injection of diesel fuel. This dual fuel operation mode has been demonstrated as a reliable and stable combustion mode for gaseous fuels, including H2 in compression ignition (CI) engine. Detailed information of the dual fuel engines can be found in the literature [12e15].
* Corresponding author. E-mail address: [email protected] (H. Li). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.06.023
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Extensive research has been conducted to explore the effect of H2 addition on the brake thermal efficiency and exhaust emissions of CI engines using diesel as either the pilot or main fuel. Varde and Frame [16] examined the effect of H2 addition on the brake thermal efficiency and the exhaust emissions of a direct injection (DI) single cylinder diesel engine. The addition of H2 was found to reduce the emissions of PM. However, the effect of H2 addition on the brake thermal efficiency to a large extent depended on the amount of H2 added. The addition of a relatively large amount of H2 improved the brake thermal efficiency. With the substitution of 12.5% diesel by H2 at full load operation, the brake thermal efficiency increased from 30.5 to 33.7%. However, the addition of a small amount of H2 (<5% of total intake energy) reduced the brake thermal efficiency. It was believed that this was due to the extremely lean H2eair mixture, which could not support the flame propagation and resulted in a low H2 combustion efficiency. The addition of H2 may affect the thermal efficiency not only through changing the overall combustion efficiency of the intake fuel, but also by changing the phasing of the combustion process with respect to piston top dead center position. The over retarding or advancing of the combustion phasing could reduce the brake thermal efficiency even when combustion was finished in a much shorter period indicating a dramatic improvement to the combustion process. However, Saravanan et al. [17] demonstrated that the addition of H2 improved the brake thermal efficiency for the range of engine loads explored. The negative effect of the addition of H2 on the brake thermal efficiency was not observed. Recently, the effect of H2 addition on the performance and exhaust emissions of diesel engines has been examined using light-duty multi-cylinder diesel engines [18e20]. Shirk et al. [18] investigated the effect of H2 addition on the thermal efficiency and exhaust emissions of a light-duty, four cylinder, diesel engine with common rail fuel injection system. The substitution of 5 and 10% diesel fuel with H2 was shown to slightly reduce the emissions of NOx while its effect on the brake thermal efficiency was negligible. McWilliam et al. [20] investigated the effect of H2 addition on the thermal efficiency and exhaust emissions of a light-duty, four cylinder, diesel engine with exhaust gas recirculation (EGR). The addition of H2 was shown to substantially increase the NOx emissions and dramatically improve the thermal efficiency. When operated at 5.4 bar brake mean effective pressure (BMEP) with 10% EGR, the mixing of 6% H2 (vol.) into the intake mixture increased the thermal efficiency from 23.8% to 30.7%. The heat release process provides fundamental information to explain the effect of H2 addition on the brake thermal efficiency and exhaust emissions of NOx. For example, Kumar et al. [21] investigated the effect of H2 addition on the combustion process of a small single-cylinder diesel engine with a rated power of 3.7 kW. The addition of H2 was shown to elongate the ignition delay period but enhance the combustion process indicated by the substantially increased the peak heat release rate. When operated at full load, the addition of 17% H2 (of total fuel mass) increased the maximum heat release rate from 38 J/ CA to 66 J/ CA. When operated at 40% load, the addition of 27% H2 (of total fuel mass) increased the maximum heat release rate from 23 J/ CA to 42 J/ CA. Roy et al. [22] investigated the combustion process of a single cylinder
dual fuel engine operated mainly on gaseous fuel mixtures of H2 and CO. After the gaseous fueleair mixture was ignited by a small diesel pilot (3.5e6.5% of the total intake energy), a singleestage combustion process, similar to that of the SI engines, was usually observed under normal operating conditions. However, when operated at high load, a two-stage combustion phenomenon was observed. The second heat release peak was observed at the middle of combustion process, which substantially increased the maximum heat release rate and enhanced the combustion process as indicated by the dramatically reduced combustion period. The rapid combustion period, defined as 10e80% mass fraction burned, took less than half the time of the normal single-stage combustion. Further increasing the engine load through the addition of more H2-rich gas mixtures was expected to result in severe engine knock. Correspondingly, the development of the featured second-stage combustion was considered as a precursor of knocking combustion, an indicator of the maximum addition of the reformed gases and maximum engine load. The authors observed that high increases in heat release, coupled with unchanged timing, resulted in higher incylinder pressures, and might increase indicated efficiency if optimized. Abu-Jrai et al. [23,24] investigated the influence of H2-rich gases, consisting of H2, CO and diluents, on the combustion and emissions characteristics of a single cylinder diesel engine with a rated power of 7.5 kW. The addition of the H2-rich gas mixture at moderate engine load was found to substantially enhance the premixed combustion and dramatically reduce the combustion duration. However, the addition of the H2-rich mixture at low load was demonstrated to retard the combustion phasing and prolong the combustion duration, which reduced the indicated thermal efficiency. The combustion phasing was retarded well beyond the point of obtaining the maximum efficiency. The increased emissions of the unburned gaseous fuels at low load might also contribute to the deteriorated thermal efficiency. Although extensive emissions and limited combustion data have been obtained using small single cylinder and lightduty multi-cylinder diesel engines, the corresponding research on the addition of H2 to heavy-duty diesel engines has not been reported. Existing experimental data indicated to some extent the dependence of the improvement to the brake thermal efficiency on the engine load and the amount of H2 added [16,24]. However, more research is required to explore the engine operation and H2 addition strategies of heavy-duty diesel engines. A well designed experiment will help to explain the effects of the addition of H2 on the thermal efficiency observed at different engine loads. For example, the different effect on the engine performance and emissions of the addition of H2 at low and high flow rate may be due to the success or failure of the development of turbulent flame that can burn all or at least most of the gaseous fuels supplied into the intake mixture. It was well known that a flame cannot propagate through a fueleair mixture leaner than the lean flammability limit of the gaseous fuel [25]. Such a limit of dual fuel engine may substantially vary with the changes in engine load, the amount of pilot diesel injected, the injection timing [26], intake pressure boosting, the application of EGR and even the pressure of diesel fuel injection. Detailed research on the operation of H2ediesel dual fuel engine near the lean
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flammability limit of H2eair mixture may provide valuable information to explain the different effects of H2 addition on the engine performance, combustion and exhaust emissions reported in the literature. For better understanding of the potential of H2 in affecting the brake thermal efficiency and exhaust emissions of heavyduty diesel engines, there is a need to explore the detailed effect of H2 addition and engine load on the combustion process including the ignition delay, combustion duration, heat release rate and combustion phasing. The correlation of the combustion process of H2ediesel dual fuel engines with the fundamental combustion properties (such as the flammability limits) of H2 may help to explain the undesirable effects of the addition of H2 on engine performance, combustion process and exhaust emissions. The data describing the minimum H2 addition needed to improve the brake thermal efficiency and enhance the combustion process could provide guidelines for the development of H2ediesel dual fuel engine and the establishment of H2 addition and diesel injection control strategies. The combustion data obtained with the addition of a very small amount of H2 may also provide guideline to the integration of a small H2O-electolyzer with the diesel engines [27]. In this paper, the effect of H2 addition on the combustion process was presented.
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2. Experimental set-up and combustion analysis
and pressure regulation system used to provide steady H2 flow to the engine test cell; (b) a low pressure H2 metering and delivery system designed to control and measure the flow rate of H2 and its delivery to the main stream of the intake air; and (c) a H2 safety system designed to eliminate the unexpected high pressure hazards in the event of backfire. The high pressure H2 fuel storage and pressure regulation system consisted of two sets of pressure regulators, each connected with six high pressure H2 tanks and used to provide steady H2 flow with gauge pressure of 45 psi. There were two valves employed to shut down the H2 flow in case of emergency and also when the flow of H2 was not needed. One valve installed after the pressure regulator was located outside the laboratory and the other installed prior to the H2 filter was located near the engine test cell. The low pressure H2 fuel system consisted of a H2 filter, a mass flow controller for controlling and measuring the flow rate of H2, and a H2eair mixer used to continuously deliver H2 into the main stream of the intake air. The safety system consisted of a stainless steel flame arrester and a pressure relief valve, installed between the intake manifold and flame arrester. In case of backfire, the flame arrester could quench the propagation of the flame into the intake mixture. The pressure relief valve could quickly relieve excessive pressure in the intake system resulting from the burning of the H2eair mixture in case of backfire. Detailed description of the H2 fuel system and the safety approach can be found in the literature [28].
2.1.
2.3.
Test engine and dynamometer
The engine used was a 1999 Cummins ISM370 diesel engine with a rated power of 370 horsepower. This turbocharged, 6-cylinder, diesel engine was widely used to power Class-8 heavy-duty trucks and was certified by the EPA as having emissions at or below 4.0 g/bhp-hr NOx and 0.1 g/bhp-hr PM. EGR management was introduced into the ISM engines three years after this model. Detailed specifications for this engine can be found in Table 1. The test engine was coupled to a 550 hp General Electric (GE) Direct Current (DC) dynamometer used to absorb the engine load and control engine speed. Torque was controlled by supplying an electronic throttle signal to the engine and controlling the amount of diesel fuel injected. This was the same experimental arrangement used to gather emissions data for Ref. [28].
2.2.
H2 fuel system
As shown in Fig. 1, the H2 fuel system consisted of the following three sub-systems: (a) a high pressure H2 storage
Table 1 e Specifications of the test engine. Engine Model Displacement Bore Stroke Power Rating Torque Rating Configuration
1999 Cummins ISM370 10.8 L 125 mm 147 mm 276 kW @2100 RPM 1830 Nm @1200 RPM Inline 6-cylinder
Pressure acquisition
In-cylinder pressure was acquired using a Kistler model 6125C pressure transducer at 0.25 degree crank angle (CA) resolution. In each test, 200 consecutive cycles were collected and averaged. A low-pass filter with a cut-off frequency of 2500 Hz was applied to the measured in-cylinder pressure to reduce the high frequency combustion noise. The dynamic pressure of the in-cylinder pressure transducer was referenced using a constant polytropic coefficient without the need of measuring the intake manifold air pressure (MAP) [29]. The averaged in-cylinder pressure was processed to obtain a set of cylinder pressure parameters such as the peak cylinder pressure (PCP) and peak cylinder pressure rise rate, and their phasing. Using the averaged cylinder pressure, the indicated engine performance metrics such as the indicated work per cycle and the indicated mean effective pressure (imep) could also be calculated [29e31].
2.4.
Measurement of exhaust emissions
The raw engine exhaust was ducted to a full-scale dilution tunnel with flow control accomplished using a critical flow venturieconstant volume sampler (CFVeCVS) system. A 25.4 cm diameter orifice, located 1 m from the tunnel entrance, ensured that the exhaust was thoroughly mixed with dilution air before reaching the emissions sampling zone, located ten tunnel diameters downstream. The CFVeCVS controlled the dilute exhaust flow at a nominal 68 standard cubic meter per minute (scmm) throughout this test program. Instantaneous measurement of CFVeCVS flow was
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Fig. 1 e Schematic diagram of H2 fuel system.
accomplished using a fast-response thermocouple and pressure transducer installed at the venturi entrance. The diluted exhaust gas was sampled from the full flow dilution tunnel through heated sample probes, lines and finally fed into the individual emissions analyzers. The emissions of CO and CO2 were measured using non-dispersive infrared (NDIR) analyzers. The HC emissions were measured using a heated flame ionization detector (HFID). The emissions of CO2, CO and HC measured were further processed to obtain the combustion efficiency of diesel fuel, defined as the percentage of carbon burned to CO2. The emissions of NOx and PM were also measured but not reported in this paper. The raw exhaust gas for measurement of the emissions of H2 was dried by flowing it through a chiller to cool the exhaust gas to 4 C and so remove the moisture contained in the exhaust gas. The dried exhaust gas was then sampled into 10-L sampling bags at a rate of 4 L per minute for the later analysis of the H2 concentrations. Prior to the sampling of the exhaust gas, the sampling system was purged with exhaust gas for 40 s to remove any residual gas that may have been present in the sampling system. In this research, the exhaust emissions of H2 were measured using an Electron Pulse Ionization (EPI) Mass Spectrometer (MS). The H2 emissions values measured were further processed to obtain the combustion efficiency of H2, defined as the percentage of H2 burned in the engine relative to that added to the intake mixture.
2.5.
Combustion analysis
In this research, the heat release rate was calculated using the well-known single zone zero-dimensional heat release model
[32]. With the assumptions of a uniform pressure, uniform temperature and ideal gas, the heat release rate can be calculated using the equation (1). dQ g dV 1 dP dQ ¼ P þ V þ dq gross g1 dq g1 dq dq ht
(1)
Where g is the specific heat ratio calculated according to the composition and temperature of the bulk mixture calculated using ideal gas equation with the in-cylinder pressure, cylinder volume at the intake valve closing as a reference condition. The well-known Woschni equation [33] was used to calculate the heat transfer from the bulk mixture to the coolants, the last term in equation (1), which allows the differencing of net and gross heat release rates. The temperature of the combustion chamber was considered as the same of the coolants. In this research, the gross heat release rate was reported. The heat release rate was further processed to obtain a set of combustion parameters such as the peak heat release rate (PHRR) and its phasing, the mass fraction burned (although time-dependent combustion efficiency is not known), start of combustion (SOC) (defined as the crank angle when the heat release rate reached 0.05 kJ/ CA in premixed combustion), and combustion duration defined as the crank angle period between the 10 and 90% of the total heat released through combustion. Other combustion parameters such as the ratio of the heat release in premixed combustion relative to the total heat released can also be calculated. In this research, the effects of H2 addition and engine load on the cylinder pressure, heat release process and combustion
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3.
Experimental results and analysis
The combustion efficiency of diesel engine was extremely high (>99.5%). As shown in Fig. 2, the addition of H2 at 70% load did not further improve but slightly deteriorated the combustion efficiency of diesel fuel. In comparison, the combustion efficiency of H2 was dramatically affected by the amount of H2 added. With the addition of 6% H2 into the intake air, the combustion efficiency of H2 observed was 97.9%, which was much lower than that diesel fuel. When a small amount of H2 was added, the relatively low combustion efficiency of H2 as shown in Fig. 2, might deteriorate the brake thermal efficiency of the diesel engine. The possible
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H2/(H2+Air), vol. % Fig. 2 e Effect of H2 addition on combustion efficiency of diesel and H2, 70% load.
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improvement to the brake thermal efficiency of diesel engine by the addition of H2 can only be obtained through the improvement of the heat release process such as the release of the heat at optimized phasing and short period noted as the combustion duration. Fig. 3 shows the effect of H2 addition on the cylinder pressure at 70% load. With the addition of a relatively small amount (<3%) of H2, the cylinder pressure prior to the initiation of the combustion was higher than diesel operation. This might be due to the widely divergent thermodynamic properties of H2 and air, and the variation of the increased intake pressure. As shown in Fig. 4, the addition of a small amount of H2 increased the intake pressure, which might due to the changes in exhaust temperature prior to the turbo-changer resulted from the variation of the combustion process, coupled with the behavior of the turbocharger and waste gate. The maximum intake pressure was observed with the addition of 2% H2. Further increasing the addition of H2 gradually reduced the intake pressure. With the addition of H2 beyond 3.5%, the intake pressure observed was lower than diesel operation. Correspondingly, the cylinder pressure prior to the initiation of combustion was lower than diesel operation with
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Diesel
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Fig. 3 e Effect of H2 addition on cylinder pressure, 70% load.
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Intake Pressure, kPa
Combustion Efficiency of Diesel and H2 %
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120 Cylinder Pressure, bar
duration of a heavy-duty diesel engine were experimentally investigated. The amount of H2 added was expressed as the volumetric fraction of H2 in the intake mixture [H2/(Air þ H2), vol. %]. The engine load was varied from 15 to 70% when operated at 1200 RPM. With the increasing addition of H2, the flow rate of diesel fuel was gradually reduced to maintain the constant load operation. With the addition of up to 6.0% H2 (equivalence ratio of 0.15) in the intake air, the energy of H2 contributed as much as 77% of the total intake energy when operated at 15% load and as little as 31% when operated at 70% load. To address the feasibility of the small H2O electrolyzers in improving the combustion of diesel engines [27], the effect on the combustion process of the addition of a very small amount of H2 (0e1% in intake mixture) was also investigated. In order to better understand the effect of H2 on oxidation of diesel fuel, the effect of H2 on the combustion efficiency of diesel fuel and H2 was also evaluated. The combustion efficiencies of diesel fuel and H2, defined as the percentage of corresponding fuel completely burned in the combustion chamber, were calculated using the exhaust emissions of CO, CO2, unburned HC and H2 measured in this research [28]. In this research, the diesel fuel used was ultra low sulfur pump diesel. The supplemented H2 fuel used was industry purity H2 having a purity of over 99.5%.
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Fig. 4 e Effect of H2 addition on the intake pressure, 70% load.
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Peak Heat Rel. Rate 12
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Fig. 5 e Effect of H2 addition on peak cylinder pressure and its phasing, 70% load.
Phasing of PHRR, CA ATDC
15 Phasing of PCP Peak Cyl. Pressure
Phasing of PCP, CA AT DC
Peak Cylinder Pressure, bar
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Peak Heat Release Rate, kJ/CA
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H2/(H2+Air), vol. % Fig. 7 e Effect of H2 addition on peak heat release rate and its phasing, 70% load.
Heat Release Rat e, kJ/CA
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0.5 0.4 Combination of H 2 Premixed comb. and diesel diffu. comb.
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Early and late stage diesel diffu. comb.
0
observed. Compared to the premixed combustion, the addition of H2 was found to have more significant effect on the diffusion combustion. As shown in Fig. 6, the addition of H2 gradually enhanced the heat release rate observed at the middle of the diffusion combustion. Following the initiation of the diffusion combustion, a sudden increase in heat release rate beyond that of the featured diffusion combustion of diesel engine was observed when a relatively large amount of H2 was added. The increased heat release rate most likely represented the combination of the diesel diffusion combustion and the H2eair combustion, which burned the H2 quickly through the propagation of multiple turbulent flames initiated by the combustion of the diesel. The featured multiple turbulent flames propagated through the H2eair mixture presented outside the diesel spray plume, enhanced the heat release process of the H2ediesel dual fuel engine and might have accelerated the combustion process of the diesel fuel. When the multi turbulent H2eair combustion completed or quenched due to the slow reaction rate, the relatively slow diesel diffusion combustion continued until its completion, noted as the late diesel diffusion combustion. Unfortunately, it is difficult to hypothesize how much synergy exists between
Combustion Duration,CA
the addition over 3.5% H2. As shown in Fig. 3, the addition of H2 increased the cylinder pressure after the combustion was initiated. This was further demonstrated by examining the effect of H2 addition on the peak cylinder pressure (PCP). As shown in Fig. 5, the addition of a small amount of H2 increased the PCP without altering the phasing when PCP was observed. Further increasing the addition of H2 beyond 3.5% substantially increased the value of PCP observed at gradually advanced the phasing. The addition of 6% H2 increased the PCP from 106.4 to 120.5 bar and advanced its phasing by 1.5 CA. As indicated by the substantially increased PCP, the addition of H2 to a diesel engine at high load should be limited to govern the PCP for both safety and mechanical durability. Fig. 6 shows the effect of H2 addition on the heat release process. When operated at pure diesel, the diesel engine featured two-stage combustion process was observed. A premixed combustion was followed by the diffusion combustion which was dominated by the diesel fuel injection, vaporization and mixing process. The addition of H2 was shown to slightly retard the initiation of the premixed combustion and reduce the peak heat release rate obtained in the premixed combustion. Compared to diesel operation, the addition of 6% H2 retarded SOC by 0.8 CA. The substantial retarding effect of H2 on the SOC of this heavy-duty diesel engine was not
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Crank Angle, CA ATDC Fig. 6 e Effect of H2 addition on heat release process, 70% load.
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H2/(H2+Air), vol. % Fig. 8 e Effect of H2 addition on combustion duration, 70% load.
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Fig. 9 e Effect of H2 addition on cylinder pressure, 30% load.
the combustion of the diesel and H2, as opposed to their burning separately in different modes. At the molecular level, there must surely be interaction of the H2 with the intermediate species associated with diesel combustion, but the extent of the interaction is unclear and must depend on both combustion chemistry and the physics of local mixing. Of course, the term “diffusion combustion” may account both for traditional diesel fuel diffusion combustion and the rapid development of flame fronts in the H2eair mixture simultaneously. The latter is similar to the combustion of lean burn SI H2 engine but burns much faster considering the multi-point ignition features of the pilot diesel. Fig. 7 shows the effect of H2 addition on the peak heat release rate (PHRR) and the phasing when PHRR was observed. The addition of a small amount of H2 gradually increased the PHRR without altering the phasing when the PHRR was observed. Further increasing the addition of H2 beyond 3.5% substantially increased the PHRR observed during diffusion combustion and started to advance the phasing when PHRR was observed. With the addition of 6% H2, the peak heat
Fig. 11 e Effect of H2 addition on cylinder pressure, 15% load.
release rate of 0.53 kJ/ CA was obtained. Compared to that PHRR of 0.25 kJ/ CA obtained with diesel operation, the addition of 6% H2 increased the PHRR by 109%. The PHRR phasing was advanced by 2.5 CA. As shown in Fig. 8, the addition of H2 beyond 2% started to reduce dramatically the combustion duration, indicating the enhancing effect of H2 on the diffusion combustion process. With the addition of 6% H2, the combustion duration observed was 23.8 CA, a 23% reduction compared to the diesel operation. This also indicated the need for the optimization of the combustion phasing in a dedicated H2ediesel dual fuel engine design especially when a relatively large amount of H2 was added. Fig. 9 shows the effect of H2 addition on cylinder pressure when operated at 30% load. The addition of H2 slightly increased the cylinder pressure with its peak value observed at slightly advanced phasing. As shown in Fig. 10, the peak heat release rate was observed at the premixed combustion stage. The addition of H2 was shown to retard slightly, but enhance the premixed combustion. The addition of H2 also enhanced the heat release process during the transition
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Heat Release Rat e, kJ/CA
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Peak Heat Release Rat e, kJ/CA
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Fig. 13 e Effect of H2 addition and engine load on peak heat release rate.
period from the premixed combustion to the diffusion combustion. Compared to 70% load operation, the effect of H2 addition on the peak cylinder pressure, peak heat release rate and their phasing was relatively small. Fig. 11 shows the effect of H2 addition on the cylinder pressure at 15% load. Although having mild effect on the cylinder pressure prior to the initiation of combustion, the addition of H2 substantially reduced the cylinder pressure after the combustion was initiated when a large amount of H2 was added. As shown in Fig. 12, the addition of H2 at 15% load substantially reduced the heat release rate observed during the premixed combustion. However, the diffusion combustion process was slightly enhanced and elongated. The deteriorated premixed and elongated diffusion combustion made the combustion process lasted for a longer time and correspondingly deteriorated the engine performance. Fig. 13 compares the effect of H2 addition on the PHRR observed at different load. The significant effect of H2 on the PHRR was only observed with the addition of relatively large amounts of H2. The addition of over 3% H2 substantially
Combust ion Durat ion, CA
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7
Fig. 14 e Effect of the addition of H2 and engine load on combustion duration.
enhanced the PHRR at 70% load. In comparison, the addition of over 4% H2 was found to substantially deteriorated combustion when operated at 15% load. In comparison, the addition of H2 at 30% load was shown to slightly increase the PHRR. Fig. 14 compares the effect of H2 addition and engine load on the combustion duration. The addition of a small amount of H2 had negligible effect on the combustion duration. However, as indicated by the gradually reduced combustion duration, the addition of a relatively large amount of H2 at 70% and 30% load enhanced the combustion process. In comparison, the addition of a large amount of H2 at 15% load was shown to elongate the combustion duration indicating the deteriorated combustion process. It was also noted that the dramatic changes in combustion duration was observed with the addition of 2% H2 at 70% load. With the decrease in engine load, such a substantial change was observed at increasingly higher concentration of H2 in the intake mixture. When operated at 30% load, the positive effect on combustion duration was observed with the addition of over 3% H2. This may help to explain the requirement for the addition of relatively a large amount of H2 to obtain the improved brake thermal efficiency when operated at a lower load. When operated at 15% load, the substantial change to the combustion duration was observed with the addition of over 5% H2, which was mainly due to the inhibited premixed combustion and elongated diffusion combustion as shown in Fig. 12. As indicated by the variation of the PHRR and combustion duration shown in Figs. 13 and 14, the addition of small amount of H2 (less than 2%) had negligible effect on the combustion process. The integration of a small water-electrolyzer may not be able to significantly improve the performance of the diesel engines.
4.
Conclusions
The effect of the addition of H2 on combustion process of this heavy-duty diesel engine depended on the load and the amount of H2 added. The addition of a small amount of H2 had only mild or negligible effect on the cylinder pressure and combustion process. A substantial effect on the cylinder pressure and combustion process was only observed with the addition of a relatively large amount of H2. The addition of a relatively large amount of H2 at 70% load substantially increased the peak cylinder pressure. Accordingly, the addition of H2 at high load should be limited to govern the peak cylinder pressure, for both safety and mechanical durability. Compared to the combustion process of diesel engine, the addition of a relatively large amount of H2 at 70% load dramatically reduced the combustion duration and enhanced the peak heat release rate observed at the middle of diffusion combustion. The extremely high peak heat release rate observed represented a combination of fast turbulent combustion of H2 and the diffusion combustion of diesel. The addition of a relatively large amount of H2 at 15% load substantially deteriorated the premixed combustion but slightly enhanced and elongated the diffusion combustion.
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When operated at 30% load, the addition of H2 had a small effect on the cylinder pressure and combustion process. The addition of H2 had no positive effect in enhancing the combustion efficiency of diesel fuel. The combustion efficiency of H2 added was much lower than that of diesel fuel especially when the amount of H2 added was small.
Acknowledgement The preparation of this paper is based on the work funded by the State of Texas through a grant from the Texas Environmental Research Consortium, with funding provided by the Texas Commissions on Environmental Quality.
references
[1] Das LM. Hydrogen engines: a review of the past and a look into the future. Int J Hydrogen Energy 1990;15:425e43. [2] Bauer CG, Forest TW. Effect of hydrogen addition on the performance of methane-fueled vehicles, part I: effect on S.I. engine performance. Int J Hydrogen Energy 2001;26:55e70. [3] Li H, Karim GA. "An experimental investigation of S.I. engine operations on lean gaseous fuels mixtures". SAE Paper 200501-3765, 2005. [4] Topinka J, Gerty JB. Heywood, Knock behavior of a lean-burn, H2 and CO-enhanced, SI gasoline engine concept. SAE Paper 2004-01-0975, 2004. [5] Ma F, Wang Y, Liu H, Li Y, Wang J, Zhao S. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine. Int J Hydrogen Energy 2007;32:5067e75. [6] Munshi SR, Nedelcu C, Harris J, Edwards T, Williams JR, Lynch F, et al. Hydrogen blended natural gas operation of a heavy duty turbo-charged lean burn spark ignition engine. SAE Paper 2004-01-2956, 2004. [7] Natkin RJ, Denlinger A, Younkins M, Weimer AL, Hashemi S, Vaught AT. Ford 6.8L hydrogen IC engine for the E-450 shuttle van. SAE Paper 2007-01-4096, 2007. [8] Jaura AK, Ortmann W, Stuntz R, Natkin B and Grabowski T. Ford’s H2RV: an industry first HEV propelled with a H2-fueled engine e a fuel efficient and clean solution for sustainable mobility. SAE Paper 2004-01-0058, 2004. [9] Furuhama S. State of the art and future trends in hydrogenfuelled engine. Int J Vehicle Design 1983;4:359e85. [10] Karim GA. Hydrogen as a spark ignition engine fuel. Int J Hydrogen Energy 2003;28:569e77. [11] White CM, Steeper RR, Lutz AE. The hydrogen-fuelled internal combustion engine: a technical review. Int J Hydrogen Energy 2006;31:1292e350. [12] Weaver CS, Turner SH. Dual fuel natural gas/diesel engines: technology, performance, and emissions. SAE Paper 940548, 1994. [13] Boehman AL, Corre OL. Combustion of syngas in internal combustion engines. Combust Sci Technol 2008;180: 1193e206. [14] Karim GA. Combustion in gas fueled compression ignition engines of the dual fuel type. Trans ASME, J Eng Gas Turbines Power 2003;125:827e36.
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[15] Mustafi NN, Raine RR. A study of the emissions of a dual fuel engine operating with alternative gaseous fuels. SAE Paper 2008-01-1394, 2008. [16] Varde KS, Frame GA. Hydrogen aspiration in direct injection type diesel engine-its effect on smoke and other engine performance parameters. Int J Hydrogen Energy 1983;8: 549e55. [17] Saravanan N, Nagarajan G, Sanjay G, Dhanasekaran C, Kalaiselvan C. Combustion analysis on a DI diesel engine with hydrogen in dual fuel mode. Fuel 2008;87:3591e9. [18] Shirk MG, McGuire TP, Neal GL, Haworth D. Investigation of a hydrogen-assisted combustion system for a lightduty diesel vehicle. Int J Hydrogen Energy 2008;33: 7237e44. [19] Bika AS, Franklin LM, Kittelson DB. Emissions effects of hydrogen as a supplemental fuel for diesel and biodiesel. SAE Paper 2008-01-0648, 2008. [20] McWilliam L, Megaritis T, Zhao H. Experimental investigation of the effects of combined hydrogen and diesel combustion on the emissions of a HSDI diesel engine. SAE Paper 2008-01-0156, 2008. [21] Kumar MS, Ramesh A, Nagalingam B. Use of hydrogen to enhance the performance of a vegetable oil fuelled compression ignition engine. Int J Hydrogen Energy 2003;28: 1143e54. [22] Roy MM, Tomita E, Kawahara N, Harada Y, Sakane A. Performance and emissions comparison of a supercharged dual-fuel engine fuelled by producer gases with varying hydrogen content. Int J Hydrogen Energy 2009;34:7811e22. [23] Abu-Jrai A, Tsolakis A, Megaritis A. The influence of H2 and CO on diesel engine combustion characteristics, exhaust gas emissions, and after treatment selective catalytic NOx reduction. Int J Hydrogen Energy 2007;32:3565e71. [24] Abu-Jrai A, Rodrı´guez-Ferna´ndezb J, Tsolakisa A, Megaritisc A, Theinnoia K, Cracknelld RF, et al. Performance, combustion and emissions of a diesel engine operated with reformed EGR. Comparison of diesel and GTL fuelling. Fuel 2009;88:1031e41. [25] Bade Shrestha SO, Karim GA. The operational limits in engines fuelled with alternative gaseous fuels. Trans ASME, J Energy Resour Technol 2006;128:223e8. [26] Li H, Karim GA, Sohrabi A. An experimental and numerical investigation of spark ignition engine operation on H2, CO, CH4, and their mixtures. Trans ASME, J Eng Gas Turbines Power 2010;132:032804. [27] Barton P. Study of heavy duty vehicle exhaust emissions and fuel consumption with the use of a JetStar hydrogen gas generator. ERMD Report # 2004-032. Environment Canada: Ottawa, ON, Canada; January 14, 2005. [28] Liew C. "An investigation of H2 supplementation on the combustion and emissions of heavy-duty diesel engines". MSc Thesis. West Virginia University; 2010. [29] Brunt M, Pond C. Evaluation of techniques for absolute cylinder pressure correction. SAE Paper 970036, 1997. [30] Brunt M and Platts K. Calculation of heat release in direct injection diesel engines. SAE Paper 1999-01-0187, 1999. [31] Brunt M and Emtage A. Evaluation of burn rate routines and analysis errors. SAE Paper 970037, 1997. [32] Heywood JB. Internal combustion engine fundamentals. New York, NY: McGraw-Hill; 1988. [33] Woschni G. A universally applicable equation for the instantaneous heat transfer coefficient in the internal combustion engine. SAE Paper 670931, 1967.
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An experimental investigation of the combustion process of a heavy-duty diesel engine enriched with H2 C. Liew, H. Li*, J. Nuszkowski, S. Liu, T. Gatts, R. Atkinson, N. Clark Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506-6106, USA
article info
abstract
Article history:
This paper investigated the effect of hydrogen (H2) addition on the combustion process of
Received 6 April 2010
a heavy-duty diesel engine. The addition of a small amount of H2 was shown to have a mild
Received in revised form
effect on the cylinder pressure and combustion process. When operated at high load, the
10 June 2010
addition of a relatively large amount of H2 substantially increased the peak cylinder
Accepted 11 June 2010
pressure and the peak heat release rate. Compared to the two-stage combustion process of
Available online 7 August 2010
diesel engines, a featured three-stage combustion process of the H2ediesel dual fuel engine was observed. The extremely high peak heat release rate represented a combination of
Keywords:
diesel diffusion combustion and the premixed combustion of H2 consumed by multiple
Hydrogen
turbulent flames, which substantially enhanced the combustion process of H2ediesel dual
Diesel
fuel engine. However, the addition of a relatively large amount of H2 at low load did not
Dual fuel engine
change the two-stage heat release process pattern. The premixed combustion was
Combustion process
dramatically inhibited while the diffusion combustion was slightly enhanced and elongated. The substantially reduced peak cylinder pressure at low load was due to the deteriorated premixed combustion. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen has long been recognized as a fuel having excellent combustion and desirable emissions characteristics when burned in internal combustion (IC) engines [1]. The addition of H2 to spark ignition (SI) engines, known as H2-enrichment, was demonstrated to accelerate the flame propagation, extend the lean operational region, enhance the combustion stability at lean operation, improve the brake thermal efficiency, and reduce the exhaust emissions of carbon monoxide (CO), unburned hydrocarbons (HC) and possibly nitrogen oxide (NOx) if operated at an extremely lean mixture [1e5]. The mixture of H2enatural gas was used as fuel to power transit buses [6]. The lean burn SI H2 engines were developed to power shuttle van and passenger cars [7,8]. The detailed discussions of the positive features of H2 and H2-rich mixture
as clean alternative fuel of SI engines have been reported in the literature [9e11]. The use of H2 in diesel engines was initiated by its perceived potential to substantially reduce the emissions of particulate matter (PM) and the desired thermal efficiency of diesel engines. Also, the reduction in carbon dioxide (CO2) would clearly occur because the burning of H2 supplies energy without bringing carbon into the engine. The reduced CO2 emissions may also occur due to the improved thermal efficiency benefiting from the improved combustion. When burned in diesel engines, H2 is usually added into the intake mixture and burned with external ignition assistance, such as pilot injection of diesel fuel. This dual fuel operation mode has been demonstrated as a reliable and stable combustion mode for gaseous fuels, including H2 in compression ignition (CI) engine. Detailed information of the dual fuel engines can be found in the literature [12e15].
* Corresponding author. E-mail address: [email protected] (H. Li). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.06.023
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Extensive research has been conducted to explore the effect of H2 addition on the brake thermal efficiency and exhaust emissions of CI engines using diesel as either the pilot or main fuel. Varde and Frame [16] examined the effect of H2 addition on the brake thermal efficiency and the exhaust emissions of a direct injection (DI) single cylinder diesel engine. The addition of H2 was found to reduce the emissions of PM. However, the effect of H2 addition on the brake thermal efficiency to a large extent depended on the amount of H2 added. The addition of a relatively large amount of H2 improved the brake thermal efficiency. With the substitution of 12.5% diesel by H2 at full load operation, the brake thermal efficiency increased from 30.5 to 33.7%. However, the addition of a small amount of H2 (<5% of total intake energy) reduced the brake thermal efficiency. It was believed that this was due to the extremely lean H2eair mixture, which could not support the flame propagation and resulted in a low H2 combustion efficiency. The addition of H2 may affect the thermal efficiency not only through changing the overall combustion efficiency of the intake fuel, but also by changing the phasing of the combustion process with respect to piston top dead center position. The over retarding or advancing of the combustion phasing could reduce the brake thermal efficiency even when combustion was finished in a much shorter period indicating a dramatic improvement to the combustion process. However, Saravanan et al. [17] demonstrated that the addition of H2 improved the brake thermal efficiency for the range of engine loads explored. The negative effect of the addition of H2 on the brake thermal efficiency was not observed. Recently, the effect of H2 addition on the performance and exhaust emissions of diesel engines has been examined using light-duty multi-cylinder diesel engines [18e20]. Shirk et al. [18] investigated the effect of H2 addition on the thermal efficiency and exhaust emissions of a light-duty, four cylinder, diesel engine with common rail fuel injection system. The substitution of 5 and 10% diesel fuel with H2 was shown to slightly reduce the emissions of NOx while its effect on the brake thermal efficiency was negligible. McWilliam et al. [20] investigated the effect of H2 addition on the thermal efficiency and exhaust emissions of a light-duty, four cylinder, diesel engine with exhaust gas recirculation (EGR). The addition of H2 was shown to substantially increase the NOx emissions and dramatically improve the thermal efficiency. When operated at 5.4 bar brake mean effective pressure (BMEP) with 10% EGR, the mixing of 6% H2 (vol.) into the intake mixture increased the thermal efficiency from 23.8% to 30.7%. The heat release process provides fundamental information to explain the effect of H2 addition on the brake thermal efficiency and exhaust emissions of NOx. For example, Kumar et al. [21] investigated the effect of H2 addition on the combustion process of a small single-cylinder diesel engine with a rated power of 3.7 kW. The addition of H2 was shown to elongate the ignition delay period but enhance the combustion process indicated by the substantially increased the peak heat release rate. When operated at full load, the addition of 17% H2 (of total fuel mass) increased the maximum heat release rate from 38 J/ CA to 66 J/ CA. When operated at 40% load, the addition of 27% H2 (of total fuel mass) increased the maximum heat release rate from 23 J/ CA to 42 J/ CA. Roy et al. [22] investigated the combustion process of a single cylinder
dual fuel engine operated mainly on gaseous fuel mixtures of H2 and CO. After the gaseous fueleair mixture was ignited by a small diesel pilot (3.5e6.5% of the total intake energy), a singleestage combustion process, similar to that of the SI engines, was usually observed under normal operating conditions. However, when operated at high load, a two-stage combustion phenomenon was observed. The second heat release peak was observed at the middle of combustion process, which substantially increased the maximum heat release rate and enhanced the combustion process as indicated by the dramatically reduced combustion period. The rapid combustion period, defined as 10e80% mass fraction burned, took less than half the time of the normal single-stage combustion. Further increasing the engine load through the addition of more H2-rich gas mixtures was expected to result in severe engine knock. Correspondingly, the development of the featured second-stage combustion was considered as a precursor of knocking combustion, an indicator of the maximum addition of the reformed gases and maximum engine load. The authors observed that high increases in heat release, coupled with unchanged timing, resulted in higher incylinder pressures, and might increase indicated efficiency if optimized. Abu-Jrai et al. [23,24] investigated the influence of H2-rich gases, consisting of H2, CO and diluents, on the combustion and emissions characteristics of a single cylinder diesel engine with a rated power of 7.5 kW. The addition of the H2-rich gas mixture at moderate engine load was found to substantially enhance the premixed combustion and dramatically reduce the combustion duration. However, the addition of the H2-rich mixture at low load was demonstrated to retard the combustion phasing and prolong the combustion duration, which reduced the indicated thermal efficiency. The combustion phasing was retarded well beyond the point of obtaining the maximum efficiency. The increased emissions of the unburned gaseous fuels at low load might also contribute to the deteriorated thermal efficiency. Although extensive emissions and limited combustion data have been obtained using small single cylinder and lightduty multi-cylinder diesel engines, the corresponding research on the addition of H2 to heavy-duty diesel engines has not been reported. Existing experimental data indicated to some extent the dependence of the improvement to the brake thermal efficiency on the engine load and the amount of H2 added [16,24]. However, more research is required to explore the engine operation and H2 addition strategies of heavy-duty diesel engines. A well designed experiment will help to explain the effects of the addition of H2 on the thermal efficiency observed at different engine loads. For example, the different effect on the engine performance and emissions of the addition of H2 at low and high flow rate may be due to the success or failure of the development of turbulent flame that can burn all or at least most of the gaseous fuels supplied into the intake mixture. It was well known that a flame cannot propagate through a fueleair mixture leaner than the lean flammability limit of the gaseous fuel [25]. Such a limit of dual fuel engine may substantially vary with the changes in engine load, the amount of pilot diesel injected, the injection timing [26], intake pressure boosting, the application of EGR and even the pressure of diesel fuel injection. Detailed research on the operation of H2ediesel dual fuel engine near the lean
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flammability limit of H2eair mixture may provide valuable information to explain the different effects of H2 addition on the engine performance, combustion and exhaust emissions reported in the literature. For better understanding of the potential of H2 in affecting the brake thermal efficiency and exhaust emissions of heavyduty diesel engines, there is a need to explore the detailed effect of H2 addition and engine load on the combustion process including the ignition delay, combustion duration, heat release rate and combustion phasing. The correlation of the combustion process of H2ediesel dual fuel engines with the fundamental combustion properties (such as the flammability limits) of H2 may help to explain the undesirable effects of the addition of H2 on engine performance, combustion process and exhaust emissions. The data describing the minimum H2 addition needed to improve the brake thermal efficiency and enhance the combustion process could provide guidelines for the development of H2ediesel dual fuel engine and the establishment of H2 addition and diesel injection control strategies. The combustion data obtained with the addition of a very small amount of H2 may also provide guideline to the integration of a small H2O-electolyzer with the diesel engines [27]. In this paper, the effect of H2 addition on the combustion process was presented.
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2. Experimental set-up and combustion analysis
and pressure regulation system used to provide steady H2 flow to the engine test cell; (b) a low pressure H2 metering and delivery system designed to control and measure the flow rate of H2 and its delivery to the main stream of the intake air; and (c) a H2 safety system designed to eliminate the unexpected high pressure hazards in the event of backfire. The high pressure H2 fuel storage and pressure regulation system consisted of two sets of pressure regulators, each connected with six high pressure H2 tanks and used to provide steady H2 flow with gauge pressure of 45 psi. There were two valves employed to shut down the H2 flow in case of emergency and also when the flow of H2 was not needed. One valve installed after the pressure regulator was located outside the laboratory and the other installed prior to the H2 filter was located near the engine test cell. The low pressure H2 fuel system consisted of a H2 filter, a mass flow controller for controlling and measuring the flow rate of H2, and a H2eair mixer used to continuously deliver H2 into the main stream of the intake air. The safety system consisted of a stainless steel flame arrester and a pressure relief valve, installed between the intake manifold and flame arrester. In case of backfire, the flame arrester could quench the propagation of the flame into the intake mixture. The pressure relief valve could quickly relieve excessive pressure in the intake system resulting from the burning of the H2eair mixture in case of backfire. Detailed description of the H2 fuel system and the safety approach can be found in the literature [28].
2.1.
2.3.
Test engine and dynamometer
The engine used was a 1999 Cummins ISM370 diesel engine with a rated power of 370 horsepower. This turbocharged, 6-cylinder, diesel engine was widely used to power Class-8 heavy-duty trucks and was certified by the EPA as having emissions at or below 4.0 g/bhp-hr NOx and 0.1 g/bhp-hr PM. EGR management was introduced into the ISM engines three years after this model. Detailed specifications for this engine can be found in Table 1. The test engine was coupled to a 550 hp General Electric (GE) Direct Current (DC) dynamometer used to absorb the engine load and control engine speed. Torque was controlled by supplying an electronic throttle signal to the engine and controlling the amount of diesel fuel injected. This was the same experimental arrangement used to gather emissions data for Ref. [28].
2.2.
H2 fuel system
As shown in Fig. 1, the H2 fuel system consisted of the following three sub-systems: (a) a high pressure H2 storage
Table 1 e Specifications of the test engine. Engine Model Displacement Bore Stroke Power Rating Torque Rating Configuration
1999 Cummins ISM370 10.8 L 125 mm 147 mm 276 kW @2100 RPM 1830 Nm @1200 RPM Inline 6-cylinder
Pressure acquisition
In-cylinder pressure was acquired using a Kistler model 6125C pressure transducer at 0.25 degree crank angle (CA) resolution. In each test, 200 consecutive cycles were collected and averaged. A low-pass filter with a cut-off frequency of 2500 Hz was applied to the measured in-cylinder pressure to reduce the high frequency combustion noise. The dynamic pressure of the in-cylinder pressure transducer was referenced using a constant polytropic coefficient without the need of measuring the intake manifold air pressure (MAP) [29]. The averaged in-cylinder pressure was processed to obtain a set of cylinder pressure parameters such as the peak cylinder pressure (PCP) and peak cylinder pressure rise rate, and their phasing. Using the averaged cylinder pressure, the indicated engine performance metrics such as the indicated work per cycle and the indicated mean effective pressure (imep) could also be calculated [29e31].
2.4.
Measurement of exhaust emissions
The raw engine exhaust was ducted to a full-scale dilution tunnel with flow control accomplished using a critical flow venturieconstant volume sampler (CFVeCVS) system. A 25.4 cm diameter orifice, located 1 m from the tunnel entrance, ensured that the exhaust was thoroughly mixed with dilution air before reaching the emissions sampling zone, located ten tunnel diameters downstream. The CFVeCVS controlled the dilute exhaust flow at a nominal 68 standard cubic meter per minute (scmm) throughout this test program. Instantaneous measurement of CFVeCVS flow was
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Fig. 1 e Schematic diagram of H2 fuel system.
accomplished using a fast-response thermocouple and pressure transducer installed at the venturi entrance. The diluted exhaust gas was sampled from the full flow dilution tunnel through heated sample probes, lines and finally fed into the individual emissions analyzers. The emissions of CO and CO2 were measured using non-dispersive infrared (NDIR) analyzers. The HC emissions were measured using a heated flame ionization detector (HFID). The emissions of CO2, CO and HC measured were further processed to obtain the combustion efficiency of diesel fuel, defined as the percentage of carbon burned to CO2. The emissions of NOx and PM were also measured but not reported in this paper. The raw exhaust gas for measurement of the emissions of H2 was dried by flowing it through a chiller to cool the exhaust gas to 4 C and so remove the moisture contained in the exhaust gas. The dried exhaust gas was then sampled into 10-L sampling bags at a rate of 4 L per minute for the later analysis of the H2 concentrations. Prior to the sampling of the exhaust gas, the sampling system was purged with exhaust gas for 40 s to remove any residual gas that may have been present in the sampling system. In this research, the exhaust emissions of H2 were measured using an Electron Pulse Ionization (EPI) Mass Spectrometer (MS). The H2 emissions values measured were further processed to obtain the combustion efficiency of H2, defined as the percentage of H2 burned in the engine relative to that added to the intake mixture.
2.5.
Combustion analysis
In this research, the heat release rate was calculated using the well-known single zone zero-dimensional heat release model
[32]. With the assumptions of a uniform pressure, uniform temperature and ideal gas, the heat release rate can be calculated using the equation (1). dQ g dV 1 dP dQ ¼ P þ V þ dq gross g1 dq g1 dq dq ht
(1)
Where g is the specific heat ratio calculated according to the composition and temperature of the bulk mixture calculated using ideal gas equation with the in-cylinder pressure, cylinder volume at the intake valve closing as a reference condition. The well-known Woschni equation [33] was used to calculate the heat transfer from the bulk mixture to the coolants, the last term in equation (1), which allows the differencing of net and gross heat release rates. The temperature of the combustion chamber was considered as the same of the coolants. In this research, the gross heat release rate was reported. The heat release rate was further processed to obtain a set of combustion parameters such as the peak heat release rate (PHRR) and its phasing, the mass fraction burned (although time-dependent combustion efficiency is not known), start of combustion (SOC) (defined as the crank angle when the heat release rate reached 0.05 kJ/ CA in premixed combustion), and combustion duration defined as the crank angle period between the 10 and 90% of the total heat released through combustion. Other combustion parameters such as the ratio of the heat release in premixed combustion relative to the total heat released can also be calculated. In this research, the effects of H2 addition and engine load on the cylinder pressure, heat release process and combustion
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3.
Experimental results and analysis
The combustion efficiency of diesel engine was extremely high (>99.5%). As shown in Fig. 2, the addition of H2 at 70% load did not further improve but slightly deteriorated the combustion efficiency of diesel fuel. In comparison, the combustion efficiency of H2 was dramatically affected by the amount of H2 added. With the addition of 6% H2 into the intake air, the combustion efficiency of H2 observed was 97.9%, which was much lower than that diesel fuel. When a small amount of H2 was added, the relatively low combustion efficiency of H2 as shown in Fig. 2, might deteriorate the brake thermal efficiency of the diesel engine. The possible
100
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96
185
Hydrogen
92
90 0
1
2
3
4
5
6
H2/(H2+Air), vol. % Fig. 2 e Effect of H2 addition on combustion efficiency of diesel and H2, 70% load.
7
4
8
12
16
20
24
improvement to the brake thermal efficiency of diesel engine by the addition of H2 can only be obtained through the improvement of the heat release process such as the release of the heat at optimized phasing and short period noted as the combustion duration. Fig. 3 shows the effect of H2 addition on the cylinder pressure at 70% load. With the addition of a relatively small amount (<3%) of H2, the cylinder pressure prior to the initiation of the combustion was higher than diesel operation. This might be due to the widely divergent thermodynamic properties of H2 and air, and the variation of the increased intake pressure. As shown in Fig. 4, the addition of a small amount of H2 increased the intake pressure, which might due to the changes in exhaust temperature prior to the turbo-changer resulted from the variation of the combustion process, coupled with the behavior of the turbocharger and waste gate. The maximum intake pressure was observed with the addition of 2% H2. Further increasing the addition of H2 gradually reduced the intake pressure. With the addition of H2 beyond 3.5%, the intake pressure observed was lower than diesel operation. Correspondingly, the cylinder pressure prior to the initiation of combustion was lower than diesel operation with
190
Diesel
0
Fig. 3 e Effect of H2 addition on cylinder pressure, 70% load.
98
94
-4
Crank Angle, CA ATDC
Intake Pressure, kPa
Combustion Efficiency of Diesel and H2 %
100
0.0% 2.0% 3.0% 4.0% 5.0% 6.0%
120 Cylinder Pressure, bar
duration of a heavy-duty diesel engine were experimentally investigated. The amount of H2 added was expressed as the volumetric fraction of H2 in the intake mixture [H2/(Air þ H2), vol. %]. The engine load was varied from 15 to 70% when operated at 1200 RPM. With the increasing addition of H2, the flow rate of diesel fuel was gradually reduced to maintain the constant load operation. With the addition of up to 6.0% H2 (equivalence ratio of 0.15) in the intake air, the energy of H2 contributed as much as 77% of the total intake energy when operated at 15% load and as little as 31% when operated at 70% load. To address the feasibility of the small H2O electrolyzers in improving the combustion of diesel engines [27], the effect on the combustion process of the addition of a very small amount of H2 (0e1% in intake mixture) was also investigated. In order to better understand the effect of H2 on oxidation of diesel fuel, the effect of H2 on the combustion efficiency of diesel fuel and H2 was also evaluated. The combustion efficiencies of diesel fuel and H2, defined as the percentage of corresponding fuel completely burned in the combustion chamber, were calculated using the exhaust emissions of CO, CO2, unburned HC and H2 measured in this research [28]. In this research, the diesel fuel used was ultra low sulfur pump diesel. The supplemented H2 fuel used was industry purity H2 having a purity of over 99.5%.
180
175
170
0
1
2
3 4 5 H2/(H2+Air),vol.%
6
7
Fig. 4 e Effect of H2 addition on the intake pressure, 70% load.
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14
115 13 110 12
105 100
11 0
1
2
3
4
5
6
7
H2/(H2+Air),vol.%
14
0.6 Phasing of PHRR 0.5
Peak Heat Rel. Rate 12
0.4 0.3
10
0.2 8
0.1 0
Fig. 5 e Effect of H2 addition on peak cylinder pressure and its phasing, 70% load.
Phasing of PHRR, CA ATDC
15 Phasing of PCP Peak Cyl. Pressure
Phasing of PCP, CA AT DC
Peak Cylinder Pressure, bar
125
Peak Heat Release Rate, kJ/CA
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1
2
3
4
5
6
7
H2/(H2+Air), vol. % Fig. 7 e Effect of H2 addition on peak heat release rate and its phasing, 70% load.
Heat Release Rat e, kJ/CA
0.6
0.0% 2.0% 3.0% 4.0% 5.0% 6.0%
0.5 0.4 Combination of H 2 Premixed comb. and diesel diffu. comb.
0.3 0.2 0.1
Early and late stage diesel diffu. comb.
0
observed. Compared to the premixed combustion, the addition of H2 was found to have more significant effect on the diffusion combustion. As shown in Fig. 6, the addition of H2 gradually enhanced the heat release rate observed at the middle of the diffusion combustion. Following the initiation of the diffusion combustion, a sudden increase in heat release rate beyond that of the featured diffusion combustion of diesel engine was observed when a relatively large amount of H2 was added. The increased heat release rate most likely represented the combination of the diesel diffusion combustion and the H2eair combustion, which burned the H2 quickly through the propagation of multiple turbulent flames initiated by the combustion of the diesel. The featured multiple turbulent flames propagated through the H2eair mixture presented outside the diesel spray plume, enhanced the heat release process of the H2ediesel dual fuel engine and might have accelerated the combustion process of the diesel fuel. When the multi turbulent H2eair combustion completed or quenched due to the slow reaction rate, the relatively slow diesel diffusion combustion continued until its completion, noted as the late diesel diffusion combustion. Unfortunately, it is difficult to hypothesize how much synergy exists between
Combustion Duration,CA
the addition over 3.5% H2. As shown in Fig. 3, the addition of H2 increased the cylinder pressure after the combustion was initiated. This was further demonstrated by examining the effect of H2 addition on the peak cylinder pressure (PCP). As shown in Fig. 5, the addition of a small amount of H2 increased the PCP without altering the phasing when PCP was observed. Further increasing the addition of H2 beyond 3.5% substantially increased the value of PCP observed at gradually advanced the phasing. The addition of 6% H2 increased the PCP from 106.4 to 120.5 bar and advanced its phasing by 1.5 CA. As indicated by the substantially increased PCP, the addition of H2 to a diesel engine at high load should be limited to govern the PCP for both safety and mechanical durability. Fig. 6 shows the effect of H2 addition on the heat release process. When operated at pure diesel, the diesel engine featured two-stage combustion process was observed. A premixed combustion was followed by the diffusion combustion which was dominated by the diesel fuel injection, vaporization and mixing process. The addition of H2 was shown to slightly retard the initiation of the premixed combustion and reduce the peak heat release rate obtained in the premixed combustion. Compared to diesel operation, the addition of 6% H2 retarded SOC by 0.8 CA. The substantial retarding effect of H2 on the SOC of this heavy-duty diesel engine was not
36 32 28 24 20
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Crank Angle, CA ATDC Fig. 6 e Effect of H2 addition on heat release process, 70% load.
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H2/(H2+Air), vol. % Fig. 8 e Effect of H2 addition on combustion duration, 70% load.
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60
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Crank Angle, CA ATDC
Fig. 9 e Effect of H2 addition on cylinder pressure, 30% load.
the combustion of the diesel and H2, as opposed to their burning separately in different modes. At the molecular level, there must surely be interaction of the H2 with the intermediate species associated with diesel combustion, but the extent of the interaction is unclear and must depend on both combustion chemistry and the physics of local mixing. Of course, the term “diffusion combustion” may account both for traditional diesel fuel diffusion combustion and the rapid development of flame fronts in the H2eair mixture simultaneously. The latter is similar to the combustion of lean burn SI H2 engine but burns much faster considering the multi-point ignition features of the pilot diesel. Fig. 7 shows the effect of H2 addition on the peak heat release rate (PHRR) and the phasing when PHRR was observed. The addition of a small amount of H2 gradually increased the PHRR without altering the phasing when the PHRR was observed. Further increasing the addition of H2 beyond 3.5% substantially increased the PHRR observed during diffusion combustion and started to advance the phasing when PHRR was observed. With the addition of 6% H2, the peak heat
Fig. 11 e Effect of H2 addition on cylinder pressure, 15% load.
release rate of 0.53 kJ/ CA was obtained. Compared to that PHRR of 0.25 kJ/ CA obtained with diesel operation, the addition of 6% H2 increased the PHRR by 109%. The PHRR phasing was advanced by 2.5 CA. As shown in Fig. 8, the addition of H2 beyond 2% started to reduce dramatically the combustion duration, indicating the enhancing effect of H2 on the diffusion combustion process. With the addition of 6% H2, the combustion duration observed was 23.8 CA, a 23% reduction compared to the diesel operation. This also indicated the need for the optimization of the combustion phasing in a dedicated H2ediesel dual fuel engine design especially when a relatively large amount of H2 was added. Fig. 9 shows the effect of H2 addition on cylinder pressure when operated at 30% load. The addition of H2 slightly increased the cylinder pressure with its peak value observed at slightly advanced phasing. As shown in Fig. 10, the peak heat release rate was observed at the premixed combustion stage. The addition of H2 was shown to retard slightly, but enhance the premixed combustion. The addition of H2 also enhanced the heat release process during the transition
0.4
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Heat Release Rat e, kJ/CA
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Crank Angle, CA ATDC Fig. 10 e Effect of H2 addition on heat release process, 30% load.
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Crank Angle, CA ATDC Fig. 12 e Effect of H2 addition on heat release rate, 15% load.
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Peak Heat Release Rat e, kJ/CA
0.6 70% Load 0.5
30% Load 15% Load
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3 4 5 H2/(H2+Air), vol. %
6
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Fig. 13 e Effect of H2 addition and engine load on peak heat release rate.
period from the premixed combustion to the diffusion combustion. Compared to 70% load operation, the effect of H2 addition on the peak cylinder pressure, peak heat release rate and their phasing was relatively small. Fig. 11 shows the effect of H2 addition on the cylinder pressure at 15% load. Although having mild effect on the cylinder pressure prior to the initiation of combustion, the addition of H2 substantially reduced the cylinder pressure after the combustion was initiated when a large amount of H2 was added. As shown in Fig. 12, the addition of H2 at 15% load substantially reduced the heat release rate observed during the premixed combustion. However, the diffusion combustion process was slightly enhanced and elongated. The deteriorated premixed and elongated diffusion combustion made the combustion process lasted for a longer time and correspondingly deteriorated the engine performance. Fig. 13 compares the effect of H2 addition on the PHRR observed at different load. The significant effect of H2 on the PHRR was only observed with the addition of relatively large amounts of H2. The addition of over 3% H2 substantially
Combust ion Durat ion, CA
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Dramatic Change to Combustion Duration 30
20
70% Load
30% Load
15% Load
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3 4 5 H2/(H2+Air), vol. %
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Fig. 14 e Effect of the addition of H2 and engine load on combustion duration.
enhanced the PHRR at 70% load. In comparison, the addition of over 4% H2 was found to substantially deteriorated combustion when operated at 15% load. In comparison, the addition of H2 at 30% load was shown to slightly increase the PHRR. Fig. 14 compares the effect of H2 addition and engine load on the combustion duration. The addition of a small amount of H2 had negligible effect on the combustion duration. However, as indicated by the gradually reduced combustion duration, the addition of a relatively large amount of H2 at 70% and 30% load enhanced the combustion process. In comparison, the addition of a large amount of H2 at 15% load was shown to elongate the combustion duration indicating the deteriorated combustion process. It was also noted that the dramatic changes in combustion duration was observed with the addition of 2% H2 at 70% load. With the decrease in engine load, such a substantial change was observed at increasingly higher concentration of H2 in the intake mixture. When operated at 30% load, the positive effect on combustion duration was observed with the addition of over 3% H2. This may help to explain the requirement for the addition of relatively a large amount of H2 to obtain the improved brake thermal efficiency when operated at a lower load. When operated at 15% load, the substantial change to the combustion duration was observed with the addition of over 5% H2, which was mainly due to the inhibited premixed combustion and elongated diffusion combustion as shown in Fig. 12. As indicated by the variation of the PHRR and combustion duration shown in Figs. 13 and 14, the addition of small amount of H2 (less than 2%) had negligible effect on the combustion process. The integration of a small water-electrolyzer may not be able to significantly improve the performance of the diesel engines.
4.
Conclusions
The effect of the addition of H2 on combustion process of this heavy-duty diesel engine depended on the load and the amount of H2 added. The addition of a small amount of H2 had only mild or negligible effect on the cylinder pressure and combustion process. A substantial effect on the cylinder pressure and combustion process was only observed with the addition of a relatively large amount of H2. The addition of a relatively large amount of H2 at 70% load substantially increased the peak cylinder pressure. Accordingly, the addition of H2 at high load should be limited to govern the peak cylinder pressure, for both safety and mechanical durability. Compared to the combustion process of diesel engine, the addition of a relatively large amount of H2 at 70% load dramatically reduced the combustion duration and enhanced the peak heat release rate observed at the middle of diffusion combustion. The extremely high peak heat release rate observed represented a combination of fast turbulent combustion of H2 and the diffusion combustion of diesel. The addition of a relatively large amount of H2 at 15% load substantially deteriorated the premixed combustion but slightly enhanced and elongated the diffusion combustion.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 1 3 5 7 e1 1 3 6 5
When operated at 30% load, the addition of H2 had a small effect on the cylinder pressure and combustion process. The addition of H2 had no positive effect in enhancing the combustion efficiency of diesel fuel. The combustion efficiency of H2 added was much lower than that of diesel fuel especially when the amount of H2 added was small.
Acknowledgement The preparation of this paper is based on the work funded by the State of Texas through a grant from the Texas Environmental Research Consortium, with funding provided by the Texas Commissions on Environmental Quality.
references
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