Study on cycle-by-cycle variations in a diesel engine with dimethyl ether as port premixing fuel

Applied Energy 143 (2015) 58–70

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Study on cycle-by-cycle variations in a diesel engine with dimethyl ether as port premixing fuel Ying Wang ⇑, Fan Xiao, Yuwei Zhao, Dongchang Li, Xiong Lei School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China

h i g h l i g h t s  Cyclic variations of diesel engine with DME as premixing fuel were examined.  Coefficients of cyclic variations of main combustion parameters were determined.  Port DME quantity drastically influenced cyclic variations of DME-diesel engine.  Cyclic variations in maximum in-cylinder pressure increased with a rise of DME.

a r t i c l e

i n f o

Article history: Received 20 March 2014 Received in revised form 7 August 2014 Accepted 29 December 2014

Keywords: Cycle-by-cycle variations Premixed charge DME Diesel

a b s t r a c t Cycle-by-cycle variations in some parameters of the combustion and performance during a dual-fuel premixed charge compression ignition (PCCI) combustion process were investigated on a modified direct injection diesel engine with dimethyl ether (DME) as premixing fuel. Experiments were conducted under the two different loads with the different DME quantities at a constant engine speed (1700 r/min). Data of in-cylinder pressure of 100 consecutive combustion cycles for each test condition were recorded. The cycle-by-cycle variations of the maximum pressure (pmax), the maximum mass-averaged temperature (Tmax), the maximum rate of pressure rise ((dp/du)max), the maximum rate of heat release (dq/du)max and the indicated mean effective pressure (IMEP) were analyzed and evaluated using the coefficient of variation (COV) of each parameter. The results show that both COVpmax (<3%) and COVTmax (<3%) are rather small with the different DME quantities. The varying trend of (dp/du)max is similar to that of (dq/du)max. The COVs of (dp/du)max and (dq/du)max decrease slightly but then increase with an increase in DME quantity. With a large DME quantity, a relatively wide range of cycle-by-cycle variations in IMEP and a higher COVIMEP can be observed in the engine operated in a DME-diesel dual fuel PCCI combustion mode, limiting an operating range of the engine. The results indicate that the DME quantity should be optimized under the different conditions to minimize the cycle-by-cycle variations and concurrently extend the operating range of the diesel-DME PCCI engines. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Limit in the future petroleum supply and high cost of oil refining and production call for an effort to use the alternative fuels through a whole area of transportation. Stringently imposed air pollution standards also dictate a shift to move toward the decentralized, diffused and renewable energy sources and the highefficiency combustion technologies. Port premixed charge compression ignition (PCCI) combustion technologies have received widespread attentions in recent decades because NOx and particulate matter (PM) emissions can be ⇑ Corresponding author. Tel.: +86 29 82668726; fax: +86 29 82668789. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.apenergy.2014.12.079 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

reduced simultaneously in the PCCI engines. Moreover, almost the same high thermal efficiency as that of the traditional diesel engines can be maintained in the PCCI engines during most operation conditions. In port PCCI operation, the mixture of fuel, air and residual gas is close to homogeneous, and diesel as well as the mixture ignites on the compression concurrently at several locations without any external ignition sources. In such a combustion mode, NOx may be reduced through a local lean mixture and EGR. Meantime, PM can be decreased by promoting air/fuel mixing process before the direct-injection compression ignition (DICI) combustion. Many kinds of fuels including liquefied petroleum gas (LPG) [1], natural gas [2,3], hydrogen [4], and diethyl ether [5], etc. have respectively been studied as the premixing fuels. Numerous studies [5–7] have proved that the port PCCI combustion technology

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Nomenclature COV DME PCCI DICI HCCI SI TDC ROHR ROPR

coefficient of variation dimethyl ether premixed charge compression ignition direct injection compression ignition homogeneous charge compression ignition spark ignition top dead center rate of heat release rate of pressure rise

is one of the effective means to realize low exhausts and high thermal efficiency in the engines. Recently, dimethyl ether (DME) has become a promising alternative fuel for its wide and renewable resource. It has been reported that when DME fuel partially substitutes diesel fuel in the engines, the low emissions as well as the high brake thermal efficiency can still be maintained in the engines [8–11]. With DME as a kind of premixing fuel in the diesel engine, there is almost no large modification in the engine’s structure because DME easily mixes homogenously with air in an intake port. Oxygenate DME’s introduction from the port also brings no obvious negative influence on volumetric efficiency. Meanwhile, the characteristic of no C–C bond in DME molecule may be advantageous to the reduction in PM emission. Chapman and Boehman [12] investigated the pilot ignited premixed combustion of DME in a turbo-diesel engine. The research showed that when DME was fumigated in an intake pipe, it ignited during a premixed combustion process and in-cylinder pressure exhibited an evidence of both an earlier start of combustion and an increased peak pressure. Furthermore, the energy conversion efficiency was increased, and NOx emission was reduced with the DME’s induction. Wang et al. [13] found that with port premixing DME, the heat-release process would be composed of the premixed charge homogeneous charge compression ignition (HCCI) combustion and the diffusive combustion at some conditions. In-cylinder fuel injection timing and port premixing DME quantity played important roles in a control of combustion and emission. They had little impact on the position of maximum rate of heat release (ROHR) during a low-temperature reaction (LTR) phase. However, they had great effects on the values and crank-angle positions corresponding to the maximum ROHR during the high-temperature reaction (HTR) phase and the diffusive combustion phase. According to previous studies, in a typical DME-diesel dual fuel PCCI engine, the DME HCCI combustion occurs before the diesel DICI combustion due to DME’s higher cetane number and superior autoignition ability. Thus, the DME-diesel dual fuel PCCI combustion can be considered as a compromise between the HCCI and the conventional DICI combustion. It is well accepted that the HCCI combustion process is dominated by chemical kinetics of in-cylinder mixture charge. At a higher load, an earlier start of combustion and a faster burning rate possibly result in high pressure oscillations and resultant knock. At a lower load, a lower initial in-cylinder temperature and relatively leaner mixture usually lead to partial combustion or misfire. Consequently, larger cycle-by-cycle variations are frequently found in the HCCI combustion engines compared to DICI engines, and they even can be significant especially at certain operating conditions. The cycle-by-cycle variations in a combustion process have long been concerns to the researchers because it adversely affects engine’s performance, causes losses in power and efficiency as well as deterioration in emissions. It is well known that the cycle-bycycle variations are the result of variations in the combustion.

LTR low-temperature reaction HTR high-temperature reaction IMEP indicated mean effective pressure pmax maximum in-cylinder pressure (dp/du)max maximum rate of pressure rise Tmax maximum mass-averaged in-cylinder temperature (dq/du)max maximum rate of heat release qDME port DME quantity

Generally mentioned cycle-by-cycle variations related factors include mixture composition, cyclic cylinder charging and in-cylinder mixture motion and so on. However, causes for the cycle-tocycle variations from the SI, DICI and HCCI engines may be different. Ozdor et al. [14] reviewed the researches on cyclic variations in the SI engines and concluded that the cyclic variations are generally attributed to the results of random fluctuations in equivalence ratio and flow field due to the turbulent nature of the flow. These fluctuations contribute to an imperfect charge mixing, partial stratification, variations in laminar speed, random heat transfer from the burning kernel to the spark electrodes, etc., and these causes are interrelated. Due to the lower cycle-by-cycle variations in the diesel engines, the analyses on cycle-by-cycle variations for diesel engines do not synchronize with the work for SI engines. Shoji [15] reported spray direction variations are the main sources for the cycle-to-cycle variations of performance at the low speed and low load. Zhang et al. [16] pointed out the cycle-to-cycle variations of in-cylinder flow field could be one source of performance variations in the diesel engines. Zhong et al. [17] reported the variations in the Pmax, Tmax and IMEP were correlated with the variations in the injection rate for a common rail DI diesel engine. However, Kouremenos et al. [18] used the stochastic analysis technique to interpret of the fluctuation phenomena in the diesel combustion and found the injection timing and ignition delay had no impact on the cyclic variation in pmax. Rakopoulos et al. [19] revealed the stochastic nature of the combustion fluctuation in the diesel engine through the analysis of probability density functions of the parameters related to combustion. In the HCCI engines, the mixture is never truly homogeneous [20], and auto-ignition initiates at the locations with the high reactivity. Ignition timing of HCCI combustion is totally controlled by the chemical kinetics and is thus influenced by the equivalence ratio, fuel composition and thermodynamic nature of air–fuel mixture. It is generally regarded that the major sources causing the cyclic variations in the HCCI engines include temperature inhomogeneity, inhomogeneity of mixture composition, fluctuations in air fuel ratio, diluents and turbulence intensity and so on [21]. Furthermore, in the HCCI engines, the chemical kinetics and aerodynamics of combustion have coupled controlling roles of the cyclic variations. The combustion of the current cycle is generally affected by the combustion completeness of the prior cycle in the HCCI combustion [22]. Up to present, a great number of systematic studies have revealed mechanisms and control strategies on the cycle-by-cycle variations of SI [14,23–25], HCCI [20,21,26–28] and DICI [15–19,29] engines. However, relatively little work has been reported on the cycle-by-cycle variations of a dual-fuel PCCI combustion, especially for a DME-diesel dual fuel PCCI combustion. Systematic study of the cycle-by-cycle variations of diesel-DME dual fuel PCCI combustion engines is essential to understand such a kind of combustion mode and successfully implement an advanced

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combustion control algorithm in the engines. Therefore, some combustion and performance parameters such as the maximum pressure (pmax), maximum mass-averaged temperature (Tmax), maximum rate of heat release ((dp/du)max), maximum rate of pressure rise ((dq/du)max) and indicated mean effective pressure (IMEP) were used to quantify cycle-by-cycle variations in this study. The chief goal of the study reported in this paper is to obtain the understanding of cycle-by-cycle variations of the diesel engine with the different DME premixing quantities under test conditions.

Table 1 Accuracies of measurement and uncertainties of test parameters. Parameter

Instrument accuracy

Uncertainty

Engine speed Engine torque Cylinder pressure Crankangle Fuel mass consumption rate NOx Smoke

±1 r/min ±0.01 N m ±0.0005 MPa ±0.5 °C – ±1 ppm ±0.01 m1

±1% ±0.4% ±1% – ±2% ±0.5% ±1%

2. Experimental setup Tests were conducted in a two-cylinder, four-stroke, naturally aspirated, DICI diesel engine with a bore of 105 mm, stroke of 120 mm, compression ratio of 17. DME was inducted to an intake port, and an approximately homogeneous DME/air charge was formed prior to a start of combustion. Diesel was injected at the timing of 7°CA BTDC by the conventional diesel DICI fuel system. The injection nozzle had 4 holes with the diameter of 0.32 mm and the injector opening pressure was 19 MPa. A schematic of the experimental setup was shown in Fig. 1. The diesel and DME fuels used in this study were both the commercial products. Experiments were carried out under the operating conditions at the speed of 1700 r/min, the brake mean effective pressure (BMEP) of 0.24 MPa and 0.36 MPa. The previous study indicated that audible knocking would be observed in the case of a large port DME quantity and a high load. Consequently, at the BMEP of 0.24 MPa, the port DME quantity was selected as 0 g/s, 0.2 g/s, 0.3 g/s and 0.5 g/s respectively, and at the BMEP of 0.36 MPa, the port DME quantity was set as 0 g/s, 0.3 g/s, 0.4 g/s and 0.6 g/s respectively. If the fuel heat input is considered, the port DME energy ratio (the ratio of the port DME heat energy to the total fuel heat energy) was thus 0%, 19%, 28% and 40% respectively at the BMEP of 0.24 MPa, and was 0%, 21%, 29% and 39% respectively at the BMEP of 0.36 MPa. In the experiment, an oil temperature remained at about 90 °C by an oil temperature control device (FC2430) and a cooling water temperature was kept at about 80 °C by a water temperature control system. In the experiment, the accuracies of measurement and uncertainties of some parameters were presented in Table 1. Pressure measurement can reflect a combined influence of combustion on the air–fuel mixing process and compression process of the cylinder volume. Consequently, the pressure as well as the related combustion parameters is widely used to judge the level of combustion cyclic variations in an engine. In this paper, a water-cooled Kistler 6125A piezoelectric pressure transducer was installed into a combustion chamber to record a history of 100 in-cylinder pressure cycles for each test condition at a resolution of 0.5 crank-angle. A crank angle signal was obtained from Kistler 2629B shaft encoder mounted on a main shaft. Adequate care

cylinder pressure signal diesel intake

has been taken to position Kistler 5011 amplifier to minimize fluctuations in signal noise. Information about in-cylinder pressure data and crank-angle position was recorded in DL750 Scopecorder digital data-acquisition system. Smoothing for the pressure signal was applied that was based on a cubical smoothing algorithm with five-point approximation. ROHR was obtained from the acquired in-cylinder pressure data using a 0-dimensional heat release model, in which heat transfer was calculated based on Woschni correlation [30]. In-cylinder mass-averaged gas temperature was calculated based on ideal gas law by assuming an even in-cylinder temperature distribution. Coefficient of variation (COV) is the most frequently used indicator to investigate the cycle-by-cycle variations in an engine. The COV of any parameter (x) can be calculated by Eq. (1) [18,26,28,30].

COV ¼

r x

 100%;

x is an average of x. where r is calculated from Eq. (2). 

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 i¼1 ðxi  xÞ r¼ ; n1

3. Experimental results and discussions 3.1. Combustion process of a diesel engine with DME as port premixing The combustion process for DME-diesel dual fuel PCCI case was not typical of DICI combustion case and seemed complicated to some extent. Fig. 2 gives the curves of pressure, mass-averaged temperature, ROPR and ROHR for the diesel engine with the different DME quantities at the speed of 1700 r/min and the load of

exhaust

NOx analyzer

smoke meter

dynamometer crank angle encoder Fig. 1. Schematic of the experimental setup.

ð2Þ

where n is the number of cycles in a sample.

DME

data acquisition system

ð1Þ

Fig. 2. The combustion process for a DME-diesel dual-fuel case.

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0.36 MPa. Obviously, the DME-diesel dual fuel PCCI combustion process usually demonstrates a staged combustion behavior, especially with a larger DME quantity. In Fig. 2 with a low DME quantity (qDME = 0.3 g/s), a faint DME LTR phase is observed, however HTR stage is not obvious because of a relatively lean premixed mixture. After the LTR stage, a traditional DICI combustion from directinjected fuel is displayed. Nevertheless, with a large DME quantity (qDME = 0.6 g/s), a typical DME two-stage HCCI combustion event distinctly appears. The diffusion combustion emerges with a three-stage heat release behavior, similar three stage behavior has also been reported by [31,32]. ROPR generally shows a level of pressure oscillation and also reveals some combustion characteristics. Like a curve of heat release process, the curve of ROPR also presents a staged characteristic. There is one peak in the curve of ROPR for the DICI case. However, because the HCCI and the DICI combustion emerge sequentially, the number of peaks in the curves of ROPR for the DME-diesel dual fuel PCCI cases is not one. With a larger DME quantity (qDME = 0.6 g/s), corresponding to the three-stage heat release behavior, ROPR also exhibits the staged behavior. It is also observed From Fig. 2 that pmax and Tmax increase with an increase in DME quantity; meanwhile, the positions of crank-angle corresponding to pmax and Tmax gradually advance. This is mainly because a self-ignited combustion of premixed homogenous charge leads to an advanced start of combustion and an elevated temperature, promoting diesel’s complete combustion and finally causing higher pmax and higher Tmax. NOx and smoke emissions for a DME-diesel dual-fuel operation at the speed of 1700 r/min and the load of 0.36 MPa are exhibited in Fig. 3. Due to a decrease in the diffusion phase for DME-diesel PCCI combustion and the smokeless nature of DME, smoke emission is observed to be reduced with a rise of qDME in Fig. 3. NOx emission of diesel-DME dual fuel operation is slightly lower compared with that of diesel DICI operation, and shows a decrease with a rise of qDME initially but this decreasing trend discontinues with a higher qDME. 3.2. Cycle-by-cycle variations in the maximum in-cylinder pressure (pmax) pmax is an important mechanical constraint in the engine design so that it is critical to analyze the variations in pmax. Fig. 4 illustrates the cycle-by-cycle variations in pmax for 100 consecutive test cycles at all test conditions. In Fig. 4(a) and (b), the highest average value of pmax corresponds to the operation with the largest port DME quantity and the lowest average value of pmax corresponds to the operation without port DME premixing. Based on analysis of the diesel-DME dual fuel combustion process, an ignition timing

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of DME is prior to that of diesel and the initial burning advances with the introduction of DME. Consequently, pmax elevates with an increase in DME quantity. It can also be clearly observed from Fig. 4 that larger DME quantity leads to a relatively higher COV of pmax, and as the DME quantity becomes less, the COV of pmax decreases. This is because it has tendency to knock with a large qDME, which can result in higher pressure oscillations. A similar varying trend in pmax was found in the study of DEE premixed compression ignition diesel engine, carried out by Cinar et al. [5]. At any BMEP, the COV of pmax increases with an increase in DME quantity as the increased DME quantity increases knock tendency. However, it can also be observed from all test points in Fig. 4 the COVs of pmax are rather small (COVpmax < 3%), therefore the overall cyclic variations in pmax are smaller. As a further discussion, the frequency distribution of pmax under the different DME quantities is shown in Fig. 5. It can also be noticed from this figure that the value of pmax shows a scattering distribution with an increase in DME quantity. This phenomenon seems to be particularly obvious with a larger DME quantity. In Fig. 5(a), the value of pmax varies from 7.01 to 7.41 MPa, and the maximum repeatability for the same value of pmax is about 32% for DICI operation without port DME premixing; whereas the value of pmax varies from 7.37 to 8.29 MPa, and the maximum repeatability for the same value of pmax falls to 23% for the DME-diesel PCCI operation with qDME = 0.5 g/s. In Fig. 5(b), the value of pmax is between 7.3 to 7.8 MPa, and the maximum repeatability for the same value of pmax is about 39% for the operation without port DME premixing; whereas the value of pmax keeps from 7.8 to 8.8 MPa, and the maximum repeatability for the same value of pmax is down to only 26% for the DME-diesel PCCI operation with qDME = 0.6 g/s. Apart from the cycle-by-cycle variations in the value of pmax, it is also important to note the variations in the crank angle corresponding to pmax. It is essential to make the crank angle corresponding to pmax close to TDC for the optimum thermal efficiency of an engine [29]. The power generated in the engine deteriorates in both cases due to either too much advance or too much delay in pmax. Hence, this parameter is also important in evaluating the cyclic variations. Fig. 6 gives the interdependency between pmax and crank angles corresponding to pmax under the different DME quantities. It can be seen from this figure that the distribution of crank angle is concentrated more near an average value of crank angle corresponding to pmax for the diesel operation and scattered around an average value with a large DME quantity as expected. The scattering distribution can be attributed to that because the amount of HCCI combustion increases with a rise of qDME for DME-diesel dual fuel operation, it has the possibility of the cycle-to-cycle variations in positions of the start of combustion and these variations even become more observable with a rise of qDME, which in turn affects the position of pmax in the cycle. For example, in Fig. 6(b), for the DME-diesel PCCI operation with a larger DME quantity (qDME = 0.6 g/s), an overall distribution region of the crank angle corresponding to pmax is between 2 and 6°CA; whereas the crank angles corresponding to pmax are concentrated in a relatively narrow region between 4 and 6.5°CA for DICI operation. It can also be observed from Fig. 6 that as the DME quantity increases, the crank angle corresponding to pmax advances due to DME’s early combustion. 3.3. Cycle-by-cycle variations in the maximum mass-averaged temperature (Tmax)

Fig. 3. NOx and smoke emission for a DME-diesel dual-fuel case.

It is worth discussing the cycle-by-cycle variations in a massaveraged gas temperature inside a cylinder because, to some extent, it is related to emissions from the engines [30]. Furthermore, cycleby-cycle variations in a mass-averaged gas temperature also affect the variations in heat transfer from cylinder liner. Therefore, it is

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Fig. 4. Cycle-by-cycle variations of the cylinder peak pressure under different port DME quantities.

Fig. 5. The frequency distribution of pmax.

necessary to investigate the variations in Tmax. Fig. 7 shows the cycle-by-cycle variations in Tmax for the 100 consecutive combustion cycles. Like the varying trend of pmax, an average value of Tmax increased with an increase in DME quantity at any given condition due to DME’s early combustion. COV of Tmax also increases with an increase in DME quantity. As DME quantity increases, knock tendency as well as the resultant pressure oscillation increases, leading to more heat transfer from the liner. The variations in a distribution of cylinder temperatures results in a higher COVTmax in the case with a larger DME quantity. COVs of Tmax at all test points are less than 3%, so the statistical variation in Tmax is rather small in the DME-diesel dual-fuel PCCI combustion. Smaller COVs of Tmax are perhaps due to that there is almost no flame propagation as

observed in the SI engine hence the Tmax deviates in a quite small range. Fig. 8 shows the frequency distribution of Tmax under the different DME quantities. Similar to the frequency distribution of pmax, the value of Tmax also shows a scattering distribution with an increase in DME quantity, and this scattering seems to be particularly remarkable in the case with a larger DME quantity. In Fig. 8(a) the value of Tmax varies from 1575 to 1715 K, and the maximum repeatability for the same value of Tmax is about 35% for a diesel operation; whereas the value of Tmax varies from 1675 to 1895 K, and the maximum repeatability for the same value of Tmax lowers to about 15% for the DME-diesel PCCI operation with qDME = 0.5 g/s. In Fig. 8(b), the value of Tmax varies from 1855 to 1955 K, and the max-

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Fig. 6. Interdependency between pmax and crank angles corresponding to pmax.

Fig. 7. Cycle-by-cycle variations in Tmax under the different port DME quantities.

imum repeatability for the same value of Tmax is about 40% for a diesel operation; whereas the value of Tmax varies from 1855 to 2115 K and the maximum repeatability for the same value of Tmax is down to only 17% for the DME-diesel PCCI operation with qDME = 0.6 g/s. It is important to determine the variations in crank angle corresponding to Tmax to completely understand cycle-by-cycle variations of diesel engine with DME as premixing fuel. Consequently, the interdependency between the Tmax and crank angles corresponding to Tmax under the different DME quantities is depicted in Fig. 9. Like the varying trend of the crank angle corresponding to pmax, as DME quantity increases, the crank angles corresponding to Tmax advance due to DME’s early combustion. It can also be observed that the distribution of crank angles for the diesel operation is relatively more concentrated than that for DME-diesel

dual fuel operation with a larger DME quantity as knock tends to occur at this condition. 3.4. Cycle-by-cycle variations in (dq/du)max ROHR is a measure of how fast fuel chemical energy is converted to thermal energy during a combustion process, so it directly affects power generation [30]. In general, cycle-by-cycle variations in ROHR should be within an optimum limit for smooth operations in an engine. Fig. 10 exhibits cycle-by-cycle variations in (dq/du)max at all engine test conditions. During the DME-diesel dual-fuel PCCI combustion process (shown in Fig. 2), with a rise of DME quantity, the amount of HCCI combustion increases and the amount of diesel combustion reduces, consequently leading

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Fig. 8. The frequency distribution of Tmax.

Fig. 9. Interdependency between Tmax and crank angles corresponding to Tmax.

to a decrease in a value of (dq/du)max. Moreover, an increased pressure and temperature results in a more complete and faster diesel combustion, causing a reduction in cycle-by-cycle variations in (dq/du)max for the DICI combustion. Thus, the cycle-by-cycle variations in (dq/du)max decrease with a rise of DME quantity. However, when the HCCI combustion gradually dominates the combustion, the amount of HCCI combustion perhaps exceeds the amount of diesel DICI with further increase in DME port quantity. Consequently, (dq/du)max appears in the HCCI HTR process. In this situation, the cycle-by-cycle variations in (dq/du)max do not decrease but increase with an increase in DME quantity. The frequency distribution of (dq/du)max presented in a form of histogram also exhibits this varying trend, as shown as in Fig. 11. The wide spread of low frequency bars is observed for the diesel operation in this figure. The value of (dq/du)max shows a relatively concen-

trating distribution with an increase in DME quantity at first, but the frequency distribution of (dq/du)max does not continue to become more concentrating with a further increase in DME quantity. In contrast, the frequency distributing range with the 0.5 g/s DME quantity is same or even a little wider than the range with 0.3 g/s in Fig. 11(a), and the frequency distributing range with the 0.6 g/s DME quantity is same or even a little wider than the range with 0.4 g/s in Fig. 11(b). Fig. 12 gives the interdependency between (dq/du)max and crank angles corresponding to (dq/du)max under the different DME quantities. It is observed that crank angles corresponding to (dq/du)max advance with a rise of DME quantity and shows a dispersal distribution. This dispersion is especially remarkable in the case with a larger DME quantity because of the increased HCCI combustion. At the condition of BMEP = 0.24 MPa, the overall crank

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Fig. 10. Cycle-by-cycle variations in (dq/du)max under different port DME quantities.

Fig. 11. The frequency distribution of (dq/du)max.

angles are concentrated in a relatively narrow range between 3.5 and 4.5°CA without an introduction of port DME, but the crank angles move to a wider region between 7.5 and 4.5°CA for the DME-diesel dual fuel PCCI operation with qDME = 0.5 g/s. Similarly, at the condition of BMEP = 0.36 MPa, the overall crank angles are concentrated in a relatively narrow range between 3.5 and 4.5°CA without an introduction of port DME; whereas they move to a wider region between 9 and 6.5°CA for the DME-diesel dual fuel PCCI operation with qDME = 0.6 g/s. 3.5. Cycle-by-cycle variations in (dp/du)max The cyclic variation in ROPR is directly related to combustion noise, and cyclic variation of (dp/du)max is also frequently used to

qualify the cyclic variations of engines. Fig. 13 illustrates the cycle-by-cycle variations in (dp/du)max for the 100 consecutive test cycles at all test conditions. During the DME-diesel dual-fuel PCCI combustion process (shown in Fig. 2), the curve of ROPR shows the same staged behavior as that of the curve of ROHR. Thus, it can be expected that cycle-by-cycle variations in (dp/du)max have a similar varying trend with cycle-by-cycle variations in (dq/du)max; COV of (dp/du)max first decreases with an increase in DME quantity but increases with a further increase in DME quantity. It also indicates that the variations of (dp/du)max can roughly reflect the variations of maximum rate of heat release although ROPR is sensitive to noise. Thus, the crank angle position of (dp/du)max can be used as an estimation of the fast early calculation of a combustion phase. The chief advantage of using the position of (dp/du)max is that more time will

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Fig. 12. Interdependency between (dq/du)max and crank angles corresponding to (dq/du)max.

Fig. 13. Cycle-by-cycle variations of (dp/du)max under different port DME quantities.

be available for a controller to obtain values of other control parameters and activate the actuators to control the next cycle [26]. The frequency distribution of (dp/du)max presented in a form of histogram is shown in Fig. 14. Like the variations in (dq/du)max, the wide spread of low frequency bars is observed for the diesel operation, and the frequency distribution of (dp/du)max is concentrated more with a rise of DME quantity for the DME-diesel dual fuel PCCI operation. However, the frequency distribution of (dp/du)max will not sequentially become more concentrating with a further increase in DME quantity. In contrast, the frequency distributing range with the 0.5 g/s DME quantity is almost same to the range with 0.3 g/s in Fig. 14(a), and the frequency distributing range with the 0.6 g/s DME quantity is same or even a little wider than the range with 0.4 g/s in Fig 14(b).

Fig. 15 gives the interdependency between (dp/du)max and corresponding crank angles to (dp/du)max under the different DME quantities. Similar to the interdependency between (dq/du)max and corresponding crank angles to (dq/du)max, it is observed that crank angles corresponding to (dp/du)max move forward with a rise of DME quantity because an early combustion leads to an advance of pressure rise. In addition, the distribution of crank angles becomes dispersal with an increase in DME quantity, and this scattered distribution of crank angles is especially remarkable in the case of a higher DME quantity. For example, at the condition of BMEP = 0.24 MPa, the overall crank angles are concentrated in a relatively narrow range between 2.5 and 3.5°CA without DME’s introduction, but they move to a wider region between 8 and 5.5°CA when the DME quantity increases to 0.5 g/s; at the

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Fig. 14. The frequency distribution of (dp/du)max.

Fig. 15. Interdependency between (dp/du)max and crank angles corresponding to (dp/du)max.

condition of BMEP = 0.36 MPa, the overall crank angles are relatively concentrated in a narrow range between 1.5 and 2.5°CA without DME’s introduction, whereas they move to a wider range between 8.5 and 5.5°CA when the DME quantity is up to 0.6 g/s. 3.6. Cycle-by-cycle variations in IMEP Cycle-by-cycle variations in IMEP directly affect engine’s drivability. It is widely reported that drivability problems in automobiles normally arise when COVIMEP exceeds 10% [30]. Fig. 16 illustrates the cycle-by-cycle variations in IMEP at all test points for the 100 consecutive combustion cycles. IMEP shows the obvious fluctuations and is distributed in a wider range with an

increase in DME quantity because an increased DME quantity enlarges the amount of HCCI combustion, therefore increasing the cycle-by-cycle variations in the ignition timing and burning rate, and finally leading to the variations in IMEP. It also means COVIMEP increases with an increase in DME quantity. Fig. 17 further demonstrates the frequency distribution of cycle-by-cycle variations in IMEP over the 100 cycles presented in a form of histogram at all test conditions. The narrow spread of high frequency bars is obviously observed for the diesel operation, and the frequency distribution of IMEP is scattered more with a rise of IMEP for the DME-diesel dual fuel PCCI operation. At the BMEP of 0.24 MPa, the values of IMEP are concentrated in a region between 0.45 and 0.55 MPa, and the maximum repeatability for the same value of IMEP is about 39% for the diesel operation;

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Fig. 16. Cycle-by-cycle variations of IMEP under different port DME quantities.

Fig. 17. The frequency distribution of IMEP.

whereas he values of IMEP are distributed in the range between 0.43 and 0.55 MPa, and the maximum repeatability for the same value of IMEP declines to 25% or so for the DME-diesel PCCI operation with qDME = 0.5 g/s. At the BMEP of 0.36 MPa, the values of IMEP are concentrated in the region between 0.61 and 0.65 MPa, and the maximum repeatability for the same value of IMEP is about 44% for the diesel operation; whereas they are distributed in the range between 0.55 and 0.71 MPa and the maximum repeatability for the same value of IMEP is down to 26% for the DME-diesel PCCI operation with qDME = 0.6 g/s. 3.7. Analysis of correlation coefficient For examining the influence of combustion phasing on the cyclic pressure variation, a correlation analysis was carried out. The

correlation coefficient (r), showed in Eq. (3), is a statistical analysis indicator disclosing the correlation between the two parameters [33,34].

Pn   i¼1 ðxi  xÞðyi  yÞ ffi; ffiqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn Pn 2  2 i¼1 ðxi  xÞ i¼1 ðyi  yÞ

ð3Þ

 represent the ith-value and mean value respecx and yi, y where xi,  tively in two characteristic parameter samples. In the study pmax and (dp/du)max are both used as the measures of the cyclic variation (the ‘effect’). The crank angle corresponding to (dp/du)max and the crank angle corresponding to pmax are chosen as potential ‘cause’ of any influence of the combustion phasing. The degrees of correlation between the crank angle corresponding to (dp/du)max and (dp/du)max, between the crank angle corresponding

Y. Wang et al. / Applied Energy 143 (2015) 58–70

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initial flame development stage and a good feedback for combustion control. However, in the DME-diesel dual-fuel combustion, combustion generally starts before TDC, hence the position of peak pressure is no longer a better representative of combustion phasing which is especially obvious with a large DME quantity. Thus, crank angle corresponding to pmax seems not to be a good feedback for controlling such combustion in all test points. (5) IMEP shows more obvious fluctuation and is distributed in a wider range with a rise of DME quantity. Consequently, COV of IMEP increases with a rise of DME quantity under all test conditions.

Acknowledgements Fig. 18. Correlation coefficients between pmax and upamx,and between (dp/du)max and u(dp/du)max as a function of DME quantity.

to pmax and pmax are shown in Fig. 18. It can be observed that these parameters show relatively strong correlations for the diesel/DME PCCI operation with a large DME quantity; whereas for the operation with low DME quantity and pure diesel operation, the absolute values of correlation coefficients are much less than 0.4. The absolute values of correlation coefficients between the crank angle corresponding to pmax and pmax increase with a rise of DME quantity; whereas the absolute values of correlation coefficients between the crank angle corresponding to (dp/du)max and (dp/du)max firstly decrease a little (BMEP = 0.24 MPa) or show no obvious variation (BMEP = 0.36 MPa), but finally increase with a large DME quantity. 4. Conclusion The study of cycle-by-cycle variations of some parameters such as pmax, Tmax, (dq/dh)max, (dp/dh)max and IMEP in a diesel-DME dual fuel PCCI combustion process were investigated on a two-cylinder, four-stroke DICI engine. Under the different port DME quantities, the cycle-by-cycle variations were examined at the speed of 1700 r/min. The conclusions are as follows: (1) COVpmax and the average value of pmax increase with a rise of DME quantity. Meanwhile, the crank angles corresponding to pmax advance and are more scattered around an average value with an increase in DME quantity. Under all test conditions, COVpmax is less than 3%, and the overall values of cyclic variations in pmax are relatively smaller. (2) Similar to the variations of pmax, COVTmax and the average value of Tmax increases with a rise of DME quantity. The crank angles corresponding to Tmax advance and a distribution of crank angle is scattered more around the average value with an increase in DME quantity. Under all test conditions, the overall values of cyclic variations in Tmax are also smaller (<3%). (3) COV of (dq/du)max decreases at first, but increases with a rise of DME quantity. The crank angles corresponding to (dq/ du)max advances and their distribution becomes more scattered with a larger DME quantity. (dp/du)max has almost the same varying trend as (dq/du)max. Thus, the variations of (dp/du)max can roughly reflect the variations of maximum rate of heat release. Furthermore, the overall values of cyclic variations in (dp/du)max and (dq/du)max are higher than those in pmax. This indicates that port DME quantity has a greater influence on (dp/du)max and (dq/du)max compared to that on pmax. (4) In a conventional SI engine, the crank angle corresponding to pmax is the most suitable indicator for cyclic variations in the

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