Effect of n-Heptane Premixing on Combustion Characteristics of

In the partial HCCI system, homogeneously premixed charge is supplied in the diesel engine like an HCCI engine. In the meantime, a small amount of dir...
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Energy & Fuels 2005, 19, 2240-2246

Effect of n-Heptane Premixing on Combustion Characteristics of Diesel Engine Dae Sik Kim and Chang Sik Lee* Department of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea Received March 4, 2005. Revised Manuscript Received August 16, 2005

This work focuses on investigating a partial homogeneous charge compression ignition (HCCI) combustion as a control mechanism for HCCI combustion. In the partial HCCI system, homogeneously premixed charge is supplied in the diesel engine like an HCCI engine. In the meantime, a small amount of directly injected diesel fuel is used for the control of combustion rate. For supplying the premixed fuel, the port fuel injection system is equipped in the intake port of diesel engine. Normal heptane is chosen as a premixed fuel. Experimental results show that the nitrogen oxides (NOx) emissions decrease at a lower premix ratio; however, at higher premix ratios, NOx emissions start to increase due to the increase of combustion temperature by an excessively advanced auto-ignition of premixed charge. When the air-fuel charge is heated to temperature higher than the boiling point of premixed fuel, soot is dramatically decreased at a high premix ratio. This work also examines the factors that limit the operational range of partial HCCI combustion and then provides suggestions for increasing the range.

1. Introduction Over the last several years, a consensus has developed as to the nature of a homogeneous charge compression ignition (HCCI) combustion. It is known that HCCI engines have substantially lower emissions of nitrogen oxides (NOx) and particulate matters (PM). Although HCCI combustion is desirable for clean engine emissions and improved thermal efficiency, achieving acceptable HCCI combustion over a wide operating range is difficult due to several practical problems. The most difficult hurdles may be the ignition control and the limitation of operational range. Fuel chemistry and gas mixture temperature are identified as the two major factors that determine the autoignition process.1 Also, many attempts to control incylinder gas temperature have been tried through several potential methods such as high EGR rate,2,3 variable compression ration (VCR),4 and variable valve actuation (VVA).5 However, none of these methods for controlling the ignition show the satisfactory results over a wide range of engine speeds and loads. In this viewpoint, many studies have focused on the application of partial HCCI engine as a control mechanism for HCCI combustion, while the improved emission characteristics of HCCI are maintained. These * Corresponding author. Phone: +82-2-2220-0427. Fax: +82-22281-5286. E-mail: [email protected]. (1) Xu, H.; Fu, H.; Williams, H.; Shilling, I. SAE Tech. Pap. Ser. 2002, 2002-01-0114. (2) Nakano, M.; Mandokoro, Y.; Kubo, S.; Yamazaki, S. Int. J. Engine Res. 2000, 1, 269-279. (3) Caton, P. A.; Simon, A. J.; Gerdes, J. C.; Edwards, C. F. Int. J. Engine Res. 2003, 4, 163-177. (4) Christensen, M.; Hultqvist, A.; Johansson, B. SAE Tech. Pap. Ser. 1999, 1999-01-3679. (5) Allen, J.; Law, D. SAE Tech. Pap. Ser. 2002, 2002-01-0422.

engines can be grouped according to the methods used to the type of premixing employed: by the early incylinder direct injection and by port fuel injection. Neely et al.6 and Hasegawa et al.7 realized the partial HCCI concept by injecting the fuel directly into the cylinder at the early compression stroke. Then the main injection of fuel is made at the late compression stroke or early expansion stroke for controlling the combustion. Their results showed that the combustion control of the premixed charge mixture is more feasible with lower cylinder pressures and temperatures. However, previous work showed that the improvement of NOx and PM emissions could be achieved only in narrow operation conditions because the fuel injected at low ambient pressure and temperature in the cylinder did not mix with air sufficiently and impinged on the cylinder wall or piston directly. Accordingly, premixing by port fuel injection is suggested because more sufficient time for the evaporation of premixed fuel and mixing with air in this type of partial HCCI system can be ensured than by in-cylinder direct injection. Our previous work8-10 shows that premixing by port fuel injection has an excellent effect on NOx and soot reduction with premixed gasoline and diesel fuels. However, gasoline premixing is a less effective approach from a practical viewpoint because a dual fuel system is required, although gasoline is regarded to be superior premixed fuel for partial HCCI (6) Neely, G. D.; Sasaki, S.; Leet, J. A. SAE Tech. Pap. Ser. 2004, 2004-01-0121. (7) Hasegawa, R.; Yanagihara, H. SAE Tech. Pap. Ser. 2003, 200301-0745. (8) Lee, C. S.; Lee, K. H.; Kim, D. S. Fuel 2003, 82, 553-560. (9) Kim, D. S.; Kim, M. Y.; Lee, C. S. Energy Fuels 2004, 18, 12131219. (10) Kim, D. S.; Kim, M. Y.; Lee, C. S. Combust. Sci. Technol. 2005, 177, 107-125.

10.1021/ef050055s CCC: $30.25 © 2005 American Chemical Society Published on Web 09/23/2005

n-Heptane Premixing on Combustion of Diesel Engine

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Table 1. Engine Specifications combustion chamber number of cylinders compression ratio displacement (cc) bore (mm) × stroke (mm) combustion chamber injection nozzle (directly injected)

direct injection 1 19 673 95 × 95 toroidal 0.28 mm × 4 holes

engine due to its excellent vaporization and mixing with air.8,9 In the meantime, diesel premixing showed the limited premixing effects in the emission characteristics and operation range due to the incomplete vaporization of diesel fuel despite charge preheating.10 Normal heptane was chosen as a premixed fuel in this work. Because its boiling point is much lower than that of diesel fuel, the problem of fuel vaporization can be eliminated. Previous work on n-heptane-fueled HCCI engines shows that port injection of n-heptane can provide comparatively homogeneous charge without charge heating.4,11 The objectives of this work are to experimentally analyze the combustion and emission characteristics of a direct injection (DI) diesel engine according to nheptane premixing. Through this controlled HCCI combustion system, the simultaneous reductions of NOx and PM are verified from diesel engine and the factors influencing the combustion and emission characteristics of partial HCCI engine are investigated. This work also examines the factors that limit the operational range of partial HCCI combustion and then provides suggestions for increasing the range. 2. Experimental Apparatus and Procedure 2.1. Experimental Apparatus. The research engine was based on a single-cylinder, direct-injection, and four-stroke cycle diesel engine with 673 cm3 of displacement volume. This engine has the toroidal-type combustion chamber with a cavity diameter of 50 mm. The fuel was directly injected through a nozzle with four 0.28-mm diameter holes. The specifications and dimensions of the test engine are listed in Table 1. The experimental apparatus is composed of the combustion pressure acquisition system, the exhaust gas analyzer system, the electronic air-temperature control system, the EGR system, and the premixed fuel injection system. Figure 1 shows the schematic diagram of the experimental apparatus. A premixture chamber with a volume of 9000 cm3 and 20-cm diameter is installed in upstream of the intake port for more effective premixing of air and fuel. The cooled EGR system is applied in this work as it has been reported to be superior to the hot EGR in its emission characteristics and operational range.12 2.2. Experimental Procedure. The injection amount of premixed fuel is controlled by a premixed fuel injection system in the intake system under the test conditions as summarized in Table 2. The premix ratio rp is defined as the ratio of energy of premixed fuel Qp to total energy Qt. The premix ratio can be obtained from the following equation.

rp )

Qp mphup ) Qt mphup + mdhud

(1)

where mp is the mass of premixed fuel, md is the mass of directly injected fuel, hu is the lower heating value, and (11) Zaidi, K.; Andrews, G. E.; Greenhough, J. SAE Tech. Pap. Ser. 2002, 2002-01-1156. (12) Hiltner, J.; Fiveland, S.; Agama, R.; Willi, M. SAE Tech. Pap. Ser. 2002, 2002-01-0417.

Figure 1. Schematic diagram of the experimental apparatus. Table 2. Test Conditions engine speed (rpm) engine load (Nm) intake air temperature (K) EGR rate (%, based on volumetric flow rate) fuel premixed directly injected injection pressure (MPa) premixed directly injected injection timing premixed directly injected premix ratio

1200 20 293, 373 0-30 n-heptane diesel 5.5 22 TDC at compression stroke -22 ATDC 0 ∼ max operating range

subscripts p and d denote premixed and directly injected fuel, respectively. Charge heating of n-heptane premixed fuel is maintained at inlet temperature (Tin) ) 373 ( 0.5 K, which is higher than its boiling point (371.4 K). Also, when both cooled EGR and charge heating are applied simultaneously, heating is controlled until the mixture temperature of fresh charge and exhaust gas becomes 373 K. For each operating condition, the cylinder pressures recorded at each crank angle are averaged over 100 cycles for the experiment. For all data presented, the 0° crank angle is defined as TDC at the compression stroke. Intake gas temperature is measured with K-type thermocouples mounted in the manifolds as close as possible to the cylinder head with the thermocouple junctions protruding into the center of the flow. This provided temperatures at streamwise locations of approximately 130 mm from the intake valve seat.

3. Results and Discussions 3.1. Overall Engine Performance. Our previous work10 showed, using the light oil as a premixed fuel, that premixing of diesel fuel with poor vaporization characteristics caused an accumulation of liquid fuel in the premixture chamber in addition to high hydrocarbon (HC) and carbon monoxide (CO) emissions, which resulted in the increase of fuel consumption. From these causes, diesel premixing by a port fuel injection had a limitation in investigating the effects of premixed fuels on combustion and emission characteristics of DI diesel engine obviously. As a result, n-heptane with a lower boiling point than diesel fuel is tried as a premixed fuel in the current work. Figure 2 is a plot of fuel consump-

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Figure 2. Comparison of specific fuel consumption between diesel and n-heptane premixing (Tin ) 293 K).

Figure 3. Effects of the premix ratio on the combustion characteristics (Tin ) 293 K).

tion rate with the premix ratio both for these current experiments using n-heptane for the premixed fuel and for results obtained previously using diesel for the premixed fuel. Premixing of n-heptane can diminish a portion of unburned fuel brought in diesel premixing, and thus more fuel can take part in combustion at the same injection rate. Accordingly, we observed a much lower increase in fuel consumption with n-heptane than with diesel premixing at high premix ratios. Figure 3 shows the effects of the premix ratio on the combustion pressure and the rate of heat release at an engine load of 20 Nm and Tin of 293 K. Also, the equivalence ratios of premixed fuel (φp) and total fuel (φt) are indicated at each condition. Normal heptane has a greater effect on combustion of partial HCCI compared to our previous works using the gasoline or diesel premixed fuels.9,10 For n-heptane premixing, there are much more low-temperature reactions despite no charge heating. As the premix ratio is increased, heat-producing reactions of premixed fuel are advanced prior to DI fuel injection and the combustion pressure rise. The premature start of combustion and increased combustion pressure before TDC become the limiting factors of operating conditions at high premix ratio due to the severe knocking and unstable combustion. Also, this advanced auto-ignition of premixed n-heptane is distinguished from the gasoline premixing,9 which showed that the combustion of premixed gasoline is controlled

Kim and Lee

Figure 4. Effects of the premix ratio on the combustion characteristics (Tin ) 373 K).

by DI fuel injection. It results from the greatly activated low-temperature reactions of n-heptane. Many researchers13-16 have investigated the auto-ignition of n-heptane in premixed charge. It is generally agreed that n-heptane exhibits a high reactivity characterized by a relatively short ignition delay and by a relatively low ignition limit, and ignition occurs in two stages with an intermediate cool flame and high-temperature reactions under a certain ambient critical temperature. For Minetti et al.,13 this critical temperature was 800 K in their rapid compression machine. Yang et al.14 showed a critical temperature of 900 K at ambient pressure of 1 MPa. For more detailed description about low and high temperature reactions of n-heptane, refer to the previous works.13-16 Figure 4 shows the effects of the premix ratio on combustion characteristics of n-heptane-fueled HCCI engine in Tin ) 373 K. The shape of heat release curve for n-heptane premixing shows the three-stage combustion discussed by Simescu et al.17 The first two stages are a typical shape of an HCCI combustion curve for diesel surrogate fuel, and it is the product of the burning of premixed fuel. The final stage is the diffusive combustion region controlled by the mixing of DI fuel. An interesting phenomenon is the negative temperature coefficient (NTC) as the intermediate temperature regions between the low-temperature and high-temperature reaction paths, which are regarded to be the general characteristics in fully or partially premixed n-heptane mixture. In this region, though reactions among in-cylinder species are still going on, the heat release is near zero. Yang et al.14 have observed that the duration of NTC decreases greatly with increasing pressure and cool flame temperature because these accelerate the activation of the hot flame region. In the current work, the increase of the amount of heat released in the cool flame region due to the increase of equivalence ratio of premixed fuel is also found to cause higher cool flame temperature and in-cylinder pressure and, thus, results in a shortened NTC. Figures 5 and 6 (13) Minetti, R.; Carlier, M.; Ribaucour, M.; Therssen, E.; Sochet, L. R. Combust. Flame 1995, 102, 298-309. (14) Yang, J. R.; Wong, S. C. Combust. Flame 2003, 132, 475-491. (15) Viggiano, A.; Magi, V. Combust. Flame 2004, 137, 432-443. (16) Xue, H.; Aggarwal, S. K. Combust. Flame 2003, 132, 723-741. (17) Simescu, S.; Fiveland, S. B.; Dodge, L. G. SAE Tech. Pap. Ser. 2003, 2003-01-0345.

n-Heptane Premixing on Combustion of Diesel Engine

Figure 5. Effects of the premix ratio on mean gas temperature (Tin ) 293 K).

Figure 6. Effects of the premix ratio on mean gas temperature (Tin ) 373 K).

show the in-cylinder mean gas temperature profiles calculated from the combustion pressures in various premix ratios of each inlet temperature. The cool flame by low-temperature reactions is found in almost constant ambient temperature of ∼615-630 K regardless of the premix ratio and inlet temperature. It shows that the onset of cool flame depends on the in-cylinder gas temperature, not the equivalence ratio of premixed fuel. Thus, in the case of Tin ) 373 K, low-temperature reactions are found much earlier than in Tin ) 293 K. In the meantime, high-temperature reactions in Figures 5 and 6 are found in almost constant ambient temperature of 830 K regardless of the premix ratio. However, high-temperature reactions are advanced in proportion to the increase of the premix ratio. The same trend was also found in test results of the n-heptane-fueled HCCI engine by Peng et al.18 Unlike the cool flame by lowtemperature reactions, the onset of HCCI combustion is considered to be affected mainly by the equivalence ratio of the premixed fuel. The advanced reactions by early auto-ignition of the premixed fuel result in the increase of in-cylinder gas temperature for the case of the premix ratios higher than 0.3. Effects of EGR rate on combustion characteristics at the premix ratio of 0.5 are plotted in Figure 7. The auto(18) Peng, Z.; Zhao, H.; Ladommatos, N. SAE Tech. Pap. Ser. 2003, 2003-01-0747.

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Figure 7. Effects of EGR on the combustion characteristics (Tin ) 373 K).

Figure 8. Effects of n-heptane premixing on NOx emissions.

ignition of the low-temperature reaction stage is affected greatly by EGR rates. This could be caused by the dilution effect of exhaust gases. In addition, the higher heat capacity of the exhaust gas can cause a reduction of charge temperature at the end of compression stroke, which would tend to retard ignition as well.19,20 In the meantime, duration of NTC was not much affected by the EGR rate. Also, this figure shows that the increase of the premix ratio delayed the start of HCCI combustion near TDC. These results suggest that EGR can control the onset of HCCI combustion. However, a significant difference in diffusive combustion is not found because DI fuels start to burn immediately on terminating the injection with little ignition delay. 3.2. NOx and Soot Emissions. NOx emission at various premix ratios with and without heating is shown in Figure 8. NOx emissions for n-heptane premixing show the decreasing trends up to approximately a premix ratio of 0.3. The NOx emission increases almost linearly with further increase of the premix ratio. This is caused by the elevated combustion temperature and pressure due to the excessively advanced start of combustion as shown in Figures 5 and 6. Soot emission as a function of the premix ratio and inlet temperature is shown in Figure 9. In this experi(19) Kim, M. Y.; Kim, D. S.; Lee, C. S. Energy Fuels 2003, 17, 755761. (20) Zheng, M.; Reader, G. T.; Hawley, J. G. Energy Convers. Manage. 2004, 45, 883-900.

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Figure 9. Effects of n-heptane premixing on soot emissions.

ment, the soot emissions are measured as the unit of percent in the gray level. Soot emissions at Tin ) 293 K show a small fluctuation but almost constant value regardless of the premix ratio, although HCCI combustion, in itself, of premixed charge should produce low soot emissions. Previous work on diesel-fueled HCCI showed similar results at low intake temperature.4,17,21,22 There may be several causes for these constant soot emissions despite the increase of HCCI combustion. Christensen et al.4 investigated diesel-fueled HCCI with port fuel injection, similar to the method used in this work, and reported that the increased soot emission was due to poor vaporization of the premixed fuel which creates an inhomogeneous mixture. In a different analysis, Simescu et al.17 explained, using their dieselfueled partial HCCI engines, that soot formed during the premixed combustion period before DI fuel injection is not oxidized completely and emitted as exhaust gas. Zhang22 also shows the similar soot increases in the flame visualization results according to the pilot injection quantity. They reported that, when main injection starts, if there exists more pilot flame, the interference of formed soot with spray of main injection may cause poor mixing of main spray and also poor oxidization of soot of pilot combustion. On the basis of these previous analyses and the results of current work, it is believed that the soot emissions in this work may be caused by the compound influences of factors mentioned above. Accordingly, further research is required to investigate the detailed correlation between soot results and influential factors through optical diagnostics. However, at Tin ) 373 K, soot concentration increases immediately up to rp ) 0.2, compared to diesel engine (rp ) 0). Then soot emissions start to decrease linearly with increase of the premix ratio. This linear decrease of soot emission at high premix ratio can be obtained due to the reduction of diffusive combustion by the decreased amount of DI fuel and the increased HCCI combustion of premixed fuel. It is generally known from previous work that HCCI engines with an ideally lean and homogeneous mixture show almost zero soot emission.23 In this work, charge heating higher than the (21) Gray, A. W., III; Ryan, T. W., III. SAE Tech. Pap. Ser. 1997, 971676. (22) Zhang, L. SAE Tech. Pap. Ser. 1999, 1999-01-3493. (23) Aceves, S. M.; Flowers, D. L.; Martinez-Frias, J.; Smith, J. R.; Westbrook, C. K.; Pitz, W. J.; Dibble, R.; Wright, J. F.; Akinyemi, W. C.; Hessel, R. P. SAE Tech. Pap. Ser. 2000, 2000-01-1027.

Kim and Lee

Figure 10. Effects of EGR and the premix ratio on NOx emissions (Tin ) 373 K).

Figure 11. Effects of EGR and the premix ratio on soot emissions (Tin ) 373 K).

boiling temperature of fuel is also found to be remarkably effective in reducing the soot emissions of an HCCI system. NOx and soot emissions according to the EGR rate are investigated at Tin ) 373 K in Figures 10 and 11. A typical tradeoff relationship between NOx and soot emissions according to EGR rate is shown like previous results. At an EGR rate of 30%, NOx concentrations are decreased below 100 ppm irrespective of the premix ratio. At the same time, soot emission is increased up to 33% at zero premix ratio of this EGR rate. However, the increase of the premix ratio can reduce the soot emissions at high EGR rate like the case without EGR. Accordingly, an appropriate combination of EGR rate and the premix ratio is important for optimal NOx and soot emissions. 3.3. Comparative Emission Behavior. Figure 12 provides more detailed information about the optimal operating conditions in the aspect of NOx and soot emissions. At a low EGR rate, a suitable combination of the premix ratio and EGR rate makes it possible to reduce the NOx and soot simultaneously, compared with measurement in conventional diesel combustion. When EGR rate is increased to 30%, NOx is decreased below one-tenth of DI diesel engine at highest premix ratio, while soot is measured to be similar to value of diesel engine in this condition.

n-Heptane Premixing on Combustion of Diesel Engine

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Figure 12. Relationship between NOx and soot emissions at various EGR rates (Tin ) 373 K).

of diesel fuel as the ignition source and a heterogeneous premixed charge that resulted in the sudden increase of COV. This large cycle variation leads to the deterioration of specific fuel consumption. This trend of high premix ratio disappeared with further heating the intake air temperature at 373 K. Hiraya et al.24 and Kim et al.10 have found that increasing the intake air temperature could expand the stable operation region of the HCCI engine. Heating of the intake air causes an increase of the combustion temperature and results in a higher chemical reaction rate. Also, higher intake air temperature facilitates the vaporization of premixed fuels, and it can be more efficient than lower intake temperature in forming a homogeneous charge. Hence, a stable ignition and fast combustion can be obtained despite a small amount of diesel fuel at a high premix ratio. However, if the premix ratio is too large at a high inlet temperature, the combustion temperature will rise, which will lead to knock combustion. At this knock condition, introduction of exhaust gas can lower the combustion temperature and expand the operating region limited by knock. Excessive cycle-to-cycle variation and knock can be factors that limit the partial HCCI operation region. In this study, the unstable combustion region is determined as the combustion region where the COV of IMEP exceeds 7, and the knock is determined as the combustion where the maximum rate of pressure rise exceeds 1 MPa/deg. 5. Conclusions

Figure 13. Relationship between COV and the premix ratio at each condition.

Figure 13 shows the coefficient of variation of indicated mean effective pressure (COVIMEP) at various operating conditions to evaluate the cycle-to-cycle variation of the engine combustion process. In this work, COVIMEP can be calculated as N

IMEP )

x∑

IMEPi ∑ i)1

N

σIMEP )

IMEPi

(IMEPi - IMEP)2

i)1

COVIMEP )

(2)

N

σIMEP IMEP

N × 100(%)

(3)

(4)

where IMEPi is IMEP of ith cycle, N is the number of cycles, IMEP is the arithmetic mean value of IMEP, and σIMEP is the standard deviation of IMEP. The smallest variations of IMEP occur at the premix ratio below 0.5 for the case of the low intake temperature conditions. At this operation condition, however, further increases in the premix ratio cause unstable ignition and combustion due to an insufficient amount

The effects of n-heptane premixing on the combustion and emission characteristics of the DI diesel engine were experimentally investigated at various premix ratios, intake temperatures, and EGR rates. On the basis of the results obtained, the following conclusions can be drawn: 1. The heat release rate curve for n-heptane premixed fuel shows a three-stage combustion: cool flame, HCCI combustion, and diffusive combustion of DI fuel. 2. The onset of cool flame depends on the in-cylinder gas temperature, not the equivalence ratio of premixed fuel. High-temperature reactions take place in almost constant ambient temperature regardless of the premix ratio. However, onset of high-temperature reactions is advanced in proportion to the increase of the premix ratio. 3. The NOx emissions decrease at lower premix ratio. However, at higher premix ratios, NOx emissions start to increase due to the increase of combustion temperature by an excessively advanced auto-ignition of premixed fuel. 4. Without charge heating, the increase of the premix ratio has no significant effect in reducing the soot emissions. However, when the air-fuel charge is heated to temperature higher than the boiling point of premixed fuel, soot is dramatically decreased at high premix ratio. 5. The onset of the low-temperature reaction is affected greatly by EGR rates. EGR can control the reaction rates in the stage of HCCI combustion. Also, (24) Hiraya, K.; Hasegawa, K.; Urushigara, T.; Iiyama, A.; Itoh, T. SAE Tech. Pap. Ser. 2002, 2002-01-0416.

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supply of premixed charge and EGR reduces NOx lower than one-tenth of conventional diesel engine. 6. The premix ratio and inlet temperature are the main factors influencing the combustion of partial HCCI. The severe operation condition of partial HCCI combustion is roughly characterized by two combustion states. One is heavy knocking combustion caused by the sudden combustion of a large amount of premixed fuel at high premix ratios and intake air temperatures, but the operational region limited by knocking can be

Kim and Lee

expanded by EGR. The other combustion state is the combustion instability and deterioration of specific fuel consumption caused by excessive cycle-to-cycle variation at a high premix ratio and low inlet temperature. Acknowledgment. This work is supported by the BK21 Program of Korea Research Foundation. EF050055S