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Effects of Inlet Pressure and Octane Numbers on Combustion and Emissions of a Homogeneous Charge Compression Ignition (HCCI) Engine Haifeng Liu, Mingfa Yao,* Bo Zhang, and Zunqing Zheng State Key Laboratory of Engines, Tianjin UniVersity, Tianjin, 300072, China ReceiVed August 25, 2007. ReVised Manuscript ReceiVed April 23, 2008
The influence of inlet pressure (Pin) and octane numbers on combustion and emissions of a homogeneous charge compression ignition (HCCI) engine was experimentally investigated. The tests were carried out in a modified four-cylinder direct injection diesel engine. Four fuels with different research octane number (RON) were used during the experiments: 90-RON, 93-RON, and 97-RON primary reference fuel (PRF) blend and a commercial gasoline, 94.1-RON(G). The inlet pressure conditions were set to give 0.1, 0.15, and 0.2 MPa of absolute pressure. The results indicate that, with the increase of inlet pressure, the start of combustion (SOC) advances and the cylinder pressure increases. The effects of the PRF octane number on SOC are weakened as the inlet pressure increased. However, the difference of SOC between gasoline and PRF is enlarged with the increase of the inlet pressure. The successful HCCI operating range is extended to the upper and lower load as the inlet pressure increased. The maximum achievable load of gasoline is higher than that of PRF with the cases of supercharging. The HC and NOx emissions of the HCCI engine decrease when supercharging is employed, while CO emissions increase remarkably. The PRF octane number has little effect on HC, CO, and NOx emissions when supercharging is employed. Nevertheless, the HC and CO emissions of gasoline are higher than those of PRF with supercharging.
1. Introduction Driven by its potential for high efficiency operation with significantly lower NOx and particulate emissions than conventional spark-ignited (SI) and diesel engines, homogeneous charge compression ignition (HCCI) has been received as one of the most promising internal combustion engine concepts for the future. In a HCCI engine, the lean locally homogeneous air-fuel mixture is compressed in the cylinder and auto-ignites simultaneously at multiple locations within the cylinder. The heat release from these regions compresses the remaining charge, promoting further auto-ignition events, and rapidly, the mixture combusts without any flame propagation.1–3 However, HCCI combustion still poses some challenges that must be overcome before it can be integrated into practical applications. The main limitations of HCCI combustion are the narrow operating window that results from the lack of directignition control and the limited power density. Ignition timing is mainly controlled by the chemical kinetics of the air-fuel mixture.4 The power density is limited by an excessive energy release rate and the associate noise, vibration, and harshness * To whom correspondence should be addressed: State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China. Telephone: 86-2227406842 ext. 8014. Fax: 86-22-27383362. E-mail:
[email protected]. (1) Onishi, S.; Jo, S. H.; Shoda, K.; Jo, P. D.; Kato, S. Active thermoatmosphere combustion: A new combustion progress for internal combustion engines. SAE Tech. Pap. Ser. 1979, 790501. (2) Kim, D. S.; Lee, C. S. Effect of n-heptane premixing on combustion characteristics of diesel engine. Energy Fuels 2005, 19, 2240–2246. (3) Stanglmaier, R. H.; Roberts, C. E. Homogeneous charge compression ignition (HCCI): Benefits, compromises, and future engine applications. SAE Tech. Pap. Ser. 1999, 1999-01-3682. (4) Shigeyuki, T.; Ferran, A.; James, C. K. A reduced chemical kinetic model for HCCI combustion of primary reference fuels in a rapid compression machine. Combust. Flame 2003, 133, 467–481.
effects. Under some operating conditions, an HCCI engine can produce higher HC and CO emissions than a SI engine. Because the homogeneous mixture auto-ignites, combustion starts more or less simultaneously in the whole cylinder. To limit the rate of combustion under these conditions, the mixture must be highly diluted. Without sufficient mixture dilution, problems associated with extremely rapid combustion and knocking-like phenomena will occur, as well as excessive NOx production. Thus, charge dilution is provided in the form of excess air (very lean air/fuel ratios) or by exhaust gas recirculation (EGR). This dilution effectively slows down the rate of combustion. However, the high dilution also limits the amount of fuel that can be added for a given mass of charge; i.e., engine power is low relative to the mass flow through the engine. The necessity of running the engine with higher loads has prompted the investigation of supercharged HCCI. Examples of other studies that have applied boosting methods to increase the speedload window of HCCI include boosting with a turbocharger on a PFI six-cylinder engine5 and boosting a single-cylinder engine by a stand-alone compressor.6,7 In addition, the SI, naturally aspirated(NA) HCCI, and supercharged HCCI are investigated in the Gharahbaghi et al.8 paper, which results show that a smaller supercharger with a moderate boost is preferable to avoid an unacceptable penalty in fuel economy. All of these results (5) Olsson, J.-O.; Tunestal, P.; Johansson, B. Boosting for high load HCCI. SAE Tech. Pap. Ser. 2004, 2004-01-0940. (6) Yap, D.; Megaritis, A. Applying forced induction to bioethanol HCCI operation with residual gas trapping. Energy Fuels 2005, 19, 1812–1821. (7) Christensen, M.; Johansson, B.; Amneus, P.; Mauss, F. Supercharged homogeneous charge compression ignition. SAE Tech. Pap. Ser. 1998, 980787. (8) Gharahbaghi, S.; Wilson, T. S.; Xu, H.; Cryan, S.; Richardson, S.; Wyszynski, M. L.; Misztal, J. Modelling and experimental investigations of supercharged HCCI engines. SAE Tech. Pap. Ser. 2006, 2006-01-0634.
10.1021/ef800197b CCC: $40.75 2008 American Chemical Society Published on Web 06/17/2008
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Table 1. Engine Specifications displacement bore stoke compression ratio inlet valve open inlet valve close exhaust valve open exhaust valve close
1.3 L 112 mm 132 mm 17.5 13.5° BTDC 38.5° ABDC 56.5° BBDC 11.5° ATDC
are showing satisfactory improvement in terms of load and NOx over aspirated HCCI. On the other hand, fuel properties are an important factor to determine the ignition timing. Many efforts have been taken to find better fuel for HCCI engine operation to control the autoignition process and expand the operation window. Since 1990s, a wide range of fuels, such as diesel, n-heptane, dimethyl ether (DME), gasoline, isooctane, ethanol, and methanol, etc., have been tried in HCCI combustion.9–13 Tanaka et al.9 investigated the auto-ignition characteristics, according to the fuel components (such as paraffins, cyclic paraffins, olefins, cyclic olefins, and aromatic hydrocarbon), using a rapid compression machine (RCM) and, as a consequence, proposed a combustion control method using various fuels and additives, according to the running conditions of a HCCI engine. Hou et al.10 investigated the effect of the addition of high-octane oxygenated fuel, including methyl tertiary butyl ether (MTBE), ethanol, and methanol, on combustion phasing and the combustion rate of HCCI combustion fueled with n-heptane as a base fuel. The results show that MTBE has more advantages over methanol and ethanol in the potential of extending the operating range of n-heptane-fueled HCCI combustion. A study on the controlling strategies of HCCI combustion with using the fuel of dimethyl ether and methanol has been carried out in the previous work.11 The results show that the maximum indicated mean effective pressure (IMEP) of HCCI operation can reach 0.74 MPa using DME/methanol dual fuel. However, the maximum IMEP of pure DME can only reach 0.44 MPa. Moreover, numerous experimental and numerical studies have been investigated using the PRF, n-heptane, and iso-octane.12–15 These investigations have been very useful for probing the effect of the fuel octane numbers on HCCI combustion. In previous works, the effects of research octane number (RON) of primary reference fuel (PRF) on HCCI combustion have been investigated.13 The results have shown that the maximum IMEP of high octane number is higher than that of the low octane number. However, if the octane number is too (9) Tanaka, S.; Ayala, F.; Keck, J. C.; Heywood, J. B. Two-stage ignition in HCCI combustion and HCCI control by fuels and additives. Combust. Flame 2003, 132, 219–239. (10) Hou, Y.; Lu, X.; Zu, L.; Ji, Li.; Huang, Z. Effect of high-octane oxygenated fuels on n-heptane-fueled HCCI combustion. Energy Fuels 2006, 20, 1425–1433. (11) Yao, M.; Chen, Z.; Zheng, Z.; Zhang, B.; Xing, Y. Study on the controlling strategies of homogenous charge compression ignition combustion with fuel of dimethyl ether and methanol. Fuel 2006, 85, 2046–2056. (12) Lim, O. T.; Sendoh, N.; Iida, N. Experimental study on HCCI combustion characteristics of n-heptane and iso-octane fuel/air mixture by the use of a rapid compression machine. SAE Tech. Pap. Ser. 2004, 200401-1968. (13) Yao, M.; Zhang, B.; Zheng, Z.; Chen, Z.; Xing, Y. Effects of exhaust gas recirculation on combustion and emissions of a homogeneous charge compression ignition engine fueled with primary reference fuels. Proc. Inst. Mech. Eng., Part D: J. Automob. Eng. 2007, 221, 197–213. (14) Zheng, Z.; Yao, M. Numerical study on the chemical reaction kinetics of n-heptane for HCCI combustion process. Fuel 2006, 85, 2605– 2615. (15) Jia, M.; Xie, M. A chemical kinetics model of iso-octane oxidation for HCCI engines. Fuel 2006, 85, 2593–2604.
Figure 1. Experimental setup: 1, engine; 2, dynamometer; 3, air compressor; 4, air tank; 5, air flow meter; 6, electric heater; 7, pressure transducer; 8, encoder; 9, charge amplifier; 10, data acquisition system; 11, fuel tank; 12, fuel flow meter; 13, fuel pump; 14, fuel injector; 15, electronic control unit; and 16, exhaust analyzer.
high, the engine cannot run smoothly at low IMEP and the HCCI operating speed is also limited (e.g., RON 90). Therefore, to improve the maximum IMEP, the research for high octane number (RON > 90) is necessary. In this work, the tests were carried out on a modified fourcylinder direct-injection diesel engine using high octane numbers (RON > 90) PRFs and commercial gasoline in different inlet pressures. The purpose of this research is to obtain a basic understanding of the influence of fuel octane numbers on combustion characteristics, engine performance, and emissions of a HCCI engine at different inlet pressures. 2. Experimental Apparatus and Methods A diesel engine (Yu Chai 4112 series, YC4112ZLQ) was converted to run in HCCI mode. The four-cylinder engine was, however, modified to operate in one cylinder only. This arrangement gives a robust and inexpensive single-cylinder engine but at the cost of the reliability of the brake-specific results. With a pressure transducer, indicated results can be used instead. The most important engine parameters are shown in Table 1. The boost pressure was generated by an external air compressor that could be used to control the inlet pressure up to 0.3 MPa. A large tank was added as a pressure stabilizer in the inlet system to reduce the effect of inlet air cycle variation on the measurement of the air flow rate. The air flow meter was mounted in the rear of the tank. Figure 1 illustrates the experimental setup. An electric heater was installed in the inlet system upstream of the injector to keep the stable combustion. According to the previous experiment results,16 the gasoline and the 93-RON fuel can be fired until the inlet temperature (Tin) reaches 363 K. Therefore, the Tin was set to 363 K during all experiments. A pressure transducer (Kistler 6125A) was fitted flush with the wall of the cylinder head, connected via a charge amplifier (Kistler 5011) to a data acquisition board (National Instruments) fitted in a compatible PC. The cylinder pressure data were recorded in half crank-angle increments, triggered by an optical shaft encoder. An electronic port fuel injector, typical of those used in modern SI engines, was also installed in the inlet pipe at a location approximately 30 diameters upstream of the inlet valve. This injector was used to inject fuel into the inlet air for operation of the engine in HCCI mode. An injector controller was used to drive the injector, controlling both the injection timing and the fuel quantity by the pulse width of the injection event. The concentrations of CO2, CO, O2, NOx, and THC in the exhaust gas were measured by an exhaust analyzer (Horiba MEXA7100DEGR), which measures HC by a hydrogen flame ionization (16) Zhang, B.; Yao, M.; Yang, D.; Zheng, Z.; Chen, Z.; Zhang, Q.; Xing, Y. Experimental study on the effect of fuel properties on HCCI combustion characteristics at various intake temperatures. Trans. CSICE 2008, 26, 1–10.
Inlet Pressure and Octane Numbers of a HCCI Engine Table 2. Fuel Properties molecular formula boiling point (°C) distillation (°C) T10 T50 T90 density (kg/m3) low heating value (MJ/kg) RON benzene (% v/v) alkene (% v/v) aromatics (% v/v)
n-heptane
iso-octane
C7H16 98.4
C8H18 99.2
688 44.93 0
692 44.65 100
gasoline (G94.1)
66.7 89.5 154.9 733 43.9-44.4 94.1 1.3 28.0 25.5
(FID) method, CO and CO2 by a nondispersive infrared (NDIR) method, and NOx by a chemiluminescent NOx analyzer (CLA). This system was used to start and warm up the engine in standard diesel configuration until the lubricating oil temperature reached 85 °C and the cooling water temperature reached 80 °C, then it was switched into HCCI mode. The engine speed was set to 1400 rpm during all experiments. Four fuels were used during the experiments: 90-RON, 93-RON, and 97-RON primary reference fuel blend and a commercial gasoline, 94.1-RON(G), which was bought from the Chinese market. We use the code PRF90, PRF93, PRF97, and G94.1 as the marker of four fuels in this paper, which the numbers are the RON of the fuel. Table 2 shows brief properties of the fuels. Although the grade of the gasoline was 93, the real RON was 94.1 from the authoritative test by North Institute of China Petrochemical Corporation. The RON of the gasoline was measured in the standardized research method (see American Society for Testing Materials Designation, D 2699-01a, 2001) tests with cooperative fuel research (CFR) engines. Three different levels of inlet pressure were used in the experiments presented. At first, the engine was operated NA, and then a boost pressure of 0.5 and 1 bar was used, giving 0.1, 0.15, and 0.2 MPa of absolute pressure.
3. Results and Discussion 3.1. Effects of Inlet Pressure and Octane Numbers on Combustion Characteristics. The cylinder pressure and heat release rate trace is presented in Figure 2, which the fueling rate investigated is 29 mg/cycle. Each pressure trace is the mean pressure trace for 50 engine cycles. The cylinder pressure data was analyzed using a single-zone heat-release model with an assumption that the mixture of air and fuel and the temperature is homogeneous in the whole cylinder volume. In addition, there is no mass leakage from the cylinder. The heat-transfer coefficient was obtained via Woschni’s correlation. The rate of heat release (ROHR) and mean gas temperature are calculated by this model, which has been used in previous research.11,13 In fact, the single-zone model is an simple effective method to the combustion analysis. Heywood stated17 that, in comparison to a single-zone model, the advantage of a two-zone analysis is that the thermodynamic properties of the cylinder contents can be quantified more accurately. However, the two-zone model has its disadvantages that the unburned and burned zone heattransfer areas must both now be estimated and a model for the composition of the gas flowing into the crevice region must be developed. Therefore, we choose the single-zone model to analyze the combustion process. As can be seen from the Figure 2, the peak of cylinder pressure is increased and the heat release rate is advanced with increasing inlet pressure. The reason is that the collision frequency among molecules increases with the increase of the boost pressure, which leads to the increase of the combustion (17) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill Book Company: New York, 1988; pp 148-154, 388-389.
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reaction velocity. Figure 2a shows that the fuel chemistry has a remarkable effect on the HCCI combustion progress in different boost pressures. In comparison to the PRF93, the SOC of G94.1 is earlier without boosting, while it is later with boosting. The research of Kalghatgi18,19 shows that the sensitive fuel will become more resistant to auto-ignition by boosting the inlet. On the other hand, if Tcomp15 (the pressure reaches 15 bar during the compression stroke) is increased, the sensitive fuel will become more prone to auto-ignition compared to the PRF. As can be seen from Table 2, the G94.1 is a sensitive fuel (RON > MON). It contains less paraffins and more aromatics and olefins compared to PRF fuels. Therefore, as the inlet pressure increases, the G94.1 becomes more resistant to auto-ignition compared to PRF93, a nonsensitive fuel. On the other hand, without boosting, the compression temperature is increased (which will be discussed in the next section), which lead to the earlier SOC of the G94.1. This illuminates that the auto-ignition depends upon the fuel as well as the pressure and temperature development in the unburnt mixture. Parts b-d of Figure 2 show the effect of RON on the combustion process at a given inlet pressure. The PRF97 is not shown in Figure 2b, because it can not reach stable combustion at this operating condition. It is either a misfire or knock. As parts b-d of Figure 2 show, with the increase of the inlet pressure, the influence of the RON gap on the SOC is less and less. This suggests that a small RON gap has a different effect on the auto-ignition at different boost pressures. Without boosting, the gap of SOC is nearly 5 CAD between PRF90 and PRF93, while it is nearly the same at 1 bar boost pressure. On the one hand, the SOC is advanced too early at the case of higher boost pressure, which weakens the gap of SOC between three PRF. On the other hand, with the increase of RON, the peak of ROHR increases after supercharging. It suggests that the PRF with a higher RON occurs in the more precombustion reaction, resulting in the increase of the combustion rate. Finally, the SOC is advanced, and the peak of ROHR is increased. This may be another reason that the gap of SOC between three PRF changes is small, except the too early SOC. The mean gas temperature trace is presented in Figure 3, which the fueling rate investigated is also 29 mg/cycle. The in-cylinder mean gas temperature was calculated from the cylinder pressure using the single-zone mode, and it is an average value. Parts a and b of Figure 3 show the mean gas temperature of PRF93 and G94.1 at different inlet pressures. As can be seen from the figure, the peak of the mean gas temperature is diminished with an increasing inlet pressure. The reason is that the excess air from the supercharger reduces the peak combustion temperature within the cylinder and thus decreases the knock occurrence. A low combustion temperature is preferable to allow for heat release without reaching the NOx critical temperature. However, Sjo¨berg20 suggested that reaching a peak in-cylinder temperature of 1500 K is necessary to have a sufficient OH concentration for the CO oxidation. The low cylinder temperature in higher boosting will lead to increased amounts of CO emissions. In addition, parts a and b of Figure 3 also show that the mean gas temperature with starting HCCI combustion for PRF93 and G94.1 are all between 1050 and (18) Kalghatgi, G. T. Auto-ignition quality of practical fuels and implications for fuel requirements of future SI and HCCI engines. SAE Tech. Pap. Ser. 2005, 2005-01-0239. (19) Kalghatgi, G. T.; Head, R. A. The available and required autoignition quality of gasoline-like fuels in HCCI engines at high temperatures. SAE Tech. Pap. Ser. 2004, 2004-01-1969. (20) Sjo¨berg, M.; Dec, J. E. An investigation into lowest acceptable combustion temperatures for hydrocarbon fuels in HCCI engines. Proc. Combust. Inst. 2005, 30, 2719–2726.
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Figure 2. Cylinder pressure and rate of heat-release traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of RON without boosting fueling PRF90 and PRF93, (c) effects of RON at 0.5 bar boost pressure fueling PRF90, PRF93, and PRF97, and (d) effects of RON at 1 bar boost pressure fueling PRF90, PRF93, and PRF97.
Figure 3. Mean gas temperature traces: (a) effects of inlet pressure fueling PRF93, (b) effects of inlet pressure fueling G94.1, and (c) effects of RON at 1 bar boost pressure.
1100 K. The temperature of SOC drops somewhat with the increase of the boost pressure. The chemical kinetics modeling of HCCI combustion has concluded that HCCI ignition is controlled by hydrogen peroxide (H2O2) decomposition.21 The main ignition needs the mixture up to 1050-1100 K, necessary for H2O2. Figure 3c shows the mean gas temperature of three PRFs at 0.2 MPa inlet pressure. It indicates that, with the increase of RON, the peak in-cylinder temperature increases a
little. The main reason may be that the ignition delay will be longer with the increase of RON, resulting in the more precombustion reaction and increased combustion rates and the peak temperature. (21) U.S. Department of Energy Efficiency and Renewable Energy Office of Transportation Technologies. Homogeneous charge compression ignition (HCCI) technology. A Report to the U.S. Congress, April 2001; p 19.
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Figure 4. Combustion efficiency traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of fuel RON at 0.5 bar boost pressure, and (c) effects of fuel RON at 1 bar boost pressure.
3.2. Effects of Inlet Pressure and Octane Numbers on Combustion Efficiency and Indicated Efficiency. The combustion efficiency was evaluated from the exhaust gas composition, and it is a measure of how complete the combustion is. Referring to the literature,17 the combustion efficiency equation is presented as follows:
(
ηc ) 1 -
∑x Q 1
1
[qf/(qa + qf)]Qf
)
× 100%
(1)
where xi values are the mass fractions of HC, CO, and H2, respectively, the Qi values are the lower heating values of these species, Qf is the lower heating value of the fuel, and qf and qa are the mass flow rate of fuel and air, respectively. The heating value of CO and H2 is 10.1 and 120 MJ/kg, respectively, according to ref 17. The composition of the unburned HC is not usually known. However, the heating values of hydrocarbons are closely comparable; therefore, the fuel heating value is used. In addition, the particulate emission can be ignored here because it is very low in the HCCI combustion mode. The calculated combustion efficiencies are presented in Figure 4. The main trend is that the combustion efficiency increases with an increase of the cycle fueling rate. This is due to the use of a richer mixture and hence higher temperature. Figure 4a shows that, with the increase of the inlet pressure, combustion efficiency increases at the 0.15 MPa case and then decreases at the 0.20 MPa case for a given fueling rate. The reason is that the collision frequency among molecules increases with the increase of the boost pressure, which leads to the increase of the combustion reaction velocity. Therefore, more fuel can be combusted at 1.5 bar inlet pressure. However, on the other hand, the higher inlet pressure leads to the lower mixture concentration for a given fueling rate, which results in the decline of the cylinder temperature and hence the combustion efficiency. The influence of the cylinder temperature on the combustion efficiency is dominant at 0.2 MPa inlet pressure.
Figure 4a also shows that the combustion efficiency of G94.1 and PRF93 are nearly identical without boosting. However, the combustion efficiency of G94.1 is lower than that of PRF93 with boosting. Moreover, the higher the inlet pressure, the larger the difference of combustion efficiency. These result from the lower cylinder temperature of G94.1. Parts b and c of Figure 4 show that the RON of PRF has little effect on combustion efficiency with boosting. Nevertheless, in comparison to the PRF, the combustion efficiency of G94.1 is lower after boosting. The G94.1, as a sensitive fuel, is harder to auto-ignite than that of PRF after boosting. The peak of ROHR of G94.1 is lower than that of PRF93, which results in the lower cylinder temperature and higher unburned HC and CO emissions. Therefore, the combustion efficiency of the G94.1 is the lowest. In addition, Figure 4b also shows that, with the increase of RON, the combustion efficiency of PRF decreases a little especially at a lower fueling rate. However, the Figure 4c shows that the combustion efficiency increases a little as the RON increases. This also indicates that, with the increase of the inlet pressure, the more complete combustion reaction occurs for higher RON, resulting in a higher peak temperature and thus a higher combustion efficiency. The gross indicated efficiency was evaluated by measuring the fuel flow and the indicated mean effective pressure during the compression and expansion strokes only. This means that the effect of supercharging on the gas exchange process is absent. Figure 5 shows the gross indicated efficiency for the different cases. As shown in Figure 5a, with an increased inlet pressure, the efficiency obtained for PRF93 and G94.1 are increased at the 0.15 MPa case and then reduced at the 0.2 MPa case. There are two dominated factors, combustion efficiency and SOC, that affect the indicated efficiency. With the increase of the inlet pressure, the SOC advances. However, the SOC is too early at the 0.2 MPa case, which can cause the decrease of the indicated efficiency. On the other hand, with the increase of the inlet pressure, combustion efficiency increases at the 0.15
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Figure 5. Gross indicated efficiency traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of fuel RON at 0.5 bar boost pressure, and (c) effects of fuel RON at 1 bar boost pressure.
Figure 6. Operating range traces: (a) effects of inlet pressure fueling PRF93 and G94.1 and (b) effects of inlet pressure fueling PRF90, PRF93, and PRF97.
MPa case and then decreases at the 0.20 MPa case for a given fueling rate. In comparison to the PRF93, the maximum achievable gross indicated efficiency of G94.1 is higher. That is the result of the more favorable combustion timing for the G94.1 fuel. Parts b and c of Figure 5 show that the maximum gross indicated efficiency is increased with increasing of RON. The maximum gross indicated efficiency of G94.1 is higher than that of PRF. In addition, they also show that the octane numbers have less effect on the gross indicated efficiency at the higher inlet pressure. 3.3. Effects of Inlet Pressure and Octane Numbers on the HCCI Operating Range. The operating range in terms of load is tested. The limits for the operating range are defined by some chosen variables. The criterion for the maximum achievable load is the maximum pressure rise rate of 1.0 MPa/CA degree, beyond which combustion tends to become “knocky”. The limits for the minimum load are misfiring, defined as COV(IMEPgross) exceeding 10% for the engine. The minimum and maximum engine loads that could be achieved for each inlet pressure under HCCI conditions are presented in Figure 6. The operating range of gross IMEP can become broad as the inlet pressure increases. On the one hand, the use of a higher inlet pressure can bring in more air, which leads to an increase of
the amount of fuel that can be injected and an increase of the maximum load that can be achieved. On the other hand, the engine can operate stably at a leaner air fuel ratio with an increasing inlet pressure, which leads to lower values of load. However, the benefits of intake boost would have been much greater if the combustion phasing had been controlled independently using different intake temperature, EGR, or other methods. Figure 6a shows that the achievable minimum load is nearly identical to the G94.1 and PRF93, but the achievable maximum load of G94.1 is higher than that of PRF93. The reason is that the main combustion of G94.1 is approximately at TDC after boosting, which benefits to increase the gross IMEP. Figure 6b shows that the achievable maximum load increases as the RON increased for a given boost pressure. At higher inlet pressure, the RON has less influence on the achievable maximum load. The achievable minimum load is nearly identical to three PRFs. 3.4. Effects of Inlet Pressure and Octane Numbers on Emissions. NOx formation is very sensitive to the temperature history during the cycle. At temperatures over 1800 K, the NOx formation rate increases rapidly with increased temperature. With a homogeneous combustion of a premixed mixture, the temperature is expected to be the same in the entire combustion
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Figure 7. NOx emissions traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of fuel RON at 0.5 bar boost pressure, (c) effects of fuel RON at 1 bar boost pressure, and (d) effects of maximum cylinder temperature fueling PRF93 and G94.1.
chamber, except near the walls. This, in combination with very lean mixtures, gives a low maximum temperature during the cycle. Therefore, Figure 7 shows that the NOx emissions are overall very low in these tests. The trend observable from Figure 7a is a remarkable increase in NOx for the richest mixtures when running on the cases of without supercharging. However, there is only a little increase or no increase in NOx for the richest mixtures at 0.15 or 0.2 MPa inlet pressure. The reason is that, with the increase of inlet pressure, more air can be drawn into the cylinder. When the fueling rate is constant, the engine will run in leaner operation to help reduce the high-temperature regions. For the cases of supercharging, the reduction of mean gas temperature makes the NOx emissions very low, even at the richest mixtures. Figure 7d shows that, because cylinder temperature is not beyond 1800 K, the temperature almost has no influence on NOx emissions. The low homogeneous combustion temperature related to HCCI can reduce the NOx emissions, but the combustion temperature becomes too low to fully oxidize the fuel completely. The low combustion temperature leads to incomplete combustion and high emissions of unburned hydrocarbons. Figure 8 shows that the major trend is that the HC emissions decrease with an increased cycle fueling rate. Keeping the boost pressure constant, an increased fueling rate means a less diluted mixture and higher combustion temperature and therefore higher combustion quality. It can also be noted that the emissions of HC are reduced after supercharging. The reason may be that the collision frequency among molecules increases after supercharging, which leads that the fuel in the crevices and walls quenching can be combusted better than without supercharging. Figure 8a shows that the HC emissions of G94.1 are higher than those of PRF93 when supercharging is employed. The reason is that the mean gas temperature of G94.1 is lower than that of PRF93. Parts b and c of Figure 8 show that the RON for PRF has little effect on HC emissions with the cases of
supercharging, especially at the 0.2 MPa inlet pressure. Nevertheless, the HC emissions of G94.1 are higher than those of PRF when supercharging is employed. Figure 8d shows that, although the HC emissions decrease with the increase of the maximum cylinder temperature, HC emissions strongly depend upon fuel chemistry and inlet pressure. Therefore, the supercharging is beneficial to reduce the HC emissions. The formation of CO is much more complex. CO is believed to be formed close to the walls where the temperature is high enough for the oxidation of HC to start, but the cooling from the walls prevents complete oxidation to CO2.17 On the basis of this mechanism, wall temperature history and bulk temperature history have a strong impact on CO emissions. The higher combustion temperature results in less CO. It can be seen from Figure 9 that the CO emissions decrease with an increased cycle fueling rate. Figure 9a shows that the CO emissions increase with the increase of the inlet pressure. The CO emissions are very high at the lower cycle fueling rate. The reason is that peak in-cylinder temperature can not reach 1500 K, which is too low to have a sufficient OH concentration for the CO oxidation. Parts b and c of Figure 9 show that the PRF octane number has little effect on CO with the cases of supercharging. The CO emissions of G94.1 are higher than those of PRF for a given inlet pressure when supercharging is employed. In addition, Figure 9b shows that, with the increase of RON, the CO emissions increase a little, especially at a lower fueling rate. However, Figure 9c shows that the CO emissions decrease a little as the RON increases. This also indicates that the more complete combustion reaction occurs for higher RON, resulting in the higher peak temperature and thus less CO emissions. Figure 9d shows that the CO emissions decrease with the increase of the maximum cylinder temperature. It indicates that CO emissions strongly depend upon the cylinder temperature.
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Figure 8. HC emissions traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of fuel RON at 0.5 bar boost pressure, (c) effects of fuel RON at 1 bar boost pressure, and (d) effects of maximum cylinder temperature fueling PRF93 and G94.1.
Figure 9. CO emissions traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of fuel RON at 0.5 bar boost pressure, (c) effects of fuel RON at 1 bar boost pressure, and (d) effects of maximum cylinder temperature fueling PRF93 and G94.1.
It exhibits a good agreement with the CO formation mechanism. That is to say, CO emissions are chemical kinetics products. 4. Summary and Conclusions The effects of inlet pressure and octane numbers on combustion and emissions of a HCCI engine were experimentally investigated. Four fuels were used during the experiments: 90-RON, 93-RON, and 97-RON PRF blend and a commercial gasoline, 94.1-RON(G). The inlet pressure conditions were set to give 0.1, 0.15, and 0.2
MPa of absolute pressure. The most important results presented in this paper can be summarized as follows: (1) The octane number has different effects on the combustion process at different inlet pressures. In comparison to the PRF93, the start of combustion (SOC) of 94.1-RON(G) occurs earlier without boosting, while occurring later with boosting. With the increase of the inlet pressure, the effects of PRF octane number on SOC are weakened and a more precombustion reaction occurs for PRF with a higher RON. (2) The successful HCCI operating region is extended to the upper
Inlet Pressure and Octane Numbers of a HCCI Engine
and lower load with the increase of the inlet pressure. The octane numbers have less influence on the achievable maximum load at the higher inlet pressure, because of the too early SOC. Therefore, the SOC should be controlled independently, unless it becomes too advanced with the increase of the inlet pressure. (3) With the increase of the inlet pressure, the combustion efficiency and gross indicated efficiency increases and then decreases for a given fueling rate. The combustion efficiency of 94.1-RON(G) is lower than those PRFs when supercharging is employed. (4) The HC and NOx emissions of a HCCI engine decrease with supercharging, while CO emissions increase remarkably. The PRF octane numbers have little effect on HC, CO, and NOx emissions with supercharging. However, the HC and CO emissions of 94.1-RON(G) are higher than those of PRF with supercharging for a given fueling rate. Acknowledgment. The authors gratefully acknowledge the Minister of Science and Technology (MOST) of China through its project 2007CB210002 and the National Natural Science Found of China (NSFC) through its project 50676066.
Energy & Fuels, Vol. 22, No. 4, 2008 2215
Nomenclature ABDC ) after bottom dead center ATDC ) after top dead center BBDC ) before bottom dead center BTDC ) before top dead center COV(IMEP) ) cycle-to-cycle variation of indicated mean effective pressure EGR ) exhaust gas recirculation HCCI ) homogeneous charge compression ignition IMEP ) indicated mean effective pressure NA ) naturally aspirated Pin ) inlet pressure PRF ) primary reference fuel RON ) research octane number SI ) spark ignition SOC ) start of combustion Tin ) inlet temperature TDC ) top dead center EF800197B
