Maximum In-cylinder Temperature Research Articles

Experimental Study on Low-Temperature Oxidation Characteristics and Ignition Boundary Conditions of Gasoline/Hydrogenated Catalytic Biodiesel

Article Experimental Study on Low-Temperature Oxidation Characteristics and Ignition Boundary Conditions of Gasoline/Hydrogenated Catalytic Biodiesel Sicheng Lai 1, Wenjun Zhong 1,*, Tamilselvan Pachiannan 2,3, Zhixia He 1,2, and Qian Wang 1 1 School of Energy Power Engineering, Jiangsu University, Zhenjiang 212013, China 2 Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China 3 School of Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China * Correspondence: wj_zhong@ujs.edu.cn Received: 25 September 2023 Accepted: 8 December 2023 Published: 14 December 2023 Abstract: This study investigates the ignition characteristics of Hydrogenated Catalytic Biodiesel (HCB) with pure gasoline, and diesel. The experiment aims to enhance the ignition characteristics of gasoline fuel by blending it with high-reactivity HCB. It provides experimental data for the fuel blends and offers dependable support for gasoline compression ignition mode applications. To achieve this, the ignition characteristics of various fuels were studied on a variable compression ratio test bench. Experiments were conducted by varying the equivalence ratio and intake temperature of the fuel blends. By analyzing CO emissions and the maximum in-cylinder temperature, we investigated the low-temperature oxidation characteristics of the fuels. Simultaneously, we identified the critical compression ratio and critical temperature as indicators of the ignition boundaries. Finally, this study examined the heat release behavior of the fuels at low temperatures and discussed their combustion characteristics under high-temperature conditions through the heat release rate analysis. The study indicates that with the increase of HCB in the blend, the low-temperature oxidation characteristics are significantly enhanced. However, the ignition boundaries are lowered, and more pronounced secondary heat release combustion behavior is observed. When the blending ratio reaches 50% namely G50H50, it exhibits low-temperature oxidation characteristics and a secondary heat release rate similar to diesel. Their critical compression ratios are 6.8 and 6.5, and the critical temperatures are 838.74 K and 881.41 K respectively. Hence the G50H50 blend holds the potential to serve as a substitute for diesel fuel in compression ignition engines. The increase in intake temperature and equivalence ratio can also enhance low-temperature oxidation characteristics and lower the ignition boundaries. Simultaneously, they exert a certain influence on the peak and phase of the heat release rate.

CFD simulation of in cylinder combustion of multi-cylinder diesel engine

The simulation /reproduction of in cylinder combustion for power stroke of multicylinder Diesel Engine is appropriate/fundamental to work upon for determination of temperature inside the cylinder. The current examination incorporates estimating of approximate temperature of in chamber gases during power stroke and accordingly liner temperature from gas side. The current investigation incorporates mathematical simulations dependent on Inbuilt ICE Combustion model in ANSYS 15.0 form, where burning space above cylinder during power stroke which is time transient is utilized. Correct averaged boundary limits were determined for different sections for the ignition model. The mesh modular is created. The prevailing of different ranges of temperature recorded in literature during power stroke along with differential liner temperatures from gas side are surveyed. Estimations are done for maximum and minimum temperatures also. The maximum in cylinder temperature was around 2150 K and least temperature was discovered to be 800 K. Likewise the most extreme temperature on liner from gas side along stroke was discovered to be 470 K during power stroke. It has been additionally discovered that the greatest temperature of in chamber gases and liner from gas side endures just during early power stroke.

Effect of Miller cycle and exhaust gas recirculation on combustion characteristics of a gasoline direct-injection engine

ABSTRACT A numerical method is used to investigate the effect of variation Miller cycle and Exhaust gas recirculation (EGR) on fuel economy and combustion characteristics of a turbocharged gasoline direct-injection (GDI) engine, in which one-dimensional (1D) and three-dimensional (3D) model are established in GT-Power and CONVERGE software, respectively. The simulation values of the model are validated well with experimental results at part load and full load conditions. The results demonstrate that fuel economy is improved with Miller degree increasing at 50% loads because of the reduction of pumping loss. Further studies have shown that, especially at ±58°CA Miller degrees, early intake valve closure (EIVC) has a better performance than late intake valve closure (LIVC) on fuel economy. In addition, EGR can further decrease fuel consumption based on miller cycle because of further decline for pumping loss and combustion phase optimization. Meanwhile, LIVC can match wider EGR rates than EIVC in terms of good fuel economy. Compared to the baseline scheme, the brake specific fuel consumption is reduced by 4.3% with LIVC58 + 7% EGR. And combustion process of the LIVC58 + 7% EGR also gets optimized which result in lower maximum in-cylinder temperature due to the advance of CA50 and delay of combustion duration.

Effects of Injection Overlap and EGR on Performance and Emissions of Natural Gas HPDI Marine Engine

ABSTRACT Due to the increasingly stringent emission regulations and the growing concern about energy hazard, natural gas (NG) has emerged as an alternative fuel with abundant reserves, low cost, high combustion performance and low pollutant emissions. In this study, a NG high pressure direct injection (HPDI) model is embedded in a Cummins ISX marine medium-speed four-stroke natural gas-diesel dual fuel (NGDDF) engine model. The strategies with different injection overlap of NG and pilot diesel oil (PDO) and different exhaust gas recirculation (EGR) were investigated, the following conclusions are drawn. At the same EGR rate, the injection start point of NG (SOING) has impact on the compression end pressure to some extent, while the injection end point of the pilot diesel oil (EOIPDO) makes little difference to the compression end pressure. With the increase of SOING, the maximum pressure (PMax) rises gradually. When the SOING remains unchanged, the longer the pilot diesel oil (PDO) injection lasts, the higher the PMax. The heat release rate (HRR) of HPDI engine has two peaks. The first one decreases as the EGR rate rises, while the other is inversely proportional only to the EGR rate. As the EGR rate increasing, the position of the second peak of HRR is increasingly delayed, irrespective of the injection strategy used. At the same EGR rate, the overlapping injection strategy has only a limited impact on the maximum in-cylinder temperature (about 42 K) and the soot emission (about 0.003 g/kwh). The first stage of nitrogen oxides (NOX) generation is closely related to the EGR rate. The higher the EGR rate, the longer the interval between the first and second stage of NOX generation. Though both reasonable injection overlap and delay of EOIPDO are conducive to reduce pollutant emissions, it remains necessary to delay the position of the overlap angle as much as possible, and the emission reduction effect of injection overlap will diminish as the EGR rate increasing. Either the injection overlap formed by delaying EOIPDO or advancing SOING has a positive impact on indicated mean effective pressure (IMEP). The introduction of EGR will increase indicated specific fuel consumption (ISFC) of the engine, while the injection overlap strategy can be used to reduce it by 5.35 g/kwh. In order to meet the Tier III standard, the combination of injection strategy B and 18% EGR can be considered as the most effective solution for soot, ISFC and IMEP factors.

Characteristics of Diesel/N-Butanol Blend on a Common Rail Diesel Engine with Exhaust Gas Recirculation

20% n-butanol is blended in diesel by volume (noted as D80B20) and experiment has been carried out to study the effect on the combustion and emission characteristics based on a common rail diesel engine with exhaust gas recirculation (EGR) system. The results reveal that D80B20 has longer ignition delay, shorter combustion duration and higher maximum in-cylinder temperature than pure diesel (noted as D100). Further, the number concentration and volume concentration of ultrafine particles decrease significantly while NOX emissions increase a little with the addition of n-butanol. When the exhaust gas is induced into cylinder, NOX emissions significantly decrease and ultrafine particles emissions increase. The number geometric mean diameters and volume geometric mean diameters of ultrafine particles increase with EGR ratio. Compared to D100 without EGR, D80B20 with 20% EGR ratio can reduce both NOX and ultrafine particles emissions at 0.14 MPa BMEP and 0.56 MPa BMEP.

Waste to energy conversion: Pyrolytic oil and biodiesel as a renewable fuel blends on diesel engine combustion, performance, and emissions

ABSTRACT The use of pyrolytic oil (WTPO) obtained from waste vehicle tires in Turkey is evaluated and encouraged within the scope of YEKDEM (Turkey Renewable Energy Resources Support Mechanism). In order to benefit from this incentive, WTPO must be used as the main fuel in diesel engines. For this reason, in this study, unlike all other studies in the literature, the use of WTPO at a high rate was examined. The effects of biodiesel and WTPO mixtures containing high WTPO on engine performance and emissions were investigated. The experiments were carried out at 2800 rpm engine speed, where the maximum power was obtained, and at 4 different loads as 100%, 75%, 50%, and 25%. When the obtained results are examined. The lowest brake thermal efficiency (BTE) value was obtained in P100 fuel at 25% load and its value was 16.21%. Brake specific fuel consumption (BSFC) data were similar for all test fuels under the same load conditions. Based on the heat release rate and in-cylinder pressure data, it was noted that biodiesel reduced the ignition delay time. The maximum in-cylinder temperature was 1723.4 K and the maximum exhaust temperature was 590°C with the use of P60B40 fuel. The lowest nitrogen oxides (NOx) value results were obtained at 100% load. Hydrocarbon (HC) emissions decreased from 1.13 g/kWh to 0.13 g/kWh for P100 fuel as load increased form 25% to 100%. All these results show that although the biodiesel additive slightly worsened the emissions, positive results were obtained for the performance data.

A comprehensive study of a reactivity-controlled compression ignition engine fueled with biogas and diesel oil

Reactivity-controlled compression ignition (RCCI) engines have been introduced as a promising alternative to compression ignition engines, especially in reducing emissions. It is common practice in an RCCI engine to use two fuels with distinctly different reactivity to control the combustion process. Also, abundant biogas fuel could be considered as a potential and suitable alternative to conventional fuels. Biogas can easily be derived from renewable resources and is basically composed of methane (CH4), carbon dioxide (CO2), a small amount of hydrogen (H2), nitrogen (N2), oxygen (O2) and hydrogen sulfide (H2S). In this paper, biogas as an alternative fuel together with diesel oil is used for reactivity-controlled compression ignition in a modified heavy-duty engine. The biogas fuel with low reactivity is fed through the inlet port and diesel oil with high reactivity directly injected into the engine cylinder. Then, a detailed study of biogas composition, engine speed, intake temperature, fuel metering and injection strategies on combustion characteristics such as engine efficiency and emission levels is carried out. It is demonstrated that the amount of CO2 in biogas composition plays a significant role in combustion and emission processes. Typically, at IMEP = 9.4 bar, as the amount of CO2 in biogas composition is changed from 0 to 40%, the maximum in-cylinder temperature and pressure are reduced about 143 K and 4.6 bar, respectively, while NOx emission is reduced drastically by 75%.

Numerical study of cold-start performances of a medium compression ratio direct-injection twin-spark plug synchronous ignition engine fueled with methanol

The cold-start mixture preparation, combustion and emissions behaviors of a medium compression ratio direct-injection twin-spark plug synchronous ignition engine fueled with methanol were numerically simulated. Simulation results display that the methanol injection timing, ignition timing and the global equivalence ratio had an important influence on the concentration distribution of methanol-air mixture, the flow velocity, and the density of the flame surface during cold start. Ignition delay period reached the shortest at injection timing 70°CA BTDC. Ignition delay period for injection timing 130°CA BTDC and 50°CA BTDC increased 209% and 45.5% compared to injection timing 70°CA BTDC, respectively. Ignition delay period reached the shortest at ignition timing 21°CA BTDC. Ignition delay period of twin-spark plug was shorter than that of single-spark plug at the same equivalence ratio. There existed the maximum in-cylinder pressure, maximum heat release rate, maximum in-cylinder temperature, lowest unburned methanol and soot emissions, and highest NOX emissions at injection timing 70°CA BTDC. The maximum in-cylinder pressure, maximum heat release rate, and maximum in-cylinder temperature gradually increased with the advance of ignition timing. Injection timing 70°CA BTDC, ignition timing 21°CA BTDC, and equivalence ratio 0.9 could obtain the best cold start performance compromise using twin-spark plug ignition mode. At equivalence ratio 1.0, twin-spark plug ignition had better combustion performances, lower unburned methanol and soot emissions, and higher NOX emissions compared with single-spark plug ignition. Twin-spark plug synchronous ignition had more advantageous to cold starting ignition compared with single-spark plug ignition for a medium compression ratio direct-injection spark-ignition methanol engine.

In-depth comparison between pure diesel and diesel methanol dual fuel combustion mode

Diesel Methanol Dual Fuel (DMDF) could achieve high efficiency and low emissions simultaneously at part of engine conditions, but the reason of this has not been clarified. In order to find the reason behind this phenomenon, Pressure-Volume (P-V) map, equivalence ratio (φ)-temperature (T) map and three-dimensional simulation software were used to compare diesel and DMDF mode, and two test conditions were selected to fulfill in-depth comparison of two combustion modes. Based on the analysis of P-V map, the reason of higher thermal efficiency at DMDF mode is as follows: lower cylinder pressure in compression stroke contributes to compression work reduction. Meanwhile, higher combustion rate of DMDF mode could reduce cylinder temperature and pressure in the late power stroke, which would further decrease exhaust loss and heat loss. Based on the analysis of φ-T map, higher methanol substitution percent could reduce the intersection area between cylinder φ-T map and nitric oxide & soot generation region. And after analyzing in-cylinder temperature, hydroxy mole fraction together with the equivalent ratio three-dimensional cloud map, it is found that the inhibitory effect of methanol on diesel low-temperature combustion prolongs diesel ignition delay, which helps to form the homogeneous charge in the cylinder. Under DMDF mode, fuel rich areas are significantly reduced, so is the maximum in-cylinder temperature. This is the key to achieve ultra-low nitrogen oxides and soot emissions for DMDF mode.

Numerical study of twin-spark plug arrangement effects on flame, combustion and emissions of a medium compression ratio direct-injection methanol engine

The effects of twin-spark plug arrangement on flame, combustion and emissions of a medium compression ratio direct injection methanol engine were numerically investigated. Computational results showed that at approximately A (ratio of distance of twin-spark plug and cylinder diameter) = 0.65, the burning and exothermic were the most reasonable compared to other twin-spark plug locations. A = 1.00 was the worst twin-spark plug arrangement. The maximum in-cylinder pressure and the maximum in-cylinder temperature were decreased with increasing A. Ignition delay were increased with increasing A. The A = 0.65 had the shortest combustion duration and the maximum heat release rate. The maximum in-cylinder pressure of A = 1.00 was approximately 57.5% lower than A = 0.25. The ignition delay of A = 1.00 was approximately 2.4 times longer than A = 0.25. The combustion duration of A = 0.67 was approximately 12% and 52.2% lower than A = 0.25 and A = 1.00, respectively. Unburned methanol and soot were increased gradually with increasing A, and at the A > 0.75, the unburned methanol and soot emissions were increased rapidly as increasing A. The unburned methanol of A = 1.00 was approximately 4.6 times higher than A = 0.25. The variations in emitted NOX emissions showed opposite tendencies with the variations in unburned methanol and soot emissions. The optimum arrangement of the twin-spark plug for application of actual engine was approximately A = 0.65.

Experimental investigation the impacts of injection strategies coupled with gasoline/ethanol blend on combustion, performance and emissions characteristics of a GDI spark-ignition engine

The injection timing under single injection strategy and double injection strategy consisting of an optimal timing for the first injection and a later timing for the second injection were investigated on a turbocharged GDI engine fuelled with gasoline and ethanol blend under stoichiometric air/fuel ratio. The combustion phasing, performance and emissions characteristics were comprehensively analyzed. The results indicated that the peak combustion pressure, maximum heat release rate and maximum in-cylinder temperature first increased and then decreased with sweeping the injection timing from 340 °CA bTDC to 280 °CA bTDC, reaching a peak value at 300 °CA bTDC. While the peak combustion pressure, maximum heat release rate and the maximum in-cylinder temperature gradually decreased with decreasing the percentage of first fuel injection distribution. In addition, the 50% combustion location was gradually stayed away from the top dead center, the combustion duration was prolonged, and the gasoline equivalent brake thermal efficiency decreased with decreasing the percentage of the first fuel distribution. Moreover, the injection timing under single injection strategy had no apparent impact on the COVIMEP. However, the fuel distribution under double injection strategy remarkably affected the COVIMEP. Lastly, the concentration of the HC increased with decreasing the percentage of the first fuel injection distribution. While the NOX emissions decreased with decreasing the percentage of the first fuel injection distribution.

A Novel NOx Correlation for CI Engine Using Multidimensional Detailed Chemistry

This paper presents a numerical investigation on a single cylinder direct injection compression ignition engine. Calculations are achieved with kiva3v2 code involving a reduced C7H16 reaction mechanism and a set of sixty species. The study has been carried out on predicted NO and NOx emission levels at the exhaust phase during the late slow diffusion combustion using the asymptotic approach. Simulations have been achieved for five loads (20%, 40%, 60%, 80% and 100%) at 1500 rpm. Predicted values of averaged pressures, O2, CO and CO2 levels in the exhaust manifold have been validated by the ones measured on engine test bench. Algebraic correlations proposed for predicted NO and NOx levels use fuel, O2, N2 concentrations and the maximum in-cylinder temperature. The numerical results of NO and NOx concentrations at the exhaust are in perfect concordance with the measured ones. In addition, numerical investigations help analyzing and predicting the injection timing effect on engine performances as well as NOx emissions. According to the results obtained, the closer the start of injection is to top dead center, the lower the NOx levels at the exhaust are.

The influences of ignition modes on the performances for a motorcycle single cylinder gasoline engine at lean burn operation: Looking inside interaction between flame front and turbulence

In this work, the effect of ignition modes on engine performance is discussed in detail. Torque, ECR (energy consumption rate), emissions and knock are investigated by using combining experiment with CFD (computational fluid dynamics). The results are as following: the torque of synchronous twin-spark is more excellent (for example, the torque improves by 2.3% under 60% engine load and 5000 rpm) relative to single-spark or the asynchronous ignition mode of twin-spark; the synchronous twin-spark ignition mode has shorter combustion period and less heat transfer loss, resulting in the lower ECR (under 60% engine load and 5000 rpm, it decreases by 3.7%).; unexpectedly all the emissions of synchronous twin-spark are higher (increased by 12%, 53.1% and 9.4% for HC, CO and NOx, respectively under 60% engine load, 5000 rpm) than those of single-spark. Then, we used CFD to reveal the seemingly contradictory phenomenon. And found that, under synchronous twin-spark mode, due to the specifically interaction form between flame and turbulence, the maximum in-cylinder temperature is higher and it decreases faster. This results in specifically pollutants formation patterns. The investigation provides a deeply understanding of in-cylinder working process under multi-ignition, and gives us a foundation for the optimization of ignition modes.

Investigation on combustion and emission performance of a common rail diesel engine fueled with diesel-ethylene glycol dual fuel

EG (ethylene glycol) is a good oxygenated fuel for diesel due to structure and thermal conductivity. Based on dual fuel experiment on a modified diesel engine, combustion and emission characteristics are investigated at 1400 r/min with four low EG energy ratio (EGER) (named EG0, EG5, EG10 and EG15 respectively). Combustion results show that the peak cylinder pressure, the peak heat release rates (HRRs) and pressure rise rate increase when EG is used in fumigation mode. However, the maximum in-cylinder temperature changed little. Owing to the auto-ignition character, EG is ignited before diesel injection at 1.29 MPa BMEP, ignition delay is shorten from 0.97 MPa to 1.29 MPa BMEP, and postpone slightly from 0.48 MPa to 0.64 MPa BMEP. Combustion duration decreases with the increase of EGER. The brake thermal efficiency (BTE) is lower and the diesel equivalent brake specific fuel consumption (BSFC) is higher followed by the rise of EGER. For emissions, the NOX, THC, CO2 and soot emissions decrease followed by the rise of EGER at the same time, while CO emissions increase. EG reduce ultrafine particles (UFPs) and decrease the average diameter of UFPs particularly. In conclusion, EG is found to be a good choice for dual fuel diesel engine from 0.81 MPa to 1.29 MPa BMEP in the aspect of performance and emissions.

Numerical investigation of in-cylinder flow characteristics of hydrogen-fuelled internal combustion engine

This paper addresses the computational fluid dynamics (CFD) simulation to investigate the in-cylinder flow characteristics of 2D combustion chamber for a hydrogen-fuelled four-stroke internal combustion engine. CFD simulation has been carried out using commercial CFD codes. The engine speed was varied from 1000 to 3000 rpm, the range of equivalent ratio from 0.6 to 1.0 and the crank angle from 0 to 720 degrees in this study. The effect of the engine speed and equivalence ratio on the flow-field characteristics and volumetric efficiency are investigated in the motoring condition. The increase of engine speed gives a more efficient diffusion process for hydrogen and gives a more homogeneous air–fuel mixture structure. The characteristics of the flow-field are represented by the in-cylinder pressure and temperature distribution as well as the contours of the hydrogen mass fraction for different engine speeds. The acquired results show the maximum in-cylinder temperature and pressure obtained of 650 K and 1.143 MPa at the engine speed of 3000 rpm respectively. It can be seen that the engine speed and equivalence ratio are strongly related to the volumetric efficiency. The results show that the volumetric efficiency increases linearly with increase of the engine speed, but decreases with increase of the equivalence ratio. The results obtained from the simulation can be employed to examine the homogeneity of the air–fuel mixture structure for a better combustion process and engine performance.

Impact of n-butanol and hydrous ethanol fumigation on the performance and pollutant emissions of an automotive diesel engine

This work studied the impact of hydrous ethanol and n-butanol fumigation on the combustion characteristics, performance, pollutant emissions, particle number concentration and size distribution of an automotive diesel engine. Independently of engine load, both alcohols exhibited higher premixed combustion peaks, faster combustion process, and higher coefficient of variation of indicated mean effective pressure (imep) and reduced maximum in-cylinder temperature, in comparison with ultra low sulfur diesel (ULSD). Neither n-butanol nor hydrous ethanol presented better brake thermal efficiency (bte) and brake specific fuel consumption (bsfc) than ULSD. Engine performance with alcohol fumigation is highly susceptible to operating mode. Therefore, it is necessary to optimize the specific thermal conditions for implementing a fumigation strategy. Both alcohols increased carbon monoxide (CO) and total hydrocarbons (THC) and reduced nitrogen oxides (NOx) and particulate matter (PM), in comparison with ULSD fuel. However, the magnitude of this reduction was markedly affected by engine operating mode. The n-butanol showed the best trade-off (PM vs NOx + THC) among all fuels. In comparison with ULSD, hydrous ethanol fumigation decreased the total number concentration of particles while maintaining or increasing the geometric mean diameter, depending on the engine load. In comparison with ULSD, n-butanol maintained or reduced the total number concentration of particles and exhibited the opposite trend for the geometric mean diameter. The particle number concentration (PNC) and size distribution were not affected by engine load for n-butanol.

Transient in-Cylinder Gas Flow Characteristics of Single Cylinder Port Injection Hydrogen Fueled Engine

Problem statement: The variation of the in-cylinder gas flow characteristics for single cylinder port injection hydrogen fueled internal combustion engine was investigated through transient state simulation. Approach: One dimensional gas dynamics was described the flow and heat transfer in the components of the engine model. Special attention is paid to selection and correction of heat transfer correlation which describe of in-cylinder heat transfer to coincide with the practical observations. The engine model was simulated with variable engine speed and Air Fuel Ratio (AFR). Engine speed varied from 2000-5000 rpm with increment equal to 1000 rpm, while AFR changed from stoichiometric to lean limit. Results: The acquired results showed that the maximum in-cylinder temperature and pressure obtained of 2753 K and 49.62 bar at 24°CA ATDC and 13°CA ATDC for AFR = 34.33 respectively, while the minimum in-cylinder temperature and pressure obtained of 1366 K and 29.14 bar at 18 deg CA of ATDC and 8 deg CA of ATDC for AFR = 171.65 respectively. The obtained results show that AFR has a crucial effect on characteristics variation during the power cycle whilst engine speed has minor effects. Conclusion: These results utilized for study the combustion process, fuel consumption, emission production and engine performance.

Performance and Exhaust Emission in a Natural-Gas Fueled Homogeneous Charge Compression Ignition Engine

An experimental study was performed to clarify the operation condition on natural gas fueled homogeneous charge compression ignition engine. Using a single cylinder engine, operation range, engine performance and exhaust emissions were measured under various intake air temperature, compression ratio and equivalence ratio. Under the high intake air temperature, combustion efficiency is high, but under lower temperature, engine was operated in narrow A/F range. The use of high compression ratio, engine was operated at lower intake air temperature. To keep combustion efficiency over 80% and NOx emission under 10 ppm, maximum in-cylinder temperature should be controlled within the narrow range between 1 500 K and 1 800 K. Natural gas auto-ignited over 1 000 K and the effect of A/F was low. Auto ignition temperature was decreased according to the decline of intake air temperature.