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Lean burn is one method for improving thermal efficiency in spark ignition (SI) engines. Suppression of knocking provides higher thermal efficiency, and ethanol blending is considered an effective way to suppress knocking due to its high octane and high latent heat of evaporation. We investigate the effect of ethanol blending on knocking in an SI engine under lean operating conditions. The Livengood-Wu (LW) integral was performed based on ignition delay duration estimated from a zero-dimensional detailed chemical reaction calculation with pressure and temperature histories. Knocking was suppressed and thermal efficiency increased with ethanol-gasoline blending fuel, even at 0.5 equivalence ratio. Decrease in unburned gas temperature by latent heat of evaporation had a comparable influence on knocking suppression, which was supported by LW integral analysis.

To adhere to stringent environmental regulations, SI (spark ignition) engines are required to achieve higher thermal efficiency. In recent years, EGR (exhaust gas recirculation) systems and lean-burn operation has been recognized as key technologies. Under such operating conditions, reducing CCV (cycle-to-cycle variation) in combustion is critical to the enhancement of overall engine performance. Flow-field CCV is one of the considerable factors affecting combustion in engines. Conventionally, in research on flow fields in SI engines, the ensemble average is used to separate the measured velocity field into a mean component and a fluctuation component, the latter of which contains a CCV component and a turbulent component. To extract the CCV of the flow field, previous studies employed spatial filter, temporal filter, and POD (proper orthogonal decomposition) methods.

Super lean burn technology is conceived as one of methods for improving the thermal efficiency of SI engines[1][2]. For lean burn, reduction of heat loss and the due to decrease in flame temperature can be expected. However, as the premixed gas dilutes, the combustion speed decreases, so the combustion fluctuation between cycles increases. Also, to improve the thermal efficiency, the ignition timing is advanced to advance the combustion phase. However, when the combustion phase is excessively advanced, knocking occurs, which hinders the improvement of thermal efficiency. Knocking is a phenomenon in which unburned gas in a combustion chamber compressed by a piston and combustion gas suffer compression auto-ignition. It is necessary to avoid knocking because the amplitude of the large pressure wave may cause noise and damage to the engine. Also, knocking is not a steady phenomenon but a phenomenon that fluctuates from cycle to cycle.

The thermal efficiency of a spark-ignition (SI) engine must be improved to reduce both environmental load and fuel consumption. Although lean SI engine operation can strongly improve thermal efficiency relative to that of stoichiometric SI operation, the cycle-to-cycle variation (CCV) of combustion increases with the air dilution level. Combustion CCV is caused by CCVs of many factors, such as EGR, spark energy, air-fuel ratio, and in-cylinder flow structure related to engine speed. This study focuses on flow structures, especially the influence of a tumble structure on flow fluctuation intensity near ignition timing. We measured the flow field at the vertical center cross section of an optically accessible high-tumble flow engine using time-resolved particle image velocimetry. There are many factors considered to be sources of CCV, we analyzed three factors: the intake jet distribution, distribution of vortex core position and trajectory of the fluid particle near the spark plug.

Knock is a factor hindering enhancement of the thermal efficiency of spark ignition engines, and is an unsteady phenomenon that does not necessarily occur each cycle. In addition, the heat release history of the flame also fluctuates from cycle to cycle, and the auto-ignition process of the unburned mixture (end-gas), compressed by the global increase in pressure due to release of chemical energy, is affected by this fluctuation. Regarding auto-ignition of the end-gas, which can be the origin of knock, this study focused on the fluctuation of the flame heat release pattern, and used a zero-dimensional (0D) detailed chemical reaction calculation in an attempt to analyze and examine the consequence on the end-gas compression and auto-ignition process of changes in the i) start of combustion, ii) combustion duration and iii) center of heat release of the flame.

HCCI (Homogeneous Charge Compression Ignition) engine has a problem which causes knocking when the maximum PRR (Pressure Rise Rate) reaches a certain level because it takes the form of combustion of simultaneous multi-point ignition by compression of the air-fuel pre-mixture. This study focused on stratified charge of fuel in combustion chamber. This method disperses the timing of local ignition. The distribution of fuel concentration is measured by using LIF (Laser Induced Fluorescence). As a result, the maximum PRR is reduced by stratified charge of fuel. In addition, it is confirmed that the dispersion of combustion timing depends on the dispersion of fuel concentration.

The characteristics of cycle-to-cycle variations of indicated mean effective pressure (IMEP) with combustion-phasing retard have been investigated experimentally and computationally in an homogeneous charge compression ignition (HCCI) engine using dimethyl ether (DME). The experiments were conducted in a single-cylinder HCCI research engine equipped with an exhaust gas recirculation (EGR) passage for external EGR and a two-stage exhaust cam for rebreathed EGR. To understand the chemical effects of rebreathed EGR, which is assumed to contribute to the autoignition enhancement, the computations were performed with a single-zone model of CHEMKIN using a chemical-kinetic mechanism developed by combining DME mechanism and NOx submechanism.

Nowadays, highly super charging is required corresponded to downsizing concept for improving thermal efficiency in direct-injected spark ignition (DISI) engine. However, highly super charging increases the possibility of super-knock caused by pre-ignition. Recently, in many studies, the reason of pre-ignition has been investigated but the reason why pre-ignition leads such strong knocking called super-knock has not been investigated. In DISI engine, it is estimated that there is more inhomogeneity of equivalence ratio and temperature of air-fuel mixture than it in port injection SI engine. In this study, factors which decide self-ignition timing was reviewed and the influence of inhomogeneity of air-fuel mixture to super-knock was investigated based on numerical calculation.

This study tried to find a potential of dedicated EGR (d-EGR) system added to the four-cylinder spark ignition (SI) engine to decrease heat loss (Qheatloss) and improve thermal efficiency (ηth). Test fuels were chosen by methane and propane. PREMIX code in CHEMKIN-PRO was employed to calculate laminar burning velocity (SL) and flame temperature (Tf). Wiebe function and Wocshni's heat transfer coefficient were considered to calculate ηth. The results show that the d-EGR system increased ηth and it was higher than that of stoichiometric combustion of conventional SI engines due to the low Tf and fast SL.

The dynamics model and model-based controller (LQG servo controller) have been constructed to improve performance of diesel engine in transient condition. The input parameters of the model are fuel quantity of main injection, timing of main injection, fuel quantity of pilot injection, timing of pilot injection, external EGR ratio and boost pressure. The parameters that are succeeded between cycles to express transient condition are residual gas temperature and of residual oxygen. In the model, one cycle is discretized into 10 representative points. The precision of the accuracy of the model and the responsiveness of the controller were confirmed.

Recently, a potentiality of Dedicated EGR (D-EGR) concept SI engine has been studied. This concept engine had four cylinders and operated with exhaust gas supplied from the single cylinder to the intake manifold. Compared with conventional SI engines, it was able to increase thermal efficiency and decrease CO, HC, and NOx emission by the high D-EGR ratio 0.25. In this study, numerical analysis of a SI engine with D-EGR system with various D-EGR ratios was conducted for detailed understanding the potentiality of this concept in terms of thermal efficiency and NOx emission. #1 cylinder of assumed engine was used as D-EGR cylinder that equivalence ratio varied from 0.6 to 3.4. Entire exhaust gas from #1 cylinder was recirculated to the other cylinders. The other cylinders run with this exhaust gas and new premixed air and fuel with various equivalence ratios from 0.6-1.0.

This study investigated effects of gas inhomogeneity induced by droplets of fuels and oils on the auto ignition timing and temperature in the direct-injection spark ignition (DISI) engine by means of detailed numerical calculation using multi zone model. Recent researchers pointed out that droplets are made of fuels and oils which mix on the cylinder liner and released from the cylinder liner [1]. During the compression stroke released droplets reach the auto ignition temperature before flame propagation induced by spark ignition. It is called Pre-ignition. In combustion chamber, there is inhomogeneity caused by temperature and mixture distribution. In this study, the effects of gas inhomogeneity produced by droplet on the auto ignition timing and temperature have been investigated using Multi-Zone model of CHEMKIN-PRO by changing initial temperature and initial equivalence ratio. Especially, the volume of first ignition zone is focused on.

To approach realization of Homogeneous Charge Compression Ignition (HCCI) combustion without external combustion ignition trigger, it is necessary to construct HCCI engine control system. In this study, HCCI research engine equipped with the EGR passage for external EGR and the two-stage exhaust cam for exhaust rebreathed. This system can control the mixing ratio of four gases (air, fuel, rebreathed EGR gas, external EGR gas) of in-cylinder by operating four throttles and fuel injection duration while maintaining acceptable pressure rise rate (PRR) and cycle-to-cycle variation of Indicated Mean Effective Pressure (IMEP), closed-loop control system designed by applying feedback variables (equivalence ratio, combustion-phasing, IMEP) for feedback control. Those control inputs (four throttles and fuel injection) has correlation mutually, control inputs cause interference, response become low and hunching occurs.

This study has been computationally investigated how the DME autoignition reactivity is affected by EGR and intake-pressure boost over various engine speed. CHEMKIN-PRO was used as a solver and chemical-kinetics mechanism for DME was utilized from Curran's model. We examined first the influence of EGR addition on autoignition reactivity using contribution matrix. Investigations concentrate on the HCCI combustion of DME at wide ranges of engine speeds and intake-pressure boost with EGR rates and their effects on variations of autoignition timings, combustion durations in two-stage combustion process in-detail including reaction rates of dominant reactions involved in autoignition process. The results show that EGR addition increases the combustion duration by lowering reaction rates.

This study experimentally investigates the control system and the algorithm after constructing a HCCI combustion control system for the development of a small HCCI engine fuelled with Dimethyl Ether (DME). This system can control four throttles for the mixing ratio of three gases of in-cylinder (stoichiometric pre-mixture, hot EGR gas and cold EGR gas). At first, the combustion behavior for combustion phasing retarded operation with cold and hot EGR was examined. Then, the potential of model-based and feed back control for HCCI combustion with change of the demand of IMEP was investigated. In the end, the limit of combustion-phasing retard for IMEP and PRR was explored. Results shows that to get high IMEP with acceptable PRR and low coefficient of variation of IMEP, crank angle of 50% heat release (CA50) should be controlled at constant phasing in the expansion stroke. CA50 can be controlled by changing the ratio of pre-mixture, hot EGR gas and cold EGR gas with throttles.

The charge stratification has been thought as one of the ways to reduce the sharp pressure rises of HCCI combustion. The objective of this study is to evaluate the potential of equivalence ratio, initial temperature, and EGR gas stratifications for reducing pressure-rise rate of HCCI combustion. Using rapid compression machine, the stratified pre-mixture is charged, and compressed to analyze the change of in-cylinder gas pressure and temperature traces during compression process. Based on the experiment results, numerical calculations by CHEMKIN are conducted to more specifically analyze the potential of equivalence ratio, initial temperature, and EGR gas stratifications on the reduction of pressure rise rate. Multi-zone model is used to simulate the thermal stratification, fuel stratification and EGR gas stratification of in-cylinder charge as like real engine.

Problem of HCCI combustion is knocking due to a steep heat release by the ignition that is occurred in each local area at the same time. It is considered that dispersion of auto-ignition timing at each local area in the combustion chamber is necessary to prevent this problem. One of technique of this solution is to make thermal stratification. It could be made by using two-stage ignition fuel, which has large heat release at low temperature reaction. Dispersion of fuel concentration leads to difference of temperature histories while combustion phasing is dispersed at each local area. Also, EGR gas stratification could make difference of temperature histories at each local area because of that of the characteristics. This study examines the effect of mixing stratification by stratifying the charge of fuel and CO2. A single-cylinder engine equipped with optical access was used in experiments, and numerical analysis was executed.

This study computationally investigates the combined effects of EGR and boost pressure on HCCI autoignition using iso-octane, PRF50 and n-heptane. The computations were conducted using the single-zone model of CHEMKIN included in CHEMKIN-PRO with detailed chemical-kinetics mechanisms for iso-octane, PRF and n-heptane from Lawrence Livermore National Laboratory (LLNL). To better reproduce the state of EGR addition in real engine, the EGR composition is determined after several combustion cycles under the constant amount of fuel. All data points were acquired with a CA50 of 5°CA aTDC by adjusting initial temperature to remove the effect of combustion phasing, which can influence on HCCI autoignition from any effect of the EGR and boost pressure themselves. The results show that EGR increases the burn duration and reduces the maximum pressure-rise rate with lower peak of maximum heat-release rates for all fuels even for a boost pressure, which accelerates a HCCI autoignition propensity.

CNG/diesel dual-fuel engine is using CNG as a main fuel, and injects diesel only a little as an ignition priming. In this study, remodeling an existing diesel engine into dual-fuel engine that can inject diesel with high pressure by CRDI (Common Rail Direct Injection), and injecting CNG at intake port for premixing. The results show that CNG/diesel dual-fuel engine satisfied coordinate torque and power with conventional diesel engine. And CNG alternation rate is over 89% in all operating ranges of CNG/diesel dual-fuel engine. PM emission is lower 94% than diesel engine, but NOx emission is higher than diesel engine. The output of dual fuel mode is 95% by the diesel mode. At this time, amount of CO₂ and PM are decreased while CO, NOx, and THC are increased. In NEDC mode, exhaust gases except NOx are decreased.

This work has been investigated the potential of in-cylinder EGR stratification for reducing the pressure rise rate of DME HCCI engines, and the coupling of both thermal stratification and fuel stratification. The numerical analyses were done by using five-zone version of CHEMKIN-II kinetics rate code, and kinetic mechanics for DME. The effects of inert components were used for the presence of EGR in calculation. Three cases of EGR stratification were tested on both thermal stratification and fuel stratification at the fixed initial temperature, pressure and fueling rate at BDC. In order to explore the appropriate stratification of EGR, EGR width was employed from zero to thirty percent. Firstly, EGR homogeneity case which means EGR width zero was examined. Secondly, EGR is located densely in hotter zone for combining with thermal stratification or in richer zone for a combination with fuel stratification. Lastly, the case was judged inversely with the second case.