Injection Homogeneous Charge Compression Ignition Research Articles

Comparison of Combustion Parameters and Homogeneity of Full and Direct Injection Homogeneous Charge Compression Ignition Engine under Supercharger Conditions

Comparison of Combustion Parameters and Homogeneity of Full and Direct Injection Homogeneous Charge Compression Ignition Engine under Supercharger Conditions

Experimental and Numerical Analysis on the Influences of Direct Fuel Injection Into Oxygen-Depleted Environment of a Homogeneous Charge Compression Ignition Engine

Abstract The oxygen-depleted environment in the recompression stroke can convert gasoline fuel into light hydrocarbons due to thermal cracking, partial oxidation, and water-gas shift reactions. These reformate species can influence the combustion characteristics of gasoline direct injection homogeneous charge compression ignition (GDI-HCCI) engines. In this work, the combustion phenomena are investigated using a single-cylinder research engine under a medium load. The main combustion phases are experimentally advanced by direct fuel injection into the negative valve overlap (NVO) compared with that of intake stroke under single/double-pulse injections. NVO peak in-cylinder pressures are lower than that of motoring due to the limited O2 concentration, emphasizing that endothermic reactions occur during the overlap. This phenomenon limits the oxidation reactions, and the thermal effect is not pronounced. The zero-dimensional chemical kinetics results present the same increasing tendencies of classical reformed species of rich mixture such as C3H6, C2H4, CH4, CO, and H2 as functions of injection timings. Predicted ignition delays are shortened due to the additions of these reformed species. The influences of the reformates on the main combustion are confirmed by three-dimensional computational fluid dynamics (CFD) calculations, and the results show that OH radicals are advanced under NVO injections relative to intake stroke injections. Consequently, earlier heat release and cylinder pressure are noticeable. Parametric studies on the effects of injection pressure, double-pulse injection, and equivalence ratio on the combustion and emissions are also discussed experimentally.

Fuel Stratification and Partially Premixed Combustion With Neat n-Butanol in a Compression Ignition Engine

Homogeneous charge compression ignition (HCCI) has been considered as an ideal combustion mode for compression ignition (CI) engines due to its superb thermal efficiency and low emissions of nitrogen oxides (NOx) and particulate matter. However, a challenge that limits practical applications of HCCI is the lack of control over the combustion rate. Fuel stratification and partially premixed combustion (PPC) have considerably improved the control over the heat release profile with modulations of the ratio between premixed fuel and directly injected fuel, as well as injection timing for ignition initiation. It leverages the advantages of both conventional direct injection compression ignition and HCCI. In this study, neat n-butanol is employed to generate the fuel stratification and PPC in a single cylinder CI engine. A fuel such as n-butanol can provide additional benefits of even lower emissions and can potentially lead to a reduced carbon footprint and improved energy security if produced appropriately from biomass sources. Intake port fuel injection (PFI) of neat n-butanol is used for the delivery of the premixed fuel, while the direct injection (DI) of neat n-butanol is applied to generate the fuel stratification. Effects of PFI-DI fuel ratio, DI timing, and intake pressure on the combustion are studied in detail. Different conditions are identified at which clean and efficient combustion can be achieved at a baseline load of 6 bar IMEP. An extended load of 14 bar IMEP is demonstrated using stratified combustion with combustion phasing control.

Stand-alone single- and multi-zone modeling of direct injection homogeneous charge compression ignition (DI-HCCI) combustion engines

Applying the homogeneous charge compression ignition (HCCI) combustion concept leads to advantages regarding fuel economy and particular emissions. One strategy to prepare an in-cylinder homogeneous charge is the very early direct fuel injection referred to as direct-injection (DI) HCCI combustion. In this study, single- and multi-zone models for DI-HCCI engines are developed; a single-zone approach and a novel hybrid approach sequentially applying single- and multi-zone models. Experimental results are compared with model predictions to validate the main assumption of the modeling approach which considers charge homogeneity after fuel entrainment. Results indicate that for DI-HCCI cases, the assumption of neglecting injection-induced stratification is valid. Both approaches show acceptable results which reveals their capability to adequately well capture DI-HCCI combustion engine performance. However, the single-zone approach, as expected, leads to over-prediction of maximum pressure and delivered power. Furthermore, maximum pressure rise rate as an indicator of smooth engine operation is highly over-estimated and combustion phasing is predicted earlier than the experimental measure. Combustion phasing is estimated by single- and multi-zone approaches with average errors of 0.4 and 0.1 crank angle degrees, respectively. Whereas, the power is estimated with average errors of 10 and 1% for single- and multi-zone approaches, respectively.

Combustion characteristics and operation range of a RCCI combustion engine fueled with direct injection n-heptane and pipe injection n-butanol

An experimental study of n-butanol pipe injection homogeneous charge compression ignition (HCCI) in combination with n-heptane in-cylinder direct injection (DI) is conducted on a modified engine. Since n-butanol does not have a good ignition property, it is preferable to utilize a better ignition fuel (n-heptane) to avoid HCCI combustion only controlled by chemical kinetics and to improve the ignition stability and combustion process. The effects of DI pressure, quantity, timing and intake temperature on RCCI combustion characteristics, thermal efficiency and emissions are studied. The results show that, the optimum DI pressure is 6 MPa. With the increase of DI quantity, peak in-cylinder pressure, peak pressure rise rate and peak heat release rate all increase and occur in advance, CA10 and CA50 occur earlier and combustion duration is shortened. DI timing has an obvious influence on combustion phase. It is concluded that, by adding n-heptane DI, HCCI combustion of n-butanol is improved, HC and CO emissions are reduced, and NOx emission is kept at a very-low level. A smaller cyclic variation is detected and more stable operation is achieved. To some extent, the combustion phase may be controlled, the indicated thermal efficiency is improved, and the operation range has been extended.

Effect of main injection timing for controlling the combustion phasing of a homogeneous charge compression ignition engine using a new dual injection strategy

Homogeneous charge compression ignition combustion of diesel fuel is implemented using a novel dual injection strategy. A new experimental technique is developed to modify a single cylinder direct injection diesel engine to run on homogeneous combustion mode. Effect of main injection timing is investigated covering a range from 26 to 8 crank angle degrees before top dead center with an interval of 3°. Retarded main injection timing is identified as a control strategy for delaying combustion phasing and a means of controlled combustion phasing of direct injection homogeneous charge compression ignition combustion. Two load conditions were investigated and it was observed that at higher load, start of combustion depends more on fuel air equivalence ratio than main injection timing, whereas at low load, it significantly varies with varying main injection timing. Significant improvements in smoke and oxides of nitrogen emissions are observed when compared with the baseline conventional combustion. By studying different combustion parameters, it is observed that there is an improvement in performance and emissions with marginal loss in thermal efficiency when the main injection timing is 20° before top dead center. This is identified as the optimum main injection timing for such homogeneous combustion under the same operating condition.

Experimental investigation of the effects of different injection parameters on a direct injection HCCI engine fueled with alcohol–gasoline fuel blends

In this study, the effects of a two stage direct injection strategy were investigated on the combustion of a homogeneous charge compression ignition (HCCI) engine fuelled with alcohol–gasoline blends. For each injection, the injection timings and fuel quantity were adjusted to get the desired mixture formation in the cylinder, and engine performance was investigated at a high equivalence ratio and constant engine speed. The first injections were selected during the intake stroke and the second injections at the end of the compression stroke by using different injection ratios. Five different fuels (gasoline, E10, E20, M10 and M20) were used at the same energy input conditions. The results showed that the maximum pressure rise rate (MPRR) increased as an earlier start of the first injection (SOI1) timing by using alcohol–gasoline blends. It was found that the start of second injection (SOI2) timing has strong effects on the HCCI combustion and performance parameters as compared to the SOI1 timing although injection ratio (IR) was changed. The maximum cylinder gas pressure (Pmax), indicated that mean effective pressure (imep) and thermal efficiency can be directly controlled by using the SOI2 timing. The operating range of E10, E20 and M10 test fuels extended, as the MPRRs of these fuels were decreased with the use of optimum injection parameters.

An integrated model for negative valve overlap early injection HCCI combustion

Using exhaust top dead center injection (ETCI) mode to realize homogeneous charge compression ignition (HCCI) combustion would cause fuel wall wetting and combustion efficiency reduction, and there is no effective one-dimension simulation model to calculate and analyze the fuel wall wetting and evaporation process. In the research, a new type of integrated model is developed for the new application, HCCI combustion simulation based on ETCI mode. The model includes the following sub-models: fuel injection model, impingement model, film evaporation model, and combustion model. The improved Hiroyasu model and the Bai's model are coupled with homogenous combustion model and film evaporation model to calculate impingement fuel evaporation. The developed model is validated by experiment in a single cylinder diesel engine with ETCI combustion mode. The simulation and experimental results show that, film evaporation is mainly related to cylinder gas temperature and wall temperature, and there is an evident two stage heating process. The cumulate heat release rate is linear with exhaust valve closing (EVC) timing and wall temperature. The burned fuel fraction increases by 6 percent as the EVC timing advances by every 10 °CA, and increases by 4 percent as the wall temperature increases by every 50 K. The cumulate heat release rate decreases with the increase of boosting pressure, and the higher the boosting pressure, the smaller the decrease degree of cumulate heat release rate is.

An experimental study of negative valve overlap injection effects and their impact on combustion in a gasoline HCCI engine

This study pertains to negative valve overlap (NVO) phenomena and subsequent combustion of gasoline in a direct injection homogeneous charge compression ignition (HCCI) engine. Experiments were performed at variable valvetrain settings, which resulted in NVO crank angle (CA) values ranging from 157° to 182°. Fuel was injected directly into the cylinder in a single dose applied during the NVO period, and injection timings were varied from the early stage of exhaust compression to the later stage of exhaust expansion. During the experiments, air excess ratio values were varied from a stoichiometric mixture to lean mixtures, limited by misfires occurrence. A Fourier transform infrared analytical system and simple gas sampling method from the intake port were applied in order to analyze changes in fuel composition resulting from the reforming process during the NVO period. As a result of reforming, which took place after fuel injection during exhaust compression, up to 10.2% of fuel carbon was converted into CO carbon. Auto-ignition promoting species were also identified in considerable quantities, where fuel conversion into acetylene was 26.5g, ethylene – 133.7g and formaldehyde – 13.5g per kilogram of fuel. Certain quantities of other light unsaturated hydrocarbons and methane were identified in the fuel as well. Furthermore, reforming products were present in the exhaust gases, where the fractional composition of unburned hydrocarbons was correlated with the composition of reformed fuel after NVO. Also, the influence of both NVO injection timing and air excess ratio on combustion was studied. It was found that retard of fuel injection during exhaust compression advanced combustion timing, while retard of fuel injection during exhaust expansion retarded combustion due to thermal NVO effects. The examination of combustion timings at a variable air excess ratio and two injection strategies: early and late NVO injections, showed the effect of mixture strength and NVO thermochemistry on the main event. At late NVO injection, applied 20°CA after top dead center (TDC), the combustion timing was retarded both for the stoichiometric and lean mixtures, due to the mixture strength itself, while for early NVO injection, applied 40°CA before TDC, the thermal effects of NVO played a dominant role in combustion control. However, the presence of reforming products in the mixture increased the heat release rate.

Combustion characteristics of a DI-HCCI gasoline engine running at different boost pressures

Homogeneous charge compression ignition (HCCI) combustion mode has some benefits compared to the most popular conventional combustion forms used in the internal combustion (IC) engines: spark ignition (SI) and compression ignition (CI). This combustion mode provides low oxides of nitrogen (NOx) emissions and high thermal efficiency. However, it can produce higher unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions than those of conventional engines due to the lower combustion temperatures. In the naturally aspirated HCCI engines, the low engine output power limits its use in the current engine technologies. Intake air pressure boosting is a common way to improve the engine output power which is widely used in the high performance SI and CI engine applications. Therefore, in this study, the effect of inlet air pressure on the combustion characteristics and exhaust emissions of a direct injection homogeneous charge compression ignition (DI-HCCI) gasoline engine was investigated. For this purpose, a heavy-duty diesel engine was converted to a HCCI direct-injection gasoline engine. The experiments were performed at three different inlet air pressures while operating the engine at the same equivalence ratio and intake air temperature as in normally aspirated HCCI engine condition at different engine speeds. The start of injection (SOI) timing was set dependently to achieve the maximum engine torque at each test condition. The effects of inlet air pressure both on the combustion characteristics (such as cylinder pressure, heat release rate, engine efficiencies, and mean effective pressure) and on the exhaust emissions (such as CO, UHC and NOx) were discussed. The coefficients of variation (COV) of the indicated mean effective pressure (IMEP) were also provided.

Homogeneous charge compression ignition (HCCI) combustion of diesel fuel with external mixture formation

Abstract HCCI (Homogeneous Charge Compression Ignition) has been touted for many years as the alternate technology of choice for future engines, preserving the inherent efficiency of CIDI (Compression Ignition Direct Injection) engines while significantly reducing emissions. The current direction for all published diesel HCCI research is mixture preparation using the direct injection – system, referred to as internal mixture formation. The benefit of internal mixture formation is that it utilizes an already available direct injection system. Direct injected diesel HCCI can be divided into two areas, early injection (early in the compression stroke) and late injection (usually after Top Dead Center (aTDC)). Early direct injection HCCI requires carefully designed fuel injector to minimize the fuel wall-wetting that can cause combustion inefficiency and oil dilution. Late direct injection HCCI requires a long ignition delay and rapid mixing rate to achieve the homogeneous mixture. The ignition delay is extended by retarding the injection timing and rapid mixing rate was achieved by combining high swirl with toroidal combustion-bowl geometry. There is a compromise between Direct Injection (DI) and HCCI combustion regimes. Even under ideal conditions, it can prove difficult to form a truly homogeneous charge, which leads to elevated emissions when compared to true homogenous charge combustion and also strongly contribute to the high sensitivity of the combustion phasing to external parameters. The alternative to the internal mixture formation is, predictably, external mixture formation. By introducing the fuel external to the combustion chamber one can use the turbulence intake process to create a homogeneous charge regardless of engine conditions. This eliminates the need for combustion system changes which were necessary for the internal mixture formation method. With this method, the combustion system remains fully optimized for direct injection and also capable of running in HCCI combustion mode with nearly ideal mixture preparation. The key to the external mixture formation with diesel fuel is proper mixture preparation. In the present investigation a fuel vapouriser was used to achieve excellent HCCI combustion in a single cylinder air-cooled direct injection diesel engine. No modifications were made to the combustion system. In this study a vaporized diesel fuel was mixed with air to form a homogeneous mixture and inducted into the cylinder during the intake stroke. To control the early ignition of diesel vapour–air mixture, cooled (30 °C) Exhaust Gas Recirculation (EGR) technique was adopted. Experiments were conducted with diesel vapour induction without EGR and diesel vapour induction with 10%, 20% and 30% EGR and results are compared with conventional diesel fuel operation (DI @ 23° before Top Dead Center (bTDC) and 200 bar injection pressure).

Effect of injection timing on gasoline homogeneous charge compression ignition particulate emissions

Particulate emissions from homogeneous charge compression ignition (HCCI) engines are often considered as negligible and the measurement of particulate matter (PM) with HCCI combustion systems has been rare. An earlier publication in the literature and the authors' own recently published work suggest that PM emissions from gasoline direct injection (DI) HCCI engines should not be neglected. It has been shown that PM emissions from HCCI engines, although generally lower, can be similar to spark ignition (SI) levels for certain engine conditions, especially in the accumulation mode. The present work shows that although injection timing is effective for control of HCCI, it can have a significant impact on PM emissions. Engine speed also appears to have an impact on PM emissions for the HCCI combustion mode. Typically for HCCI operating at 1500 r/min, the total mass and total number of the PM emissions could be up to six times higher when the fuel is injected during the recompression period compared with the case for injection during the inlet valve open period. Similarly, when the engine was operated in SI mode, an attempt to inject fuel close to induction top dead centre (TDC) made the PM emission noticeably higher, reaching unacceptable levels. The split injection strategy for HCCI mode has been investigated for its effect on PM emissions. It has been found that the PM number concentration for split injection lies between the maximum concentration measured for very early injections (near TDC at recompression) and the minimum concentration measured for the latest injections used (at crank angles near to the inlet valve maximum opening time), with the magnitude of number density varying approximately from 0.2 to 1.2 million/cm3. It is also observed that for split injection the number of particulates in nucleation mode varies less, while the number in accumulation mode level varies within a limit of one order of magnitude; both are dependent on the injection strategy. The shape of the PM distribution is such that the nucleation mode exceeds the number concentration of 1×107 (d N d log Dp) and the accumulation mode falls parabolically to approximately 1×106. For comparison, it can be said that the PM number concentration measured in the authors' engine is approximately one order of magnitude lower in accumulation mode than the level for a typical modern diesel engine.

A modelling study into the effects of variable valve timing on the gas exchange process and performance of a 4-valve DI homogeneous charge compression ignition (HCCI) engine

Homogeneous charge compression ignition (HCCI) combustion mode is a relatively new combustion technology that can be achieved by using specially designed cams with reduced lift and duration. The auto-ignition in HCCI engine can be facilitated by adjusting the timing of the exhaust-valve-closing and, to some extent, the timing of the intake-valve-opening so as to capture a proportion of the hot exhaust gases in the engine cylinder during the gas exchange process. The effects of variable valve timing strategy on the gas exchange process and performance of a 4-valve direct injection HCCI engine were computationally investigated using a 1D fluid-dynamic engine cycle simulation code. A non-typical intake valve strategy was examined; whereby the intake valves were assumed to be independently actuated with the same valve-lift profile but at different timings. Using such an intake valves strategy, the obtained results showed that the operating range of the exhaust-valve-timing within which the HCCI combustion can be facilitated and maintained becomes much wider than that of the typical intake-valve-timing case. Also it was found that the engine parameters such as load and volumetric efficiency are significantly modified with the use of the non-typical intake-valve-timing. Additionally, the results demonstrated the potential of the non-typical intake-valve strategy in achieving and maintaining the HCCI combustion at much lower loads within a wide range of valve timings. Minimizing the pumping work penalty, and consequently improving the fuel economy, was shown as an advantage of using the non-typical intake-valve-timing with the timing of the early intake valve coupled with a symmetric degree of exhaust-valve-closing timing.

Dual injection homogeneous charge compression ignition engine simulation using a stochastic reactor model

Multiple direct injection (MDI) is a promising strategy to enable fast-response ignition control as well as expansion of the homogeneous charge compression ignition (HCCI) engine operating window, thus realizing substantial reductions of soot and NOx emissions. The present paper extends a zero-dimensional-probability-density-function-based stochastic reactor model (SRM) for HCCI engines in order to incorporate MDI and an improved turbulent mixing model. For this, a simplistic spray model featuring injection, penetration, and evaporation sub-models is formulated, and mixing is described by the Euclidean minimal spanning tree (EMST) sub-model accounting for localness in composition space. The model is applied to simulate a gasoline HCCI engine, and the in-cylinder pressure predictions for single and dual injection cases show a satisfactory agreement with measurements. From the parametric studies carried out it is demonstrated that, as compared with single injection, the additional second injection contributes to prolonged heat release and consequently helps to prevent knock, thereby extending the operating range on the high load side. Tracking the phase space trajectories of individual stochastic particles provides significant insight into the influence of local charge stratification owing to direct injection on HCCI combustion.

Experimental optimization of a direct injection homogeneous charge compression ignition gasoline engine using split injections with fully automated microgenetic algorithms

Homogeneous charge compression ignition (HCCI) is receiving attention as a new low-emission engine concept. Little is known about the optimal operating conditions for this engine operation mode. Combustion under homogeneous, low equivalence ratio conditions results in modest temperature combustion products, containing very low concentrations of NOx and particulate matter (PM) as well as providing high thermal efficiency. However, this combustion mode can produce higher HC and CO emissions than those of conventional engines. An electronically controlled Caterpillar single-cylinder oil test engine (SCOTE), originally designed for heavy-duty diesel applications, was converted to an HCCI direct injection (DI) gasoline engine. The engine features an electronically controlled low-pressure direct injection gasoline (DI-G) injector with a 60° spray angle that is capable of multiple injections. The use of double injection was explored for emission control and the engine was optimized using fully automated experiments and a microgenetic algorithm optimization code. The variables changed during the optimization include the intake air temperature, start of injection timing and the split injection parameters (per cent mass of fuel in each injection, dwell between the pulses). The engine performance and emissions were determined at 700 r/min with a constant fuel flowrate at 10 MPa fuel injection pressure. The results show that significant emissions reductions are possible with the use of optimal injection strategies.

Application of detailed chemistry and CFD for predicting direct injection HCCI engine combustion and emissions

The present study applied a detailed chemical kinetic mechanism to simulate the combustion and emissions in a direct-injection homogeneous charge compression ignition (HCCI) engine. The engine is a heavy-duty diesel engine equipped with a pressure-swirl injector using gasoline as the fuel. The intake air was heated to help fuel vaporization to achieve HCCI conditions. The CHEMKIN code was implemented into KIVA-3V so that the chemistry is solved in the context of the engine computational fluid dynamics simulation. The effects of turbulent mixing on the reaction rate were considered. The computations started from intake valve closure with the initial conditions provided by a one-dimensional cycle simulation code that can simulate the gas-exchange process. Good levels of agreement in combustion and emissions were obtained using the present model. Predicted cylinder pressures and heat release rates agreed well with measurements. The computational results showed that a lean and fairly homogeneous mixture was obtained under the current engine configurations. Relatively low gas temperature (with peak value only about 2000 K) was observed in the present HCCI combustion that produced low NOx emissions. The model also predicted the correct trends in unburned hydrocarbon and carbon monoxide emissions as the start-of-injection timing and engine load were varied. It was also found that the unburned hydrocarbons and carbon monoxide emissions increased drastically if the overall equivalence ratio was less than a certain limit, for example, 0.15, due to poor combustion.