Improved emission characteristics of HCCI engine by various premixed fuels and cooled EGR

This work investigates partial HCCI (homogeneous charge compression ignition) combustion as a control mechanism for HCCI combustion. The premixed fuel is supplied via a port fuel injection system located in the intake port of DI diesel engine. Cooled EGR is introduced for the suppression of advanced autoignition of the premixed fuel. The premixed fuels used in this experiment are gasoline, diesel, and n-heptane. The results show that with diesel premixed fuel, a simultaneous decrease of NO_x and soot can be obtained by increasing the premixed ratio. However, when the inlet charge is heated for the improved vaporization of diesel fuel, higher inlet temperature limits the operational range of HCCI combustion due to severe knocking and high NO_x emission at high premixed ratios. Gasoline premixing shows the most significant effects in the reductions of NO_x and soot emissions, compared to other kinds of premixed fuels.     

An experimental and numerical analysis of the HCCI auto-ignition process of primary reference fuels,toluene reference fuels and diesel fuel in an engine,varying the engine parameters

For a future HCCI engine to operate under conditions that adhere to environmental restrictions, reducing fuel consumption and maintaining or increasing at the same time the engine efficiency, the choice of the fuel is crucial. For this purpose, this paper presents an auto-ignition investigation concerning the primary reference fuels, toluene reference fuels and diesel fuel, in order to study the effect of linear alkanes, branched alkanes and aromatics on the auto-ignition. The auto-ignition of these fuels has been studied at inlet temperatures from 25 to 120 °C, at equivalence ratios from 0.18 to 0.53 and at compression ratios from 6 to 13.5, in order to extend the range of investigation and to assess the usability of these parameters to control the auto-ignition. It appeared that both iso-octane and toluene delayed the ignition with respect to n-heptane, while toluene has the strongest effect. This means that aromatics have higher inhibiting effects than branched alkanes. In an increasing order, the inlet temperature, equivalence ratio and compression ratio had a promoting effect on the ignition delays. A previously experimentally validated reduced surrogate mechanism, for mixtures of n-heptane, iso-octane and toluene, has been used to explain observations of the auto-ignition process.     

A soot formation embedded reduced reaction mechanism for diesel surrogate fuel

A reduced diesel surrogate fuel chemical reaction mechanism of n-heptane/toluene was developed, the reduced mechanism (referred as the “THU mechanism”) includes 60 species and 145 reactions, and it contains soot formation reactions. The THU mechanism was developed from the existing n-heptane/toluene mechanism (70 species and 313 reactions) of Chalmers University of Technology (referred as the “CTH mechanism”). SENKIN and XSENKPLOT were used to analyze the important reactions and species during n-heptane, toluene oxidation and soot formation processes to formulate the reduced mechanism. Ignition delays of n-heptane and toluene predicted by the THU mechanism match well with the CTH mechanism and shock-tube test data under different conditions. The THU and CTH mechanisms also show similar soot concentration prediction. The global reaction of diesel fuel decomposed into n-heptane and toluene with mole fraction 7:3 was built to accelerate the decomposition and advance ignition timing. Kinetic constants of soot oxidation reactions were adjusted to reduce the soot oxidation rate. The THU mechanism was coupled with the KIVA-3V Release 2 code to model diesel combustion processes in the constant-volume combustion vessel and optical diesel engine of Sandia. The predicted ignition delay, in-cylinder pressure and heat release rate match the experimental results well. The predicted spatial and temporal soot concentration distributions have similar trend with the experiments.     

A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR. Part 2. Effect of operating conditions and EGR on HCCI combustion

In Part 1, the effects of octane number of primary reference fuels and equivalence ration on combustion characteristics of a single-cylinder HCCI engine were studied. In this part, the influence of exhaust gas recirculation (EGR) rate, intake charge temperature, coolant temperature, and engine speed on the HCCI combustion characteristics and its emissions were evaluated. The experimental results indicate that the ignition timing of the first-stage combustion and second-stage combustion retard, and the combustion duration prolongs with the introduction of cooled EGR. At the same time, the HCCI combustion using high cetane number fuels can tolerate with a higher EGR rate, but only 45% EGR rate for RON75 at 1800 rpm. Furthermore, there is a moderate effect of EGR rate on CO and UHC emissions for HCCI combustion engines fueled with n-heptane and RON25, but a distinct effect on emissions for higher octane number fuels. Moreover, the combustion phase advances, and the combustion duration shorten with the increase of intake charge temperature and the coolant out temperature, and the decrease of the engine speed. At last, it can be found that the intake charge temperature gives the most sensitive influence on the HCCI combustion characteristics.     

Biodiesel engine performance and emissions in low temperature combustion

Engine performance and emission comparisons were made between the use of soy, Canola and yellow grease derived B100 biodiesel fuels and an ultra-low sulphur diesel fuel in the high load engine operating conditions. Compared to the diesel fuel engine-out emissions of nitrogen oxides (NO_x), a high-cetane number (CN) biodiesel fuel produced comparable NO_x while the biodiesel with a CN similar to the diesel fuel produced relatively higher NO_x at a fixed start of injection. The soot, carbon monoxide and un-burnt hydrocarbon emissions were generally lower for the biodiesel-fuelled engine. Exhaust gas recirculation (EGR) was then extensively applied to initiate low temperature combustion (LTC) mode at medium and low load conditions. An intake throttling valve was implemented to increase the differential pressure between the intake and exhaust in order to increase and enhance the EGR. Simultaneous reduction of NO_x and soot was achieved when the ignition delay was prolonged by more than 50% from the case with 0% EGR at low load conditions. Furthermore, a preliminary ignition delay correlation under the influence of EGR at steady-state conditions was developed. The correlation considered the fuel CN and oxygen concentrations in the intake air and fuel. The research intends to achieve simultaneous reductions of NO_x and soot emissions in modern production diesel engines when biodiesel is applied.     

Development and calibration of a reduced chemical kinetic model of n-heptane for HCCI engine combustion

A new reduced chemical kinetic model for the Homogeneous Charge Compression Ignition (HCCI) combustion of n-heptane in an engine has been developed. The new model is based on two previous reduced kinetic models for alkane oxidation, from which some reactions have been eliminated and with enhanced treatment of the oxidization of CO and CH₃O. The kinetic parameters of the key reactions in the new model were adjusted by using a genetic algorithm optimization methodology to improve ignition timings predictions over the range of equivalence ratios from 0.2 to 1.2, temperature from 300 to 3000 K. The final model contains 40 species and 62 reactions and was validated under HCCI engine conditions. The results showed the well-known two-stage ignition characteristics of n-heptane, which involve low and high temperature regimes followed by a branched chain explosion. The optimized reduced model generally agrees well with those of the detailed chemical kinetic model (544 species and 2446 reactions); the computational time of using the former is less 1/1000 that of the latter.     

Investigations on surrogate fuels for high-octane oxygenated gasolines

Gasoline is a complex mixture that possesses a quasi-continuous spectrum of hydrocarbon constituents. Surrogate fuels that decrease the chemical and/or physical complexity of gasoline are used to enhance the understanding of fundamental processes involved in internal combustion engines (ICEs). Computational tools are largely used in ICE development and in performance optimization; however, it is not possible to model full gasoline in kinetic studies because the interactions among the chemical constituents are not fully understood and the kinetics of all gasoline components are not known. Modeling full gasoline with computer simulations is also cost prohibitive. Thus, surrogate mixtures are studied to produce improved models that represent fuel combustion in practical devices such as homogeneous charge compression ignition (HCCI) and spark ignition (SI) engines. Simplified mixtures that represent gasoline performance in commercial engines can be used in investigations on the behavior of fuel components, as well as in fuel development studies. In this study, experimental design was used to investigate surrogate fuels. To this end, SI engine dynamometer tests were conducted, and the performance of a high-octane, oxygenated gasoline was reproduced. This study revealed that mixtures of iso-octane, toluene, n-heptane and ethanol could be used as surrogate fuels for oxygenated gasolines. These mixtures can be used to investigate the effect of individual components on fuel properties and commercial engines performance.     

Study on the controlling strategies of homogeneous charge compression ignition combustion with fuel of dimethyl ether and methanol

The controlling strategies of homogeneous charge compression ignition (HCCI) fueled by dimethyl ether (DME) and methanol were investigated. The experimental work was carried out on a modified single-cylinder diesel engine, which was fitted with port injection of DME and methanol dual fuel. The results show that exhaust gas recirculation (EGR) rate and DME percentage are two important parameters to control the HCCI combustion process. The ignition timing and combustion duration can be regulated in a suitable range with high indicated thermal efficiency and low emissions by adjusting the DME percentage and EGR rate. EGR cannot extend the maximum indicate mean effective pressure (IMEP) of HCCI operation range with dual fuel, but can enlarge the DME percentage range in normal combustion. The combustion efficiency largely depends on DME percentage, and EGR can improve combustion efficiency. The results also show that HC emissions strongly depend upon DME percentage, and CO emissions have good coherence to the peak mean temperature in cylinder. In normal combustion, adopting large DME percentage and high EGR rate can attain an optimal HCCI combustion.     

An experimental and numerical analysis of the influence of the inlet temperature,equivalence ratio and compression ratio on the HCCI auto-ignition process of Primary Reference Fuels in an engine

In order to understand better the auto-ignition process in an HCCI engine, the influence of some important parameters on the auto-ignition is investigated. The inlet temperature, the equivalence ratio and the compression ratio were varied and their influence on the pressure, the heat release and the ignition delays were measured. The inlet temperature was changed from 25 to 70 °C and the equivalence ratio from 0.18 to 0.41, while the compression ratio varied from 6 to 13.5. The fuels that were investigated were PRF40 and n-heptane. These three parameters appeared to decrease the ignition delays, with the inlet temperature having the least influence and the compression ratio the most. A previously experimentally validated reduced surrogate mechanism, for mixtures of n-heptane, iso-octane and toluene, has been used to explain observations of the auto-ignition process. The same kinetic mechanism is used to better understand the underlying chemical and physical phenomena that make the influence of a certain parameter change according to the operating conditions. This can be useful for the control of the auto-ignition process in an HCCI engine.     

Experimental study on the auto-ignition and combustion characteristics in the homogeneous charge compression ignition (HCCI) combustion operation with ethanol/n-heptane blend fuels by port injection

This article investigates the auto-ignition, combustion, and emission characteristics of homogeneous charge compression ignition (HCCI) combustion engines fuelled with n-heptane and ethanol/n-heptane blend fuels. The experiments were conducted on a single-cylinder HCCI engine using neat n-heptane, and 10%, 20%, 30%, 40%, and 50% ethanol/n-heptane blend fuels (by volume) at a fixed engine speed of 1800 r/min. The results show that, with the introduction of ethanol in n-heptane, the maximum indicated mean effective pressure (IMEP) can be expanded from 3.38 bar of neat n-heptane to 5.1 bar, the indicated thermal efficiency can also be increased up to 50% at large engine loads, but the thermal efficiency deteriorated at light engine load. Due to the much higher octane number of ethanol, the cool-flame reaction delays, the initial temperature corresponding the cool-flame reaction increases, and the peak value of the low-temperature heat release decreases with the increase of ethanol addition in the blend fuels. Furthermore, the low-temperature heat release is indiscernible when the ethanol volume increases up to 50%. In the case of the neat n-heptane and 10% ethanol/n-heptane blends, the combustion duration is very short due to the early ignition timing. For 20–50% ethanol/n-heptane blend fuels, the ignition timing is gradually delayed to the top dead center (TDC) by the ethanol addition. As a result, the combustion duration prolongs obviously at the same engine load when compared to the neat n-heptane fuel. At overall stable operation ranges, the HC emissions for n-heptane and 10–30% ethanol/n-heptane blends are very low, while HC emissions increase substantially for 40% and 50% ethanol/n-heptane blends. CO emissions show another tendency compared to HC emissions. At the engine load of 1.5–2.5 bar, CO emissions are very high for all fuels. Beside this range, CO emissions decrease both for large load and light load. In terms of operation stability of HCCI combustion, for a constant energy input, n-heptane shows an excellent repeatability and light cycle-to-cycle variation, while the cycle-to-cycle variation of the maximum combustion pressure and its corresponding crank angle, and ignition timing deteriorated with the increase of ethanol addition.     

Study of HCCI-DI combustion and emissions in a DME engine

HCCI combustion demonstrates the capability of simultaneously reducing NO_x and PM emissions and having a high brake thermal efficiency. However, there are still many challenges such as combustion control to overcome before full HCCI operation can be used reliably over the full engine operation range. Recently, the HCCI-DI compound combustion concept is presented, which is a compromise to full HCCI in that only a portion of the fuel is premixed and a portion of combustion is still controlled by the direct injection timing. Investigations towards HCCI-DI combustion in a DME engine were carried out in this paper. HCCI engine performances were presented to make a comparison. The peak in-cylinder pressure and the maximum heat release rate for HCCI-DI were lower than those for HCCI combustion and they decreased with a decrease in port DME aspiration quantity. Moreover, combustion duration was longer for HCCI-DI combustion and it would elongate with a decrease in port DME aspiration quantity. Engine experimental results showed HCCI-DI combustion could extend the operating range with a comparatively high brake thermal efficiency in comparison to HCCI combustion. CO and HC emission for HCCI-DI were lower than those of HCCI engine. As for NO_x emissions for HCCI-DI operation, it decreased remarkably at low loads with an increase in port DME aspiration quantity, while showed an increasing trend at high loads. To control the ignition and combustion phase of HCCI, the effect of cooled EGR on HCCI-DI was evaluated. As a result, NO_x emission decreased and the engine’s operating range enlarged for HCCI-DI combustion with cooled EGR.     

Effects of ethyl tert-butyl ether addition to diesel fuel on characteristics of combustion and exhaust emissions of diesel engines

The effects of ethyl tert-butyl ether (ETBE) addition to diesel fuel on the characteristics of combustion and exhaust emissions of a common rail direct injection diesel engine with high rates of cooled exhaust gas recirculation (EGR) were investigated. Test fuels were prepared by blending 0, 10, 20, 30 and 40 vol% ETBE to a commercial diesel fuel. Increasing ETBE fraction in the fuel helps to suppress the smoke emission increasing with EGR, but a too high fraction of ETBE leads to misfiring at higher EGR rates. While the combustion noise and NO_x emissions increase with increases in ETBE fraction at relatively low EGR rates, they can be suppressed to low levels by increasing EGR. Though there are no significant increases in THC and CO emissions due to ETBE addition to diesel fuel in a wide range of EGR rates, the ETBE blended fuel results in higher aldehyde emissions than the pure diesel fuel at relatively low EGR rates. With the 30% ETBE blended fuel, the operating load range of smokeless, ultra-low NO_x (<0.5 g/kWi h), and efficient diesel combustion with high rates of cooled EGR is extended to higher loads than with the pure diesel fuel.     

The development and experimental validation of a reduced ternary kinetic mechanism for the auto-ignition at HCCI conditions,proposing a global reaction path for ternary gasoline surrogates

To acquire a high amount of information of the behaviour of the Homogeneous Charge Compression Ignition (HCCI) auto-ignition process, a reduced surrogate mechanism has been composed out of reduced n-heptane, iso-octane and toluene mechanisms, containing 62 reactions and 49 species. This mechanism has been validated numerically in a 0D HCCI engine code against more detailed mechanisms (inlet temperature varying from 290 to 500 K, the equivalence ratio from 0.2 to 0.7 and the compression ratio from 8 to 18) and experimentally against experimental shock tube and rapid compression machine data from the literature at pressures between 9 and 55 bar and temperatures between 700 and 1400 K for several fuels: the pure compounds n-heptane, iso-octane and toluene as well as binary and ternary mixtures of these compounds. For this validation, stoichiometric mixtures and mixtures with an equivalence ratio of 0.5 are used. The experimental validation is extended by comparing the surrogate mechanism to experimental data from an HCCI engine. A global reaction pathway is proposed for the auto-ignition of a surrogate gasoline, using the surrogate mechanism, in order to show the interactions that the three compounds can have with one another during the auto-ignition of a ternary mixture.     

Detailed approach for apparent heat release analysis in HCCI engines

Homogeneous charge compression ignition (HCCI) combustion is a spontaneous multi-site auto-ignition of a lean premixed fuel-air mixture, which has high heat release rate, short combustion duration and no evidence of flame propagation. In HCCI engines, there is no direct control method for the time of auto-ignition. Auto-ignition timing should be controlled in order to make heat release process take place at the appropriate time in the engine cycle. Heat release analysis is a diagnostic tool which aids engine experimenters. It facilitates the endeavors being conducted in obtaining a control method by investigating heat release rate and also cumulative heat release. This study can be divided into two parts. First, traditional first law heat release model which is widely used in engine combustion analysis was presented and the applicability of this model in HCCI engines was investigated. Second, a new heat release model based on the first law of thermodynamics accompanying with a temperature solver was developed and assessed. The model was applied in four test conditions with different operating conditions and a variety of fuel compositions, including i-octane, n-heptane, pure NG, and at last, a dual fueled case of NG and n-heptane. Results of this work indicate that utilizing the modified first law heat release model together with a solver for temperature correction will guarantee obtaining a well-behaved and accurate apparent heat release trend and magnitude in HCCI combustion engines.     

Charge stratification to control HCCI: Experiments and CFD modeling with n-heptane as fuel

An optimized reduced mechanism of n-heptane including 42 species and 58 elementary reactions adapted to charge stratification combustion is developed first in this study. Some engine experiments and a fully coupled CFD and reduced chemical kinetics model with n-heptane as fuel are adopted to investigate the combustion processes of HCCI-like charge stratification combustion aimed at diesel HCCI application. For premixed/direct-injected stratification combustion, the low temperature reaction occurs in the regions with homogeneous fuel first and high temperature reaction begins from high fuel concentration regions involved in the spray process. With the increase of the injection ratio, the high temperature reaction occurs in advance, the pressure rise rate reduces, UHC emissions decrease and CO emissions increase. At larger injection ratio, the onset of the high temperature reaction advances and the maximum pressure rise rate decreases with the retarding of injection timing. UHC and CO emissions have relation to the fuel spray penetration at different injection timings. NO_x emissions increase rapidly with the increase of the stratification degree.     

Effect of a narrow fuel spray angle and a dual injection configuration on the improvement of exhaust emissions in a HCCI diesel engine

The aim of this work was to investigate the effect of narrow fuel spray angle injection and dual injection strategy on the exhaust emissions of a common-rail diesel engine. To achieve successful homogeneous charge compression ignition by an early timing injection, a narrowed spray cone angle injector and a reduced compression ratio were employed. The combination of homogeneous charge compression ignition (HCCI) combustion and conventional diesel combustion was studied to examine the exhaust emission and combustion characteristics of the engine under various fuel injection parameters, such as injection timings of the first and second spray.The results showed that a dual injection strategy consisting of an early timing for the first injection for HCCI combustion and a late timing for the second injection was effective to reduce the NO_x emissions while it suppress the deterioration of the combustion efficiency caused by the HCCI combustion.     

A computational study of direct injection gasoline HCCI engine with secondary injection

The detailed intake, spray, combustion and pollution formation processes of compression ignition engine with high-octane fuel are studied by coupling multi-dimensional computational fluid dynamic (CFD) code with detailed chemical kinetics. An extended hydrocarbon oxidation reaction mechanism used for high-octane fuel was constructed and a modeling strategy of 3D-CFD/chemistry coupling for engine simulation is introduced to meet the requirements of execution time acceptable to simulate the whole engine physicochemical process including intake, compression, spray and combustion process. The improved 3D CFD/chemistry model was validated using the experimental data from HCCI engine with direct injection. Then, the CFD/chemistry model has been employed to simulate the intake, spray, combustion and pollution formation process of gasoline direct injection HCCI engine with two-stage injection strategy. The models account for intake flow structure, spray atomization, droplet evaporation and gas phase chemistry in complex multi-dimensional geometries. The calculated results show that the periphery of fuel-rich zone formed by the second injection ignited first, then the fuel-rich zone ignited and worked as an initiation to ignite the surrounding lean mixture zone formed by the first injection. The two-zone HCCI leads to sequential combustion, this makes ignition timing and combustion rate controllable. In addition, HCCI load range can be extended. However, the periphery of fuel-rich zone leads to fierce burning, which results in slightly high NO_x emissions.     

Ignition control of homogeneous-charge compression ignition (HCCI) combustion through adaptation of the fuel molecular structure by reaction with ozone

The present paper describes a method of controlling the time of ignition in homogeneous-charge compression ignition (HCCI) combustion. In the described experiments some control of ignition timing in HCCI combustion is achieved through alteration of the fuel molecular structure using a chemical reaction of the fuel with ozone, prior to introduction of the fuel into the combustion chamber. Controlling ignition timing is essential, in achieving high thermal efficiency and low pollutant emission in HCCI engine operation. To this end, ignition should occur in the vicinity of piston top-dead-centre (TDC), the point of maximum compression of the fuel-air charge. The present paper proposes a method of controlling the time of ignition of the fuel-air charge by adapting the ignitability of the fuel through prior chemical reaction of the fuel with ozone. Ozone can be readily produced using air in conjunction with a corona discharge ozoniser and may be brought into contact with the fuel in a reaction chamber before its injection into the engine. It was shown through experiments that an acetal fuel which has undergone treatment with ozone, ignites earlier during the engine cycle in HCCI combustion, than fuel which has not undergone treatment with ozone, as a result of changes in its molecular structure prior to combustion. The observed changes in molecular structure consisted primarily in the formation of peroxides within the fuel. This method can be used to operate an engine in HCCI combustion mode with some control over the point of ignition of the fuel-air charge by varying the proportions of fuel previously treated with ozone and fuel not treated with ozone. The experiments showed that the time of ignition could be controlled, whilst keeping other parameters such as the load and speed of the engine, and pressure and temperature of the intake air and fuel, constant.     

Numerical and experimental study of water/oil emulsified fuel combustion in a diesel engine

Numerical and experimental studies were made on some of the chemical and physical properties of water/oil emulsified fuel (W/OEF) combustion characteristics. Numerical investigations of W/OEF combustion's chemical kinetic aspects have been performed by simulation of water/n-heptane mixture combustion, assuming a model of a homogenous reactor's concentric shells. The injection and fuel spray characteristics are analyzed numerically also in order to study indirectly the physical effects of water present in diesel fuel during the combustion process. The experimental results of W/OEF combustion in the DI diesel engine are also presented and discussed. The results of engine testing in a broad field of engine loads and speeds have shown a significant pollutant emission reduction with no worsening of specific fuel consumption.     

Homogeneous charge compression ignition of LPG and gasoline using variable valve timing in an engine

The combustion characteristics and exhaust emissions in an engine were investigated under homogeneous charge compression ignition (HCCI) operation fueled with liquefied petroleum gas (LPG) and gasoline with regard to variable valve timing (VVT) and the addition of di-methyl ether (DME). LPG is a low carbon, high octane number fuel. These two features lead to lower carbon dioxide (CO₂) emission and later combustion in an LPG HCCI engine as compared to a gasoline HCCI engine. To investigate the advantages and disadvantages of the LPG HCCI engine, experimental results for the LPG HCCI engine are compared with those for the gasoline HCCI engine. LPG was injected at an intake port as the main fuel in a liquid phase using a liquefied injection system, while a small amount of DME was also injected directly into the cylinder during the intake stroke as an ignition promoter. Different intake valve timings and fuel injection amount were tested in order to identify their effects on exhaust emissions and combustion characteristics. Combustion pressure, heat release rate, and indicated mean effective pressure (IMEP) were investigated to characterize the combustion performance. The optimal intake valve open (IVO) timing for the maximum IMEP was retarded as the λ_TOTAL was decreased. The start of combustion was affected by the IVO timing and the mixture strength (λ_TOTAL) due to the volumetric efficiency and latent heat of vaporization. At rich operating conditions, the θ_90-20 of the LPG HCCI engine was longer than that of the gasoline HCCI engine. Hydrocarbon (HC) and carbon monoxide (CO) emissions were increased as the IVO timing was retarded. However, CO₂ was decreased as the IVO timing was retarded. CO₂ emission of the LPG HCCI engine was lower than that of the gasoline HCCI engine. However, CO and HC emissions of the LPG HCCI engine were higher than those of the gasoline HCCI engine.