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.     

Vapour-phase and particulate-bound PAHs profile generated by a (SI/HCCI) engine from a winter grade commercial gasoline fuel

The vapour-phase and particulate-bound Aromatic Hydrocarbons, PAHs, generated by a V6 gasoline engine working in spark-ignition (SI) and homogeneous charge compression ignition (HCCI) modes were collected and analysed. All data were obtained during steady-state, fully warmed-up operation at different engine power levels (low and medium loads and mid-speed), and two different engine operation modes (SI and HCCI). The fuel used in this study was winter grade commercial gasoline fuel.The vapour-phase exhaust gases were passed through stainless-steel cartridges containing XAD-2 resin to capture PAHs. The PAHs were extracted from the resin with dichloromethane in an ultrasonic bath, the obtained extracts were later analysed qualitatively and quantitatively by GC-MS. The vapour-phase PAHs compounds observed from HCCI mode operated in low load were Naphthalene, Acenaphthylene and Acenaphthene only, while that obtained from SI mode under low load were Naphthalene, Acenaphthylene, Acenaphthene, Fluorene, Anthracene, Phenanthrene, Fluoranthene and Pyrene.The PAHs bound to particulates were trapped by using a complex of dilution tunnels with filter papers. The soluble organic fractions (SOF) of the trapped particulates were separated from the insoluble fraction (ISF) with the help of ultrasonic elution, and analysed by GC-MS method. The most abundant PAHs detected under selected operation condition for HCCI mode was Benzo[a]anthracene, followed by Chrysene, then Pyrene and pursued by Benzo[b]fluoranthene, in SI mode under same operation condition the highest PAH detected was Benzo[a]anthracene followed by Pyrene, Benzo[b]fluoranthene and Chrysene. Probable mechanisms for the production of some of the pyrosynthetic PAH were discussed.     

A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR. Part 1. The basic characteristics of HCCI combustion

This article investigates the basic combustion parameters including start of the ignition timing, burn duration, cycle-to-cycle variation, and carbon monoxide (CO), unburned hydrocarbon (UHC), and nitric oxide (NO_x) emissions of homogeneous charge compression ignition (HCCI) engines fueled with primary reference fuels (PRFs) and their mixtures. Two primary reference fuels, n-heptane and iso-octane, and their blends with RON25, RON50, RON75, and RON90 were evaluated. The experimental results show that, in the first-stage combustion, the start of ignition retards, the maximum heat release rate decreases, and the pressure rising and the temperature rising during the first-stage combustion decrease with the increase of the research octane number (RON). Furthermore, the cumulative heat release in the first-stage combustion is strongly dependent on the concentration of n-heptane in the mixture. The start of ignition of the second-stage combustion is linear with the start of ignition of the first-stage. The combustion duration of the second-stage combustion decreases with the increase of the equivalence ration and the decrease of the octane number. The cycle-to-cycle variation improved with the decrease of the octane number.     

A chemical kinetics model of iso-octane oxidation for HCCI engines

The necessity of developing a practical iso-octane mechanism for homogeneous charge compression ignition (HCCI) engines is presented after various different experiments and currently available mechanisms for iso-octane oxidation being reviewed and the performance of these mechanisms applied to experiments relevant to HCCI engines being analyzed. A skeletal mechanism including 38 species and 69 reactions is developed, which could predict satisfactorily ignition timing, burn rate and the emissions of HC, CO and NO_x for HCCI multi-dimensional modeling. Comparisons with various experiment data including shock tube, rapid compression machine, jet stirred reactor and HCCI engine indicate good performance of this mechanism over wide ranges of temperature, pressure and equivalence ratio, especially at high pressure and lean equivalence ratio conditions. By applying the skeletal mechanism to a single-zone model of HCCI engine, we found out that the results were substantially identical with those from the detailed mechanism developed by Curran et al. but the computing time was reduced greatly.     

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.     

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 study on the spray structure and evaporation characteristic of common rail type high pressure injector in homogeneous charge compression ignition engine

The purpose of this study was to investigate the spray structure and evaporation characteristics of common rail high pressure injector for use in a direct injection type HCCI (Homogeneous Charge Compression Ignition) engine. In this study, we measured the injection rate and visualized the spray structure of a HCCI injector according to injection conditions. The CFD simulation of the spray and the air fuel mixture formation in real engine conditions was also conducted using the VECTIS commercial code. In addition, we compared simulation results to experimental results.

From the spray experiment and simulation results, we found that the spray penetration was proportional to the back pressure by an exponent of 1/4. This is similar to Hiroyasu's experimental result. The fuel evaporation and air fuel mixture result indicate that the influence of the spray impingement with the ambient density was bigger than that of the intake pressure and temperature conditions in evaporation rate when the fuel was injected at the early stage of the compression stroke. The results also reveal that the fuel was uniformly distributed in the combustion chamber at this early injection time and the air fuel mixture was enhanced in this relatively rich region. However, when ambient density was kept constant, the fuel evaporation was sensitive to the influence of the intake temperature and pressure. As the fuel was injected at the later stage of the compression stroke, the fuel tended to concentrate in the bowl zone and to generate the lean air fuel mixture. From these results, it was confirmed that the air fuel mixture characteristics are sensitive to the impingement position of the injected fuel.     

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.     

Effects of valve events on the engine efficiency in a homogeneous charge compression ignition engine fueled by dimethyl ether

Combustion characteristics of a homogeneous charge compression ignition (HCCI) engine were investigated with regard to the residual gas, i.e. internal exhaust gas recirculation (IEGR), by changing the intake and exhaust maximum opening points (MOP) and the exhaust cam lifts. Three different exhaust camshafts were used and had 2.5 mm, 4.0 mm and 8.4 mm exhaust valve lift. In-cylinder gas was sampled at the intake valve immediately before ignition to measure the IEGR rate. The heat release, fuel conversion efficiency and combustion efficiency were calculated using the in-cylinder pressure and composition of exhaust gases to examine the combustion features of the HCCI engine. The negative valve overlap (NVO) was increased as exhaust valve lift was reduced. Longer NVO made an increased IEGR through exhaust gas trapping. The IEGR rate was increased as the exhaust valve timing advanced while it was affected more by exhaust valve timing than by intake valve timing. Combustion phase was advanced by lower exhaust valve lift and early exhaust and intake MOP. It was because of higher amount of IEGR gas and effective compression ratio. The fuel conversion efficiency with higher exhaust valve lift was higher than that with lower exhaust valve lift. The late exhaust and intake MOP made the fuel conversion efficiency improve.     

Air-fuel mixing and combustion in a small-bore direct injection optically accessible diesel engine using a retarded single injection strategy

In this paper, the air-fuel mixing and combustion in a small-bore direct injection optical diesel engine were studied for a retarded single injection strategy. The effects of injection pressure and timing were analyzed based on in-cylinder heat release analysis, liquid fuel and vapor fuel imaging by Laser induced exciplex fluorescence technique, and combustion process visualization. NOx emissions were measured in the exhaust pipe. Results show that increasing injection pressure benefits soot reduction while increases NOx emissions. Retarding injection timing leads to simultaneous reduction of soot and NOx emissions with premixed homogeneous charge compression ignition (HCCI) like combustion modes. The vapor distribution in the cylinder is relatively homogeneous, which confirms the observation of premixed combustion in the current studies. The postulated path of these combustion modes were analyzed and discussed on the equivalence ratio-temperature map.     

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.     

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.     

催化燃烧中表面反应-气相反应间相互作用及其对均质压燃发动机着火特性的影响

通过对微元管中甲烷在铂表面的催化燃烧过程的数值计算,分析了当混合气入口压力很高时气相反应对整个催化燃烧过程的影响;通过敏感度分析,找出了对异相着火及气相着火起主要作用的基元反应步.结果表明,在异相着火过程中起主要作用的基元反应步为甲烷与氧气在催化剂表面的吸附反应及氧气的解吸反应,在气相着火过程中起主要作用的基元反应步为OH·及水的吸附与解吸反应.对活塞顶涂有铂催化剂的均质压燃（HCCI）发动机的燃烧过程进行了数值模拟,分析了催化效应及关键表面反应基元步对HCCI发动机着火时刻以及燃烧过程中中间组分的影响,结果表明,催化反应能促进混合气的着火,缩短着火延迟时间,对HCCI发动机着火时刻起主要影响的表面反应为OH·及水的吸附与解吸反应.     

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.     

An investigation into propane homogeneous charge compression ignition (HCCI) engine operation with residual gas trapping

Propane is available commercially for use in conventional internal combustion engines as an alternative fuel for gasoline. However, its application in the developing homogeneous charge compression ignition (HCCI) engines requires various approaches such as high compression ratios and/or inlet charge heating to achieve auto ignition. The approach documented here utilizes the trapping of internal residual gas (as used before in gasoline controlled auto ignition engines), to lower the thermal requirements for the auto ignition process. In the present work, with a moderate engine compression ratio the achievable engine load range was controlled by the degree of internal trapping of exhaust gas supplemented by inlet charge heating. Increasing the compression ratio decreased the inlet temperature requirements; however, it also resulted in higher pressure rise rates. Varying the inlet valve timing affects the combustion phasing which can help to decrease the maximum pressure rise rates. NOx emissions were characteristically low due to the nature of homogeneous combustion.     

Impact of fuel characteristics on HCCI combustion: Performances and emissions

The aim of this study is to assess how the fuel, through its characteristics, could increase the maximum load achievable in HCCI condition. First a specific procedure has been developed. Second using this procedure, a set of fuels using a Jet B as base fuel and having different cetane numbers (from 33 to 40) and different chemical compositions (via the addition of reactive products which are olefinic and naphtenic compounds) have been tested. We conclude that a fuel having a low cetane number, a high volatility and an appropriate chemical composition could improve the HCCI operating range of more than 30% without a too large decrease of the performance under conventional Diesel combustion mode.     

Effects of charge temperature and exhaust gas re-circulation on combustion and emission characteristics of an acetylene fuelled HCCI engine

In this work, experiments were conducted on a homogeneous charge compression ignition (HCCI) engine with acetylene as the sole fuel at different power outputs. Initially, the intake air was heated to different temperatures in order to determine the optimum level at every output. Charge temperatures needed were in the range of 40-110 °C from no load to a BMEP (Brake Mean Effective Pressure) of 4 bar. Subsequently, exhaust gas re-circulation (EGR) was done at the identified charge temperatures and brake thermal efficiency was found to improve. At high BMEPs, use of EGR led to knocking. Thus, fine control over charge temperature and EGR quantity is needed at these conditions. Nitric oxide and smoke levels were very low. However, HC levels were high at about 1700-2700 ppm. Brake thermal efficiencies were comparable to or even better than the compression ignition mode of operation.     

Controlled three-stage heat release of stratified charge compression ignition (SCCI) combustion with a two-stage primary reference fuel supply

This paper discusses the heat release mode and its effect on combustion characteristics of stratified charge compression ignition (SCCI) combustion with a two-stage fuel supply. To create and control the fuel concentration stratification, composition stratification, and temperature stratification, primary reference fuels or their mixtures were supplied from the intake port, while n-heptane was directly injected into the cylinder near the top dead center of the compression stroke. To achieve a controllable staged heat release and to optimize the thermal efficiency and emissions, important factors, including premixed fuel properties, direct injection timing, the overall equivalence ratio, and the premixed ratio were tuned to modulate the heat release pattern. The experimental results revealed that, with the port fuel injection of a two-stage reaction fuel, the heat release curve of the SCCI combustion exhibits a three-stage heat release pattern. The in-cylinder fuel delivery advance angle plays an important role in the indicated thermal efficiency, and the earlier fuel delivery angle has a positive effect on the indicated thermal efficiency. It should be noted that an excessively advanced fuel delivery angle will lead to a sharp increase of NOx emissions. With the port fuel injection of PRF50, both fuel efficiency and ultra-low NOx emissions were obtained over wide ranges of the premixed ratio and the equivalence ratio. Moreover, the experimental results suggest that a higher premixed ratio for low-to-medium equivalence ratios and a smaller premixed ratio for larger equivalence ratios are preferred. The maximum thermal efficiency was observed at the zone with the earlier CA50 but with shorter burn duration. NOx levels were determined not only by CA50 and burn duration but also by the heat release mode. One-stage SCCI combustion, which was dominated by the diffusion burn, exhausted considerable NOx emissions, compared to the staged heat release mode.     

The relationship of knock during controlled autoignition to temperature inhomogeneities and fuel reactivity

Experiments have been performed in a rapid compression machine (RCM), to investigate the conditions for and the origins of ‘knock’ in controlled autoignition (CAI), or homogeneous charge compression ignition (HCCI). Ignition in an RCM is the closest approach to that in a CAI engine without engendering the full complexity of reciprocating motion and fuel+air charge induction. As a representative fuel of intermediate reactivity, the combustion of n-pentane in air was studied at the compositions φ=1.0, 0.75 and 0.6 at end-of-compression pressures of 0.80-0.86 MPa (7.9-8.4 bar) and 1.4-1.5 MPa (13.8-14.8 bar), respectively, over the compressed gas temperature range 690-820 K. Autoignition is characterised by a two-stage development in these ranges of conditions, a ‘cool flame’ being followed by hot stage combustion.Filtered Rayleigh scattering from a planar laser sheet was used to characterise the temperature field that developed in the combustion chamber following rapid compression. High resolution pressure records, combined with image intensified, natural light output originating from chemiluminescence, were used to characterise the transition from non-knocking to knocking reaction and the evolution of the spatial development of chemical activity in this temperature field. It appears that knock originates from a localised development of the incandescent hot stage of ignition. Even though non-homogeneities prevail in the non-knocking reaction, it is associated with a relatively benign development, in which the cool flame is followed by a second stage, blue flame rather than the normal incandescent hot flame. The kinetics that may contribute to this distinction are discussed.