Controlled Auto-ignition Combustion Research Articles

Validation of a ternary gasoline surrogate in a CAI engine

This experimental study validated in a piston engine the European gasoline surrogate from [Pera and Knop, Fuel 96 (2012) 59–69], consisting of a ternary mixture of n-heptane, iso-octane, and toluene. Because only the gas phase properties of gasoline were emulated with the selected mixture, this validation was deliberately limited to port fuel injection operating points. By considering engine operation under controlled autoignition (CAI) combustion mode, the validation focused on fuel autoignition characteristics (autoignition delay and rate of heat release). A direct comparison of gasoline and its surrogate over the entire CAI operating range permitted a comprehensive evaluation of the surrogate adequacy under purely kinetically controlled combustion mode. The acquired data include autoignition timings, rate of heat release, exhaust gas temperatures, pollutant emissions, operating point stability, and operating ranges under CAI combustion mode. Good agreement between gasoline and its surrogate was obtained for all quantities, indicating similar behavior for the two fuels. Experimental results showed that a mixture of 13.7mol% n-heptane, 42.9mol% iso-octane, and 43.4mol% toluene is a satisfactory surrogate for a European unleaded gasoline with a research octane number of 95, conforming to the EN 228 specification.

CAI combustion of gasoline and its mixture with ethanol in a 2-stroke poppet valve DI gasoline engine

Controlled Auto Ignition (CAI), also known as Homogeneous Charge Compression Ignition (HCCI), is one of the most promising combustion technologies to reduce the fuel consumption and NOx emissions. Currently, CAI combustion is constrained at part load operation conditions because of misfire at low load and knocking combustion at high load, and the lack of effective means to control the combustion process. Extending its operating range including high load boundary towards full load and low load boundary towards idle in order to allow the CAI engine to meet the demand of whole vehicle driving cycles, has become one of the key issues facing the industrialisation of CAI/HCCI technology. Furthermore, this combustion mode should be compatible to different fuels, and can switch back to conventional spark ignition operation when necessary. In this paper, the CAI operation is demonstrated on a 2-stroke gasoline direct injection (GDI) engine equipped with a poppet valve train. The results shown that the CAI combustion can be readily achieved in the 2-stroke cycle of a poppet valve engine and the range of CAI combustion can be significantly extended compared to the 4-stroke cycle operation. In addition, the effects of ethanol concentration on 2-stroke CAI operational range, combustion process, emissions and efficiencies are studied and presented.

Performance and emissions of a 4-cylinder gasoline engine with Controlled Auto-Ignition

Advanced combustion modes such as Controlled Auto-Ignition or Homogeneous Charge Compression Ignition have been under much attention due to their ability to reduce both emissions and fuel consumption. Thus, this paper aims at demonstrating the achievement of Controlled Auto-Ignition combustion on a standard 4-cylinder gasoline engine with negative valve overlap (NVO) and analyzing its performance and emissions. The engine remained with substantially original components. The only major modification was the replacement of the camshafts for a new set of bespoke ones. The results showed a fair range of load and speed under CAI combustion with reduced brake specific consumption and ultra-low levels of NOx emissions. CO was also reduced, while HC emissions showed increased values. The results also point out some of the drawbacks of CAI combustion and the technological challenges of this advanced combustion process.

Analysis of the effect of direct injection of alcohol fuel on minor heat release reactions and controlled autoignition combustion

In this study, systematic investigation was carried out on the effect of alcohol on heat release reactions during the negative-valve-overlap period in a single-cylinder direct-injection gasoline engine equipped with an air-assisted injector. The fuels used included gasoline, gasoline–ethanol blends, gasoline–methanol blends, and pure ethanol and methanol. To achieve controlled autoignition combustion, the negative-valve-overlap method was employed to trap hot exhaust gases into which fuel was injected directly. An air-assisted injector injected fuel together with a small amount of air immediately after the exhaust valves were closed. The in-cylinder pressure was recorded and then the heat release was analysed. The occurrence of the heat release reactions were identified through the in-cylinder pressure and heat release analysis. In particular, the effect of the alcohol fuels on the degree of heat release reactions during the negative-valve-overlap period was investigated. In addition, its influence on the initiation and heat release of the main combustion process was analysed. Alcohol fuels were found to produce more heat release reactions during the negative-valve-overlap period. Methanol exhibited the most pronounced heat release. This resulted in advanced autoignition and faster combustion of alcohol–gasoline blended fuels, except for pure ethanol and methanol. Gasoline and low-alcohol fuel blends were characterised by hybrid combustion of initial slow flame propagation and faster autoignition combustion later on. The autoignition combustion became dominant as the percentage of alcohol increased.

Measurement of short-circuiting and its effect on the controlled autoignition or homogeneous charge compression ignition combustion in a two-stroke poppet valve engine

Controlled autoignition or homogeneous charge compression ignition combustion in four-stroke spark ignition engines has been the subject of extensive research over the last decade. In order to extend the load range of controlled autoignition combustion for automotive applications, systematic research has been carried out on these combustion processes in a single-cylinder poppet valve engine that is capable of both four-stroke operation and two-stroke operation through flexible variable valve actuation. Since short-circuiting is a common issue for two-stroke engine operation, it is necessary to quantify the amount of short-circuiting. In this paper, a new method of measuring the short-circuiting rate during two-stroke engine operation is presented and applied to investigate the effect of short-circuiting on two-stroke controlled autoignition or homogeneous charge compression ignition operation. The cycle-resolved air short-circuiting rate is derived from instantaneous measurement of the carbon dioxide concentration near an exhaust valve using a Cambustion fast-response non-dispersive infrared analyser. It is found that the short-circuited air leads to fuel-rich combustion and hence high carbon monoxide and hydrocarbon emissions under closed-loop λ = 1 control. A method to calculate the in-cylinder air-to-fuel ratio based on the measured short-circuiting rate is presented and used to understand better the in-cylinder combustion and pollutant formation process during two-stroke operation of a direct-injection four-valve engine.

Combustion phase detection algorithm for four-cylinder controlled autoignition engines using in-cylinder pressure information

For several decades, the primary goal of the automotive industry has been to reduce harmful emissions and improve fuel economy. Gasoline engines are clean and powerful propulsion systems, but have poorer fuel economy than that of diesel engines. However, due to the development of new technologies such as variable valve timing and lift and direct gasoline injection, controlled autoignition (CAI) combustion can be realized. CAI engines combine the advantages of cleaner emissions and lower fuel consumption than conventional spark-ignition gasoline engines. In this study, a cylinder-pressure-based combustion phase detection method for CAI combustion is proposed. This method utilizes a normalized difference pressure (NDP), which is defined as the normalized pressure difference between the firing and motoring in-cylinder pressures. The proposed method was developed and validated with steady-state experimental data from an inline 4 cylinder, 2 L gasoline direct injection (GDI) CAI engine. Because the calculations in the NDP method are faster and simpler than in the conventional combustion phase detection method in CAI engines, this method can be embedded in a real-time controller. Furthermore, the proposed method displayed good accuracy in detecting the combustion phase and thus stabilized CAI combustion. Finally, the detailed experimental results are presented.

In-cylinder studies of hybrid combustion in a direct injection single-cylinder optical engine

Over the last decade, controlled autoignition (CAI) combustion, or homogeneous charge compression ignition (HCCI), has been the subject of numerous studies and has shown its potential to reduce fuel consumption and NO x emissions simultaneously from gasoline engines. However, there are still some fundamental questions that are yet to be addressed, such as combustion control and underlying physical and chemical processes. In the present study, a single-cylinder optical gasoline engine was operated with negative valve overlap to investigate CAI and hybrid spark ignition (SI)–CAI combustion. The effects of direct fuel injection and spark timings were studied by means of a simultaneous in-cylinder heat release study and high-speed visualization of total chemiluminescence, OH and CHO radicals.

Effects of spark-assistance on controlled auto-ignition combustion at different injection timings in a multicylinder direct-injection gasoline engine

Controlled auto-ignition (CAI) combustion, also known as homogeneous charge compression ignition (HCCI) can be achieved by trapping residuals with early exhaust valve closure in a direct fuel injection in-cylinder four-stroke gasoline engines. CAI combustion is achieved by auto-ignition independent of spark discharge. However, it is found that, at loads with reduced trapped residuals, the presence of spark influences combustion. Therefore the effects of spark timing on the CAI combustion process were investigated through the introduction of spark. The effect on engine performance and the emission specific values were evaluated. The engine speed was maintained at 1500 r/min and lambda was kept constant at 1.2. It was found that with spark-assisted CAI, indicated mean effective pressure (IMEP), and indicated specific oxides of nitrogen (ISNO x) values increased as compared with CAI without spark. ISHC and ISCO values were lower for spark-assisted CAI as compared with CAI without spark. Heat release data were analysed to better understand this phenomenon.

Investigation of transition between spark ignition and controlled auto-ignition combustion in a V6 direct-injection engine with cam profile switching

Controlled auto-ignition (CAI) combustion, also known as homogeneous charge compression ignition (HCCI), can be achieved by trapping residuals with early exhaust valve closure in a direct-fuel-injection in-cylinder four-stroke gasoline engine (through the employment of low-lift cam profiles). Because the operating region is limited to low-load and midload operation for CAI combustion with a low-lift cam profile, it is important to be able to operate spark ignition (SI) combustion at high loads with a normal cam profile. A 3.0l prototype engine was modified to achieve CAI combustion, using a cam profile switching mechanism that has the capability to switch between high- and low-lift cam profiles. A strategy was used where a high-lift profile could be used for SI combustion and a low-lift profile was used for CAI combustion. Initial analysis showed that for a transition from SI to CAI combustion, misfire occurred in the first CAI transitional cycle. Subsequent experiments showed that the throttle opening position and switching time could be controlled to avoid misfire. Further work investigated transitions at different loads and from CAI to SI combustion.

Detection of combustion start in the controlled auto ignition engine by wavelet transform of the engine block vibration signal

The CAI (controlled auto ignition) engine ignites fuel and air mixture by trapping high temperature burnt gas using a negative valve overlap. Due to auto ignition in CAI combustion, efficiency improvements and low level NOx emission can be obtained. Meanwhile, the CAI combustion regime is restricted and control parameters are limited. The start of combustion data in the compressed ignition engine are most critical for controlling the overall combustion. In this research, the engine block vibration signal is transformed by the Meyer wavelet to analyze CAI combustion more easily and accurately. Signal acquisition of the engine block vibration is a more suitable method for practical use than measurement of in-cylinder pressure. A new method for detecting combustion start in CAI engines through wavelet transformation of the engine block vibration signal was developed and results indicate that it is accurate enough to analyze the start of combustion. Experimental results show that wavelet transformation of engine block vibration can track the start of combustion in each cycle. From this newly developed method, the start of combustion data in CAI engines can be detected more easily and used as input data for controlling CAI combustion.

Quantitative measurements of in-cylinder gas composition in a controlled auto-ignition combustion engine

One of the most effective means to achieve controlled auto-ignition (CAI) combustion in a gasoline engine is by the residual gas trapping method. The amount of residual gas and mixture composition have significant effects on the subsequent combustion process and engine emissions. In order to obtain quantitative measurements of in-cylinder residual gas concentration and air/fuel ratio, a spontaneous Raman scattering (SRS) system has been developed recently. The optimized optical SRS setups are presented and discussed. The temperature effect on the SRS measurement is considered and a method has been developed to correct for the overestimated values due to the temperature effect. Simultaneous measurements of O2, H2O, CO2 and fuel were obtained throughout the intake, compression, combustion and expansion strokes. It shows that the SRS can provide valuable data on this process in a CAI combustion engine.

Spontaneous Raman scattering for simultaneous measurements of in-cylinder species

A technique for multi-species mole fraction measurement in internal combustion engines is described. The technique is based on the spontaneous Raman scattering. It can simultaneously provide the mole fractions of several species of N2, O2, H2O, CO2 and fuel. Using the system, simultaneous measurement of air/fuel ratio and burnt residual gas are carried out during the mixture process in a Controlled Auto Ignition (CAI) combustion engine. The accuracy and consistency of the measured results were confirmed by the measured air fuel ratio using an exhaust gas analyzer and independently calculated mole fraction values. Measurement of species mole fractions during combustion process has also been demonstrated. It shows that the SRS can provide valuable data on this process in a CAI combustion engine.

Managing controlled auto-ignition combustion by injection on a direct-injection gasoline engine

Controlled auto-ignition (CAI) combustion in gasoline engines has great potential for reducing both NO x emissions and fuel consumption, but its application is still hindered by the lack of direct control of combustion phasing and by the limited CAI operation range. In this paper, the effect of injection timing and split injection on CAI combustion is presented in a single-cylinder direct-injection gasoline engine with an air-assisted injector. The CAI combustion was achieved by trapping some of the burned gases within the cylinder by using low-lift short-duration camshafts and early closure of the exhaust valves. During the experiments, the engine speed was varied from 1200 to 2400 r/min and the air-fuel ratio was altered from stoichiometric to the misfire limit. Both single and split injections were investigated at different injection timings and fuel quantities. The experimental results show that injection timing has an important effect on CAI combustion for single and split injections. Early injection produces faster and more stable combustion, less hydrocarbon and CO emissions, but very rapid heat release rates and higher NO x emissions. The CAI operation range could be extended significantly by early injection. Split injection gives even further extension of the CAI range in both stoichiometric and lean mixture operations. These results indicate that optimizing the injection timing and using split injection is an effective way to control and extend CAI operation in a direct-injection gasoline engine.

An Experimental Study of the Extent of the Operating Region and Emission Characteristics of Stratified Combustion Using the Controlled Autoignition Method

Controlled autoignition (CAI) combustion, also known as homogeneous charge compression ignition (HCCI), offers the potential to simultaneously improve fuel economy and reduce emissions. CAI combustion was achieved in a single-cylinder gasoline direct-injection (DI) engine with a cylinder running in CAI mode. Standard components were used in the camshafts, which had been modified to restrict the gas exchange process. We investigated the effects of the air/fuel ratio, residual exhaust gas recirculation (EGR) rate, and injection timing such as early and late injections on the attainable CAI combustion region. The effect of injection timing on factors such as the start of combustion, its duration, and its heat release rate was also investigated. The results show that early injection caused the mixture to ignite earlier and to burn more quickly because of the exothermic reaction during the recompression and gave rise to thorough mixing of the fuel and air. Late injection extended the operation region more than...

CAI 연소 방법을 이용한 직분식 가솔린 엔진내의 조기 분사시 연소 및 배기 특성에 관한 실험적 연구

Controlled auto ignition (CAI) combustion, also known as HCCI (homogeneous charge compression ignition), offers the potential to simultaneously improve fuel economy and reduce emission. CAI-combustion was achieved in a single cylinder gasoline DI engine, with a cylinder running in a CAI mode. Standard components were used the camshafts which had been modified in order to restrict the gas exchange process. The effects of air-fuel ratio, residual EGR rate and injection timing such as early injection and late injection on the attainable CAI combustion region were investigated. The effect that injection timings on factor such as start of combustion, combustion duration and heat release rate was also investigated. From results early injection caused the mixture to ignite earlier and burn more quickly due to the exothermic reaction during the recompression and gave rise to good mixing of the fuel-air.

CAI 연소 방법을 이용한 성층 연소를 통한 운전 영역 확대, 연소 및 배기 특성에 관한 실험적 연구

Controlled auto-ignition(CAI) combustion, offers the potential to improve fuel economy and reduce emission simultaneously. In this study, CAI-combustion was achieved in a single cylinder gasoline DI engine with modified camshafts in order to restrict the gas exchange process. We investigated the effects of air-fuel ratio, residual EGR rate and injection timing such as early injection and late injection on the attainable CAI combustion region. The effect of injection timings on combustion characteristic such as start of combustion, combustion duration and heat release rate was also investigated. From the result early injection causes the mixture to ignite earlier and burn more quickly due to the exothermic reaction during the recompression and gives rise to good mixing of the fuel/air. On the other hand, late injection extended the operation region more than early injection but the emissions of HC and NOx were more or less increased than early injection.

Numerical Investigation Into Effect of Fuel Injection Timing on CAI/HCCI Combustion in a Four-Stroke GDI Engine

The Controlled Auto-Ignition (CAI) combustion, also known as Homogeneous Charge Compression Ignition (HCCI), was achieved by trapping residuals with early exhaust valve closure in conjunction with direct injection. Multi-cycle 3D engine simulations have been carried out for parametric study on four different injection timings in order to better understand the effects of injection timings on in-cylinder mixing and CAI combustion. The full engine cycle simulation including complete gas exchange and combustion processes was carried out over several cycles in order to obtain the stable cycle for analysis. The combustion models used in the present study are the Shell auto-ignition model and the characteristic-time combustion model, which were modified to take the high level of EGR into consideration. A liquid sheet breakup spray model was used for the droplet breakup processes. The analyses show that the injection timing plays an important role in affecting the in-cylinder air/fuel mixing and mixture temperature, which in turn affects the CAI combustion and engine performance.

Mixture formation and controlled auto-ignition combustion in four-stroke gasoline engines with port and direct fuel injections

Controlled auto-ignition (CAI) combustion, also known as homogeneous charge compression ignition (HCCI) can be achieved by trapping residuals with early exhaust valve closure in both port and direct fuel injection four-stroke gasoline engines. A multi-cycle three-dimensional engine simulation program has been developed and applied to study the effect of injection on in-cylinder mixing and CAI combustion. The full engine cycle simulation, including complete gas exchange and combustion processes, was carried out over several cycles in order to obtain the stable cycle for analysis. The combustion models used in the present study are based on the Shell auto-ignition model and the characteristic-time combustion model, both of which have been modified to take the high level of residual gas into consideration. A liquid sheet break-up spray model was used for the droplet break-up processes. The analyses show that the injection timing plays an important role in affecting the in-cylinder air/fuel mixing and mixture temperature, which in turn affects the CAI combustion and engine performance. In comparison with the port fuel injection case, an early direct injection at exhaust valve closure can lead to higher load and lower fuel consumption.

Understanding the infiuence of valve timings on controlled autoignition combustion in a four-stroke port fuel injection engine

Controlled autoignition (CAI) combustion, also known as homogeneous charge compression ignition (HCCI), was achieved through the negative valve overlap approach by using small- lift camshafts. Three-dimensional multicycle engine simulations were carried out in order better to understand the effects of variable intake valve timings on the gas exchange process, mixing quality, CAI combustion, and pollutant formation in a four-stroke port fuel injection (PFI) gasoline engine. Full engine cycle simulation, including complete gas exchange and combustion processes, was carried out over several cycles in order to obtain the stable cycle for analysis. The combustion models used in the present study are a modified shell ignition model and a laminar and turbulent characteristic time model, which can take high residual gas fraction into account. After the validation of the model against experimental data, investigations of the effects of variable intake valve timing strategies on the CAI combustion process were carried out. These analyses show that the intake valve opening (IVO) and intake valve closing (IVC) timings have a strong infiuence on the gas exchange and mixing processes in the cylinder, which in turn affect the engine performance and emissions. Symmetric IVO timing relative to exhaust valve closing (EVC) timing tends to produce a more stratified mixture, earlier ignition timing, and localized combustion, and hence higher NO x and lower unburned HC and CO emissions, whereas retarded IVO leads to faster mixing, a more homogeneous mixture, and uniform temperature distribution.

Influence of variable valve timings on the gas exchange process in a controlled auto-ignition engine

The controlled auto-ignition (CAI) engine concept has the potential to be highly effcient and to produce low NOx and particulate matter emissions. However, the problem of controlling the combustion over the entire load/speed range limits its practical application. The CAI combustion is controlled by the chemical kinetics of the charge mixture, with no influence of the flame diffusion or turbulent propagation. Therefore, to achieve successful control of the CAI process, the composition, temperature and pressure of the charge mixture at the inlet valve closure (IC) point have to be controlled. The use of the variable valve timing strategy, which enables quick changes in the amount of trapped hot exchaust gases, shows the potential for control of CAI combustion. The aim of this paper is to analyse the influence of the variable valve timing strategy on the gas exchange process, the process between the first valve open event (EO) and the last valve closing event (IC), in a CAI engine fuelled with standard gasoline fuel (95RON). The gas exchange process affects the engine parameters and charge properties and therefore plays a crucial role in determining the control of the CAI process. Analysis is performed by experimental and modelling approaches. A single-cylinder research engine equipped with a fully variable valvetrain (FVVT) system was used for the experimental study. A combined code consisting of a detailed chemical kinetics code and one-dimensional fluid dynamics code was used for the modelling study. The results obtained indicate that the variable valve timing strategy has a strong influence on the gas exchange process, which in turn influences the engine parameters and the cylinder charge properties, and hence the control of the CAI process. The EC timing has the strongest effect, followed by the IO timing, while the EO and IC timings have minor effects.