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 3.0 l 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.
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.
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.
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