The present turbo-charged direct injection 660cm3 engine achieves low engine-out emissions and low fuel consumption with high engine output because of synergies of direct injection combined with turbo-charging. The fuel mixture in the combustion chamber is slightly stratified and is slightly richer than stoichiometric in the vicinity of the spark plug at the time of ignition, thereby yielding stable combustion. This reduces the unburned HC at cold start operation and makes is possible to retard spark timing at cold start operation, which activates the catalyst quicker and reduces exhaust emissions. Also, the stable combustion allows introduction of higher EGR(Exhaust Gas Recirculation) rates, which reduces NOx emission and improves fuel economy resulting from low pumping loss. Due to charge cooling, the compression ratio can be increased, which has inherent fuel economy advantage as well.
Key challenges for small-displacement spark-ignition direct-injection (SIDI) engine such as the 660 cm3 with 68 mm bore are lack of space for installing the injectors and the smallness of the distance between the injector and the piston. The latter is a key parameter in influencing wall wetting. Wall wetting tends to increase smoke emissions, and could have negative effects on HC and fuel economy.
The present effort is a part of an investigation to reduce smoke emission through experimental study including visualization work evaluating injector spray pattern, injection timing, and spark timing. Through this study, it was found that the generation of smoke is related to the fuel wetting amount at the start of combustion. The fuel wetting amount at the start of combustion is dependent mainly on the injection timing, the spray drop size, the piston temperature and spark ignition timing.