A new concept DISC engine equipped with a two-stage injection system was developed. The engine was modified from a single cylinder DI diesel engine with large cylinder diameter (135mm). Combustion characteristics and exhaust emissions with regular gasoline were examined, and the experiments were also made with gasoline-diesel fuel blends with higher boiling temperatures and lower octane numbers. To realize stratified mixture distribution in combustion chamber flexibly, the fuel was injected in two-stages: the first stage was before the compression stroke to create a uniform premixed lean mixture and the second stage was at the end of the compression stroke to maintain stable ignition and faster combustion. In this paper, the effect of the two-stage injection on combustion and exhaust emissions were analyzed under several operating conditions.
The results showed that stable and smooth combustion without knocking over a wide range of operation could be realized in the new concept DISC engine, and compared with stoichiometric homogeneous combustion at a BMEP of 0.61 MPa, simultaneous 30% SFC and 50% NOx reductions were achieved. The SFC and NOx were reduced remarkably not only in the lean conditions but also in the stoichiometric conditions by the stratified charge combustion with the two-stage injection. Optimization for spark timings, secondary fuel injection timings, the proportion of two-stage fuel injection, and the number of nozzle holes was essential for efficient combustion and ignition. Smokeless and knockfree operation could also be achieved with higher boiling temperature and lower octane number fuels. Soot was not detected even with gasoline containing 50vol.% diesel fuel.
The NOx emission from compression ignition CI engines has become a serious problem in urban environments and globally, and strict regulations may force the conversion of some CI engines to spark ignition SI engines if CI engine emission countermeasures remain insufficient. Such conversion presents two problems in SI engines however: (1) a deterioration in thermal efficiency, and the exhaust contributes to the greenhouse effect, (2) limitations in cylinder diameter resulting in knocking especially in large size engines. Also the middle distillation component of petroleum, the diesel fuel, will be in excess, so that utilization of wide cut gasoline with higher boiling temperatures and lower octane numbers will be desired. To improve the thermal efficiency of SI engines, lean combustion assisted by tumbling or swirling air motion in the combustion chamber has been reported and already put to practical use[1, 2 and 3]. A DISC combustion is usually more effective to improve thermal efficiency and also offers the potential of knockfree operation and utilization of a wide-range of fuels. Despite much work, ordinary types of DISC engines have difficulty in achieving stable and efficient combustion. However, if the fuel injection is flexible, a possibility with today's technology, DISC engines can be expected to achieve more stable combustion, lower exhaust emissions, higher thermal efficiency, and higher output over the whole range of operation.
In this research, a new concept DISC engine with a “two-stage fuel injection system” was developed. To realized desirable combustion characteristics, lower emissions, and higher thermal efficiency and power, the stratified mixture distribution in the combustion chamber was controlled flexibly with two-stage fuel injection: the first stage was set before the compression stroke to create a uniform premixed lean mixture and the second stage at the end of the compression stroke to maintain stable ignition and efficient combustion. Combustion characteristics and exhaust emissions with regular gasoline were examined, and the experiments were extended to the gasoline-diesel fuel blends with higher boiling temperatures and lower octane numbers.
