Investigation on Spray Characteristics of a Pumpless Pneumatic Fuel Injection System for Two-Cycle, S.I. Engines

The adoption of a system of pneumatic direct fuel injection in two-cycle, S.I. engines can radically reduce their most important drawbacks, which consist of a poor energetic efficiency and of great amounts of unburnt hydrocarbon exhaust emissions. The main cause of these drawbacks is the loss of fuel from the exhaust port during the scavenging process, loss which can be eliminated with the adoption of any system of direct fuel injection. Nevertheless, it should not be forgotten a second cause, which is poor combustion and/or misfire at light loads, for the excessive ratio of residual gas to fresh gas within the cylinder. This second cause is enhanced if the fuel atomisation is not complete at the ignition time, and this is what normally happens with the liquid fuel injection (excepted the only case of very expensive high-pressure systems), also owing to the very short time elapsing between injection and ignition. The pneumatic fuel injection (i.e. the introduction of the fuel into the cylinder by means of an air stream) allows better results, at the condition that the fuel is injected already well atomised. Of course, it is more difficult to respect this condition if the air pressure is low, like in the very simple system conceived and realized some years ago by one of the writers. In this case system configuration and tuning should be carefully optimized and for this purpose a research has been undertaken with the aims: a) to distinguish the spray characteristics by means of a special method of photographic image electronic processing; b) to investigate both the dependence of such characteristics on some parameters of the injection system, and their influence on engine performance, with special regard to fuel economy and exhaust emissions