ABSTRACT
Conventional approach of 4-stroke engines to down-sizing using direct injection and turbocharging has several limitations, such as maximum cylinder pressure, rapid heat release rate and so on. 2-stroke engines can produce higher power densities compared to a 4-stroke cycle counterpart because of faster combustion frequency. To expand engine’s operation ranges, 2/4-stroke switchable multi-cylinder gasoline engine was proposed by Ricardo. Considering advantages of 2/4-stroke engine, same concept was used in a single-cylinder diesel engine in this paper aiming to obtain high output power. This 2/4-stroke engine was built on a traditional 4-stroke diesel engine by using a switchable camshaft system. Although high power could be achieved, only 63.9% trapping ratio and 79% scavenging efficiency was put out on 2-stroke mode due to inadequate exchange process. In this paper, a method to measure cycle-resolved scavenging efficiency based on CO2 concentration is proposed. Combining experiment with CFD simulation to optimize scavenging process and engine performance. It is found that under the limitation of head height, the optimization of port structure only has 0.8-2% increasing of scavenging efficiency and 2.6-3.6% increasing of trapping ratio. And the effect of structure weaker with speed increasing. In the prototype engine, intake and exhaust valves open at the same time, which lead exhaust gas backflow, affects the scavenging process badly. By optimizing the 100 ° CA valve overlap can guarantee the best trapping ratio and scavenging efficiency. Finally, trapping ratio of 73.3% and scavenging efficiency of 84.3% enable the engine to achieve 111kW power when the air flow rate of 661kg/h, 14.3% lower than the air flow rate of the original 771kg/h. 14.3% lower than the air flow rate of the original 771kg/h.
