Cycle-to-cycle variation is one of the main factors for high fuel consumption and emissions of a two-stroke engine during the low-load and low-speed running. The increase of residual gas ratio due to the lower delivered amount of fresh scavenging air leads to a lower flame front speed and, therefore, to a slow combustion or even misfiring. The consequence is a very high level of unburnt hydrocarbons, since a large amount of fuel does not take part in the combustion process.
The use of a direct injection system allows a more flexible management of the injection of fuel over subsequent engine cycles. Under a low-load condition, the low request in terms of brake mean effective pressure (BMEP) can be achieved by performing a load control based on an intermittent injection, thus reducing the need for intake throttling and avoiding the loss of fresh fuel resulting from cycles without combustion. In more detail, the supply of fuel to the combustion chamber can be skipped for one or more cycles, thus performing a number of consecutive scavenging cycles with only fresh air. As a result, the fresh air is less diluted by the residual gas and the combustion efficiency increases.
This paper presents the results of a preliminary experimental activity on the use of an intermittent injection strategy with a Low Pressure Direct Injection (LPDI) system. In more detail, the effect of skipping one cycle - thus operating the two-stroke engine in a four-stroke-like mode - was investigated at part load conditions by considering four BMEP levels (i.e. from 1.0 bar to 2.5 bar). The benefits of such strategy were evaluated at the test bench and compared with the performance of the standard operation mode. In particular, the cycle-to-cycle variation was drastically reduced and the combustion misfire was avoided with the intermittent injection, thus leading to a strong reduction of both hydrocarbon emissions and brake specific fuel consumption.
