Differential air/fresh charge demand, supply and utilisation in two-stroke spark ignition engines

Abstract

In a two-stroke engine, the net quantity of air/fuel delivered to the cylinder is usually less than that demanded amount due to flow constraints, the nature of working fluid, port timings and pressure waves. Central to all these losses is the limited time for
combustion relative to the intake ports’ open down periods. This research utilises a manifold differential-pressure model to establish a loss of fresh-charge due to the supply time-lag and cycle-overlaps which are chiefly dependent on the ports’ positions and
engine speeds. Three different throttle angles and engine speeds were utilised to establish differential flow rates between the ports’ opening and closing intervals. Test results show that at low intake pressure and engine speed, 12.5 ms is required to fulfil
the engine demand with a maximum value of 16.5 g/s occurring at 4 ms. The corresponding supply is 5.5 g/s lagging by about 7.5 ms and resulting in a maximum loss of 13 g/s. At high speed with the same initial pressure, the maximum demand of 40
g/s occurs at 2.5 ms while the maximum supply of 32.5 g/s lags behind by 5 ms, resulting in a maximum loss of 22.5g/s. Thus, the differential pressure model is a useful tool for estimating the difference between the charge demanded and that supplied.