Influence of Engine Parameters on ATAC Behaviour in a GDI Two-Stroke Engine

In two-stroke S.I. engines, direct fuel injection allows avoiding fuel loss from the exhaust port, since the cylinder is scavenged only with air. However, only if injection produces charge stratification, also combustion difficulties at light loads, due to the excessive presence of residual in the cylinder, can be removed. An alternative solution for this problem is ATAC (Active Thermo Atmosphere Combustion), which turns the effect of residual gas from negative to positive, since its thermal energy is used to prime the combustion of fresh gas. ATAC leads to very good combustion stability with small cycle-to-cycle variation and to good fuel economy and low exhaust emissions of unburned hydrocarbons. This paper deals with ATAC combined with direct fuel injection, both of the air-assisted medium-pressure type and of the liquid highpressure type. In the case of liquid injection, also ATAC dependence on relevant parameters, such as injection timing, injection pressure and exhaust timing has been investigated, with the intent of expanding ATAC existence field towards the lowest speeds and loads, as it is required for an appropriate application of this unconventional combustion to production engines.