What is the Future of the Two-Stroke S.I. Engine?

The conventional two-stroke S.I. engine is simple and low-cost, however its scavenging process is unfavourably characterised by the loss of a great part of the fresh charge from the exhaust port. Besides, bad combustion and/or misfire occur at light loads, because of the excessive ratio of residual gas to fresh gas within the cylinder. These drawbacks, because of the oncoming new limits for exhaust emission, might cause the death of this engine in few years, at least in Europe and in the other developed countries. However, innovative solutions are expected saving this engine. The main one is direct fuel injection, which allows to avoid fuel short circuiting to the exhaust port during scavenging, even in two stroke engines of simple construction with loop scavenging, since the cylinder is scavenged with air alone. Moreover, some fuel injection systems can generate charge stratification, thus solving the problems occurring at light loads or, in the case of air-assisted fuel injection which is not able to perform late injection, the adoption of ATAC combustion should be the solution for light loads problems. Thanks to direct fuel injection and to their intrinsic internal EGR (which gives rise to low CO and NOX emissions), the small two-stroke engines might satisfy the regulation ECE 2002 even without catalytic converter (the four-stroke ones need the converter). The increasing diffusion of direct-injection, stratified-charge induces to consider evolved two-stroke engines in the automobile field too, since, on the one hand, direct injection and charge stratification equalise four-stroke and two-stroke-engines as regards exhaust emissions and, on the other hand, two-stroke engines have remarkable advantages of torque regularity and availability at low engine speeds. Of course, in this case, valved two-stroke engines without cylinder ports should be considered, to reduce cylinder wear and thermal deformation and to decrease lubricant.