Low Emission Small Two-Stroke Engines

The scavenging process of two-stroke engines is unfavorably characterized by the loss of large part of the fresh charge from the exhaust port. Besides, bad combustion and misfire occur at light loads, because of the excessive ratio of residual gas to fresh gas within the cylinder. To avoid fuel loss from the exhaust port, the solution is direct fuel injection, as well as charge stratification is the solution for light-load operation. With direct fuel injection and charge stratification, the simple two-stroke engine with loop-and-crankcase scavenging for small motorbikes can satisfy present exhaust emission limits even without catalytic converter. Direct injection and charge stratification equalize two-stroke and four-stroke engines as regards exhaust emissions and fuel economy, or give a slight superiority to two-stroke engines thanks to their intrinsic EGR and low friction. Besides, two-stroke engines deliver higher torque, especially at low engine speeds and, of course, have double firing frequency that improves torque regularity. High-pressure liquid fuel injection is able to control the mixing process inside the cylinder for getting either stratified charge at partial loads or quasi-homogeneous conditions, as it is required at full load. The most difficult aims are keeping stable stratification when engine operating conditions change and, at very light loads, avoiding excessive dilution and spreading of fuel vapour in consequence of burned gas expansion. New-concept engine designs are shown in this paper. Shapes of piston and head, together with scavenging-duct orientation have been optimised to obtain stable in-cylinder flow field features (independently of engine speed) and proper fuel distribution at ignition time. Computational Fluid Dynamics (CFD) predictions at different loads and speeds are reported and discussed.