An Innovative Solution for Two-Stroke Engines to Reduce the Short-Circuit Effects

Two-stroke engines complete the process cycle in one crankshaft revolution: the scavenging process takes place when the piston is close to the bottom dead center, with the opportunity to open and close the cylinder ports by means of the piston motion, greatly reducing the number of moving parts. This solution however, typically used in small engines, imposes a symmetrical timing with respect to the bottom dead center, leading to a lower scavenging efficiency than a four-stroke engine. Except for the short rpm range of dynamic tuning, two-stroke engines are affected by the short-circuit of fresh air-fuel mixture during the scavenging process: this phenomenon results in a fuel loss, subsequent lower torque and higher specific consumption, and also in an inevitable increase in pollutant emissions. This paper presents one possible mechanical solution to reduce the short-circuit in the whole rpm engine range, to keep the typical advantages of two-stroke engines (simple construction, high specific power and working regularity for a single cylinder engine of a given displacement) and, at the same time, to avoid the usual problems of the two-stroke cycle. An asymmetric timing of the exhaust port is certainly a benefit, and for this reason an innovative design solution was conceived: a rotating valve, directly driven from the crankshaft, was positioned in correspondence with the exhaust port. During every cycle, this valve prevents the leakage of the fresh charge from the exhaust port in the last phase of the scavenging process. At the same time, thanks to its particular geometry, it allows the exhaust flow during the discharge. In other words, the valve converts the typical fluid dynamic effect of the outlet overpressure wave into a mechanical system for all rpm range and not only for the tuning speed. The benefits of this solution were analyzed both in terms of global performance with a 1-D simulation code, and of fluid dynamics behavior of the system through 3-D CFD simulations. The main results, presented in this paper, show significant improvements when compared to analogous traditional two-stroke engines.