An experimental and theoretical investigation to minimise the hydrocarbon emissions from a 25 cm3 two-stroke engine with finger transfer ports is described. Finger ports have the side of each passage closest to the cylinder axis open to the cylinder bore making it possible to produce high-pressure die castings with the simplest of dies. Cylinders utilising this type of porting are believed to have inferior scavenging characteristics compared to those using closed or cup-handle porting.
The effects of cylinder scavenging characteristics and port optimisation on engine performance were examined using a computer simulation. It is concluded that there is potential for a 70% reduction in exhaust hydrocarbon emissions through scavenging efficiency improvements and port optimisation, provided the cylinder scavenging can be developed to match that of the best existing unconventional crossflow scavenged designs.
Stereolithography models of loop and cross-scavenged cylinders, all with finger ports, were evaluated using the QUB single-cycle gas testing apparatus to determine their idealised constant volume scavenging behaviour. The results from these tests are presented in the form of constant volume scavenging and trapping efficiency characteristics. The best crossflow designs are found to be superior to the loop-scavenged designs.
A 38% reduction in brake specific hydrocarbon emissions is predicted for a projected crossflow design employing the best scavenging characteristic achieved with finger type transfer passages in this study.