Increasingly stringent emission regulations force manufacturers of two wheelers to develop low emission motorcycle concepts. Especially for small two-stroke engines with symmetrical port timing structure, causing high HC-emissions due to scavenge losses, this is a challenging demand that can only be met with alternative mixture formation strategies and by intensifying the use of modern development tools. Changing from EU4 to EU5, emission legislation will not only have an impact on the improvement of internal combustion but will also drastically change the after-treatment system.
Nowadays, small two-stroke engines make use of a simple carburetor for external mixture preparation. The cylinders are scavenged by air/fuel mixtures. Equipped with exhaust gas after-treatment systems, such as secondary air with two or three catalytic converters, the emission limits for EURO 4 homologation can be achieved with carbureted engines. An increased number of catalytic converters in the exhaust system reduces the performance of a carbureted two stroke engine and has therewith no advantage in comparison to a four stroke engine.
Electronically controlled direct injection systems with low pressure (SETC 2008-32-0059), reducing the untreated emissions by minimizing the typical scavenge losses can also be found in this market segment. Approaches to reduce the exhaust emissions with high pressure direct injection systems have been investigated in 50cm3 two-stroke applications, but they are not present as mass products on the market yet. Due to the advantage of a direct injection system, a simple oxidation catalyst, the same as in EURO 2 engines, can fulfill the EURO 4 emission standard without any performance losses.
But only focusing on injection technology, it is not possible to achieve the next legislation step with two stroke engines. There are two main characteristic limits. First, a drastic reduction of cold start HC emission is necessary and second, a lambda = 1 application is forced to fulfill the EU5 emission limit in terms of NOx.
A novel approach to achieve low emissions in this engine category is the main subject of this publication. By analyzing different strategies and technologies done with two stroke vehicles on the roller dynamometer, an estimation in terms of performance, exhaust emissions and costs will show a possible way to reach EU 5 emission stage. As the biggest disadvantage of two stroke engines is still the high level of scavenge losses, especially at cold start, the time of the respective effect of the different solutions is of great importance.
Additionally, a combination of solutions for a 50cc Scooter shows the potential to fulfill the future emission targets.