Are loop scavenged 2-stroke engines for recreational purposes able to fulfil future emission limits?

摘要

The present technology applied in two-wheeler two-stroke engines is realized by external mixture formation with carburettor. This engine concept - used in displacements between 50cm~3 and 500cm~3 per cylinder - has to put up with heavy drawbacks concerning torque characteristics and maximum achievable power in order to cope with emission limitations. Due to high scavenging losses, the applied exhaust gas after treatment concepts, often realized via an oxidation catalyst, reach their endurance limit. More severe future emission limits, discussed for 2012 (EURO IV), as well as the planned evidence of endurance, strive for the inevitable change of the system to four-stroke or two-stroke engines with internal mixture formation to achieve a reduction of scavenging losses. High system costs, complexity and space prevent the application of known automotive high pressure direct injection systems in small capacity engines. Due to these circumstances, different systems and technologies have been developed to cope with the specific requirements for these engine types. A system comparison gives a survey on the mode of operation, the field of application, and the achievable emission limits. Extensive test bench investigations with an optical analysis of the mixture preparation and scavenging process, supported by 3D CFD simulations, lead to new findings concerning mixture formation in loop scavenged two-stroke engines. Different novel mixture formation variations were scrutinized. The most promising variation was realized in a 50cm~3 and a 250cm~3 two-stroke engine respectively. The detailed documentation of the development on the engine test bench proves that the targeted use of charge movement during the scavenging process, even with low pressure injectors, is able to realize a sufficient internal mixture formation in the two-stroke engine. The separation of air and fuel trapping efficiency enables a significant reduction of fuel scavenging losses as well as an optimization of the oxidation process in the exhaust gas after treatment system. The application in prototype vehicles could further confirm a decrease of emissions and a reduction of fuel consumption. The new low pressure direct injection concept shows the high potential to fulfil future emission regulations.