Two-stroke Compared with four-stroke engines

Two-stroke engines have several marked disadvantages that have largely precluded their use in automobiles (although there was some use, such as in historic Saabs and DKWs and until recently in several automobiles produced in the Eastern bloc, including Trabants and Wartburgs, among others) and are reducing their prevalence in the above applications. Firstly, they require much more fuel than a comparably powerful four-stroke engine due to less efficient combustion. The burning oil, and the less efficient combustion, makes their exhaust far smellier and more damaging than a four-stroke engine, thus struggling to meet current emission control laws. They are noisier, partly due to the more penetrating high-frequency buzzing and partly due to the fact that muffling them reduces engine power far more than on a four-stroke engine (high-performance two-stroke engine exhausts are tuned by determining the resonant frequency of the exhaust systems and exploiting it to top-up the fuel air charge just before the cylinder port closes). Finally, they are considered less reliable and durable than four stroke engines.

Two-stroke diesel engines were used in trucks, with a notable example being the 1954 Commer. This engine was the Rootes TS3 (TillingStevens), a horizontal, opposed piston, three-cylinder. The General Motors EMD diesel powered locomotives have been using 2-stroke engines since the 1930s. These engines have up to 16 cylinders (with a total displacement of approximately 11,000 cubic inches). Typical power output would be 4,300 hp. The Wartsila-Sulzer RTA96-C turbocharged two-stroke diesel engine is the most powerful and most efficient power plant in the world today and is used in ships.

A notable area of use today is in small displacement motorcycles, mostly in off-road "dirt-bikes", and scooters, where their higher power-to-weight ratio, and smaller size outweigh their aforementioned disadvantages.

There are more elaborate possible two-stroke engine configurations, but these often have enough complications that they do not outperform comparable four-stroke engines. New two-stroke designs rely on electronically-controlled fuel injection, oil injection and other design improvements to reduce pollution and increase fuel efficiency. However, such systems increase the cost of the engines to the point that for small systems simple four-stroke engines are most cost-effective. Many large manufacturers, including Ford and Honda are still actively researching ways to build practical and clean two strokes for automotive use.