emissions

The 2 stroke engine has always had a problem with hydrocarbon (unburnt gas) emissions. The amount of overlap, the time when the intake and exhaust are both open, can not be controlled as well as a 4 stroke can with it’s valves. This is the same problem that has plagued the rotary engine until recently, when a new port configuration was used on them. Unfortunately another problem with two strokes is that most people want to build them up and make more horsepower. Normally a tuned exhaust is one of the first modifications done, due to a large increase in power for a reasonable amount of money. The purpose of an expansion chamber is to reflect a pressure wave back to the cylinder. If timed correctly in a certain rpm range, this strong pulse will be reflected away from the cylinder which creates a vacuum at the cylinder. The overlap period takes atvantage of this situation as the fresh intake air rushes into the cylinder. As the air fuel mixture fills the cylinder it does a great job of increasing the volume which translates to power.

 

The problem being that some of the air fuel mixture overshoots the cylinder and ends up in the exhaust. That is why 2 stroke engine smokes and has very little to nothing to due with the pre-mix oil added to the gas or injected in. New 2 stroke engines, named direct injection engines, are infact very clean and many boat engines use this technology to keep the light weight atvantage of a two stroke. On the down side the technology isn’t cheap and can be very complicated. An injector is placed in the cylinder head and waits until the overlap process has been completed to inject the fuel leaving no opportunity to escape out of an exhaust port. Some of these engines are very clean, cleaner than their 4 stroke counterpart, but preconcieved notions about 2 strokes stop many people from embracing this new technology. The interesting thing is that along with this direct injection technology is the fact that an engine could have the capabilities to burn other fuels such as jet fuel, kerosene, and diesel. I have participated in running and developing these engines and they have potential. Some companys currently have mechanical injected engines which are fairly simple, lightweight and burn any fuel.