2-STROKE ENGINE

                             2-STROKE ENGINE

Why Two Stroke?

Because a 2-stroke engine fires twice for every firing of a 4-Stroke engine. This means that the 2-stroke engine has potentially up to twice the output of a 4-stroke engine of the same displacement. Because it does twice the work at the same rpm, it also makes does it with half the parasitic friction (all else being equal). Eg. when both types were available on the market, no 125cc 4-stroke motorcycle ever makes as much power and/or weigh as little as a 125cc 2-stroke bike.

The problem with 2-Stroke designs...

However, the 2-stroke engine has many traditional short comings, some of them utter show stoppers. To begin with 2-stroke engines usually burn a premix of lubricant oil and fuel. This is because they tend to use the crank case is a piston pump to push the intake charge into the combustion chamber when both the intake and exhaust ports are open. This means that instead of having lubrication oil in the crankcase, they must fill it with a fuel, air and oil mixture. This is a serious problem because with enough oil content in the mixture to lubricate the load bearing main bearings, journal bearings and wrist pins, the mixture will burn in a smoky manner and is guaranteed to fail modern emission standards. Despite this, lubrication is still poor compared to 4-stroke engines leading to 2-stroke motors wearing out twice to three times as quickly as 4-cycle engines. To make matters worse, because both the intake and exhaust ports must be open concurrently at some point, 2-stroke cycles must either exhale the exhaust gases incompletely or over aspirate the intake charge such at a portion of the fuel-air mixture escapes into the exhaust. The former leads to reduced power output from not having enough air to burn all the fuel in the mixture. The latter leads to wasted fuel going straight into the exhaust. Both further compromises hydrocarbon emissions and lead to reduced fuel efficiency. As if that is not enough, the use of tuned exhaust systems provide a back pressure pulse to help achieve a balance between the two aforementioned problems also leads to a very narrow rpm range where the engine is optimally powerful, efficient and clean running. Even when everything is perfect, at the ideal rpm and load range, because both ports are on the lower lower of the cylinder scavenging is never as complete as a 4-stroke engine due to dead spots on the upper part of the combustion chamber and some degree of inefficiency cannot be avoided. All it all, inferior fuel economy, lousy emissions, poor longevity and narrow power bands have condemned the traditional 2-stroke engine to garden blowers and RC models. In fact, in many countries 2-stroke engines are outright banned not just on cars, but motorcycles and Jet Skis alike.



Direct Injected SPOHV Engine Changes Everything

Here I am presenting a concept that changes everything while retaining the advantageous of a 2-stroke design. The engine uses an overhead valve and side exhaust ports. Fueling is by means of direct gasoline injection during the compression stroke. It uses a wet sump lubrication system for the main bearings, journal bearings, wrist pins and part of the cylinder walls. The crankcase is filled with oil like a 4-stroke engine and is not used to pump a fuel-air-oil charge into the combustion chamber. Instead aspiration is enabled by an external centrifugal supercharger. The engine operates on a hybrid 2-stroke / Miller Cycle in that the intake valve stays open during a good portion of upward travel of the piston after the exhaust ports have closed. This results in an air charge that is above atmospheric pressure when the intake valve closes making this a true force induction engine. It also creates an asymmetrical compression and power stroke with the latter being longer than the former for superior combustion efficiency very much like Atkinson and Miller Cycle 4-stroke engines.