Two-stroke Power Valve System - Engineering Design Improvements

Engineering Design Improvements

The only moving parts inside simple two-stroke engines are the crankshaft, the connecting rod, and the piston. It is the same simplicity in design, however, that causes a two-stroke engine to be less fuel-efficient and produce high specific levels of undesirable exhaust gas emissions. At the bottom of the power stroke, the transfer ports, which deliver fresh fuel, are open at the same time as the exhaust port. This can allow a significant amount of fresh fuel to run straight through the engine without ever being available for power production. Properly designed exhaust systems help minimize the amount of raw fuel loss in the exhaust process, but a two-stroke engine will always waste some fuel (modern direct injected engines being an exception to this).

Many producers of two-stroke performance bikes fit them with the exhaust valve systems. These valves act to vary the height (and width) of the exhaust port thereby broadening power delivery over a wider rev range. Exhaust ports with fixed dimensions only produce usable power in a narrow rev range, which also affects fuel consumption and emissions.

In a race bike, this is not a problem as the engine will be operating at high RPM almost all the time. However, in a road/commuter bike, the limited power range is a problem. To provide more low RPM power, as well as enable the engine to produce a lot of high RPM power, a power valve system is used.

All power valve systems vary the duration of the exhaust port open time, which gives the engine usable low end power combined with excellent top end power. Manufacturers have also included sub exhaust chambers that extend the 'tuned length' of the expansion chamber.

Power valve actuation can be by mechanical (RPM dependent) or electric (servo motor) means increasingly with electronic control. Electronic control offers a greater degree of accuracy as well as being able to vary the opening of the valve and be tuned to conditions.