There are four basic actions that any internal-combustion-engine cycle must contain: intake, compression, power, and exhaust. In a four-stroke engine, each of these actions has its own stroke (up or down movement of the piston). This means that it is comparatively fuel-efficient because no fuel is wasted. However, it does mean that it needs a complex arrangement of valves to regulate the intake and exhaust, and the distributor is marginally more complicated.

In a two-stroke, on the other hand, almost everything is in the power stroke, leaving just two strokes (surprise surprise!): power and compression. This greatly simplifies the mechanism, although it does reduce fuel efficiency as some of the fuel goes straight out the exhaust (this is much reduced with bifurcated cylinders).

One big advantage of many two-stroke mechanisms is that, instead of using a complicated oil pump, sump, pipes, etc it is possible to mix the oil directly in with the fuel, as this goes through the crank case. However, contrary to popular opinion, it is not strictly technically necessary to mix the oil with the fuel, but all two-strokes that have been manufactured in the last eighty years or so will require it! It is technically possible to have a two-stroke with a conventional four-stroke oil arrangement. If anyone wants any more information about engines, e-mail me at HYPERLINK "mailto:davidr.morris@virgin.net" davidr.morris@virgin.net.