Four-Stroke Model Airplane Engines

The four-stroke glow or nitro engine is an elegant concept, though not as simple as its two-stroke counterpart. While there are many variations on the design, the basic elements remain unchanged.

There is a piston moving back and forth in a cylinder, that turns a rotating crankshaft. There is a combustion event on every fourth stroke of the piston at the beginning of every other down-stroke. The combustion is caused by a platinum element in a heated glow plug, combined with a compressed fuel-air and oil mixture. There are intake and exhaust ports, as well as a carburetor to mix air with fuel in the required ratio.

This engine has separate intake and exhaust valves. These are operated by a cam arrangement. The most common design is overhead valves that are opened by pushrods, and closed by return springs. The fuel carries its own oil. Being able to remove the battery after the engine starts, due to the self-sustaining glow-plug, reduces weight considerably. This is very desirable in a model airplane engine.

Compared to two-strokes, four-strokes are generally quieter and have a more realistic deeper pitched sound. They generate much more torque and will swing larger props at lower rpm. These advantages make them a good choice for scale models. Their disadvantages are related to their increased number of moving parts: they tend to be trickier to adjust, and are generally more expensive than comparable two-strokes.

If you need more info about starting or troubleshooting these engines, visit the

RC Airplane Advisor.