Radial Engine Gear Reduction

Radial engines powered most aircraft through World War II.

Traditional internal combustion engines idle at a faster rate than what can be used when starting from a standing stop. Gear reduction reduces the revolutions-per-minute output by means of transmission systems that transfer motor power between gears of progressively larger sizes.

Radial Engines

A radial engine is a direct-drive motor primarily used in propeller-driven airplanes. Radial engines have no transmission and thus can neither idle nor reduce RPM by means of gears. Radial engines were the standard design for airplane engines through World War II. Development of liquid-cooled engines ended the dominance of the radial engine

Radial Engine Basics

As the name implies, radial engines are round in shape. The cylinders encircle a short crankshaft that is housed inside the engine block. One piston rod, the master rod, is permanently affixed to the crankshaft. The remaining rods catch a ring protruding from the permanent master rod linkage. Radials use an odd number of cylinders. This allows a staggered firing order of 1-3-5-2-4-1. This means the engine turns the crankshaft twice for every complete firing sequence, allowing each piston to complete all four strokes -- intake, compression, ignition and exhaust.

Rotary Engines

Among laypeople, radial engines are sometimes confused with rotary engines. In rotary engines, the cylinders actually rotate around the crankshaft.

References

• Aircraft Engine Historical Society: Development of the Diesel Aircraft Engine
• Understanding Gear Reduction, Brett "Buzz" Dawson, 2002