The radial engine is a type of reciprocating internal combustion engine in which the cylinders radiate outward from a central crankcase like the spokes of a wheel. When viewed from the front, radial engines resemble a star. For this reason, they are sometimes called star engines. The radial configuration was commonly used for aircraft engines prior to the rise of gas turbine engines. In this blog, we will discuss the operation of radial engines in detail.
As the axes of the cylinders are in the same plane, the connecting rods cannot all be directly attached to the crankshaft unless mechanically complex forked connecting rods are used. Instead, the pistons are connected with a master-and-articulating rod assembly. In this configuration, the uppermost piston has a master rod with a direct attachment to the crankshaft. The remaining pistons pin their connecting rod’s attachments to rings around the edge of the master rod. Extra rows of radial cylinders can be added to increase the capacity of the engine without adding to its diameter.
In four stroke radial engines, there is an odd number of cylinders per row, allowing for a consistent every-other-piston firing order to be maintained, providing smooth operation. For example, on a five-cylinder engine, the firing order is 1, 3, 5, 2, 4, and back to 1. This also leaves a one-piston gap between the piston on its combustion stroke and the piston on compression. The active stroke assists in the compression of the next cylinder, creating a more uniform motion. If there was an even number of cylinders, this cycle would not be possible.
As in most four-stroke engines, the crankshaft takes two revolutions to complete the four strokes of each piston (those being intake, compression, combustion, exhaust). The camshaft ring is geared to spin slower and in the opposite direction of the crankshaft. The cam lobes are divided into two rows: one for the intake valves and one for the exhaust valves. In general, radial engines use fewer lobes than other engines. For example, in a radial engine, four cam lobes serve all ten valves across five cylinders, while a typical inline engine would require ten.
The majority of radial engines use overhead poppet valves driven by pushrods and lifters on a camplate which is circular in the crankshaft with smaller radials, using individual camshafts within the crankcase for each cylinder. Some engines use sleeve valves which are quieter and run more smoothly but often require much tighter manufacturing tolerances. Although radial engines are considered an older technology, they still have many uses in modern aircraft.
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