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The Radial Engine

Warbird engines · Demo

Nine, fourteen or eighteen cylinders round one crankpin: the master rod, the odd firing order, the B-17’s turbo.

1234567899-cylinder radialmaster rod in colour — the others hang from its big end

Cyclone R-1820 · B-17

Readout

One crankpin, one master rodEvery cylinder’s rod pushes on the same pin: one master rod directly, the rest through knuckle pins on its big end.
Firing order1-3-5-7-9-2-4-6-8
Firing interval80.0° (even)
Articulated TDC error±3.8°
Weight per hp0.50 kg/hp
Merlin 61 + radiators0.53 kg/hp

The drawing turns slowly so you can follow the rods; the numbers are the real engines’.

Hear it in The Sound of Engines →

In plain terms

A radial puts its cylinders in a ring around a short crankshaft with a single crankpin per row. Only one connecting rod — the master rod — actually sits on that pin; the others hang from pins around the master rod’s big end, so they all push the same crank through it. That makes the engine short and light, and because the cylinders stick out into the airflow they cool themselves: no radiator to carry or to be shot through. The odd number of cylinders per row lets the engine fire every other cylinder round the ring at even intervals. Stack a second row behind the first, turned half a pitch, and you have the fourteen- and eighteen-cylinder engines that hauled the transports and bombers.

More detail

The master piston is an ordinary crank-slider. An articulated piston on cylinder axis u hangs from a knuckle pin K on the master rod’s big end, so its distance from the crank axis along its own axis is

s=K⋅u+ l2−∣K∣2+(K⋅u)2 s = \mathbf{K}\cdot\mathbf{u} + \sqrt{\,l^2 - |\mathbf{K}|^2 + (\mathbf{K}\cdot\mathbf{u})^2\,}

K moves with the master rod, which swings about the crankpin, so s is not the simple crank-slider curve: the piston reaches its own top dead centre when K, not the crankpin, is farthest out along u — a few degrees before or after the crank is aligned — and the stroke differs slightly from 2r. The readout gives the worst angular error and the stroke spread for the engine chosen. The firing interval for n cylinders in a row is 720°/n; with two rows on pins 180° apart and the rear row turned half a pitch, the rows interleave to 720°/2n.

The turbo loop holds a rated manifold pressure by opening the waste gate as much as needed; at the critical altitude the gate is shut, and above it the manifold follows the atmosphere at the turbine’s maximum ratio. Turbine speed is taken as proportional to that ratio — enough to show the trend, not a map of the General Electric unit.