The Merlin’s Supercharger

Warbird engines · Demo

Two impellers at seven times crank speed, tips near Mach 1, and the boost each fuel grade allows.

994hp at 6.0 km

ENGINEER MODE

Altitude6000 m

Readout

Full throttle — the blower is flat outAbove the critical altitude the impellers cannot make up for the thinning air; from here power falls with altitude.

Drive gearhigh · 7.06 : 1

Impeller speed21180 rpm

Tip speed324 m/s · M 1.02

Manifold pressure1.68× sea level

Shaft power994 hp

Critical altitude3.9 km

Blower drive cost12% of power

Intercoolerbetween the stages

Impeller and tip figures are at the Merlin’s rated 3,000 rpm; the drawing turns slowly so you can see the ratio.

In plain terms

An engine can only burn as much fuel as it has air for, and the air runs out with altitude. The Merlin’s answer was a compressor bolted to the back of the engine and geared to the crankshaft — a supercharger — that squeezes thin air back to sea-level pressure or well beyond. One impeller can only spin so fast before its rim reaches the speed of sound, so Rolls-Royce added a second impeller behind the first and a radiator between them to cool the twice-compressed charge. Two gear ratios match the blower to the height: low gear near the ground, high gear upstairs. And how hard it may squeeze is set by the petrol — better fuel resists knock, so it allows more boost from the very same engine.

More detail

The impeller rim speed and its Mach number in the intake air at temperature T are

v_{tip} = \frac{\pi D N}{60}, \qquad M = \frac{v_{tip}}{\sqrt{\gamma R T}}

With D = 292 mm and N = 3,000 × 7.06 rpm the tip runs at about 320 m/s; at 6 km the speed of sound is near 316 m/s. That is the ceiling of one stage. Pressure ratio scales with tip speed squared, so the second stage — smaller, at the same shaft speed — multiplies rather than adds:

\frac{p_m}{p(h)} = PR_1 \cdot PR_2, \qquad p_m \le p_{knock}

Compression heats the charge by roughly PR to the power 0.28 in temperature ratio; the intercooler between the stages takes most of that back, which is why it is worth its weight. The manifold pressure is held to the knock limit by the throttle below the critical altitude and follows the atmosphere above it, exactly as in the Supercharger vs Altitude demo, which shares this model.