The Most Complex and Powerful Engine of the Postwar Era

Mark Vaughn
·4 min read
Photo credit: Napier/NPHT/IMechE
Photo credit: Napier/NPHT/IMechE
  • The Napier E124 Two-Stroke H-24 Diesel airplane engine was designed in the ‘30s.

  • The dual horizontally opposed engine displaced 75 liters and had 24 cylinders and a gas turbine.

  • It was superseded by the Nomad 1, which made half the horsepower of the H-24, with half the displacement: 3,000 hp from 12 cylinders.

There have been some wacky engine designs in the 125 or so years of “modern” internal combustion: rotaries, sleeve valves, various variable-compression engines, all of which made perfect sense to their designers at the time. Even among all those, however, you would be hard-pressed to find something more magnificently complex than the Napier E124 Two-Stroke H-24 Diesel airplane engine.

Just trying to describe all the parts that made up an E124 is a difficult task. At its heart it was a 75-liter diesel. Yes, 75 liters, 4,577 cubic inches, more than two and a half cubic feet. To harness all that displacement, Napier distributed it among 24 cylinders. The cylinders were arranged in two blocks of horizontally opposed configurations. So you have two 12-cylinder engines facing each other and sharing a common crankshaft. Think of the BRM H16 Grand Prix engine, then add eight more cylinders.

The engines had no traditional valves, employing instead two-stroke sleeve valves to get the exhaust out and the fuel/air mix in. To help push those gases in and out there was a gas turbine used to compress the air that went into the 24 cylinders, with further help from energy harnessed from various exhaust gases flowing around the big block. Nothing was wasted on a Napier. All of this would ultimately spin two counter-rotating propeller shafts through a variable-speed transmission.

The setup made 6,000 delirious horsepower. At least that’s what I gathered from many different sources I read. However, it also looks like the H-24 configuration itself might never have been built, struggling mightily to get off the drawing board but ultimately being superseded by an only slightly less-complex 12-cylinder version dubbed the Nomad 1.

Photo credit: Napier/NPHT/IMechE
Photo credit: Napier/NPHT/IMechE

Consider the route that air molecules had to travel to get through a Nomad 1, as laid out by

  1. Enter a 10-stage axial flow compressor.

  2. Enter a bifurcated duct.

  3. Enter a supercharger.

  4. Enter the engine-driven centrifugal supercharger.

  5. Enter either the left or right cylinder banks at 95.5 psi.

  6. Enter one of the 12 cylinders through a sleeve valve.

  7. Get blown up in the combustion chamber.

  8. Exit the cylinder through the sleeve valve.

  9. Get spritzed with fuel, ignited, and sent to the turbine, if more power is deemed necessary for the plane to take off.

  10. Pass through the exhaust nozzle and generate some thrust.

  11. Or, if more power was being harnessed by injecting fuel into the exhaust, a valve allowed the gases to flow into a secondary axial flow turbine between the engine and primary turbine.

  12. From there, the gases would power the primary turbine and thence to the exhaust nozzle.

Circuitous, eh?

Nomad 1 made half the horsepower of the H-24, with half the displacement: 3,000 hp from 12 cylinders.

“The Nomad 1 was a liquid-cooled, horizontally opposed, 12-cylinder, two-stroke, valveless, diesel engine that incorporated a gear-driven, two-speed supercharger and an exhaust-driven turbine that drove a compressor integral with the bottom of the engine,” is how described it. Still wonderfully complex. You couldn’t go into aNAPA Auto Parts store and get anything for this.

A single Nomad 1 actually powered a British Avro Lincoln, the plane’s usual phalanx of four Rolls-Royce Merlin engines being shut down during the flight. Development of that engine lasted until 1952, when its complexity and temperamental nature made even Napier give up on it. The 41-liter Nomad II was a new design—a horizontally opposed 12-cylinder layout incorporating a turbine and compressor, but without the contra-rotating propellers and mechanically driven centrifugal supercharger. That engine made 3,570 hp. Jet engines and straight-up turbines were taking over aviation by then and the Nomad II was shelved in 1955.

When I become Minister of Motors, I will revive the original Napier H-24 and insist that it be the sole powerplant for any number of planes, trucks, and anything else it could possibly fit in.

What's your favorite little-known engine design? Share in the comments below.