The patent. US 608,845, "Internal-Combustion Engine", granted 9 August 1898 to Rudolf Diesel of Berlin, following his German patent of 1892 and his 1893 treatise Theory and Construction of a Rational Heat Engine. Diesel was a refrigeration engineer by trade and a thermodynamicist by inclination; his stated goal was an engine approaching the Carnot efficiency limit, and compression ignition was the means, not the end. The first working engine ran in 1897 at about 26 % efficiency — roughly double the best steam engines of the time.

The physics in one line. Compress a gas quickly and no heat escapes, so the work done on it goes into internal energy and the temperature rises: T₂/T₁ = (V₁/V₂)^(γ−1). With γ ≈ 1.4 and a compression ratio around 20, air at 300 K reaches roughly 1000 K, comfortably above the autoignition temperature of diesel fuel. A fire syringe demonstrates the whole principle for the cost of a tube and a plunger, and it is the same effect that makes a bicycle pump warm.

Why compressing air alone changes everything. A petrol engine compresses a fuel-air mixture, so raising compression risks the mixture detonating spontaneously before the spark — knock — which caps the ratio near 10:1 and therefore caps efficiency. A diesel compresses pure air, where there is nothing to pre-ignite, so the ratio can go to 20:1 or beyond. Fuel is then injected into air already above ignition temperature, and combustion begins on arrival. This also means ignition timing and fuelling are the same control, that there is no throttle and hence no pumping loss at part load, and that a runaway is possible if fuelling is not governed.

Diesel's own intentions, and his end. He demonstrated the engine on peanut oil at the 1900 Paris Exposition and argued that agricultural oils could give farmers independence from petroleum — compression ignition genuinely does not care much what the fuel is, which is why biodiesel needs no engine redesign. Diesel disappeared overboard from a Channel steamer in September 1913 in circumstances never established. Within a decade the engine bearing his name dominated shipping, and it went on to take rail, heavy road transport and stationary power, on the strength of the efficiency he set out to find.

Engines

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Diesel Engine

Rotative Beam Engine

Centrifugal Governor

Savery's Miner's Friend