Diesel Engine

The 1893 prototype blew apart on its first firing. Build only the demonstration apparatus described here at low pressure, behind a shield, and never pressurise a vessel you have not proof-tested.

Diesel claims gradual introduction of fuel into air already compressed to ignition temperature. Power is set by how much fuel is injected — there is no throttle on the air.

Make a thick-walled clear tube with a close-fitting plunger, drop in a wisp of tinder, and strike the plunger down sharply. It ignites with no flame and no spark. That is the entire Diesel cycle in one stroke.

For adiabatic compression T₂/T₁ = (V₁/V₂)^(γ−1), with γ ≈ 1.4 for air. A 20:1 squeeze from 300 K gives roughly 1000 K — hot enough to light fuel unaided.

Petrol runs around 10:1 because higher ratios make the mixture detonate before the spark. A diesel compresses AIR ALONE, so there is nothing to pre-ignite.

Bore a cylinder and fit a piston with rings. Blow-by past the piston is fatal here — you cannot reach ignition temperature if the charge leaks.

Shape the piston crown to spin the air as it is compressed. Fuel is injected as a spray and must find oxygen fast — mixing time is the limit on how quickly a diesel can burn.

Force fuel through a fine orifice at high pressure so it leaves as a mist. A stream of liquid fuel will not burn fast enough — surface area is everything.

Inject as the piston nears the top. There is a short ignition delay while the spray heats and vaporises, so injection must lead the intended burn.

If the delay is long, a lot of fuel accumulates and lights at once — a pressure spike you hear as the characteristic clatter. Shortening the delay is most of diesel refinement.

The engine always draws a full charge of air. Power is set purely by fuel quantity, which is why diesels run very lean at light load and lose no energy pumping against a closed throttle.

Unloaded and over-fuelled, a diesel accelerates until it destroys itself. A mechanical governor limiting fuel is not an accessory — it is a safety-critical part.

Thermal efficiency rises with compression ratio. That is why Diesel chased high compression at all, and why his engines beat the steam plants of the day by a wide margin.

Compression ignition is indifferent to fuel chemistry in a way a spark engine is not. Diesel ran demonstrations on peanut oil — the cycle cares about ignition temperature, not petroleum.

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.