The diesel engine

A diesel engine burns its fuel inside the cylinder like any internal combustion engine, but it lights that fuel in a different way. A petrol engine draws in a mixture of fuel and air, compresses it modestly, and sets it off with a spark. A diesel compresses *air alone*, and compresses it far harder — to a fifteenth or twentieth of its volume rather than an eighth. Squeezing a gas that hard makes it hot, several hundred degrees Celsius, and at the top of the stroke a fine spray of fuel oil is injected straight into that furnace-hot air, where it ignites the instant it enters. There is no spark plug, no carburetor, and no explosive pre-mixed charge sitting in the cylinder waiting to fire.

Thermodynamics says a heat engine gets more work the further its gas expands and cools across the power stroke, and the higher you compress before burning, the wider that fall. By pushing compression to the point where the air ignites the fuel unaided, Diesel built the most efficient common engine there is. His 1897 machine reached about 26 percent efficiency at a time when steam engines managed roughly 10; modern diesels exceed 40. It also runs on a cheaper, heavier, less volatile fuel that will not flash from a stray spark, so it is safer to store and handle in bulk. That combination — efficiency, cheap fuel, and ruggedness — made it the engine of heavy haulage.

The diesel became the workhorse of the industrial world. It replaced the coal-fired steam engine in ships, so that today almost all the world's ocean cargo moves behind large diesels; it replaced the steam locomotive with the diesel-electric; and it powers the trucks, buses, tractors, generators, and construction and mining machines that move heavy things overland. It let the early submarine run on the surface and charge its batteries. The same efficiency that made it indispensable also tied a century of freight to a fuel whose exhaust — soot and nitrogen oxides — became one of the defining pollution problems of the modern city.