Direct injection gave German fighters an edge, then took over ordinary cars
In the Second World War, the Messerschmitt Bf 109's engine kept running when the pilot pushed into a sudden dive, while carburetted rivals like the Rolls-Royce Merlin could sputter. The secret was spraying petrol straight into the cylinders. Half a century later, that same idea spread to half of all new American cars.
Gasoline direct injection squirts fuel directly into each combustion chamber, rather than into the intake manifold where it mixes with air beforehand. Done well, it raises efficiency and power output and can cut emissions. The first production engine to use it was the Swedish Hesselman engine of 1925, a low-compression design that could burn diesel, kerosene, ethanol or even tar oil. Since petrol cost more, operators typically used it only to start the engine and then switched to something cheaper.
Military aviation pushed the technology forward. The Daimler-Benz DB601 V12, fitted to the Bf 109 from 1936, used Bosch mechanical injection, letting designers run very high compression and boost on low-grade 87-octane fuel. Rival carburetted engines needed far higher octane aviation spirit for similar power, and they could lose power in negative-G manoeuvres, a weakness enemy pilots could exploit until Rolls-Royce fixed it around late 1943. A few German cars adopted mechanical direct injection in the 1950s, but it stayed rare until Mitsubishi introduced an electronic system in mass-produced cars in 1996. In the United States its share jumped from 2.3 percent of model year 2008 vehicles to about half by 2016.
Engines can distribute fuel in two ways. In homogeneous mode, fuel is injected early in the intake stroke and mixes evenly, which suits a conventional catalytic converter and helps small engines deliver more power. In stratified mode, fuel is injected late so that a rich pocket forms around the spark plug while the rest of the cylinder holds mostly air. That ultra-lean burn promises savings at light loads, but in practice gains have been modest and it produces more nitrogen oxides, sometimes requiring extra exhaust treatment.
There are trade-offs. With no fuel washing over the intake valves, carbon deposits can build up, and some owners fit oil catch tanks to limit them. Because fuel can only be injected during intake and compression, delivering enough at very high revs is harder. Petrol also lubricates poorly compared with diesel, so injection pressures are usually capped around 20 megapascals to limit injector wear.
Source: Gasoline direct injection
