Diesel fuel systems were entirely mechanical until the mid-1980s, when manufacturers of vehicle engines turned to computers to give more precise control over fuel delivery and timing. Mechanical systems continue to be specified for many agricultural, construction, and marine applications.
Early engines used compressed air to force fuel into the cylinders against compression pressure. Injectors received fuel from a low-pressure pump and air from a common manifold, which was pressurized to between 800 and 1000 psi. The engine camshaft opened the injectors, appropriately enough called “valves,” to admit a blast of compressed air and atomized fuel to the cylinders. A throttling valve regulated engine speed by controlling the amount of fuel delivered; air required no metering, since surplus air is always present in diesel cylinders.
Air injection had serious drawbacks. The air/fuel mixture could not penetrate deeply into cylinders that were themselves under 450–500 psi of compression pres- sure. Attempts to increase engine power by admitting more fuel merely dampened the flame. The compressors were parasitic loads that absorbed 15% and sometimes more of engine output. Nor did air injection make for a compact package: starting and four-stage air-injection compressors accounted for a third of the length 1914-era Krupp submarine engines.1