The data picked common rail in 1990. VW, the unit injector’s biggest backer in cars, switched to it 15 years later.
A diesel engine gets its fuel into the cylinder in one of three ways. Pump-line-nozzle: one pump makes the pressure and feeds each cylinder through its own pipe. Unit injector: each cylinder has its own small pump in the injector, driven by the camshaft. Common rail: one pump keeps a shared pipe at high pressure, and a computer opens each injector when it chooses. For each we measured how much better per dollar it got every year, using only the patents that existed at the time.
% better per dollar each year. Readings every two years. Dashed: common rail while it was still a handful of patents, an early sign.
Common rail was ahead from the 1980s. By 1990 it was improving about 23% a year, against 16% for pump-line-nozzle and 13% for the unit injector, and it stayed ahead at every reading since. The unit injector improved slowest of the three throughout, even as car and truck makers fitted it in the 2000s.
In the other two systems the camshaft makes the pressure, so it rises and falls with engine speed and each stroke gets one squirt. Common rail holds the pressure at any speed and splits each injection into several small ones, which cleaner, quieter engines needed. VW’s stated reason for switching: common rail now reached much higher pressures.
Our data: common rail improving faster than both other systems.
mtu files its first common rail patent.
mtu Series 4000 launches with common rail as standard.
Common rail goes into its first cars, from Alfa Romeo and Mercedes.
Cummins and Scania team up to replace their unit injector with common rail.
VW, the unit injector’s biggest backer in cars, switches to common rail.
Volvo moves its D13 truck engine to common rail, one of the last holdouts.
It would not have said which supplier would win, or how fast emission rules would tighten. It would have said the diesel fuel system of the future was a pressurised rail with computer-timed injectors, five years before the first one went into production.
For a maker of large diesel engines, that is the window to design a new engine series around it and file early. mtu did both.
Data centres now ask the next question: diesel, gas engines, fuel cells or batteries. The same reading works for that race.
TIR — the Technology Improvement Rate — measures how fast a technology gets better each year, computed from global patent data. Not who is filing, or how much: how fast the route is actually moving. Historically, the fastest-improving technology always wins.
Provenance. Every patent carries a publication date, so the record can be replayed: every improvement rate on this page uses only the patents that were published at the time. Chart data: GetFocus platform exports, October 2026.
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