How does direct injection work and why does it tickle the Horsepower Unicorns?

Cars today have never been faster but also friendlier to drive around town. Technology plays a huge part in this, as manufacturers try to squeeze more power, better driving dynamics, and friendlier tailpipe emissions out of their vehicles, and a big piece of this has been the uptake in the move from port fuel-injection to direct-injection.

Both Marty's GR Yaris (CLICK HERE) and MOOG's Mk8 Golf R (CLICK HERE) feature direct-injection (DI) engines, though there are some key differences in how they apply the technology.

At its heart, direct-injection (DI) is simple: fuel injectors are moved from above the intake port (AKA, port-injection) down to squirt directly into the combustion chamber. This improves the whole combustion process as the air/fuel mixture is put exactly where it needs to be, at exactly the right time, and can be controlled at a much more precise level than older-style port injection.   

The newest Golf R uses the Evo4 variant of the long-running "EA888" turbocharged two-litre four-cylinder engine. Built in Gyor in Hungary the 9.3:1 compression ratio engine in his Golf R produces 315hp, runs 0-100km/h under 5-seconds, and still gets approximately 9.4L/100km fuel economy. All of that from one engine is pretty remarkable.

The key difference between this engine and the Yaris, is the Toyota has a set of traditional port-injectors in addition to the high-pressure Squirtles mounted in the head. Other manufacturers, like Chevrolet, have also used "dual injection" on high-performance cars like the 7th-generation ZR1 Corvette.

The supercharged 6.2-litre V8 uses both port- and direct-injection to produce a staggering 755hp, as seen in the image above. While Chevrolet used the second set of injectors for when the V8 was being beaten like it owed the driver money, Toyota have the GR Yaris' port injectors set-up for a different purpose. 

DI engines do have a reputation for having issues with carbon build-up on the backs of the intake valves. This isn't an issue on port-injection engines as the fuel/air mixture keeps the backs of the valves clean, but DI engines don't get that wash, and therefore need to be cleaned (often a labour-intensive process involving removing the intake and sometimes cylinder head to media blast the carbon deposits off).

Upgrading the fuel system isn't super simple with DI, either. Because it works in such a precise way, direct-injection runs at huge pressure. While carburettor engines run at less than 10psi fuel pressure, and most port-injected engines between 45-60psi, the Golf R engine sees over 5000psi! 

Thankfully most DI cars use a traditional-style electric pump to get the fuel to the engine, where a mechanical high-pressure pump takes over to supercharge the fuel pressure. Some cars, like the GR Yaris, have the luxury of adding fuel headroom through turning the port injectors into supplementary injectors. 

All in all, DI is one piece of technology that probably won't be retrofit to older engines, but a DI engine could make for an excellent, fast and efficient way to re-power an old street car.