Technology explained: Direct Fuel Injection

In a traditional petrol injection engine, fuel is injected into the intake manifold where it mixes with air. This air-fuel mixture is then transferred into the combustion chamber when the intake valves open.

This basic fuelling principle has been used on Porsche 911s since the introduction of the ‘E’ in 1968, the first road-going offering from Zuffenhausen to feature Bosch fuel injection.

For the second-generation 997 though, Porsche made the move to direct fuel injection (DFI), the first major revision to the sports cars injection technology in 40 years. DFI, as the name suggest, injects the petrol straight into the combustion chamber, leaving pure air to be fed through the intake manifold.

DFI technology first made it onto the second generation of Porsche 997 Carrera.
DFI technology first made it onto the second generation of Porsche 997 Carrera.

By injecting petrol directly into the cylinder (at pressures up to nearly 2000psi) better homogeneity of the air-fuel mixture is achieved. This creates a leaner mixture than standard fuel injection, resulting in improved fuel economy.

Similarly, the cone angles of in the 9A1 engine’s multi-hole injectors have been specifically calculated to optimise torque, power output, fuel consumption and emissions. DFI is able to inject the fuel closer to the source of ignition (the spark plugs) producing a more even flame front.

Because of this, the fuel mixture is burnt more completely, increasing efficiency, reducing emissions and improving power output. DFI also enhances combustion chamber cooling, allowing greater compression ratios to be used, again resulting in increased efficiency and power.

The DFI flat six seen in 997.2 and 991 models (including Turbos and, latterly, the new GT3) is codenamed the 9A1.
The DFI flat six seen in 997.2 and 991 models (including Turbos and, latterly, the new GT3) is codenamed the 9A1.

Injecting the fuel straight into the combustion chamber allows the DFI system to engage multiple injector pulses during a single combustion phase. In the 9A1 engine, Porsche utilises up to three pulses during cold starts and high engine loads, the former to speed up catalytic converter heating, the latter to optimise torque.

The engine management system is able to individually regulate the injection timing for each cylinder in the flat six, while the injection rate for each bank can also be controlled.

To make sure that the fuel is being burnt as efficiently as possible, emissions are monitored by Lambda sensors in the exhaust system which feed back to the control unit, allowing for accurate adjustments to be made to the mixture.

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Comments (1)

  • John Morreira

    I have heard horror stories, about high carbon build up on the valves of DI engines. Stemming from the fact that the backs of the valves are no longer “washed” in the