What is direct injection?

Direct injection has been used for decades on diesel engines. So why do we hear so much about direct injection as an emerging technology from car manufacturers? Simple, the direct injection they are talking of is Gasoline Direct Injection.

Also known as GDi (Gasoline Direct Injection), petrol is injected directly into the combustion chamber. Major improvements in emissions, fuel economy, and performance.

It would seem at first that GDi is a new innovation, an evolution of the now commonplace multipoint fuel-injection systems. In fact, when Mercedes-Benz introduced its iconic fuel-injected 300SL sportscar in 1955, the two-door used a direct injection system! Less expensive and less complex systems became more popular as mainstream manufacturers introduced fuel injection over the following decades.

In recent years direct injection has made a comeback. The need for fuel efficient, low emission cars has prompted several manufacturers to concentrate on this technology which is, in essence, an evolution of lean burn technology.

Essentially DI sees petrol injected directly into the combustion chamber instead of the inlet manifold. Concave shaped pistons concentrate the mixture near the sparkplug and the engine management system can choose between three modes of combustion depending on the load on the engine.

Engine power output is controlled by the fuel injection and the system does not have a throttle on the air intake, like a diesel engine. Therefore the air intake is smooth and unrestricted allowing improved power outputs.

During low engine loads, or cruising, the mixture is injected during the compression stroke and is concentrated around the sparkplug. The rest of the cylinder remains filled with 'unfuelled' air. The mixture is therefore very lean, often achieving air/fuel ratios of up to 65:1!

The emissions and fuel consumption in this mode are very low compared to a traditional system.

When under low load, such as light acceleration, the injection timing changes. The fuel is now injected during the intake stroke. The mixture tumbles and mixes and is uniform throughout the cylinder. It's richer but very closely controlled to be at the most efficient ratio to allow the exhaust's catalytic converter to remove as many pollutants from the exhaust gas as possible.

The final mode activates at high load conditions, such as full acceleration, allowing the engine to produce its maximum power and torque. Again the fuel is injected during the intake stroke, though now at its richest air/fuel ratio to maximise power and avoid preignition.

Of course, any new technology isn't without its disadvantages. The main one is that GDI's extremely high air/fuel ratios produce large amounts of NOx pollutants. There are catalytic converters that can easily deal with this but they are prone to damage from high sulphur fuels.

Therefore in some markets such as the USA and Australia where petrol has high sulphur content, DI technology cannot be offered by most manufacturers. In the meantime, those that choose to do so often must 'erode' the benefits such technologies offer by using more conservative air/fuel ratios.