Engine Tech: What is Direct Injection?
The latest development in engine tech confers some real benefits - so check whether your next new car has direct injection
QUESTION
You always talk about direct injection on your website. I thought all cars were fuel injected. What gives? - Alex
ANSWER
In practise, all modern engines are fuel injected, but they come in two flavours: Direct (newer) and Multi-point (older). Direct injection of petrol engines is different to ‘multi-point’ injection. Direct is the latest development, and better.
Direct injection is a newer development offering greater power and/or economy than in a multi-point injected engine, all other things being equal.
WHAT'S THE DIFFERENCE?
The big difference is where the fuel mixes with the inlet air inside the engine.
With conventional multi-point injection, there is a low-pressure fuel injector for every cylinder, located deep inside the inlet port in the cylinder head. Fuel is sprayed from that injector right onto the top/back of the inlet valve. So, technically, the fuel and the air mix (just) inside the inlet port. That is, they mix outside the combustion chamber, and they enter the chamber together when the inlet valve is open.
With direct injection, only air enters the engine through the inlet port. Fuel enters the combustion chamber directly. The fuel injector is relocated to the combustion chamber in these kinds of engines. It’s a much more sophisticated, high pressure injector, which is fed by a common fuel rail (often called simply a ‘common rail’).
Typical of the car industry: they can’t standardise the name of anything - and direct injection conforms to this convention. It is called, variously, FSI (fuel stratified injection), GDI (gasoline direct injection), and even SIDI (spark ignition direct injection). They’re all the same thing.
WHAT DOES DIRECT INJECTION DO FOR ME?
As a consequence of the relocation of the injector and the higher operating pressure of the fuel, more precise control of the delivery of fuel is possible. This means, effectively, that more precise control of the combustion event is possible.
This allows engineers to achieve one very important improvement.
There is a significant improvement in efficiency. Essentially, this means doing more with less. Here’s the back story: In the context of engine operation, every combustion event delivers pressure on the crown of the piston as the carbon monoxide and water (steam) produced by the combustion of the fuel expand rapidly in the enclosed space. Because of the orientation of the piston, crank and con-rod, the result is torque.
In a direct injection engine, all other things being equal, more torque results from the combustion of a given mass of fuel. The flip-side of this proposition is: less fuel is required to deliver any given amount of torque (all other things being equal).
For the driver or owner, direct injection delivers either greater absolute performance, or better economy - depending on how the car is driven. For the carmaker, direct injection allows the car to achieve the required performance using a smaller displacement - hypothetically saving a little weight. It also allows carmakers to conform to regulatory pressure to reduce fuel consumption across the range of engines in the fleet. This is done mainly to reduce CO2 emissions.
PETROL ENGINES ARE CATCHING UP
Interestingly, diesel engines have had direct injection for some time. The advent of direct injection in petrol engines represents a kind of ‘catch up’ for petrol engines. (Diesels remain the only ‘autoignition engines’ commonly used in cars - the fuel/air mix ignites spontaneously in a diesel engine, as a consequence of the high compression environment. Petrol engines - even with direct injection - remain spark ignition engines, meaning they still use spark plugs.)
Direct injection engines require significantly more computing power than multi-point engines in the engine ECU. Some direct injection engines don’t use an air-type throttle to control output. (This reduces what engineers call ‘pumping losses’ - even though an engine’s not a pump.) In engines sans-throttle the engine ECU controls output.
See also my full report on petrol versus diesel >>
THREE OPERATING MODES
Direct injection engines actually operate in three basic modes: ‘Lean burn’, ‘stoichiometric’ and ‘rich’. Lean burn mode occurs on a trailing type throttle where no acceleration is required. In this mode a very small amount of fuel is delivered, and the combustion event is confined to the centre of the chamber near the spark plug (often there is an annular ring in the piston crown to constrain the burn near the plug).
In stoichiometric mode the fuel and the air mix in the theoretically ideal ratio for efficient combustion, and in rich mode, for heavy acceleration, a little more than the ideal amount of fuel is added. (This reduces the likelihood of detonation, or ‘pinging’, which damages engines at high rpm and under load.)
Because the fuel pressure is so high, and the injectors so responsive in the time domain, several fuel delivery events can occur per combustion event. (This happens really fast - for example, at 6000rpm, the engine is doing 100 revs per second. That’s 50 combustion events per second…) Delivering precise quantities of fuel quickly can help manage emissions.
DARK SIDE OF DIRECT INJECTION
High pressure injection allows all of this to be effectively achieved, but there are a couple of negatives. The firs, and most obvious, is cost - a high pressure fuel rail and advanced piezo-electric injectors are much more expensive than a multi-point injection setup, and the extra lines of computing code and faster processors aren’t free, either. Second is acoustic noise - common-rail is just noisier, which means more time and effort in the R&D centre keeping noise, vibration and harshness under control, as well as a greater amount of acoustic insulation required to attenuate the noise from within the cabin.
Direct injection right now is a good barometer of how much money particular carmakers have invested in drivetrain development over the past decade. Those manufacturers which offer many direct injection engines in their ranges (Mazda, Hyunda-Kia, etc.) have been keeping themselves up-to-date, whereas those whose ranges are bereft of direct injection virtually across the board are obviously dragging the chain. (These include Honda, Toyota, Mitsubishi and Nissan).
CONCLUSION
The bottom line: When you see a Toyota Corolla, a Honda Civic and a Mazda3 for sale at the same price, and two have a 1.8-litre multi-point engine from the dark ages (the Corolla and Civic) and the other has a cutting-edge 2.0-litre direct injected engine (Mazda3) then you’d need a compelling case of supplementary reasons to justify buying the Corolla with the inferior engine tech. It really is that simple.
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