Fuel injection: Why a little squirt is now a big deal

Published: 10 March 2000

NOT so long ago, fuel injection was the sole preserve of tiny firms which couldn’t get the big carburettor manufacturers to make parts tailored to their small production runs.

Companies such as Moto Guzzi, Ducati, Bimota and to some extent BMW, found it more cost effective to buy injection systems and programme them to suit their different engines. Of course, that was before the greenies came to prominence, in the days when you could empty oil into a stream and kill a few fish without the fear of prosecution (only joking).

But nowadays, with ever-tightening emissions laws, more and more firms are turning to fuel injection as a means of making their bikes legal without losing too much power and driveability.

Many of Britain’s best superbikes have it, including the GSX-R750, 2000 FireBlade and 996, and their riders take it for granted. Bikes like the R1 and CBR600 are likely to get it in future.

Injection is still more expensive than carbs as far as large production runs are concerned, and it makes little difference in terms of outright horsepower. But it is the best way to keep an engine responsive and smooth while cutting down on ozone-busting pollutants.

The various systems all operate on the same principles. Controlling everything is an Electronic Control Unit (ECU) which is constantly fed a whole range of information about the engine and the atmospheric conditions.

Typically, it has sensors measuring throttle position, engine revs, engine temperature, crankshaft position, camshaft position (so the ECU knows if an engine is on its power stroke or intake stroke, for example), intake air pressure, intake air temperature and atmospheric pressure.

This data constantly changes, sometimes hundreds of times a second, so to make sense of it all the ECU is pre-programmed with every foreseeable combination of the more important sensor readings. This information is stored on " maps " . When the engine is running the ECU goes to the point on the map which corresponds closest to the particular combination of figures it’s being fed at the time, then reads off a pre-determined amount of fuel to squirt into the inlet tract. This means that whether you’re queuing to get into Brands Hatch or caning it down your favourite back road, the optimum amount of fuel is delivered every time you open the throttle. Impressive stuff.

The Suzuki Hayabusa’s two-stage system is typical of the set-up used on most modern sports bikes. Under light load conditions (partial throttle openings and low rpm) the injection is controlled primarily by a map based on air intake pressure and engine rpm. At higher revs and heavy loads (larger throttle openings) it switches to a second map based on throttle position and engine rpm.

The reason for the switch is that Suzuki’s engineers found they were getting better throttle response this way. Each of the Suzuki’s four cylinders has its own version of these two basic maps, to allow for the slightly different operating temperatures, which means the engine depends on eight different maps throughout the rev range.

The information on these maps is then modified by additional information. For example, when the engine is cold, the coolant temperature and intake air temperature are used to correct the amount of fuel injected, making it richer. While at high altitudes the atmospheric pressure information is used to make the mixture leaner.

All this information is painstakingly accumulated by the engine design teams during the development stage.

First of all, the new engine is run on a dyno and a basic ECU calibration is worked out to get it running reasonably well. For example, the engine will first be run at 3000rpm with a very small throttle opening, and the optimum amount of fuel noted for the best combination of power and exhaust emissions. Then the throttle is opened slightly (and the load on the engine increased to keep it at 3000rpm) and, once again, the optimum amount of fuel determined. This will be repeated at 3000rpm with increasing throttle openings right up to full throttle, before the whole process is repeated at, say, 3100rpm. Yes, it takes a very long time, especially with modern engines running up to 14,000rpm or more. Ask an injection engineer what his reaction is when halfway through this process a decision is made to change the airbox shape, alter the camshaft timing or something similar, and you stand a good chance of being hurt!

The ECU controls the amount of fuel being injected by altering the amount of time the injectors are open. The fuel lines are held at a fixed pressure by the fuel pump and pressure valves, usually about three bar in cars and slightly more on motorcycles. So if an injector is open for a certain time, the amount of fuel which will flow through it will always be the same.

Or it is until the limits of the injectors start to be reached. At very small throttle openings, the time taken for the fuel in the fuel line to accelerate from a standstill starts to have an influence and you can no longer assume that halving the opening time will give you half the fuel delivery – it will be slightly less. This non-linear relationship is usually dealt with by the injection map, but a bigger problem can occur at the other end of the scale.

The flow rate through a typical injector is around 150 grams per minute, but some high-performance machines with large cylinders, like Ducati’s V-twins, might demand a lot more fuel, even as much as 300 grams per minute.

Purpose-built high-flow injectors would be prohibitively expensive, even for factory race teams, so the solution is to use more than one injector per cylinder. This introduces other possibilities, too. One is that low rev, small throttle opening mixture control can be more accurate, as one injector can be switched off or set to a controlled minimum while the other varies the flow rate. This is of no use on race bikes, of course, but is already used on some cars and will eventually come to road bikes. Another variation used by Ducati is two main injectors positioned conventionally, close to the throttle butterfly, with a third, smaller injector farther up the inlet tract to provide more control over the mixture.

Most of the more refined fuel injection systems are sequential, which means the injection pulses are timed to coincide with a particular engine position. So, for example, the fuel is squirted into the inlet tract just as the inlet valve is opening.

This sequential control also allows extra injection pulses on occasion, particularly when the throttle is suddenly snapped open, when extra fuel can be added to prevent a sudden leaning of the mixture (the same job done by accelerator pumps, where fitted, on carburettors).

The alternative is non-sequential injection, often called banked injection, where all the injectors in a multi-cylinder engine squirt together. This works well enough in many applications, but doesn’t offer as much control. While the injectors themselves are off-the-shelf items produced by the specialist industry, the angle they’re positioned at in the inlet tract comes down as much to experience and experimentation as pure science.

The problem is that the injector has to squirt fuel across the air flow at an angle, from the side of the inlet tract. With a fast air flow it is rapidly carried downstream, atomising from a fine spray into a partial vapour (ideal for efficient combustion) on the way. But at lower throttle openings the air flow is reduced, leading to a problem known as " wall-wetting " , when some of the fuel is squirted across to the other side of the inlet tract where it forms a puddle. This starts to vaporize while more fuel is being injected, upsetting the carefully-calculated mixture values.

Several manufacturers deal with this by using flapper valves in the airboxes, which restrict the cross-sectional area the air can flow through, in turn speeding it up through the inlet tract.

Suzuki has taken this a stage further with its new GSX-R750 by adding a second electronically-controlled butterfly to each inlet tract, which closes down as the airflow falls, maintaining the speed of air past the injectors. As a result, it has been able to increase the angle of the injectors from 32° to 60° (which improves atomisation) without inducing wall-wetting problems.

So what of the future? With advances in fuel injection technology being made all the time, it seems inevitable that just about every motorcycle will have it within a few years. The only reason it hasn’t happened already is cost, but that will come down once firms have carried out the initial development work – and people start choosing injected models over their carbed relations.