Know This – Motorcycle Fuel-Injection

For several years now the majority of bikes have sported the electrickery of fuel injection rather than carburettors. It’s clever stuff, but fuel injection is only as good as the electronic system managing it. So, if you want to up the IQ of your bike in order to boost performance, increase fuel economy, or match other modifications, then you’ll need to invest in some electronic brainpower.

Know the basics

A fuel management system contains a microprocessor that gathers information from sensors around the bike and then triggers the fuel injectors, situated in the bike’s inlet tracts (just before the cylinder head’s inlet valve). These squirt an appropriate amount of fuel at just the right time to keep an engine at its most efficient.

At least that’s the theory. In reality standard production bikes come with a sealed injection unit, set-up for a compromise that takes into account not only the range of disciplines that might be asked of a specific model, but noise and emissions regulations too.

So, if you want your fuel injection system to work at the absolute optimum for your individual bike and riding needs, then you’ll need a plug-in unit that will allow the adjustment of how much fuel your injectors deliver at any given time. By far the most popular of these is Dynojet’s Power Commander.

What is a Power Commander?

It’s a microprocessor that plugs into a bike’s stock wiring, linking it to the machine’s electronic control unit (ECU). It modifies the signals sent by the ECU to the injectors across various revs, loads and throttle openings (the ‘fuelling map’). On some models, it will simultaneously change ignition timing to suit.

Fitting

A Power Commander can be ordered with a number of different fuelling maps to suit various set-ups. If you’ve fitted an aftermarket exhaust, high flow air filter, or just want your standard set-up optimised then there is likely to be an ‘off-the-peg’ map to suit. As the unit plugs straight into a bike’s standard wiring connectors, fitting one should take less than 15 minutes for even the least skilled mechanic.

Do I need a Power Commander?

For between £270 and £315, an optimised fuelling map will certainly improve the peak power, torque and power delivery of any standard fuel-injected bike. It should make the bike smoother and more driveable than can be expected of spending the same cash on an end-can.

But it is when changing the exhaust, or undergoing any other performance modification, that a Power Commander really comes into its own, allowing the fuelling to be changed in much the same way as when in the past we ‘re-jetted’ bikes. A loud pipe will change the dynamics of the way the engine shifts air and so the amount of fuel delivered should also be changed to keep the air/fuel ratio in proportion.

The only way to do this on most bikes is by fitting a Power Commander and failure to do so can cause the engine to run lean (not enough fuel) and overheat, doing serious damage. On a less serious note, you are unlikely to realise the value of any other engine modification you make unless you also change the fuelling to suit.

Be an expert

PC World

An out of the box Power Commander will arrive with a ‘base’ map stored in its processor. This map will have been developed by Dynojet to optimise a stock bike’s fuelling, or to match it to an exhaust/air filter combination specified upon ordering.

But this base map can be reprogrammed with a new map to suit any further changes. Either yourself or your dealer can download maps from Dynojet’s website (www.powercommander.com) via a simple cable hook-up to a PC.

Mapping the changes

The ideal air/fuel mixture for efficient combustion is around 13 parts air to one part fuel, depending on factors like combustion chamber design and air temperature. A perfect fuelling map is aiming to keep the air/fuel ratio at its optimum at all revs and all throttle positions, so the engine can make the best possible power for that combination. It’s not all about top-end, an optimised bike should make substantially more go at all throttle openings.

And your sensors are…

Engine speed

A sensor on the crankshaft or camshaft – usually a gear with a tooth missing so that tdc can be identified, too.

Throttle position

A simple potentiometer on the throttle valve spindle.

Intake air pressure

Positioned in the airbox this will monitor any changes in pressure due to speed, ram-air effect, ambient air pressure or a clogged filter.

Coolant temperature

Indicates engine temp’ and triggers cold running systems and sometimes also the fan system.

Be a boffin

Why fuel injection?

Fuel injection scores points over carburetion in may departments, not least because it’s mechanically less complicated. Injectors, without float bowls and carb-tops, are far more compact, allowing more radical engine designs. This is especially useful in allowing inlet tracts to become steeper and straighter.

The use of squirters with a brain also allows wide throttle bodies for big peak power without the risk of fuel ‘drop out’. Drop out happens in big carbs at low revs because the wider a throttle, the slower the air will travel, causing fuel droplets suspended in the mixture to literally fall to the floor. This can be tuned out with injection systems by rapidly reducing the amount of fuel introduced and it is for this reason injection found favour early on big twins with huge throttles.

How it works

A pump pressurises the fuel and a regulator keeps the pressure stable as it is fed by a fuel rail (a tube) to the injectors. Injectors are either ‘top feed’ with fuel flowing down through the whole injector or ‘bottom feed’ where it sits in a pool of cooling fuel which enters half way up.

Some bikes with dual injectors feature one of each per pot, the bottom feed injectors firing fuel straight into the back of the throttle body at high engine speeds. For low engine speeds and general running an injector near the inlet valve is best to avoid drop out.

Injectors themselves are fairly basic valves with a needle being held closing a hole by spring pressure. The stem of the needle goes through a solenoid which pulls the valve from its seat (by about 50 one thousandths of a millimetre) when a current is passed through it. When this happens, fuel obviously flow out…

As fuel pressure, nozzle size and fuel density are all constant, the only factor determining how much fuel is delivered is the time for which the valve is open. Which is where our fuel management comes in… The ECU calculates the amount of fuel required (from gear, throttle position etc) and sends an appropriate electric current to the injector solenoid.

More power requires more ‘on-time’ until eventually the on-time, plus ‘dead-time’ (the time it takes the system to react) equals almost a whole engine revolution. When this happens a second injector must be used if the engine is to continue gaining power without the mixture leaning out.