In my two previous articles about the future of sportbike engine design (here and here), I discussed different valve control systems and their potential for making more horsepower on street-ridden sportbikes. This time, I’m going to talk about fuel injection, and how new advances in the way fuel is delivered could result in more powerful production sportbikes.

First of all, I thought I’d offer a quick explanation of how fuel injectors work, for those of you still stuck in the carburetor era. A fuel injector is basically just an electronically-controlled valve – a signal from the ECU opens the injector, and pressurized fuel sprays out through a nozzle which is designed to atomize the fuel (turn it into an extremely fine mist that can mix homogenously with the intake air). The fuel comes from the fuel rail, which is nothing more than a tube which gets pressurized fuel from the fuel pump and runs it into the back end of the injectors.

For optimum power production and efficiency, the goal is to get the fuel to atomize as finely as possible and for it to mix evenly throughout the intake charge, thus becoming distributed evenly throughout the cylinder before being ignited by the spark plug. Uneven distribution of fuel, or fuel that falls out of atomization and clusters into droplets on the inside of the combustion chamber will result in weak power and poor mileage, or even pre-ignition or detonation.

Advances are made every year in the design of the injector’s atomizing nozzle, which usually results in small power increases. In recent years, many sportbike engines have picked up bigger gains by switching to a dual-injector setup, with the second set of injectors positioned above the throttle body (prior to the butterfly) pointing directly down towards the ‘throat’ of the intake tract (traditionally, injectors are placed in the sidewall of the intake port or manifold just after the throttle body butterfly). At high RPM, when the engine is producing a huge vaccum and sucking a massive volume of air into the throttle bodies, placing the injectors further back in this airstream aids atomization and helps distribute the fuel more evenly. This can provide horsepower increases in the 4-5% range – but only in the upper RPM.

So now you know what’s being done right now, but the question is: what will happen next? Most experts believe that the next big step in fuel delivery will be the switch to direct injection, where special injectors spray the fuel directly into the combustion chamber rather than delivering it into the intake tract.

Several car companies have sold production automobiles with direct injection gasoline engines, with Audi being one of the biggest users of this new technology. Because direct injection provides the ECU with more precise control of fuel delivery, Audi has been able to run higher compression ratios in its direct injection engines than was previously possible when using traditional fuel injection. This can result in higher horsepower and torque outputs, in the range of 10% in some cases.

Another beneficiary of direct injection is fuel mileage. During light-load conditions (part-throttle cruising), the engine can be run in an extremely lean condition, again due to the more precise control of fuel delivery which is offered by direct injection. This opens the doors to mileage increases of as much as 50% when spending long periods of time cruising at part-throttle, as when traveling on the freeway.

Does direct injection have any drawbacks? Well, it does require more expensive hardware than traditional injection, as both the injectors and the ECU are more advanced than those found on current production bikes. However, this is a minor hurdle, and we expect to see direct injection appear on large volume production motorcycles within the next two years.

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