Almost all on-road motorcycles are fuel injected now, but we are still behind the automotive world when it comes to the induction side of the four-stroke engine. More and more cars are fueled by direct injection – with fuel injected right into the cylinder – while motorcycles make do with port injection, where fuel is injected into the intake port or throttle body. Direct injection offers more power and improved emissions, as the fuel can be better mixed with air in the combustion chamber over a wider variety of conditions.
There are two-stroke scooters using direct injection, and the ill-fated Bimota V-Due was originally equipped with DI. Currently, however, the Motus MST is the only four-stroke motorcycle with direct injection, although it is not in production. The company claims power and torque are increased by 10 percent compared to port fuel injection, and emissions are reduced by as much as 25 percent during cold starts, the time most harmful emissions are generated.
There are multiple reasons direct injection is not used on more motorcycles, at least for now. Most high-performance motorcycles – where you would expect direct injection to first appear – have crowded cylinder heads, with four valves and a centrally located spark plug. Fitting a fuel injector in there as well is a difficult proposition. An injector located in the cylinder head is under much harsher conditions than in the relatively safe throttle body, and must also overcome the pressure of compression. Higher fuel pressure – as much as 40 times higher – must be used. This makes the individual components and the entire fuel system much more expensive. Note in the picture of the MST engine how solid the fuel rails are, and compare that to the plastic bits on most motorcycles.
Another consideration is the volume of fuel that must be injected into the cylinder, and how much time is available for that fuel to be injected. A fuel injector has only two states, on or off, and the amount of fuel delivered varies according to how much time the injector is on during each engine cycle. As rpm and/or load increase, the injector is opened for longer to deliver more fuel. In port fuel injection, a limit is reached when the injector is on all the time and can deliver no more fuel. You can use a larger injector to get around this limit, but a large injector has difficulty metering the tiny amounts of fuel required at idle – just like you can put more water on your garden in the same amount of time by using a bigger hose, but would have trouble measuring out just a couple of drops of water. Some manufacturers remedy this by using two injectors, one for the low load/rpm accuracy and one for the high load/rpm volume.
With direct injection, fuel can only be injected into the cylinder on the intake and compression strokes, rather than at any time during the cycle – half as long as with port injection. And, for best emissions, it’s best to wait until after the exhaust valve closes during the intake stroke to open the injector, further limiting the time available. This is the stumbling block for direct injection currently. Motorcycles have such high-revving engines that the injectors can’t deliver enough fuel in the reduced time available for each cycle. And the approaches used in traditional port fuel injection to increase delivery are difficult to implement: A bigger or additional injector would use up even more space in an already crowded cylinder head; increasing fuel pressure even further is also not an option, as it is already extremely high.
The sport-touring Motus engine is well suited to direct injection. With two valves per cylinder, there is room in the combustion chamber for the injector, and with a relatively low rev limit of 8000 rpm there is time in each cycle for the fuel to be injected. It will be interesting to see what develops in the next few years with more mainstream production motorcycles. With increasingly stringent emissions requirements and improving components and technology, direct injection will certainly make its way onto more motorcycles; the question is when rather than if.
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