I’ve asked a lot of people how the KX250F’s dual injectors work, but their answers don’t make a lot of sense. Can MXA explain how Kawasaki’s dual fuel injectors work?
On each intake cycle an engine draws a breath of air from the intake tract, which can be viewed as a long rubber tube of air. At low rpm, the engine is taking quick little breaths, and only using a fraction of that long column of air. A single fuel injector nozzle must be placed next to the intake port to deliver fuel at low rpm. At high-rpm, however, when the engine is drawing in the whole long columns of air, a secondary injector, like on the KX250F, can take advantage of the added volume of the long column of air. Atomization is what counts and dual injectors can keep up with the needs of both low and high rpm running.
On the first version of the Dual Injector system, the second injector was located just above the velocity tube of the air boot.
Spraying fuel with a secondary injector nozzle as far away of the intake port as possible gives the fuel time to atomize while being tumbled in the natural turbulence of the intake tract. It is also claimed that the upstream injector cools down the intake charge to make the mixture denser with oxygen—which translates into more power. This is the reverse of Honda’s Dual-Timing injection. The Honda system does not use two injector nozzles, nor does it metered the amount of fuel between separate systems. Instead, Honda’s Dual-Timing separates the spray from its single injector into two charges. The first charge is a short-duration spray that is used to cool the back of the intake valves heads for more oxygen.
In 2013 Kawasaki's engineers repositioned the secondary injector above and behind the velocity tube in search of more air mixture and misting.
Both of Kawasaki's injector nozzles have the same output capacity. In the KX250F system only the first injector is running until 7000 rpm,. At 7000 rpm the upstream injector kicks in and gradually increases fuel delivery while the first injector gradually tapers off. When the bike is running wide open, only the upstream injector is working—because the rpm is high and the demand for air is great—so the longer column provides the extra volume of fuel and air.
Dual injectors make CPU programming more critical. The two injectors must blend together seamlessly to optimize fuel delivery. When several non-Kawasaki race teams tried using a home-made secondary injector simultaneously with a downstream injector, it didn't work because they didn’t have the correct mapping. The secondary injector needs to come into action when the engine rpm is high enough to utilize it. The extra time it takes for the upstream injector's fuel to reach the combustion chamber must be calculated into the equation. The amount of fuel delivered by current motocross EFI systems depends on how long the injector is turned on, which is called “pulse width.” Obviously, two injectors add more dimension to the pulse width equation.
MXA test riders love the KX250F engine. It has the best racing-style powerband of any 250 four-stroke made—which is why it wins the majority of 250 Shootouts.
There is a certain amount of lag time from when the CPU sends the signal to turn the injector on, and when it actually turns on. This lag can be a millisecond or more. When the pulse width is only two milliseconds at best, a delay of a millisecond is substantial. On a "single injector" engine the engineers must figure out an average lag time and increase the pulse width accordingly to cover all the bases. With two simultaneously operating injectors the pickup delay can easily be covered up by the difference between fuel traveling a short distance and fuel coming from farther away.
The takeaway from the dual-fuel story is that the KX250F has one injector nozzle located close to the combustion chamber for low rpm running (when the need for air volume is less) and a second injector that is mounted farther away so that it can deliver a great volume of air down the intake tract.