Injection Intricacies

INJECTION INTRICACIES

Suzuki leads the motocross EFI revolution

Carburetion strangely resembles politics. The center takes care of itself, but the trouble is on the fringes. Suzuki’s new fuel-injected RM-Z450 is designed to correct carburetion’s rough edges.

Carburetors work great at rpm where exhaust-pipe wave action makes suction pulses crisp and strong. But below that, fuel flow weakens along with that suction “signal.” And at rpm points where the exhaust pipe works against the engine, blowing exhaust back into the cylinder through the intake port and even into the airbox, the Great Evil known as “multiple carburetion" rears its shaggy head.

Carburetors carbúrate whichever way gas rushes through them so that exhaust-gas blowback picks up fuel going out and coming back in, making the engine rich and burbly on the bottom and in the midrange. That’s right where you need smooth, predictable power to get your drive going. And you also have to carry extra fuel so the engine can waste it in such richness.

Electronic fuel-injection has banished that nonsense on streetbikes, so why can’t we have its benefits off-road? Weight and cost

have been the big reasons. Electronic systems usually mean batteries, which are heavy. Developing new systems costs money that off-road buyers might not have.

Suzuki’s answer is no battery. An 18-pole single-phase alternator on the crankshaft charges a capacitor pack. Power for the fuel-injector’s control circuitry, the electromagnetic injector valve in the 43mm throttle body and the in-tank fuel pump is taken from that. Stab the kickstart lever once to charge the capacitors and the second stab starts the engine. Engine stalled on the course? Charge remains in the system, enabling an immediate restart.

A problem of earlier single-cylinder EFI was that if rpm was sensed at one point in crank rotation, rather than averaged over the whole 720-degree cycle, the light crank’s large rpm flutter (faster after firing, slower coming up on compression) could give false information to the injection computer. Suzuki’s magneto-style alternator provides crank-position sensing, measures rpm and signals which stroke the engine is on. With that information, throttle angle and a stored fuel map, the system has all it needs. —Kevin Cameron