Fuel Injection the Best Motorcycle Fueling


A few months back we talked about carburetors. Now we take a look at the fueling system that's ruled them antiquated; Fuel Injection.

Your first question is probably, what's the advantage of fuel injection over carburetors. Fuel injection allows for more accurately controlled fuel metering. Whereas carburetors deliver fuel dependent only on the velocity of air being drawn into the engine and throttle position, fuel injection can deliver and accurately metered amount of fuel based on many engine and environmental factors, such as; engine rpm, throttle position, engine coolant temperature, intake are pressure, exhaust oxygen (lamda) content and road speed.

Basing fuel delivery on so many parameters allows the fuel injection system to deliver just the right amount of fuel to the engine at any given time. The rather excellent benefits of doing this are; increased engine power, lower fuel consumption, less engine wear and lower exhaust emissions.

Fuel injection is user friendly too, as coolant temperature is monitored, cold start enrichment can be catered for automatically and as atmospheric pressure is monitored, fuel injection systems are able to automatically make the required fueling adjustment when traveling at high altitudes, they even make tiny alterations to fuel delivery based on day to day changes in the weather. Fuel injection systems that monitor exhaust oxygen content with lamda sensors can even adjust the fueling to compensate for engine wear.
Unlike fuel injection systems, carburetors need to present a certain amount of restriction to the intake air to allow fuel to be drawn off into the engine. Fuel injection's strengths can also be its weakness though; accurate transient fueling can lead to snatchy throttle response, especially compared to the smooth and steady throttle response rider's used to enjoy with CV carburetors.

Early fuel injection systems controlled air-flow into the engine with just a single throttle butterfly, with this system, opening the twist-grip rotates a disc (butterfly) in the throttle body through 90 degrees; this allows the engine to draw in as much air as it can past the butterfly. As these systems also used primitive single-hole injectors that struggled to accurately meter fuel flow at very low delivery rates and injection control modules with relatively small memories, correct fuel delivery at low engine speed and rapidly changing airflow rates wasn't always achieved.

Motorcycle fuel injection systems have come a long way in recent years. Many systems now utilize two sets of throttle butterflies; one controlled by the rider and a secondary set controlled by the ECU (Electronic/Engine Control Unit). When the rider opens the throttle, the ECU opens the secondary butterflies too, but at a per-determined rate, much like the slides on a CV carburetor. This allows the ECU to gradually increase engine load and more accurately meter fuel delivery.

Fuel injector design has improved also the number of holes found in fuel injectors have increased from one in the early years, to 12 now. Multi-hole injectors offer improved fuel atomization and distribution, this improves combustion and allows the injectors to be placed closer to the engine which also improves transient throttle fueling.

Most motorcycle fuel injections systems are very similar; almost all are now multi-point closed loop systems with either ECU controlled secondary butterflies of 'fly-by-wire' throttle control. The vast majority of the bigger machines now have two injectors per cylinder. The second set located above the inlet trumpets in the airbox; at very high rpm the secondary injectors spray fuel into the trumpet mounts giving the fuel the longest possible time to become evenly distributed in the intake charge and form a nice homogeneous easily burnt mixture.

Some modern motorcycles have diagnostic ports on their ECU's that allow them to be re-programmed, the internal fuel map can altered at will. More older motorcycles and lower budget motorcycles don't have this facility; the best way to adjust the fueling on these motorcycles is with an interceptor device like a Dynojet Power Commander. These devices intercept the fuel injector signal from the motorcycle's ECU and then apply a programmer to add or remove a percentage of fuel, any engine load or speed the tuner desires.

Most motorcycle on the market today would benefit from having their stock fueling 're-mapped'.

Exhaust pollution and noise emission legislation places constraints on motorcycle manufacturers. For example, some motorcycles fuel quite poorly at the rpm range that corresponds to the top-gear drive-by noise emission test speed. This often hinders the motorcycle with poor throttle response and rough running at these rpm ranges.

Many motorcycles also run too rich at very high rpm, the reason for this is engine Preservation; a slightly rich mixture keeps the pistons cool at high rpm and provides a buffer in case any modifications made to the motorcycle by the owner result in a dangerous weakening of the fuel mixture, like fitting a aftermarket exhaust system for example.

Direct fuel injection. Squirting the fuel directly into the combustion chamber, not the intake tract, opens up some pretty exciting opportunities. Starter-less starting for one; provided the engine is always stopped with at least one cylinder on its compression stroke, the engine could be restarted by injecting fuel directly into that combustion chamber and igniting it.

More power and improved fuel efficiency for another, direct injection allows engines to run with higher compression ratios and leaner mixtures at cruising speeds.

Direct fuel injection is pretty well accepted as the future for petrol powered cars, once the systems become compact and light enough to be fitted to motorcycles, direct fuel injection could provide a significant leap forward in motorcycle performance and economy.
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