The Fueling of our Motorcycle's Engine

Off all the bits we borrowed from the first combustion engine, it is probably the carburetor which looks most prehistoric. I mean, little bowls of petrol? Brass holes and tapered needles? It's like something off an industrial era steam engine, not a modern motorcycle. Compare with the dual-valve, dual-injector engine management system on the average 2012 big bike. There are some physical similarities – you have a barrels leading into the cylinder head, with an air valve controlled by the throttle twist-grip, and a means of introducing petrol into the air flow. But that's where any similarities end.

The move from carburetors to fuel injection is like the move from analog music to digital – you have an analog passive, self-regulating mechanism replaced by an active, digitally-regulated system. Old motorcycles where fueled by predictable physical actions on suitably-sized components: the air is drawn through a venturi which speeds up its flow, and dropping its pressure. This pressure drop sucks petrol up through small brass jets in an appropriate ratio. And as the rider opens or closes the throttle, more or less fuel/air mixture is introduced.

Compare this elegant, almost clockwork system with a modern motorcycle's electronic fuel injection. A computer is frantically watching every major parameter on the engine – from RPM, temperature, crank position, throttle opening and air pressure, comparing it to look-up tables of fuel injector opening times, then sending an electrical signal to the injectors to open and close for that time. This all happens in tiny fractions of a second, and happens constantly. Even when the motorcycle is just idling, the ECU is making millions of calculations a second, it never gets a rest.
The result is good – but as an audiophile would tell you, analog has its strengths too. The old motorcycle's carburetors may look prehistoric, but they gave smooth, progressive fueling. Accelerator pumps add extra fuel to compensate for sudden opening of the slides, the tapered needle gradually meters more fuel out until full throttle, and the brass jets inside the body are carefully selected to give optimum fueling.

Where the electronic fuel injection wins out is in two main areas – easier emissions control, and adjustability. The engine management carefully manages the mixture so there's no excess fuel in the exhaust, improving economy and cutting emissions. Electronic fuel injection also allows a catalyst to be used in the exhaust, further reducing hydrocarbons and carbon monoxide in the exhaust.
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