The basic chemistry inside your motorcycle engine is to take petrol, break it up and then allow the carbon to combine with oxygen in the air to form carbon dioxide (CO2), and the hydrogen to combine with oxygen to form water – releasing energy in the process. But how much of each do you need?
Petrol is complex, but it has a basic structure of four carbon to 10 hydrogen atoms. Each such unit when burned would make four CO2 molecules and five water molecules, consuming 13 oxygen atoms in the process. If you stir in the weight of each type of atom, you get a weight ratio around three of oxygen to one of petrol.
Only about 20 percent of air is oxygen, the other 80 percent – mostly nitrogen – is just an inert filler as far as an engine is concerned. Therefore, the optimum air-fuel ratio is approximately 15 of air to one of petrol. In practice, sometimes you need it richer – during starting, for example, when the engine is cold and fuel condenses in the intake manifold. You also want a slightly rich mixture at full throttle/high rpm because it limits maximum temperatures and flame duration inside the cylinder, and helps prevent some of the bits in there burning out.
You can easily tolerate a slightly weaker mixture for cruising and enjoy the benefit of a better consumption. Achieving this at all throttle positions and over the full engine-speed range is not as simple as it might seem. Up until relatively recently, carburetors managed to do the job but emission control, fuel economy, performance and economic factors are leading to the deployment of electronic fuel injection on more and more motorcycles. A fuel injection system has two basic functions: measuring mass airflow and metering fuel accordingly. Let's have look at both of these aspects in turn.
Early electronic injection systems sensed airflow by having a spring-loaded vane in the induction system and measuring the deflection of the vane. This was a bit complicated since it also involved measuring air temperature to get its density. The engine controlled did the sums to multiply air speed by density to get mass airflow. That's not a problem, but the real drawback of a vane is that it's an expensive moving part that has to be made precisely and it can gum up. It also partly obstructs airflow and therefore limits efficiency and power to some extent.
A better and more precise way to measure mass airflow directly is to pass the airstream over a fine platinum wire that is heated by an electric current. The air passing over the wire cools it, reducing its resistance and causes it to take more current. The magnitude of the current forms forms a signal proportional to airflow. Apart from accuracy, the benefits include no moving parts and almost zero obstruction of the airflow. The wire is quite delicate though, so air-filters are an absolute must with these systems.
Some more recent systems simply measure pressure difference either side of the throttle body, throttle position and inlet air temperature. These measurements can then be looked-up by the motorcycle in a table stored inside the engine control unit (ECU) of measured airflow from test engines. These Manifold Air Pressure (MAP) sensors have the benefit of being easy to package – they can be mounted anywhere and just connected by hoses to the manifold.
How about metering? That's the simple part really. All you need to do is supply fuel at a constant pressure to the injection nozzles in the engine inlet ports. The nozzles have needle valves that are opened by little, build-in solenoids (electromagnets that move a mechanical actuator). The important point is that the injector has only two states – fully open and fully closed, there is nothing in between. Fuel metering is then just a matter of controlling how long the injector is open.
Fuel pressure is typically quite high, around five to 10-bar, so the plumbing definitely needs to be kept in good shape. The characteristic fast-slow noise from the fuel pump on an injection-equipped motorcycle when you turn on the ignition comes from the fuel system building up to working pressure.
The link between the measuring and metering functions is the ECU. It's a specialized computer that calculates injector opening periods and timing based on mass airflow and a whole host of other inputs from sensors – engine temperature, throttle position, engine speed and crank position being the principal ones. This allows automatic cold start enrichment, idle speed stabilization, rev-limiting and over-run fuel cut-off as simple, software-driven functions. The computations inside the ECU are a combination of direct calculation and look-ups from stored data from test engines in standard form. These are both amenable to tweaking, for more performance at the expense of fuel economy, for example, by swapping controller chips. Just to make it more fun, engine controllers usually also control ignition timing based on the same sensor inputs.
This is all very clever, but it is still what's called an open-loop system – basically a dumb computation of fuel metering according to simple sensor input predicting what will happen in the engine.
The final tweak in the latest systems is to measure actual combustion characteristics by looking at the amount of oxygen in the exhaust, and fine-tuning the fueling based on that.
It's easy to think of electronic fuel injection as a performance-related innovation but, these days, it's as much about minimizing emissions, maximizing fuel economy and lowering production costs as anything else.