EFI - Electronic Fuel Injection

For the most part, motorcycles and carburetors have come to a parting of the ways. While a few carburetor-equipped bikes are still on the market, the vast majority of street bikes are now equipped with some form of electronic fuel injection, which despite the complication it adds, is both a better way to go, and undeniably here to stay.

As a rule, internal combustion engines achieve their best performance with air-fuel ratios that hover in the 14.7 to 1 range. Although there's always some latitude, Leaner mixtures tend to degrade performance, while richer ones waste fuel and increase emissions, without substantially increasing performance.

Because lean settings create drivability problems, motorcycles in the pre-emission law days were normally set up on the rich side. This made them easy to start, quick to warm up and provided good performance. True, economy wasn't what it might have been and tail pipe emissions were off the charts, but at the time, gas was cheap and emissions unregulated. As long as the bike ran decently and raw fuel wasn't dripping from the pipes, no one cared how rich their jetting was.

Unfortunately those days are coming to end, and here's why; Because carburetors rely on fixed orifice jets, overlapping fuel circuits, and volumetric pressure to deliver the correct fuel/air mixture there's not a whole lot of adaptability to them, at least not until you break out the screwdrivers and start changing things, so jetting is often a compromise between slightly lean at some throttle openings and slightly rich at others. This leads to things like slow warm ups, surges at small throttle openings and emission outputs that are borderline legal, and may be pushed over the edge by even the slightest adjustment or change in jetting.

On the other hand, electronic fuel injection systems employ a variety of sensors that tell a computer exactly what the engine is doing at any given moment. After comparing that information to a set of known parameters called a map, the computer determines exactly how much fuel is required to maximize power while creating the lowest emissions, then adjusts the air/fuel ratio accordingly.

Provided the map is accurately written, and with some minor exceptions they're usually pretty good, this allows the engine to receive the ideal mixture under every circumstance, neither so lean that it creates problems nor so rich that it exceeds emission standards, and it's for that reason that they've become the fuel delivery system of choice on everything from scooters to superbikes.

EFI Basics

In its simplest terms, an electronic fuel injection (EFI) system is a computer controlled fuel delivery system. This electronic control unit (ECU) reads various sensors located on the vehicle and makes the determination of how much fuel to allow the engine to have based on this information. The computer will open and close the injectors allowing gasoline into the engine based on the sensor inputs and the fuel map programmed into the computer. The various sensors (RPM, engine temperature, air temperature, throttle position, manifold pressure, crankshaft position) provide information on operating conditions and load on the engine. Figure 1 is a detailed drawing of a typical fuel injection system and the sensors that might be in place. The EFI Components table provides a detailed description of each of the major components.

We will limit our discussion to the types of fuel injection systems that are likely to be encountered on motorcycles. One of the major differences between fuel injection systems is how the sense the amount of load that is placed on the engine. The load can be sensed by how far the rider has twisted the throttle and MAP (manifold absolute pressure). EFI systems that sense engine load by the use of the throttle or TPS (throttle position sensor) are referred to as an Alpha-N system. EFI systems that sense engine load by the use of a MAP sensor are referred to as a Speed-Density system. Harley-Davidson has used both types of EFI systems on its vehicles. While many of the sensors can be the same in both types of EFI systems, there is a significant difference in the way that each system determines what the load on the engine actually is. The Magneti-Marelli system used on EVO and Twin Cam FLH's until 2001 and Buell XL engines is an Alpha-N EFI system. On these bikes, load sensing is determined by the throttle position. The newer Delphi EFI system used on current Twin Cam engines is a Speed Density system. Speed Density EFI systems determine the engine load based on the intake manifold vacuum.

It is a sad day in Hogville.

The mighty Harley is slowly loosing its carburetor. For those of you who haven't noticed or are stuck back in the 80's, a large number of the bikes coming from Milwaukee have fuel injection installed. International environmental concerns have slowly forced the change from carburetor to electronic fuel injection. Harley purists are mourning the loss of the venerable carburetor. You can be sure that it will not be many years before all V-twins have fuel injection as standard equipment.

Sources: Motorcycle Cruiser, Nightrider