MEFI Fuel Injection Systems

Fuel Injection

The injectors are usually fitted into the inlet manifold and inject pressurized fuel into the air stream entering the combustion chamber.

The electronic fuel injector is normally closed, and opens to inject pressurized fuel as long as electricity is applied to the injector's solenoid coil. The duration of this operation, called pulse width, is proportional to the amount of fuel desired. The electric pulse may applied in closely-controlled sequence with the valve events on each individual cylinder (in a sequential fuel injection system), or in groups of less than the total number of injectors (in a batch fire system).

Since the nature of fuel injection dispenses fuel in discrete amounts, and since the nature of the 4-stroke-cycle engine has discrete induction (air-intake) events, the ECU calculates fuel in discrete amounts. In a sequential system, the injected fuel mass is tailored for each individual induction event. Every induction event, of every cylinder, of the entire engine, is a separate fuel mass calculation, and each injector receives a unique pulse width based on that cylinder's fuel requirements.

It is necessary to know the mass of air the engine "breathes" during each induction event. This is proportional to the intake manifold's air pressure/temperature, which is proportional to throttle position. The amount of air inducted in each intake event is known as "air-charge", and this can be determined using a MAF/MAP sensor.

The three elemental ingredients for combustion are fuel, air and ignition. However, complete combustion can only occur if the air and fuel is present in the exact stoichiometric ratio, which allows all the carbon and hydrogen from the fuel to combine with all the oxygen in the air, with no undesirable polluting leftovers. Oxygen sensors monitor the amount of oxygen in the exhaust, and the ECU uses this information to adjust the air-to-fuel ratio in real-time.

One benefit of a well-presented fuel delivery to the air is that the fuel will evaporate (turn to vapor) as it enters the air stream, removing heat from the surrounding air. This leads to both a slightly denser air charge – allowing more fuel to be burned and hence giving more torque – while also helping to reduce the likelihood of detonation within the engine.

If it’s done properly, with the enough time for the fuel to vaporize, then this can improve overall efficiency by up to 3 per cent. This is why running an engine slightly rich (excessive fuel for the amount of air) can sometimes help to give a cooling effect. The ideal ratio of air to fuel, where the fuel will burn completely (known as the Stoichiometric ratio) is 14.6:1 air/fuel.

However, high performance and race engine tend to run slightly rich to exploit this cooling effect. In addition, turbo engines also tend to have a slight excess of fuel for the same reason. An engine running lean can suffer from detonation, though, as it can’t generally handle as much spark advance.

Something else to consider is that fuel in the air stream will displace air, so it’s important to get the balance between the two correct. This is one of the reasons that direct engine is good – all the air is in the engine before you introduce the fuel, so it can’t displace air. However, it also means that there’s a very short amount of time for the fuel to vaporize, and hence reduce the temperature by a significant degree.

Additionally the octane rating of the fuel is of importance. The octane rating for the fuel basically indicates the amount it can be compressed before it ignites spontaneously, or detonates. So a higher octane-rated fuel will be able to deal with higher compression ratios or boost pressures better than a lower one, and will resist detonation much better.

Different fuels can also oxygenate the mixture as more air is pumped in. The best pump fuel is rated at 93 octane (Premium) but for motor-sport applications, fuels with octane ratings up to 117 (C17) can be used.

Supercharger

The supercharger is run mechanically by the engine. Its main function is to compress air and force it into engine, thereby more fuel can be burnt and thus more power can be generated. The supercharger has been around for many years, almost as long as the internal combustion engine itself.

The term supercharging technically refers to any pump that forces air into an engine - but in common usage, it refers to pumps that are driven directly by the engine as opposed to turbochargers that are driven by the pressure of the exhaust gases. A supercharger can be powered mechanically by belt, gear, or chain-drive from the engine's crankshaft.

Superchargers draw their power directly from the crankshaft. Most are driven by an accessory belt, which wraps around a pulley that is connected to a drive gear. The drive gear, in turn, rotates the compressor gear. The rotor of the compressor can come in various designs, but its job is to draw air in, compress the air into a smaller space and discharge it into the intake manifold. To compress the air, a supercharger must spin rapidly -- more rapidly than the engine itself. Making the drive gear larger than the compressor gear causes the compressor to spin faster. Superchargers can spin at speeds as high as 50,000 to 65,000 rotations per minute (RPM).

A compressor spinning at 50,000 RPM translates to a boost of about six to nine pounds per square inch (psi). That's six to nine additional psi over the atmospheric pressure at a particular elevation (atmospheric pressure at sea level is 14.7 psi).
There are three types of superchargers:

  • Roots
  • Twin-screw
  • Centrifugal

Supercharger Installed

The main difference is how they move air to the intake manifold of the engine. Roots and twin-screw superchargers use different types of meshing lobes, and a centrifugal supercharger uses an impeller, which draws air in. Although all of these designs provide a boost, they differ considerably in their efficiency. Each type of supercharger is available in different sizes, so that a match can be done to the engine attributes, such as engine displacement.

The biggest disadvantage of superchargers is that they steal power as it helps generate additional power. Because the crankshaft drives them, they must steal some of the engine's horsepower. A supercharger can consume as much as 20 percent of an engine's total power output. But because a supercharger can compress air and help increase more power, most think the trade-off is worth it.

An additional benefit is, compared to a turbocharger, is the instant response. As the supercharger is driven off the crankshaft, it will increase speed in the same rate as the engine, thereby having instant response and no lag. The Turbo Charger on the other hand must spool up as exhaust pressures increase thus providing non-parasitic horse power increase.

Fuel Control

All fuel injection systems feature the same basic components: a device to determine the amount of air going into the engine (usually a ‘mass of air flow’ – MAF- sensor), an ECU (to determine how much fuel to add) and the injectors themselves (to squirt the fuel into the inlet airstream).

A MAF sensor does as its name suggests. It’s a device in the upstream air flow of the air intake system that measures exactly how much air, by mass, as it enters the induction system of the engine. The signal is fed into the ECU. The typical variable that are used by the ECU to calculate a corrected air mass value are based on additional signals furnished by:

  • (IAT) Intake Air Temperature
  • (ECT) Engine Coolant Temperature
  • (RPM) Engine Speed
  • (MAP) Manifold Air Pressure 1 BAR, 2 BAR, or 3 BAR

From these values at a specific time it will look up, in a series of pre-set and pre-stored 2D and 3D maps, what amount of fuel needs to be injected (injector pulse width) and when, it will deliver the required amount of fuel.

A MAF sensor is a very accurate measure of the air entering an engine, but it’s also a relatively slow-reacting one, since it needs to measure the air flow where it is located and send the information back to the ECU. Therefore there can be a slight delay between the driver snapping throttle open and the increase in airflow being registered. However, most