Fuel Injection Basics
Fuel injection started to replace carburetors in the 1980s and completely took over in the 1990s. Fuel injection is much more precise than carburetors. The injectors are controlled by the PCM which considers many inputs before calculating the amount of fuel to be injected. For more on management systems see our management system page. This allows for better fuel economy, lower emissions, better warm-up/cold starting and even longer engine life. There are a few different kinds of fuel injection systems, some work better than others.
Multi-Port Fuel Injection
Mutli-port fuel injection is the most common fuel injection system on the road today. In this system, fuel is pumped by the electronic fuel pump at about 25-45psi through the fuel filter to the fuel rail. The fuel pressure regulator sits at the end of the fuel rail, its job is to adjust fuel pressure in the fuel rail. The injectors sit between the fuel rail and the intake manifold/runners. When the injectors are opened by the PCM, pressurised fuel is prayed into the intake manifold/runners. This is done during a cylinders intake stroke to pull fuel and air into the engine. The fuel that the fuel pressure regulator allows through to control rail pressure is returned to the tank through the return line.
Return-less systems are a common variation of the multi-port fuel injection system. This system keeps rail pressure constant, the PCM has to compensate for intake manifold pressure/vacuum when deciding how long to keep the injectors open. Fuel pressure can be regulated at the fuel filter or in the sending unit itself (both pictured). The only real advantage to this setup is that the return line is not needed.
Throttle Body Injection
A simple explanation of throttle body injection is a computer controlled carburetor. These systems can use a single or multiple injectors located above the throttle body that injects fuel according to PCM commands. These systems acted as a transition from carburetors to port fuel injection. They were not around for very long and were quickly replaced by port fuel injection.
Gasoline Direct Injection (GDI)
In a gasoline direct injection system, fuel is injected directly into the combustion chamber under high pressure like a diesel. Fuel must be injected under high pressure to overcome cylinder compression pressure. GDI has several advantages over other injection systems. GDI engines can run very lean, (30:1 air/fuel ratios are possible) this increases fuel economy and improves emissions (other than NOx). Injecting fuel directly into the cylinder decreases the possibility of engine knock, so higher compression ratios are possible. Since fuel is not sprayed into the intake manifold, the only thing coming in the intake port is air, which increases the engines volumetric efficiency. Fuel can also be injected whenever the system demands it, for instance, just before ignition. Fuel is pumped by the in-tank pump under moderate pressure to the high pressure fuel pump (HPFP) which is a mechanical pump usually driven by a separate camshaft lobe which steps the pressure up to anywhere from 500 to 2000+psi. Fuel is then directed to the fuel rail and injectors.
Pieces of a Fuel Injection System
Fuel tanks can be metal or plastic, metal tanks tend to rust over time, contaminating the fuel system for months before actually leaking. Tanks are designed to make sure the fuel pump pick-up is submerged in fuel even when the vehicle takes a hard corner, or whenthe fuel level is getting low.
Fuel Pump/Sending Unit
The fuel pump makes the pressure and flow that the system needs for fuel injection. They have a screen at the fuel pick-up it protect the pump from large particles in the fuel but this is not enough protection for the injectors, which is why a fuel filter is needed. The pump is usually one piece with the sending unit, which is a float attached to a potentiometer or rheostat, which detects fuel level. The pump will also have a pressure relief valve, if system pressure approaches dangerous levels, the pressure relief valve will open, allowing some fuel to return to the tank.
Fuel is supposed to be filtered before it goes into your tank but sometimes that is not enough. Debris can end up in your fuel system from a rusting metal fuel tank, debris from the in ground tanks at the fuel station, or from rusting metal fuel lines. The fuel filter filters the fuel before it gets to the injectors, particles in the fuel can clog the injectors and cause many driveability problems. Manufactures will have a set interval for replacing the fuel filter.
The fuel rail houses the top of the injectors and supplies fuel to them. The fuel pressure regulator sits at the end of the rail on a return system.
Fuel Pressure Regulator
The fuel pressure regulator sits at the end of the fuel rail. Its job is widely misunderstood among automotive enthusiasts. Under no circumstances does the fuel pressure regulator “add fuel” to the mixture, that is the job of the PCM. The fuel pressure regulators job is to make sure that when the computer demands an injector open for say 3ms that “x” amount of fuel is sprayed into the manifold. If the manifold has high vacuum, more fuel will be sprayed/pulled into the manifold, if there is pressure made by the turbo, less fuel will be sprayed in. The fuel pressure regulator has a vacuum hose connected to the intake manifold to sense manifold vacuum. Manifold vacuum moves a diaphragm which controls how much fuel is allowed to return to the tank, which regulates fuel rail pressure.
Fuel lines carry fuel from one component to the next. They can be plastic or metal, metal lines tend to rust like the metal fuel tanks.
The injectors sit between the fuel rail and the intake manifold, they are electronically controlled by the PCM. The PCM will supply constant power to the injectors and give ground when it wants them to open. Inside the injector is a solenoid which will move a pintle or valve when current flows through it, allowing fuel into the intake manifold. The injectors must also atomize the fuel for combustion.
Throttle Body Injector
Throttle body injectors sit above the throttle plate and spray fuel down into the intake manifold in a TBI system. These units are PCM controlled and contain the fuel pressure regulator.
High Pressure Fuel Pump
The HPFP is found on GDI systems and step up the fuel pressure necessary for injection directly into the cylinder. They are usually driven off an extra lobe on the camshaft.