3.2 Gasoline Fuel Injection Systems

A modern gasoline injection system uses pressure from an electric fuel pump to spray fuel into the engine intake manifold. Like a carburetor, it must provide the engine with the correct air-fuel mixture for specific operating conditions. Unlike a carburetor, however, PRESSURE, not engine vacuum, is used to feed fuel into the engine. This makes the gasoline injection system very efficient. A gasoline injection system has several possible advantages over a carburetor type of fuel system. Some advantages are as follows:

• Improved atomization. Fuel is forced into the intake manifold under pressure that helps break fuel droplets into a fine mist.

• Better fuel distribution. Each cylinder receives an equal flow of fuel vapors.

• Smoother idle. Lean fuel mixture can be used without rough idle because of better fuel distribution and low-speed atomization.

• Lower emissions. Lean, efficient air-fuel mixture reduces exhaust pollution.

• Better cold weather drivability. Injection provides better control of mixture enrichment than a carburetor.

• Increased engine power. Precise metering of fuel to each cylinder and increased air flow can result in more horsepower output.

• Fewer parts. Simpler, late model, electronic fuel injection systems have fewer parts than modern computer-controlled carburetors.

There are many types of gasoline injection systems. Before studying the most common ones, you should have a basic knowledge of the different classifications. Systems are classified as either single- or multi-point injection and as either indirect or direct injection.

The point or location of fuel injection is one way to classify a gasoline injection system. A single-point injection system, also call throttle body injection (TBI), has the injector nozzles in a throttle body assembly on top of the engine. Fuel is sprayed into the top center of the intake manifold.

A multi-point injection system, also called port injection, has an injector in the port (air- fuel passage) going to each cylinder. Gasoline is sprayed into each intake port and toward each intake valve. Thereby, the term multi-point (more than one location) fuel injection is used.

An indirect injection system sprays fuel into the engine intake manifold. Most gasoline injection systems are of this type. Direct injection forces fuel into the engine combustion chambers.

There are several basic configurations of gasoline fuel injection we will discuss the timed, and throttle body.

Timed Fuel Injection System

Timed fuel injection systems for gasoline engines inject a measured amount of fuel in timed bursts that are synchronized to the intake strokes of the engine. Timed injection is the most precise form of fuel injection but is also the most complex. There are two basic forms of timed fuel injection: mechanical and electronic.


Figure 3-7 — Mechanical-timed injection system.

The basic operation of a mechanical-timed injection system (Figure 3-7) is as follows:

A high-pressure electric pump draws fuel from the fuel tank and delivers it to the metering unit. A pressure relief valve is installed between the fuel pump and the metering unit to regulate fuel line pressure by bleeding off excess fuel back to the tank.

The metering unit is a pump that is driven by the engine camshaft. It is always in the same rotational relationship with the camshaft so it can be timed to feed the fuel to the injectors just at the right moment.

Each injector contains a spring-loaded valve that is opened by fuel pressure, injecting fuel into the intake at a point just before the intake valve opens.

The throttle valve regulates engine speed and power output by regulating manifold vacuum, which, in turn, regulates the amount of fuel supplied to the injectors by the metering pump.


Figure 3-8 — Electronic fuel injection.

The more common type of timed fuel injection is the electronic-timed fuel injection, also known as electronic fuel injection, or EFI (Figure 3-8). An electronic fuel injection system can be divided into four subsystems:

• Fuel delivery system

• Air induction system

• Sensor system

• Computer control system

The fuel delivery system of an EFI system includes an electric fuel pump, a fuel filter, a pressure regulator, the injector valves, and the connecting lines and hoses.

The electric fuel pump draws fuel out of the tank and forces it into the pressure regulator.

The fuel pressure regulator controls the amount of pressure entering the injector valves. When sufficient pressure is attained, the regulator returns excess fuel to the tank. This maintains a preset amount of fuel pressure for injector valve operation.

The fuel injector for an EFI system is a coil or solenoid-operated fuel valve. When not energized, spring pressure keeps the injector closed, keeping fuel from entering the engine. When current flows through the injector coil or solenoid, the magnetic field attracts the injector armature. The injector opens, squirting fuel into the intake manifold under pressure.

The air induction system for the EFI typically consists of a throttle valve, sensors, an air filter, and connecting ducts.

The throttle valve regulates how much air flows into the engine. In turn, it controls engine power output. Like the carburetor throttle valve, it is connected to the gas pedal. When the pedal is depressed, the throttle valve swings open to allow more air to rush into the engine.

The EFI sensor system monitors engine operating conditions and reports this information to the computer. A sensor is an electrical device that changes circuit resistance or voltage with a change in a condition (temperature, pressure, position of parts, etc.). For example, the resistance of a temperature sensor may decrease as temperature increases. The computer can use the increased current flow through the sensor to calculate any needed change in the injector valve opening. Typical sensors for an EFI system include the following:

• Exhaust gas or oxygen sensor

• Manifold pressure sensor

• Throttle position sensor

• Engine temperature sensor

• Air flow sensor

• Inlet air temperature sensor

• Crankshaft position sensor

Since some of these sensors were discussed in the section on computerized carburetor systems, we will concentrate only on the sensors that are particular to the EFI system. These sensors are as follows:

The throttle position sensor is a variable resistor connected to the throttle plate shaft.

When the throttle swings open for more power or closes for less power, the sensor changes resistance and signals the computer. The computer can then enrich or lean the mixture as needed.

The air flow sensor is used in many EFI systems to measure the amount of outside air entering the engine. It is usually an air flap or door that operates a variable resistor.

Increased air flow opens the air flap more to change the position of the resistor. Information is sent to the computer indicating air inlet volume.

The inlet air temperature sensor measures the temperature of the air entering the engine. Cold air is denser, requiring a little more fuel. Warm air is not as dense as cold, requiring a little less fuel. The sensor helps the computer compensate for changes in outside air temperature and maintain an almost perfect air-fuel mixture ratio.

The crankshaft position sensor is used to detect engine speed. It allows the computer to change injector openings with changes in engine rpm.

The signal from the engine sensors can be either a digital or an analog type output. Digital signals are on/off signals. An example is the crankshaft position sensor that shows engine rpm. Voltage output or resistance goes from maximum to minimum, like a switch. An analog signal changes in strength to let the computer know about a change in condition. Sensor internal resistance may smoothly increase or decrease with temperature, pressure, or part position. The sensor acts as a variable resistor.

Basic operation of an electronic-timed injection system is as follows:

Fuel is fed by a high-pressure electric fuel pump to the injectors that are connected in parallel to a common fuel line.

The fuel pressure regulator is installed in line with the injectors to keep fuel pressure constant by diverting excess fuel back to the tank.

Each injector contains a solenoid valve and is normally in a closed position. With a pressurized supply of fuel behind it, each injector will operate individually whenever electric current is applied to the solenoid valve.

The electronic computer sends the electric impulses and provides the proper amount of fuel. The computer receives a signal for the ignition distributor to establish the timing sequence.

By sending electric current impulses to the injectors in a sequence timed to coincide with the needs of the engine, the system will supply fuel to the engine as it should.

Throttle Body Injection System

The throttle body injection (TBI) system uses one or two injector valves mounted in a throttle body assembly (Figure 3-9). The injectors spray fuel into the top of the throttle body air horn The TBI fuel spray mixes with the air flowing through the air horn. The mixture is then pulled into the engine by intake manifold vacuum. The throttle body injection assembly typically consists of the following: throttle body housing, fuel injectors, fuel pressure regulator, Idle air control sensor, throttle position sensor, and throttle plates.


Figure 3-9 — Throttle body injection.

The throttle body housing, like a carburetor body, bolts to the pad on the intake manifold.

It houses the metal castings that hold the injectors, the fuel pressure regulator, and the throttle plates. The throttle plates are located in the lower section of the body. A linkage or cable connects the throttle plates with the accelerator pedal. An inlet fuel line and an outlet return line connect to the fittings on the body.

The throttle body injector consists of an electric solenoid coil, armature or plunger, ball or needle valve and seat, and injector spring. Wires from the computer connect to terminals on the injectors. When the computer energizes the injectors, a magnetic field is produced in the injector coil. The magnetic field pulls the plunger and valve up to open the injector. Fuel can then squirt through the injector nozzle and into the engine.

The throttle body pressure regulator consists of a fuel valve, a diaphragm, and a spring. When fuel pressure is low, the spring holds the fuel valve closed, causing pressure to build as fuel flows into the regulator from the fuel pump. When a preset pressure is reached, pressure acts on the diaphragm. The diaphragm compresses the spring and opens the fuel valve. Fuel can then flow back to the fuel tank, limiting the maximum fuel pressure at the injectors.

Although throttle body injection does not provide the precise fuel distribution of the direct port injection, it is much cheaper to produce and provides a much higher degree of precision fuel metering than a carburetor.

Multi-Port Injection

Multi-port injection systems use a computer, engine sensors, and one solenoid injector for each cylinder. This is the most common fuel injection system in late model vehicles. The multi-port injection system operates similar to that of a throttle body injection system, except that the fuel is injected at each intake port instead of at the top of the intake manifold.

A fuel rail feeds fuel to the injectors. It connects the main fuel line to the inlet of each injector. The injector is pressure fit into a port in the intake manifold. Each injector is aimed to spray fuel toward the engine’s intake valve. Each injector is connected to the computer and is electronically fired just before the intake valve is to open.

Direct Injection

Direct injection has been around for many years on a diesel engine. With gasoline direct injection (GDI), the gasoline is injected directly into the combustion chamber (Figure 3-10). To make this possible, specially designed injectors deliver fuel into the high pressures and temperatures in the cylinders. To prevent heat from igniting the fuel in the injector, the injectors are designed to completely seal after the fuel is sprayed. The injectors must also be able to spray the fuel at a higher pressure than what is in the cylinder.


Figure 3-10 — Gasoline direct injection.

GDI allows for very lean operation during cruising. When the engine is operating under heavy loads, the system provides near perfect air-fuel mixture. The ability to run at such lean mixtures allows for increased fuel economy, nearly 30%.

Spraying the fuel directly into the cylinder also increases volumetric efficiency. GDI decreases an engine’s tendency to knock, allowing for a higher compression ratio without the need for a higher octane gasoline.

With GDI, fuel can be injected at any time, not just during intake. The injectors can pulse twice during the transition from the compression stroke to combustion. The two pulses promote complete combustion when the PCM senses operating conditions may prevent a complete burning of fuel.

The fuel is injected into the cylinder under enormous pressure, typically between 400 and 1,500 psi. The injector delivers a relatively small, precisely shaped burst of fuel around the spark plug just before ignition. This means only the area around the spark plug has air and fuel to begin combustion; the rest of the chamber is filled with air.

The fuel pump that delivers this high pressure is driven by the engine. The pump is fed by an in-tank electric fuel pump. A PCM controls the timing of injection and ignition for each cylinder.

Test your Knowledge

4. A gasoline injection system has all these advantages over a carburetor, except which one?

A. Improved atomization
B. Better fuel distribution
C. Lower horse power
D. Lower emissions

5. What gasoline fuel injection system is the most precise and also the most complex?

A. Hydraulic-timed
B. Throttle body fuel
C. Timed fuel
D. Continuous fuel