Most electronic fuel injection systems only inject fuel during part of the engine's combustion cycle. The engine fuel needs are measured by intake airflow past a sensor or by intake manifold pressure (vacuum). The airflow or manifold vacuum sensor converts its reading to an electrical signal and sends it to the engine control computer. The computer processes this signal (and others) and calculates the fuel needs of the engine. The computer then sends an electrical signal to the fuel injector or injectors. This signal determines the amount of time the injector opens and sprays fuel. This interval is known as the injector pulse width.

Throttle body injection systems have a throttle body assembly mounted on the intake manifold in the position usually occupied by a carburetor. The throttle body assembly usually contains one or two injectors.

On port fuel injection systems, fuel injectors are mounted at the back of each intake valve. Aside from the differences in injector location and number of injectors, operation of throttle body and port systems is quite similar with regard to fuel and air metering, sensors, and computer operation.

Port-type continuous injection systems (CIS) have been used on many import vehicles. These systems deliver a steady stream of pressurized fuel into the intake manifold. The injectors do not pulse on and off as in port and throttle body systems. In CIS, the amount of fuel delivered is controlled by the rate of airflow entering the engine. An airflow sensor controls movement of a plunger that alters fuel flow to the injectors. When introduced, CIS was a mechanically controlled system. However, oxygen sensor feedback circuits and other electronic controls have been added to the system. CIS systems that have electronic controls are commonly referred to as CIS-E systems.

ELECTRONIC FUEL INJECTION SYSTEM Electronic fuel injection (EFI) has proven to be the most precise, reliable, and cost effective method of delivering fuel to the combustion chambers of today's vehicles. EFI systems must provide the correct air/fuel ratio for all engine loads, speeds, and temperature conditions. To accomplish this, an EFI system uses a fuel delivery system, air induction system, input sensors, control computer, fuel injectors, and some sort of idle speed control.

In a typical EFI fuel delivery system, fuel is drawn from the fuel tank by an in-tank or chassis-mounted electric fuel pump. Before it reaches the injectors, the fuel passes through a filter that removes dirt and impurities. A fuel line pressure regulator maintains a constant fuel line pressure that may be as high as 50 psi in some systems. This fuel pressure generates the spraying force needed to inject the fuel. Excess fuel not required by the engine returns to the fuel tank through a fuel return line.

THROTTLE BODY VERSUS PORT INJECTION

Throttle Body Fuel Injection

For some auto manufacturers, TBI served as a stepping stone from carburetors to more advanced port fuel injection systems. TBI units were used on many engines during the 1980s and are still used on some engines. The throttle body unit is similar in size and shape to a carburetor and, like a carburetor, mounts on the intake manifold. The injector(s) spray fuel down into a throttle body chamber leading to the intake manifold. The intake manifold feeds the air/fuel mixture to all cylinders.

TBI Operation The basic TBI assembly consists of two major castings: a throttle body with a valve to control airflow and a fuel body to supply the required fuel. A fuel pressure regulator and fuel injector are integral parts of the fuel body. Also included as part of the assembly is a device to control idle speed and one to provide throttle valve positioning data.

The throttle body casting has ports that can be located above, below, or at the throttle valve depending on the manufacturer's design. These ports generate vacuum signals for the manifold absolute pressure sensor and for devices in the emission control system, such as the EGR valve, the canister purge system, and so on.

The fuel pressure regulator used on the throttle body assembly is similar to a diaphragm-operated relief valve. Fuel pressure is on one side of the diaphragm and atmospheric pressure is on the other side. The regulator is designed to provide a constant pressure on the fuel injector throughout the range of engine loads and speeds. If regulator pressure is too high, a strong fuel odor is emitted and the engine runs too rich. On the other hand, regulator pressure that is too low results in poor engine performance or detonation can take place, due to the lean mixture.

The fuel injector is solenoid operated and pulsed on and off by the vehicle's engine control computer. Surrounding the injector inlet is a fine screen filter where the incoming fuel is directed. When the injector's solenoid is energized, a normally closed ball valve is lifted. Fuel under pressure is then injected at the walls of the throttle body bore just above the throttle plate.

TBI Advantages Throttle body systems provide improved fuel metering when compared to carburetors. They are also less expensive and simpler to service. TBI units also have some advantages over port injection. They are less expensive to manufacture, simpler to diagnose and service, and don't have injector balance problems to the extent that port injection systems do when the injectors begin to clog.

However, throttle body units are not as efficient as port systems. The disadvantages are primarily manifold related. Like a carburetor system, fuel is still not distributed equally to all cylinders, and a cold manifold may cause fuel to condense and puddle in the manifold. Like a carburetor, throttle body injection systems must be mounted above the combustion chamber level, which eliminates the possibility of turning the manifold design for more efficient operation.

Port Fuel Injection

PFI systems use one injector at each cylinder. They are mounted in the intake manifold near the cylinder head where they can inject a fine, atomized fuel mist as close as possible to the intake valve. Fuel lines run to each cylinder from a fuel manifold, usually referred to as a fuel rail. The fuel rail assembly on a PFI system of V-6 and V-8 engines usually consists of a left- and right-hand rail assembly. The two rails can be connected either by crossover and return fuel tubes or by a mechanical bracket arrangement. Fuel tubes crisscross between the two rails. Since each cylinder has its own injector, fuel distribution is exactly equal. With little or no fuel to wet the manifold walls, there is no need for manifold heat or any early fuel evaporation system. Fuel does not collect in puddles at the base of the manifold. This means the intake manifold passages can be tuned or designed for better low-speed power availability. The port-type systems provide a more accurate and efficient delivery of fuel. Some engines are now equipped with variable induction intake manifold that have separate runners for low and high speeds. This technology is only possible with port injection.

The throttle body in a port fuel injection system controls the amount of air that enters the engine as well as the amount of air that enters the engine as well as the amount of vacuum in the manifold. It also houses and controls the idle air control (IAC) motor and the throttle position sensor (TPS). The TPS enables the ECU to know where the throttle is positioned at all times.

The throttle body is a single, cast aluminum housing with a single throttle blade attached to the throttle shaft. The TPS and the IAC valve/motor are also attached to the housing. The throttle shaft is controlled by the accelerator pedal. The throttle shaft extends the full length of the housing. The throttle bore controls the amount of incoming air that enters the air induction system. A small amount of coolant is also routed through a passage in the throttle body to prevent icing during cold weather.

Port systems require an additional control system that throttle body injection units do not require. While throttle body injectors are mounted above the throttle plates and are not affected by fluctuations in manifold vacuum, port system injectors have their tips located in the manifold where constant changes in vacuum would affect the amount of fuel injected (at a given pulse width). To compensate for these fluctuations, port injection systems are equipped with fuel pressure regulators that sense manifold vacuum and continually adjust the fuel pressure to maintain a constant pressure drop across the injector tips at all times.

Pressure Regulator The pressure regulator in port injection systems is similar to the regulator used in TBI systems. A diaphragm and valve assembly is positioned on the center of the regulator, and a diaphragm spring seats the valve on the fuel outlet.

When fuel pressure reaches the setting of the regulator, the diaphragm moves against the spring tension, and the valve opens. This action allows fuel to flow through the return line to the fuel tank. The fuel pressure drops slightly when the pressure regulator valve opens, and the spring closes the regulator valve. In many systems, the regulator maintains fuel pressure at 39 psi.

A vacuum hose is connected from the intake manifold to the vacuum inlet on the pressure regulator. This hose supplies vacuum to the area where the diaphragm spring is located. This vacuum works with the fuel pressure to move the diaphragm and open the valve. When the engine is running at idle speed, high manifold vacuum is supplied to the regulator. Under this condition, the specified fuel pressure opens the regulator valve. When the engine is running under heavy load and/or wide-open throttle, a very low vacuum is supplied to the regulator. During these times, the vacuum does not help open the regulator valve and a higher fuel pressure is required to open the valve.

The change in fuel pressure allows the fuel to be sprayed into the manifold with same effect. regardless of the pressure present in the manifold. When there is a high vacuum in the manifold, a very low pressure exists and the pressure difference between the fuel spray and the vacuum is the same as when there is a higher pressure in the manifold (low vacuum) and a higher fuel pressure.

Port Firing Control While all port injection systems operate using an injector at each cylinder, they do not fire the injectors in the same manner. This one statement best defines the difference between typical multiport injection systems (MPI) and sequential fuel injection systems (SFI).

SFI systems control each injector individually so it is opened just before the intake valve opens. This means the mixture is never static in the intake manifold and adjustments to the mixture can be made almost instantaneously between the firing of one injector and the next. Sequential firing is the most accurate and desirable method of regulating port injection.

In MPI systems, the injectors are grouped together in pairs or groups, and these pairs or groups of injectors are turned on at the same time. When the injectors are split into two equal groups, the groups are fired alternately, with one group firing each engine revolution.

Since only two injectors can be fired relatively close to the time when the intake valve is about to open, the fuel charge for the remaining cylinders must stand in the intake manifold for varying periods of time. These periods of time are very short; therefore, the standing of fuel in the intake manifold is not that great a disadvantage of MPI systems. At idle speeds, this wait is about 150 milliseconds; at higher speeds, the time is much less. The primary advantage of SFI is the ability to make instantaneous changes to the mixture.

In SFI systems, each injector is connected individually into the computer, and the computer completes the ground for each injector, one at a time. In MPI systems, the injectors are grouped and all injectors within the group share the same common ground wire.

Some injection systems fire all of the injectors at the same time for every engine revolution. This type of systems offers easy programming and relatively fast adjustments to the air/fuel mixture. The injectors are connected in parallel so the ECU sends out just one signal for all injectors. They all open and close at the same time. It simplifies the electronics without compromising injection efficiency. The amount of fuel required for each four-stroke cycle is divided in half and delivered in two injections, one for every 360 degrees of crankshaft rotation. The fact that the intake charge must still wait in the manifold for varying periods of time is the system's major drawback.