Knowledgebase: General
Fuel Injection Basics
Posted by Alex (Im) E. on 01 February 2013 01:08 AM

FUEL INJECTION

Electronic fuel injection (EFI) comes in several basic varieties.

A Throttle Body Injection (TBI) system has a single throttle body which contains one or two fuel injectors. The injector(s) spray fuel into the intake manifold. TBI systems were used in the late 1980s and early 1990s.

In a Multipoint Fuel Injection (MPI) system, each cylinder has its own individual fuel injector. Each injector is mounted on the intake manifold and sprays fuel directly into the intake port for its cylinder. This provides better cylinder-to-cylinder fuel distribution for improved engine performance, emissions and fuel economy.

Direct Injection is similar to multipoint injection except that special high pressure injectors are used to spray fuel directly into the combustion chambers of the engine. Direct injection allows very lean fuel mixtures for improved fuel economy and emissions.

Another type of fuel injection is General Motor's Central Port Injection. Here, there is a single centrally located master injector that routes fuel through lines to mechanical injectors at each cylinder.

 

HOW IT WORKS

Fuel injectors do not spray continuously, but are "pulsed" or fired in short bursts -- usually once every revolution of the camshaft or crankshaft (depending on how the system is triggered). In some engines, all the injectors fire simultaneously at the same time.

In newer vehicles, each injector if usually fired separately in the same firing sequence (called "sequential" injection) as the spark plugs. This allows more rapid changes in the air/fuel mixture to improve fuel economy, emissions and performance even more.

The amount of air entering a fuel injected engine is controlled by the throttle body. But to maintain the correct air/fuel ratio, the amount of air must be measured or estimated.
In "Mass Air Flow" EFI systems, an airflow sensor located ahead of the throttle body measures incoming air.

The sensor may be a flap connected to an electronic rheostat (a vane airflow sensor), or it may be a heated wire or filament (mass airflow sensoer) that senses air flow by how much of a cooling effect the airflow has on the sensing element.

Mass airflow EFI systems are very sensitive to air and vacuum leaks. Leaks allow unmetered air to enter the engine which usually causes the air.fuel mixture to go lean. This may cause hard starting, rough idling, lean misfire and elevated hydrocarbon emissions in the exhaust.

In "Speed-Density" EFI systems, no airflow sensor is used. Airflow is estimated by the Powertrain Control Module using inputs from the throttle position sensor, manifold absolute pressure (MAP) sensor, air temperature sensor, engine coolant sensor and oxygen sensor.

This type of system is less ensitive to air or vacuum leaks, and is a less expensive system because it requires no mass airflow sensor.

 

FEEDBACK FUEL CONTROL LOOP

To keep the air/fuel mixture at the proper ratio, an oxygen (O2) sensor in the exhaust manifold monitors the amount of unburned oxygen in the exhaust. On V6 and V8 engines, there are usually two upstream O2 sensors (one for each bank of cylinders).

Most oxygen sensors are designed to give a RICH or LEAN signal indication when the air/fuel mixture varies from the ideal mid-point of 14.7:1 (called the "stoichiometric" or ideal ratio). This is where the air/fuel mixture burns the cleanest and produces the lowest emissions.

It is also called the "Lambda" point. When Lambda equals one, the air/fuel mixture is neither RICH nor LEAN but is perfectly balanced.

The powertrain control module uses the rich/lean signal from the oxygen sensor to constantly rebalance the fuel mixture.

When the O2 sensor says LEAN, the computer responds by increasing the on time of the injectors to add more fuel.

This makes the fuel mixture go RICH. When the O2 sensor signals back that the mixture has indeed gone RICH, the computer responds by reducing the on time of the injectors to make the mixture go LEAN.

A couple of things you need to know about O2 sensors: One is that they must be hot (about 600 degrees F) to produce a RICH or LEAN voltage signal.

To reduce the time it takes them to warm up and reach operating temperature, most O2 sensors in newer vehicles have a built-in heater circuit.

This can cut warmup time to 15 to 30 seconds. Oxygen sensors are sensitive to contamination and can be ruined by an internal coolant leak inside the engine (silicone contamination), or by excessive oil burning (phosphorus contamination).

They also tend to become sluggish and less responsive as they age. Because of this, replacing the oxygen sensor(s) may be necessary if the sensors are not reading accurately.

On some of the newest vehicles, a new type of oxygen sensor called a "wide band" O2 sensor is used for the same purpose.

But instead of producing a rich/lean signal that flipflops back and forth, the wide band sensor tells the computer the exact air/fuel ratio.

 

FUEL DELIVERY

The amount of fuel delivered by each injector depends on how long it is energized.

The "on-time" of the injector signal controls how much fuel is sprayed into the engine, which also affects the air/fuel ratio. The longer the injector remains on, the more fuel it sprays into the engine.

Fuel delivery must be increased when engine speed and load are higher. It can be reduced under light load or when decelerating to improve fuel economy.

Fuel delivery through the injectors is further controlled by the amount of fuel pressure in the system.

A "fuel pressure regulator" maintains system pressure at a preset level. A diaphragm inside the regulator is connected to engine vacuum so the regulator can increase or decrease fuel pressure in proportion to engine load.

On some newer systems, the fuel pressure regulator is mounted in the fuel tank and there is no fuel return line from the engine. These are called "returnless" EFI systems, and are found on Chrysler and other makes.

Fuel pressure is generated by a high pressure pump usually located inside the fuel tank. When the ignition is turned on, the engine computer energizes a relay that turns the pump on.

The pump runs continuously while the vehicle is being driven. A bad pump relay, wiring problem or pump motor can cause a loss of fuel pressure which may cause driveability problems, hard starting -- or prevent starting if there is no pressure.

AS pluged fuel filter can also preventnormal fuel pressure from reachign the injectors.

 

COLD START INJECTORS

Older fuel injection systems often have an additional "cold start" injector that aids starting by sprayign extra fuel into a cold engine when it is first cranked. A "warm-up regulator" provides additional fuel enrichment while the engine is warming up.

Newer EFI systems do not use either of these devices and rely instead on longer injector pulses from the computer for fuel enrichment during starting and warm-up.

The powertrain control module monitors coolant temeprature and adjusts the fuel mixture as the engine warms up.

 

IDLE SPEED CONTROL

Idle speed in fuel injected engines is not controlled by the opening of the throttle, but rather by an air bypass circuit.

On most EFI applications, idle speed is computer-controlled by an "idle air control" (IAC) valve.

A small electric motor opens and closes the bypass circuit valve so air can bypass the throttle plates.

Vacuum leaks can interfere with the normal operation of the idle control system, which typically causes the PCM to close the idle air control valve to limit idle speed.

Over time, this may burn out the idle control speed motor.

 

FUEL INJECTORS

Fuel injectors come in two types: mechanical and electronic. Mechanical injectors that are essentially spring-loaded poppet valves are found in many older import applications, including Bosch CIS/K-Jetronic systems.

All electronic fuel injection systems on domestic and newer import applications have injectors with solenoid operated valves. In both varieties, fuel sprays out of the injector nozzle when the valve is opened.

Varnish buildup in the nozzle orifices of the fuel injectors can restrict fuel delivery and cause driveability problems such as lean misfire, rough running and hesitation.

Cleaning the fuel injectors can often restore injector performance, but replacement is sometimes necessary if the injector is completely clogged or fails to respond to cleaning efforts.

Replacement is also necessary if the injector is shorted or open.

Two things typically cause dirty injectors: frequent short trip driving, and using gasoline that does not contain adequate levels of fuel detergents to keep the injectors clean. Adding a can of fuel injector cleaner to the fuel tank every 5,000 or so miles can help keep injectors clean.

But if the injectors are really dirty, they may have to be professionally cleaned using special on-car or off-car cleaning equipment that flushes solvent through the injectors.

The flow characteristics, spray pattern and calibration of a particular set of injectors is matched to the engine and has an important affect on engine performance and emissions.

So it's extremely important to always use the correct replacement injectors for the application. Injectors can't be interchanged from one application to another even when they look the same on the outside.

Installing the wrong injectors may make the fuel mixture run excessively rich or dangerously lean.

The only exception would be if an engine were modified to make more power needed more fuel than teh stock injectors could deliver. In such cases, injectors with a higher flow capacity would be necessary --along with reprogramming the fuel curves in the computer.

 

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