Fuel injection

Fuel injection is a system of delivering fuel to the combustion chamber in an internal combustion engine. Fuel injection has almost completely replaced the carburetor in use on engines. with only exception being chainsaws and other small engines. Fuel injection has it roots in the early 20th century when it was designed for use on diesel engines, however it was not seen in usage until the 1930's when German scientists designed a fuel injection system for petrol powered combat aircraft. Carburetor powered aircraft could risk starving the engine in a dive, fuel injection gave German aircraft an advantage for some time. After the war mechanical fuel injection saw limited use in racing cars and aircraft, but was not common on the vast majority of petrol powered vehicles, due to its complexity and cost. Electronic fuel injection also had limited use from 1957 but was also uncommon. After strict emissions regulations were imposed by the U.S government in the 1970s and 1980s Car makers started to remove complex carburetor systems and implement fuel injection, by the early 90s nearly every car being made had fuel injection.

Alfa romeo 6C 2500 One of the first cars with mechanical fuel injection

Fuel injection systems require a large amount of sensors to ensure proper operation, these sensors make sure that the right amount of fuel is being delivered at the right time, here are some examples:

What is the Electronic control unit and how does it work?

The Electronic control unit serves as the control center for all of the sensors on a car. The engine control unit is responsible for monitoring and controlling all the sensors on a cars engine. The ECU monitors the performance of the engine and makes adjustments according to where the problem is. The ECU also does calculations of pulse length (injector time opening) and adjustments to any change in the engine. The ECU can be upgraded and programmed to enhance the performance of the engine.

What is the mass air flow sensor and how does it work?

The mass air flow sensor is a device that is used in conjunction with an oxygen sensor to accurately measure the flow of air into a fuel injection engine. The air flow sensor sends this data to the ECU and it calculates how long the pulse wave should be, this is the amount of time the injector is open regulating the amount of fuel sent to the engine. There are a few ways to measure air flow, the most common in fuel injection engines is the Vane meter sensor and the hot wire sensor. the vane meter uses a spring attached to a potentiometer which measure the voltage and air flow is calculated by the ECU. The hot wire type uses a wire suspended in the engines air stream, an electrical current through the wire heats it up and the air flow cools it down  the air flow is calculated by the ECU using the current required to keep the wire at a stable temperature.

This is a hot wire type sensor

What is the air temperature sensor and how does it work?

The air Temperature sensor is used to measure the temperature of the incoming air in the engines air stream. Warm air is less dense than cold air which means more fuel needs to be added to maintain the air/fuel mixture. The ECU measure the temperature and density of the air and adjust the fuel delivery accordingly.

Air temperature sensor

What is the purpose of the Throttle Position sensor (TPS) and how does it work?

The purpose of the throttle position switch is to relay the position of the the throttle butterfly valve to the ECU. A potentiometer on the butterfly valve measures what position it is in by providing resistance when the valve is opening, the more the valve opens the more the resistance. The ECU monitors this resistance and is able to monitor the position of the valve. The ECU can now keep other sensors on the engine in sych with the opening of the throttle valve. If the valve is opened rapidly more fuel can be injected to prevent the engine from stalling.

Throttle position sensor mounted on throttle body

What is the purpose of the throttle body and how does it work?

The throttle body is a part of the air intake system on a fuel injection engine. The body acts as control for the amount of air entering the engine, by metering the air flow with a butterfly valve called the throttle plate. This valve is connected to the throttle pedal via a wire. The butterfly valve is opened and closed by the wire which allows more or less air into the engine.On a single point fuel injection engine the single injector is positioned inside the throttle body.

What is the purpose of the Temperature sensor and how does it work?

There are various temperature sensors in a fuel injection engine, their purpose is to measure the temperatures of fluids or parts in the engine and report it to the ECU. There are temperature sensors for coolant, air, oil, water and more. The sensors work by recording the temperature and relaying that temperature to the ECU, the ECU can now make adjustments to decrease or increase the temperature of the fluid. An engine has a optimum running temperature which is maintained by the ECU.

Coolant temperature sensor
 

What is the purpose of the fuel rail?

The Fuel rail is a part found on electronic fuel injection engines, its purpose is to provide fuel to the individual cylinder injectors, it also provides a seat for the injector. Fuel is pumped into the rail through a regulator and the right amount of fuel is released by the injectors, if there is too much pressure in the fuel rail then the regulator releases pressure with a return valve.

Fuel rail with injectors

What is the purpose of the fuel pressure regulator and how does it work?

A EFI engine needs a constant fuel pressure in the fuel rail to feed the injectors, the purpose of the fuel pressure regulator is to maintain that pressure. It controls the pressure and releases pressure when it is high.

What is the purpose of Fuel injectors and how do they work?

Fuel injectors are responsible for delivering an accurate amount of fuel to the engine from the fuel rail or lines. Injectors need to respond to changes in engine loading by entering more or less fuel in the engine, for this they are connected to the ECU which monitors engine loading and changes the injectors pulse width. this pulse width is the length of time that the injectors are open for. There are different types of injectors like indirect and direct injection. Indirect places the injectors in the intake manifold and fuel is sprayed just before the intake valve is opened, direct injection means the injectors are positioned in the combustion chamber, High pressure injectors spray fuel directly into the cylinder, this is usually used on diesel engines. There are also mechanical and electronic fuel injection systems, in mechanical fuel injection a high pressure pump is used to pump fuel to the injectors when the fuel pressure in the injector is high enough the fuel overcomes a spring system and is injected into the cylinder. In EFI engines a solenoid in the injector which is controlled by the ECU opens the injector at the right time and for the right amount of time. Different spray patterns are also used on injectors to increase combustion efficiency.

What is the idle air control valve and how does it work?

The idle air control valve controls the amount of air entering the engine while idling. The valve is positioned in the throttle body and opens to allow air to bypass the throttle plate when idling and closes when the engine is under load. The ECU controls the opening of the valve this helps regulate the idle speed.

What is the purpose of the oxygen sensor and how does it work?

In all internal combustion engines the right amount of fuel/air ratio is necessary for proper combustion. The oxygen sensor is a device positioned in the exhaust stream which is tasked with measuring the make up of the exhaust whether it is running too lean or too rich. It works in conjunction with other sensors in the engine like the air flow sensor to measure the air/fuel ratio. This information is then sent to the ECU and changes in fuel/air intake are made. The sensor is essentially a voltmeter the amount of fuel in the air/fuel mix changes the voltage, a rich mixture brings a voltage build up, and a lean mixture causes a low voltage.

What is the purpose of the MAP sensor and how does it work?

The purpose of the MAP sensor or Manifold absolute pressure sensor is to provide information about the air pressure in the intake manifold to the ECU. The ECU can use this data to calculate the density of the air in the system, and in turn calculate the amount of fuel needed to achieve efficient combustion. The MAP sensor is placed on or above the intake manifold, it essentially operates as a barometer.

What is the purpose of the plenum chamber and how does it work?

The plenum chamber in internal combustion engines is a chamber situated between the throttle body and attached to the intake manifold, it's purpose is to act as a stabiliser for air flow into the engine. Air intake pulses inside the intake manifold as air is only need on 1 of four strokes, the plenum chamber helps maintain constant air flow by storing air for the next intake and helps dampen the pulsing effect.

What is the purpose of the Crankshaft sensor?

The crankshaft sensor is used to relay the position and speed of the crankshaft to the ECU. Using the data from the crank sensor the ECU can control the ignition and other aspects of engine timing. The crank sensor can also relay the speed of the engine in RPM to the ECU.

What is the purpose of the camshaft sensor?

The Camshaft sensor works on similar principles to the crankshaft sensor. It measures the position and speed of the camshaft to aid the ECU in engine timing.

Explain how Single and Multi injection systems work?

In a Fuel injected engine the fuel is delivered to the intake manifold via injectors. In a single point injection system one injector is positioned inside of the throttle body and provides the fuel for all of the cylinders. This system was used earlier in the development of fuel injection when carburettor engine designs were given small modifications to incorporate the throttle body for fuel injection. Many modern engines use multi point fuel injection, this type of injection has a injector for every cylinder, these injectors are opened by the ECU at the right time. They can be sequential meaning that the injectors open before intake or simultaneous meaning they open all at the same time.

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Carburettor assignment

The Carburettor is a mechanical device that is designed to mix air and fuel on an internal combustion engine. For an internal combustion engine to work a very precise amout of air/fuel mixture is needed. the carburettor works with simple principles to obtain this mix. It was invented in the late 19th century for this purpose.

How does a carburettor work?

Two stroke: A two stroke Carburettor is much more simple than a four stroke carburettor. It is essentially a tube that mixes and pressurisers the air and fuel., Air enters the carbuerttor through an adjustable plate called a throttle plate, this is controlled by a wire attached to the throttle pedal. Inside the carburettor is a narrowing tube called a venturi, this pressurises the air by forcing it into a smaller space. Inside the venturi is a jet or jets which use the vaccum created to allow fuel to enter the venturi and mix with air to create the right air fuel mix. This mixture then leaves the carburettor and goes to the engine. A choke can be used to increase fuel flow into the carburettor.

Four stroke: A Four stroke carburettor works on the same priciples as a two stroke carburettor, however it is more complicated as it has to operate under different circumstances. The main differences are: More parts, more complication, different operating demands. A four stroke carburattor contains an accelerator pump which assists in clean fuel delivery and smooth acceleration.


Identify the components in a carburettor:

How does an emulsion tube work? and What are the holes in the tube for?:

The emulsion tube meters air and fuel entering the carburettor, the holes regulate the amout of air entering by the change in fuel levels in the tube, under acceleration the holes are uncovered by the fuel going down allowning more oygen into the tube.

How does the manual choke and automatic choke work?

A choke is used to help an engine start when cold. A manual choke works by closing a valve over the air intake of the carburettor, this valve contains a small hole which resctricts tthe amount of air entering the carb, this creates a vaccum so more fuel can be pulled through for startup. An automatic choke works in the same way, but the choke valve is controlled automatically by a temperature sensor.

 Holes on a choke valve

What is the purpose of the float in the Carburettor?

 The float in a carburettor acts to keep the level of fuel in the fuel bowl constant, if the level of fuel drops the float lowers and exposes a pin valve which allows more fuel in.

What is the purpose and how does a mechanical lift pump work?

 The mechanical lift pump is a vaccumm pump that uses pressure to suck fuel from the petrol tank up towards the carburettor. It works by pumping with a diaphram that is run by a camshaft and cam roller.

How does a vacuum fuel pump work on a two stroke engine?
A vaccum fuel pump uses vaccumm created by the movement of the pistons from T.D.C to B.D.C to pump fuel from the fuel tank to the carburettor.


What pressures would you get from these types of pump?
A vaccumm pump depending on its size can be anything from 1 kPa - 50 kPa.

What the difference between petrol and diesel fuel?
Diesel and petrol are both derived from crude oil, using different refining techniques. Diesel fuel has different properties to petrol, the main one being that dielsel is far less volatile than petrol it ignites at a much higher temperature which is required in a Compression ignition engine. Diesel behaves differently to petrol in extreme temperatures with diesel clumping up in the cold.

How does the fuel shut off solenoid work?
A fuel shut off solenoid works by shutting off the supply of fuel to the diesel engine when high temperatures or malfunctions are detected. This is to stop a diesel engine running accidentilly regardless of the throttle.

What is the four stroke cycle?
The four stroke cycle is the cycle of a piston in a four stroke internal combustion engine. The first stroke is intake, the piston travels from T.D.C to B.D.C this creates a vaccum, at the same time the intake valve opens and allows fuel and air mixture to enter the cylinder. The second stroke is compression, the intake valve closes and the piston rises compressing the air fuel mixture into the combustion chamber. The Third stroke is power, the compressed air fuel mix is ignited by a spark plug and expands the air in the chamber forcing the piston down. The last stroke is exhaust, the piston moves up towards T.D.C and the exhaust gas is removed out the open exhaust valve. The strokes are repeated.

What does S.I mean?
S.I means spark ignition. where the fuel in the combustion chamber is ignited by a spark plug.

What does C.I mean?
C.I means Compression ignition. Meaning as the air in the cylinder is compressed by the piston it becomes a very high temperature, fuel is then injected into thishot air and it ignites imediately.

When dismantling a carb what precaution should you take?

Always make sure that the float bowl is facing downwards, if turned upwards too much fuel can enter from the float bowl through the jets. and flood the carburettor. Always remember where the parts go! 

How does a bimetallic spring work in a automatic choke work?

The bimetallic spring is a temperture sensitive metal spring that is part of the automatic choke. The spring is attached to the choke butterfly valve. When the engine is cold the choke is on but as the engine warms up the spring unwinds and slowly opens the air intake butterfly valve.

What is the purpose of the venturi and how does it work?
The venturi is a narrowing in the body of the carburettor, it works by compressing the air that is flowing into the carburettor into a smaller space. Its purpose is to compress the incoming air and create a low pressure area around the fuel nozzle which allows fuel to come out.

An engine lacks power. What is likely to cause this problem?
It is likely a problem with fuel delivery, Not enough fuel going through the carburettor to the intake manifold, possibly a sticky valve or dirty fuel nozzle.

How do you adjust the float in a carburetor?
The float tabs on the inside of the float resivoir tank will need to be adjusted to the right height, so that the float is sitting at the right level. A Carburettor float should sit at 16mm

What is the purpose of a power valve?
The power valve is an extra valve which opens and adds more fuel to the circuit when the carburettor is using more fuel, like acceleration or loading.

What are some of the causes of high fuel consumption?
More fuel can be used when there is a problem with the choke like a sticking butterfly valve. Possibly the carburettor float is set at the wrong height which is allowing too much fuel into the combustion chamber, which in turn overfills the fuel nozzle using more petrol.

How is the the idle and mixture screw adjusted?
The idle mixture screw sets the amount of petrol being used when the car is at idle. The screw is simply turned to adjust the amount of petrol used at idle.

When refitting a three mutiple carburettor what should be done after installing them?
Make sure they are all opening together, and that the choke works on all three.