ENGINE Chapter 9 Fuel System

ENGINE 

Chapter 9.

Fuel System

1. Carburetor

In the gasoline engine, the carburetor or the ECM is used for mixing the air and fuel. The most 4-cylinder engine uses the ECM system, only some small engines use the carburetor.

Carburator fuel system

Electronic fuel system

The carburetor uses spraying principle for mixing the gasoline and the air. That is, a fuel spraying hole is made at the venturi and the air flow at the venturi is high. So the pressure at the fuel spraying hole is decreased. So the gasoline will be following the air flow in the fog form and it is mixed with air.

This phenomena is called as the Venturi effect named by the inventor. To enhance this effect, the ventury is made in narrow as possible. This inside diameter is called the main bore size indicating the size of the carburetor. By define the diameter of the venturi about the main bore size, the gasoline amount can be controlled according to the air flow, so the air-fuel ratio can be maintained uniformly.

The pipe for supplying the gasoline is the main jet (fuel spraying hole). By selecting proper jet according to the driving state, it is possible to cover the wide range of rpm. However, when the engine is rotating with low speed such as idling state, the pressure in venturi is lowered so the proper amount of gasoline is not injected. When the engine is rapidly accelerated, the amount gasoline should be larger than normal state. However, it is not so easy to control these variations. To overcome these hindrances, various carburetors have been developed. Especially, in the motor sports, as the displacement volume is determined, the carburetor is most important device for enhancing the engine performance.

The operation of carburetor is like that; at first, the gasoline from the fuel tank is contained into the floating chamber; when the amount of gasoline in the float chamber is reduced then the float goes down and the more gasoline is supplied from the fuel tank. When a driver presses the accelerator pedal, the throttle valve is opened and air flow passing the venturi part so that the gasoline will be taking out from the main jet.

The carburetor has this simple structure, so the cost is low and it has fewer defects. However, for the modern engines which need to enhance the emission, the fuel efficiency and the power output, the carburetor has many problems.

2. Mechanical Fuel Injecting Device

Basically, as regarding the negative pressure as the air amount injected, the carburetor supplies proper amount of gasoline using only mechanical device. Therefore, it can not maintain the air-fuel ratio uniformly. It is need to control the injected fuel amount exactly using the predetermined the air-fuel ratio to prevent from wasting the fuel or to enhance the engine response.

A new system is developed in which the inhaled air amount is measured directly, and the gasoline is sprayed to the intake manifold with the optimized the air-fuel ratio using the mechanical device. This is the K-jetronic invented by Bosch.

By comparing with the carburetor, the fuel injecting device is injecting the fuel to the manifold directly. Therefore, it has good response at the start and at accelerating and decelerating. It is hard for this device to make the vapor lock in which flow of fuel is broken by vaporizing the fuel in the middle of fuel pipe. Additionally, it does not make icing that the carburetor is frozen.

The main feature of the K-jetronic is that installing a circular plate named sensor plate in front of the throttle valve, the gasoline amount is controlled using that the opening state of this plate is changed by the air amount. When the throttle valve is open, the air presses the sensor plate installing in the air flow meter. The lever supporting this plate is connected to the device for controlling the injected fuel amount, and the gasoline is injected to the fuel injecting device by a response to the plate operating.

As a system replacing the carburetor, the K-jetronic has better reliance. However, it sometimes injects the fuel in the same manner of carburetor and controls the fuel amount mechanically. Therefore, it is hard to control the mixture ratio precisely.

In some cases, by accepting an ECM to a portion of the system, the KE-jetronic supplementing these week points is developed. However, step by step, the full ECM system is replacing these mechanical systems in order to regulate the exhaust gas and to enhance the fuel efficiency.

3. Electrical Fuel Injection System

The main part of the electrical fuel injecting system is the fuel injecting device deciding the amount of fuel to make proper mixing ratio by measuring the inlet air amount. This system comprises of the device of measuring the air amount, the device of injecting the fuel, and the device of controlling theses operations.

The representative mechanical fuel injecting device, the K-jetronic, uses the sensor plate for device measuring air amount, and transmits the movement of sensor plate to the valve controlling fuel supplying. Contrarily, the electrical fuel injecting system the measured air amount by the air flow sensor is sent to computer as an electric signal to decide the fuel amount with the result from the sensor for checking the engine status. By these devices, it is possible to control the air-fuel ratio precisely.

The structure of the fuel injector is differed according to the manufacturer and the applied engine type. Except the air flow sensor, most parts are similar. Here, we will explain about the mass air flow type using the air flow meter as the air flow sensor.

The filtered air through the air cleaner is measured its amount at the air flow meter, and taken into the surge tank (intake collector) via the throttle body including throttle valve connecting to the accelerator pedal. This air is distributed to the intake manifold of each cylinder, and inhaled into the cylinder with the gasoline injected from the fuel injecting valve (injector) to the manifold or intake port.

At this time, a controller decides the gasoline amount optimized to the driving status and running condition of the vehicle. This is called as the ECM, Electronic Control Module. The driving status is the electrical signal from the water temperature sensor, the intake air temperature sensor, and the throttle position sensor, and the running condition is the signal from the speed sensor and the signal indicating the air conditioning working status. These signals are sent to the ECM.

By memorizing the data concerning to some combinations of these signals and the control method which indicate what amount of gasoline is injected according to the combination to the computer installed into the ECM, the computer can inject proper amount fuel decided by the computer according to the accelerator operating, from the injector.

4. Fuel Supplying System

The gasoline, the fuel, is contained in the fuel tank and sent to the fuel injecting device after eliminating dust and water by filter.

The fuel tank is made of galvanized steel to prevent rust, as well as the plastic tank is more used. To protect the fuel from leaning one side, there are some partitions, called separator, and level gauge is attached.

The fuel pump has various types. The carburetor uses mechanical pump, ECM system uses electrical pump using electric motor mostly.

A regulator, pressure controller, is attached for sending the gasoline to the injector after controlling the gasoline pressure within the specific range.

For injecting the gasoline, an injector is used. The injector has a needle valve closing the front of the nozzle so the valve is open by flowing an electric current to the solenoid to injecting gasoline.

In the injecting method, there are a Single Point Injection (SPI) injecting gasoline to the collecting portions of manifold, and a Multi Point Injection (MPI) injecting gasoline to each manifold corresponding with each cylinder. The SPI has the injector at the same place as the carburetor does, but this makes the mixture more effectively than carburetor does.

The MPI can be classified to point injection, group injection and bank injection according to the injecting timing. Each manifold has injector. The point injection performs injecting operation at the intake stroke of each cylinder according to the engine rotation. The group injection performs the injecting operation with a group of cylinders which have the sequential intake stroke.

Of course, the point injection can inject the gasoline with most optimized timing and amount. However, the electrical circuit for driving the injector is more complicated. So, some commercial car accepts the group injection.

Simplifying the group injection is the bank injection. At the intake stroke and the combustion stroke in which the piston goes down, the needed gasoline is injected after dividing into two turns and taken into the cylinder after gathering at the intake stroke. Due to the simple structure, it ensures the most efficiency in the gasoline injection. So it is the most used method in the gasoline engine.