Advanced Ic Engine Notes.docx

Carburetor Carburetor is a device which is used for atomizing and vaporizing thefuel (petrol) and mixing it with the air in varying proportions, to suit the changing operating conditions of the engine. Atomization is the breaking up the liquid fuel (petrol) into very small particles so that it is properly mixed with the air. But vaporization is the change of state of the fuel from liquid to vapour. Carburetor performs both the process i.e., atomization of the fuel and vaporization of the fuel. SIMPLE CARBURETOR A simple carburetor refer Fig. 1.6 consists of following 1. float and float chamber, 2. venturi and throttle valves and 3. choke valve. 1. Float and Float chamber The petrol is supplied to the float chamber from the fuel tank through the filter and fuel pump. When the petrol in float chamber reaches a particular level, the needle valve blocks the inlet passage and thus cuts off the petrol supply. On the fall of the petrol level in the float chamber, the float descends down and inlet passage opens. The petrol is supplied to the chamber again. Thus a constant fuel (petrol) level is maintained in the float chamber. The float chamber supplies the petrol to the main discharge jet placed in venturi tube. The level of fuel in the float chamber is kept slightly below the top of the jet to prevent the leakage when not operating. 2. Venturi and Throttle valve The carburetor consists of a narrower passage at its centre, called venturi. One end of the carburetor is connected with the intake manifold of the engine. During the suction stroke, vacuum is created inside the cylinder. Due to vacuum, the air is sucked to the carburetor. The velocity of the air increases as it passes through the venturi where the area of cross section is minimum. Due to increased velocity of air at the venturi, the pressure at the venturi decreases. Therefore a low pressure zone is created in the venturi. So the jet (nozzle) located at the venturi is in the zone of low pressure. The fuel comes out from jet (nozzle) in the form of fine spray. This fuel spray is mixed with air and the mixture is supplied to the intake manifold of the engine. The throttle valve is placed between the jet (nozzle) and the intake manifold of the engine. The quantity of the mixture is controlled by means of throttle valve. 3. Choke Valve While starting in cold weather the engine needs extra rich mixture. So a choke valve is introduced in the air passage before the venturi. When the choke valve is closed it creates high vacuum near the fuel jet and small quantity of air is allowed, to get rich mixture. The fuel flow increases as the vacuum near the jet increases. GASOLINE INJECTION SYSTEM In a multicylinder engine with a carburetor, it is difficult to obtain uniform mixture in each cylinder. The various cylinders receive the gasoline mixture in varying quantities and richness. This problem is called mal-distribution and the above mentioned problem can be solved by using gasoline injection system. By adapting gasoline injection, each cylinder can get the same richness of the air-gasoline mixture and the mal-distribution can be avoided to a great extent. 1.4.1 Reasons for adopting gasoline injection system To have uniform distribution of fuel in a multi-cylinder engine. To reduce (or) eliminate detonation.

To improve volumetric efficiency. To improve fuel atomization by forcing fuel under pressure into the cylinder. To prevent fuel loss during scavenging in case of two-stroke engines. Fuel-injection system in SI engine can be classified as follows: Fuel-injection systems 1) Indirect injection (IDI) A) Mono-Point Injection (MPI) B) Multi-point fuel injection 2) Direct injection (D I) In indirect injection, fuel is injected into the air stream prior to entering the combustion chamber. In direct injection, fuel is injected directly inside the combustion chamber. The gasoline fuel injection system used in a spark-ignition engine can be either of continuous injection or timed injection.

1.4.1 Continuous fuel injection system (CIS) In continuous injection system, the injection nozzle and its valves are permanently opened while the engine is running, so that the fuel is injected continuously into the combustion chamber. This system usually employs rotary pumps for fuel injection. This pump maintains a fuel line gauge pressure of about 0.75 to 1.5 bar. The timing and duration of the fuel injection is determined by the Electronic Control Unit (ECU) depending upon the load and speed. 1.4.2 Timed fuel injection system In this system, the fuel is sprayed from the injector nozzle in pulses at certain time i.e usually during the early part of the intake (or) suction stroke. This system has a fuel supply pump which sends fuel at a low pressure at about 2 bar when the engine is running at maximum speed. The length of time for fuel injection is determined by Electronic Control Unit (ECU) depending on input signals from various engine sensors. 1.5 MONOPOINT FUEL INJECTION SYSTEM A monopoint injection system is also called as throttle body injection system (TBI). In this system, the injector nozzle is mounted just above the throat of the throttle body as shown in Fig. The throttle body assembly resembles like a carburetor except that there is no fuel bowl float (or) metering jets. Here, the injector nozzle sprays gasoline into the air in the intake manifold so that the gasoline mixes with air. This air-fuel mixture then passes through the throttle valve and enters into the intake valve. Moreover, this system requires only one circuit in the computer to control injection which simplifies the construction of electronic control unit (ECU). Thus it reduces the cost of the system.In this system, injection pressure is higher compared to carburetor discharge pressure, which speeds up and improves the atomization of the liquid fuel. However, maldistribution of fuel cannot be avoided. To overcome maldistribution of fuel, multi-point fuel injection system can be used. Advantages 1. Monopoint injection system meters fuel better than a carburetor. 2. Reduced fuel consumption. 3. Less expensive and easier to service.

1.6 MULTIPOINT-INJECTION SYSTEM A multi-point fuel injection system is also called as port-injection system. Refer Fig. 1.11. In this system, the injector nozzle is placed on the side of the intake manifold. Here each cylinder is provided with separate fuel injector. The injector nozzle sprays gasoline into the air inside the intake manifold so that the gasoline mixes with air. This air-fuel mixture then passes through the intake valve and enters into the each cylinder. The seperate fuel injector used in this system supplies the correct quantity of fuel to each of the engine cylinders by a fuel-rail according to the Firing order or in a ‘particular sequence’. This system provides further precision by varying the fuel quantity and injection timing by governing the each injector separately and thereby improving the performance and controlling the emissions. This technology consists of following parts: 1. Injectors 2. Fuel Pump 3. Fuel Rail 4. Fuel Pressure Sensor 5. Engine Control Unit 6. Fuel Pressure Regulator 7. Various Sensors - Crank/Cam Position Sensor, Manifold Pressure Sensor, Oxygen Sensor The advantages of Multi point fuel injection are Increased power and torque through improved volumetric efficiency. More uniform fuel distribution to each cylinder. More rapid engine response to changes in throttle position. 1.7 DIRECT INJECTION SYSTEM Direct Injection In a direct injection engine, fuel is injected directly into the combustion chamber. Modern gasoline engines may utilize direct injection using electronic control, which is referred to as Gasoline Direct Injection (GDI). In IC engines, GDI is also known as petrol direct injection, direct petrol injection, spark ignition direct injection (SIDI) or fuel stratified injection (FSI). This is the next step in evolution from multi-point injection. It reduces emissions and fuel consumption. 1.8 ELECTRONICLLY CONTROLLED GASOLINE INJECTION SYSTEM 1.8.1 Description The Bosch D-Jetronic electronic fuel injection system Fig. 1.16 is composed of 3 major subsystems: The air intake system The fuel system, and The electronic control system. The D-Jetronic system uses constant fuel pressure and flow, so that only injection duration time needs to be modified to control air/fuel mixture. The D-Jetronic system measures incoming airflow by monitoring intake manifold pressure. Engine speed, temperature, and other factors are monitored for the purpose of fine-tuning injection duration. An auxiliary air valve, cold start injector and thermo time switch are useful aid in cold starting and operation. The simple layout of electronic fuel injection system is shown in Fig. 1.17