Presentation on theme: "Fuel Injection System Dr Jehad Yamin. It is the most effective component that affects the performance and operation mainly of CI and modern SI engines."— Presentation transcript:
Fuel Injection System Dr Jehad Yamin
It is the most effective component that affects the performance and operation mainly of CI and modern SI engines. Basically, both fuel injection and carburetion systems have the same aim, which is the preparation and supply of combustible mixture to the engine as per the engine requirements, however, they perform this task differently. For example :
Fuel Atomization Both systems have to atomize the fuel before igniting it for proper mixing with the air. For the Carburetor, this depends on the incoming air velocity, which has to be greater than the fuel velocity. For the injector, the fuel speed at the point of injection has to be greater.
Fuel supply They both supply the engine with exact amount as per the engine requirements. For the case of carburetor, the amount of fuel supply depends on the amount of air velocity passing through the throat. Therefore it is called Quantitative Control. For the injector it depends on the pump pressure. Therefore it is called Qualitative Control since the amount of air is fixed.
Problems of Carburetor Maldistribution for multi-cylinder engines. Loss of volumetric efficiency duel to passage restrictions. Freezing at low temperatures. Surging of the secondary tank due to tilting. Risk of fire due to back fire. Vapor leakage in hot summer days. Excessive supply of fuel.
Advantages of Injectors Correct and uniform mixture distribution. Improved volumetric efficiency. Freedom from icing and drawbacks. Better starting and acceleration. Absence of tilting problems. Better control on fuel.
Disadvantages Higher initial cost. Complicated design. Maintenance cost is high. Bulk and weight. More noisy. Requires specific type of fuel.
Objectives of Fuel Injection Systems Especially for C.I. engines, injection system must satisfy the following requirements: 1.Meter the correct amount of fuel as per engine demand, 2.Equally distribute this fuel among all cylinders, 3.Inject this fuel inside the cylinder at the correct time and rate, 4.Inject this fuel into the cylinder with a proper pattern and atomization as the combustion chamber design demands, 5.Start and end of injection should be sharp without dribbling or after injection, 6.Injection timing should vary as per engine load and speed and 7.It should maintain all the above requirements throughout its operational life with higher degree of reliability.
Functional Elements To accomplish these requirements, the injection system needs: 1.Pumping Element (to move the fuel from the tank to the cylinder), 2.Metering Element (to measure and supply the fuel at rate as per the engine demand), 3.Metering Control (to adjust the rate of fuel metering element as per engine demands), 4.Timing Control (to adjust the start and end of the injection process), 5.Mixing Element (to atomize and distribute the fuel inside the combustion chamber).
Types of Injection systems For producing the required pressure for preparation of the fuel, either air or mechanical means are used. Air Injection System : in this system, fuel is forced into the cylinder by means of compressed air. This is rarely used nowadays as it is bulky and requires multi-stage compressors. It has certain advantages of producing good mixing between the air and fuel and it allows the use of cheaper and higher viscosity fuels. Solid Injection System : this is also called “Air-Less Mechanical Injection”. In this system, liquid alone is injected into the cylinder.
Fuel Injection Systems
Injection pump governor As the fuel delivered is dependent on the injection pump characteristics and air intake, some means of control on the amount to be supplied by the pump has to be applied to correct the amount of fuel supplied as per the engine demand. This is done by means of a governor, which may be either mechanical or pneumatic type. Example: As the engine speed increases, the amount of the fuel delivered by the pump also increases at the same time the engine’s volumetric efficiency decreases, if no correction is made on the amount of fuel supply this results in excess fueling at higher speeds. At idling speeds the opposite situation occurs and engines stalls due to fuel deficiency. Further, as the engine load increases, the fuel supplied by the pump increases causing excessive carbon deposits. These variations of engine demand as well as fuel supply by pump must be adjusted and controlled. This is done by the governor.
Fuel Injector The design of the fuel injector affects the rate and degree of completeness of the fuel combustion process. By atomizing the fuel to very fine droplets, its surface area increases and hence more rapid evaporation and better mixing of fuel with air inside the cylinder takes place. This results in a better and fast combustion of the fuel. The proper atomization of the fuel depends on the injection pressure.
The main requirements of the injection nozzle are: Proper atomization of the fuel, Distribution of the fuel as per the combustion chamber design (this is affected by injection pressure, fuel physical properties like viscosity and S.I.T. and finally on the density of air inside cylinder), Allow for proper mixing with the air and Prevention of impingement of fuel on the cylinder walls.
Nozzle Nozzle is that part of an injector through which the liquid fuel is sprayed into the combustion chamber. The nozzle should fulfill the following functions: Atomization: This is a very important function since it is the first phase in obtaining proper mixing of the fuel and air in the combustion chamber. Distribution of Fuel: Distribution of fuel to the required areas within the combustion chamber.
Factors affecting the distribution of the fuel are: (a) Injection Pressure: Higher the injection pressure better the dispersion and penetration of the fuel into all the desired location in combustion chamber. (b) Density of Air in the cylinder: If the density of compressed air in the combustion chamber is high then the resistance to the movement of the droplets is higher and dispersion of the fuel is better. (c) Physical Properties of Fuel: The properties like self-ignition temperature, vapor pressure, viscosity, etc.
Prevention of impingement on walls: Prevention of the fuel from impinging directly on the walls of combustion chamber’s piston. This is necessary because fuel striking the walls decomposes and produces carbon deposits. Mixing: Proper mixing between the air and fuel in case of non-turbulent combustion chamber should be taken care of by the nozzle. Now, at the exit of the fuel jet, the fuel velocity U f is of the order of 400 m/s and is given by the following equation :
Larger droplets provide a higher penetration into the chamber but smaller droplets are required for quick mixing and evaporation of the fuel. The diameter of most of the droplets in a fuel spray is less than 5 microns. The droplet sizes depends on various factors which are listed below: Mean droplet size decreases with increase in injection pressure. Mean droplet size decreases with increase in air density. Mean droplet size increases with increase in fuel viscosity. Size of droplets increases with increase in the size of the orifice.
The volume of the fuel injected per second, Q, is given by : Q=(Area of all orifices) * (fuel jet velocity) * (time of one injection) * (number of injections per second for one orifice) This can be mathematically written as follows :
The rate of fuel injected/degree of crankshaft rotation is a function of injector camshaft velocity, the diameter of injector or plunger and flow area of the tip orifice. Increasing the rate of injection decreases the duration of injection for a given fuel input and subsequently introduces a change in injection timing. A higher rate of injection may permit injection timing to be retarded from optimum value. This helps in maintaining fuel economy without excessive smoke emission. However, increase in injection rate requires an increase in injection pressure and increase in the load on the injector push rod and the cam. This may affect the durability and life of the injector and engine.