From many years, scientists and engineers have been striving for innovation, to develop fuel efficient cars and control the emissions. There has been a lot of change over the years in the fuel system. Earlier the cars had carburetor system and in present electronic fuel injection system is utilized.
Fuel Injection system was there from mid 1920s but it was used for diesel running engines. It went through a lot of developments and then around late 1950s it started coming in petrol driven engines. It was phased in through in the late '70s and '80s at a fast pace, with the US, German and French markets leading and somewhat lagging back were the UK and Commonwealth. Since 1990s all the gasoline passenger cars sold in first world markets were equipped with electronic fuel injection (EFI).
The technology behind fuel injection system has evolved significantly since its usage from mi 1980s. The current EFI system provides precise, dependable and cost-effective method of fuel metering. They provide maximum engine efficiency keeping emissions in control. This is the reason of EFI systems having replaced carburetors in the market place.
Before we start talking about EFI it is useful for us to know about the carburetor system and its downfall.
2. Carburetor
A carburetor is a device used in earlier cars for mixing of air and fuel in the right proportion for the efficient combustion. A basic carburetor consists of the following parts:
Air Horn
It is also called the throat or barrel. It transfers outside air into the engine intake manifold. It contains the throttle valve the venturi, and the outlet end of the main discharge tube.
Throttle Valve
This disc shaped valve controls the flow of air entering into the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. When opened it hardly restricts the flow of air, and when closed it completely blocks the flow of air.
Venturi
The venturi produces sufficient suction to pull fuel out of the main discharge tube. It narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part.
Main Discharge Tube
The main discharge tube is also called the main fuel nozzle. It uses venturi vacuum to feed fuel into the air horn and engine. It is a passage that connects the fuel bowl to the center of the venturi.
3. Fall of the carburetor
As time passed by and engineers led innovation, there was a downfall of the carburetor system and electronic fuel injection was developed.
EFI
Carburetor
Above is the graph comparing the performance of a carburetor and EFI driven car.
There are several reasons for the downfall of the carburetor which are sated below:
As automobile evolved with new technology and it getting more sophisticated, the carburetor got more and more complicated trying to handle all of the operating requirements. That way the simple design of EFI was able to handle a lot of varied operations without any complications.
A fuel injection driven car can work at very low ambient temperatures. Whereas in case of a carbureted operated a separated device called choke is operated. It restricts the flow of air for a small amount of time and allows more fuel to pull in, as a result providing a rich mixture necessary for the operation to sustain at low temperatures.
A fuel injection system delivers more precisely and equal amount of fuel is injected to each cylinder than carburettor does. This improves the cylinder-to-cylinder distribution. As a result engine fuel efficiency increases as less fuel is needed to obtain the same power output. This criterion is not met in a carburettor system. In carburettor system some cylinders receive excess fuel as a result of ensuring that sufficient amount of fuel is present in all cylinders.
Many standards are set by governments to control the emissions from a vehicle which are to be taken in to consideration by the designer. In order to meet these stricter emissions requirements, catalytic converters were introduced. It reduces the toxicity of emissions through chemical reaction. But for this vary careful control of air-fuel ratio is required for it to be effective. And since carburetor system is mechanically operated it was not feasible. In EFI everything is electronically controlled, and it is attached to various sensors, therefore emission control was very much feasible because of accurate fuel metering in EFI system.
Fuel injection systems are preferable because they can react to any kind of inputs such as sudden change in throttle, and also the amount of fuel injected can be controlled to match the engines needs corresponding to wide range of operating conditions such as engine load, ambient air temperature, engine temperature, fuel octane level, and atmospheric pressure. This is not possible in carburetor system to that extent.
4. Electronic Fuel Injection System
When we hear the term fuel injection, we assume it to be a simple process of injecting the fuel. But it is not true; it requires several peripheral components in order to imitate all the functions performed in carburetor. The complete EFI system can be divided in to three basic sub-systems:
Fuel delivery system
Air induction system
Electronic control system
4.1 Fuel delivery system
The fuel delivery system consists of the following components:
Fuel tank
Fuel pump
Fuel pipe and in line filter
Fuel delivery pipe
Pulsation damper
Fuel injectors
Fuel pressure regulator
Fuel return pipe
Fuel Tank
Fuel Pump
Fuel Pipe
Fuel Filter
Pulsation Damper
Fuel Rail
Cold Start Injector
Injectors
Cylinders
Pressure Regulator
Return Pipe
High pressure
Low pressure
Layout of Fuel Delivery System
The fuel is pumped from the Fuel Tank by the Fuel Pump, from where the fuel flows through the fuel filter to the fuel rail (fuel delivery pipe) till the pressure regulator where it is held under constant pressure. In the pressure regulator a constant pressure drop is maintained across the fuel injectors regardless of engine load. If any excess fuel is consumed by the engine operation, it is returned via the fuel return line. To absorb pressure variations in the fuel rail due to injectors opening and closing, pulsation damper is also mounted.
The fuel injectors, in which fuel metering is directly controlled, are pulsed by the ECU (engine control unit). The ECU calculates the pulse width (the amount of time fuel injector opens) through the various sensors to which it is attached.
The operation and functioning of few of the components of fuel delivery system will be discussed below.
4.2 Fuel pump
In automobiles generally two types of fuel pumps are used mechanical and electrical. The purpose of the fuel pump is to pump the fuel from the fuel tank and to the fuel injectors of the EFI system.
Prior to the widespread usage of the EFI system, most carburetor operated automobiles employed mechanical fuel pumps to pump fuel from the fuel tank into the fuel bowls of the carburetor. Mechanical fuel pumps are diaphragm pumps, which is positive displacement type. These pumps are mounted on the engine and operated by an eccentric cam usually on the camshaft. A rocker arm attached to the eccentric moves up and down flexing the diaphragm and pumping the fuel to the engine. These pumps work in the pressure range of 4-6 psi.
In all the modern cars which are incorporated with EFI system, they are operated by the electric fuel pump. These pumps are usually mounted inside the fuel tank, though they can be mounted outside the tank also. The electric fuel pump being used now is roller cell type pump. This type of pump uses rollers which are mounted on an offset disc that rotates inside a steel ring. The fuel is drawn in from the pump inlet into the spaces (cells) between the rollers and pushed to the outlet. They support high pressure and high volume fuel delivery. They operate with less discharge pulsation and run quieter compared to other types of fuel pumps.
In roller cell pump, the fuel enters the pump and being compressed by rotating cells which force it through the pump at a high pressure. The pump can produce a pressure of 8 bar (120 psi) with a delivery rate of approximately 4 to 5 litres per minute. Within the pump is a pressure relief valve that lifts off its seat at 8 bar to arrest the pressure if a blockage in the filter or fuel lines or elsewhere causes it to become obstructed. The other end of the pump (output) is home to a non-return valve which, when the voltage to the pump is removed, closes the return to the tank and maintains pressure within the system. The normal operating pressure within this system is approximately 2 bar (30 psi), at which the current draw on the pump is 3 to 5 amps.
4.3 Fuel Pressure Regulator
The fuel pressure regulators job is to maintain a constant pressure of 30 psi. It does so with spring-loaded diaphragm that controls a valve. It is electromagnetically-operated and receives the signal in milliamp from ECU. This valve when opened due to excessive pressure in the fuel lines, it uncovers a fuel line that returns excess fuel back to the fuel tank. a constant pressure differential is maintained in the fuel rail across the fuel injector. The specified pressure differential is either 36 psi or 41 psi depending on the engine application.
4.4 Pulsation damper
Although fuel pressure is maintained at a constant value by the pressure regulator, the pulsing of the injectors causes minor fluctuations in rail pressure. To ensure accurate fuel metering, pulsation damper acts as an accumulator to smooth out these pulsations. It is employed when the level of flow fluctuation or force variation is unacceptably high and for the safety of the pipe work.
The design of pulsation damper will be discussed later in the design component.
4.5 Fuel Injector
Fuel injectors are the most important part of the complete EFI system. They are electro-mechanical devices that supply a desired quantity of fuel at a controlled time to the engine. When the fuel injector is energized by the signals from the ECU, an electromagnet moves a plunger that leads to the opening of a valve, allowing the fuel at a high pressure to flow out of the nozzle. The fuel injectors are installed in the fuel delivery pipe that supplies fuel to it at high pressure.
Types of fuel injector
Pintle
The pintle type design is the most common type that is being used and turns out to be the best. It has been well prove for more than 30 years and is still in use. In this design, the needle at the tip of the injector is tapered. The core and the needle is pulled back when the solenoid is energized, permitting the fuel to discharge.
The pintle profile dramatically affects the ability of the fuel injector to atomise the fuel. This is because it has a direct affect on the operating pressure it is designed to operate under. The effect of imbalanced pressure affects the way fuel injector is designed to operate. Only when the fuel injector is operating in the correct pressure range it will result in a well atomised spray. If the pressure is insufficient or excessive it will result in the hosing effect. Excessive pressure also has the effect of resulting in a spray angle that is too large for the targeted area. The consequence of it is that the fuel mixture the engine receives may be leaner; resulting in the fuel being injected not atomised and enters the cylinder as a liquid mass.
Disc
In this type the mechanism is same as in the pintle type of injector, only that instead of the pintle there is a flat disc and a plate on which there are tiny holes. In this type the solenoid acts directly on the flat disc through the core of the injector body. The flow of the fuel is determined by the size of the holes in the disc. This design ensures long service life as there is less deposit build-up at the orifice.
Bosh disc type Lucas disc type
Hole
In this type a ball and socket arrangement is used to control the fuel flow by raising the ball off its seat. This permits the flow of fuel through the orifice and then out through a fixed director plate with several holes drilled on it. This design offers better fuel atomization compared to the pintle design and offers better resistance to deposit build-up.
4.6 Electrical Differences
The injector windings used in the solenoid of the fuel injector can be classified in to two groups:
Low resistance fuel injector
They are also called as peak and hold injectors. The resistance for this type of injector measure between 2-5 Ω @ 70'F. Initially when the injector is energized it draws a large amount of current. This initial period is called the peak portion of the injector. After the injector is open the overall current drawn is reduced to maintain the open condition of the injector.
High resistance fuel injector
They are also referred to as saturation injectors. The resistance for this type of injector measures in the range of 12-16 Ω @ 70'F. Due to high resistance the current flow in the injector remains low keeping all the components cool and offers long life. But its disadvantage is that the response time is slower when compared to peak and hold type injector.
5. Different types of fuel injection
The pulsation in fuel injector can be of different types. These types are:
Single-point or throttle body injection (TBI)
Port or multi-point fuel injection (MPFI)
Continuous injection
Direct injection
Single point injection
They are commonly known as Throttle Body Injection or TBI. This was the earliest and the simplest type of fuel injection that was developed by the automotive world. In this system the fuel injector injects the fuel at the throttle (throat of the engine's air intake manifold) body much like the carburettor. TBI system was the stepping stone for various automakers and led to the development of presently used continuous or multi-point fuel injection systems. Though TBI system is not as precise and advantageous in many ways compared to other systems used presently, when compared to carburettor system it meters fuel better and proves to be less expensive.
Throttle body injection
Continuous injection
In this system, the injectors spray the fuel continuously at variable rates. This is in contrast to other fuel injection system where the fuel is pulsed at varying duration having a constant flow during each pulse. This is a most common type of injection in piston aircraft engines.
View of the CIS-E fuel supply system and components.
Multi-point injection
In this system for each there is an independent fuel injector situated at the intake port of the cylinder which is why this kind of system is often calledpot injection. This way it holds true that for a four cylinder engine no. of injectors are 4, for V6 there are and correspondingly 8 for V8.
This makes the multi-point fuel injection expensive due to added number of injectors. But it also tends to be advantageous, when performance is taken into consideration. An engine with multi-point fuel injection will produce more power than one with throttle body injection. This is because there is cylinder-cylinder fuel distribution.
The other advantage is that since the fuel is directly injected into the intake ports, it eliminates the possibility of condensing the fuel and accumulates in the intake manifold. In TBI systems a separate consideration has to be made to preheat the intake manifold to vaporize the liquid fuel. Thus for multi-point injection system different material can be used for making the intake manifold and even lighter material such as plastic. This offers engineers flexibility in designing.
In multi-point fuel injection there are different ways in which the injector is pulsed. In some systems there is simultaneous pulsing i.e. all the injectors are wired together and fuel is injected to all the cylinders simultaneously. In other systems all the injectors are wired separately and they are pulsed sequentially and all the injectors are timed corresponding to each cylinders intake stroke. There is one more way in which injectors are pulsed, but it is not in much use. In this system the injectors are batched, and injector injects the in to the cylinders in groups without any synchronization in relation to each cylinder's stroke.
Direct injection
This kind of injection system is mostly used for diesel engine, but as technology is improving and emissions control is becoming everyone's concern, this system has also been employed in petrol engines. In this system the injectors are placed in the combustion chamber past the valves. This system costs more than the other injection system discussed above as the injectors are exposed to more pressure and heat from the combusting fuel. Thus more costly materials are used for manufacturing the injectors and more precise electronic management is required.
Apart from the cost direct fuel injection system has its own advantages. Since the injectors are placed close to the valve it helps in emission control as the "wet" portion of the induction system along the inlet tract is eliminated. Direct fuel injection system offers better dispersion and homogeneity which imparts more compression ration and aggressive ignition timing which results in enhanced power output. Homogeneity in the fuel mixture allow the fuel-air ratio to be leaner which with precise ignition timing improves greatly the fuel efficiency.
6. Air Induction System
The first step in fuel injection system is the inlet of air into the engine. This is the purpose of air induction system i.e. to filter, meter and measure intake air flow into the engine. This system consists of air cleaner, air flow meter, throttle valve, air intake chamber, intake manifold runner, and intake valve.
6.1 Air Flow Meter
These are devices that measure the amount of air that has entered into the engine and sends the signal to the ECU, which then accordingly calculates the amount of fuel required to get a proportionate mixture for that specific throttle opening.
There are two types of air flow meters being used - vane air flow meter and hot wire sensor.
6.1.1 Vane air flow meter
It measures the volume of air entering the engine through a spring-loaded mechanical flap. This flap is also called the measuring plate as the volume of air is measured by the opening of the flap. This flap is connected to a potentiometer (variable resistor or rheostat). The incoming air pushes or exerts force on the flap that is proportional to the volume of air entering the engine. The movement of the plate is transferred through a shaft to the arm of the potentiometer, which produces a voltage signal i.e. proportional to the volume of air entering the engine. As more air flows into the engine, the flap opens further producing a variable voltage output.
Vane air flow meter has the disadvantage that it measures the volume of air and not mass. As the air temperature changes so does the mass of air, so some changes are needed on the air fuel ratio depending on the temperature of the air entering. This is somewhat compensated by the air temperature sensor installed in vane air flow meter.
6.1.2 Karman Vortex Air Flow Meter
They measure air flow by creating a swirling effect or vortex or turbulence in the airflow downstream. The sensor measures this turbulence which is created behind a small object that is placed in the path of the inlet air which generates air-flow signal. The greater the air flow, greater is the turbulence.
This turbulence is measured by passing light or sound waves through the incoming air and detects the change in the pressure or by measuring the frequency of the air turbulence. Then a signal is generated by the sensor which is proportional to the flow of air intake.
The whole system works in this way. On either side of the chamber are located a transmitter and a receiver that sends and receives a signal. In this system the transmitter is a LED and the receiver are mirror and photo receptor. The mirror is mounted over a spring located near the hole where the vortices are formed. As the intake air flows through the hole vortices are formed, and this drop in pressure wiggles the spring placed near the hole. This wiggling of the spring causes the LED light falling on the mirror to flicker and this flickering is picked up by the photo receptor. This flickering is proportionate to the air flow. The photo receptor receiving this flickering light creates an electric signal which varies in frequency proportionate to the air flow.
Karman vortex air flow meter is just like vane air flow meter, it measures the volume and not the mass. It must be synchronized with other sensors such as air density and temperature sensors to enable the ECU to make the necessary calculations for determining the air mass.
But they have their own disadvantages; these devices are very sensitive and wear out as time passes. They also cause funny driveability problems such as surging, hesitation, poor idle, cutting out and non-start which is mostly noticeable when the engine is running hot or at normal operating temperature.
6.1.3 Hot wire air flow meter
This unlike the other flow meters discussed measures the mass of air flowing into the engine. There is a hot wire that is suspended between the walls of the measuring tube. This meter works on the principle of constant-temperature where the hot wire is an integral part of the circuit. This wire is heated with electric current which is suspended in the intake air stream. As the temperature of the wire increases due to heating the electrical resistance of it falls which limits the further flow of the current through the wire. As the air flows into the engine there is a temperature drop in the hot wire, as a result its resistance decreases correspondingly. This results in the more flow of the current into the hot wire, and the temperature of the hot wire increases till it reaches equilibrium i.e. to the prior original temperature. Thus the mass of air is determined by measuring the amount of current needed to heat the hot wire back to its original temperature.
Hot wire air flow meter has its advantages when compared to other air flow meter. Since there are no moving parts it's wear-free. It responds very quickly to any change in the flow rate of the air s the control has no effect on the heat balance of the hot wire. Unlike other flow meters it measures mass directly, therefore additional pressure and temperature sensors are not required. But it has one drawback, during its operation deposits can form on the hot wire which has negative effect on the measured results.
6.2 Throttle Body
It is a part of the air induction system that controls the quantity of air that enters in to the engine's combustion chamber. The throttle body consists of a throttle valve (butterfly valve) which controls the amount of air inlet and indirectly having control over the fuel burned during each cycle as constant air/fuel ratio is maintained through out the cycle.
The throttle valve is controlled by the driver by the amount of force he applies on the accelerator pedal. This pedal is connected to the sensor which sends signals to ECU about the pedal position. Then the ECU controls the valve through a motor connected to it.
When the accelerator is pressed by the driver, the throttle plate rotates within the throttle body, which opens the throttle passage allowing more air to enter in to the intake manifold. When the valve is wide open, the intake manifold is usually at atmospheric pressure. This change in pressure is communicated to the ECU by airflow sensors. After receiving this signal, the ECU in order to maintain a constant air/fuel ratio the amount of fuel being sent to the fuel injectors is increased. In order to determine the throttle position if it is in idle position, wide-open throttle (WOT) position, or in between these extremes, an additional throttle position sensor is connected to the shaft of the throttle plate.
Components of throttle body
7. Design Component
In this article design of few of the components of EFI system will be discussed.
7.1 Fuel Pressure Regulator
The fuel pressure regulator is a diaphragm operated pressure relief valve. To maintain precise fuel metering, the fuel pressure regulator maintains a constant pressure differential across the fuel injector.
The specified pressure differential is either 36 PSI or 41 PSI depending on engine application.
Below is the calculation for the appropriate fuel supply from the fuel pump and the pressure regulator. The following fuel supply calculation can be worked backward or forward.
Suppose we have a 300hp engine
Flow = Horsepower x BSFC (Brake Specific Fuel Consumption)
Assuming this is a turbocharged engine having BSFC of 0.55. Therefore,
Flow = 300 x 0.55
Flow = 165 lb/hr
= 27.5 gallons/hr
Below is the schematic diagram of fuel pressure regulator.
Mathematical model description of fuel pressure regulator
7.1.1 Motion equation: Forces acting on the diaphragm during a transient process are mainly due to the pressure difference across the diaphragm, the spring and its preload force and the inertia forces required to accelerate the diaphragm, Damping forces as compared to the spring and pressure forces. The motion equation for the diaphragm is given by:
7.1.2 Flow equation
7.1.2.1 Fuel delivery to the regulator:
The fuel flow delivered from the fuel pump to the regulator is given by:
7.1.2.2 By pass flow:
The excess flow returned to the tank is given by:
Where, Y is the diaphragm displacement.
Air flow to air chamber:
The term sgn (P3-P4) is used to indicate the air flow in both directions during the transient process.
7.1.2.3 Injectors Flow:
7.1.3 Continuity Equations:
Pressure P of a fuel rail volume is a function of the volume included between the pump outlet and regulator inlet (fuel rail volume), the effective bulk modulus of elasticity of the fuel (β) and the net influx of fuel to the volume:
7.1.4 Diaphragm Fuel Chamber Volume:
Variation of fuel pressure in bottom side of diaphragm, has an important role in regular operation. This variation is given by
where, Ad is the diaphragm effective area.
7.1.5 Diaphragm air chamber volume:
The pressure in the upper volume of diaphragm is described by:
7.2 Pulsation Damper
Although fuel pressure is maintained at a constant value by the pressure regulator, the pulsing of the injectors causes minor fluctuations in rail pressure. The pulsation damper acts as an accumulator to smooth out these pulsations, ensuring accurate fuel metering. It is a vessel with gas inside, normally Nitrogen.
When a pulsation damper has been installed the volume supplied by the pump during a complete rotation or work cycle is divided in two parts; one is going to the circuit needs and the other part goes into the pulsation dampener. This volume stored into the dampener is returned immediately to the circuit while the pump is in its suction cycle.
To the amount of liquid going into and out of the dampener in each cycle or pump revolution we will call "dv".
When "dv" is introduced into the dampener the gas filled inside will reduce its volume and increase its pressure, the final gas volume plus the volume of liquid introduced will be equal to the initial gas volume.
The initial gas volume is the total dampener volume or the dampener size. The dampener size is the unknown value to calculate and that will depend in all cases on the pump performances. To the dampener size we will call "Vo"
We can establish that: V2 + dv = Vo, (V2 is the final gas volume)
The initial gas volume is the total dampener volume or the dampener size. The dampener size is the unknown value to calculate and that will depend in all cases on the pump performances. To the dampener size we will call "Vo"
We can establish that: V2 + dv = Vo, (V2 is the final gas volume)
Each dampener has a constant which depends on the charging gas value and its size; Po x Vo = constant.
When the dampeners are working is not convenient that all the liquid stored goes out in each cycle keeping the dampener empty of liquid, this will damage prematurely the bladder or the membrane when the insert fixed on it is hammered against the dampener internal bottom.
We will have a new formula: V2 + dv + v =Vo
Where, "v" is a non used volume of liquid inside the dampener; as a norm this volume is the 10% of the total dampener volume the former formula will change to:
V2 + dv + 0,1Vo = Vo; and from this Vo = (V2 + dv) / 0.9
The following graph and the figure representing the three states of gas volume inside the dampeners will make clearer everything exposed above.
This graphs shows:
At charging gas value "Po" there is no liquid inside the dampener. The curve cuts the ordinate axis in the point where corresponds a zero value in the abscissa axis.
The pressure "P1" is the gas pressure when the volume "v" has been introduced into the dampener; the pressure "P2" is the value reached by the gas when the additional volume "dv" is into the dampener.
From this curve we can see that for a fixed dampener size if the value "dv" increases then the pressure value "P2" will also increase or if we increase the dampener size keeping constant the value "dv" the final pressure gas value "P2" will be lower.
The data needed to calculate the dampener size are :
"dv" = volume of liquid that the dampener must store ( in the different types of pumps described below we will see the relation between "dv" and the capacity per revolution of each type of more used pumps )
"P1" y "P2" are the mini. and maxi. pressure values that are accepted in the circuit.
Lets see an example: If the theoretical or work pressure is "Pt" and the residual pulsation admitted is
+, - 5% of this pressure, the P1 y P2 values will be:
P1 = Pt - (5/100) x Pt, and P2 = Pt + (5/100) x Pt
Note: The "Pt" pressure is that created at the outlet port of the pump
Knowing all this data dv ,P1 and P2 we can already calculate the dampener size "Vo"
From the gases compressed law (we will made some comments about this equality for this application) in isothermal conditions called Boyle law we have the expression;
Po x Vo = P1 x V1= P2 x V2= Constant. (1)
If V1 = Vo - v, and v = 0.1 x Vo
We have, V1 = 0.9 x Vo (2)
And also, V2 =V1- dv (3)
From (1) and (2) we obtain
Po=0.9 x P1 (4)
Finally from; (1) (2)(3)and(4) we will obtain
Po x Vo = P2 x V2; 0.9P1 x Vo=P2 x (V1-dv) =P2 (0.9Vo-dv)
from the underlined equalities we have the final formula
P2 x dv
Vo= ------------------
0.9 (P2-P1)
This is the theoretical simplified formula to calculate the dampener volume in function of; dv, P1 and P2.
As we have already said, that the charging gas "Po" = 0.9P1, this relation between Po and P1 has been taken to avoid the complete liquid empty from the dampener in each work cycle. Having this extra quantity of liquid "v" into the dampener it will compensate the possible variations of gas pressures due to external temperatures variations, and consequently the theoretical "dv" calculated could not be introduced or stored into the dampener.
The former formula (1) Po x Vo=P1 x V1= constant does not comply in the practice ,because when a volume of gas is compressed (in a short time) the temperature rises making an extra increase of the pressure and when the gas expands its pressure drops an extra value because the temperature is reduced-effect refrigerator-
This effect is produced in all the majority of gases, Nitrogen and air included which are the more common used for charging the dampeners ( the atmospheric air can be used for pressures less than 10 Bar ,and always when there is not any chemical reaction between the oxygen in the air and the liquid pumped )
The formula (1) will be transformed;
g g
Po x Vo=P1 x V1= constant
g= specific heat relation of the gas at constant pressure and volume . For the majority of gases, g = 1.41 .This constant is also theoretical. In the practice the value that can be taken is g= 1.25
But in order not to complicate the calculation formula of dampener size we will use a new constant (0.8) that will give the same result.
P2 x dv
Vo= ------------------------
(0.8) x 0.9x (P2-P1)
This formula can be used in practice in all applications needed in the industry. The volume calculated with this formula many times will not be those of one standard manufacturer dampener; except in very exigent applications we can recommend to use the manufacturer standard lower volume, for cost reasons obviously
Note: we have not considered a possible temperature variation of the fluid or environment. This will change the charging gas value at 20° (take note that for a 10°C of temperature variation the gas pressure will change approx. a 3%).
7.3 Fuel Injectors
Fuel injectors are the valves that regulate the amount of fuel that enters the engine of a vehicle. The amount of time that the fuel injector stays open is referred to as the injector pulse width (IPW). Fuel injector duty cycle is a term used to describe the length of time each individual fuel injector remains open relative to the amount of time that it is closed. For example if, during each of the fuel injector's pulses, the injector is open for 75 milliseconds and closed for 25 milliseconds, the injector duty cycle (IDC) would be 75%. This is because the injector remains open for 75% percent of the time that it takes to complete one pulse. Knowing a fuel injector's duty cycle is important because it helps to determine if the injector is still functioning correctly and if injector is of the appropriate size.
7.4 Selecting the proper injector size
In order to select the correct size of the injector for your application, the following formulas are to be used.
Fuel injector size = Horsepower x B.S.F.C
No.of Injectors x IDC
Assuming a 200 hp four cylinder engine, having BSFC of 0.5 lbs/hr and IDC of 80% we get,
Fuel injector size = 200 x 0.5 = 31.25 lbs/hr (per injector) = 328 cc/min
4 x .8
Thus we get that for the above specifications the appropriate fuel injector size is 31.24 lbs/hr or 328 cc/min.
We can also calculate the fuel injector pulse width. It is the amount of time, measured in milliseconds (ms), a fuel injector stays open (delivers fuel) during a cylinder intake cycle. Typical injector pulse width for an idling engine at normal operating temperature is between 2.5 and 3.5 ms.
7.5 Pulse width can be calculated from the air flow as follows:
Firstly the the air mass flow rate is determined by the ECU from the sensors - Massair / Minute.
- Minutes / Revolution is the reciprocal of engine speed (RPM).
- A four stroke engine has Revolutions / Cycle = 2, and 1 / (Strokes / Cycle) = 1 / 4.
- 1 / (MassFuel / Minute) is the flow capacity of the injector, or its size.
Combining the above three terms . . .
Substituting values for 5.0 L engine at idle we get:
Substituting values for 5.0 L engine at maximum power output we get:
The range of injector pulse width is from 4 ms/engine-cycle at idle, to 35 ms per engine-cycle at wide-open throttle. The accuracy of pulse width is approximately around 0.01 ms.
7.6 Calculate fuel-flow rate from pulse width
(Fuel flow rate) = (pulse width) Ã- (engine speed) Ã- (number of fuel injectors)
Looking at it another way:
(Fuel flow rate) = (throttle position) Ã- (rpm) Ã- (cylinders)
Looking at it another way:
(Fuel flow rate) = (air-charge) Ã- (fuel/air) Ã- (rpm) Ã- (cylinders)
Substituting values for 5.0 L engine at idle we get.
(Fuel flow rate) = (2.0 ms/intake-stroke) Ã- (hour/3,600,000 ms) Ã- (24 lb-fuel/hour) Ã- (4-intake-stroke/rev) Ã- (700 rev/min) Ã- (60 min/h) = (2.24 lb/h)
Substituting values for 5.0 L engine at maximum power we get:
(Fuel flow rate) = (17.3 ms/intake-stroke) Ã- (hour/3,600,000-ms) Ã- (24 lb-fuel/hour) Ã- (4-intake-stroke/rev) Ã- (5500-rev/min) Ã- (60-min/hour)
= (152 lb/h)
Thus we observe that there is an increase in maximum engine output by 68% when compared to engine in idle.
8. Conclusion
In this report we have discussed the timeline of fuel injection system and the fall of the carburettor system. Keeping in mind about the emission control EFI system was developed and there is still enough room to make it more efficient. With scientists in search for alternate fuels EFI is the only system that can cope with such changes. In present it presents a great use in emission control, fuel efficiency and performance also.
The complete EFI system and its components have been discussed briefly stating the advantages if each of them. In the end of the report a small section has been allocated for the design component in which design and calculations for few of the components of EFI system has been discussed briefly.
