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Fuel Injection

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Fuel Injection Fuel injection • Fuel injection is a system for mixing fuel with air in an internal combustion engine. It has become the primary fuel delivery system used in gasoline automotive engines, having almost comppyletely replaced carburetors in the late 1980s. • The carburetor was invented by Karl Benz (founder of Mercedes‐Benz) in 1885 and patented in 1886. • Carburetors were the usual fuel delivery method for almost all gasoline (petrol)‐ fuelled engines up until the late 1980s, when fuel injection became the preferred method of automotive fuel delivery. In the U.S. market, the last carbureted cars were the 1990 Oldsmo bile CtCustom CiCruiser, BikBuick EttEstate Wagon, and SbSubaru Justy, and the last carbureted light truck was the 1994 Isuzu. Elsewhere, Lada cars used carburetors until 1996. A majority of motorcycles still use carburetors due to lower cost and throttle response problems with early injection set ups, but as of 2005, many new models are now being introduced with fuel injection. Internal combustion Engines: Carburetor, Fuel injection Carburetors are still found in small engines and in older or specialized automobiles, such as those designed for stock car racing. Dr. Primal Fernando • A fuel injection system is designed and calibrated specifically for the type(s) of [email protected] fuel it will handle. Most fuel injection systems are for gasoline or diesel Ph: (081) 2393608 applications. 1 2 Gas Review November 1913 Well, lets see if we can figure it out…… Used on tractors, boats, and stationary engines, including the Waterloo Boy Gas Review September 1917 and MdlModel D tttractors 3 4 Venturi‐type Carburetor Carburetor Theory Air/Fuel Mixture To Engine Bernoulli Effect: 2 • Venturi P+1/2 V = Constant Throttle Plate – What is it? Atomized Fuel • Wind blowing in downtown Chicago Fuel Inlet Valve Stem – always stronger in the smaller areas between 2 buildings Float Venturi • River currents Bowl Choke Plate – always faster in a narrower, shallower place than Constant level is Inlet Air maintained in bowl -as Fuel deep, wide pools float moves down, NlNozzle valve stem moves down, allowing more fuel into Metering Orifice bowl, float moves up and closes valve Ref. Obert 5 6 Carburetor Theory Carburetor Theory • It’s all due to Air Pressure (or lack thereof) • Close to sea level pressure is 14. 7 psi –Air has weight –88 lbs in a 12x12x8 ft room • “Vacuum” is a pr essure less than 14.7 psi –Often measured in inches of mercury 14.7 psi ~ 30 in Hg • As engine runs, intake strokes create “vacuum” or lower air pressure in manifold – NlNormal ~10 psi (20(~20 in H)Hg) • With throttle plate open, carburetor throat • Carburetors operate on the venturi effect exposed to manifold pressure • The venturi is a narrowing of the bore 7 8 Calculation of Air‐fuel Ratio The fuel is a liquid before mixing with the air, it can be taken to be incompressible. We can apply Bernoulli’s equation between the atmospheric conditi ons prevailing at the top of the fuel surface in the float bowl, which corresponds to point 1 and the point where the fuel will flow out, at the venturi, which corresponds to point 2. Fuel flow will take place because of the drop in pressure at point 1 due to the venturi effect. 2 2 2 P V P1 V1 P2 V2 gz C (Constant) or gz1 gz2 2 1 2 2 2 9 10 Calculation of Air‐fuel Ratio Calculation of Air‐fuel Ratio p p V 2 Fuel flow will take place because of 1 2 f gz the drop in pressure at point 1 due f f 2 to the venturi effect. P V 2 P V 2 From above equation we can obtain an expression for the fuel velocity at 1 1 gz 2 2 gz 1 2 the jet exit as 1 2 2 2 P P V 2 1 2 2 gz p1 p2 2 V 2 gz 1 2 2 f f P P V 2 AliApplying the continu ity equation for the flfuel, we can obta in the 1 2 f gz (2) 2 theoretical mass flow rate, f f (1) . 3 where ρf isthe didensity of the flfuel in k/kg/m , Vf isthevelilocity of the flfuel mf f Af Vf at the exit of the fuel nozzle (fuel jet), and z is the depth of the jet exit below the level of fuel in the float bowl. This quantity must always be above zero otherwise fuel will flow out of the jet at all times. The value A 2 p p gz of z is usually of the order of 10 mm. f f 1 2 f 11 12 Calculation of Air‐fuel Ratio Air‐fuel ratio neggglecting comppyressibility of air • If we assume air to be incompressible, then we can apply 2 where Af is the exit area of the fuel jet in m .IfCd,f is the Bernoulli’s equation to air flow also. Since initial velocity is coeffic ien t of disc harge of the flfuel nozzle (je t) given by assumed zero, we have 2 2 P1 V1 P2 V2 gz1 gz2 1 2 2 2 . m f 2 C m C A 2 p p gz p1 p2 v2 d , f . f d , f f f 1 2 f Thus mf a a 2 p1 p2 Thus v2 2 a 13 14 Applying the continuity equation for the fuel, we can obtain the Air‐fuel ratio neggglecting comppyressibility of air theoretical mass flow rate, • If we assume air to be incompressible, then we can apply . Bernoulli’s equation to air flow also. Since initial velocity is ma a A2C2 A2 2a p1 p2 assumed zero, we have 2 Thus where A2 is the venturi in m .IfCd,a is the coefficient of discharge of the 2 venturi given by . p1 p2 v2 ma Cd ,a . a a 2 ma . p p then ma Cd ,a A2 2a p1 p2 Thus v 2 1 2 2 . a Air A m a Since Fuel F . m f 15 16 Fuel injection systems • Gasoline fuel injection A C A p p d ,a 2 a 1 2 – Inject the fuel into the engine intake system – F Cd , f Af f p1 p2 f gz Required one injector per cylinder – There are both mechanical and electronic injector systems – Increased power and torque, uniform fuel distribution, rapid engine response to throttle position, precise control of equivalence ratio‐‐‐‐‐ A C A p p d ,a 2 a 1 2 • Diesel fuel injection F Cd , f Af f p1 p2 f gz – Fuel sprayed in cylinder near TDC – Atomization, vaporization & mixing delay ignition If we assume z = 0, then – Ignition occurs wherever conditions right – Combustion rate controlled by injection characteristics (injection rate, spray angle, injection pressure, nozzle size and shape), chamber A Cd ,a A 2 a shape, mixture motion, & turbulence F Cd , f Af f – Glow plug may be used to aid cold starting – Power output controlled only by amount of fuel injected 17 18 Limitations of Petrol Injection Gasoline Fuel Injection • High Initial Cost/High Replacement Cost • Increased Care and Attention/More Servicing Problems • In SI engines the air and • Requires Special Servicing Equipment to Diagnose Faults and Failures fuel are usually mixed • Special Knowledge of Mechanical and Electrical Systems Needed to together in the intake Diagnose and Rectify Faults system prior to entry to • Injection Equipment Complicated, Delicate to Handle and Impossible to the engine cylinder. Service by Roadside Service Units • Ratio of air to fuel ≈ 15 : 1 • Contain More Mechanical and Electrical Components Which May Go • Fuel is injected to trough Wrong individual injectors from a • Increased Hydraulic and Mechanical Noise Due to Pumping and low‐pressure fuel supply Metering of Fuel system into the intake • Very Careful Filtration Needed Due to Fine Tolerances of Metering and Discharging Components port. • More Electrical/Mechanical Power Needed to Drive Fuel Pump and/or Injection Devices • More Fuel Pumping/Injection Equipment and Pipe Plumbing Required‐ May be Awkwardly Placed and Bulky 19 20 Gasoline Fuel Injection System Components Gasoline Fuel Injection‐Injector types 1. Electric Fuel Pump 2. Fuel Accumulator – Maintains Fuel Line Pressure When Engine is • Mechanical injection using an injection Shut Off and Quietness the Noise Created by the Roller Cell Pump pump driven by the engine. 3. Fuel Filter ‐ A Pleated Paper or Lint‐of‐fluff Type Plus Strainer 4. Primary Pressure Regulator – Maintains Output Delivery Pressure to • Mec han ic al, didriv eless, cotiuouontinuous be About 5 Bar injection. 5 Push Up Valve –Prevents Control Pressure Circuit Leakage. It is a Non‐return Valve Placed at Opposite End of Pressure Regulator • Electronically controlled driveless 6. Fuel Injection Valve – Valves are Insulated in Holders to Prevent Fuel Vapor Bubbles Forming in the Fuel Lines Due to Engine Heat. injection. Valves Open at about 333.3 Bar and Spray Fuel. Valve Oscillates About 1500 cycles per second and so Helps in Atomization 21 22 Fuel Injection (electronic, multi‐port) MitdEiMonitored Engine Operating Conditions: TRIGGER COMPUTER Manifold Pressure Engine Speed Air Temperature Coolant Temperature Acceleration INJECTOR DRIVE UNIT Pressure Regulator Fuel Fuel 50 psi typical Filter Pump Injectors FUEL TANK 23 24 Engine Information‐valve timing and volume relationship 25.
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