22-11-2012, 01:34 PM
MULTI –POINT FUEL INJECTION
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Introduction
I was commuting with my friends back from college one evening when one of them asked me: What is an MPFI Engine?
You must have seen cars with specifications which mention words like MPFI and CRDI or CRDE. To an automotive engineer or enthusiast, it means something, but for a common man, it may not make much sense.
MPFI means – Multi-point Fuel-Injection (also called fuel-injection system)
The term MPFI is used to specify a technology used in Gasoline/petrol Engines. For Diesel Engines, there is a similar technology called CRDI. MPFI System is a system which uses a small computer to control the Car’s Engine. A Petrol car’s engine usually has three or more cylinders or fuel burning zones. So in case of an MPFI engine, there is one fuel –injector installed near each cylinder, that is why they call it Multi-point (more than one points) Fuel Injection.
In plain words, to burn petrol in an Engine to produce power, Petrol has to be mixed with some air, ignited in a cylinder (also called combustion chamber), which produces energy and runs the engine.
Before MPFI system was discovered, there was a technology called “Carburetor”. Carburetor was one chamber where petrol and air was mixed in a fixed ratio and then sent to cylinders to burn it to produce power. This system is purely a mechanical machine with little or no intelligence. It was not very efficient in burning petrol; it will burn more petrol than needed at times and will produce more pollution. But with the advancement of technology this was about to change.
Based on various inputs from the sensors, the computer in the MPFI system decides what amount of fuel to inject. Thus it makes it fuel efficient as it knows what amount of petrol should go in. To make things more interesting, the system also learns from the drivers driving habits. Modern car’s computers have memory, which will remember your driving style and will behave in a way so that you get the desired power output from engine based on your driving style. For example, if you have a habit of speedy pick-up, car’s computer will remember that and will give you more power at low engine speeds by putting extra petrol, so that you get a good pick-up. It will typically judge this by the amount of pressure you put on accelerator.
Objectives
The functional objectives for fuel injection systems can vary. All share the central task of supplying fuel to the combustion process, but it is a design decision how a particular system will be optimized. There are several competing objectives such as:
• power output
• fuel efficiency
• emissions performance
• ability to accommodate alternative fuels
• reliability
• driveability and smooth operation
• initial cost
• maintenance cost
• diagnostic capability
• range of environmental operation
• Engine tuning
History and Development
Herbert Akroyd Stuart developed the first system laid out on modern lines to meter out fuel oil at high pressure to an injector. This system was used on the hot bulb engine and was adapted and improved by Robert Bosch for use on diesel engines.
The first use of direct gasoline injection was on the Hesselman engine invented by Swedish engineer Jonas Hesselman in 1925. Hesselman engines use the ultra lean burn principle; fuel is injected toward the end of the compression stroke, and then ignited with a spark plug. They are often started on gasoline and then switched to diesel or kerosene. Fuel injection was in widespread commercial use in diesel engines by the mid-1920s. Because of its greater immunity to wildly changing g-forces on the engine, the concept was adapted for use in gasoline-powered aircraft during World War II, and direct injection was employed in some notable designs like the Junkers Jumo 210, the Daimler-Benz DB 601, the BMW 801, the Shvetsov ASh-82FN (M-82FN) and later versions of the Wright R-3350 used in the B-29 Superfortress.
Mechanical
The term Mechanical when applied to fuel injection is used to indicate that metering functions of the fuel injection (how the correct amount of fuel for any given situation is determined and delivered) is not achieved electronically but rather through mechanical means alone.
In the 1940s, hot rodder Stuart Hilborn offered mechanical injection for racers, salt cars, and midgets.
One of the first commercial gasoline injection systems was a mechanical system developed by Bosch and introduced in 1952 on the Goliath GP700 and Gutbrod Superior 600. This was basically a high pressure diesel direct-injection pump with an intake throttle valve set up. This system used a normal gasoline fuel pump, to provide fuel to a mechanically driven injection pump, which had separate plungers per injector to deliver a very high injection pressure directly into the combustion chamber.
Electronic
The first commercial electronic fuel injection (EFI) system was Electrojector, developed by the Bendix Corporation and was to be offered by American Motors (AMC) in 1957. A special muscle car model, the Rambler Rebel, showcased AMC's new 327 cu in (5.4 L) engine. The Electrojector was an option and rated at 288 bhp (214.8 kW). With no Venturi effect or heated carburetor (to help vaporize the gasoline) AMC's EFI equipped engine breathed easier with denser cold air to pack more power sooner and it reached peak torque 500 rpm quicker. This was to have been the first production EFI engine, but Electrojector's teething problems meant only pre-production cars were so equipped: thus, very few cars so equipped were ever sold and none were made available to the public. The EFI system in the Rambler was a far more-advanced setup than the mechanical types then appearing on the market and the engines ran fine in warm weather, but suffered hard starting in cooler temperatures.
Supersession of carburetors
The ultimate combustion goal is to match each molecule of fuel with a corresponding number of molecules of oxygen so that neither has any molecules remaining after combustion in the engine and catalytic converter. Such a balanced condition is known as stoichiometry. Extensive carburetor modifications and complexities were needed to approach stoichiometric engine operation in order to comply with increasingly-strict exhaust emission regulations of the 1970s and 1980s. This increase in complexity gradually eroded and then reversed the simplicity, cost, and packaging advantages carburetors had traditionally offered over fuel injection systems.
FUEL INJECTION SYSTEM
The carburetor remains in use in developing countries where vehicle emissions are unregulated and diagnostic and repair infrastructure is sparse. Fuel injection is gradually replacing carburetors in these nations too as they adopt emission regulations conceptually similar to those in force in Europe, Japan, Australia and North America.