26-03-2011, 12:15 PM
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INTRODUCTION
A multi-point injection system, also called port injection, has an injector in the port (air-fuel passage) going to each cylinder. Gasoline is sprayed into each intake port and toward each intake valve. Thereby, the term multi-point (more than one location) fuel injection is used. Multipoint fuel injection (MFI) locates an injector immediately upstream of each inlet valve, which enables better control of the air/fuel mixture to each cylinder.
Ever since the World saw the first single cylinder petrol engine driven Car, a Carburetor became synonymous with it. Later on with the advent of multi-cylinder Engines, so did the Distributors. During the last Century, both evolved considerably till the ‘Euro-II’ like Pollution Norms got the better of them. For, the Carburetors worked on a fundamental principle of ‘reaction’, which could easily get stifled for so many reasons. Whereas man, as always, wanted to be literally in the Driver’s Seat, all the way along.
Thus, as the cliché of necessity being the mother of invention goes, a System of controlled-feeding of Fuel + Air + Ignition Spark to the ICE Cylinders in the early 80s evolved. A system like that had to be based on its ‘needs of the moment’, such as the load on it, engine rpm/road speed etc.
Such a ‘system’ had to ensure that the Pollutants were kept to a bare minimum all over its operating range. Not to mention squeezing the maximum mileage out of the last drop of fuel that went into it. Thus Fuel Injection systems were born replacing nearly a century old Carburetors.
It all started with Single Point Fuel Injection Systems virtually an electronic replica of the Carburetors but soon led to Multipoint Fuel Injection Systems.The 1990 Subaru Justy was the last car sold in the United States to have a carburetor; the following model year, the Justy had fuel injection. But fuel injection has been around since the 1950s, and electronic fuel injection was used widely on European cars starting around 1980.
The Fall of the Carburetor :
For most of the existence of the internal combustion engine, the carburetor has been the device that supplied fuel to the engine. On many other machines, such as lawnmowers and chainsaws, it still is. But as the automobile evolved, the carburetor got more and more complicated trying to handle all of the operating requirements. For instance, to handle some of these tasks, carburetors had five different circuits:
• Main circuit - Provides just enough fuel for fuel-efficient cruising.
• Idle circuit - Provides just enough fuel to keep the engine idling.
• Accelerator pump - Provides an extra burst of fuel when the accelerator pedal is first depressed, reducing hesitation before the engine speeds up.
• Power enrichment circuit - Provides extra fuel when the car is going up a hill or towing a trailer
• Choke - Provides extra fuel when the engine is cold so that it will start.
In order to meet stricter emissions requirements, catalytic converters were introduced. Very careful control of the air-to-fuel ratio was required for the catalytic converter to be effective. Oxygen sensors monitor the amount of oxygen in the exhaust, and the engine control unit (ECU) uses this information to adjust the air-to-fuel ratio in real-time. This is called closed loop control. It was not feasible to achieve this control with carburetors. There was a brief period of electrically controlled carburetors before fuel injection systems took over, but these electrical carbs were even more complicated than the purely mechanical ones.
At first, carburetors were replaced with throttle body fuel injection systems (also known as single point or central fuel injection systems) that incorporated electrically controlled fuel-injector valves into the throttle body. Gradually, as new engines were designed, throttle body fuel injection was replaced by multi-port fuel injection (also known as port, multi-point or sequential fuel injection). These systems have a fuel injector for each cylinder, usually located so that they spray right at the intake valve. These systems provide more accurate fuel metering and quicker
HOW MPFI WORKS?
The MPFI system consists of one fuel injector placed near every intake valve and directed towards it, in the fuel intake manifold. Fuel is supplied to the injector through a common rail. The amount of air intake is decided by the car driver by pressing the gas pedal, depending on the speed requirement. The air mass flow sensor near throttle valve and the oxygen sensor in the exhaust sends signal to ECU. ECU determines the air fuel ratio required ,hence the pulse width. Depending on the signal from ECU the injectors inject fuel right into the intake valve. The fuel sprayed at high pressure gts atomized into fine particles and get mixed with air. The air fuel mixture is sucked ito the engine cylinder and the combustion takes place.
FUNCTIONAL DIVISIONS OF MPFI SYSTEM:
The mpfi system can be functionally divided into the following three main components :
1. Electronic control unit
2. Fuel system
3. Air induction system
These functional divisions are described in the following sections
MPFI- Electronic control system
The MPFI electronic control system is show in the form of block diagram.
The sensors that monitor intake air temperature, the oxygen, the water temperature, the starter signal and the throttle positioned signal to the ECU the air flow sensor sends signal to the ECU regarding the intake air volume.
The RPM sensor sends signal about the engine speed.
The ECU processes all these signals and sends appropriate commands to the injectors, to control the volume of the fuel for fuel injection. When necessary the cold start injector timing switch off the ECU operates the cold start injector which is a part of the fuel system.
MPFI-Fuel system:
The MPFI fuel system is shown in the form of block diagram. In this system, fuel is supplied by fuel pump. At the time of starting, the cold start injector is operated by the cold start injector time switch. The cold start injector injects fuel into the air intake chamber, thus enriching the air fuel mixture . the pressure regulator regulates the pressure of the fuel. The injectors receive signal from the ECU and inject fuel into the intake manifold.
