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24-09-21 1 Hits

How Fuel Injection Systems Work

    In trying to keep up with emissions and fuel efficiency laws, the fuel system used in modern cars has changed a lot over the years. 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. Now, all cars sold in the United States have fuel injection systems.

    In this article, we'll learn how the fuel gets into the cylinder of the engi-ne, and what terms like "multi-port fuel injection" and "throttle body fuel injection" mean.

    -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 FIAT fuel injector systems (also known as single point or central fuel injection systems) that incorporated electrically controlled fuel-injector valves into the throttle body. These were almost a bolt-in replacement for the carburetor, so the automakers didn't have to make any drastic changes to their engine designs.

    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 response.

    When You Step on the Gas

    The gas pedal in your car is connected to the throttle valve -- this is the valve that regulates how much air enters the engine. So the gas pedal is really the air pedal.

    When you step on the gas pedal, the throttle valve opens up more, letting in more air. The engine control unit (ECU, the computer that controls all of the electronic components on your engine) "sees" the throttle valve open and increases the fuel rate in anticipation of more air entering the engine. It is important to increase the fuel rate as soon as the throttle valve opens; otherwise, when the gas pedal is first pressed, there may be a hesitation as some air reaches the cylinders without enough fuel in it.

    Sensors monitor the mass of air entering the engine, as well as the amount of oxygen in the exhaust. The ECU uses this information to fine-tune the fuel delivery so that the air-to-fuel ratio is just right.

    -In order to provide the correct amount of fuel for every operating condition, the e-ngine control unit (ECU) has to monitor a huge number of input sensors. Here are just a few:

    Nox sensor - Tells the ECU the mass of air entering the engine

    Oxygen sensor(s) - Monitors the amount of oxygen in the exhaust so the ECU can determine how rich or lean the fuel mixture is and make adjustments accordingly

    Throttle position sensor - Monitors the throttle valve position (which determines how much air goes into the engine) so the ECU can respond quickly to changes, increasing or decreasing the fuel rate as necessary

    Coolant temperature sensor - Allows the ECU to determine when the engine has reached its proper operating temperature

    Voltage sensor - Monitors the system voltage in the car so the ECU can raise the idle speed if voltage is dropping (which would indicate a high electrical load)

    Manifold absolute pressure sensor - Monitors the pressure of the air in the intake manifold

    The amount of air being drawn into the engine is a good indication of how much power it is producing; and the more air that goes into the engine, the lower the manifold pressure, so this reading is used to gauge how much power is being produced.

    Engine speed sensor - Monitors engine speed, which is one of the factors used to calculate the pulse width

    There are two main types of control for multi-port systems: The fuel injectors can all open at the same time, or each one can open just before the intake valve for its cylinder opens (this is called sequential multi-port fuel injection).

    The advantage of sequential vw fuel injector is that if the driver makes a sudden change, the system can respond more quickly because from the time the change is made, it only has to wait only until the next intake valve opens, instead of for the next complete revolution of the engine.

    Engine Controls and Performance Chips

    --The algorithms that control the engine are quite complicated. The software has to allow the car to satisfy emissions requirements for 100,000 miles, meet EPA fuel economy requirements and protect engines against abuse. And there are dozens of other requirements to meet as well.

    The engine control unit uses a formula and a large number of lookup tables to determine the pulse width for given operating conditions. The equation will be a series of many factors multiplied by each other. Many of these factors will come from lookup tables. We'll go through a simplified calculation of the fuel injector pulse width. In this example, our equation will only have three factors, whereas a real control system might have a hundred or more.

    Pulse width = (Base pulse width) x (Factor A) x (Factor B)

    In order to calculate the pulse width, the ECU first looks up the base pulse width in a lookup table. Base pulse width is a function of engine speed (RPM) and load (which can be calculated from manifold absolute pressure). Let's say the engine speed is 2,000 RPM and load is 4. We find the number at the intersection of 2,000 and 4, which is 8 milliseconds.

    From this example, you can see how the control system makes adjustments. With parameter B as the level of oxygen in the exhaust, the lookup table for B is the point at which there is (according to engine designers) too much oxygen in the exhaust; and accordingly, the ECU cuts back on the fuel.

    Real control systems may have more than 100 parameters, each with its own lookup table. Some of the parameters even change over time in order to compensate for changes in the performance of engine components like the catalytic converter. And depending on the engine speed, the ECU may have to do these calculations over a hundred times per second.

    Performance Chips

    This leads us to our discussion of performance chips. Now that we understand a little bit about how the control algorithms in the ECU work, we can understand what performance-chip makers do to get more power out of the engine.

    Performance chips are made by aftermarket companies, and are used to boost engine power. There is a chip in the ECU that holds all of the lookup tables; the performance chip replaces this chip. The tables in the performance chip will contain values that result in higher fuel rates during certain driving conditions. For instance, they may supply more fuel at full throttle at every engine speed. They may also change the spark timing (there are lookup tables for that, too). Since the performance-chip makers are not as concerned with issues like reliability, mileage and emissions controls as the carmakers are, they use more aggressive settings in the fuel maps of their performance chips.

    For more information on RENAULT fuel injector systems and other automotive topics, check out the links on the next page.

    The call for reduction in pollution has been mandated by government′s policies worldwide. This challenges the engine manufacturer to strike an optimum between engine performance and emissions. However with growing technology in the field of fuel injection equipment, the task has become realizable. For past few years it has been the hot topic to improve combustion and emissions of compression ignition engines through optimizing the fuel injection strategies. Choosing between various injection strategies are potentially effective techniques to reduce emission from engines as injection characteristics have great influences on the process of combustion. For example, increasing the fuel injection pressure can improve the fuel atomization and subsequently improve the combustion process, resulting in a higher brake thermal efficiency, producing less HC, CO, PM emissions, but more NOx emission. Pilot injection help in reducing combustion noise and NOx emissions and immediate post injection may help in soot oxidation and late post injection helps in regeneration of diesel particulate filter. This article aims at a comprehensive review of various fuel injection strategies viz varying injection pressure, injection rate shapes, injection timing and split/multiple injections for engine performance improvement and emissions control. Although every strategy has its own merits and demerits, they are explained in detail, in view of helping researchers to choose the better strategy or combination for their applications.


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