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Mastering fuel injection

Diagnostic techniques that will lead you to the root cause quickly.
Monday, October 27, 2014 - 07:00
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Since the introduction of fuel injection on the internal combustion engine (ICE) more than 100 years ago, there have been many changes to this system. Early fuel injection systems were based on mechanical principals, these systems used a jerk pump that metered high pressure fuel to a mechanical injector. Later came electronic fuel injection systems that work with both mechanical and electronic principles; these systems use a microprocessor that calculates the fuel delivery needed and commands the fuel injector solenoid. With either of these systems the end result is to meter the correct amount of fuel and deliver this in an atomized format to the cylinder. This is better accomplished with the modern electronic fuel control system so this is the system used on the vast majority of newer vehicles.

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These modern electronic controlled engines are high tech marvels that have more computer processing power than the space shuttle. With this type of sophistication present within the vehicle, many shops struggle when it comes to repairing the modern fuel injection system. When diagnosing electronic engine control systems, 80 percent of these types of problem are moderately easy, 13 percent of these types of problems are difficult, 5 percent of these types of problems are extremely difficult and 2 percent of these types of problem might be unable to be diagnosed by the shop. Many shops are losing all of their profits from the 80 percent that are easy by trying to fix the 20 percent that are hard. What we need are techniques to diagnose these sophisticated vehicles quickly and accurately, and do so in a way that allows the shop to make money.

Diagnosing drivability problems can, and should, be very profitable. In order to diagnose fuel injection problems fast and accurately, you must think outside the box. Fuel injection is about the fuel, right? The fuel is pressurized and delivered in a controlled manner to the cylinder. Fuel injection is part of the engine control system. This system will regulate the intake air, fuel and spark timing in order to achieve the desired performance in the form of torque or power output. The driver of the vehicle will determine the power output from the engine by depressing the accelerator pedal. This is a request to the microprocessor from the Throttle Position Sensor (TPS) or from the Accelerator Pedal Position Sensor (APPS). These sensors convey the driver intent in the form of voltage.

As with all sensors, the TPS and APPS take a physical quantity and convert this quantity to an electrical output signal (voltage). The microprocessor then uses a circuit to change the sensor voltage into a binary code (1s and 0s). This binary code allows the microprocessor to read the voltage change produced from the sensor. In order for the microprocessor to use this voltage, a programmer will program a look up table, otherwise known as a transfer function table. This transfer function table takes the voltage and converts it to a physical quantity. One example of a transfer function table would be used for a Mass Air Flow (MAF) sensor. This sensor may produce an analog voltage that will need to be converted into air weight (grams per second). The transfer function table will have all of the voltage readings that can be produced from the MAF sensor on a particular engine and will provide an actual air weight for each of the voltage readings. In this way the voltage can be used to represent the actual air weight.

So what is the fuel injection system doing as the engine is running? It is calculating the amount of air entering the engine and commanding the fuel injector on-time, the fuel injector sequence, the ignition spark timing and the ignition spark sequence, as well as other functions. We think of fuel injection as being about the fuel; however fuel injection is about the air. You must think outside the box because before the microprocessor can do any control function, it must first calculate the air weight entering the engine.

The fuel weight is a known factor; the air weight is the unknown factor. The fuel injector flow rate is known and is rated based on the restriction size of the discharge orifice and the fuel system pressure. This orifice size is rated by what is known as the Injector Slope or the Injector Flow Rate. This flow rate is the weight of fuel the injector can deliver in a set amount of time. As seen in Figure 1, the fuel weight delivered in a set amount of time can be calculated.

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