METHODS OF INJECTION

LEARNING OBJECTIVE: Describe the principles and operation of the different diesel fuel systems.

You have probably heard the statement that "the fuel injection system is the actual heart of the diesel engine." When you consider that indeed a diesel could not be developed until an adequate fuel injection system was designed and produced, this statement takes on a much broader and stronger meaning.

In this section, various methods of mechanical injections and metering control are described. There have been many important developments in pumps, nozzles, and unit injectors for diesel engines over the years with the latest injection system today relying on electronic controls and sensors.

FUEL INJECTION SYSTEMS

Diesel fuel injection systems must accomplish five particular functions-meter, inject, time, atomize, and create pressure. A description of these functions follows:

1. METER - Accurately measure the amount of fuel to be injected.

2. INJECT - Force and distribute the fuel into the combustion chamber.

3. TIME - Injection of the fuel must start and stop at the proper time.

4. ATOMIZE - Break the fuel up into a fine mist.

5. CREATE PRESSURE - Create the necessary high pressure for injection.

You can remember these functions by the initials, MITAC. All five of these functions are necessary for complete and efficient combustion

Metering

Accurate metering or measuring of the fuel means that, for the same fuel control setting, the same quantity of fuel must be delivered to each cylinder for each power stroke of the engine. Only in this way can the engine operate at uniform speed with uniform power output. Smooth engine operation and an even distribution of the load between the cylinders depend upon the same volume of fuel being admitted to a particular cylinder each time it fires and upon equal volumes of fuel being delivered to all cylinders of the engine.

Injection Control

A fuel system must also control the rate of injection. The rate at which fuel is injected determines the rate of combustion. The rate of injection at the start should be low enough that excessive fuel does not accumulate in the cylinder during the initial ignition delay (before combustion begins). Injection should proceed at such a rate that the rise in combustion pressure is not to great, yet the rate of injection must be such that fuel is introduced as rapidly as possible to obtain complete combustion. An incorrect rate of injection affects engine operation in the same way as improper timing. When the rate of injection is too high, the results are similar to those caused by an injection that is too early; when the rate is too low, the results are similar to those caused by an injection that is too late.

Timing In addition to measuring the amount of fuel injected, the system must properly time injection to ensure efficient combustion so that maximum energy can be obtained from the fuel. When the fuel is injected too early in the cycle, ignition may be delayed because the temperature of the air, at this point, is not high enough. An excessive delay, on the other hand, gives rough and noisy operation of the engine. It also permits some fuel to be lost due to the wetting of the cylinder walls and piston head. This, in turn, results in poor fuel economy, high exhaust gas temperature, and smoke in the exhaust. When fuel is injected too late in the cycle, all the fuel will not be burned until the piston has traveled well past top center. When this happens, the engine does not develop enough power, the exhaust is smoky, and fuel consumption is high.

Atomization of Fuel

As used in connection with fuel injection, atomization means the breaking up of the fuel, as it enters the cylinder into small particles, which form a mistlike spray. Atomization of the fuel must meet the requirements of the type of combustion chamber in use. Some chambers require very fine atomization; while 5-15