The fuel injection equipment used on Navy diesel engines is the mechanical type. Some engine manufacturers make and install their own fuel injection equipment. Others rely on manufacturers who specialize in fuel injection equipment and who design and modify their products to meet the requirements of the engine manufacturer. This equipment will vary in construction and method of forcing fuel into the combustion chamber; however, in every case, the fuel injection equipment for any diesel engine must accomplish several basic functions.

FUNCTIONS

The five general functions that fuel injection equipment must accomplish are metering the fuel, injecting the fuel, timing the injection process, atomizing the liquid fuel, and creating pressure for dispersion of the fuel. Each of these functions is defined as follows:

1. METERING-To measure accurately the amount of fuel to be injected, according to engine speed and load

2. INJECTING-To force and distribute the fuel into the combustion chamber

3. TIMING-To allow fuel injection into each cylinder to start and stop at the proper time

4. ATOMIZING-To break liquid fuel up into tiny particles

5. CREATING PRESSURE-To create the high pressure required to force fuel into the pressurized combustion chamber

You can easily recall these functions by remembering the initials MITAC. All five functions are required for effective combustion of fuel in the cylinders of a diesel engine. Now we will discuss the role of each of these factors and how all of the factors work together.

Metering

Accurate metering, or measuring, of fuel means that for a given engine speed, setting, and load, the same quantity of fuel must be delivered to each cylinder just before each power stroke of the engine. If this does not happen, engine speed will be erratic and the horsepower output of the engine will not be uniform. Smooth engine operation and even distribution of the load between cylinders requires that the same amount (volume) of fuel is delivered to a particular cylinder each time it fires, and that equal volumes of fuel are delivered to all cylinders of the engine.

Injecting

The fuel system on the engine must control the rate of injection which, in turn, determines the rate of combustion. The rate of fuel injection at the start must be low enough so that excessive fuel does not accumulate in the cylinder during the first phase (physical) of injection delay (before combustion begins). Injection should then proceed at such a rate that the rise in pressure in the combustion chamber is not too great. However, the rate of fuel injection must be such that the fuel is introduced as rapidly as possible to obtain complete burning of the fuel-air mixture. An incorrect rate of injection affects engine operation in the same way as improper timing. When the rate of injection is too high, the symptoms are similar to those caused when fuel injection is too early. When the fuel injection rate is too low, the symptoms are similar to those caused when fuel injection is too late.

Timing

In addition to measuring the amount of fuel and rate of fuel injection, the fuel injection equip-ment must cause these events to occur at the proper time. Correct timing is vital to ensure that complete combustion takes place and that maximum energy is obtained from the fuel. (In other words, the engine develops rated horsepower for each pound of fuel burned.) When fuel is injected too early in the cycle, ignition may be delayed because the temperature of the air charge in the cylinder is not high enough. On the other hand, late injection results in rough, noisy operation of the engine.

Noisy engine operation occurs when the engine cannot convert as much energy from the fuel into the horsepower required to move the load. Late injection permits some fuel to be wasted by wetting of the cylinder walls and piston crown. This condition, of course, results in poor fuel economy, higher than normal exhaust gas temperatures, and smoky exhaust.

Atomizing

Shortly before the top of the compression stroke, at a point controlled by the mechanical injection timing arrangement, one or more jets of fuel are introduced into the combustion chamber. As explained previously, ignition of fuel does NOT occur immediately on injection. The fuel droplets absorb heat from the compressed air swirling around the combustion chamber. This process is necessary because it causes the liquid fuel to VAPORIZE so it can burn! The duration of the second phase (chemical) of ignition delay is controlled by the design (shape) of the combustion chamber, fuel and air inlet temperatures, degree of atomization of the fuel, and the quality of the fuel. When the fuel-air mixture reaches a temperature at which self-ignition occurs, the flame begins to spread. Injection of the remaining volume of fuel for the cylinder continues during this time. The ignition delay period must be short so that diesel knock can be avoided.

Once the flame has been completely initiated, the fuel being delivered to the cylinder is that which is being injected into the burning mixture. This fuel vaporizes and burns almost instantaneously. This process is the third phase of fuel injection. Liquid fuel must be injected into each cylinder in the form of a fine spray. Proper atomization increases the surface area of the fuel, which must be exposed to oxygen molecules in the air so that complete burning of the fuel can take place and the rated horsepower can be developed. To avoid simultaneous combustion of all droplets of the fuel spray (detonation), the injected spray is usually in the form of fine droplets to start ignition (beginning of second phase) and larger droplets later in the phase. The degree of atomization of the fuel is controlled by the diameter and shape of the nozzle orifice(s) or opening(s), injection pressure, and the density of the air charge in the combustion chamber.