Basic Principles
Purpose of Fuel Injection System
The performance of diesel engines is heavily influenced by their injection system design. In fact, the most notable advances achieved in diesel engines resulted directly from superior fuel injection system designs. While the main purpose of the system is to deliver fuel to the cylinders of a diesel engine, it is how that fuel is delivered that makes the difference in engine performance, emissions, and noise characteristics.
Unlike its spark-ignited engine counterpart, the diesel fuel injection system delivers fuel under extremely high injection pressures. This implies that the system component designs and materials should be selected to withstand higher stresses in order to perform for extended durations that match the engine’s durability targets. Greater manufacturing precision and tight tolerances are also required for the system to function efficiently. In addition to expensive materials and manufacturing costs, diesel injection systems are characterized by more intricate control requirements. All these features add up to a system whose cost may represent as much as 30% of the total cost of the engine.
The main purpose of the fuel injection system is to deliver fuel into the cylinders of an engine. In order for the engine to effectively make use of this fuel:
Fuel must be injected at the proper time, that is, the injection timing must be controlled and
The correct amount of fuel must be delivered to meet power requirement, that is, injection metering must be controlled.
However, it is still not enough to deliver an accurately metered amount of fuel at the proper time to achieve good combustion. Additional aspects are critical to ensure proper fuel injection system performance including:
Fuel atomization-ensuring that fuel atomizes into very small fuel particles is a primary design objective for diesel fuel injection systems. Small droplets ensure that all the fuel has a chance to vaporize and participate in the combustion process. Any remaining liquid droplets burn very poorly or are exhausted out of the engine. While modern fuel injection systems are able to produce fuel atomization characteristics far exceeding what is needed to ensure complete fuel evaporation during most of the injection process, some injection system designs may have poor atomization during some brief but critical periods of the injection phase. The end of the injection process is one such critical period.
Bulk mixing-While fuel atomization and complete evaporation of fuel is critical, ensuring that the evaporated fuel has sufficient oxygen during the combustion process is equally as important to ensure high combustion efficiency and optimum engine performance. The oxygen is provided by the intake air trapped in the cylinder and a sufficient amount must be entrained into the fuel jet to completely mixed with the available fuel during the injection process and ensure complete combustion.
Air utilization-Effective utilization of the air in the combustion chamber is closely tied to bulk mixing and can be accomplished through a combination of fuel penetration into the dense air that is compressed in the cylinder and dividing the total injected fuel into a number of jets. A sufficient number of jets should be provided to entrain as much of available air as possible while avoiding jet overlap and the production of fuel rich zones that are oxygen deficient.
The primary purposes of the diesel fuel injection system are graphically represented in Figure 1.
Figure 1. Main Functions of Diesel Fuel Injection System
Definition of Terms
Many specialized concepts and terms are used to describe the components and the operation of diesel fuel injection systems. Some of the more common of these include [Breitbach 2002][Bosch 2004]: