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In general, the control system of a diesel engine is responsible for maintaining performance at its optimum while at the same time keeping the engine from exceeding certain emission limits. For instance, good engine performance may be had when injection timing is relatively advanced. Yet, this timing may not be appropriate for maintaining NOx emission below the mandated limit. The control action will then be to retard timing to a place where the engine can comply with NOx emission limits without necessarily exceeding particulate emission limits.
To perform its function, the control system needs three components. Sensors that obtain a measurement of a physical variable through direct measurement or a combination of measurement and computation. For instance electromagnetic sensors can produce an electrical signal any time their magnetic field is interrupted. Gear teeth on the perimeter of a flywheel interrupting the magnetic field of a sensor can be used to indicate speed which is proportional to the frequency of the gear teeth interrupting the sensor’s magnetic field. A “soft”or “virtual” sensor delivers a value through an intermediate computation . These sensors should be able to measure a range of physical and chemical quantities in a time short enough to meet the control requirements of high speed diesel engines. In addition, sensors have to survive the environment in which they are to perform their function. Yet, they need to be produced within a reasonable cost and automotive type durability.
Electrical signals produced by sensors are relayed to the second major component of the control system, the controller. The controller is often described as the brain of the control system where a control action is determined on the basis of calculations that will keep system performance at the required level. The controller can be electronic, but many of these controllers can simply be mass-spring devices that control basic functions such as speed in engines. However, controllers based on purely mechanical or hydraulic devices tend to be cumbersome and bulky. For this reason, the majority of modern control systems are now fitted with electronic controllers built around microprocessors that are also programmable for added flexibility.
The third of the three components of a control system is the actuator. An actuator is a device that receives an order from the controller to perform a certain function or a required control action. In most cases this control function requires that the actuator either close or open a flow path or move a system control component a specific distance. Because of this function actuators are generally likened to muscles in the human body. A very obvious and fundamental actuator in diesel engines is its fuel injection system which controls fueling to each cylinder. In the past fueling was adjusted by setting the pump rack, an action that controlled fueling to all cylinders simultaneously. Modern systems allow full-authority control over injection timing as well as fuel metering to each cylinder independently on a cycle-by-cycle basis.