Diesel Engine Computer Systems Computers Control Many Aspects of Engine Operation

Study Unit Diesel Engine Computer Systems Computers control many aspects of engine operation. Controlled systems range from more-obvious applications such as fuel injection systems to less- Preview Preview likely ones like engine cooling fans. The precise nature of computer control systems is largely responsible for the increased fuel efficiency and reduced pollution emissions that characterize today’s engines. This study unit explains how engine-controlling computers interface with other engine systems. Since this interface occurs through the use of input and output devices, which are more-general terms used to describe sensors and actuators, much of this study unit identifies common devices, explains how they work, and then describes their specific role in one or more engine systems. When you complete this study unit, you’ll be able to • Identify the functional components that make up an engine- controlling computer system and describe the role of each • Explain the difference between analog and digital signals as they relate to an ECM system • Describe the operating principles and common engine-control applications for various types of temperature, pressure, position, and speed sensors • Describe the operating principles and common engine- control applications for various types of output devices including relays, solenoids, stepper motors, and piezo actuators • Understand the operating principles of various ECM-controlled engine systems including the fuel injection, engine braking, emissions control, and fault indication systems iii Contents Contents COMPUTER SYSTEM BASICS 1 Basic Computer Operation 1 Control Fundamentals 14 SENSORS 20 Temperature Sensors 20 Pressure Sensors 26 Position and Speed Sensors 31 Specialized Sensors 38 OUTPUT DEVICES 41 Solenoids and Relays 41 Stepper Motors 44 Piezo-Actuated Injectors 45 Common Applications of Output Devices 47 Indicator Lights and Fault Codes 58 SELF-CHECK ANSWERS 63 v Diesel Engine Computer Systems COMPUTER SYSTEM BASICS Like anything unfamiliar, an engine’s computer system may seem strange and confusing at first. Yet, vehicle-computer systems can be easier to troubleshoot and repair than engine or electrical-system failures. Today’s computers use state-of-the-art technology to control a vehicle’s fuel, ignition, emission, transmission, engine brake, and service braking systems. Also, computers are used to control diagnostic, climate control, and many other systems with which drivers interface. Basic Computer Operation Unlike human beings, computers have no true intelligence. A computer is only an electronic device that processes information very swiftly. For any given task, a computer requires much more instruction than a person does. For example, get- ting a glass of water is a very simple task for most people. In contrast, a computer-controlled robot would require millions of instructions to get the same glass of water. A computer is, however, a marvel in the speed at which it can process information and instructions. Most computers can process millions of instructions every minute. The speed with which it processes information is what makes a computer ideal for controlling a vehicle’s operation. The functions of the computers found in our vehicles can be broken down into three categories: • Receiving data • Processing data and/or storing 1 • Outputting signals The computer in Figure 1 controls a very basic system. Here, an input device called a sensor sends a value to the computer. This value, in electronic terms, can be a voltage or digital num- ber. This input value tells the computer the condition of the monitored system. The computer compares the value received against a preprogrammed value in the computer’s memory and makes a decision. The computer then converts this decision into an output signal that either controls an output device or reports information to a dashboard gauge, warning light, or other diagnostic system. Input Signal Output Signal Sensor Computer Output Device FIGURE 1—A computer system receives inputs from sensors and other devices, evaluates and otherwise processes the input, then sends a signal to an output device. A sensor measures a physical property (such as temperature) and converts that measurement to an electrical signal. The voltage level of the signal emitted by the sensor corresponds with the measured value. For instance, one type of temperature sensor represents the temperature to which it’s exposed by emitting a small voltage (several millivolts) that’s proportional to the measured temperature. A type of pressure sensor measures fluid pressure, such as the pressure of oil inside the crankcase, then converts that measurement into a voltage level it supplies to the computer. The computer uses this voltage to generate the pressure reading displayed on the dashboard, illuminate a warning light when the level is too low, and control output devices. Most vehicle sensors employ either a change in voltage or resistance to “report” to the computer what they observe while monitoring an engine system. You’ll learn much more about how various types of sensors function later in this study unit. 2 Diesel Engine Computer Systems Of course, a real-world engine- or vehicle-control computer is much more complex than the one shown in Figure 1. An engine-control computer, often referred to as an electronic control module (ECM) or by a similar name, receives data from and outputs signals related to many engine systems including: • Common rail fuel injection systems—These very common ECM-controlled fuel injection systems include a crank- shaft-driven fuel pump that provides pressurized fuel to a storage component known as the rail (Figure 2). Distribution lines transmit fuel from the rail to injectors serving each cylinder. The ECM turns on fuel flow from each nozzle, either as part of the regular injection process or, in some engines, to support the diesel- particulate filter washdown process. Very high fuel pressure in the rail is key to the successful operation of these systems. However, since the rail supplies fuel to all cylinders at a rate that varies dramatically with operating conditions, and the incoming fuel supply comes from a pump that’s not electronically controlled, the ECM constantly monitors data from a rail pressure sensor to determine when rail fuel pressure drops below or rises above the target value. When pressure exceeds the acceptable limit, the ECM adjusts a metering valve that increases the flow of fuel from the rail into a drain line until the pressure sensor data indicates rail pressure has fallen into the acceptable measurement range. Similarly, if pressure in the rail is too low, the ECM adjusts the metering valve to allow less fuel to flow from the rail into the drain line. When a controller, like the ECM, adjusts a controlled device like a metering valve based on the input from a sensor that’s monitoring the effect of the adjustment, the control system is referred to as a closed-loop system. Diesel Engine Computer Systems 3 High-Pressure Fuel Low-Pressure Fuel Electrical Signals ECM Independently Commands Each Nozzle Engine Control Module ECM Output Controls Metering Valve Position Fuel Rail Pressure Input to ECM Fuel Pump Delivers Pressurized Fuel to Rail Metering Valve Rail Drain Line Fuel Rail to Fuel Tank Pressure Sensor Fuel Nozzles Tank FIGURE 2—Much of an ECM’s computing power focuses on the supply of fuel for the combustion process. This common rail fuel injection system is an example of a computer-controlled engine system you’ll often encounter. Shortly in this program • Electronic unit fuel injection (EUI) systems—Relying on you’ll learn much more data the OEM loaded into the ECM, throttle position, about the engine sys- engine and outdoor temperature, and even altitude, the tems that are briefly described here as well as computer system energizes and de-energizes the sole- the components they noids that control the injector’s spill and needle-control include. Your studies valves. This process precisely controls the pressure at will focus on input which fuel is injected and the exact timing (start point) devices, known as sensors, computer-con- and duration of the injection process. trolled valves as well as other output devices, • Hydraulically actuated unit injector (HEUI) systems—The and the important role ECM controls HEUI unit injector operation in a way played by a feedback cir- that’s similar to the EUI described above. However, in cuit in closed-loop control system operation. these systems the extremely high fuel pressures achieved prior to injection are accomplished through the 4 Diesel Engine Computer Systems use of a hydraulic pumping element. The pumping element’s pressurization of engine oil, monitored by the injection control pressure (ICP) sensor, is electronically controlled by an injection pressure regulator (IPR) whose spool is positioned by the ECM. Throughout this process, the ECM receives and processes pressure- measurement data from the ICP sensor, which it uses to determine whether or not to change the injection pressure. • Engine braking systems—Often referred to as jake brakes, these systems help reduce vehicle speed without the use of wheel brakes. As you’ll shortly read in more detail, this is accomplished by adjusting the valve open- ing and fuel-injection behavior of one or more cylinders (Figure 3). The intention of these adjustments is to use power from the drive wheels to turn the cylinder(s) into a compressor. The ECM is responsible for halting fuel injection to the cylinder(s) involved in the engine braking process, and for changing the position of the cylinder valves to facilitate the power-absorbing compression process. • Exhaust gas recirculation (EGR) systems—One of several computer-controlled systems whose purpose is limiting pollution levels. In this case, the pollutant is known as NOx (oxides of nitrogen). The EGR system includes an ECM-controlled valve, which routes some of the exhaust gas to mix with intake air, reducing the amount of oxygen available for combustion.

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