Engine Management
Today’s car engines must offer low exhaust emissions, good fuel economy and excellent driving performance under all driving conditions. Many factors are important in achieving this objective. Improvements in the mechanical design of the engine, such as the shape of the combustion chamber, location of the spark plug and number of intake valves are very significant. However, precise control of the air-fuel mixture ratio and spark timing are of central importance in maximizing an engine’s power and efficiency, and minimizing its emissions. For a modern engine this task is now considered to be beyond the capabilities of simple mechanical control systems and hence electronic management system must be incorporated. Such a system uses a microprocessor-based electronic control unit (ECU) and a large number of electronic and electromechanical sensors and actuators. The major tasks of an electronic engine management system are to:
• Control of the air-fuel mixture ratio accurately through a fuel injection system.
• Maintain accurate and precise ignition timing for all engine operating conditions.
• Monitor and control several additional parameters such as idle speed, exhaust-gas recirculation, air conditioner operation and fuel evaporative emissions to ensure consistently good performance under all circumstances.
Prior study of chapters 16 and 17 may provide better understanding of this chapter.
18.1.
Engine System
A system is consisted of a collection of interacting parts. These parts are connected together to perform a particular function, which may be complete in itself or it may form just a part of a larger system. A system may also be divided into a number of smaller systems, called sub-systems. In an engine, ignition arrangements and fuel supply layouts are two examples of systems. Along with other systems they form a larger system, named the engine or power system. A carburettor and a distributor are examples of sub-systems.
It is difficult to investigate the behavior of a large family of mechanical components in a complete assembly, because each part responds in a different way to a given change. To simplify this study the assembly is split up into smaller sections, so that the function of each part can
be analyzed. When investigations of these parts are completed, it is possible to visualise the effect, or interaction of individual parts on the complete system.
18.1.1.
System Function and Performance
An investigation of a system often constitutes an analysis to determine its function and study of its behavior to ascertain its performance. A functional study, known as a qualitative analysis, establishes the basic qualities of a system and indicates the role to be full filled by each part of the system. The manner, in which the system performs its role, and the effectiveness of a system in respect to its performance, is called a quantitative analysis. A qualitative study provides the basic information regarding the fundamental duties to be performed by each main part of a system. The quantitative analysis provides its operational details and the measurement of the effectiveness of its operation.
A qualitative analysis of an electronic ignition system is presented in Fig. 18.1. The main parts are represented by blocks in this diagram indicating the function to be performed by each part. This layout in the form of blocks is a simple method commonly used in electronics to indicate the arrangement of the various sub-systems and the paths followed by the control signals in the system.
A quantitative analysis involving performance of a component is generally expressed in a mathematical equation. By comparing the equations of alternative parts, the designer is able to make an accurate, non-subjective judgement, which helps him to select the most suitable system or part for a particular application.
18.1.2.
System Modelling
The representation of the performance of individual parts through mathematical equations can be extended to the complete system. The mathematical representation of particular system or subsystem is called a mathematical model. Combining the models and entering the mathematical data of individual model to a computer makes it possible to understand the overall performance. In addition to this the response of the system can also be studied, when any part of the system is altered.
A modern electronic system has to function over a wide range of parameters and conditions. With the aid this modelling facility, it is possible to vary the signal from each subsystem and study the effect of the change on the complete system. These design aids enable to predict accurately the performance of a system before it is put into production and actual use. This also allows numerous alternatives and modifications, which can be tried to determine if it is possible to improve the performance or to decide on a simpler and cheaper system. Prior to the use of the computer as a design tool, the introduction of a new system required a series of practical tests on the actual components, which was time consuming.
18.1.3.
Control Systems
A control system directs the operation of a main system. The commands given by the control system should ensure that the main system performs according to a given program, which has been devised to achieve a desired performance. In the case of a power unit, this objective may be the production of a given power, the achievement of a set economy or the limitation of a given exhaust product.
Fig. 18.1. Block diagram of an ignition system.
To fulfill a set program within the parameters in the scope of the control system, the system must respond quickly and accurately to changes in the operating conditions, must maintain a stable control and must separate valid input signals from other that are induced into the sensing lines by electrical disturbances (i.e. the system should have noise immunity). The two main systems of control are open-loop and closed-loop.
18.1.4.
Open-loop Control
This type of control system senses commands to the main systems but does not have the ability to check or monitor the actual output of the main system (Fig. 18.2). Considering an engine as the main system, if a signal from an open-loop control system has been given, the engine produces its output, but this output may vary if the engine-operating conditions alter. For example if the engine control system does not take into account of ambient air temperature, then any variation in this temperature alters the power output, which is not corrected by the control system.
Although an open-loop control system is suitable for many applications, however it can not be used where an output is desired within narrow limits such as the case of exhaust emissions, which must be controlled precisely to meet environmental legislation standards. Open-loop controls are still commonly used for fuel supply and ignition systems. In these applications the air-fuel mixture and ignition timing settings follow independent programs. Ideally any change in the air-fuel ratio should be accompanied by an alteration in the timing, but in many cases this does not happen. Consequently, the engine performance becomes lower than expected, economy becomes poor and a high exhaust emission results. This problem is over come by incorporating closed-loop control systems in many new vehicles.
18.1.5.
Closed-loop Control
This control system works in similar way to an open-loop system, but has one important feature of measuring the output from the system and feeding back a signal for a comparison between the command signal and the output produced.
Figure 18.3 illustrtes the principle of operation of a closed-loop system. It can be seen in the diagram that the feedback signal from the output sensor is passed back to the input where it is compared using an error amplifier, which intensifies and processes the signal. If the output differs from the input, the command signal is altered until the required output is achieved.
Fig. 18.2. Open-loop control.
Fig. 18.3. Closed-loop control.
When this control system is used in an engine, the feedback facility corrects any variation in the output, so that a more accurate and stable output is produced, which is not possible with an open-loop system. Moreover the system with closed-loop control can be made to respond quickly and correctly for any changes in the operating conditions. If these conditions are not compensated, then not only the output can be much different from that intended, but in some cases this can cause an extensive damage to the engine.
Two forms of closed-loop control are proportional control and limit cycle control. A closed-loop proportional control system has a sensor in the output, which generates a signal proportional to the output, so that the magnitude of the feedback signal indicates the output of the system. A closed-loop limit cycle control system uses the feedback to signal when a present limit is exceeded. The output sensor is inoperative during the normal operating range, but when the limit is exceeded, the feedback circuit passes a signal back to the input, which permits an alteration to be made to the command input.
