Open-loop Engine Control Systems (Automobile)

18.3.

Open-loop Engine Control Systems

Prior to about 1935, only manual adjustment of the ignition timing and setting of the air-fuel mixture were carried out in addition to manipulating the main control. This demanded considerable skill since it involved the setting of the ignition to a point where the engine was just knock-free and keeping the mixture at the point where it just rich enough to develop maximum power. In later years, these activities were taken over by open-loop systems, which automatically controlled the ignition and fuel-mixture settings in accordance with signals received from engine indicating its speed and load.
18.3.1.

Ignition Timing Control

A centrifugal timer was first introduced for automatic timing. This speed-sensitive unit advanced the spark to maintain maximum cylinder pressure at about 12 degrees after TDC over the full speed range. This type of automatic control works with the assumption that the burn-time between spark and maximum pressure remains constant in time. A weak mixture
increases this burn-time. Therefore the weaker mixture delivered by economy carburettor during part-load operation needed a larger advance than that provided by the centrifugal timer. This requirement was fulfilled by a load-sensitive vacuum control unit. The spring-loaded diaphragm used the manifold depression to sense engine load and the carburettor also used this source for the same purpose, so that the actions of the ignition timing control and the carburettor were harmonised.
Subsequently electronic control unit (ECU) was used to meet the requirement of greater precision of the timing. The timing requirements are stored in a memory of ECU. The timing data must relate the timing of the spark to the two factors, the engine speed and engine load (Fig. 18.10).
Although the modern open-loop system has undergone considerable improvements over earlier designs, but still carries one draw back that it assumes the engine in the same condition as used when the memory was programmed. When this is not correct, the timing provided by the look-up table in the ECU becomes unsuitable for the engine as a result emission, economy and power all suffer from inaccuracy to certain extent.
18.3.2.


Fuel-mixture Control

The constant-choke carburettor was used in the system, which has mixture compensation systems to bring about correction of mixture enrichment with increase in load/speed. Sub­sequently economy systems were introduced to weaken the mixture during part-load operation. Although methods for improving mixture distribution and poor atomization at low speeds received considerable attention from carburettor designers, but the benefits of fuel injection with respect to these problems did not become attractive until stricter emission controls came into force.
Both engine load and engine speed must be measured by the main sensing system, whether it is carburettor system or injection system. Mechanical as well as electronic sensing systems suffer from the same drawback as the ignition unit that is they can only follow the program on the engine condition introduced in the beginning. Variable factors such as air leaks past the pistons, valve guides, throttle spindles, etc. have not been considered, as a result the output suffers, which can be improved by using more sensors. Introduction of electronic systems can take into account many more variables when compared with mechanical systems.
18.3.3.

Combined Ignition and Fuel-supply Systems

As indicated earlier both the ignition timing and the fuel-supply systems incorporate sensors to measure engine speed and engine load. It is uneconomical to duplicate the basic sensors and control electronics. Therefore in many cases the two systems are combined to form a single Engine Management System.
Combination of the two sensors also permits other sensing signals to be used jointly by ignition and fuel systems, providing greater precision of control. These additional sensing signals include the measurement of ambient and engine temperatures and other factors that affect the operation of the engine (Fig. 18.11).
Ignition timing system.
Fig. 18.10. Ignition timing system.
Combined ignition and fuel system.
Fig. 18.11. Combined ignition and fuel system.
18.3.4.

Use of Engine Maps

To achieve precise control of an engine without any means for feeding-back output data, the control system program must be fed with very accurate information relating to the setting required for each condition under which the engine is expected to perform. This program should also ensure that the input command produces the expected engine response. Control maps drawn during the engine development provide the settings of the main system with respect to the parameter, which affect the particular system. The control data indicated by the maps is programmed into the ECU’s memory unit forever.

The computer uses the stored control data in several ways. In one way the computer calculates the ignition and fuel mixture settings each time the speed or load conditions change. This computation may take as long as 100 milliseconds to execute. Since this operation may have to be repeated at 50 millisecond intervals, a quicker method is necessary, which is achieved by using look-up tables.
18.3.5.

Look-up Tables

A look-up table is a list of related data stored in the memory of a computer. This table relates the output settings provided by the computer to the input signals received from a particular sensor. To understand the principle of performing this task by the computer, consider the example of the spark-timing look-up table shown in Fig. 18.12, which has been complied from the engine map illustrated in Fig. 18.8. To simplify the process further only the speed parameter is considered at this stage.
Spark timing look-up table.
Fig. 18.12. Spark timing look-up table.
Consider the engine is running at 32 rps (1920 rpm). The appropriate sensor signals this speed to the ECU. In the computer the signal initially is converted by an encoder into the 8-bit digital form, 00100000, which is the binary code for the number 32. This code is then stored in one of the registers of the CPU and the memory is searched until a similar code is recognized. When the search has been completed the memory unit then issues another binary code, such as 00110100. This is the code that the computer has been programmed to write whenever it reads the code 00100000, in other words the matching value in its look-up table. The 8-bit code is the spark timing instruction to the ignition control unit, so, after it has been deciphered, the control unit sets the spark to occur at 52 degrees before TDC.
This example is limited to a table of 10 values, but a modern computer has a much larger table. In addition to the spark advance/speed table, more tables covering other variables are also stored in the memory unit. Since spark advance depends mainly on load and speed, the advance indicated in the load look-up table must be added to the value indicated by the speed look-up table. The computer performs this task very quickly as it involves only the addition of the values obtained from the look- up tables.
To obtain this method of control, the following basic steps must be followed. (i) Mapping of the prototype engine to determine the best settings.
(H) Construction of maps to indicate graphically the required settings to suit the varying operating conditions.
(Hi) Programming the computer’s look-up tables using the data contained in the maps. (iv) Verification through engine testing that the control instructions provided by the computer are in accordance with the requirements for the control.
18.3.6.

Detailed Maps

For the test possible engine performance, the various maps should closely follow the requirements of the engine. To achieve this, maps should be used with many more reference points than those used in Fig. 18.8. Also an engine management computer has to be installed which has a memory unit of sufficient capacity to store the detailed look-up tables. Figure 18.13 may be referred for an example of a detailed map.
Detailed map for electronic spark advance.
Fig. 18.13. Detailed map for electronic spark advance.

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