The device consists of a permanent magnet, a winding, and a soft iron core. It is mounted in proximity to a reluctor disc. The disc has 34 teeth, spaced at 10 ° intervals around the periphery of the disc. It has two teeth missing, 180 ° apart, at a known position before TDC (BTDC). Many manufacturers use this technique with minor differences.
As a tooth from the reluctor disc passes the core of the sensor, the reluctance of the magnetic circuit is changed. This induces a voltage in the winding, the frequency of the waveform being proportional to the engine speed. The missing tooth causes a ‘missed’ output wave and hence the engine position can be determined.
Engine load – manifold absolute pressure sensor
Engine load is proportional to manifold pressure in that high load conditions produce high pressure and lower load conditions – such as cruise – produce lower pressure. Load sensors are therefore pressure transducers. They are either mounted in the ECU or as a separate unit, and are connected to the inlet manifold with a pipe.
Detonation – knock sensor
Combustion knock can cause serious damage to an engine if sustained for long periods. This knock, or detonation, is caused by over-advanced ignition timing. At variance with this is that an engine will, in general, run at its most efficient when the timing is advanced as far as possible. To achieve this, the data stored in the basic timing map will be as close to the knock limit of the engine as possible. The knock sensor provides a margin for error.
The sensor itself is an accelerometer often of the piezoelectric type. It is fitted in the engine block between cylinders two and three on in-line four-cylinder engines. Vee engines require two sensors, one on each side. The ECU responds to signals from the knock sensor in the engine’s knock window for each cylinder – this is often just a few degrees on each side of TDC. This prevents clatter from the valve mechanism from being interpreted as a knock.
Electronic control unit
As the sophistication of systems has increased, the information held in the memory chips of the ECU has also increased. The earlier versions of the programmed ignition system produced by Rover achieved accuracy in ignition timing of 1.8 ° whereas a conventional distributor is
8 °. The information, which is derived from dynamometer tests as well as running tests in the vehicle, is stored in ROM. The basic timing map consists of the correct ignition advance for 16 engine speeds and 16 engine load conditions.
A separate three-dimensional map is used that has eight speed and eight temperature sites. This is used to add corrections for engine coolant temperature to the basic timing settings. This improves drivability and can be used to decrease the warm-up time of the engine. The data are also subjected to an additional load correction below 70 ° C. Figure 8.22 shows a flow chart representing the logical selection of the optimum ignition setting. Note that the ECU will also make corrections to the dwell angle, both as a function of engine speed to provide constant energy output and corrections due to changes in battery voltage. A lower battery voltage will require a slightly longer dwell and a higher voltage a slightly shorter dwell.
Another important function of the mounting plate is to prevent the build-up of harmful gases such as ozone and nitric oxide by venting them to the atmosphere. These gases are created by the electrolytic action of the spark as it jumps the air gap between the rotor arm and the cap segment. The rotor arm is also made of Crasline and is reinforced with a metal insert to relieve fixing stresses.