Programmed ignition
Programmed ignition is the term used by some
manufacturers; others call it electronic spark
advance (ESA). Constant energy electronic igni-
tion was a major step forwards and is still used
on countless applications. However, its limita-
tions lay in still having to rely upon mechanical
components for speed and load advance characteristics. In many cases these did not match ideally the requirements of the engine.
Programmed ignition systems have a major
difference compared to earlier systems in that
they operate digitally. Information about the operating requirements of a particular engine is programmed in to memory inside the ECU. The datafor storage in ROM is obtained from rigorous testing on an engine dynamometer and further development work in the vehicle under variousoperating conditions. Programmed ignition hasseveral advantages.
● The ignition timing can be accurately matched to the individual application under a range of operating conditions.
● Other control input can be utilised such as coolant temperature and ambient air
temperature.
● Starting is improved, fuel consumption is
reduced as are emissions and idle control is
better.
● Other inputs can be taken into account such as engine knock.
● The number of wearing components in the
ignition system is considerably reduced.
Programmed ignition or ESA can be a separate
system or included as part of the fuel control system. In order for the ECU to calculate suitable timing and dwell outputs, certain input information is required.
The crankshaft sensor 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 to
periphery. It has two teeth missing 180° apart, at
a known position BTDC. Many manufacturers
use this technique with minor differences. As a
tooth from the reluctor disc passes the core of the densor 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 engine position can be determined.
Engine load is proportional to manifold pres-
sure in that high load conditions produce high
pressure and lower load conditions, such as cruise, produce lower pressure. Load sensors are there fore pressure transducers. They are either mounted in the ECU or as a separate unit and are condected to the inlet manifold with a pipe. The pipe often incorporates a restriction to damp out fluctuations and a vapour trap to prevent petrol fumes reaching the sensor.
Coolant temperature measurement is carried
out by a simple thermistor. In many cases the
same sensor is used for the operation of the temperature gauge and to provide information to the fuel control system. A separate memory map is used to correct the basic timing settings. Timing may be retarded when the engine is cold to assist in more rapid warm up.
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 in general will 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 engine’s 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 each side of TDC. This prevents clatter from the valve mechanism being interpreted as knock. The signal from the sensor is also filtered in the ECU to remove unwanted noise. If detonation is detected the ignition timing is retarded on the fourth ignition pulse after detection (four cylinder engine), in steps until knock is no longer detected. The steps vary between manufacturers but about 2° is typical. The timing is then advanced slowly in steps of say 1° over a number of engine revolutions, until the advance required by memory is restored. This fine control allows the engine to be run very close to the knock limit without risk of engine damage.
Correction to dwell settings is required if the battery voltage falls, as a lower voltage supply to the coil will require a slightly larger dwell figure.
This information is often stored in the form of a
dwell correction map.
As the sophistication of systems has increased
the information held in the memory chips of the
ECU has also increased. The earlier versions of
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 dynamom-
eter 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
which has eight speed and eight temperature
sites. This is used to add corrections for engine
coolant temperature to the basic timing settings.
This improves driveability and can be used to
decrease the warm-up time of the engine. The
data is also subjected to an additional load cor-
rection below 70°C. Figure 7.14 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 due to changes in battery voltage. A lower battery voltage will require a slightly longer dwell and a higher voltage a slightly shorter dwell. A Windows® shareware program that simulates the ignition system (as well as many other systems) is available for download from my web site.
The output of a system such as this programmed ignition is very simple. The output stage, in common with most electronic ignition, consists of aheavy-duty transistor which forms part of, or is driven by, a Darlington pair. This is simply to allow the high ignition primary current to be controlled. The switch off point of the coil will control ignition timing and the switch on point will control the dwell period.
The high tension distribution is similar to a
more conventional system. The rotor arm, how-
ever, is mounted on the end of the camshaft with the distributor cap positioned over the top.
7.15 shows a programmed ignition system.