It’s five in the afternoon, and thousands of cars are firing up to leave the city. Among them is one of the latest luxury imports. Like most modern cars, this one is fitted with the very latest engine control modules able to monitor the engine’s state of operation and modify the fuel mixture and timing to optimise the complete energy conversion process.
Let’s imagine we can eavesdrop on the reports coming in to the ECU (electronic control unit) from the various sensors. Most of these reports will be sent to the CPU (central processing unit), which is inside the ECU, for processing. The processing consists of calculations or comparisons with stored values, to enable the ECU to initiate action at various points.
At start-up, when the ignition key is turned to the ON position, a high-pressure electric pump, inside the fuel tank, starts to rotate, pumping fuel into the fuel lines, which are kept at a reasonable pressure by a non-return valve. This ensures there is sufficient pressure inside the fuel lines to feed the injectors. At the same time, the ECU is activated, and the various sensors come alive. By the time the starter motor starts to rotate, the first reports are coming in from the various sensors:
o Airflow sensor: intake air flow five per cent of maximum.
o Intake air temperature sensor: 18 degrees celsius.
o Crankshaft position sensor: number one piston 45 degrees before top dead centre on the compression stroke.
o Battery voltage sensor: 12,5 volts.
o Engine cooling water temperature sensor: 34 degrees celsius.
o Throttle angle sensor: three per cent open.
o Lambda oxygen sensor: oxygen levels indicate no ignition.
o Knock sensor: no knock.
The data is evaluated and the ECU recognises that the engine is being rotated but hasn’t started, and this causes it to retard the ignition timing and enrich the mixture according to a table or map of values, stored in its memory, that relates the temperatures and airflow values to the required settings. It therefore sends the following signals:
o To number one fuel injector: open for 0,003 milliseconds NOW!
o To the ignition unit: send a spark to number one cylinder NOW!
This is followed by signals, at the correct times, to the other cylinders. As soon as the engine speed picks up, the ECU knows that the engine is running, and advances the ignition timing slightly and leans out the mixture.
The arrival of the data, as well as the evaluation and the subsequent action, has taken only a few milliseconds. As the engine warms up, and the car pulls away into the traffic, the flow of data from the various sensors continues, and the combined CPU/ECU unit is busy comparing the temperature and airflow values with the key values in the memory map. When the cooling water sensor reports a significant rise in water temperature, and the rise in underbonnet air temperature is confirmed by the intake air temperature sensor, the mixture will be made leaner by opening the injectors for a shorter period. In addition, the ignition timing in each cylinder will be advanced until it just starts to knock, and then retarded 0,75 degrees, to keep the engine at an optimum operating level.
Once the car reaches the open road, and the temperature and airflow values are steady, signalling a constant cruising speed and normal operating temperature, the ECU will be guided by the oxygen levels in the exhaust, as reported by the lambda sensor, to keep the mixture as close to the chemically-correct level as possible.
This is necessary because the catalytic converter is only efficient under this condition, combined with a temperature of over 350 degrees celsius. Starting, cold running, acceleration and full throttle running require a richer mixture, so it is only on journeys of more than 20 kilometres that the catalytic converter makes a significant contribution to delivering a clean exhaust.
Let’s take a look at the various sensors in the system, and the data they gather:
o The engine temperature sensor is usually a resistor that responds to engine coolant temperature. The ECU monitors the voltage and converts the voltage to a temperature reading by using a reference table stored in its memory. In the near future, some engines will have electric water pumps, and when this happens the ECU will also have to speed up or slow down the water pump to keep the temperature constant.
Intake airflow temperature is needed to enable the CPU to calculate the mass of air entering the system so that the correct amount of fuel to be injected can be calculated.o The airflow sensor tells the ECU how much air is flowing through the engine, and the values transmitted are combined with the intake air temperature to calculate the mass of air inhaled. This obviously has a bearing on the amount of fuel that has to be injected. Airflow measurements are done in a number of ways. The older systems use a spring-loaded flap inside the manifold, between the air filter and the throttle valve. The angle of the flap, which depends on the amount of air flowing past, is sensed by a potentiometer whose voltage is transmitted to the ECU. Here, the voltage is compared with the voltage initially sent to the potentiometer, and the resulting ratio is used as an indication of the amount of air flowing past. In this way, the ECU compensates for potentiometer aging and initial temperature.
o Air-mass meters are used on many later systems. They embody a heated element, either wire or a film, installed inside the intake manifold, where it is cooled by air entering the system. The wire, or film, carries an electric current, controlled by a modulating circuit that tries to keep the temperature differential between the air and the wire at a constant level. The current required is a measure of the mass airflow, and it even compensates for changes in air density, because, for example, less dense air will have less of a cooling effect.
o The intake manifold pressure sensor monitors the absolute pressure in the intake manifold, which depends on the throttle opening and the engine speed.
Engine position sensors usually employ a ferromagnetic gear mounted on the crankshaft designed to have 60 square-cut teeth, but two are deliberately left off at top dead centre of number one cylinder, to provide a bigger-than-normal gap to act as a crankshaft position sensor. The pickup is a permanent magnet and a soft-iron core with a copper winding. When the engine rotates, the magnetic field at the sensor responds to the gaps in the teeth by generating an AC voltage whose amplitude decreases where the teeth are missing and increases as the engine speed increases.
This enables the ECU to use a drastic lowering in amplitude as a top dead centre trigger, and the regular lows and highs in amplitude (which occur 2 x 60 times per revolution) as the square teeth pass by to register when the crankshaft has rotated by another 360/120 = 3 degrees. The ECU further divides this angle by
four to enable it to advance or retard the spark by steps of 0,75 degrees.
o The crankshaft position sensor is often augmented by a camshaft position sensor so that the ECU can keep control of where each piston is in terms of the crank position as well as its relation to the four working strokes.
o A camshaft position sensor is usually monitored by a sensor triggered by a toothed wheel on one of the camshafts. This voltage is very weak, and is usually amplified inside the sensor before being sent to the ECU to enable it to determine which cylinder needs a spark.
In addition, a constant reporting of position allows the CPU to pick up a slight wavering in piston movement that signals a misfire in one cylinder. The result will be an immediate closure of the relevant fuel injector, so that very little unburnt fuel reaches the catalytic converter. This is done to prevent damage to the converter in the event of unburnt fuel catching alight inside it.
Battery voltage is also monitored constantly, because fuel delivery by the injectors is done on a time basis, and not by varying the injector opening lift. These times are measured in milliseconds, which means that a low battery could affect mixture strength, and the ECU must be able to compensate for this.
Throttle angle obviously tells the ECU what the driver’s demands are. For example, a slowly opening throttle will change very few settings, but a suddenly opened throttle will cause the ECU to enrich the mixture by keeping the injector open for perhaps two thousandths of a second longer. If a drive-by-wire system is used, the throttle angle is even easier to measure and control.
The Lambda sensor is named after the Greek letter Lambda that is used to denote excess intake air. A rich mixture, ie, not enough air, is equivalent to lambda values less than one, while a lean mixture would have a value above one. A catalytic converter is most efficient at a stoichiometric or chemically-correct mixture where Lambda = 1.
The knock sensor uses the piezo-electric effect to send a signal to the ECU when the engine starts to knock. The sensor has a small element that’s free to vibrate, housed in a bigger unit in close proximity to the piezo-electric material. When any of the cylinders start to knock, the vibrating element will apply pressure to the piezo-electric unit, causing it to send a signal to the ECU.
The ECU will then retard the ignition of that particular cylinder in steps of 0,75 degrees until the knocking stops. If no knock has occurred for three ignitions in the offending cylinder, the ECU will advance the timing in steps of 0,75 degrees as long as no knock occurs or until a pre-determined limit is reached.
There’s no doubt that the modern engine management system is very efficient, and has become very reliable, but many people lament the passing of the familiar carburettor, coil and distributor engine management system. It worked very well, despite the crude way that engine mixture and ignition timing were adjusted, and was relatively easy to understand and repair. By contrast, modern electronic systems are so complicated that most manufacturers don’t even try to trace faults in individual units. It’s far easier, and more lucrative, to simply replace them.
Why was the change made, and why has just about everybody adopted the new systems? The reason is simple: pollution. The regulations concerning the levels of pollution that can be tolerated in most heavily populated countries are so restrictive that they’ve forced automotive engineers to drastically reduce the amount of harmful substances that leave the exhaust pipe.
This has only become possible because a way has been found to control the combustion process electronically so that one or more catalyst-driven converters in the exhaust system are able to reduce the pollutants.
