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Electronic Engine Management Pt.7: Other Input Sensors – by Julian Edgar In addition to the airflow and exhaust oxygen sensors pre­viously discussed, engine management systems run other input sensors to allow the system to monitor changing engine and envi­ronmental parameters. For example, the temperature of various parts of the engine is another factor that influences fuel and ignition requirements. This is especially so at engine start-up, as a cold engine requires substantially more fuel to run satis­factorily. Temperature sensors The engine coolant temperature plays a major role in deter­ mining the amount of fuel enrichment. The lower the engine tem­ perature, the greater the fuel correc­tion applied to the base injector opening time. Sometimes this correction factor, which is A potentiometer type throttle position sensor. It meas­ures the precise amount of throttle opening and feeds this data to the ECM (electronic control module) to control fuel enrichment. 4  Silicon Chip also tied to idle speed, is applied in a series of discrete steps. As a result, the engine idle speed reduces in a corresponding series of abrupt steps as the water temperature rises. The coolant temperature sensor also plays a major role, even when the engine is up to operating temperature. In one system, for example, when the coolant temperature is over 95°C and the throttle position switch idle contacts are open (ie, the throttle is applied), fuel injection is increased by 10% over the base quantity. This enriches the mixture to counteract possible detonation. If the same high engine temperature exists at start-up, the fuel pressure is increased to avoid possible vapour-lock problems. The ignition timing control is also affected by the engine coolant temperature. For example, in one engine management sys­ t em, the ignition timing is advanced by about 7° when the coolant temperature is below 0°C. This allows greater time after ignition for maximum combustion pressures to occur. Pollution control mechanisms may also be influenced by coolant temperature. In one car, for example, the evaporated fuel from the fuel tank is purged from its absorption canister by being vented to the intake manifold –but only when the engine is sufficiently warmed-up to burn it without further emissions release. Inside a switch-type throttle position sensor – note the contacts for idle & full-throttle positions. The movable arm (centre) follows the track in the guide cam (see also Fig.9). Fig.1: cross-section of Holden VL Commodore optical crankshaft position sensor. It uses two LEDs and two matching photodiodes to sense slots cut into a rotating disc mounted in the base of the distributor. Other temperature sensing which may be carried out includes the intake air temperature (especially with engines running vane-type airflow meters), cylinder head temperature and – in some programmable injection systems – engine and gearbox oil tempera­ture. Invariably, temperature sensing is carried out by a ther­mistor mounted within a heat-conductive body. Road speed sensor A vehicle road speed sensor is also generally used to feed data to the ECM. This data may be used in several ways. First, many vehicles feature over-run fuel injector cut-off. This means that when the throttle is lifted, fuel injector operation ceases, resuming only when the engine rpm approaches idle speed. This reduces exhaust emissions and improves fuel economy. An example of fuel shut-off occurs in the Nissan 6-cylinder engine used in the VL Commodore. In this case, the fuel injectors are shut off if the throttle position switch contacts are closed (ie, if your foot is taken off the accelerator) at any engine speed above 2000 rpm. The proviso here is that the engine coolant must have reached normal operating temperature. Fuel injection resumes when the engine speed falls below 2000 rpm. In some cars, however, the injector-resume speed is as low as 1500 rpm and a slight jerk can often be felt by the sensitive driver when the injection starts again. The road speed sensor input is relevant here because injector cut-off operation occurs only above a certain speed – 8km/h in the VL Commodore. A second use for road speed data occurs in those cars which run a speed limiter as part of the engine man­ agement sys­tem. Its job is to cut off the fuel or ignition when a certain road speed is reached. This is often well above the speeds reached in normal conditions – even in the Northern Territory! However, domestic Japanese cars run either a 145 or 180km/h speed limiter. The road speed sensor is usually built into the back Fig.2: the rotating disc in the VL Commodore’s distributor has 360 1° slots around its periphery to provide a signal that’s proportional to engine speed. Also on the disc are six slots at 60° intervals to indicate the crankshaft posi­tion. The large slot at the top indicates the position of the number one piston. Fig.3: this diagram shows how the rotating disc & the optical sensor are mounted in the base of the distributor. April 1994  5 The crankshaft position sensor is often built into the base of the distributor, as in this Holden 4-cylinder engine. This distributor-based system uses an optical pick-up but an inductive pick-up system using a coil & a magnet to sense protrusions on a crankshaft sprocket can also be employed. of the speedometer and so uses the speedo cable to drive it. Other systems mount the sensor on the gearbox. Crankshaft position sensor One very important sensor is the crankshaft (or camshaft) position sensor. This provides vital inputs to the ECM so that it can provide the correct injection and ignition timing. Fig.1 shows a cross-section of the optical sensor used in the Holden VL Commodore engine. It uses two LEDs, two photodiodes and a rotating disc. The rotating disc is built into the base of the distributor and has 360 tiny slots near its outside edge (see Fig.2). These slots rotate between one LED and its corresponding photodiode and provide a signal to the ECM that’s proportional to engine speed. In addition, there are a further six slots in the disc but fur­ther towards the disc’s centre. Five of these are of the same size but the sixth is much larger. Fig.4: the Subaru Liberty uses an inductive pick-up sensor to determine the crankshaft position. This sensor consists of a magnet & coil assembly & is mounted close to a toothed crankshaft sprocket. 6  Silicon Chip These six slots are placed 60° apart and are used to signal the crankshaft angle (or piston position) to the ECM. The large cutout is used to show the position of number one piston. Fig.3 shows the whole assembly. Other manufacturers use an inductive system, whereby a crankshaft sprocket with specifically located protrusions rotates past a moulded pick-up containing a magnet and coil. Fig.4 shows the cross-section of the inductive sensor used by Subaru in the Liberty. Fig.5 shows the layout of the system. Note that the pick-up is separated from the toothed sprocket by only a small air gap. In operation, the magnet briefly magnetises the sprocket protrusion as it passes the sensor and a voltage is then induced in the coil as the air gap changes. An AC waveform (Fig.6) is emitted by the pick-up, with the pulses occurring at different crankshaft positions. Camshaft position sensors often work in the same way. Knock sensor Knock (or detonation) occurs when fuel in the combustion chamber ignites before the progressively-moving flame front actually reaches it. When this happens, a sudden increase in combustion pressure occurs and this blow to the piston is the “tinking” sound heard inside the car. The fact that this sound is produced by a detonation hitting the crown of the piston Fig.5: as each protrusion on the crankshaft sprocket passes the sensor, a voltage is induced in the pick-up coil. This voltage is then fed to the ECM to indicate the crankshaft position. This late 1980s Holden 4-cylinder engine is fitted with six major input sensors for the ECM plus three minor sensors. Fig.6: the shape of the output waveform from an inductive pick-up sensor. Fig.7: cross-section of a typical knock sensor. It uses a piezoe­lectric transducer as the sensing element. April 1994  7 Fig.8: the knock sensor control process, as developed by Bosch. A filtering & evaluation system is needed to differentiate detona­tion noise from ambient engine noises. deep inside the engine indicates the violence of this phenomenon! Detonation can occur because the ignition timing is too advanced, the fuel octane rating is too low, or the tur­ bocharger boost pressure is too high – or due to a combination of these factors. However, maximum efficiency is often gained by running engines very near to the onset of detonation and so knock sensors are now being used in some engine management sys­tems to prevent engine damage. Knock sensors employ piezoelectric elements, with elaborate filtering and Fig.9: the layout of switch-type throttle position sensor. The movable contact is controlled by a guide cam & closes with the power contact when the throttle is opened. comparison circuits to differ­ entiate knock from normal engine noise. Fig.7 shows a typical knock sensor, while Fig.8 shows the control process carried out by the ECM. The sensor itself is usually screwed into the block near to the head (some systems use separate knock sensors for each cylin­der but most road-going engines make do with one). When knock is sensed, the ECM usually retards ignition timing and then, when the problem has gone, slowly advances the timing back to its original figure. Knock sensors are notoriously A typical intake air-temperature sensor. It is bolted into one of the intake runners. Temperature sensors invariably use a thermistor mounted inside a heat-conductive body. 8  Silicon Chip prone to false-alarming. In one car, the fault-code indicating a problem with the knock sensor is almost sure to be registering – with no apparent fault present! Shielding of the input cable is generally used to prev­ent interference but problems have continued to plague this device in production cars. Throttle position sensors Throttle position sensors (TPS) do just that – they in­dicate to the ECM the opening of the throttle valve. In the past, these sensors were invariably simple switches, with contacts for idle and full load. Current cars can run switches of this sort or can use a combination of an idle-position switch and a potentiom­eter to indicate the precise throttle opening. Other cars use just a potentiometer. Fig.9 shows a switch-type throttle position sensor. Input data from the throttle position sensor is used to indicate when full load and/or acceleration injection enrichment is needed, and to signal injector cut-off on the over-run. This sensor also sometimes causes the air-conditioner clutch to be switched off under full throttle, thereby allowing maximum road performance. Those cars using a potentiometer TPS also often use an ECM that’s programmed to take note of the speed of the throttle opening, as well as its angle. Rapidly flooring your right foot will then give different ignition advance and fuel rates compared to gentle acceleration to full throttle. SC