The sensor inputs I will be discussing are the Coolant Temperature Sensor (ECT), Throttle Position Sensor (TPS), Oxygen Sensor (O2), Manifold Absolute Pressure Sensor (MAP), Mass Air Flow Sensor (MAF), Camshaft Position Sensor and the Crankshaft Position Sensor. First of all, it should be stated that all of these sensors are electrical switches which are linked to the ECU and send information which contribute to the adjustment of engine output at any given time.
The Coolant Temperature Sensor, is located in the coolant passage just before the thermostat, monitors just what it suggests, the coolant temperature of the engine. This sensor is a thermistor, which is really a resistor, and depending on the temperature of the coolant in the engine, changes the resistance of the switch which therefore changes the voltage output to the ECU. For example, if the temperature of the coolant increases, the resistance of the switch decreases and the voltage output will increase. This sensor contributes to the fuel efficiency of the engine. A basic reading from this switch would be about 2-3k ohms at 20⁰C, this will change due to the temperature.
When coolant is cold resistance through the sensor is high and less voltage can pass, but once coolant properly warms up resistance through the sensor is reduced and more voltage can pass.
Resistance | 0.95 kΩ |
Voltage, engine cold | 3.3 V |
Voltage, engine properly warm up | 1.3 V |
The Throttle Position Sensor, located externally on the throttle body, senses the position of the throttle butterfly and the rate of throttle opening.It measures the position of the throttle valve, whether it’s wide open or closed and sends it to the ECU.
This sensor is a variable resistor that’s resistance changes as the throttle butterfly opens and closes. The more the throttle butterfly opens, the more acceleration applied to the engine which will mean that the engine will need to alter fuel output, controlled by the ECU. Generally when testing this sensor, at closed throttle the reading should be approximately -1.25volts and at wide open throttle should read approximately 5volts, as specified by the manufacturer.
Resistance | 0.54 kΩ |
Voltage, engine idle / throttle close | 0.6 V |
Voltage, wide-open throttle | 1.8 V |
The Manifold Absolute Pressure Sensor is mounted on the intake manifold and monitors the pressure in the manifold vacuum. When the pressure changes the sensor will change its voltage output to the ECU for adjustments to be made. The reading of the sensor will, again, change depending on the vacuum, for example, a reading of 0.5-1.5volts means there is a high vacuum and low pressure which will suggest that the vehicle is idling or decelerating. Whereas a reading of 3.0-4.5volts means there is a low vacuum and high pressure which suggests that the vehicle is accelerating hard or perhaps there could be a mechanical fault.
The Mass Air Flow Sensor measures the air entering the engine at any given time.It Measures the quantity of air flowing to the throttle body and sends it to the ECU There are different types of these sensors but the one to be discussed currently is the hot wire air flow sensor. This works by air passing by a hot element and cooling this element, which then generates more voltage to maintain the heat. The voltage increase is how this is measured. Output at idle can range from 0.4-0.8volts whereas at wide open throttle the voltage will generally be around 4.5-5volts.
Now, the Camshaft Position Sensor is positioned either directly on or next to the Camshaft. This sensor picks up the motion of the magnetic field generated by the movement of pins on the camshaft. The different speeds this camshaft turns will allow the magnetic field to fluctuate which in turn will alter the speed of the engine. This sensor is used with the Crankshaft Position Sensor to ensure correct ignition timing as well as operation and performance of the engine.
The Crankshaft sensor is made out of 3 segments, centre part is a magnet, and the other 2 are sensors. Magnetic field is induced between the sensors and the magnet, as the magnetic field is broken, pulse signal are produced. These pulses are send to the ECU to determine the position of the pistons. The ECU then calculates the engine speed and controls ignition timing. But if the knock sensor detects detonation the ECU will retard the ignition timing up to when the detonation is ceased. It measures the RPM of the crank-shaft and the position at which the pistons are at, and sends it to the ECU.
As the seven inputs have now been discussed, it is only fitting to move on to the three actuator outputs selected for this report. The spark plugs, the fuel injectors and the Variable Valve Timing with Intelligence ( VVT-i) system.
Firstly, the spark plug, a small plug fitted into the cylinder head of the engine. The number of spark plugs used on an engine will depend upon the number of cylinders this engine will have. Spark plugs use a ceramic insert which isolates the high voltage to the end of the electrode making sure that the spark is only generated at this point. If the spark were to be created anywhere else it would cause a massive fault with the spark plug and will need replacing. The spark plug must be properly insulated so that the voltage generated can jump the gap, be conducted into the engine block and be grounded. If the gap of the spark plug is too great, it could cause the voltage to become weak or non-existent. Now spark plugs in the engine are used to ignite the air fuel mixture in the combustion chamber. This mixture is compressed and when ignited causes an explosion which pushes the piston down at a great force. The spark can range from 400,000-1,000,000volts each.
Second, the Fuel Injector, sits on the fuel rail of the engine and delivers fuel to be mixed with the air in the combustion chamber. Each injector will have an electrical plug to be attached to it, which is also attached to the ECU. The fuel however is pushed into the injector and when the pressure is at a certain point, the spring will lift the pintle off its seat and cause a passage way for the fuel to be sprayed into the chamber. Now, there are no specifications from the manufacturer on what an injector should read but if testing all these injectors and one voltage is signal is significantly different from the others then there is more than likely a fault with that particular injector.
Lastly, the VVT-i system. This system maximises engine performance by making the air intake and exhaust valves as efficient as possible. This system is able to adjust overlap time between when one valve shuts and another opens, which ensures power when you need it and optimal fuel efficiency. From this will also create a cleaner exhaust as less unburnt gases will be emitted.
Now that both the inputs and outputs have been explained, this can all be related to a closed loop circuit. A closed loop circuit is when a control system is able to get feedback. In this case, the ECU is able to receive information in order to alter faults in the engine where ever necessary. To put it simply, the sensor inputs send information to the ECU which evaluates this information and sends information to the actuator outputs accordingly. The oxygen sensor being at the end of the cycle is able to send feedback to the ECU, this will then enable the ECU to make adjustments and after the cycle is repeated, see the outcome of the adjustments and if any further adjustments are necessary to maintain the air fuel ratio and maximum fuel efficiency. This circuit is continuously working and operating as the engine is.
In this report, I have explained the function of the sensor inputs and actuator outputs of a Toyota petrol engine. Also I have explained how this system relates to a closed loop circuit and why
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