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Most modern car engines are fitted with an array of different sensors, which gather data to calculate a perfect air-fuel mixture for the best fuel economy in all situations.
Since the late 1980s and early 1990s, most vehicle's engines and drivetrains (the system that connects the transmission to the drive axles) have been controlled by a central processor and sensors. This might include spark advance, fuel metering and delivery, transmission shift points, emission control, traction control, and antilock braking.
Below, the standard sensors found in modern cars are described in more detail.
Oxygen Sensors
Oxygen sensors measure the proportion of oxygen in the exhaust gases coming from the engine. There are usually two: one in front of the catalytic converter to measure oxygen and adjust the fuel mixture, and one in the back.
The sensors compare the detected oxygen concentration, to the oxygen content of the ambient air. They determine whether the engine is running on a rich or lean fuel ratio. The engine's computer utilizes this information to determine a fuel metering strategy and emissions control.
Engine Speed Sensor
This sensor measures the rotational speed of the crankshaft in revolutions per minute (RPM). It consists of a serrated disc and a magnetic coil. As the crank spins, a current and magnetic field are created around the coil, which are then disrupted by the disc. This disruption serves as a count of RPMs.
Problems with this sensor could manifest as a cruise control or speedometer issue, or as fuel or ignition problems.
Mass Air Flow Sensor (MAF)
Located near the air filter or on the air filter box, this sensor monitors the amount of air that enters the engine. It often employs a hot wire that cools down as the air flows over it.
The resistance to airflow is converted to airflow mass and provides information to the engine control unit. The drivetrain computer utilizes this data to regulate fuel metering and delivery.
Failing MAF sensors result in either rich or lean running conditions, rough idle, hesitation or stalling and a switched-on engine check light.
Fuel Temperature Sensor
The fuel temperature sensor sends information to the engine control unit to increase or decrease the amount of required fuel depending on the fuel temperature.
A failed sensor will cause the check engine light to illuminate and lead to a drop-off in fuel economy.
Manifold Absolute Pressure Sensor (MAP)
This sensor, fitted to the manifold or vacuum hose from the intake manifold, measures the pressure inside the intake manifold to indicate the engine load.
The computer uses this information to map out spark advance and fuel delivery.
Crankshaft Sensor
This sensor sends signals to the engine control about the position of the crankshaft. It is essential to get the right signal when it is time for the injectors and ignition coil to fire.
A faulty sensor, located at the engine block, can cause the engine to stop and introduce hard starting conditions.
Other Sensors
Coolant Temperature Sensor (CTS)
Measures the temperature of the coolant and is located high in the engine bay at the coolant hose.
Intake Air Temperature Sensor (IAT)
Measures the air temperature flowing into the 'car's cylinders through the intake pipe. It is often integrated with the MAF but could be externally fitted on the intake pipes, at the manifold or close to the air filter box.
Knock Sensors
Monitors detonations or ''knock'' from the cylinder combustion chamber. The sensor recognizes a misfire and will adjust the ignition or fuel to ensure the engine is not detonating.
Camshaft Position Sensor
Measures the position of the camshafts to let the engine control unit know which the engine position.
Fuel Pressure Sensor
Measures the fuel pressure on the fuel pressure line. Measuring this is important, as an increase in pressure could result in a richer fuel mixture while a low pressure could result in a leaner fuel mixture.
Voltage Sensor
Measures the voltage in the 'car's electrical system.
NOx Sensor
Measures the NOx volume in the exhaust gases and is fitted on the exhaust pipe and control unit.
Exhaust Temperature Sensor
Often fitted on diesel engines at the exhaust pipe and exhaust manifold. These are used to measure the exhaust temperature before and after the particle filter to optimize particle filter regeneration.
Boost Pressure Sensor
Measures the boost pressure on pressure pipes in cars with turbo or superchargers.
Throttle Position Sensor
Measures the angle of the throttle body to see how much you are accelerating the car.
Conclusion
These sensors are critical to drivability, performance, and emissions controls. Abnormal signals from any sensors can be enough to trigger a trouble code in the 'engine's computer and illuminate the check engine light.
Sources
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