Subject: Automobile Engineering

Topic: Recent Trends in Automobiles

Difficulty: Medium

Explain the position and working of:

i. MAP

ii. Position Sensor

iii. Knock Sensor

iv. Oxygen Sensor

v. Optical Sensor

vi. Light Sensor

vii. MAF

viii. Hot wire MAF Sensor

ix. SAW

x. Lambda

xi. Rain Sensor

xii. Methanol Sensor

1) MAP Sensor:
MAP stands for manifold absolute pressure. A MAP sensor senses air pressure or vacuum in the intake manifold. The Sensor measures manifold air pressure against an absolute pressure. The MAP sensor uses a perfect vacuum as a reference pressure. The MAP sensor measures changes in the intake manifold pressure that result from changes in the engine load & speed. The PCM ( Powertrain Control Module) sends a voltage reference signal to the MAP sensor. As the pressure changes, the sensor’s resistance also changes. The control module determines manifold pressure by monitoring the sensor output voltage.

The MAP is the second most important sensor in the fuel management system. The basic injector pulse width is set according to MAP signal. A defective MAP can cause a number of problems, including a no-start conditions, reduced power output & heavy engine surging.

2) Position Sensor: Position Sensors are used to monitor the position of something from totally closed to totally open position. Position Sensors are basically potentiometers & all operate in the same way. A wiper arm is connected to a moving part of one end & the other end is in contact with a resistor, the resistor is supplied with the reference voltage. As the part moves, so does the wiper arm. As the wiper arm moves on the resistor, the available voltage at the point where the wiper arm contacts the resistor is the signal voltage sent to the PCM. The PCM then interprets the parts position according to voltage.

3) Knock sensor:
The Knock sensor tells the PCM that detonation is occurring in the cylinders. In turn, the computer retards the timing. The Knock sensor is a piezoelectric device that works like a microphone & converts engine knock vibrations into a voltage signals. Piezoelectric devices generate a voltage when pressure or a vibration is applied to them. Engine knock typically is within a specific frequency range & a knock sensor is set to detect vibrations within that range. The knock sensor is located in the engine block, cylinder head, or intake manifold.

4) Oxygen sensor:
The exhaust gas oxygen sensor $O_2S$ or air fuel ratio sensor, is the key sensor in the closed loop mode. The oxygen sensor is threaded into the exhaust manifold or into the exhaust pipe near the engine. The PCM uses an $O_2S$ to ensure that the air fuel ratio is correct for the catalytic converter. The PCM adjusts the amount of fuel injected into the cylinders based on the oxygen sensor signal.
Oxygen sensor generate a voltage signal based on the amount of oxygen in the exhaust gas. They compare the oxygen content in the exhaust gas with the oxygen content of the outside air. As the difference between the exhaust & atmosphere increases, the voltage signal also increases. Lean mixtures releases large amount of oxygen & voltage signal from the oxygen sensor is low. Rich mixtures release lower amounts of oxygen & resulting $O_2$ voltage signal is high. When the sensor reading is lean, the computer enriches the air- fuel mixture to the engine. When the sensor reading is rich, the computer leaves the air- fuel mixture.

5) Optical sensor:
Optical sensor (normal Cameras) are usually referred to as passive sensors because they acquire date in a non lustrusive way. One advantage of passive sensor over active sensor is cost. With the introduction of inexpensive cameras, we can have both forward & rearward facing cameras on a vehicle, enabling a nearly 360° field of view.

Optical sensors can be used to track more effectively cars entering a curve or moving from one side of the road to another. Also, visual information can be very important in a number of related applications, such as Iane detection, traffic sign recognition, or object identification, without requiring any modifications to road infrastructure.

6) Light Sensor:
Light sensor provide light-sensitive, automatic on-off control of the light normally controlled by the regular headlight switch. It consists of a light sensitive photocell sensors/amplifier assembly & a headlight control relay. Turning the regular headlight switch on overrides the automatic system. In other words, automatic operation is not possible until the regular headlight switch is turned off.

7) MAF:
A mass air flow sensor is located between the air cleaner & the throttle body. It is used by the control module to measure the volume of air entering the engine. The Control module uses this information to calculate fuel delivery. The typical MAF sensor contains a mesh screen to break up air flow & prevent debris from entering the sensor. A thermal element is placed in the air stream & is heated to a specified temperature by the flow of electrical current.
A mass air flow sensor is used to find out the mass flow rate of air entering a fuel injected internal combustion engine. The air mass information is necessary for the engine control unit (ECU) to balance & deliver the correct fuel mass to the engine. Air changes its density as it expands & contracts with temperature & pressure. A MAF is used in conjunction with an oxygen sensor, the engine’s air fuel ratio can be controlled very accurately. The MAF sensor provides the opens loop controller predicted air flow information to the ECU & the oxygen sensor provides closed- loop feedback in order to make mirror corrections to the pedicited air mass.

8) Hot Wire MAF Sensor:
A hot wire MAF sensor determines the mass of air flowing into the engine's air intake system. The theory of operation of the hot wire mass airflow sensor is similar to that of the hot wire anemometer (which determines air velocity). This is achieved by heating a wire with an eletric current that is suspended in the engines air stream, like a toaster wire. The wire's electrical resistance increases as the wire's temperature increases, which limits electrical current flowing through the circuit. When air flows past the wire, the wire cools, decreasing its resistance, which in turn allows more current to flow through the circuit. As more current flows, the wire’s temperature increases until the resistance reaches equilibrium again. The amount of current required to maintain the wire’s temperature is directly proportional to the mass of air flowing past the wire. The integrated electric circuit converts the measurement of current into a voltage signal which is sent to ECU.

9) SAW sensor:
The torque sensors is so called SAW sensor as SAW stands for Surface Acoustic Wave. Torque sensors for automotive applications help to improve fuel efficiency. Torque sensors that allow for exact metering of driving power, shifting operations & steering movements that increases the driving comfort & reduce fuel consumption, as a result Automatic transmissions are becoming progressively smoother. The SAW sensor consists of Base, Ring & cover. The unique housing of SAW provides a combine a metal with high elastic limit for transmitting the torque to the sensor with an annealed metal suitable for hermetic glass fitted electrical feed through.

10) Lambda:
The exhaust gas oxygen sensor (EGO or $O_2$) or lambda sensor, is the key sensor in the engine fuel control feedback loop. The computer uses the Oxygen ($O_2$) sensors input to balance the fuel mixture, leaning the mixture when the sensor reads rich & enriching the mixture when the sensor reads lean.
Lambda sensors produce a voltage signal that recognizes the amount of unburnt oxygen in the exhaust.The lambda sensors output voltage does not remain constant. However, it flip-flops back & forth from rich to lean. Every time the voltage reverse itself & goes high to low or vice versa called as cross counts.
The lambda sensor is also grounded through the exhaust manifold. If rust & corrosion of the manifold gaskets & bolts is creating resistance, it may affect the sensors output.

11) Rain Sensor:
Modern Rain Sensors are based on the principle of total internal reflection. An infrared light is beamed at a 45° angle in the windshield from the interior. If the glass is wet, less light makes it back to the end & the wipers turn on.Most vehicles with this feature have an "AUTO" position on the stalk.
During driving in rain, show or windshield just got soaked by vehicle that passed , the rain tracker automatically senses the water on the windshield, & activates the wipers at just the right speed. When the weather is clear, the rain tracker keeps the wiper off, ready for the next raindrop or snowflake. Get the convenience & safety of automatic windshield wipers for vehicle.
The rain tracker as cruise control for the vehicle wipers. The rain tracker senses water water hitting to windshield & runs the wipers at just the right speed. A dash mounted switch activates the rain tracker. Simply leave the switch off when it is not raining, or when cleaning the windshield or going through a car wash.

12) Methanol sensor:
Methanol sensor is designed for detecting volatile organic compounds in a diverse range of application. The sensor stabilizes in less than 1 minute & zero drift thereafter is less than 1% 1 year. The humidity response is small & the sensor can operates in both high & low gas concentrating. The sensor has a cross sensitivity to other oxidizable molecules such as solvents & organic vapours.
Methanol injection helps to reduce inlet air charge temperature & suppress detonation. Methanol is a cool burning; high anti-knock rated fuel that also helps to cool & control combustion.