For blog 3 which is the part 2 of the sensors and actuators on-vehicle I am going to explain these 3 things shown below:
- Mass Air Flow sensor.
- Oxygen sensor.
- Manifold Absolute Pressure sensor.
Background:
A Mass Air Flow sensor is a device to find out and calculate the air flow rate,it is normally used in a fuel injected internal combustion engine.Different manufacturer and factory use different way to get air flow rate information for ECU, if there is a MAP(Manifold absolute pressure)sensor is used,then a MAF sensor is not necessarily needed, so is in opposite.
A MAF sensor is always mounted on the intake housing after the air cleaning device(air filter/air box) and before the throttle body, if the engine is turbo-charged, a MAF sensor is mounted before the turbocharger close to the air filter. If you see a round black plastic device on a intake pipe that has a plug and some wires going onto it, yes, that is a MAF sensor.
This is a diagram showing where a MAF sensor should be in the engine system:
There are two common types of MAF sensor in use on a automotive engine, they are Vane type and Hot wire type. The vane type of MAF sensor is invented before the hot wire type which you will still probably see them on older cars, however modern cars are using hot wire type now.The reason that the vane type MAF sensor is no longer in use is because they are bigger than hot wire ones, more smaller the sensor is clears more spaces and rooms, and the more important reason is the hot wire ones measure the air flow rate more correctly than a vane type and the IAT(Intake air temperature)sensor is always built in the hot wire MAF sensor so it doesn't need to run IAT sensor somewhere else.
A typical vane type MAF sensor is shown below:
A typical hot wire type MAF sensor is shown below:
The General Motors (GM) was the first car company to use the hot wire type MAF sensor,the mesh is always the side where air flows in as it can stop dust or other things damaging the materials inside,if it is mounted the other way around,the sensor will not working or working probably.
Because the vane type is hard to see nowadays so I will be talking more about hot wire type here.
Theory:
In a fuel injected internal combustion engine, the ECU must know the flow rate of the air coming in to the engine to balance and deliver the correct signal to fuel system to make the best air/fuel ratio. Air density varies depend on the speed,temperature and altitude, a MAF sensor is in a very important position to make a car running properly.
The hot wire in a hot wire type MAF sensor is made by platinum which will heat up itself when current is flowing through, normally the resistance of the wire increases when its temperature increases. The air will cool the hot wire down once it starts to flow and decreasing the wire's resistance, which in turn allows more current to flow through the circuit. As more current flows, the wire heats up again and the resistance will meet equilibrium again. The electrical unit inside will convert the amount of current that changes to a voltage signal and send back to ECU,the voltage should increases when there is more air flowing through.
Beside of the hot wire usually there is a thermistor which measures the temperature of the air flow works together with the hot wire sending signal to ECU to help making the air/fuel ratio
Here is a simple picture give you an idea what they look like:
And also a diagram:
My experiment:
You can test a MAF sensor without taking it off the car.Here is what I did:
The vehicle I was doing the testing is a 2008 Mitsubishi Cedia and a multimeter and a oscilloscope is involved for testing.
First I located the Sensor is mounted just right behind the intake air box which the plug is easy to reach without taking out other parts. This sensor has 4 wires coming out of it which is simple to identify what they are.
Back probe the 4 wires with 4 pins and I identified which one is which by using the multimeter. Make sure the engine is OFF and the ignition On,connect the back lead to the negative side of the battery and red lead to the pins on by one, I have got a very low voltage reading close to 0 which is the ground wire and the next one has a constant 5V which this one is the voltage supply from ECU,move onto the next one and it reads just over 2V, this one should be the air temperature signal according to manufacturer's specifications and the one left must be the MAF signal to ECU.
Now,swap the multimeter to the oscilloscope, ground the black lead and connect the red lead to the signal wire,make sure anything is clear from the engine bay and start up the car, notice the stable waveform just to make sure the oscilloscope is getting the reading.Now rev the engine up a little bit and I got this waveform showing below:
From this waveform, we can see the signal is between 0-1V which is low at idling as low air flow rate, and when the engine revs up, the voltage increases to its peak at 4V and drops down below 1V when the engine backs to idle.This process happened in 500ms.
Reflection:
According to the theory and manufacturer's specifications, this particular MAF sensor is doing what it should do.
If there is no good constant reference voltage supplies to the signal,the sensor will not work and if the grounding is bad, the effect will acting like a resistor which increases the resistance to the circuit and ECU will get wrong signal.
The hot wire is weak so make sure not to damage it when testing or taking the sensor out
Sometimes spray the CRC MAF sensor cleaner to the hot wire can help it gets more correct reading as it does gets dirty from the air.
References:
http://en.wikipedia.org/wiki/Main_Page
www.autoshop101.com
Unitec moodle
Vijay Naidu
Oxygen sensor
Background:
The Oxygen sensor(known as O2 sensor) is also named as Lambda sensor which is an electronic device that measures the proportion of oxygen in the exhaust gas.
It was developed Robert Bosch during the late 1960s,the most common type of the O2 sensor is zirconia and there is also another type is titania type. Also there is a narrow or wide band differences to O2 sensors which the narrow band is more common to see now but the wide band starts taking places in modern cars.
The ECU uses a O2 sensor ensures the air/fuel ratio is correct for catalytic convertor. Based on the O2 sensor signal, ECU will adjust the amount of fuel to inject. There are two very important conditions which are not good for the engine is rich and lean, an O2 sensor is used to detect these two conditions and let the ECU know.The O2 sensor make the modern electronic fuel injection and emission control possible.
The O2 sensor is probably the easiest sensor to find on a engine, it is always mounted after the exhaust manifold on the exhaust piping before the catalytic convertor, some modern cars may have another O2 sensor mounted after the catalytic convertor but it is not common.
The O2 sensor is designed not for just making the best air/fuel ratio, it is also for stop producing more pollution to our environment.
This is a typical O2 sensor gives you an idea what they look like:
This is a diagram shows you where an O2 sensor normally is in an engine system:
Theory:
An O2 sensor works in close-loop system with long/short term fuel trim equipped.
The O2 sensor needs to reach a minimum operating temperature of aprox 300 degree celsius or more to produce an accurate voltage signal. The faster the heated oxygen sensor reaches that temperature the faster the sensor will start sending an accurate signal to the ECU.
There is a heater element in modern O2 sensor in order to achieve the require temperature. The ECU controls the heater element circuit by allowing current flow through it to ground.
This is a simple diagram shows you how a heated O2 sensor works with ECU
A simple zirconia O2 sensor is constructed with platinum electrodes and zirconia dioxide element
The O2 sensor differs than other sensor which it can produce voltage itself. Like the information I mentioned above, when the O2 sensor gets heated up to the operation temperature, it will burn the unburned oxygen in the exhaust and produces voltage which varies depends on the amount of the unburned oxygen and let the ECU know whether the engine is running rich or lean.
What is rich or lean exactly?
If there is less air than the perfect air/fuel ratio, there will be fuel and no oxygen left over after the combustion and this is a rich mixture, this will cause more pollution and a lean mixture is when there is more air or less fuel than the perfect air/fuel ratio, there will be too much oxygen and no fuel left over after the combustion, this will cause engine lacks of power or even damages the engine internal when the revolution is high.
This is a picture will speak for itself:
The ECU can determine if the ratio is rich or lean from the O2 sensor. A rich mixture consumes nearly all the oxygen and the output signal is higher in a range of 0.6-1V and a lean mixture contains more oxygen and the output signal is lower in a range of 0.4-0.1V.
My experiment:
You can test a O2 sensor without taking it off the car.Here is what I did:
The vehicle I was doing the testing is a 2008 Mitsubishi Cedia and a multimeter and a oscilloscope is involved for testing.
First I located the Sensor is mounted just right below the exhaust manifold on the piping with 2 wires coming out of it, this is a very typical heated O2 sensor.
Back probe the both wires with 2 pins and I identified which one is which by using the multimeter. Make sure the engine is OFF and the ignition On,connect the back lead to the negative side of the battery and red lead to the pins on by one, I have got a very constant 12V from one of them which this one is the voltage supply for the heater element and the other one should be the output signal wire for ECU.
Now,swap the multimeter to the oscilloscope, ground the black lead and connect the red lead to the signal wire,make sure anything is clear from the engine bay and start up the car, let the car to idle for a while as it needs to reach the operation temperature to work and then I got this waveform shown below:
The O2 sensor is working between 0-1V and in this 1 second period of time the signal varies between rich to lean 5 times.
Reflections:
We can see from this waveform above that this particular O2 sensor is working as what it should be, according to the theory, this O2 sensor is producing voltage signal to ECU from burning the unburned oxygen in the exhaust to let it know how rich or lean the mixture is.
When the O2 sensors fail the computer can no longer sense the air/fuel ratio and will end up guessing and the car will run inadequate and use more fuel.
Here are some symptoms of rich or lean:
Rich:
- Hard to start when engine is warm
- Very bad pollution
- Kill the spark plugs
- Poor fuel milage
Lean:
- Lacks of power
- Hard to start when cold
- Backfiring
- May damages engine internal components when RPM keeps going high
http://en.wikipedia.org/wiki/Main_Page
Vijay Naidu
Manifold Absolute Pressure Sensor
Background:
A Manifold Absolute Pressure sensor is used in fuel injected internal combustion engine, it can provide the manifold pressure information to ECU which the data is used to calculate the air density and flow rate which determine how much fuel to inject.
The MAP sensor has the same purpose to a MAF sensor but only one is needed to one particular engine, like the information from MAF sensor, if there is a MAF sensor is in use the MAP sensor is not necessarily needed, if there is a MAP sensor is in use you will probably not find a MAF sensor from this engine.
This a typical MAP to show you what they look like:
The MAP sensor is always mounted either on the the intake manifold or somewhere close to the intake manifold such as firewall, if it is not on the intake manifold, there must be at least one vacuum hose connected to it to allow air to flow into the sensor.
This is photo of a MAP sensor from an EG Civic where the sensor is mounted on the firewall:
Theory:
The intake manifold pressure is directly related to engine load, the ECU needs to know this pressure to calculate how much fuel to inject or when to inject.
There is a silicon chip mounted in the reference chamber to a MAP sensor, on one side of the silicon chip is a reference pressure and the other side of the chip is the pressure that actually gets measured.
This is a picture can show you the internal of a MAP sensor:
When the chip flexes with air pressure changes, the electrical resistance of the chip changes, this change also alters the output voltage signal to the ECU so ECU knows the intake pressure is changed.
This is a picture can show you how the chip flexes when air pressure changes:
When the throttle butterfly is shut, the vacuum is very high inside of the intake manifold which means less air coming in and the pressure is low.When the throttle butterfly opens or wide opens, the vacuum drops which means more air coming in and the pressure is going to be higher. There is more air comes in when accelerating or the throttle body opens wider, the output signal will be higher for the ECU to read. This fig below can show you what the signal like:
Normally there are 3 wires coming out of a MAP sensor which they are 5V reference voltage from ECU, a ground wire to the ECU ground and a output voltage signal back to ECU
This is a picture shows you the diagram of a MAP sensor:
My experiment:
You can test a MAP sensor without taking it off the car.Here is what I did:
The vehicle I was doing the testing is a 1997 Honda Civic 1.5l and a multimeter is involved for testing.
First I located the Sensor is mounted just on the firewall very close to the intake manifold with a vacuum hose connected. This sensor has 3 wires coming out of it which is simple to identify what they are.
Back probe the 3 wires with 3 pins and I identified which one is which by using the multimeter. Make sure the engine is OFF and the ignition On,connect the back lead to the negative side of the battery and red lead to the pins on by one, I have got a very low voltage reading close to 0 which is the ground wire and the next one has a constant 5V which this one is the reference voltage from ECU, the one wire left must be the voltage signal back to ECU and connect it with the red lead and it shows 2.86V on the multimeter.
Now, make sure everything is clear and start up the engine, I got a reading of 0.94V when the engine is idling and voltage increased to 1.69V after I stepped on the throttle to get a quick short acceleration.
I did not have a oscilloscope so there is no waveform can be provided to this vehicle, but I can explain what is happening to this MAP sensor using this waveform I found on the internet below:
At the beginning of this period of time the engine is idling and the sensor reads just over 1V, at about 0.5second the engine starts to rev up and there is more air flowing into the sensor which the output voltage increases, as you can see it reaches nearly to 5V between 0.7 to 1.3 second which means in this period the throttle is wide opened to the maximum and the engine rev drops down and de-acellerating after 1.3second so the voltage signal decreases below 1V.
Reflection:
From the test I have done to the Honda Civic, I can see that particular MAP sensor was in good working order according to the theory and manufacturer's specifications.
If there is no good constant reference voltage supplies to the signal,the sensor will not work and if the grounding is bad, the effect will acting like a resistor which increases the resistance to the circuit and ECU will get wrong signal.
If there's extra amount of voltage signal backs to the ECU, it will think that there is more air coming in so it will make injectors spray more fuel, the engine will run rich due to there is not that much of air
If there's less amount of voltage signal backs to the ECU, it will think that there is less air coming in so it will make injectors spray less fuel, the engine will run lean due to there is more air but less fuel to cooperate.
The MAP sensor sits in a very important position to measure the air pressure for ECU to calculate best air/fuel ratio for engine load, make sure the sensor is good and clean and all the wires are connected properly, and also the most important is to check if there is any vacuum leak to the intake system, a vacuum gauge always can help to detect problems.
http://en.wikipedia.org/wiki/Main_Page
