First the exhaust analyser had to be given time to take readings the surrounding air before it can take readings of emissions as the analyser needs to calibrate itself. This test takes about 30 seconds once it has done this test then the emissions can be analysed to see how the engine is running. Now the analyser probe can be put in the exhaust pipe of the vehicle that needs to be tested, the first test is to be done when the engine is cold so that the exhaust gases can be compared to under different circumstances of engine operation. The results that we got for the car was Carbon Monoxide (CO) was at 2.5% this is high value but is expected when the engine is cold as the engine is running a rich air/fuel mixture this means that there is a lack of oxygen when the exhaust comes out as most of the oxygen has been burnt during combustion and the oxygen molecules cannot attach itself to other oxygen atoms to make Carbon Dioxide (CO2). There is a high level of HydroCarbons (HC), HC is basically unburnt fuel and helps lead to form smog, the readings that we got where 788 parts per million (ppm), this is a high number as because the engine is cold it needs to run a rich air/fuel mixture this means that there is usually to much fuel to burn and some of it condenses on the cold surfaces of the combustion chamber so the fuel does not burn, the unburnt fuel leads to high levels of HC. CO2 levels are low around 10% as there is not enough oxygen to make more CO2, the level of CO2 refers to the efficiency of combustion when CO2 gets to around 15% the engine is running as efficiently as it can as it is burning most of the fuel at this time. With some unburnt fuel this means that there is some left over oxygen (O2) because not all of the fuel was burnt and so not all of the O2 was burnt this left 5.15% O2 in the emissions. Because the engine is still running cold the ECU ignores the O2 sensor until the engine has warmed up and the car is moving at revs this is when the O2 sensor goes into closed loop and the ECU listens to the O2 sensor so that it knows whether the engine is running lean or running rich and adjusts the fuel injected to keep the fuel ratio at stoichiometric. When the engine is at idle, cold or under heavy load the O2 sensor is on open loop and the ECU does not listen to it instead the ECU uses the other sensors to adjust how much fuel is being injected. If there is a lean air/fuel mixture and the O2 sensor is on closed loop then the O2 sensor would see a high level of oxygen and it would try and richen the air/fuel mixture by injecting more fuel into the combustion chamber. If the air/fuel mixture is to rich this means that there is lack of O2 and the voltage from the O2 sensor would be high and when there is a lean mixture there is a lot of oxygen and this would cause low voltage output from the O2 sensor, the ECU uses this to richen or lean the air/fuel mixture to keep the engine running as efficiently as possible. The purpose of the O2 sensor is to reduce the amount of harmful emissions as much as possible, and to try and keep the air/fuel ratio at stoichiometry (14.7:1, the most efficient mixture that the engine can run, producing the least harmful emissions and keeping the best fuel economy). If there is to rich or to lean of a air/fuel mixture running all the time or a constant misfire, this can cause the O2 sensor to become fouled up and it would wind up giving incorrect readings to the ECU which means the ECU will think that the engine is running to rich or to lean depending on the voltage and make the engine run improperly meaning that it could run rough or have misfire, the engine could have poor fuel economy or poor performance. These faults can occur if the O2 sensor becomes fouled up with carbon from the exhaust.
^The image above shows what a good voltage wave pattern is from a O2 sensor using an oscilloscope as it shows the engine running a little bit rich then a little bit then a little bit rich again this is what causes the variations in voltage is the increasing and decreasing voltage.^
(O2 sensor)When the engine has warmed up the next readings where taken to check how well the fuel is being burnt and how this would be making the engine run. The CO levels where at 0.001% this is because combustion is now taking place very efficiently and all of the oxygen is being burnt during combustion. HC is low and was at 37ppm this is because the air/fuel mixture is at stoichiometry or 14.7:1 when the engine is running at its most efficient. CO2 levels are at 15.16% which is normal as this means that combustion is taking place efficiently, this also shows that the car has a catalytic converter as a car that does not have a catalytic converter would never get CO2 levels as high as 15% and there would be higher levels of CO and HC without the catalytic converter. These readings are all good as it shows that there combustion is taking place efficiently and this means that there are very little harmful emissions. O2 levels are low because most of it has been burnt so the O2 level was 0.4% since there combustion burnt it all and a lot of the O2 has paired up with the CO to make CO2.
The next test was to check the emissions when the engine is revving at 2500RPM for around 30 seconds, the readings that the engine gave where that CO levels where at 0.2% which is almost nothing so it is not anything to worry about. The reason it increased a little bit is because the engine would be running a slightly richer air/fuel mixture, this means that there is a little bit more unburnt fuel as HC was at 121ppm. However CO2 levels where still at 15.10% so it means that combustion is taking place efficiently. O2 levels are lower as more fuel is being burnt which means there is more oxygen being burnt the O2 levels where at 0.3% which means all these readings are good. With the engine at revs the O2 sensor would go into closed loop and the O2 sensor would tell the ECU to adjust the amount of fuel being injected so that fuel can burn as efficiently as possible.
The next test was to run a lean air fuel mixture by creating a vacuum leak in the intake manifold this makes more air go into the engine than what the ECU thinks there is so the ECU does not add any more fuel, the vacuum leak was made by disconnecting one of the vacuum lines that recirculate exhaust gases back into the intake manifold so that they can be reburnt again. This lean air/fuel mixture has not really increased CO levels as they are still around 0.03% but HC level is high as it was at 470ppm, the HC is high because there is so much air in the combustion chamber that the fuel is not a strong enough concentration to be explosive enough for everything to burn so there is unburnt petrol. The CO2 level went down to 7.8% this is because combustion is no longer efficient as there is unburnt fuel this also results in a high O2 level of 5.9% this is because along with not all the fuel burning not all of the oxygen is burning this means that there is high level of oxygen coming out of the exhaust on the exhaust stroke. The O2 sensor would see the high level of oxygen and would try to richen the mixture if the engine is at revs, when the engine is at idle the O2 sensor is on open loop and the ECU ignores the O2 sensor.
The next test was to accelerate the engine by blipping the throttle a few times, without revving the engine to much. This creates a rich air/fuel ratio because more is going into the engine so more fuel is required but also more fuel than usual is injected as the engine needs more power to accelerate quickly. This means that there is a high level of HC at 434ppm this is because of the rich air/fuel mixture and there would be some unburnt gas, this also means there is higher CO as CO is created by a rich air/fuel mixture. CO2 is low at 10% as the engine is not running efficiently anymore this means that there is low level of O2 at 1.3% because most of the oxygen has been burnt and there is not enough oxygen for CO to turn into CO2 hence the high level of CO.
The next test was to measure what happens when a spark plug lead is disconnected, this is done by grounding the spark lead by attatching a spark tester this way voltage does not build up to much and damage the system. This test is done to see what a misfire with no spark would do to the emissions on the car, the CO levels where low as there was still unburnt fuel going into the exhaust which meant that the oxygen was not burnt this resulted in a reading of 13.6% for CO2. HC was extremely high due to the fact that one cylinder was not burning any gas this meant that HC was at 1033ppm, oxygen levels where high at 4.53% as well because of the fact that no oxygen was burnt because combustion was not taking place. Even though there is high levels of HC the oxygen sensor would see the high oxygen level in the exhaust and it would tell the ECU to richen the air/fuel mixture this would only happen if the O2 sensor was on closed loop whilst the engine is at revs, other wise whilst the engine is at idle the O2 sensor is on open loop and the ECU does not listen to the O2 sensor.
The next test was to measure the emissions when one of the injectors is disconnected so that it can be seen how this would differ from the test where no spark was taking place, now no fuel is being injected. CO levels where at 0% because there was no unburnt fuel from a rich air/fuel mixture this meant that the HC was low at 55ppm because there is no unburnt fuel however CO2 levels where lower at 11.61% because one of the cylinders is not producing any fuel this means that the engine is not running as efficiently as it could this also means that there is a high level of O2 at 5.45% because no oxygen is being burnt due to the fact that there is no fuel to let the oxygen burn this means that there is high level of O2 during the exhaust stoke. The O2 sensor would see the high level of oxygen and try to richen the air/fuel mixture as it would think that the engine is running to lean.
These are the results that you can expect to get when the engine is giving put under different conditions and the level of the different emissions the engine will produce from these different running conditions.
Reference:
Exhaust analyses on car image-http://www.google.co.nz/imgres?imgurl=http://www2.le.ac.uk/departments/engineering/people/academic-staff/aldo-rona/research/technology-development/internal-combustion-engines/liquid-petroleum-gas-lpg-conversion-of-a-1600cc-car-engine/11.gif/image_preview&imgrefurl=http://www2.le.ac.uk/departments/engineering/people/academic-staff/aldo-rona/research/technology-development/internal-combustion-engines/liquid-petroleum-gas-lpg-conversion-of-a-1600cc-car-engine/commissioning-and-testing-a-liquid-petroleum-gas-fuel-system&usg=__0C8FFcaiEOrdRFO9zU-qjitGnAQ=&h=261&w=400&sz=176&hl=en&start=603&zoom=1&tbnid=eu20S7Groi7cIM:&tbnh=119&tbnw=165&ei=y-4YTqzgJsrHmAWb1cgT&prev=/search%3Fq%3Dexhaust%2Bgas%2Banalyzer%26hl%3Den%26biw%3D1280%26bih%3D711%26gbv%3D2%26tbm%3Disch&chk=sbg&itbs=1&iact=hc&vpx=484&vpy=248&dur=552&hovh=119&hovw=165&tx=139&ty=84&page=23&ndsp=25&ved=1t:429,r:8,s:603&biw=1280&bih=711
O2 sensor image: http://www.airpowersystems.com/corvette/c5/install/lhs_front_o2_sensor.jpg
Oscilloscope wave pattern : http://www.4x4wire.com/toyota/4Runner/tech/OBDII_ECU/o2_sensor_frequency.jpg
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