Day 15 - Exhaust Gas Analysis

Theory

We can use five-gas exhaust analyser for emission testing and engine analysis. The five gases are HC(Hydrocarbons), CO(Carbon monoxide), CO2(Carbon dioxide), O2(Oxygen) and NOx(Oxides of nitrogen).

In the normal air condition:  O2(20.9%) + N2(79%)

In ideal perfect combution: H2O + CO2 + N2    <----    O2 + N2 + HC(fuel)

Bad combution: CO, NOx, HC

The Hydrocarbons are unburned gas and are measured in ppm(parts per million). Acceptable levels of HC are 100 ppm or less. If HC is more 100 ppm, it means that there are unburned gas(excessive oil consumption). The most common cause of excessive HC emissions is a fault in the ignition system. Therefore we can check the spark plugs, spark plug wires and ignition coil.

The Carbon monoxide is unstable and easy to combine with oxygen. It is a posionous gas and should be less than 0.5%. High levels of CO can be caused by clogged crankcase ventilation devices such as the PCV valve, hoses and tubes.(find the cause of rich mixture)

The Carbon dioxide is produced by combining the oxygen with the carbon of the gas. Levels of CO2 should be between 12% and 15%. If the CO2 level is low, the mixture may be either too rich or too lean.

The Oxygen should be used up during the combustion process. Therefore levels of O2 should be less than 1%. High levels of O2 at idle could be due to an exhaust system leak.

The Oxides of nitrogen is a colorless, tasteless and odorless gas when it lease the engine, but as soon as it reaches the atmosphere and mixes with more oxygen, nitrogen oxides are formed. NO and NO2 are grouped together and referred to as NOx. Acceptable level of NOx should be less than 100 ppm at idle and less than 1000 ppm at WOT.

Practical

Exhaust Gas Analyser

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1. With the analyser probe sensing normal air, what are the four gas readings?

CO: 0.001%
HC: 5 ppm
CO2: 0.01%
O2: 20.68%

In the normal atmosphere, there are 21% of oxygen the and 79% of the nitrogen.






2. Start the engine idling cold and record the four gas readings.

CO: 0.160%
HC: 174 ppm
CO2: 14.41%
O2: 1.16%









3. When the engine has warmed up, record the four gas readings.

CO: 0.004%
HC: 65 ppm
CO2: 15.25%
O2: 0.03%
Lambda: 0.998











4. Run the warm engine at 2500 RPM, record the four gas readings.

CO:  -0.00%
HC: 785 ppm
CO2: 15.21%
O2: -0.04%










5. At idle, create a lean condition with an air leak or vacuum leak, read the four gas readings.

CO: 0.014%
HC: 2276 ppm
CO2: 15.15%
O2: -0.05%

If there is a vacuum leak, the O2 should be high. There is something wrong with my measurements.





6. Accelerate the engine, by blipping the throttle a few times and record the four gas readings when the CO is highest.

CO: 1.083%
HC: 1546 ppm
CO2: 14.67%
O2: 0.03%








7. Make other changes to the engine at idle, such as turning on the air conditioning or rocking the steering wheel. Record the four gas readings.

CO: 0.032%
HC: 231 ppm
CO2: 15.10%
O2:  -0.04%



A catalytic converter is used to reduce exhaust emissions outside of the engine. The converter converts harmful exhaust gases into water vapor(H2O) and carbon dioxide(CO2).

The three-way catalytic converter first separates the NOx into nitrogen and oxygen and then converts the HC and CO into harmless water(H2O) and carbon dioxide(CO2). The nitrogen(N2) passes through the converter and exits the tailpipe and enters the atmosphere which is about 78% nitrogen.

1. Explain the different readings you would get from a vehicle with a catalytic converter and a vehicle without one and why?

Acceptable exhaust emissions are as follows:

	Without CAT	With CAT
HC	300 ppm or less	30 to 50 ppm or less
CO	3% or less	0.3% to 0.5% or less
O2	0% to 2%	0% to 2%
CO2	12% to 15% or higher	12% to 15% or higher
NOx	Less than 100 ppm at idle and less than 1000 ppm at WOT	Less than 100 ppm at idle and less than 1000 ppm at WOT

2. Explain what light off point means and what happens?

The catalytic converter does not work when cold, so it must be heated to its light off temperature of close to 260 degrees before it starts working at 50% effectiveness. When fully effective, the converter reaches a temperature range of 482 degrees to 871 degrees.