I want to analyse the impact of my fuel injection timing on HC emissions. Currently I am measuring the raw emissions at a point of 65 cms and 90 cms from exhaust port. But I cannot really see the difference in HC Emissions(~ 1550 ppm by volume)
You need to be careful measuring emissions in pipes on an engine test stand, as the exhaust flow is unsteady and if you pull a steady state sample out, it may not be properly weighted. Way back in the 1960's studies on 4 stroke cycle emissions showed that emissions measurements were a function of the distance from the exhaust port. NOx emissions are shown in SAE paper 670482, “Influence of Engine Variables on Exhaust Oxides of Nitrogen Concentrations from a Multi-Cylinder Engine”, by T. A. Huls and H. A. Nickol. HC emissions are particularly sensitive, as crevice gasses are the last to be exhausted, and they are higher near the exhaust valve in 4 stroke cycle engines. I think this is the reference for 4-stroke cycle engines: Daniel, W.A., and Wentworth, J.T., "Exhaust gas hydrocarbons — Genesis and exodus" SAE Technical Progress Series, Vol. 6, Vehicle Emissions, Society of Automotive Engineers, New York, 1964. I don't know of similar studies on 2 stroke cycle engines. We usually mixed the exhaust gasses in a tank, before sampling to get a representative average value. Measuring at fixed location may give you trends, but it might be a good idea to supplement it with a well mixed sample.
'exhaust port' of the cylinder or of the tailpipe ?
I'd suggest to measure some 10 cm from the tailpipe end INSIDE the tailpipe to eliminate ambient influences resp. dilutive effects. When the system has warmed up, there should not be any losses due to condensation. (This is about the same technique TÜV and DEKRA use when assessing emissions.)
The only drawback may be that it takes some time until the exhaust gas reaches the end of the tailpipe.
First you need to warm up the engine and then choose your steady state test condition with regard to load and speed and other parameters. When you reach the test condition place your sampling probe inside the tail pipe at a distance 5-10 cm inside the pipe and then record the measurements and repeat taking readings more than 3 times and then take the average of them for one condition.
The point at which you can sample the exhaust gases depends on the analyzer. If it is a popular 4-gas analyzer given distances are correct.
Piston ported 2 stroke is extremly sensitive to pressure changes, so the fundamental questions for a setup is to check for pressure changes due to the sampling system of the exhaust gas. Therefor I would suggest to use a CVS System for your investigastion and check the C-Balance of your measurment to make sure that you collected all. If there is no pressure change by the analyzer using raw gas, the distances look fine if your probe is at temperatures > 190°C.
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