# Homework Help: Combustion: How to find ARF and ratio of H and C (elements) from dry gas analysis

1. Apr 13, 2012

### Pammy2601

1. The problem statement, all variables and given/known data

In an engine test the dry exhaust gas analysis is CO2: 5.27%; O2: 13.38%: N2: 81.35%.
Assuming that the fuel is a pure hydrocarbon of the form CxHy, and that it is completely
burnt, calculate the ratio, by mass, of carbon to hydrogen in the fuel and the air to fuel
ratio by mass.

2. Relevant equations

combustion equation:
Cn+H2n+1 + A*(0.21O2+0.79N2) = aCO2 + bH2O + cN2

3. The attempt at a solution

I'm pretty confused, as the question states the fuel is pure hydrocarbon so therefore one can assume the combustion is stoichiometric, but the bit that confuses me is the O2 from the dry exhaust gas analysis. I could only find the Air fuel ratio as a algebraic term:

A = (2a+b+2d)/0.42

this is done by counting the oxygens on each side, 0.21*2*A = 2a + b +2d, I added '2d' because in the exhaust gas there's oxygen. I'm not really sure how to start this question. Even a nudge in the right direction would be greatly appreciated. Thanks a lot!

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