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Recently I put my car on the 4 gas analyser, and it gave me these results at idle:

CO2: 10.64 %
CO: 4.98 %
O2: 1.94 %
HC: 148 ppm
Lambda value: 0.93/13.7AFR

I have to mention that it concerns an older carbureted vehicle, so the tuning is rather usual. There is no EGR or secondary air injection, and the exhaust has no leaks. I am wondering how it is possible for the engine to leave almost 10% of the inducted oxygen unused while tuned rich. I would assume all the oxygen should be used in the combustion with this tune. Poor atomisation may cause poor combustion, but I wouldn't expect that to have such dramatic results.

The only other thing i can think of, is that intake air gets mixed in the exhaust at the time of gas exchange. But with these results, that would mean that the gas exchange here is extremely good. Is that assumption probable?

The valve timing is:

IO 16deg
IC 56deg
EO 56deg
EC 16deg

Pᴀᴜʟsᴛᴇʀ2
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Bart
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2 Answers2

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TL DR: I would suggest you have an exhaust leak up stream of the sensor location. I know you said it doesn't have a leak, but I'd suggest you just haven't recognized it.

Valve overlap only occurs between the exhaust cycle and the intake cycle. Valve overlap will only allow exhaust gasses to revert (called reversion) back into the combustion chamber during the intake cycle. You would never push part of the new intake charge through the exhaust valve opening because the cylinder would be under vacuum at the time. Overlap can be used in place of an EGR, if done correctly.

I would throw one caveat out there. All bets are off if the engine is turbo/supercharged. If there was enough pressure, it is conceivable there could be some intake charge forced out of the exhaust port if there was enough overlap. You don't mention it in your question, so I doubt this even comes into play here.

Pᴀᴜʟsᴛᴇʀ2
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There many potential reasons why all oxygen isn't consumed during the combustion process. In older engines the combustion chambers may be inefficiently shaped which doesn't allow the entire intake charge to burn.

In recent times there has been massive investment in optimizing valves, pistons, heads, airflow etc to make combustion engines more efficient, specifically combustion efficiency. Something that took a back seat to other concerns in the past.

Running rich doesn't guarantee efficient combustion, in particular fuel cools the intake charge, too much fuel may cool the combustion, foul sparkplugs, etc.

Engine development has been a long continuum that continues today and without a specific technical analysis of any given engine its difficult to pin lack of efficiency to any specific cause(s).

Tim Nevins
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