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Understanding AFR: Effect of AFR on Emissions

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Effect of AFR on Emissions


00:00 While we are generally interested in engine performance and hence engine power, particularly in countries or states with strict emissions laws, it is still necessary to understand how the AFR can effect emissions.
00:14 What we usually see is that all factory vehicles spend the majority of their operation running at a strictly controlled stoichiometric air fuel ratio.
00:24 While it would be nice to think that this is aimed at optimising fuel economy, the reality is that this is the point where the tail pipe emissions are minimised.
00:34 Emissions performance is essential for a manufacturer to get a new car on the road in the first place and a large amount of factory tuning exists in an effort to ensure emissions standards are met.
00:47 To understand emissions, we need to know what is occurring during the combustion process.
00:53 In theoretical perfect combustion, we are taking our fuel comprised of Hydrogen and Carbon, and mixing it with air which is predominantly made up of Oxygen and Nitrogen.
01:03 The result of this combustion is water, carbon dioxide and Nitrogen.
01:09 When we have an excess of either fuel or air, the emissions can vary dramatically.
01:16 The typical emissions we will measure at the tailpipe include excess hydrocarbons or unburnt fuel, carbon monoxide, and oxides of nitrogen.
01:27 These emissions can’t be measured with our typical wideband sensor although they can be with a 5 gas analyser which is used for emissions testing rather than performance tuning.
01:39 As we move the AFR richer or leaner, some of these emissions will increase and others decrease as you can see in this diagram.
01:49 What is important to note is that while some emissions increase as we move richer, and others may increase as we move leaner, all of the collective emissions are at their minimum at a stoichiometric AFR of 14.7:1 for pump gas.

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