Suck, Squish, Bang, Blow part 3- Compression Ratio and Air Fuel Ratio


Fuel Air Ratio

The fuel air ratio is the ratio of fuel to air expressed as a percentage.  Fuel alone will not burn.  It must be combined with the oxygen in air to form a combustible mixture.  The perfect balanced air to fuel ratio by mass is 14.7 to 1.  This is 14.7 parts of air to one part of fuel.  At this ratio under ideal conditions all of the fuel and oxygen will be consumed by combustion. This is called a stoichemetric ratio by chemists and engineers.

The A/F ratio for best power is actually a little leaner for a modern 4-valve per cylinder engine, around 13:1 but the general shape of the curves is about right. A Lambda of one means a stoichiometric 14.7:1 ratio.

A modern car's ECU is programmed to run at 14.7:1 during cruise and light acceleration for low emissions, keeping catalytic converters happy in these common modes of operation (the catalytic converter only works well near this ratio of air and fuel), going to a richer mixture at wide open throttle for more power.  Since most cars only spend a small amount of time at wide open throttle, they can be tuned for maximum power without hurting the environment much.

These curves show where the conversion effectiveness of a catalytic converter is the greatest,right at Lambda one or 14.7:1.  This is why modern engines run at 14.7:1 for a great deal of their operating cycle. 

Although a stoichemetric air/fuel ratio is optimal for low emissions, engines produce more power at richer air fuel ratios.  Naturally aspirated engines usually produce the best power at around 13:1 at wide open throttle although 12.5:1 is a bit safer.  This is because leaner air/fuel ratios burn hotter.  If the mixture is too lean and the combustion temps increase, the engine can start to overheat and detonate. Since a stock engine must run under a wide range of fuel qualities as a cheap ass owner may decide to run his performance car on 87 octane regular, many  stock engines run really rich at wide open throttle. We have seen air fuel ratios in the 10.5:1 range on NA stock engines. This is extremely rich.

There are a bunch of highly effective wideband air fuel ratio meters on the market that can be used for tuning like this Zetronix part.

Supercharged or turbocharged cars can run as rich as 10:1.  Rich mixtures burn cooler and can help prevent detonation in these high pressure engines.  Mixture ratios richer than 10:1 can contribute to engine damaging detonation because the excess fuel begins to take up too much room in the combustion chamber and increases cylinder pressure to the point where it can detonate or auto ignite.  Detonation is a common factor in an engine's death.  We will explain this more carefully in the next segment of this series.

Tune in next time for more basic tuning factors and terms.

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