Engine Performance Tuning and Air/Fuel Ratio
Achieving maximum engine performance for automobiles or motorcycles means delivering a sufficient amount of the proper fuel mixture to the cylinders. But what is the 'proper' fuel mixture, and how do you know if you've achieved it?
The ideal, or stoichiometric, fuel mixture contains exactly enough air to completely burn the fuel. For gasoline engines, this means 14.7 pounds of air for each pound of fuel, a 14.7:1 air/fuel ratio.
One of the most effective tools for determining an engine's air/fuel mixture is an exhaust gas analyzer. A simple CO/HC analyzer can deliver good results, while a 4-gas analyzer (CO/HC/CO2/O2) provides better accuracy.
The amount of carbon monoxide (CO) in an engine's exhaust can be used to determine the air/fuel ratio. The exhaust gas will contain approximately 1% CO when the fuel mixture is optimum. A 2-gas exhaust analyzer can be used for this purpose. This technique is effective, and has been used for many years, but is subject to errors because it doesn't account for the engine's combustion efficiency.
With a 2-gas analyzer you can estimate the air/fuel ratio by measuring exhaust CO, but a 4-gas analyzer can actually measure it. When CO2 and O2 are measured along with CO and HC, all combustion components are measured and the air/fuel ratio can be calculated more accurately than the CO based estimate. 4-gas exhaust analyzers also deliver other useful combustion statistics:
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Lambda |
The ratio of available oxygen to required oxygen. Lambda equals 1 when the fuel mixture ideal. For example, a lambda value of 0.95 means that 5% more oxygen is required for complete combustion, i.e. the fuel mixture is rich. When lamba is 1.05, the fuel mixture is 5% lean |
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Combustion Efficiency |
The degree to which the fuel is completely burned. Combustion is 100% efficient when all of the fuel mixture is burned. Modern engines can achieve 95% combustion efficiency |
Reference: Using a Model 9003 2-Gas (CO/HC) Gas Analyzer for Motorcycle Tuning
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