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Air Fuel Ratio Gauges Explained... An Air Fuel Ratio Gauge is a valuable tuning aid. By taking the output from an oxygen sensor, sometimes called a lambda sensor, in the exhaust of the car the a/f gauge is able to read the content and display the ratio of air to fuel that was present in the original mix.
A/F ratios are given by molecular weight and the 'stoichiometric' ratio is 14.7:1 - this is the ideal air fuel ratio for complete combustion. It is at this a:f ratio that all the fuel is burned in the given quantity of air.
When tuning a car, some folk tune for emissions, some for economy and some for performance. It's already been said that 14.7:1 is the ideal air/fuel ratio for clean burning but what about going rich or lean.
The economy minded tuner will be aiming to reduce the amount of fuel in the mix, or lean it. Lean air/fuel mixtures improve fuel economy but also cause a sharp rise in oxides of nitrogen (NOX). If the mixture goes too lean, it may not ignite at all causing "lean misfire" and a huge increase in unburned hydrocarbon (HC) emissions. Lean air/fuel mixtures burn hotter and can cause rough idle, hard starting and stalling, and may even damage the catalytic converter. Lean mixtures also increase the risk of spark knock (detonation) when the engine is under load.
The performance minded tuner will most likely want to richen up the mixture. A rich fuel mixture is necessary when a cold engine is first started, and additional fuel is needed when the engine is under load. Rich a:f mixtures burn cooler and decrease the risk of spark knock (detonation) when the engine is under load. But rich mixtures cause a sharp increase in carbon monoxide (CO) emissions.
So, in an ideal world your engine would run rich when you first started it, a little lean on a cruise and a little rich under hard acceleration or when pulling up a hill. It doesn't just stop there. There are many reasons why a performance tuner might want to alter the air fuel ratio of his car and with one of these air:fuel ratio meters he'd be able to see just what is going on.
There are two types of sensor available, the 'narrow band' and the 'wide band'. The narrowband is the original type and the wideband is a more recent development. Depending on your aims one or the other might be more suited for your application. If you are looking to merely diagnose a problem with your car and you don't want to go tweaking it too lean or too rich then the narrow band air:fuel ratio meter will probably suffice. If, however, you want to go delving into more radical tuning a wideband a/f gauge is a must. Take a look at the graphs below for a pictoral explanation of the differences. |
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Narrow Band Output Graph Ref: narrowbandgraph
See how the output range is only linear over a 'narrow' band. This band corresponds to the stoichiometric A/F Ratio (14.7:1). If the air fuel mix is very lean or very rich then the output of the sensor is very low or very high and it is impossible to acertain the a:f ratio.
This sensor output could be read over the narrow range of 14.25:1 to 15.25:1
Click More for a bigger picture.
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Wideband Output Graph Ref: widebandoutputgraph
See how the output range is only linear over a 'narrow' band. This band corresponds to the stoichiometric A/F Ratio (14.7:1). If the mix is very lean or very rich then the output of the sensor is very low or very high and it is impossible to acertain the ratio.
This sensor output could be read over the narrow range of 14.25:1 to 15.25:1
Click More for a bigger picture.
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Round Digital Air / Fuel Ratio Gauge - Narrowband Ref: DT001
*** This gauge is built to order, expected delivery of 2 weeks ***
for detailed product information click...
Price: £63.02 (Incl. VAT)
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Display
Face
Bezel
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Square Digital Air /Fuel Ratio Gauge - Narrowband Ref: DT002
*** This gauge is built to order, expected delivery of 2 weeks ***
for detailed product information click...
Price: £53.04 (Incl. VAT)
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Display
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Narrowband Oxygen Sensor Ref: DT017
*** Usually in stock for next day (UK Mainland) delivery ***
Price: £20.68 (Incl. VAT)
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Bung and Plug for mounting oxygen sensor Ref: DT032
Bung and plug for mounting oxygen sensor
*** Usually in stock for next day (UK Mainland) delivery ***
Price: £6.33 (Incl. VAT)
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