Air-fuel ratio meter

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A meter monitoring the air-fuel ratio of a combustion engine. Also called air-fuel ratio gauge or simply air-fuel meter or air-fuel gauge. It reads the voltage output of an oxygen sensor, sometimes also called lambda sensor, whether it be from an narrow band or wide band oxygen sensor.

The original narrow band sensors became car factory installed standard in the late 70´s and early 80´s. In recent years, a newer and much more accure 'wide band' sensor, yet more expensive, has come to availability.

Since there are two types of sensors, there are two types of meters, narrow band and wide band.

Most stand alone narrow band meters have 10 leds and some have more. Also common, narrow band meters in round housings with the standard mounting 2 1/16" and 2 5/8" diameters, as other types of car 'gauges'. These usually have 10 or 20 leds.

As above, there are wide band meters that stand alone or are mounted in round housings. Nearly all of these show the air-fuel ratio on a numeric display, since the wide band sensors provide a much more accurate reading. And since they use more accurate electronics, these meters are more expensive.

1 Different aspects of air-fuel ratio metering.
2 Oxygen sensor types.
3 Which type of air-fuel ratio meter to be used.
4 See also:
5 External links

[edit] Different aspects of air-fuel ratio metering.

Simply to be aware of the status of the oxygen sensor. If it seems aged or damaged in order to determine serviceability. A damaged or defective sensor may lead to increased fuel consumption and increased pollutant emissions.
Environment considerations. A lean air-fuel mixture lowers the amount of exhaust gas pollutants since a richer air-fuel mixture leads to exhaust gases with more unburned fuel that would cause more pollution.
Fuel economy considerations. A properly leaned air-fuel mixture will result in optimum fuel mileage, costing less per mile traveled and producing the least amount of CO2 emissions.
Performance considerations. Car tuning.

Lean mixtures improve the fuel economy but also cause sharp rises in the amount of nitrogen oxides (NOX). If the mixture becomes too lean, the engine may fail to ignite causing misfire and a huge increase in unburned hydrocarbon (HC) emissions. Lean mixtures burn hotter and may cause rough idle, hard starting and stalling, and can even damage the catalytic converter. The risk of spark knock/engine knocking (detonation) is also increased when the engine is under load. The engine needs a rich mixure under load.

Rich mixtures not only improve the performance, but are necessary when a cold engine is first started, since additional fuel is needed under engine load. Rich mixtures burn cooler and decrease the risk of spark knock/engine knocking (detonation) when the engine is under load. However, rich mixtures sharply increase carbon monoxide (CO) emissions.

[edit] Oxygen sensor types.

Being in the exhaust part of the car, attached to the engine's exhaust manifold, the sensor determines the ratio of the air-fuel mixture.

As mentioned above, there are two types of sensors available, narrow band and wide band. Narrow band sensors were the first to be introduced and the wideband sensors is a later developement.

Taking in consideration, a narrow band sensor has an unlinear output and switches between the thresholds of lean (ca 100-200 mV) and rich (ca 650-800 mV) areas very steeply, as explained below. There isn’t really any air fuel ratio difference within switching between the lean and the rich area thresholds.

Also, narrow band sensors are temperature dependent. If the exhaust gases get warmer, the output voltage in the lean area will rise and in the rich area it will be lowered. Consequently, a sensor without pre-heating which is not warmed up, has a lower lean-output and a higher rich-output, maybe even exceeding 1 Volt. The influence of temperature to voltage is smaller in the lean mode than in the rich mode. A "cold" engine makes the sensor to switch the output voltage between ca 100 to 850/900 mV and a after a while the sensor may output a switch voltage between ca 200 to 700/750mV, for turbocharged cars even less.

The ecu unit tries to keep the stoichiometric point, where the air-fuel mixture is approximately 14.7 times the mass of air to fuel. This, in order to maintain a neutral engine performance (lower fuel consumption yet decent engine power and minimal pollution.), to keep it close to the stoichiometric point.

The average level of the sensor is defined as 450 mV. Since narrow band sensors cannot output a fixed voltage level between the lean and the rich areas, the ecu tries to control the engine by controlling the mixture between lean and rich in such a sufficient fast manner, that the average level becomes ca 450 mV. Mixtures that are less than 14.7 to 1 are rich, and more than 14.7 to 1 are lean.

A wideband sensor on the other hand, has a very linear output, 0 - 5 V, and is not temperature dependent. They are also more expensive.

[edit] Which type of air-fuel ratio meter to be used.

If the purpose of the air-fuel ratio meter is to diagnose an existing or possible problem with the sensor and/or to check the general mixture performance, or to know just about the air-fuel ratio, a narrow band air-fuel ratio meter would do.

If tuning is taken into a very serious consideration, a wideband air-fuel meter is the only adequate choice.