Air-fuel ratio meter

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 a 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 accurate 'wide band' sensor, though 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. Analogue 'needle' style gauges are also available.

As stated above, there are wide band meters that stand alone or are mounted in 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.



Benefits of air-fuel ratio metering.

* Determining the condition of the oxygen sensor: A malfunctioning oxygen sensor will result in air-fuel ratios which respond more slowly to changing engine conditions. A damaged or defective sensor may lead to increased fuel consumption and increased pollutant emissions as well as decreased power, and throttle response.

* Reducing emissions: Keeping the air-fuel mixture near the stoichometric ratio of 14.7:1 (for gasoline engines) allows the catalytic converter to operate at maximum efficiency.

* Fuel economy: An air-fuel mixture at the stoichometric ratio will result in near optimum fuel mileage, costing less per mile traveled and producing the least amount of CO2 emissions. However, from the factory, cars are designed to operate at the stoichometric ratio (rather than as lean as possible while remaining driveable) in order to maximize the efficiency and life of the catalytic converter. While it may be possible to run smoothly at leaner mixtures than 14.7:1, manufacturers must focus on emissions and especially catalytic converter life (which must now be 100,000 miles on new vehicles) {fact} as a higher priority due to EPA regulations.

* Engine performance: Carefully mapping out air-fuel ratios throughout the range of rpm and manifold pressure will maximize power output in addition to reducing the risk of detonation.

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 large 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, or burn valves in the engine. The risk of spark knock/engine knocking (detonation) is also increased when the engine is under load.

Mixtures that are richer than stoichometric are necessary when the engine is under heavy load, especially under wide open throttle. The ideal mixture in this type of operation depends on the individual engine. For example, engines using forms of forced induction such as turbochargers and superchargers typically require a richer mixture under wide open throttle than naturally aspirated engines.

Cold engines also typically require more fuel and a richer mixture when first started, because fuel does not vaporize as well when cold, and because cold air is denser, and therefore requires more fuel to properly "saturate" it. Rich mixtures also 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.


Oxygen sensor types.

Oxygen sensors are installed in the exhaust system of the vehicle, attached to the engine's exhaust manifold, the sensor calculates 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. The wideband sensor was introduced much later.

A narrow band sensor has a non-linear output, and switches between the thresholds of lean (ca 100-200 mV) and rich (ca 650-800 mV) areas very steeply.

Also, narrow band sensors are temperature-dependent. If the exhaust gases become warmer, the output voltage in the lean area will rise, and in the rich area it will be lowered. Consequently, an oxygen sensor, without pre-heating, has a lower lean-output and a higher rich-output, possibly 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 switch the output voltage between ca 100 and 850/900 mV and after a while the sensor may output a switch voltage between ca 200 and 700/750mV, for turbocharged cars even less.

The ECU tries to maintain a stoichiometric balance, wherein 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.).

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 sufficiently 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.


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, and performance, a narrow band air-fuel ratio meter is sufficent.

In high-performance tuning applications, however, the wide-band system is desireable.