Air–fuel ratio meter

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An air–fuel ratio meter monitors the air–fuel ratio of an internal combustion engine. Also called air–fuel ratio gauge, 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 oxygen sensors became factory installed standard in the late 1970s and early 1980s. In recent years, a newer and much more accurate wide-band sensor, though more expensive, has become available.

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 that 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. Most engine management systems will detect a defective oxygen sensor.
  • Reducing emissions: Keeping the air–fuel mixture near the stoichiometric ratio of 14.7:1 (for gasoline engines) allows the catalytic converter to operate at maximum efficiency.
  • Fuel economy: An air–fuel mixture leaner than the stoichiometric ratio will result in near-optimal fuel mileage, costing less per distance traveled and producing the least amount of CO2 emissions. However, from the factory, cars are designed to operate at the stoichiometric ratio (rather than as lean as possible while remaining drivable) to maximize the efficiency and life of the catalytic converter. While it may be possible to run smoothly at mixtures leaner than the stoichiometric ratio, manufacturers must focus on emissions and especially catalytic converter life (which must now be 100,000 miles (160,000 km) on new vehicles[citation needed]) as a higher priority due to U.S. 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 stoichiometric allow for greater peak engine power when using vaporized liquid fuel due to the mixture not being able to reach a perfectly homogenized state so extra fuel is added to ensure all oxygen is burned producing maximum power. The ideal mixture in this type of operation depends on the individual engine. For example, engines with forced induction such as turbochargers and superchargers typically require a richer mixture under wide open throttle than naturally aspirated engines. Forced induction engines can be catastrophically damaged by burning too lean for too long. The leaner the air–fuel mixture, the higher the combustion temperature is inside the cylinder. Too high a temperature will destroy an engine – melting the pistons and valves. This can happen if you port the head and/or manifolds or increase boost without compensating by installing larger or more injectors, and/or increasing the fuel pressure to a sufficient level. Conversely, engine performance can be lessened by increasing fuelling without increasing air flow into the engine. Furthermore, if an engine is leaned to the point where its exhaust gas temperature starts to fall, its cylinder head temperature will also fall. This is only recommended in the cruising configuration, never when accelerating hard, but is becoming increasingly popular in aviation circles, where the appropriate engine monitoring gauges are fitted and the fuel air mixture can be manually adjusted.[1]

Cold engines also typically require more fuel and a richer mixture when first started (see: cold start injector), because fuel does not vaporize as well when cold and therefore requires more fuel to properly "saturate" the air. Rich mixtures also burn slower 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, and measure 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 wide-band sensor was introduced much later.

A narrow-band sensor has a nonlinear output, with ranges from 0.10v to 1.0v with .450 being ideal. 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, a sensor, without pre-heating, has a lower lean-output and a higher rich-output, possibly even exceeding 1 volt. The influence of temperature on voltage is smaller in the lean mode than in the rich mode.

A "cold" engine makes the computer change the fuel air ratio so the output voltage of the o2 sensor switches between about 100 and 850/900 mV and after a while the sensor may output a switch voltage between about 200 and 700/750 mV, for turbocharged cars even less.

The engine control unit (ECU) when operating in "closed loop" tends to maintain zero oxygen (thus a stoichiometric balance), wherein the air–fuel mixture is approximately 14.7 times the mass of air to fuel for gasoline. This ratio maintains a "neutral" engine performance (lower fuel consumption yet decent engine power and minimal pollution).

The average level of the sensor is near to 450 mV. Since catalytic coverters need a cycling a/f ratio the oxygen sensor is not allowed to hold a fixed voltage, the ECU controls the engine by providing the mixture between lean (and rich) in such a sufficiently fast manner by means of shorter (or longer) time of signal to injectors, so the average level becomes as stated about 450 mV.

A wide-band sensor, on the other hand, has a very linear output, 0–5 V, and is requires a much hotter running temperature.

Which type of air–fuel ratio meter to be used

If the purpose of the air–fuel ratio meter is to detect also an existing or possible problem with the sensor above of checking the general mixture and performance, a narrow band air-fuel ratio meter is sufficient.

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

See also


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