Air-Fuel Ratio Meter

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 AFR sensor or lambda sensor

 

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 travelled 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 drive-able) to maximise 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.

 

Engine performance: Carefully mapping out air–fuel ratios throughout the range of rpm and manifold pressure will maximise 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). 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 vaporised liquid fuel due to the mixture not being able to reach a perfectly homogenised 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 one ports 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.

Cold engines also typically require more fuel and a richer mixture when first started, because fuel does not vaporise 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.

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