Air-fuel ratio meter
Encyclopedia
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 80s. 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.
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 vapourized liquid fuels, due to the cooling effect of the evaporating fuel. This increases the intake oxygen density, allowing for more fuel to be combusted and more power developed. The ideal mixture in this type of operation depends on the individual engine. For example, engines with forced induction such as turbocharger
s and supercharger
s 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.
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.
's exhaust manifold
, the sensor
measures 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, and switches between the thresholds of lean (ca 100–200 mV) and rich (ca 650–800 mV) areas very steeply. 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 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 car
s even less.
The engine control unit
(ECU) when operating in "closed loop" tend to maintain 0 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 narrow band sensors cannot output a fixed voltage level between the lean and the rich areas, the ECU control 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 said ca 450 mV.
A wide-band sensor, on the other hand, has a very linear
output, 0–5 V, and is not temperature dependent.
In high-performance tuning applications, the wide-band system is desirable.
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
. Also called air–fuel ratio gauge, air–fuel meter, or air–fuel gauge. It reads the voltage output of an oxygen sensor
Oxygen sensor
An oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen in the gas or liquid being analyzed. It was developed by the Robert Bosch GmbH company during the late 1960s under the supervision of Dr. Günter Bauman...
, 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 80s. 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
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
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 sensorOxygen sensorAn oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen in the gas or liquid being analyzed. It was developed by the Robert Bosch GmbH company during the late 1960s under the supervision of Dr. Günter Bauman...
: 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 converterCatalytic converterA catalytic converter is a device used to convert toxic exhaust emissions from an internal combustion engine into non-toxic substances. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion are converted to less toxic substances by dint...
to operate at maximum efficiency. - Fuel economyFuel economy in automobilesFuel usage in automobiles refers to the fuel efficiency relationship between distance traveled by an automobile and the amount of fuel consumed....
: An air–fuel mixture leaner than the stoichiometric 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 stoichiometric ratio (rather than as lean as possible while remaining driveable), 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 ) as a higher priority due to U.S. EPA regulations. - Engine performanceCar tuningCar tuning is both an industry and a hobby, in which an automobile is modified in order to improve its performance, handling and appearance. As most vehicles leave the factory set up for average driver expectations and average conditions, tuning has become a way to personalize the characteristics...
: 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 detonationDetonationDetonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases...
.
Lean mixtures improve the fuel economy
Fuel economy in automobiles
Fuel usage in automobiles refers to the fuel efficiency relationship between distance traveled by an automobile and the amount of fuel consumed....
but also cause sharp rises in the amount of nitrogen oxides (NOX)
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
. If the mixture becomes too lean, the engine may fail to ignite, causing misfire and a large increase in unburned hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
(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
Engine knocking
Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...
(detonation) is also increased when the engine is under load.
Mixtures that are richer than stoichiometric allow for greater peak engine power when using vapourized liquid fuels, due to the cooling effect of the evaporating fuel. This increases the intake oxygen density, allowing for more fuel to be combusted and more power developed. The ideal mixture in this type of operation depends on the individual engine. For example, engines with forced induction such as turbocharger
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
s and supercharger
Supercharger
A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...
s 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.
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
Engine knocking
Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...
(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 engineInternal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
's exhaust manifold
Exhaust manifold
In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. The word manifold comes from the Old English word manigfeald and refers to the folding together of multiple inputs and outputs.In contrast, an inlet manifold is the part of an engine...
, the sensor
Oxygen sensor
An oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen in the gas or liquid being analyzed. It was developed by the Robert Bosch GmbH company during the late 1960s under the supervision of Dr. Günter Bauman...
measures 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, and switches between the thresholds of lean (ca 100–200 mV) and rich (ca 650–800 mV) areas very steeply. 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 to voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
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 car
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
s even less.
The engine control unit
Engine control unit
An engine control unit is a type of electronic control unit that determines the amount of fuel, ignition timing and other parameters an internal combustion engine needs to keep running...
(ECU) when operating in "closed loop" tend to maintain 0 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 narrow band sensors cannot output a fixed voltage level between the lean and the rich areas, the ECU control 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 said ca 450 mV.
A wide-band sensor, on the other hand, has a very linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
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 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
- Automobile emissions controlAutomobile emissions controlVehicle emissions control is the study and practice of reducing the motor vehicle emissions -- emissions produced by motor vehicles, especially internal combustion engines....
- Car tuningCar tuningCar tuning is both an industry and a hobby, in which an automobile is modified in order to improve its performance, handling and appearance. As most vehicles leave the factory set up for average driver expectations and average conditions, tuning has become a way to personalize the characteristics...
- Engine control unitEngine control unitAn engine control unit is a type of electronic control unit that determines the amount of fuel, ignition timing and other parameters an internal combustion engine needs to keep running...
(ECU) - Engine tuningEngine tuningEngine tuning is the adjustment, modification or design of internal combustion engines to yield optimal performance, to increase an engine's power output, economy, or durability....