Mercedes Mullins
The air-fuel ratio is the mass ratio of air to fuel present in a combustion process. This ratio is important for anti-pollution and performance-tuning reasons. The ideal ratio for a gasoline engine is 14.7 parts air to 1 part fuel, which is a compromise between optimum fuel economy and power output. This ratio is also known as the stoichiometric air-fuel ratio, which causes all the oxygen and fuel to be consumed inside the engine during combustion, resulting in only water and carbon dioxide exiting the vehicle's tailpipe. The ratio can be determined by using an air-fuel ratio meter, which reads the voltage output of an oxygen sensor. Oxygen sensors were first used in the 1970s as part of electronic fuel injection systems and have since become standard in most vehicles. Other methods of measuring the fuel-air mix include checking the colour of the spark plug and tail pipe, and monitoring the fuel trim data via a scan tool.
Characteristics and Values of Measuring Fuel-Air Mix in Car Exhaust
| Characteristics | Values |
|---|---|
| Ideal Air-Fuel Ratio | 14.7:1 (14.7 parts air to 1 part fuel) |
| Range of Operability | 8:1 to 18.5:1 |
| Measurement Tools | Portable exhaust gas analyzers, oxygen sensors, UEGO sensors, air-fuel ratio meters, scan tools |
| Indicators of Richness | Watery eyes, backfiring, spark plug and tailpipe colour, high CO concentrations in the exhaust |
| Indicators of Leanness | Backfiring, high NOX formation |
| Pollutants | PM2.5 particles, carbon monoxide, carbon dioxide, water vapour, nitrogen, unburned hydrocarbons, nitrogen oxides, sulfur oxides, volatile organic compounds |
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What You'll Learn
Oxygen sensors
The early oxygen sensors were of the Zirconia type, producing a voltage output between 0.1 and 1.0 volts. A reading of 0.45 volts is considered ideal, as it indicates a stoichiometric air-fuel ratio, which means efficient and clean burning. However, as the mixture deviates from this ideal voltage, Zirconia sensors become less accurate, only indicating whether the mixture is rich or lean without providing quantitative information.
To address this limitation, wide-band oxygen sensors, also known as ultra-wideband or UWB sensors, were introduced in the 1990s. These sensors provide a more accurate and sensitive measurement of the air-fuel mixture, allowing for better fuel efficiency and lower emissions. They are often coupled with a dash-mounted air/fuel gauge to provide real-time data on the exhaust mixture, enabling mechanics and enthusiasts to fine-tune their engine's performance.
In addition to oxygen sensors, there are also air/fuel ratio sensors, which serve a similar purpose but offer a higher level of sensitivity. These sensors provide more detailed data to the EMS, allowing for even greater accuracy in responding to the immediate air/fuel mixture requirements of the engine and further improving fuel efficiency and emissions.
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Carbon monoxide levels
The ideal air-fuel mixture for a gasoline engine is 14.7 parts air to 1 part fuel. This is known as the stoichiometric air-fuel ratio, a compromise between fuel economy and power output. A stoichiometric air-fuel ratio results in the complete combustion of oxygen and fuel inside the engine, producing only water and carbon dioxide.
However, the stoichiometric ratio is dynamic and often fluctuates between a rich mixture (less air than the stoichiometric ratio) and a lean mixture (more air than the stoichiometric ratio). A rich mixture can cause the engine to misfire and run poorly due to incomplete combustion.
Carbon monoxide (CO) is formed when combustion occurs with less than the ideal volume of oxygen, resulting in a rich fuel mixture. CO levels are a good indicator of fuel mixture richness but a poor indicator of leanness. At high combustion efficiencies, the CO level decreases as the air-fuel ratio increases, and it remains low even as the mixture is further leaned out.
Oxygen sensors are the primary input for the PCM (Powertrain Control Module) regarding mixture control. They detect molecular oxygen in the exhaust, which is used in the combustion process. If more than a trace amount of oxygen is detected, the computer adjusts the fuel injectors to spray more fuel into the mix, bringing it back to the efficient amount.
Portable exhaust gas analyzers like the Heathkit CI-1080 can be used to directly measure the percentage of carbon monoxide in the exhaust, which is closely related to the air-fuel mixture. Modern cars use a PCM to continuously monitor and adjust the air-fuel mixture, and the best way to check if the engine is running rich or lean is to use a scan tool to monitor the fuel trim data.
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Fuel injection systems
Fuel injection is the process of introducing fuel into an internal combustion engine. Fuel injection systems are now standard in most new petrol cars, replacing older carburetor motors. They are more reliable, effective, and eco-friendly.
The fundamental function of a fuel injection system is to spray pressurised fuel into the engine. The system must determine the appropriate amount of fuel to be supplied and control the fuel flow to supply this amount. The fuel injector is effectively a spray nozzle that performs the final stage in the delivery of fuel into the engine. The fuel is sprayed with the help of a nozzle that is opened and closed with a solenoid-operated needle valve.
There are several types of fuel injection systems, which differ in mechanism and working principles. They can be classified according to different characteristics, including external or internal air-fuel mixture formation, the number of injection points (nozzles), and direct or indirect injection.
Direct injection means that the fuel is injected into the main combustion chamber of each cylinder. The air and fuel are mixed only inside the combustion chamber, so only air is sucked into the engine during the intake stroke. The injection scheme is always intermittent (either sequential or cylinder-individual). This can be done either with a blast of air or hydraulically, with the latter being more common in automotive engines.
Indirect injection, on the other hand, involves injecting the fuel into a pre-chamber, where it begins to combust, before it is injected into the main combustion chamber.
Single-point injection systems are the simplest and earliest type, replacing carburetors in the 1980s. They use only one nozzle in the throttle body, which sprays fuel to an air intake manifold shared by all cylinders. This system is more precise than a carburetor and does not require significant changes to the engine. However, it is now considered below eco-standards and not very reliable, as the break of a single nozzle can crush the whole system.
Multi-point injections are more common today, with the most effective system being sequential injection. This system coordinates each nozzle separately with its cylinder phase, so each nozzle sprays the fuel just when the intake valve opens, saving maximum energy and improving emissions.
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Air-fuel ratio
The air-fuel ratio is an important measure for anti-pollution and performance-tuning reasons. The ideal ratio for a gasoline engine is 14.7 parts air to 1 part fuel, also known as the stoichiometric ratio. This ratio is a compromise between optimum fuel economy and power output.
A stoichiometric ratio causes all of the oxygen and fuel to be consumed inside the engine during combustion, resulting in only water and carbon dioxide exiting the vehicle's tailpipe. However, a perfectly stoichiometric mixture burns very hot and can damage engine components under high load. Therefore, stoichiometric ratios are only used under light to low-moderate load conditions. For acceleration and high-load conditions, a richer mixture (lower air-fuel ratio) is used to produce cooler combustion products.
A rich air-fuel mixture contains less air than the stoichiometric ratio, while a lean mixture contains more air. Rich mixtures are less efficient but may produce more power, and lean mixtures are more efficient but may cause higher temperatures, leading to the formation of nitrogen oxides. Engines with improper coolant temperatures, for example, can cause higher combustion chamber temperatures and contribute to nitrogen oxide formation.
Oxygen sensors are the primary input for the PCM (Powertrain Control Module) to control the air-fuel mixture. These sensors can detect molecular oxygen in the exhaust, which is used in the combustion process. If more than a trace amount of oxygen is detected, it indicates that the combustion is inefficient, and more fuel is injected into the mix.
To measure the air-fuel ratio, a scan tool can be used to check the fuel trim data and determine if the engine is running rich or lean. The fuel trim data is typically displayed as a percentage and can be retrieved by plugging the scan tool into the diagnostic port under the vehicle's dashboard.
In addition to scan tools, there are other methods and tools available to measure the air-fuel ratio. For example, the colour of the spark plug and tailpipe can provide indications of rich and lean running. Race shops may use dynamometers and exhaust gas analysers to directly measure the richness of the mixture under certain load conditions. Portable exhaust gas analysers, such as the Heathkit CI-1080, can also be used to directly measure the percentage of carbon monoxide in the exhaust, which is closely related to the air-fuel mixture.
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Pollutants and emissions
The ideal air-fuel mixture for a gasoline engine is 14.7 parts air to 1 part fuel, known as the stoichiometric air-fuel ratio. This ratio is a compromise between optimum fuel economy and power output. When the engine burns its fuel efficiently, the by-products are water vapour, carbon dioxide, and nitrogen, none of which are harmful. However, deviations from the stoichiometric ratio can result in a rich or lean mixture, leading to incomplete combustion and the release of harmful pollutants.
One of the primary pollutants of concern is carbon monoxide (CO), a colourless, odourless, and poisonous gas that can cause various health issues, including headaches, respiratory problems, and even death. High levels of carbon monoxide are often observed in urban areas, especially during winter when engines take longer to warm up. Another harmful pollutant is unburned hydrocarbons (HC), which can cause respiratory issues, crop damage, and contribute to smog formation. Oxides of nitrogen (NOx) are also emitted, exacerbating respiratory problems and smog. These three pollutants, often referred to as the "terrible trio," are addressed by the catalytic converter in a vehicle's exhaust system.
Particulate matter, or diesel engines' emissions of airborne particles of black soot and metal, is another significant concern. Modern cars are equipped with diesel particulate filters (DPFs) to mitigate this issue. However, older diesel vehicles, in particular, have been criticised for their harmful particulate emissions. To address this, many cities have implemented low-emission zones, discouraging older diesel vehicles from entering congested areas.
Other pollutants include benzene, a carcinogenic substance found in petrol and diesel, and trace metals such as lead and sulfur. Additionally, brakes and tyres contribute to harmful emissions, releasing tiny fragments of particulate matter, dust, and plastic particles that can impact human health and the environment.
To comply with stricter emission control standards, wide-band oxygen sensors (also called "ultra-wideband" or UWB sensors) have been employed since the 1990s. These sensors provide real-time measurements of the exhaust mixture, allowing for adjustments to carburetor settings or fuel injection to optimise the air-fuel ratio and minimise pollutant emissions.
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Frequently asked questions
The best way to tell whether your engine is running rich or lean is to monitor the fuel trim data via a scan tool that displays live data.
The ideal ratio is 14.7 parts air to 1 part fuel. This is referred to as the stoichiometric air-fuel ratio.
A stoichiometric mixture has just enough air to completely burn the available fuel. This causes all of the oxygen and fuel to be consumed inside the engine during combustion, resulting in nothing but harmless water and carbon dioxide exiting the vehicle's tailpipe.
You can use an air-fuel ratio meter or an oxygen sensor to measure the fuel-air mix in your car exhaust.
A rich air-fuel mixture contains less air than the stoichiometric ratio, whereas a lean mixture contains more air. Both can cause incomplete combustion (misfiring) and the engine will run poorly.
