Rajan Wilcox
The air-fuel ratio (AFR) is the mass ratio of air to fuel in a combustion process. It determines whether a mixture is combustible, the amount of energy released, and the amount of unwanted pollutants produced. Combustion, or burning, is the process of releasing energy from a fuel and air mixture. In an internal combustion engine (ICE), the ignition and combustion of the fuel occur within the engine itself, and the engine then partially converts the energy from the combustion to work. The expansion of the combustion gases pushes the piston, which rotates the crankshaft, ultimately driving the vehicle's wheels. The expansion ratio of an ignited gas/air mixture is approximately 8:1, with diesel mixtures having a higher ratio of about 17:1.
| Characteristics | Values |
|---|---|
| Definition of Air-Fuel Ratio | The ratio between the mass of air and the mass of fuel in the air-fuel mix at any given moment |
| Factors Affecting Air-Fuel Ratio | Oxygen content of combustion air, altitude, intake air temperature, dilution by ambient water vapour, enrichment by oxygen additions |
| Air-Fuel Ratio for Complete Combustion | Stoichiometric air fuel ratio, which is 14.7:1 for a gasoline (petrol) engine |
| Air-Fuel Ratio for Pure Octane | 15.1:1 or λ of 1.00 exactly |
| Air-Fuel Ratio for Maximum Output | 12:1 |
| Air-Fuel Ratio for Maximum Fuel Economy | 16:1 |
| Air-Fuel Ratio for Spark Ignition Engines | 12:1 (rich) to 20:1 (lean) |
| Air-Fuel Ratio for Compression Ignition Engines | 18:1 to 70:1 |
| Air-Fuel Ratio for Jet Engines | 15:1 |
| Expansion Ratio of Ignited Gas/Air Mixture | 8:1, with diesel mixtures higher at about 17:1 |
| Combustion Process | Ignition and combustion of fuel, expansion of combustion gases, conversion of energy from combustion to work |
| Combustion By-Products | Exhaust gases, nitrogen oxides, carbon monoxide, hydrocarbons, particulate matter |
| Anti-Pollution Measures | Catalytic converters, three-way catalysts, lambda control |
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What You'll Learn
The expansion of combustion gases pushes the piston
An internal combustion engine (ICE) converts the energy from the combustion of a fuel-air mixture into mechanical energy. The combustion process involves igniting a mixture of fuel and air, which produces exhaust gases and heat. The expansion of these combustion gases pushes the piston, which in turn rotates the crankshaft, ultimately driving the vehicle's wheels.
The ratio of air to fuel in the mixture is critical to the combustion process. This air-fuel ratio (AFR) determines whether the mixture is combustible, the amount of energy released, and the level of pollutant byproducts. The ideal ratio for complete combustion, where all the fuel is burned, is called the stoichiometric air-fuel ratio. For a gasoline engine, this ratio is typically around 14.7:1, meaning 14.7 kg of air is required to burn 1 kg of fuel.
In practice, achieving a perfect stoichiometric mixture is challenging due to the short time available for each combustion cycle. Most of the combustion process occurs within approximately 2 milliseconds at an engine speed of 6,000 revolutions per minute. Catalytic converters are designed to work optimally with exhaust gases resulting from near-perfect combustion.
The expansion ratio of an ignited gas/air mixture is approximately 8:1 for gasoline engines, while diesel engines have a higher expansion ratio of about 17:1. The expansion of the combustion gases is influenced by factors such as the effective compression ratio, temperature, humidity, and other variables. The expansion of these gases leads to an increase in pressure, which drives the movement of the piston during the power stroke.
The specific design of spark ignition gasoline and compression ignition diesel engines differs in how they supply and ignite the fuel. In a spark ignition engine, the fuel is mixed with air and inducted into the cylinder. The mixture is then compressed and ignited by a spark, causing combustion. On the other hand, a diesel engine inducts only air, compresses it, and then sprays fuel into the hot compressed air at a controlled rate, leading to ignition.
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The ideal air-fuel ratio for complete combustion
In theory, the stoichiometric mixture is 1 gram of fuel to 14.7 grams of air, or a ratio of 14.7:1. However, in practice, this is never quite achieved due to the very short time available in an internal combustion engine for each combustion cycle. Catalytic converters are designed to work best with exhaust gases produced by nearly perfect combustion.
The stoichiometric mixture is 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 and avoid overheating. Ratios lower than stoichiometric, where fuel is in excess, are considered "rich". These mixtures are less efficient but may produce more power.
Conversely, ratios higher than stoichiometric, where air is in excess, are considered "lean". Lean mixtures are more efficient but may cause higher temperatures, leading to the formation of nitrogen oxides. Some engines are designed to allow lean-burn. The ideal air-fuel ratio will vary depending on the operating load, and tuning involves establishing the desired ratio under various conditions.
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Spark ignition vs compression ignition engines
The two dominant types of engines are spark ignition (SI) engines and compression ignition (CI) engines. They differ fundamentally in their combustion processes, making them suitable for different applications and presenting unique characteristics and advantages.
Spark Ignition Engines
Spark ignition engines, also known as petrol engines, use spark plugs to ignite a mixture of air and fuel that is compressed in the combustion chamber. This mixture is drawn into the engine's cylinders during the intake process. The piston then compresses the fuel-air mixture, and the spark plug ignites it, causing combustion. The expansion of the combustion gases pushes the piston during the power stroke. The basic operation of an SI engine involves four strokes: intake, compression, combustion, and exhaust.
SI engines are used in most cars, motorcycles, and light-duty vehicles for transportation. They are also found in small engines like lawnmowers and chainsaws, as well as power generators, boats, and some small aircraft. SI engines are generally lighter in weight due to their lower peak pressures.
Compression Ignition Engines
Compression ignition engines, also known as diesel engines, work on the basis of the Diesel cycle or constant pressure heat addition cycle. In these engines, self-ignition occurs due to the high temperature and pressure of highly compressed air, eliminating the need for spark plugs. During the intake stroke, only air is drawn into the engine and then compressed. Diesel engines then spray the fuel into the hot compressed air at a suitable, measured rate, causing it to ignite. The basic operation of a CI engine also involves four strokes: intake, compression, combustion, and exhaust.
CI engines are commonly used in heavy-duty applications such as trucks, buses, ships, and generators. They are known for their superior fuel efficiency and torque output, making them suitable for tasks requiring robust and reliable power delivery. CI engines are generally heavier due to their higher peak pressures.
Air-Fuel Mixture Expansion
The expansion ratio of an ignited gas/air mixture is approximately 8:1, while diesel mixtures have a higher expansion ratio of about 17:1. The expansion of the combustion gases in both types of engines pushes the piston, which in turn rotates the crankshaft, ultimately driving the vehicle's wheels.
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The impact of air-fuel ratio on exhaust emissions
The air-fuel ratio (AFR) is the ratio between the mass of air and the mass of fuel in the air-fuel mix at any given moment. It determines whether a mixture is combustible, how much energy is released, and how much unwanted pollutants are produced in the reaction. The AFR is an important measure for anti-pollution and performance-tuning reasons.
The stoichiometric mixture has just enough air to completely burn all of the fuel. In practice, this is never quite achieved due to the very short time available in an internal combustion engine for each combustion cycle. Catalytic converters are designed to work best when the exhaust gases passing through them are the result of nearly perfect combustion.
Rich mixtures, where fuel is in excess, are less efficient but may produce more power and burn cooler. Lean mixtures, where air is in excess, are more efficient but may cause higher temperatures, leading to the formation of nitrogen oxides. There is no fixed air-fuel mixture for which minimum exhaust gas emissions can be obtained.
The air-fuel ratio has been studied in the context of reducing vehicle exhaust emissions pollution associated with gasoline combustion. Vehicle exhaust gases significantly contribute to air pollution in urban areas. The results of these studies can help in reducing fuel consumption, improving the quality of fuel combustion, and reducing vehicle exhaust emissions.
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The role of the lambda (oxygen) sensor
The lambda sensor, also known as the oxygen or O2 sensor, is a crucial component of modern internal combustion engines. It plays a significant role in optimising the air-fuel mixture and reducing harmful exhaust emissions.
The lambda sensor measures the volume of oxygen in the exhaust gases of an internal combustion engine. By doing so, it helps to optimise the air-to-fuel ratio, ensuring that the engine operates efficiently. This ratio is crucial in determining the engine's performance, fuel economy, and emissions.
The sensor works by detecting the difference in oxygen concentration between the exhaust gas and the external air. This information is then transmitted to the Engine Control Unit (ECU), which adjusts the fuel and gas mixture accordingly. The ECU aims to maintain a stoichiometric mixture, where there is just enough air to completely burn the available fuel. This balance ensures a compromise between power, fuel economy, and emissions.
The lambda sensor helps to prevent issues such as engine misfires, rough idling, and overheating. By optimising the air-fuel mixture, the lambda sensor also contributes to enhanced fuel economy and improved engine performance. Additionally, it aids in reducing the emission of pollutants, such as nitrogen oxides (NOx), unburnt fuel, and carbon monoxide (CO).
The development of the lambda sensor by Robert Bosch GmbH in the late 1960s revolutionised automotive technology. The sensor enabled more precise control of the air-fuel mixture, leading to improved engine efficiency and reduced environmental impact.
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Frequently asked questions
Air-fuel ratio (AFR) is the mass ratio of air to fuel in the combustion process. It determines whether a mixture is combustible, how much energy is released, and the amount of pollutants produced.
A stoichiometric mixture has just enough air to completely burn all the fuel. The ratio for pure octane is approximately 15.1:1, while for gasoline (petrol) engines, it is around 14.7:1. Modern internal combustion engines operate around this ratio for emissions reduction.
The ignited fuel-air mixture expands the combustion chamber volume, causing a drop in temperature and a rapid increase in pressure. This expansion pushes the piston, which rotates the crankshaft, ultimately driving the vehicle's wheels. The expansion ratio of a gasoline/air mixture is approximately 8:1, while diesel mixtures are higher at about 17:1.
