Rajan Wilcox
The air-fuel ratio is the ratio between the mass of air and the mass of fuel in the air-fuel mix entering a car's engine. This ratio is important as it determines whether a mixture is combustible, how much energy is released, and how much pollutant is produced in the reaction. The ideal ratio for a car engine is considered to be a stoichiometric mix, which is usually 14.7:1 for gasoline engines, though this varies for other fuels. This mixture is the best for power, emissions, and fuel efficiency.
Air and Fuel Mixture to a Car
| Characteristics | Values |
|---|---|
| Definition | The ratio between the mass of air and the mass of fuel in the air-fuel mix at any given moment. |
| Importance | Determines whether a mixture is combustible, how much energy is released, and how much unwanted pollutant is produced. |
| Ideal Ratio | 14.7:1 for gasoline engines. This is known as the stoichiometric mix. |
| Other Fuels | The ideal ratio differs for other fuels. For pure octane, the stoichiometric mixture is approximately 15.1:1. |
| Maximum Output Ratio | 12:1 |
| Maximum Fuel Economy Ratio | 16:1 |
| Rich | Too much fuel in the mixture. |
| Lean | Too little fuel in the mixture. |
| Lambda 1 | The point at which the air and fuel mixture is perfect for a petrol engine. |
| Sensors | Throttle position, mass air flow, and lambda sensors. |
| Carburetor | The first method for adjusting the air/fuel ratio. |
| Fuel Injection | A computer-controlled method to adjust the amount of fuel used. |
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What You'll Learn
Air-fuel ratio (AFR)
The air-fuel ratio (AFR) is the mass ratio of air to fuel in a combustion process. It determines whether a mixture is combustible, how much energy is released, and how much unwanted pollutants are produced. In an internal combustion engine, the AFR is an important measure for anti-pollution and performance-tuning.
The ideal AFR, also known as the stoichiometric mixture, is when there is just enough air to completely burn all of the fuel. This is usually considered 14.7:1 for gasoline engines, and is different for other fuels. For example, for pure octane, the stoichiometric mixture is approximately 15.1:1. The stoichiometric mixture is the best for both power and emissions, and improves fuel efficiency over a rich mixture.
When there is too much fuel in the mixture, it is considered a "rich" mixture, and when there is not enough fuel, it is considered a "lean" mixture. A rich mixture results in terrible fuel economy and increased emissions. A lean mixture will cause a much hotter burn, potentially damaging pistons and spark plugs.
The AFR can be monitored using an air-fuel ratio meter, also known as an oxygen sensor or lambda sensor. The meter reads the voltage output of the sensor to determine the AFR. In recent years, newer and more accurate wide-band sensors have become available, although they are more expensive than the older narrow-band sensors.
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Lambda 1
The stoichiometric mixture, often abbreviated as stoich, is when there is exactly enough air to completely burn all the fuel. This ratio is crucial for achieving the best performance and efficiency in automotive tuning. It is also essential for balancing performance and fuel economy. For pure gasoline, the stoichiometric ratio is 14.7:1, which provides the best compromise between performance, fuel economy, and emissions.
The air-fuel ratio (AFR) is the mass ratio of air to fuel present in a combustion process. It determines whether a mixture is combustible, the amount of energy released, and the level of unwanted pollutants produced. The ideal AFR can vary depending on the specific engine and its modifications, as well as geographical location. For example, a gasoline engine with a forced induction engine typically requires a richer mixture (lower Lambda) to prevent knocking and manage higher cylinder pressures.
The lambda (oxygen) sensor plays a critical role in maintaining the stoichiometric level. It measures the level of oxygen molecules in the exhaust gas and sends this information to the engine electronic control unit (ECU). Based on this data, the ECU adjusts the level of fuel mass to maintain the stoichiometric ratio.
While Lambda 1 is considered the ideal ratio for stoichiometric combustion, it is important to note that optimal engine performance and fuel efficiency may be achieved within a range of AFR values. The specific ratio that works best depends on various factors, including engine design, modifications, and environmental conditions.
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Carburetor
The primary function of a carburetor is to ensure an appropriate mixture of fuel and air so that the engine runs properly. The carburetor takes liquid gasoline from the gas tank and mixes it with air, which then travels to the combustion chamber, where the mixture is ignited by the spark plug. The energy stored in gasoline is thus converted into mechanical force.
The fuel inlet to a carburetor is quite complex. Attached to the fuel pipe is a mini fuel tank called a float-feed chamber, which has a float and valve inside it. As the chamber feeds fuel to the carburetor, the fuel level and the float both sink. When the float falls below a certain level, a valve opens, allowing fuel to enter and refill the chamber from the main gas tank. Once the chamber is full, the valve closes, and the fuel feed switches off.
The basic working principle of the carburetor has remained the same since Karl Benz's original design in 1888. Fuel from the tank enters what he called the generator, where it evaporates. The fuel vapour then passes through a pipe and meets air coming down the same pipe, which enters from the atmosphere through perforations at the top. The air and fuel mix in a chamber, then pass through a valve into the cylinder, where they burn to make power.
In modern engines, the fuel-air mixture is regulated by an electronically controlled system called fuel injection. However, many small-engine machines still use carburetors, especially in lawn mowers and chainsaws, as they are simple and inexpensive. The amount of fuel and air an engine needs varies depending on how long it has been running, the speed, and other factors. If there is not enough fuel mixed with the air, the engine "runs lean" and will either not run or damage the engine. On the other hand, if there is too much fuel, the engine "runs rich" and will not run properly, waste fuel, or produce smoke.
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Fuel injection
The primary difference between carburetion and fuel injection is that fuel injection atomises the fuel through a small nozzle under high pressure, while carburetion relies on suction created by intake air. Fuel injection is operated by spraying pressurised fuel into the engine. A device to pressurise the fuel is needed, such as a fuel pump. The system must determine the appropriate amount of fuel to be supplied and control the fuel flow to supply this amount. The amount of fuel supplied to the engine is determined by the amount of time the fuel injector stays open, which is called the pulse width.
The injector is located in the combustion chamber, inlet manifold, or throttle body. Fuel injectors that also control the metering are called injection valves, while injectors that perform all three functions are called unit injectors. 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. Therefore, 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.
There are two main functional principles of mixture formation systems for internal combustion engines: internal and external. A fuel injection system that uses external mixture formation is called a manifold injection system. There are two types of manifold injection systems: multi-point (or port) and single-point (or throttle body) injection. Internal mixture formation systems can be separated into several different varieties of direct and indirect injection, the most common being the common-rail injection, a variety of direct injection. The term electronic fuel injection refers to any fuel injection system controlled by an engine control unit (ECU).
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Anti-pollution and performance-tuning
The air-fuel ratio (AFR) is a critical measure for anti-pollution and performance-tuning reasons. It determines whether a mixture is combustible, how much energy is released, and how much unwanted pollutants are produced.
The stoichiometric mixture is where there is just enough air to completely burn all the available fuel. This mixture is abbreviated to "stoich" and burns very hot, which can damage engine components if the engine is placed under high load. This is why stoichiometric mixtures are only used under light to low-moderate load conditions. For acceleration and high-load conditions, a richer mixture is used to produce cooler combustion products and avoid cylinder head overheating.
Rich mixtures are less efficient but may produce more power and burn cooler. Ratios higher than stoichiometric, where there is excess air, are considered "lean". Lean mixtures are more efficient but may cause higher temperatures, leading to the formation of nitrogen oxides. Some engines are designed with features to allow lean-burn.
The ideal ratio of air to fuel in an internal combustion gasoline engine varies from engine to engine and fuel to fuel. However, there is a range of what is considered normal operation for high-performance applications. All new cars run at 14.7:1 air-fuel ratio at part throttle because this is the lowest emission point.
To tune an engine's air-fuel ratio, one can install an oxygen sensor (also known as a lambda sensor) in the exhaust of the car. This can be plugged into a handheld meter, data-logger, or permanently mounted gauge, to read the air-fuel ratio in real time.
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Frequently asked questions
The air and fuel mixture for a car is known as the air/fuel ratio or AFR. This is the ratio between the mass of air and the mass of fuel in the air-fuel mix.
A stoichiometric mix occurs when there are 14.7 parts of air to one part of fuel, or a ratio of 14.7:1. This is the most efficient mixture, resulting in the best fuel economy and lowest emissions.
When a car is running 'a bit rich', it means there is too much fuel in the air/fuel mixture. 'A bit lean' means there is not enough fuel, so the air/fuel ratio is incorrect and needs to be adjusted.
Modern engines use sensors to take in information on throttle position, mass air flow, and lambda sensors, among other things. They then use this information to automatically adjust the amount of fuel being delivered to maintain the correct air/fuel ratio.
