Regan Brown
Increasing airflow improves engine efficiency, meaning that the same amount of work can be done while consuming less fuel. This is because the engine doesn't have to work as hard to overcome pumping losses. For gasoline engines, this means that the same power can be achieved at a lower air throttle opening, resulting in a smaller amount of fuel being used. For diesel engines, flow improvements in the intake and exhaust systems increase efficiency, as power output is regulated by how much fuel is injected. Turbochargers can be used to increase airflow, and their efficiency can be observed by identifying the pressure ratio and referring to compressor maps.
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What You'll Learn
Airflow improvements increase power output
The power output of a gasoline engine is directly related to the total airflow through the engine at any given time. As such, airflow enhancements that decrease pumping losses can increase power output without requiring additional fuel consumption.
The amount of oxygen available to support fuel combustion is known as the "charge density". When the throttle is closed, very little air mass flows into the engine, resulting in low-density air. Conversely, when the throttle is opened, more air mass can enter, increasing the density. The more oxygen that flows into the engine, the more fuel can be burned, and the greater the power output.
For a non-supercharged engine, the pressure available to force air into the engine is simply atmospheric pressure, which is about 14.7 pounds per square inch at sea level. This pressure can be increased through supercharging, which involves using a compressor to raise the pressure above atmospheric levels.
The stoichiometric air-fuel ratio (AFR) depends on the fuel type, with alcohol having a ratio of 6.4:1 and diesel 14.5:1. A lower AFR number indicates a richer mixture with less air, while a higher number contains more air and is considered leaner.
For diesel engines, which are "fuel throttled," power output is regulated by the amount of fuel injected. As such, improvements in airflow through the intake and exhaust systems can enhance efficiency and power output. Supercharging and turbocharging can increase charge density and power output for both gasoline and diesel engines, but proper controls must be in place to prevent excessive temperature and poor fuel economy.
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Diesel engines are fuel throttled
In a gasoline engine, the throttle is typically a butterfly valve, which rotates within the throttle body when the accelerator pedal is pressed, opening the throttle passage to allow more air into the intake manifold. The mass airflow sensor measures this change and communicates it to the Engine Control Unit (ECU), which then increases the amount of fuel injected by the injectors. The ECU can also achieve better control to reduce emissions, maximize performance, and adjust the engine idle.
Diesel engines, on the other hand, do not need to control air volumes and therefore usually lack a butterfly valve in the intake tract. Instead, they rely on fuel injectors to inject more or less diesel, depending on the power required. This is because diesel engines operate at higher ratios, typically with air-fuel ratios of around 18:1 to 70:1, while gasoline engines keep the ratio within 12:1 to 18:1.
It is worth noting that some newer diesel engines meeting stricter emissions standards may use a butterfly valve to generate intake manifold vacuum, allowing the introduction of exhaust gas to lower combustion temperatures and minimize NOx production. Additionally, some diesel engines have implemented throttle controls to regulate intake manifold pressure, increasing the amount of exhaust gas recirculation.
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Gasoline engines are air throttled
The throttle response of a gasoline engine refers to how quickly the engine can increase its power output in response to a driver's request for acceleration. The throttle opening is determined by the Engine Control Unit (ECU), which takes into account the accelerator pedal's position and inputs from other engine sensors. When the driver presses on the accelerator pedal, the throttle plate rotates, opening the throttle passage and allowing more air into the intake manifold.
The mass airflow sensor measures this change and communicates it to the ECU, which then adjusts the amount of fuel injected by the injectors to maintain the required air-fuel ratio. In modern engines, the ECU controls the flow of fuel and air, rather than the driver, in order to reduce emissions, maximize performance, and adjust the engine idle. This is known as a drive-by-wire system.
Factors such as improper maintenance, fouled spark plugs, or bad injectors can reduce the throttle response of a gasoline engine. Additionally, the design of the engine, such as whether it is naturally aspirated, supercharged, or turbocharged, can also affect its responsiveness. For example, naturally aspirated gasoline engines typically have better responsiveness than supercharged or turbocharged engines with similar peak power outputs.
The amount of fuel required for a corrected airflow depends on the stoichiometric air-fuel ratio (AFR), which varies depending on the fuel type. For example, the stoichiometric AFR for alcohol is 6.4:1, while for diesel, it is 14.5:1. A lower AFR number indicates a richer mixture with less air, while a higher AFR number contains more air and is considered a leaner mixture.
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Turbocharger efficiency and pressure ratio
Turbochargers are centrifugal compressors driven by an exhaust gas turbine and are used to boost the charge air pressure in an engine. They influence important engine parameters such as fuel economy, power, and emissions. The performance of a turbocharger depends on its pressure ratio, which is the variable equation of atmospheric pressure and gauge pressure divided by atmospheric pressure. This identifies the duty cycle of the compressor and is a key component in selecting the correct turbocharger for a given application.
The pressure ratio can be calculated by taking the absolute outlet pressure and dividing it by the absolute inlet pressure. It is important to note that most gauges read in gauge pressure, which is zero psi at atmospheric pressure, while the actual pressure is around 14.7 psi. Pressure ratio is used instead of boost because atmospheric pressure changes with altitude and weather conditions.
Compressor maps are contour plots that show the efficiency of a compressor stage. They are used to determine the size of the compressor required for a specific application. The maps contain efficiency islands, which are concentric regions that represent the compressor stage efficiency at various points. The smaller islands at the center of the map represent the most efficient operating points, and as the rings move outward, the efficiency decreases.
The surge line of a compressor map represents the maximum amount of pressure the turbocharger can produce while flowing the least amount of mass (air). The choke region, on the other hand, represents the maximum amount of air that the compressor side can flow at a given pressure ratio. When a compressor enters the choke region, the compressor outlet temperatures and shaft speed rapidly increase, indicating that the maximum flow limit of the turbocharger has been reached.
While the focus is on turbocharger efficiency and pressure ratio, it is worth noting that other factors influence the overall efficiency of an engine and its fuel consumption. These include engine volumetric efficiency (how efficient an engine is at moving air through its cylinders), brake-specific fuel consumption, engine speed (RPM), elevation, and fuel quality.
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Stoichiometric AFR and rich/lean AFR
The air-fuel ratio (AFR) is a critical measure in engine tuning. It represents the ratio of air to fuel in the combustion chamber. This ratio is essential because it determines the combustion efficiency and, consequently, the engine's performance, fuel economy, and emissions.
The stoichiometric mixture is the ideal ratio of air to fuel that burns all the fuel with no excess air. For gasoline fuel, the stoichiometric air-fuel mixture is about 14.7:1, i.e., for every one gram of fuel, 14.7 grams of air are required. This ratio is also referred to as Lambda 1, where Lambda equals AFR divided by stoich. At stoichiometry, Lambda equals 1.0.
A rich mixture has a higher fuel content relative to air. For example, a gasoline AFR of 9:1 indicates a rich condition, meaning there is a lot of fuel and not much air in the exhaust stream. Running a rich AFR mixture will result in terrible fuel economy and increased emissions.
A lean mixture has more air than fuel. A gasoline AFR of 20:1 is considered lean, with high air content and low fuel content in the exhaust. A lean mixture will cause a much hotter burn, potentially damaging the engine's internals.
Forced induction engines require richer mixtures (lower Lambda) to prevent knocking and manage higher cylinder pressures. A gasoline AFR of 11.5:1 (Lambda 0.78 to 0.8) is often used.
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Frequently asked questions
The stoichiometric AFR for diesel is 14.5:1. A lower AFR number contains less air and is considered a richer mixture.
Improving airflow reduces pumping losses, meaning the engine doesn't have to work as hard. This results in improved fuel efficiency, as less fuel is required to achieve the same amount of work.
Increasing airflow can enhance power output without hurting reliability. Additional fuel can be added to match the increased airflow, resulting in substantial power gains.
