Sylvie Willis
The amount of fuel used in a combustion cycle depends on the type of engine and its efficiency. Internal combustion engines (ICEs) are typically four-stroke engines that use gasoline or diesel, but they can also run on renewable or alternative fuels such as natural gas, propane, biodiesel, or ethanol. During the combustion cycle, the engine converts only 40-45% of the supplied energy into mechanical work, with the remaining energy being wasted as heat. To improve efficiency, methods such as waste heat recovery systems and variable displacement have been devised. The most efficient cycle is the Atkinson Cycle, found in some gasoline engines, which achieves an expansion ratio larger than the compression ratio. Diesel engines, on the other hand, can achieve peak efficiencies of around 45% in large trucks, buses, and newer cars.
| Characteristics | Values |
|---|---|
| Number of piston strokes in one combustion cycle | 4 |
| Engine type | Internal combustion engine (ICE) |
| Engine components | Fixed cylinder, moving piston, crankshaft |
| Cycle steps | Intake, compression, combustion, power stroke, exhaust |
| Fuel | Gasoline, diesel, natural gas, propane, biodiesel, ethanol |
| Fuel mixture | 12-18 parts air to 1 part fuel |
| Fuel efficiency | 40-45% |
| Fuel efficiency improvement methods | Turbocharging, thermoelectric generation, firing after top dead centre, Miller cycle |
| Engine power improvement methods | Forcing more air into the cylinder |
| Engine efficiency | 30% (Otto cycle), 45% (large diesel trucks, buses, newer diesel cars), 54.4% (MAN S80ME-C7 low-speed diesel engine) |
| Engine performance improvement methods | Forced induction, injecting nitrous oxide, using nitromethane |
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What You'll Learn
Fuel-air mixture
The amount of fuel used in a combustion cycle varies depending on the type of engine and fuel. For example, the stoichiometric air-fuel ratio for a gasoline engine is around 14.7:1, meaning that for every gram of fuel, 14.7 grams of air are required for complete combustion. This ratio is important because it determines whether a mixture is combustible, how much energy is released, and how much pollutant is produced.
The air-fuel ratio, or AFR, is the mass ratio of air to fuel present in a combustion process. This ratio can be adjusted to be either richer or leaner, depending on the desired outcome. A rich mixture has a lower AFR and may produce more power and burn cooler, while a lean mixture has a higher AFR and is more efficient but can lead to higher temperatures and the formation of nitrogen oxides.
In a four-stroke engine, the first part of the piston downstroke draws in the fuel-air mixture. The inlet valve then closes, and in the remainder of the downstroke, the mixture fires. The exhaust valve opens for the piston upstroke, releasing the exhaust gases. This process is similar in a two-stroke engine, except that the exhaust and intake are performed simultaneously, with the combustion chamber at its maximum volume.
The type of engine also affects the air-fuel ratio. Compression ignition (CI) engines, commonly found in diesel fuel vehicles, always run on lean mixtures with AFRs between 18:1 and 70:1. On the other hand, spark-ignition (SI) engines run on homogeneous mixtures.
Additionally, external factors such as altitude and air temperature can affect the air-fuel ratio. At high altitudes, the mixture should be leaned to avoid over-rich roughness and power loss. Similarly, at high temperatures, a richer mixture is used to produce cooler combustion products and prevent engine damage from overheating.
Overall, the fuel-air mixture is a critical aspect of the combustion cycle, influencing not only engine performance but also fuel efficiency and emissions.
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Compression
The compression process itself involves reducing the volume of the air-fuel mixture in the cylinder, thereby increasing the pressure and temperature. This compression is performed by the piston within the cylinder. In spark ignition gasoline engines, the fuel-air mixture is inducted into the cylinder and compressed, after which a spark plug ignites the mixture, causing combustion. The expanding combustion gases then push the piston during the power stroke, converting the energy from combustion into work.
In contrast, diesel engines, which are a type of compression ignition engine, compress only air initially. This compression generates heat, and then fuel is injected into the hot compressed air, causing ignition without the need for a spark plug. The subsequent combustion pushes the piston, as in spark ignition engines.
Higher compression ratios offer several advantages. Firstly, they allow engines to extract more mechanical energy from a given mass of the air-fuel mixture due to increased thermal efficiency. This is because higher compression ratios enable the same combustion temperature to be achieved with less fuel. Additionally, higher compression ratios result in higher peak cylinder pressures and temperatures, requiring stronger engine components and more robust materials.
However, there are also challenges associated with higher compression ratios. One issue is the increased susceptibility of engines to "knocking" or "detonation," particularly when using lower-octane fuels. Knocking refers to the premature self-ignition of the fuel-air mixture before the spark, which can damage engine components and reduce efficiency. Furthermore, the thermal efficiency gains from increasing compression ratios diminish beyond approximately 10:1 due to increased friction and heat losses. As a result, in modern petrol engines, compression ratios are typically between 8:1 and 12:1.
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Ignition
The ignition of fuel is a critical aspect of the combustion cycle, and it involves the initiation of combustion through various methods, depending on the type of engine and fuel used. The two primary ignition methods are spark ignition and compression ignition.
In a spark ignition gasoline engine, the fuel is mixed with air and inducted into the cylinder during the intake process. The piston then compresses this fuel-air mixture, and an electric spark is introduced to ignite it, initiating combustion. This combustion causes a rapid expansion of gases, which pushes the piston during the power stroke. This type of engine typically has a compression ratio ranging from 6:1 to 10:1, and higher compression ratios generally lead to improved fuel efficiency.
On the other hand, compression ignition diesel engines operate differently. In these engines, only air is inducted into the engine and compressed initially. Subsequently, diesel fuel is sprayed into the hot compressed air at a carefully controlled rate, causing it to ignite and combust. Diesel fuels have a higher energy density than gasoline, and they are less volatile, which makes them harder to ignite when cold. The description of how readily diesel fuel ignites is known as the Cetane rating.
The ignition process is influenced by the properties of the fuel, such as its flash point and auto-ignition point. The flash point is the lowest temperature at which the fuel can form an ignitable mixture with air, while the auto-ignition point is the temperature at which the fuel will spontaneously ignite without an external ignition source. Gasoline has a lower flash point than diesel, making it easier to ignite with a spark.
Additionally, the compression ratio of the engine plays a role in ignition. A higher compression ratio means a higher fuel efficiency, as it allows for a more substantial extraction of energy from the fuel. However, this also increases the risk of pre-ignition, which can damage the engine. To mitigate this issue, some engines employ systems to relieve pressure during the compression stroke, making it easier to start the engine.
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Power stroke
The power stroke is the third phase of a four-stroke cycle engine, following the intake and compression strokes. It is an engine operation stroke that involves the conversion of energy from combustion into mechanical energy. During the power stroke, the piston moves from the top of the cylinder (Top Dead Center) to the bottom of the cylinder (Bottom Dead Center).
At the end of the compression stroke, the fuel injector sprays diesel fuel directly into the hot, compressed air in the cylinder. The heat ignites the fuel, causing a controlled explosion. This combustion event involves a rapid oxidizing chemical reaction where the fuel combines with oxygen in the atmosphere and releases energy in the form of heat. The spark at the spark plug initiates combustion at approximately 20° of crankshaft rotation before Top Dead Center (BTDC). The flame front, which is the boundary wall separating the charge from the combustion by-products, spreads across the combustion chamber until the entire charge has burned.
As the fuel ignites, the expanding combustion gases push the piston downward during the power stroke. This piston force and motion are transferred through the connecting rod to apply torque to the crankshaft, initiating crankshaft rotation. The amount of torque produced depends on the pressure on the piston, the size of the piston, and the throw of the engine. This conversion of energy from combustion into rotational force drives the vehicle's wheels through a system of gears in the powertrain.
During the power stroke, both valves are closed, and the combustion gases are fully compressed. As the piston moves downward, it simultaneously pressurizes the mixture in the casing. Midway through the power stroke, two new ports open into the cylinder: the exhaust port and a port to admit fuel and air. When the piston reaches the bottom of the cycle, the now-pressurized fuel-air mixture enters the cylinder and helps force the exhaust through the exhaust port.
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Exhaust
The exhaust stage is one of the four key steps in a combustion cycle, the others being intake, compression, and power stroke. During the exhaust stage, the combustion gases that have been created during the combustion process are released from the engine. This process is essential to prevent the engine from becoming overloaded with gases, which could lead to a decrease in performance or even engine failure.
The exhaust stage is particularly important in internal combustion engines (ICEs), which include spark ignition gasoline engines and compression ignition diesel engines. In an ICE, the ignition and combustion of fuel occur within the engine itself, and the resulting gases are highly pressurised. The release of these gases through the exhaust system helps to regulate the pressure inside the engine and prevent overheating.
The exhaust system in an ICE is typically comprised of a network of pipes and chambers that are designed to channel the gases away from the engine. These systems often include a range of components such as catalytic converters, mufflers, and exhaust manifolds, which serve various functions including reducing harmful emissions, minimising noise, and optimising gas flow.
The composition of the exhaust gases can vary depending on the type of fuel used and the efficiency of the combustion process. In a complete combustion process, the fuel is fully oxidised, resulting in carbon dioxide and water vapour as the primary exhaust gases. However, incomplete combustion can lead to the formation of partially oxidised compounds such as carbon monoxide, aldehydes, and ketones, which are hazardous and contribute to air pollution.
Optimising the exhaust system is an important aspect of engine design and can significantly impact the overall performance and efficiency of the combustion cycle. This includes considerations such as the size and shape of the exhaust pipes, the placement of exhaust ports, and the use of advanced exhaust technologies. By minimising back pressure, improving gas flow, and reducing emissions, the exhaust system plays a crucial role in enhancing the overall efficiency of the combustion cycle.
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Frequently asked questions
The amount of fuel used in a combustion cycle depends on the type of engine and fuel. For example, a gasoline engine burns a mix of gasoline and air, with a ratio of about 12-18 parts air to 1 part fuel.
A combustion cycle refers to the process by which an engine converts energy from the combustion of fuel into mechanical work. There are different types of combustion cycles, including the two-stroke and four-stroke cycles.
A four-stroke cycle engine utilizes four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle. During the intake stroke, a fuel-air mixture is drawn into the cylinder. The mixture is then compressed, ignited, and combusted during the compression and power strokes, respectively. Finally, the exhaust valve opens during the exhaust stroke, and the piston pushes the exhaust gases out.
There are several methods to improve fuel efficiency in internal combustion engines. One way is to increase the amount of oxygen available inside the engine by injecting nitrous oxide or using fuels that provide their own oxygen, such as nitromethane. Another method is to capture waste heat and convert it into mechanical energy, improving the overall efficiency of the cycle.
