Mercedes Mullins
The primary difference between gasoline and diesel engines is that the former takes a mixture of gas and air, compresses it, and ignites the mixture with a spark, while the latter takes air, compresses it, and then injects fuel into the compressed air. The heat of the compressed air ignites the fuel spontaneously. Diesel engines have high compression ratios, which heat the air significantly, so that when fuel is injected, the air is above the SIT, and the fuel combusts when it is injected into the cylinder. This means that diesel engines always run lean, while gasoline engines must maintain a stoichiometric ratio.
| Characteristics | Values |
|---|---|
| Air-fuel ratio | The air-fuel ratio in a diesel engine ranges between 14.5:1 and 70:1 (air to fuel). |
| Air intake | The air intake in a diesel engine is much higher than the amount of fuel injected. |
| Compression ratio | Diesel engines have a higher compression ratio than gasoline engines, ranging from 14:1 to 25:1. |
| Fuel injection | Diesel engines use direct fuel injection, injecting fuel directly into the cylinder. |
| Ignition | Diesel engines use compression ignition, igniting fuel through the heat of compressed air, unlike gasoline engines that use spark plugs. |
| Emissions | Diesel engines have higher emissions and are more difficult to control than gasoline engines. |
| Efficiency | Diesel engines have higher thermal efficiency than gasoline engines due to their high expansion ratio and inherent lean burn. |
| Usage | Diesel engines are used in larger on-road and off-road vehicles, such as trucks, buses, and heavy equipment. |
| Tuning | Diesel engines can be tuned to adjust the air-fuel ratio, improving fuel efficiency and performance. |
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What You'll Learn
- Diesel engines use compression ignition, which requires a precise balance of air and diesel fuel
- The torque produced is controlled by manipulating the air-fuel ratio
- Unlike gasoline engines, diesel engines do not use throttle plates for air control
- The higher compression ratio of diesel engines leads to better efficiency
- Diesel engines may be equipped with a glow plug to facilitate fuel ignition when the engine is cold
Diesel engines use compression ignition, which requires a precise balance of air and diesel fuel
Unlike gasoline engines, diesel engines rely on compression ignition. This requires a precise balance of air and diesel fuel. The air-fuel ratio is crucial for combustion and impacts power output, fuel economy, and emissions. The ratio in a diesel engine typically ranges between 14.5:1 and 70:1 (air to fuel), depending on conditions such as idling, acceleration, or heavy towing.
Diesel engines do not use a throttle plate for air control; instead, they always draw in the maximum amount of air, and power is regulated by the amount of fuel supplied. This is in contrast to petrol engines, which use a throttle plate to control the amount of air entering the engine, and therefore require careful air-fuel ratio control.
In a diesel engine, the compression of air must be sufficient to trigger ignition. The higher compression ratio of diesel engines, compared to petrol engines, leads to better efficiency. The diesel engine has the highest thermal efficiency of any practical internal or external combustion engine due to its very high expansion ratio and inherent lean burn, which enables heat dissipation by excess air.
However, achieving the correct air-fuel ratio is critical. Running too "rich", with too much fuel for the air, may temporarily increase power but at the expense of higher emissions and lower fuel efficiency. Running too "lean", with too much air for the fuel, results in sluggish performance and can cause overheating and increased wear and tear on engine components.
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The torque produced is controlled by manipulating the air-fuel ratio
Unlike petrol engines, diesel engines do not require maintaining an air-fuel ratio. This is because diesel engines do not use a throttle plate for air control; instead, the throttle plate is always open, allowing the engine to draw in the maximum amount of air. The power is then regulated by altering the amount of fuel supplied, thereby manipulating the air-fuel ratio.
The torque produced by a diesel engine is controlled by changing the air-fuel ratio (λ). This involves adjusting the amount of fuel injected into the engine, rather than throttling the intake of air. By manipulating the air-fuel ratio, the diesel engine can achieve its characteristic high thermal efficiency. This is due to the engine's high expansion ratio and inherent lean burn, which allows for heat dissipation by excess air.
In contrast, petrol engines require maintaining a specific air-fuel ratio, known as the stoichiometric ratio, to ensure the catalytic converter operates efficiently. Operating a petrol engine with a fuel-rich mixture can damage the converter. Additionally, petrol engines use a throttle plate to control the amount of air entering the engine, which varies depending on the load. This variation in air intake directly influences the need for air-fuel ratio control in petrol engines.
The air-fuel ratio plays a significant role in the combustion process and the resulting torque output. Studies have shown that when the air-fuel ratio is slightly smaller than 33, the engine produces maximum torque output. At lean operating conditions, the engine delivers maximum power at lower speeds. Additionally, the air-fuel ratio influences the formation of soot emissions in compression-ignition engines.
The control of the air-fuel ratio is essential for optimising engine performance and minimising pollutant emissions. Open-loop control allows for manual adjustment of the air-fuel ratio, while closed-loop control utilises an on-board central unit to regulate fuel injection based on feedback from an oxygen sensor in the exhaust pipe.
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Unlike gasoline engines, diesel engines do not use throttle plates for air control
Rudolf Diesel's original design for the diesel engine in the 1890s centred on compression. Atmospheric air is compressed to a degree that, before ignition or combustion, the highest pressure and temperature are obtained. This compression is necessary to trigger ignition.
Diesel engines do not require throttle plates for air control because they are designed to always draw in the maximum amount of air. Power is then regulated by the amount of fuel supplied. This is in contrast to gasoline engines, where the throttle plate controls the amount of air coming into the engine, which in turn varies depending on the load and the need for an air-fuel ratio.
In gasoline engines, the air-fuel ratio needs to be maintained at around 14.7:1, otherwise, the spark will not properly ignite the mixture. In diesel engines, the air-fuel ratio is usually high, and the fuel is ignited as it enters the cylinder due to the high temperatures caused by compression. This means that the engine doesn't need to regulate air intake as the flame won't go out until all the fuel or oxygen has burned.
Diesel engines are designed to run lean, meaning they don't need throttle plates to run. They use the amount of diesel fuel needed to keep the engine running and to provide the work needed. However, running lean has its drawbacks. If the burn is hotter than around 1700°F, nitrogen oxides (NOx) are formed, which is a key ingredient in acid rain and harmful to people's lungs.
Some modern diesel engines have begun to adopt throttle plates, mainly to prevent engine runaway and/or facilitate rapid shutdown.
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The higher compression ratio of diesel engines leads to better efficiency
Compression ratios are typically between 14:1 and 23:1 for direct injection diesel engines, and between 18:1 and 23:1 for indirect injection diesel engines. In comparison, gasoline engines have a compression ratio of 8:1 to 12:1. The higher compression ratio of diesel engines leads to better efficiency.
A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature.
Diesel engines are also more fuel-efficient because they rely on compression for combustion. Diesel engines do not use a throttle plate for air control, meaning they always draw in the maximum amount of air, and power is regulated by the amount of fuel supplied.
The higher compression ratio of diesel engines also means that they are ideal for use in heavy vehicles such as locomotives, ships, trucks, and other large automobiles. The large combustion rate of diesel engines means that they can generate the energy required to power such large vehicles.
Newer diesel engines are equipped with high-pressure common rail fuel systems, which help to ensure that the fuel mist is evenly distributed throughout the cylinder.
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Diesel engines may be equipped with a glow plug to facilitate fuel ignition when the engine is cold
Diesel engines are unique in that they do not require an air-fuel ratio to be maintained. Instead, they rely on altering the amount of fuel injected, and the air-fuel ratio is usually high. Diesel engines have a very high expansion ratio and inherent lean burn, which enables heat dissipation by excess air.
Diesel engines use glow plugs to facilitate ignition when the engine is cold. Glow plugs are heating elements that increase in surface temperature when an electric current passes through them. This, in turn, heats the surrounding fuel-air mixture, making ignition easier. When the engine is cold, the air and engine are also cold, and it is challenging to reach the auto-ignition temperature through compression alone. A glow plug can reach temperatures of 1,300 degrees Celsius, which is more than sufficient to initiate combustion.
Glow plugs are essential for a smooth and reliable start, reducing overall strain on the engine and potentially extending its lifespan. They also contribute to a more complete combustion process, reducing harmful emissions and making diesel engines more environmentally friendly.
Ceramic glow plugs have proven to be superior to metal ones, with higher temperatures, improved engine performance, and increased durability. They can also reduce white smoke emissions.
Diagnosing and maintaining glow plugs is crucial to ensure the diesel engine operates optimally, especially during startup. If a diesel engine takes longer than usual to start, it may be an indication of malfunctioning glow plugs. Other signs of faulty glow plugs include unstable and rough idling, excessive smoke during startup, and white smoke when the engine is cold.
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Frequently asked questions
A diesel engine is an internal combustion engine that uses compression ignition to ignite the fuel as it is injected into the engine. Diesel engines compress only air, and then inject fuel into the compressed air. The heat of the compressed air ignites the fuel spontaneously.
The air-fuel ratio in a diesel engine ranges between 14.5:1 and 70:1 (air to fuel), depending on conditions like idling, acceleration, or heavy towing. The ratio is crucial for combustion and affects the truck’s power output, fuel economy, and emissions.
An improper air-fuel ratio leads to both short-term and long-term engine difficulties. Running rich for an extended period creates carbon buildup in the system, while running lean causes overheating.
