Rajan Wilcox
Diesel engines are internal combustion engines that use compression ignition to ignite the fuel as it is injected into the engine. Unlike gasoline, diesel fuel requires extreme pressure to ignite. This is because diesel fuel evaporates into vapour at higher temperatures than gasoline. The pressure increases the temperature of the diesel, transforming it into vapour, which then self-ignites. This is why diesel engines do not need spark plugs, as the fuel ignites from the pressure in the combustion chamber.
| Characteristics | Values |
|---|---|
| Flashpoint | 62°C |
| Boiling point | Higher than gasoline |
| Compression ratio | 14:1 to 25:1 |
| Combustion type | Constant pressure cycle |
| Ignition type | Compression ignition |
| Fuel-air mixture | Unmixed |
| Fuel injection | Near the end of the compression stroke |
| Fuel atomization | Yes |
| Fuel vaporization | Yes |
| Fuel-air mixing | Yes |
| Ignition delay | Yes |
| Efficiency | Higher than gasoline engines |
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What You'll Learn
Diesel fuel ignites at a lower pressure than gasoline
Diesel fuel has a higher flashpoint than gasoline, requiring extreme pressure to ignite. Gasoline, on the other hand, has a lower flash point, and can be ignited with a single spark after compression. However, diesel fuel ignites at a lower pressure than gasoline.
Diesel engines use compression to ignite the fuel, unlike gasoline engines, which use spark plug ignition. The compression ratio in diesel engines is much higher, ranging from 14:1 to 25:1, compared to 8:1 to 12:1 in gasoline engines. This higher compression ratio leads to greater efficiency in diesel engines.
In a diesel engine, only air or a mixture of air and residual combustion gases are compressed in the cylinder. As the piston compresses the air, the temperature and pressure increase. When the temperature exceeds the fuel's ignition point, usually near the end of the compression stroke, fuel is injected into the cylinder, where it vaporizes and mixes with the hot, compressed air. This mixture then ignites spontaneously due to the high temperature and pressure, without the need for a spark plug.
The combustion process in a diesel engine is complex and involves several stages, including ignition delay, premixed combustion, and rate-controlled combustion. During ignition delay, the fuel atomizes into small droplets, vaporizes, and mixes with air. As the piston moves closer to the top dead center, the mixture reaches the fuel's ignition temperature, causing the ignition of a premixed quantity of fuel and air. This rapid ignition marks the start of combustion and is accompanied by a sharp increase in cylinder pressure. The remaining fuel that did not undergo premixed combustion is consumed in the rate-controlled combustion phase.
The higher compression ratio and absence of spark plugs in diesel engines contribute to their increased fuel efficiency compared to gasoline engines. Additionally, diesel engines produce fewer emissions of carbon monoxide and hydrocarbons due to the lack of fuel accumulation on cylinder walls during the compression stroke.
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Diesel engines use compression ignition, not spark plugs
Unlike gasoline engines, diesel engines use compression ignition, meaning they don't require spark plugs. Gasoline engines take a mixture of gas and air, compress it, and ignite the mixture with a spark. Diesel engines, on the other hand, compress air and then inject fuel into the compressed air, allowing the heat of the compressed air to ignite the fuel spontaneously. This is because diesel fuel has a higher flashpoint than gasoline, requiring higher temperatures to transform into vapour and ignite.
The German engineer Rudolf Diesel's original design for the diesel engine was replaced by a constant pressure cycle in 1893, two years before he patented the idea. This cycle relies on compression for combustion, with the cylinder and piston constructed and arranged to compress air to a degree that produces a temperature above the ignition point of the fuel.
During the compression stroke, the atomised diesel fuel is injected into the combustion chamber, where it absorbs heat from the surrounding compressed air, vaporises, and mixes with the air. As the piston moves closer to the top dead centre (TDC), the mixture's temperature reaches the fuel's ignition temperature, causing ignition.
This process is known as compression ignition and does not require spark plugs. The higher compression ratio of diesel engines compared to gasoline engines also contributes to their increased efficiency. Additionally, diesel engines form fewer emissions of carbon monoxide (CO) and hydrocarbons because the fuel does not stick to the cylinder walls during the compression stroke.
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Diesel fuel evaporates at a higher temperature than gasoline
Diesel fuel requires extreme pressure to ignite, unlike other flammable liquids such as gasoline. This is because diesel fuel evaporates into vapour at higher temperatures than gasoline. Diesel has a higher flash point than gasoline, with diesel's flash point being around 62°C and gasoline's being around -40°C.
Gasoline evaporates at considerably lower temperatures than diesel and, therefore, doesn't require extreme temperatures to create more vapour. Once compressed, gasoline only needs a single spark to ignite. However, gasoline can also self-ignite from high temperatures, although this is undesirable.
In a diesel engine, pressure is used to raise the temperature above the auto-ignition point, rapidly creating vapour that auto-ignites. This is not ideal for gasoline engines, so they have controlled pressures to prevent auto-ignition while still having enough pressure to rapidly convert gasoline to oxygen.
The difference in combustion between diesel and gasoline engines is also worth noting. A diesel engine's burn is more complex, similar to a candle, where fuel and air mix as a result of combustion. Convection currents and turbulence play a significant role in how the unmixed fuel is burned. On the other hand, gasoline engines have a higher tolerance for compression due to the presence of octane, a component of gasoline.
While diesel fuel requires extreme pressure to ignite, it is important to note that neither diesel nor gasoline needs extreme pressure to ignite when in their liquid states. If you were to drop a match into a cup of diesel fuel, the match would go out. However, if you were to drop a match into a can of diesel fuel, it would burn just like gasoline.
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Diesel engines are more fuel-efficient than gasoline engines
Diesel engines are known to be more fuel-efficient than gasoline engines. They are ideal for heavy-duty tasks and long distances. Diesel engines offer high efficiency, more torque, and better thermal efficiency. They also provide strong acceleration and engine longevity.
The combustion process in gas and diesel engines varies. Gas engines use spark plugs to ignite the fuel, whereas diesel engines use extreme compression to generate the heat required for ignition. This phenomenon is known as compression ignition. When this occurs in a gas engine, it can damage the engine.
Diesel engines are also more efficient on highways than in cities. Diesel fuel packs more energy per gallon than gasoline, making it more economical. According to a study by The Motley Fool, diesel engines were 29% more efficient on the highway and 24% more efficient in the city.
However, diesel engines have some disadvantages. They tend to be heavier and more expensive than gasoline engines. They also have lower maximum RPM ranges, making them slower in terms of acceleration. Additionally, diesel engines are noisier and have more vibrations.
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Diesel engines produce fewer carbon monoxide emissions
The difference in carbon monoxide emissions between diesel and gasoline engines can be attributed to several factors. Firstly, diesel fuel has a higher flash point than gasoline, which means it requires higher temperatures to evaporate and ignite. Gasoline, on the other hand, has a lower flash point and can ignite at lower temperatures. This results in gasoline engines having a higher risk of carbon monoxide production due to their lower ignition temperature.
Additionally, the combustion process in diesel engines is different from that of gasoline engines. In a diesel engine, the piston compresses the air inside the cylinder to a fraction of its original volume, rapidly increasing the temperature and pressure. This high temperature and pressure mixture of air and fuel vapour ignites without the need for spark plugs, similar to the way a candle burns. The combustion process in gasoline engines, on the other hand, involves controlled pressures to prevent auto-ignition while still allowing for rapid conversion to oxygen.
The design of diesel engines also contributes to their lower carbon monoxide emissions. In a diesel engine, only air is compressed, while in a gasoline engine, fuel can get stuck in the cylinder walls during the compression stroke. This results in a more efficient combustion process in diesel engines, as there is no fuel residue to interfere with the ignition process.
While diesel engines produce fewer carbon monoxide emissions, they are not without their drawbacks. Diesel exhaust contributes to the formation of ground-level ozone, which can have negative impacts on the environment, including damage to crops, trees, and other vegetation. Additionally, diesel exhaust plays a role in climate change, and the fine particles in diesel exhaust have been linked to various health concerns, including respiratory issues and cardiovascular disease.
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Frequently asked questions
Diesel fuel evaporates into vapour at higher temperatures than gasoline. Gasoline has a lower flash point and boiling point than diesel.
In diesel engines, fuel is injected into the engine cylinder near the end of the compression stroke. The liquid fuel is injected at high velocity and atomizes into small droplets. These droplets vaporise and ignite due to the high temperature and pressure in the cylinder.
Diesel engines compress only air or air combined with residual combustion gases. This increases the air temperature inside the cylinder so that the atomised diesel fuel injected into the combustion chamber spontaneously ignites.
As pressure increases in the cylinder, so does temperature. Therefore, pressure is used to increase the temperature in the cylinder to the ignition point of diesel fuel.
