Mercedes Mullins
Diesel engines are internal combustion engines that use compression to ignite the air-fuel mixture, unlike spark plug-ignition engines. This compression process increases fuel efficiency and power generation. Diesel engines have a higher compression ratio than gasoline engines, which allows them to ignite the fuel without the need for spark plugs. The absence of fuel during the compression stroke also prevents knocking, pre-ignition, or detonation issues. Additionally, diesel engines can run on various fuels, including biodiesel, cooking oil, and used motor oil. The compression of the fuel-air mixture is critical to the engine's performance and efficiency, making it a key factor in the design and operation of diesel engines.
| Characteristics | Values |
|---|---|
| Compression ratio | Higher than gasoline engines, typically 15:1 but can be as high as 23:1 or 25:1 |
| Ignition | Self-ignition due to high compression ratio; no spark plug required |
| Efficiency | Higher fuel efficiency than gasoline engines due to compression for combustion |
| RPM | Lower RPMs due to high compression |
| Torque | Higher torque output |
| Power | More power generated due to higher compression ratio |
| Maintenance | Generally requires less maintenance than gasoline engines |
| Fuel type | Can run on a variety of fuels, including biodiesel, cooking oil, used motor oil, and coal |
| Combustion | High-pressure combustion; can combust a wide variety of fuels |
| Noise | Can be made "quieter" by reducing compression ratio |
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What You'll Learn
Diesel fuel ignites differently to gasoline
Diesel fuel ignites differently from gasoline due to differences in their chemical compositions and the presence or absence of spark plugs in their respective engines.
Gasoline engines use spark plugs to ignite the gasoline-air mixture. The spark plug creates a spark that ignites the gasoline, which has a lower flash point than diesel, at around −40° C. This means that gasoline engines require less pressure to ignite the fuel-air mixture. Gasoline engines also have a lower compression ratio, typically running at 10:1, and the gasoline is mixed with additives to reduce detonation by raising octane levels.
On the other hand, diesel engines are compression-ignition engines (CI engines) that rely on the high compression of air in the cylinder to raise the temperature, causing the atomised diesel fuel injected into the combustion chamber to ignite spontaneously. This is known as auto-ignition or compression ignition. Diesel engines have a higher compression ratio, typically between 16-23:1, which results in more power generated. The absence of spark plugs and the use of direct fuel injection allow diesel engines to utilise a wider range of fuels, including biodiesel, fuel oils, and even cooking oil or used motor oil.
The different ignition methods also contribute to the distinct characteristics of the two types of engines. Gasoline engines are generally smoother, quieter, more responsive, and produce less smoke. In contrast, diesel engines are heavier, louder, have a narrower range of operating speeds, and produce more power at lower revolutions per minute (RPM).
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Diesel engines have a higher compression ratio
Rudolf Diesel's 1895 patent application describes the compression required for his cycle: pure atmospheric air is compressed to a degree that, before ignition or combustion takes place, the highest pressure and temperature are obtained. This is the temperature at which combustion takes place, not the burning or ignition point.
Diesel engines are compression-ignition engines (CI engines). This means that ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression. This is in contrast to engines using spark plug ignition of the air-fuel mixture, such as a petrol engine (gasoline engine) or a gas engine.
Diesel engines work by compressing only air, or air combined with residual combustion gases from the exhaust (known as exhaust gas recirculation, "EGR"). Air is inducted into the chamber during the intake stroke and compressed during the compression stroke. This increases air temperature inside the cylinder so that atomised diesel fuel injected into the combustion chamber ignites.
The higher compression ratio of the diesel engine leads to better efficiency. The higher the compression ratio, the more power generated.
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Diesel engines have lower RPMs
Diesel engines have a lower RPM (rotations per minute) than gasoline engines. This is due to several reasons, primarily because diesel fuel has a lower burn rate and longer stroke than gasoline.
Firstly, diesel engines have a lower RPM due to their compression ignition design. Unlike gasoline engines that use spark plugs for ignition, diesel engines rely on highly compressed air in the cylinder to increase the temperature, which then ignites the fuel. This compression process takes time, and the engine needs more time between compressions for the fuel to combust properly.
Secondly, the slow burn rate of diesel fuel also contributes to the lower RPM. If the RPM is too high, there is a risk of the exhaust valve opening while the mixture in the cylinder is still burning, which can lead to detonation and engine damage.
Thirdly, the rotating assembly in a diesel engine is built to withstand high compression, which results in heavier components such as steel pistons and longer strokes. This increased rotating mass leads to higher forces at a given RPM, limiting how high the RPM can go.
Additionally, diesel engines typically have a narrower power band due to their reliance on compression ignition. The timing of combustion is limited by when the compression event is triggered, unlike gasoline engines that can control the exact point of combustion using spark plugs.
Lastly, diesel engines are designed to compress only air or a mixture of air and residual combustion gases, which allows for a higher compression ratio and more power generated per mass of fuel. However, this higher compression ratio also contributes to the lower RPM as it requires heavier-duty parts to withstand the higher compression explosion, slowing down the speed of the piston.
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Diesel engines are more fuel-efficient
Diesel engines are generally more fuel-efficient than petrol engines, offering an average of 20-25% higher mileage. This increased fuel efficiency is due to the diesel engine's ability to compress fuel before igniting it. The more the fuel is compressed, the more energy per mass is generated.
Diesel engines use compression ignition, which means that the air in the cylinder is compressed, raising the temperature to a point where the fuel auto-ignites. This is in contrast to petrol engines, which use spark plug ignition. The absence of spark plugs in diesel engines means that knocking, pre-ignition, or detonation cannot occur, which can be detrimental to the engine.
Additionally, diesel engines have lower RPMs, which allow for more efficient combustion. The longer hydrocarbon chains in diesel fuel also contribute to its higher energy content compared to gasoline.
The higher fuel efficiency of diesel engines is particularly noticeable on highways, where they can be up to 29% more efficient than petrol engines. This makes diesel engines a popular choice for those who drive long distances or frequently travel on highways.
However, it is important to note that diesel engines have higher upfront costs and maintenance expenses. They are also less responsive and produce more emissions than petrol engines. Therefore, the decision between diesel and petrol engines depends on individual preferences and driving requirements.
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Diesel engines can use other fuels
Rudolf Diesel's original engine ran on kerosene (paraffin) and petrol. However, diesel engines can combust a wide range of fuels, including biodiesel, vegetable oil, peanut oil, shale oil, lamp oil, petrol, ligroin, coal tar creosote, paraffin oil, crude oil, gasoline, fuel oil, and several fuel oils.
The key difference between diesel and gasoline engines is that diesel engines use compression-ignited injection systems, whereas gasoline engines use spark-ignited systems. In a compression-ignited system, the diesel fuel is injected into the combustion chamber of the engine and ignited by the high temperatures achieved when the gas is compressed by the engine piston. Therefore, diesel engines can run on any fuel that can be ignited by compression.
Diesel engines can also be equipped with a glow plug, an electrically heated wire that facilitates fuel ignition when the engine is cold. This allows diesel engines to combust fuels with lower auto-ignition temperatures.
However, not all diesel engines run on diesel fuel. For example, large two-stroke watercraft engines typically use heavy fuel oils, and certain types of diesel engines are designed to run on petrol. Additionally, some older diesel engines can run on alternative fuels, such as vegetable oil, but newer diesel engines are more selective and require the use of diesel fuel.
Overall, while diesel engines are known for their ability to combust a wide range of fuels, the specific fuel used will depend on the engine's design and the availability of different fuel sources.
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Frequently asked questions
A diesel engine is an internal combustion engine that uses compression to ignite an air-fuel mixture. It does not require a spark plug to generate power.
A diesel engine compresses only air, or air combined with residual combustion gases from the exhaust (known as exhaust gas recirculation, "EGR"). Air is inducted into the chamber during the intake stroke, and compressed during the compression stroke. This increases air temperature inside the cylinder so that atomised diesel fuel injected into the combustion chamber ignites.
Diesel fuel compresses because it has a higher compression ratio than gasoline, which allows it to be ignited more easily. The compression ratio of a diesel engine is usually around 15:1, while a typical gasoline engine will have a compression ratio of about 8:1 or 12:1.
