Rajan Wilcox
The diesel engine was invented by Rudolf Diesel in 1892, and it operates quite differently from a gasoline engine. In a diesel engine, the air is compressed first, and then the fuel is injected directly into the combustion chamber. The heat of the compressed air, which has nowhere to escape, lights the fuel. This is in contrast to a gasoline engine, where fuel is mixed with air, compressed by pistons, and ignited by sparks from spark plugs. Diesel fuel is a mixture of hydrocarbons obtained from petroleum, and it has a high energy content, making it more likely to react with the concentrated air.
| Characteristics | Values |
|---|---|
| Type of engine | Internal combustion engine |
| Fuel ignition | No spark, compression of inlet air, then injection of fuel |
| Compression ratio | 14:1 to 25:1 |
| Fuel type | Liquid fuel, fractional distillate of petroleum fuel oil, biodiesel, biomass to liquid (BTL), gas to liquid (GTL) |
| Efficiency | 40% and higher |
| Sustainability | More sustainable than gasoline engines, but still produces emissions |
| Peak power | Low engine revolutions per minute (RPM), generally at speeds below 65 miles per hour |
| Injectors | Most important fuel system component, delivering precise amounts of atomized and pressurized fuel into each cylinder |
| Fuel sources | Crude oil, petroleum |
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What You'll Learn
Diesel engines have no spark plug
Gasoline engines, on the other hand, require spark plugs to create a spark and ignite the fuel-air mixture in the combustion chamber. The spark timing in gasoline engines is different from the engine timing, with the spark being "advanced" relative to the RPMs of the engine to create a proper combustion event. This combustion event pushes the piston through its stroke with a stronger force. However, the high compression and rapid flame front from the spark plug can lead to detonation and resulting damage in gasoline engines.
Diesel engines, without spark plugs, operate differently. They compress the air first, and then inject the fuel, which allows for direct mixing. This process creates a steady burn and a slower flame front, preventing detonation and reducing the risk of damage to the engine. The absence of spark plugs in diesel engines is also related to the fuel properties. Diesel fuel has a higher ignition point than gasoline, requiring higher temperatures to ignite.
The design of diesel engines also contributes to their ability to ignite without spark plugs. Diesel engines are designed to rely on compression for ignition, with the cylinders in the combustion chamber using glow plugs to heat the chamber and aid ignition in cold conditions. These glow plugs heat the vaporized diesel fuel, increasing the pressure and facilitating ignition without the need for spark plugs. The use of glow plugs and compression ignition in diesel engines improves fuel economy and produces more torque, making them a popular choice for heavy-duty vehicles and those requiring utility and power.
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Diesel fuel is injected directly into the combustion chamber
The diesel fuel is injected at high pressure into the combustion chamber. The start of injection (SOI) is the point at which injection of fuel into the combustion chamber begins. The SOI is often indicated by a signal from a needle lift sensor that indicates when the injector needle valve starts to open. Due to the mechanical response of the injector, there can be a delay between the indicated SOI and the actual SOI when the fuel exits the injector nozzle.
The injector ensures that the fuel is broken down into small droplets, which are distributed evenly. The heat of the compressed air vaporises the fuel from the surface of the droplets. The vapour is then ignited by the heat from the compressed air in the combustion chamber. The droplets continue to vaporise and burn until all the fuel in the droplets has been burnt.
The injection duration is the period during which fuel enters the combustion chamber from the injector. The rate of injection often varies during this duration, with boot, ramp and square being common injection rate shapes. Multiple injection events can be used, as opposed to a single injection event in conventional fuel injection systems.
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Diesel engines are more efficient than gasoline engines
Another factor contributing to the efficiency of diesel engines is their use of turbochargers, which pump more air into the combustion chambers at high altitudes, resulting in better performance compared to gasoline engines. This advantage is particularly noticeable in mountainous regions or high-altitude areas where the air is thinner. Additionally, diesel fuel is more energy-dense than gasoline, providing more energy per gallon. As a result, diesel-powered cars often achieve higher miles per gallon, with fuel efficiency figures of 50 mpg or higher not uncommon.
The higher torque of diesel engines also contributes to their efficiency. Diesel cars have better fuel economy and more impressive acceleration, making them well-suited for towing and hauling heavy loads. According to some sources, diesel engines can be up to 29% more efficient on highways and 24% more efficient in city driving compared to gasoline engines. However, the efficiency advantage of diesel engines is more pronounced on highways due to their higher torque and fuel efficiency characteristics.
While diesel engines generally have higher upfront costs and maintenance expenses, they tend to be more durable and last longer than gasoline engines. The average lifespan of a diesel engine is around 500,000 miles, and with proper maintenance, it can reach up to 800,000 miles. Additionally, diesel engines produce lower levels of certain pollutants, such as carbon dioxide per mile, contributing to their appeal for environmentally conscious consumers. However, it is important to note that diesel exhaust, especially from older engines, can cause health issues, and the technology is constantly evolving to improve sustainability.
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Diesel engines use direct fuel injection only
Diesel engines are internal combustion engines that use direct fuel injection. Unlike spark ignition engines, diesel engines do not require a spark plug to ignite the fuel. Instead, they rely on the compression of inlet air and the subsequent injection of fuel to initiate combustion. This process, known as compression ignition, is a key characteristic of diesel engines.
Direct fuel injection involves injecting fuel directly into the combustion chamber or a pre-combustion chamber. This method has been used in diesel engines since the first successful prototype in 1894. It allows for greater control over the fuel delivery process, atomizing the spray of fuel in the cylinder and providing better distribution throughout the chamber. As a result, direct injection enables the implementation of advanced engine management protocols such as Variable Valve Timing.
The use of direct fuel injection also contributes to the efficiency of diesel engines. Diesel engines can achieve higher efficiency compared to gasoline engines, with up to 40% efficiency or more. This makes them a popular choice for heavy vehicles like trucks, where fuel costs can be a significant factor. The higher efficiency of diesel engines is partly due to their ability to run on alternative fuels, such as biodiesel, biomass-to-liquid (BTL), or gas-to-liquid (GTL) diesel.
Direct injection diesel engines also offer advantages in terms of performance and emissions. They can achieve peak power at lower engine revolutions per minute (RPM), generally at speeds below 65 miles per hour. Additionally, modern direct injection diesel engines meet stringent global standards for particulate matter (PM), nitrogen oxides (NOx), and greenhouse gas (GHG) emissions.
While direct injection has its benefits, there are also some drawbacks to consider. One of the main issues is the buildup of carbon in the intake ports and on the back of the valves. This buildup can occur due to the aggressive atomization of fuel during the direct injection process, and it may require specialized maintenance procedures to address.
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Diesel fuel is a mixture of hydrocarbons
Diesel fuel, also known as diesel oil, heavy oil, or simply diesel, is a mixture of hydrocarbons obtained from the distillation of crude oil. It is specifically designed for use in diesel engines, which are internal combustion engines that ignite fuel through air compression and fuel injection, rather than a spark. Diesel fuel, therefore, requires good compression ignition characteristics.
The most common type of diesel fuel is a fractional distillate of petroleum fuel oil, but alternative types that are not derived from petroleum, such as biodiesel, biomass-to-liquid (BTL), and gas-to-liquid (GTL) diesel, are becoming more common. In the United States, petroleum-derived diesel is composed of about 75% saturated hydrocarbons (mainly paraffins, including n, iso, and cycloparaffins) and 25% aromatic hydrocarbons (including naphthalenes and alkylbenzenes). The average chemical formula for common diesel fuel is C12H23, with a range of approximately C10H20 to C15H28.
Diesel fuel has a boiling point range of approximately 150 to 380 °C. It is classified based on the type of engine it fuels, ranging from 1-D to 4-D. Diesel fuel specifications differ across various fuel grades and countries. For example, diesel fuel in the European Union must meet the EN 590 standard.
Before diesel fuel was standardized, diesel engines typically ran on cheap fuel oils, which were often a mixture of fuels such as petrol, kerosene, rapeseed oil, or lubricating oil. These fuel mixtures were untaxed and, therefore, more affordable. The introduction of motor-vehicle diesel engines in the 1930s, such as the Mercedes-Benz OM 138, led to the development of higher-quality fuels with improved ignition characteristics.
Diesel engines are known for their efficiency, with a 40% efficiency rate compared to gasoline engines' 20% rate. They are also more sustainable, as they produce less carbon dioxide and have better fuel mileage. Additionally, diesel engines are equipped with turbochargers, which enhance performance at high altitudes by pumping more air into the combustion chambers.
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Frequently asked questions
The heat of the compressed air in the combustion chamber lights the fuel in a diesel engine.
A diesel engine compresses air using a piston and cylinder.
The compression of air increases the temperature inside the combustion chamber, which lights the fuel.
Fuel ignition in a diesel engine occurs without a spark due to the compression of inlet air and the subsequent injection of fuel.
