The term “compression ignition” is typically used in technical literature to describe the modern engines commonly called “Diesel engines”. This is in contrast to “spark ignition” for the typical automobile gasoline engines that operate on a cycle derived from the Otto cycle. Rudolph Diesel patented the compression-ignition cycle which bears his name in the 1890s. The diesel internal combustion engine differs from the gasoline powered Otto cycle by using a higher compression of the fuel to ignite the fuel rather than using a spark plug (“compression ignition” rather than “spark ignition”).
In the diesel engine, air is compressed adiabatically with a compression ratio typically between 15 and 20. This compression raises the temperature to the ignition temperature of the fuel mixture which is formed by injecting fuel once the air is compressed. The ideal air-standard cycle is modeled as a reversible adiabatic compression followed by a constant pressure combustion process, then an adiabatic expansion as a power stroke and an isovolumetric exhaust. A new air charge is taken in at the end of the exhaust, as indicated by the processes a-e-a on the diagram. Since the compression and power strokes of this idealized cycle are adiabatic, the efficiency can be calculated from the constant pressure and constant volume processes. The input and output energies and the efficiency can be calculated from the temperatures and specific heats.
Diesel engines are more powerful and fuel-efficient than similar-sized gasoline engines (about 30-35% more fuel efficient). Today’s diesel vehicles are much improved over diesels of the past.
In order to take a deeper look at how diesel engines work first we need to take a deeper look at diesel fuel itself. Diesel is not made up of one single molecule, but rather a mixture of several different hydrocarbons. The empirical formula for diesel is C12H23. That means for every 12 moles of carbon in a given amount of diesel there are 23 moles of hydrogen. This formula does not tell us the structure of diesel, because it is not a structural formula, it is an empirical formula.
Even though that formula doesn’t tell us the structure it is still possible to talk about it. The hydrocarbons in diesel are alkanes. Alkanes have straight chains of carbon with hydrogen filling in all of other places where bonds are formed. The general formula for alkanes is CnH2n+2. You may be asking yourself if diesel is made up of alkanes why doesn’t it follow the general formula for alkanes. It is because, as stated before, diesel is made up of a mixture of hydrocarbons. If you take the weighted average of amount of carbon and hydrogen, it turns out that the formula for diesel is C12H23.
an example of an alkane.
An example of what one type of molecule from diesel looks like.
The macroscopic properties of diesel are also important to understanding how the diesel engine works. The macroscopic properties of diesel are governed by the intermolecular forces of diesel. The only intermolecular force in diesel in the London dispersion force because hydrocarbons are all non-polar. Since diesel molecules are relatively large, its intermolecular forces play a big role. Diesel can become very viscous in the cold environments. Engines are usually designed for the fuel having one viscosity, so this can be a major problem.
Other properties of diesel fuel that are important are flash point and auto ignition temperature. Diesel has a high flash point which is a safety factor. Flash point is the lowest temperature that the fuel can form an ignitable temperature with air. Since diesel has a high flash point it does not burn as easily as gasoline. Diesel also has a low auto ignition temperature, which means it will ignite without any external actions, such as a flame or a spark. This is why diesel engines do not need a spark plug. The auto ignition temperature of diesel ranges from 177-329 degrees Celsius depending on the type of diesel you are using.
Hygroscopy is the ability to absorb water molecules around it. Because there is water vapor in the air, it is not hard for things to be able to attract these water molecules. Diesel is hygroscopic. This means diesel is susceptible to attracting and holding these water molecules. This can be harmful to the fuel and make it not work as well. Diesel is not relatively hygroscopic, however it does still have the ability to absorb some water and lessen the quality of the fuel.
The process of diesel fuel absorbing water is almost inevitable, almost any storage option will cause diesel to absorb water. Obviously the shelf life of diesel can vary based on storage. The more humid the environment the fuel is stored in, the more water will be absorbed, and the shorter the shelf life. The less humid the environment is, the less water in the air to get into the fuel, the longer the shelf life. When storing diesel, it is best to store in a dark, dry place. When it is stored outside in a car, for example, the sun causes the absorption of water. When the sun is out during the day, the fuel heats up and expands. When the fuel expands, it pushes the air out of the fuel tank. At night time, the sun goes down which causes the temperature to go down. As the temperature goes down, the fuel compresses and creates a vacuum of space in the tank. This is filled with the air from outside of the tank. This new air that is coming in has water vapor in it, and this is how the water gets into the fuel. If it is stored in a dark place, the sun light does not reach it, and will have little effect on it.
Another way water can get into your fuel is when the fuel is pumped through the diesel engine from the tank to the pump and injection system where it is heated to about 100° Celsius then returned to the fuel tank. This recirculation process brings the hot diesel into the cool tank, causing condensation. This cannot be prevented by quality of storage.
Why is water in your fuel bad? With the water molecules, come bacterial and fungal spores. The spores begin growing and eating the fuel. As they grow and eat the fuel their waste products act to breakdown or destabilize the fuel. This makes it less reactive, thus making the combustion produce less energy, making it less efficient.
The reaction where water and diesel come together can be spontaneous or nonspontaneous. A reaction is spontaneous when it will just happen; it does not need energy to get the reaction started. The water that gets in due to the expansion and compression of the fuel due to the sun is spontaneous. It does take the energy of the sun however; the sun’s energy will always come to earth on its own because the sun already has the energy it needs to do things like this. The other way water gets into the fuel is obviously nonspontaneous. The engine needs to be running for this to happen which requires activation energy.
Now that you have gained a basic knowledge of the diesel engine and how it works it is time to use chemistry to analyze the processes involved. Our third post was titled “The Difference Between Diesel Fuel and Gasoline,” and talked about the differences between the two fuels. In comparison to diesel fuel, gasoline is lighter, less dense, more flammable and more volatile. When you spray gasoline into a cylinder, it starts to vaporize immediately, so that as soon as the spark plug fires, the gasoline detonates and powers the engine. Diesel fuel is heavier, denser, less flammable and less volatile. So to detonate it, it has to be compressed in a cylinder to a very high pressure and temperature, at which point it detonates without a spark.
The typical molecule of gasoline (isooctane, C8H18).
The typical molecule of diesel fuel (cetane or n-hexadecane, C16H34).
Diesel engines use Charles’ Law, which states that when a gas is compressed, its temperature rises, to ignite diesel fuel. Air is drawn into the cylinder of a diesel engine and compressed by the rising piston leading to an increase in air temperature. At the top of the piston stroke, diesel fuel is injected into the combustion chamber at high pressure mixing with the hot, high-pressure air. The resulting mixture ignites and burns very rapidly. This contained explosion causes the gas in the chamber to expand, driving the piston down with force, creating power in a vertical direction. The connecting rod transmits this motion to the crankshaft, which is forced to turn, delivering rotary power at the output end of the crankshaft.
Diesel fuel is less volatile than gasoline in comparison. The volatility of diesel fuel refers to how easily the fuel vaporizes. Volatility affects how easily you can start your car, warm it up, and how well it runs. Diesel fuel comes in two basic grades, each with a different volatility. Diesel engines do not operate well when the cylinders are cold, due to the lower volatility of the fuel. To maintain high performance some diesel engines use glow plugs inside the cylinder to warm the cylinders prior to starting, while others use resistive grid heaters in the intake manifold to warm the inlet air until the engine reaches operating temperature. Once the engine is operating, the combustion of fuel in the cylinder keeps the engine warm effectively. Engine block heaters plugged into the utility grid are often used when an engine is shut down for extended periods in cold weather to reduce startup time and engine wear. Modern electronically-controlled diesel engines also advance injection timing to improve cold startability and reduce white smoke under cold start conditions.
In very cold weather, diesel fuel thickens and increases in viscosity and forms wax crystals or a gel. This can make it difficult for the fuel injector to get fuel into the cylinder in an effective manner, making cold weather starts difficult at times, though recent advances in diesel fuel technology have made these difficulties rare. A commonly applied advance is to electrically heat the fuel filter and fuel lines.
The processes used in diesel engines correspond to numerous topics in Chemistry. Our next post will continue to illustrate the Chemistry at work in diesel engines.