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.
In this day and age people are concerned about where we will get fuel from once we run out of crude oil. Crude oil is a non-renewable resource, so eventually we are going to run out of it. This is a major problem for everyone because cars run on gasoline or diesel, which are both made from crude oil as described in a previous blog post. This means that when there is no more crude oil no one will be able to operate anything with a combustion engine. That is not a viable option because automobiles are the most common form of transportation, so some sort of alternative for fuel must be made that will be compatible with cars. This is where biodiesel comes into play
Biodiesel is made from either plant oils or animal fats as opposed to crude oil. Biodiesel is renewable and can be replenished either by farming plants or raising animals. Cooking oils that restaurants would normally throw away could also be used as biodiesel. In order to make the oils usable as biodiesel it needs to go through a chemical process. Potassium hydroxide and methanol are added to the oils, which produces glycerin and biodiesel. Glycerin sinks to the bottom because it is denser and is removed and then used to produce soap and other pharmaceuticals. Recycled greases require an additional step to purify them, but the process is mostly the same as vegetable oils. Biodiesel can be used in mixtures with petroleum based diesel. The amount is denoted by BXX, where the xx is the percent of biodiesel by volume.
There are many benefits to biodiesel. Biodiesel can be used in diesel engines with little or no modifications to the engines, which would make the transition from diesel made from crude oil to biodiesel an easy one. If the biodiesel is B20 or less then no modifications need to be made to the engine, but if a higher concentration is used then the fuel lines, gaskets, and fuel pump seals need to be modified because biodiesel has been shown to deteriorate these. When pure biodiesel is used unburned hydrocarbons drop 67%, carbon monoxide produced drops 48%, and no sulfates are made when biodiesel is used. Also biodiesel does not have as harsh of a smell since it is made from fatty acids, as seen in the picture, instead of crude oil.
There are some negative aspects to biodiesel. Biodiesel costs slightly more than conventional diesel. Pure biodiesel costs about $1 more per gallon than conventional diesel. Also as stated before high concentrations of biodiesel can degrade the fuel lines, gaskets, and fuel pump seals. Another disadvantage for biodiesel is it starts to crystalize, which diminished flow, at a relatively high temperature. Any temperatures below 0 OC will impede the flow of biodiesel, but this can be mitigated with certain additives that will lower the freezing point.
There are some advantages and disadvantages of biodiesel as an alternative fuel source. Regardless of whether biodiesel or some other form of energy is used as a replacement for crude oil is not important, but some sort of replacement is very important. There will not be enough crude oil to last forever, so there has to be some way of making fuel with renewable resources to replace crude oil. Biodiesel is definitely a possible contender for replacing crude oil.
Diesel and gasoline are the two major fuels used in cars today. They are similar; however they have differences in where they come from, how they are burned to make the cars move, and efficiency. Diesel is generally more efficient that gasoline because how it reacts. It burns at a much higher temperature allowing for more if it to be converted to energy.
Both fuels are direct products of crude oil. Crude oil is what comes straight out of the ground. This is done through the process of refining. There are three major steps to this process and there are key differences in the process for refining diesel and gasoline: separation, conversion, and purification.
The first step, separation, is when heat is applied to the crude oil, distillation occurs. The crude oil in a distillation tower separates into all of its different carbon forms in order of viscosity. The most viscos liquids settle at the bottom and the less viscos particles and gases rise to the top.
The second step, conversion, basically rearranges the hydrocarbon bonds to make them better for consumer use. This makes the fuels more valuable. This is done through the process of hydrocracking where the bonds between the hydrocarbons are reduced.
The third step, purification, is simply removing sulfur from the fuels to make them more pure. This is also called hydro treatment, or hydrodesulphurization. This is a process where the hydrocarbons react with hydrogen gas under moderate temperatures and pressures. The hydrogen gas reacts with the sulfur to make H2S (hydrogen sulphide) gas. This causes the sulfur to be removed from the hydrocarbons leaving a pure product that is ready for use.
This process that was just explained is general for all forms of hydrocarbon fuel. Diesel is different from gasoline in its viscosity. As mentioned before, in the first process, separation, the hydrocarbon fuels are separated by viscosity. Once they are separated, gas and diesel (and many others) can be extracted separately and used for different things. Gasoline is less viscos than diesel which is why it is higher up in the distillation tower.
The way the two fuels are burned is also very different. Cars that use gasoline ignite their fuel with a spark plug. Because gasoline is not compressed to higher temperatures, the spark plug is necessary. It creates a spark hot enough to ignite the fuel and burn the gas so that it can make the car move.
Cars that use diesel do not use a spark plug because diesel engines compress the fuel to points where the temperature is high enough to combust without one.
Diesel engines and gasoline engines are quite similar. Both are internal combustion engines which convert the chemical energy of fuel into mechanical energy. Internal combustion engines covert fuel into energy through a series of small explosions or combustions. The major difference between diesel and gasoline is how these explosions occur. In a gasoline engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. In a diesel engine, the air is compressed first, and then the fuel is injected.
This series of explosions or combustions is called the injection process. Most car engines use port injection or a carburetor. A port injection system injects fuel just prior to the intake stroke (outside the cylinder). A carburetor mixes air and fuel long before the air enters the cylinder.
Diesel engines use direct fuel injection, where diesel fuel is injected directly into the cylinder. The injector on a diesel engine is its most complex component and must withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. Getting the mist circulated in the cylinder so that it is evenly distributed is also a problem, so some diesel engines employ special induction valves, pre-combustion chambers or other devices to swirl the air in the combustion chamber or otherwise improve the ignition and combustion process.
Some diesel engines contain a glow plug. When a diesel engine is cold, the compression process may not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electrically heated wire, similar to the hot wires seen in a toaster, that heats the combustion chambers and raises the air temperature when the engine is cold so that the engine can start. Smaller engines and engines that do not have such advanced computer control use glow plugs to solve the cold-starting problem.
Mechanics aren’t the only difference between diesel engines and gasoline engines. There’s also the issue of the fuel itself. The upcoming posts will give other differences between diesel and gasoline engines while continuing to illustrate how a diesel engine works. As always continue reading our blog learn more about diesel engines.
In this day and age pretty much everyone has a car. It is a very important part of how people get from places and is an integral part of many people’s lives, but have you ever wondered how they work? Engines are what propel the car forward. There are two types of engines that are mainly used in cars, diesel and gasoline engines. As you can see from the images diesel engines(left) are different from gasoline engines (right) in that diesel engines do not have a spark plug, they compress the gas which causes it to heat up and self-ignite, while gasoline engines have spark plugs which ignite the gas.
You may think that they are very similar, but they do have one that key difference. This one key difference in design drastically changes many things about the engine. This blog will delve into the world of diesel engines and discuss everything about the diesel engine from how they work and efficiency to possible future technologies
First, however, a good place to start a discussion on diesel engines would be from the beginning, so the first thing that will be discussed is a short history of the diesel engine. Rudolf Diesel invented the diesel engine in the late 1800’s and patented his idea in 1894. The first prototype was not made until 1897. During the 1920’s the diesel engine was made small enough to be practical for automobile use, but it was not until 1936 that the first car with a diesel engine was built by Mercedes Benz. These vehicles were very dependable and often lasted into the 1950’s. In the 1970’s there was a massive increase in the price of oil so many companies switched over to diesel engines because they are more efficient than gasoline engines. This surge ended in the 1980’s when the price of oil had re-stabilized and to this day gasoline engines are the most common type of engine found in automobiles in America.
Since diesel engines are not the most prevalent, there are many misconceptions about diesel engines. Many people think diesel engines release a lot of emissions and are dirty, this is not true, and diesel engines are the most efficient engine in the car market. Another myth is that diesel engines are worse for the environment because they produce more carbon dioxide. Even though they have around 14% more carbon dioxide per gallon, current diesel engines are usually 20-30% more efficient than gasoline engines, so per mile driven current diesel engines release less carbon dioxide than gasoline engines. Also some facts about diesel engines that you may not know are since diesel is a better lubricant than gasoline and diesel engines are made with stronger parts, diesel engines often last longer than gasoline engines. Diesel engines are a promising piece of technology in the world today because of their durability, efficiency, and promising future technologies. Keep reading this blog to find out more about diesel engines. The next blog post will be a more detailed description of how diesel engines work and a more in depth comparison of diesel engines and gasoline engines.