Thermal Efficiency is basically a performance measure of devices, such as diesel engines, that use thermal energy. It basically measures energy output over energy input. Because you can never get more energy than you put it, thermal efficiency will always be less than one.
Maximum diesel efficiency is dependent on the cut-off ratio and the compression ratio shown by the following equation:
where nth is the thermal efficiency, α (alpha) is the cut-off ratio (V3/V2), r is the compression ratio (V1/V2), and γ is the ratio of the specific heats (Cp/Cv). This equation is based off of this graph that represents the idealized diesel cycle:
The cycle can be summarized as follows. Work is put in at point one. As this happens, volume decreases and pressure increases. Basically, the pressure compresses the gas. The heat is put in increasing the volume of the gas, expanding it, at constant pressure. From there the pressure decreases as the volume increases due to expansion of the gas, releasing work that powers the engine. Then, pressure goes down at constant volume as heat is given off to return to the beginning of the cycle.
Higher thermal efficiency is not only more cost effective than lower thermal efficiency, but it yields a longer engine life. This is why diesel engines are becoming more popular, they last longer, and fuel is cheaper. This makes them perfect for heavy hauling trucks and now some companies are even making cars that have them that everyday people can use.
Diesel engines convert the chemical energy in fuel to mechanical energy which moves pistons up and down inside cylinders. The pistons are connected to the engine’s crankshaft, which changes their linear motion into the rotary motion needed to propel the vehicle’s wheels. Energy is released in a series of small explosions (combustion) as fuel reacts chemically with oxygen from the air. The chemical equation of diesel fuel combustion is as follows, C13H28 + 20O2 → 13CO2 +14H2O. Combustion reactions are spontaneous yielding a -∆G. The reaction goes from 20 moles of O2 gas to 13 moles of CO2 yielding a -∆S. Combustion reactions break bonds between the molecules signaling an exothermic reaction or -∆H.
German engineer Rudolf Diesel theorized that fuel could be made to ignite spontaneously if the air inside an engine’s cylinders became hot enough through compression because air heats up when it’s compressed. Achieving high temperatures meant producing much greater air compression than occurs in gasoline engines, but Diesel calculated that high compression should lead to high engine efficiency. Part of the reason is that compressing air concentrates fuel-burning oxygen. A fuel that has high energy content per gallon, like diesel fuel, should be able to react with most of the concentrated oxygen to deliver more punch per explosion, if it was injected into an engine’s cylinders at exactly the right time. Diesel’s calculations were correct. As a result, although diesel engines have seen vast improvements, the basic concept of the four-stroke diesel engine has remained virtually unchanged for over 100 years.
Diesel engines had to have several design considerations taken into account because of the chemical properties of the diesel fuel. The engine had to be designed around the fuel and because of these chemical properties some considerations were made for the diesel engines.
In order to ignite the diesel engine the cylinder has to be very pressurized because there are no spark plugs. This means that the engines have to be made stronger in order to not break under such high pressures. This results in diesel engines being extremely heavy than gasoline engines and they are also much stronger as a result.
Another consideration with diesel engines is the byproducts they create. There are two main byproducts made from the combustion of diesel fuel, carbon based soot and NOX, which is a nitrogen oxide that can have different amounts of oxygen depending on the conditions. This is more of a problem with diesel engines than gasoline engines because of the way fuel is injected into the diesel engine. While a car is accelerating there is a shortage of air in diesel engines, which yields unburned soot.
Generally when one is decreased, it results in an increase in the other. As combustion temperatures go down, the amount of NOX created is much less, but that results in an increase in the production of soot. A reduction in temperature can be accomplished this by injecting the fuel later during the combustion cycle. However this is bad because if too much soot gets trapped in the oil, around a concentration 3-5%, that will results in increased wear on the engine. There are also very strict regulations on the amount of soot that diesel engines can make which are enforced by the government. To reduce the soot and NOX made there are several filters in an engine that help reduce levels of these harmful compounds. There is a diesel particulate filters which treats the exhaust of the engine after the combustion is complete. This is much easier and cheaper than reducing NOX by selective catalyst reduction, however there are such strict regulations on the amount of soot that can be created; both are usually present in modern diesel engines.
Because of the different properties chemical properties of diesel fuel, different considerations were taken into account in the design of the diesel engine. These are a few of the design considerations that were made.
Viscosity is a very important issue with diesel fuel. Viscosity is the resistance to deformities of a substance caused by stress. It is also described as how thick something is. The reason a substance, such as diesel has a higher viscosity in lower temperatures and a lower viscosity in higher temperatures is that in higher temperatures, more energy from heat is being added to the substance to break the bonds, or weaken the intermolecular forces. This makes something less viscos because the property of viscosity is due to intermolecular forces. The atoms with stronger intermolecular forces are pulling each other together making it harder to move through the substance macroscopically; this substance is thicker and more viscos.
This causes a problem with diesel fuel. In the winter time, when the temperature gets cold enough, diesel fuel becomes too viscos and will not ignite or pump. It will not ignite because diesel needs to be aerosolized to ignite. Simply, it must be easily turned into a gaseous material which is difficult to do if it is too viscos. Also it is too thick to pump throughout the engine. It is like trying to swim through honey, it would be very difficult.
Preheaters are used in diesel engines to get around the cold weather problem. This is used to heat the fuel during start up to make it less viscos so the car can start. They can only operate for a few seconds while the car is starting up. If they go for any longer they may burn out. This problem is now much easier to get around with computers that monitor things like that.