Otto cycle, Diesel cycle differences
Aug 26, The basic difference between otto cycle and diesel cycle is the heat addition process. In otto cycle it take place at constant volume and in diesel it take place at . Both diesel engines and gasoline engines convert fuel into energy through a series of small explosions or combustions. The major difference between diesel. with four hp. The first functioning and successfully designed gas-powered internal combustion engine was invented in by Jean Joseph Etienne Lenoir .
In the gasoline engine it is fairly easy. As we told already, the concentration of the air-fuel mixture is always the same no mater of the power level of the engine and so we only need to keep this concentration somewhat below optimal to decrease the combustion temperature. This ensures for clean exhaust.
The diesel engine is more problematic. As the air-fuel concentration varies greatly depending on the power level of the engine, we cannot easily control the combustion temperature. The combustion temperature will be lowered if a part of oxygen in the air is replaced with some inert gas.
To decrease the concentration of the oxygen, some of the engine exhaust concentrated with inert gases; carbon-dioxyde and water wapor is supplied back at engine air input. This controls the peek combustion temperature, but also reduces the amount of excesive oxygen available for combining with nitrogen.
The EGR is very useful, and is used on both, diesel and gasoline engines. Many of my friends with gasoline-powerd cars are puzzled with the questions "why do they measure oxygen O2 at madatory emission tests?
Four Stroke Gasoline Engine – Otto Cycle
The fact is, as we discussed above, that in the gasoline engine the oxygen to fuel ratio is always almost-ideal and aimed for full combustion, invariable to the throtle level. Therefore, if there is much oxygen left, it is a strong indication that the engine is sub-optimaly tuned.
Why is diesel engine more efficient than gasoline Is it because of a higher compression? Well, yes, it is, but only indirectly Large part of the heat energy maybe more than half will be expelled through the exhaust pipe and therefore lost. Because it uses higher compression ratio, the diesel engine will be able to extract more mechanical energy from heat that is, the diesel will attain higher efficiency. If you look at the engine in largely simplified way, you can see that the combustion stroke is a simple adiabatic process.
During the combustion stroke, you start with high pressure P1 and high temperature T1 of the combustion gas in the cylinder.
Otto cycle - Wikipedia
Then the piston travels downward for some length to its bottom position, and now you have lower pressure P2 and lower temperature T2 of the gas. Obviously, the energy contained in gas is now lower it has lower temperature and lower pressure, while its molar amount is the same.
This difference is converted to mechanical energy of piston. You can also look at the above process following way: After some time you simply have to give up and let the exhausted gas out because it would take the entire eternity for it to give all of its energy to the piston. Expansion combustion stroke of an gasoline engine - piston in top and bottom position.
The gas inside colds down as its volume increases and gives its energy to the piston. K-constant, Cp-molar specific heat for constant pressure, Cv-molar specific heat for constant volume Now, clearly it is possible to convert more heat energy to mechanical if the difference between P1,T1 and P2,T2 is larger. But, during the compression stroke the diesel spends more energy for compression than gasoline?
All that energy that was put into the gas during compression stroke will be recovered during expansion stroke plus the energy that comes from fuel. During compression it takes some, but during expansion it gives the same amount. No matter what the compression ratio is. Why then there are no engines with compression ration 1: Sure, such an engine would be efficient. In gasoline engine the limitation is because of self-ignition of the gasoline-air mixture. In diesel engines the limitation is only in mechanical strength of materials pistons, cylinders, valves, rods, crankshaft What does engine power has to do with efficiency?
Well, not that much. Efficiency and power are largely unconnected.
However, in general, a less efficient internal combustion machine can be made more powerful. Well, it takes time for gas to give its energy to the piston and the gas gives it slower and slower as it gets colder.
This is a way to improve inherently low efficiency of a gasoline engine. In gasoline engine you are limited about compression ratio. Image courtesy Baris Mengutay The diesel engine uses a four-stroke combustion cycle just like a gasoline engine. The four strokes are: Intake stroke -- The intake valve opens up, letting in air and moving the piston down. Combustion stroke -- As the piston reaches the top, fuel is injected at just the right moment and ignited, forcing the piston back down.
Exhaust stroke -- The piston moves back to the top, pushing out the exhaust created from the combustion out of the exhaust valve.
Remember that the diesel engine has no spark plug, that it intakes air and compresses it, and that it then injects the fuel directly into the combustion chamber direct injection. It is the heat of the compressed air that lights the fuel in a diesel engine. In preparation for adding heat to the air, we next compress it by moving the piston down the cylinder.
It is in this part of the cycle that we contribute work to the air. In the ideal Otto cycle, this compression is considered to be isentropic. It is at this stage that we set the volumetric compression ratio, r which is the ratio of the volume of the working fluid before the compression process to its volume after.
It will turn out that the efficiency of the Otto cycle assuming that air is an ideal gas can be described entirely in terms of this ratio. Next, heat is added to the air by fuel combustion when the piston is at it bottom dead center position.