One of the differences between spark ignition (gas) engines and compression ignition (diesel) engines is one is Constant Volume and the other Constant pressure.
- Which is which, and
- graphically, why is it important?
Both types are Internal combustion engines. Neither are V=C or P=c machines
A gasoline engine is a low compression engine. Gaspline is aspirated into the intake air stream compressed & utilizes spark ignition to ignite the fuel, thus heatig the air to increase the pressure & push the piston downward on the power stroke.
A diesel is a high compression engine which heats the air by compression. Fuel is injected into the hot air where it burns very rapidly, heating the air further to push the piston downward on the power stroke.
**Diesel Engine Principles **
Shows the INDICATOR Cards for the OTTO, DIESEL, and MODIFIED DIESEL CYCLES.
The accompanying text shpuld explain the answers to your questions better than I could.
You actually answered the question with your link. The Otto cycle is constant-volume ignition; the Diesel cycle is constant-pressure ignition.
EXACTY why I added the link to provide better details of the whole picture of ICE’s than I could.
I case someone doesn’t want to slog through the whole thing, the answers are on pages 7 and 8 of the rather good link.
Of course those are ideal cycles; IRL they don’t work exactly like that. But not too far off.
In the gas engine, the fuel and air are premixed and well mixed. When the spark ignites the mixture, it burns very fast compared with the motion of the piston, so the pressure shoots up virtually instantaneously. This is an approximately constant volume process, and is represented by the vertical B-C line on the Otto cycle diagram, page 8 of spingears’ link.
In the diesel engine, a spray of liquid fuel droplets enters the extremely hot compressed air within the top of the cylinder. There is poor mixing - the droplets themselves are fuel with no air, and in between the droplets is air with no fuel. Only in small regions near the droplet surfaces is a mixture which can burn, and further mixing and burning occurs as the piston moves down. The burning rate is therefore relatively slow compared with a gas engine. Combustion proceeds over an increasing volume interval, which is modelled as an interval of constant pressure as the piston moves down, although it is unlikely to actually be so. This is represented by the horizontal B-C line on the Diesel cycle diagram, page 8 of spingears’ link.
The diagrams allow the theoretical efficiencies of the cycles to be worked out. The analysis is beyond me, but for the same compression ratio, the gas cycle is more efficient than the diesel cycle. However, in practice you can build diesel engines with much higher compression ratios than gas engines, since the gas engine tends to prematurely compression-ignite (“knock”) at compression ratios much above 10. Diesel engines don’t have this problem since they only compress air, so diesels tend to be more fuel efficient than gas engines.
Another advantage of the diesel engine is that the power can be varied by lowering the amount of fuel - the droplets will still burn okay even if they are smaller and/or there are fewer of them. This means that diesels don’t lose efficiency at partial power output. The gas engine on the other hand has to lower both the amount of fuel and the amount of air to operate at partial power output, or the mix will be too lean to burn. This is achieved by “throttling” - sucking the intake air through a small hole, so the cylinders fill with fuel-air mix at lower than atmospheric pressure. This sucking of air through a constriction takes energy, so a gas engine at partial power suffers from “pumping losses”.
Thanks, matt, for an excellent post.
In a diesel engine the fuel injection is controlled so as to approach burning at constant pressure. In Diesel’s original engines the fuel was injected through a substantial fraction. of the power stroke.
As I recall (but college thermodynamics was a long time ago…), the efficiency of the cycle consists of the area enclosed by the upper and lower curves divided by the area under the uppermost curve. The Diesel cycle has (proportinately) more area enclosed, so is inherently more efficient.
Diesel’s idea was to duplicate as closely as possible the Carnot vapor cycle which is the theoretical best you can do. The original Diesel engines sprayed the fuel during most of the power stroke. However, at that time this required that the engine run at low RPM. Each power stroke supplied lots of energy at good efficiency but there weren’t very many power strokes/minute and so the output power was limited to relatively low values.