# Is this the reason diesel engines are less powerfull?

Im studdying PV diagrams for dieslel and petrol engines, and I've noticed that for Diesel the fuel is combusted at constant pressure as opposed to petrol (gasoline) which is at constant volume.
I know that the area inbetween the lines is the net work done by the engine.

So if you look at the graphs:

http://img861.imageshack.us/img861/5465/pvdiagram.png [Broken]

You can see that the shape of the petrol curve has a naturaly larger area than the shape of the diesel area. (Like a star shape has a naturaly larger perimeter than a square).

The petrol curve has like a extra triangle of area over the diesel curve.

Is this why diesels are generaly less powerfull than petrol engines?

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It's been a while since i've done anything engine and thermo related but. That's a rather arbitrarily drawn graph.

A generalization:
Diesels releases more energy per combustion event than a petrol engine, which is why diesels have higher torque outputs (and better bottom end power). They produce less peak power because they don't rev as high as a petrol engine.

Your graph does not look correct. Diesels have a lot higher compression than petrol engines.

256bits
Gold Member
You have to know what kind of cycle(s) you are comparing - theoretical or practical and realistic.
But yes, by having the same compression ratio, the Otto (petrol) cycle would be some percentage points greater in efficiency than the diesel cycle.

Comparing peak pressure in the cylinder, I am not sure who wins out - Otto or Diesel.

Realistically though, an Otto cycle engine as you have diagramed would have a peak pressure much higher than that of the Diesel. The engine components ( ex. block, head, piston ) would have to designed ( for the Otto versus the Diesel ) much stronger to cope.

Note this sentence
"Here, ( Diesel ) taking the compression ratio from 18 to 25, for instance, which takes our thermal efficiency to about 65%, only increases the maximum cycle pressure from under 5.7 MPa to just over 9 MPa. By comparison, the Otto cycle with a similar efficiency requires a compression ratio of about 13.5, which results in a maximum pressure of over 15 MPa. "
from
http://www.qrg.northwestern.edu/thermo/design-library/diesel/diesel.html

jack action
Gold Member
In practice, engines - whether diesel or petrol - have a similar thermo cycle that could be best represented by the mixed cycle.

All things considered, the diesel usually have a slightly smaller Brake Mean Effective Pressure (BMEP), because it's main disadvantage - power wise - is the fact that it runs very lean ($\lambda$ = 1.65 versus $\lambda$ = 0.85 for the petrol engine), meaning that not all the air that goes in the engine contributes to create power. (But it does consume a lot less fuel for almost the same BMEP). So comparing a diesel and a petrol engines with the same displacement leads to a petrol engine that produces more power because it uses more efficiently all the air that goes in (but it will consume a lot more fuel, accordingly).

You can use this calculator to make some comparisons and learn what influences power in engines.

Mech_Engineer