# Is this the reason diesel engines are less powerfull?

1. Jun 15, 2012

### CraigH

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?

Last edited by a moderator: May 6, 2017
2. Jun 15, 2012

### xxChrisxx

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.

3. Jun 15, 2012

### Bob S

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

4. Jun 16, 2012

### 256bits

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

5. Jun 16, 2012

### CraigH

6. Jun 16, 2012

### jack action

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.

7. Jun 18, 2012

### Mech_Engineer

The reason diesel engines have lower horsepower (but higher torque) all comes down to RPMs. Diesels use high compression ratios to combust the fuel (around 18:1 vs. 9:1 for gasoline); a large compression ratio means the picton needs to have a large stroke and heavier components. This in turn means the engine will have a lower maximum engine speed but higher torque due to a larger lever arm on the crankshaft.