Adobe2X

OK --- which engineer out there can answer this for me?:

The compression on a diesel engine at about twice the normal naturally aspirated gas engine (say 10 to 1 compression compared to 21.5 to 1) would be the same rate of effect/force to the piston as a turbo huffing twice the atmospheric pressure (14.7 pounds of boost) into the cylinder, right?
I know that they typically reduce the off boost compression ratio on a turbo engine.......but if the off boost compression ration is only 8.5 to one, doesn't the full boost equal 17 to one at 14.7 pounds of boost?
Or is the explosion of the diesel fuel on the power stroke THAT much more dynamic than gasoline and that is why they rattle????

EDIT: After thinking about this for a few more minutes, I realized that the diesel engine is high compression all the time --- even at idle. So I know they are going to be harder on the internals than an off boost turbo -- but the question remains as to the stress on the internals at full turbo boost --- like maybe towing up an 8% grade for 5 miles????.............and, if most diesels are turbo, does that raise their effective compression ratio to 40 to 1???....ok, my brain hurts, I am going to stop NOW!

TwinTurboFx4

Internals on a diesel engine need to be forged. Also need 4-5 studs per cylinder to prevent the head from lifting. Same goes with a turboed gas engine, must have atleast a forged crank and heavy duty head studs.

pfbz

Engine compression ratio, with or without some help from a turbo, is only one factor that contributes to engine stress... but yes, a gasoline turbo engine will definitely see higher "effective" compression ratios at max boost than a non-turbo engine. And if properly engineered (eg: not a backyard turbo build) will be designed stronger to withstand the effects of that extra compression.

Still going to be a lower compression ratio than a diesel engine, but then again, a turbo gasoline engine is going to redline at probably 40%-60% higher RPM's than a diesel, so different kinds of stress.

Gasoline engines are engineered to be lighter, spin faster, but handle less explosive force on every bang, Diesels are designed where weight is typically less of an issue, spin more slowly but for extended periods, and handle enormous cylinder explosive force.

engineermike

Of course, the loading on the bottom end is higher in turbo and turbo deisels. However, the actual stress level is a function of the load, the geometric design of the components, and the materials. In other words, if it's designed for it, it's fine. Even just saying "the pistons are forged" doesn't mean much without looking at the geometry. For instance, many many forged pistons have broken at 5 psi boost, whereas turbo Buicks have cast pistons and they live beyond 30 psi. It has to do with the geometry of the piston.

I suppose what I'm getting at is the only ones who can answer your question accurately are the guys at Ford who designed the bottom-end. The rest of us are just speculating based on a very limited pool of experience.

Edit: After I posted this I saw pfbz's post, which essentially says the same thing.

Adobe2X

Great discussion and responses folks......I am enjoying reading these!!!

NewfieFX4

Compression ratio has nothing to do with the boost provided by forced induction in regards to cylinder pressure. Compression ratio is simply the volume of a cylinder with the piston at the lowest position divided by the volume of the cylinder with the piston at top dead center. This 'compression' of gas does create pressure, but does not add linearly to the pressure added to the intake tract by a turbo or supercharger.....

You guys are talking two different things.

Adobe2X

OK, I "think" that I disagree.........if the compression ratio on an engine is 10 to 1 under no boost of any kind, but the supercharger or turbo is adding an additional atmosphere of pressure into the cylinder as the piston comes up on the compression stroke ---- doesn't that add resistance to the piston and raise the "effective" compression ration to 20 to 1? When I say effective compression ratio, I mean the effect or force on the engine internals --- not the mechanical compression ratio, which I know never changes.
The piston is now trying to compress twice the air and gasoline it normally would, right? So the effect on the engine is as if it had a non-boosted 20 to 1 compression ratio, correct?

pfbz

I'm not a physicist, but this is how I think it works... Any experts out there, feel free to chime in.

Pressure is pressure. Cylinder compression increases pressure, and a turbocharger increases pressure. Both will effect the final pressure in the combustion chamber before ignition.

The rise in pressure when gasses are compressed is not linear. For run-of-the-mill earth atmosphere:

p-final = p-initial x compression-ratio^1.4

So at sea level (14.696 psi), without any adjustment for variations in standard air pressure, on a 10:1 compression ration engine, no turbocharger, the pressure in the combustion chamber before ignition is:

p-final = 14.696 x 10^1.4 = 369 psi.

If it was a 13:1 compression ratio, it would be:

p-final = 14.696 x 13^1.4 = 533 psi.

With 10 psi of boost, back at 10:1 compression, I think the pressure would be:

p-final = (14.696+10) x 10^1.4 = 620 psi.

15 psi boost, 10:1 compression (EcoBoost at max boost):

p-final = (14.696+15) x 10^1.4 = 746 psi.

Definitely a quite high cylinder pressure!
About equal to a 16.5:1 compression ratio without a turbo! Far more than any gas engine would normally run. Even full alcohol fuel race engines top out at about 15.5:1.

p-final = (14.696) x 16.5^1.4 = 744 psi.


Just for kicks, look at the same thing at high altitude, say Denver which has about 12.1psi standard air pressure... Assuming the turbocharger measures boost against ambient air pressure rather than compressing to an absolute PSI, which is how I think it works...

p-final = (12.1+15) x 10^1.4 = 680 psi.


All of this is very approximate... As heat goes up (and a turbocharger definitely heats up the air), the gamma (that 1.4 exponent) goes down, so the resulting pressure is less, but not by so much to worry about it for this discussion..

2010FX4F150GILROY

Now add a turbo charged diesel engine.....

pfbz

The new Ford 6.7L Powerstroke in the Super Duty has a 16.2:1 compression ratio. Reported at about 20 psi max boost.

p-final = (14.696+20) x 16.2^1.4 = 1712 psi.

KABANG!

Explains why the diesel's are so damn heavy!

But again, spinning at max 4,200 rpm. BIG BANG, low speed.