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Boost and effective displacement.

Boost and effective displacement.

Boost and effective displacement.

A new Duramax 6.6 liter engine may reach 29 lps of turbo boost.
Simple arithmetic and a little rounding makes that an additional 2 X atmospheric pressure.
Simple arithmetic suggests that that would yeild an effective displacement of 11.88 liters.
But, in the real world, dynamic pressure flow due to increased flow and increased back pressure will reduce this quite a bit.
My question:
What is the ratio of swept displacement versus effective displacement in common turbo charged diesel engines?

Ohm's law
Not just a good idea;
It's the LAW!

RE: Boost and effective displacement.


What is the ratio of swept displacement versus effective displacement in common turbo charged diesel engines?
In our area, we don't think about it like that. We would think about the gain in charge density, i.e. density in the intake manifold.
Other parameters not to be neglected are
  1. volumetric efficiency, which is very likely different in an engine optimized for turbocharging vs one optimized for normal aspiration
  2. delta P from intake manifold to exhaust manifold, which is strongly influenced by turbocharging, and not always in the wrong direction!
All the above boil down to air mass flow, of course, which is what really matters. But then there's also air/fuel ratio, which most likely you won't find to be the same when comparing turbocharged and normally aspirated applications.
Numerous other variables come into play with their second order effects on power, such as compression ratio, and combustion phasing, which will not be the same for NA and boosted engines.
Such a comparison of diesel engines is quite academic these days, because only the smallest and cheapest diesel engines today are without turbocharging. I'd be very surprised if you could find a case of a normally aspirated and a turbocharged diesel engine competing head to head in the same market.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Boost and effective displacement.

That engine is also probably using considerable EGR (exhaust gas recirc) by design for emission control, and some of it may be by intentionally closing the exhaust valve before TDC to trap some exhaust in the cylinder for another go-round.

A normal decent (not exceptionally tuned, but not Flintstone era) non-turbo gasoline engine (which runs approximately at stoichiometric and won't be using much intentional EGR) normally makes 70-ish lb.ft of peak torque per litre of displacement (give or take quite a bit), so a 6.6 litre engine would be making around 462 peak lb.ft of torque (give or take quite a bit). I think the newest Duramax is claiming something like 975 lb.ft, so basically double-and-a-bit. On the one hand, the diesel ought to have higher thermal efficiency (and "all else equal" make more torque because of that). On the other, lean combustion ... EGR ... imperfect intercooling ... it isn't a race car engine. Many, many things are different between the two.

"BMEP" (brake mean effective pressure) is the proper way to look at all this.

RE: Boost and effective displacement.

Note that many turbocharged engines run on a Miller Cycle where the intake valve closes very late in the compression stroke which reduces the swept volume of the engine.

Our Caterpillar 3500 series engines (56-78L) run 35-40 psi of boost pressure. The charge air is is hot enough to to burn the paint on the ducts at 380°F.

RE: Boost and effective displacement.

Thank you all for sharing this information with me.
I appreciate the time that you have given me.

Ohm's law
Not just a good idea;
It's the LAW!

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