One point of forced induction/NA SI engine comparison

[Mark944turbo]

Hi,

I'm looking at one of aspects of SI NA/forced induction engines.
Let's say we have a defined RPM and a given amount (mass) of air for every stroke, any AFR, any engine design, WOT and optimal ignition timing (max torque or knock limited).
I wonder if a forced induction engine could produce more torque (pressure) than a NA one and WHY. I'm only focusing on the induction+combustion process and not on the overall engine efficiency. Induction would include the air compression and cooling.

 

[franzh]

Uh, your question answers itself. You defined an engine condition, air mass, AFR, RPM, etc. That sets the parameters of horsepower calculation. Change any of these and the output changes. Forcing air into an engine changes the air mass. Accompanying changes to the AFR must also take place to maintain power output/engine efficiency. You also state you are focusing in induction+combustion process, but not in overall engine efficiency, but they are one and the same.

Franz

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[schmidtj86]

Do you want to compare 2 DIFFERENT engines? Like a 454 at 10:1 NA vs a boosted 350 with an effective compression of (i.e) 13:1 - Both taking in the same air mass? Torque is more than just pressure when comparing engines of different size.

 

[Hydroformer]

Check out this "cold-air" intake.

Once open use the 'next' button to see more.

http://www.01designsystem.com/gallery/diis_gallery01.html

*Without data, you're just another person with an opinion.*

Hydroformer

 

[Mark944turbo]

franzh: I don't think parameters I set are enough to calculate the engine output. Engines could be knock limited, have different mixture temperature at ignition, ...

schmidtj86: yes, torque is more than just pressure. Let's say in both cases engines would be designed to fully exploit the pressure. Or let's talk about the torque, if there were some fundamental differences which would allow torque with less pressure.

 

[Mark944turbo]

hydroformer: I saw. I see what it's intended for, but how is it related to my question ?

 

[franzh]

If you take the basic engine displacement, air density/mass, and RPM, with the calorific value of the fuel, you can calculate the raw net power output. The rest of the infinite number of variables are refinements. Change the knock value of the fuel, the rod length, valve timing events, combustion temperatures, fuel temps, and so on, you will change the power output, but you still must have the basic inputs. Change the CR and the net power changes, along with the engine efficiency, but other variables must be in place too. You cannot simply change the CR and expect a great change without the fuel and ignition having an impact. Change the air density by forced induction without the accompanying fuel and timing and you will not achieve your desired maximum power outputs.
Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Hydroformer]

Mark944turbo,

I know it doesn't directly relate. But, it would change the air density through super-cooling, changing the air mass for a given stroke. (one of your parameters)

Of course... the local gas station doesn't carry dry-ice.

*Without data, you're just another person with an opinion.*

Hydroformer

 

[Mark944turbo]

franzh:
You say:
"If you take the basic engine displacement, air density/mass, and RPM, with the calorific value of the fuel, you can calculate the raw net power output. The rest of the infinite number of variables are refinements."
And then:
"Change the CR and the net power changes, along with the engine efficiency, but other variables must be in place too."

I don't see what CR has to do with RPM or air density.
You don't answer the question. You say the answer is in my question, but I can not find it. Could you explain me please.

 

[Mark944turbo]

Hydroformer,

I've specified the air mass does not change . So after having cooled the air, would we be able to redesign the engine in order to get more torque from the same air mass ? We probably would, by pushing farther the knock limit (at WOT).
But we don't have dry ice. In a forced induction engine we have a heat exchanger after the compressor (of any type) and we use a lower CR. Could we expect to have a cooler mixture at the end of the compression of such a "two stage" compression scheme compared to a "one stage" compression of a NA engine ? If yes, could we get a greater effective compression without knock and get a higher combustion pressure (and torque) ?

 

[franzh]

My last 2 bits worth:
Your original post offers this hypothesis:

"Let's say we have a defined RPM and a given amount (mass) of air for every stroke, any AFR, any engine design, WOT and optimal ignition timing (max torque or knock limited).
I wonder if a forced induction engine could produce more torque (pressure) than a NA one and WHY. I'm only focusing on the induction+combustion process and not on the overall engine efficiency. Induction would include the air compression and cooling."

When you force induce air, you change the air mass and density. When you change the compression and cooling, you change the mass/density.
Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Mark944turbo]

By saying
"Let's say we have a defined RPM and a given amount (mass) of
air for every stroke"
I ment we have a defined (constant) mass of air induced every stroke. Sorry for missunderstanding.

 

[richdubbya]

i believe the forced induction engine would make less power (assuming displacement is similar) for 2 reasons. one, the air introduced into the cylinder will be hotter (and limited to the same mass as the n/a engine) so the limit of the fuel octane will be reached sooner. two, there has to be a power loss to produce the forced induction. another thought is, a much smaller displacement engine, will have less friction and a smaller bore which could combine to overcome the two points above.