One point of forced induction/NA SI engine comparison
One point of forced induction/NA SI engine comparison
(OP)
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.
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.





RE: One point of forced induction/NA SI engine comparison
Franz
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RE: One point of forced induction/NA SI engine comparison
RE: One point of forced induction/NA SI engine comparison
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*Without data, you're just another person with an opinion.*
Hydroformer
RE: One point of forced induction/NA SI engine comparison
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.
RE: One point of forced induction/NA SI engine comparison
RE: One point of forced induction/NA SI engine comparison
Franz
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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: One point of forced induction/NA SI engine comparison
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
RE: One point of forced induction/NA SI engine comparison
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.
RE: One point of forced induction/NA SI engine comparison
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) ?
RE: One point of forced induction/NA SI engine comparison
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.
RE: One point of forced induction/NA SI engine comparison
"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.
RE: One point of forced induction/NA SI engine comparison