Intake temperature vs efficiency 5

[ebarba]

Hi!

Which one of the following two intake manifold conditions will lead to better efficiency, on a spark-ignited engine? Can someone say what is the ballpark efficiency increase between the best and worst condition?

A) 1.5 bar(a) pressure and 100ºC
B) 1.5 bar(a) pressure and 50ºC
Both conditions have the same mass flow.

The story: we are planning on turbocharging a spark-ignited engine burning gaseous fuel. The load is an electric generator working at constant power and speed. The reason we want to turbocharge it is efficiency as well as power increase. We just can't decide wether to install an aftercooler downstream of the compressor.

Is it ALWAYS a good idea to cool the compressor discharge?
[ul]
[li]On one hand I remember that all heat engines have better efficiency working on higher temperatures... so if the increase in mass that the compressor will deliver matches the power we want to reach, why bother in cooling?[/li]
[li]On the other, higher inlet temperatures should make the compression stroke more difficult... and we have knock lurking.[/li]
[/ul]

Thanks!

P.S. Don't know if this is really important, but the engine works great lean and we will keep it that way. Preliminary testing the selected turbocharger showed that this is OK even in the boosted operation.

 

[GregLocock]

Personally i wouldn't be shouting about my opinions if I just stuffed up by 30% or so, but maybe that's just me.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[aap2]

LOL

 

[gruntguru]

The forum needs a quoting feature.

je suis charlie

 

[Tmoose]

I'm mighty grateful for getting an EDIT button.

 

[aap2]

I guess I'll post this again to defend myself...

Changing it to kelvin makes it a 13% efficiency gain from P1,V1,T1 to P2,V2,T2. It's a law of physics so I'm positive that it could hold true. I'm done with this thread.

NERD FIGHT LOL

 

[RodRico]

Professor Paul Ronney at USC provides an Excel workbook named "AirCycles4Recips.xls" that will allow you to explore some of the basic trades in an engine including intake air pressure and temperature, compression ratio. cycle type (Otto, Diesel, etc). theoretical max and attained efficiency, etc. Google "AirCycles4Recips" and you'll find both the spreadsheet and course materials from his class (AME 436) that explain the concepts behind and operation of the Excel workbook. The workbook has some limitations (fixed 1.3 alpha for air over all temperatures, simplified heat transfer, rounded values, etc), but it's more than adequate to answer your questions.

 

[gruntguru]

Nice spreadsheet RodRico. Unfortunately it doesn't consider compressor or turbine work so can't answer the OP's question.

je suis charlie

 

[RodRico]

Grunt, it allows specification of intake pressure and temperature, so I think in can be used to explore the effects of turbo and super chargers. Simply changing intake pressure and temperature without calculating the work cost of the higher pressure, however, doesn't yield a complete answer as you note. Fortunately, the OP only asked about the impact of cooling the intake charge, and intercoolers don't do work. Thus, he might simply set the intake pressure to 1.5 bar with temperature elevated according to that pressure (which he can calculate or observe in the compression section) then modify the temperature to reflect an intercooler and note the observed change in efficiency.

 

[gruntguru]

In terms of the efficiency impact from intercooling, I see two areas:
a) combustion impacts (which are difficult to predict)
b) air cycle impacts (pumping, compression and expansion work). This is the area than can be estimated, but the model doesn't include them all. (Compression and expansion work, efficiencies, pressures and temperatures would have to be calculated outside the model then fed in)

je suis charlie

 

[RodRico]

grunt, I actually have a version of the spreadsheet all cleaned up and modified to incorporate a supercharger. Unfortunately, the model also contains many of the details of my new motor design, so I can't share it. Modifying the professor's basic spreadsheet shouldn't be too hard for the OP, and he'd learn a lot in the process, don't you think ?

 

[Panther140]

It would help to know where this engine spends most of its time as far as speed and load.



"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin