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dhanavr
Active Contributor

Join Date: Dec 2008
Location: kakinada
Posts: 17

Internal Combustion Engines

01/23/2009 4:46 AM

why does the efficiency of compression ignition engine increases with increase in compression ratio?

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Floram
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Join Date: Feb 2008
Posts: 353
Good Answers: 8
#1

Re: Internal Combustion Engines

01/23/2009 7:33 PM

Overall efficiency is the product of the three individual efficiencies. These are:

1) Thermal efficiency of the idealized cycle,

2) Efficiency factor of the real engine relative to the idealized cycle. Covers mainly heat losses.

3) Mechanical efficiency. Friction and losses to drive auxiliary components needed for engine operation water pump, fuel pump etc.

Overall engine efficiency is the product of the three.

The thermal efficiency or enthalpy efficiency is solely dependent on the compression ratio. In the spark ignited engine (gasoline) the formula is thus:

Thermal efficiency = 1 - (1 / e ^(x-1) ) where

e =compression ratio, and x is cp / cv, cp and cv are the specific heat capacities of air for pressure cp, and volume cv, x is near 1.4.

For a diesel engine the formula is more complicated. At the same CR (compression ratio) the thermal efficiency is somewhat less, however due to the far higher CR in the Diesel engine its thermal efficiency is higher.

In a diesel, thermal efficiency may be near ~ 0,70. When both other efficiencies are multiplied in, overall useful efficiency is roughly half that.

In more layman terms, if you increase the CR there is also more expansion before the exhaust valve opens and more work is extracted, the exhaust gases get cooler and the thermal efficiency goes up.

Trust that helps.

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Anonymous Poster
#2
In reply to #1

Re: Internal Combustion Engines

01/23/2009 11:58 PM

That's a great answer. GA from me!
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Stueywright
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Join Date: Jan 2008
Location: "Springwood", North Tamborine Mountain. Qld. OZ.
Posts: 837
Good Answers: 28
#3
In reply to #1

Re: Internal Combustion Engines

01/24/2009 2:10 AM

And me! GA!

Stu.

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babybear
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United Kingdom - Member - Not a new member!

Join Date: Jun 2008
Location: USA/Europe
Posts: 4547
Good Answers: 68
#5
In reply to #1

Re: Internal Combustion Engines

01/24/2009 9:57 AM

Hello Floram,

Good answer!............However you may have breached the site rules a little by explaining in such detail the 'whys and wherefores'! Check this out, it was on his profile:

I am doing my 2nd B.Tech in mechanical engineering.

This could well have been a critical question in his education. By explaining and effectively giving him the answer he does not now have to learn it. Just copy and paste?

No offence intended, but think before you help. These questions often come at the end of the week (it seems to me) which may indicate that this was home/course work.

Take care.

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Anonymous Poster
#7
In reply to #5

Re: Internal Combustion Engines

01/25/2009 11:35 AM

Hello Baby Bear,

Thanks for the advise. No offence taken.

I just jumped on the question without checking anything at all. You are right it was an easy way out for him to get an answer. On the other hand, I think, if he is so inclined he will not get far in life.

If he solely depends on asking others he can not succeed. At the end he hurts himself.

Next time I pay more attention to the background of the question and the questioner. Thanks.

BTW, do we such site rules? I must find and read them.
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Floram
Power-User

Join Date: Feb 2008
Posts: 353
Good Answers: 8
#8
In reply to #5

Re: Internal Combustion Engines

01/25/2009 11:47 AM

Hello Baby Bear,

Above answer, listed under Guest, was by me, Floram. I don't know what happened, I wrote that one from my laptop. Would that make the difference?

Greetings

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ray cockrell
Associate

Join Date: Jan 2009
Location: Black Mountain, NC
Posts: 40
#4

Re: Internal Combustion Engines

01/24/2009 8:40 AM

If your asking questions like this ....Go to your local community college and check out Thermodynamics. Totally the coolest course. The perfect gas theorem will make your day.
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Anonymous Poster
#6

Re: Internal Combustion Engines

01/25/2009 9:35 AM

good answer i like it
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Anonymous Poster (3); babybear (1); Floram (2); ray cockrell (1); Stueywright (1)

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