Compression Ratio Question..Need help!
Thread Starter
Joined: Feb 2008
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From: Ireland
Compression Ratio Question..Need help!
Doing ATPL's at the moment but cannot get this one right.
On a four cylinder engine with a total volume of 9600cc, bore area of 100sqcm and a crank throw of 10cm, what would the compression ratio be?
*Compression Ratio= Total Volume (Divided By) Clearance Volume
The book says the answer is 6:1,
Any Help?
Regards
Brian
On a four cylinder engine with a total volume of 9600cc, bore area of 100sqcm and a crank throw of 10cm, what would the compression ratio be?
*Compression Ratio= Total Volume (Divided By) Clearance Volume
The book says the answer is 6:1,
Any Help?
Regards
Brian
Joined: Jul 2006
Posts: 48
Likes: 0
From: london
Bore Area = 100cm2
Stroke = Crank Throw x 2 = 10cm x 2 = 20cm
Therefore, Swept Volume = 100 x 20 = 2000cm3
4 cyls, 9600cm3 total
Cyl volume = 9600/4 = 2400cm3
Clearance Vol = Cyl Vol - Swept Vol = 2400 - 2000 = 400cm3
Comp Ratio = Cyl Vol/Clearance Vol = 2400/400 = 6
Therefore 6:1
I suspect you're taking the crank throw as the stroke, rather than doubling it.
Stroke = Crank Throw x 2 = 10cm x 2 = 20cm
Therefore, Swept Volume = 100 x 20 = 2000cm3
4 cyls, 9600cm3 total
Cyl volume = 9600/4 = 2400cm3
Clearance Vol = Cyl Vol - Swept Vol = 2400 - 2000 = 400cm3
Comp Ratio = Cyl Vol/Clearance Vol = 2400/400 = 6
Therefore 6:1
I suspect you're taking the crank throw as the stroke, rather than doubling it.
Wunderbra
Joined: Aug 2006
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From: Bedford, UK
Had to think about that one for a second, but K12479 is right. You need to double the crank throw to get the total stroke length. It's all in the terminology. The crank throw is the distance between the centre line of the crankshaft and the pivot point of the con rod. Once you realise that's what it means it all becomes clear!
Joined: Feb 2008
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From: Wingham NSW Australia
Compression Ratio
Seems like the boffins have a hard time formulating questions which are relevant. What useful purpose does testing an ATPL candidate pilot on such useless information have? Questions for the sake of questions, it seems. Maybe questions on suitable fuel for nominated engines might be more useful. 
Joined: Jul 2001
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From: Gold Coast
On a four cylinder engine with a total volume of 9600cc, bore area of 100sqcm and a crank throw of 10cm, what would the compression ratio be?
Not enough information there to work out the compression ratio.
Avoid imitations
Joined: Nov 2000
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From: Wandering the FIR and cyberspace often at highly unsociable times
I agree, the question is over-simplified - it wouldn't work for real in an engine shop, the definitions are a bit "loose".
This "total volume" assumes a perfectly flat piston, most aren't.
This "total volume" assumes a perfectly flat piston, most aren't.
Joined: Jul 2001
Posts: 1,611
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From: Gold Coast
I agree, the question is over-simplified - it wouldn't work for real in an engine shop, the definitions are a bit "loose".
This "total volume" assumes a perfectly flat piston, most aren't.
This "total volume" assumes a perfectly flat piston, most aren't.
The piston either being domed, flat, or dished doesn't matter, we now have enough information to work out the combustion chamber volume, etc.
