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

:O

Whoopsie! I better stick to turbines...

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.

Thanks for that

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!

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. :ugh: :ugh:

Agreed, Ol' Fella, what a pointless question!

Not enough information there to work out the compression ratio.

Much like the majority of one's education from the time you start your GCSE sutdying I fear. Not really to the point and 90% of it unused in your chosen career.

k12479 worked it out correct so there is enough info given in the question. Its just the book doesn't give any examples of that type of question, only simpler versions.

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.

I missed the book answer of 6:1, yeah it can be worked out sorry.

The piston either being domed, flat, or dished doesn't matter, we now have enough information to work out the combustion chamber volume, etc.

Shy, doesn`t matter if it`s flat , countersunk,domed,,you are only interested in total and swept volume,and the stroke.