Calculating air speeds in cylinder filling

Started thinking about CTW's engine, then got to thinking about how the stroke/bore ratio effects the speed of filling of the cylinder. Then I tried to do some maths and got stuck. Anyone know how to work this out?

I want to make an excel spreadsheet....

Suppose we take a single cylinder. lets call it 1000cc for ease of use.

This cylinder could either have a larger bore and a shorter crank throw, or a smaller bore and a longer crank throw. Either way 1000cc of air is being ingested every 1/2 revolution of the engine, but does this happen at different speeds?

Clearly the fill speed is going to be slow-fast-slow (making the assumption that the valve opens as soon as the downstroke begins and closes when it ends.) But will these speeds vary according to the variables I mentioned above.

I'm currently pounding away at excel....

Your maths is better than mine if you can model that! :thumbs-up:

I did the maths.

I was talking sh1te. Lol

I think you are talking about an engine's squareness. It is considered that an engine with an over square design ie more bore diameter than stroke produces power high in the rev band and is happier to rev but the opposite is true with an under square engine like my own and most VW's engines. Also a lower crank to rod ratio will produce more power lower in the rev range.

The way I did this was to work out the radius of a piston for a given cylinder capacit and stroke. I took 1000cc (1000000mm^3) and worked out radius's for an 80mm stroke and a 100mm stroke. I needn't, if I'd just thought about this stage a bit more I could have saved myself 18 minutes of maths.

I then worked out the piston movement for each degree of rotation, and from this movement and the surface area of the piston I could work out the swept area for every degree of rotation of the crank. 180 degrees totalled 1000cc. At every degree of rotation, the swept volume remained the same.

However.

What I could now do with my spreadsheet is calculate the speed of the air through a given valve, and then look at different camshaft timings. So it might not have been a total waste of time...

ctwg60 wrote:I think you are talking about an engine's squareness. It is considered that an engine with an over square design ie more bore diameter than stroke produces power high in the rev band and is happier to rev but the opposite is true with an under square engine like my own and most VW's engines. Also a lower crank to rod ratio will produce more power lower in the rev range.

That is a factor, but different to what I was thinking about. I think that is more to do with piston velocity at certain points.

Now I've started to think about piston velocities....

Variation in pisons speeds between two given stroke legnths.

I'll sort out units and stuff tomorrow....

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Yes me too, piston acceleration is related to crank throw and rod length. I specifically "went" or should I say "had to" go for a longer crank and shorter rod ratio to gain better low end torque and knew that top end bhp would suffer but was playing to the G60 on 65mm pulley's benefits/qualities.

And I think I've achieved that considering the standard head, maybe! 210lb/ft at the wheels is good for a high loss gear and the Rolling Road characteristics.

Yeah, I've been thinking about the pistons speed being a product of the rotational speed of the crank, but actually it's the other way round isn't it?

The expansion of the burning fuel/air mix would determine the peak speed of the piston if flywheel effect and friction was ignored.

I'm going to think about this in bed...

I sat in bed last night thinking, and while the spreadsheet didn't do quite what I thought, I just wonder if I can now alter it to work out vavle opening areas and compare cams and overlaps to piston speeds etc.

Were you good at maths in school! Thats' pretty impressive what you've done at 1am on a Sunday morning!

Errr. I was alright at it. An IQ of 3765 helps.

Right. I want to play with camshafts now.

Lets use a G60 as an example.

How big is a G60 inlet valve and how much lift is there, and how long does it open for?

What does this all mean in real life?

piper 285 is a
symmetric 268/268
lobe centre 114
timing 20-68/68-20
lift 11.13/11.13

lobe centre 114 degrees from tdc? what is the 20-68 bit about?

What you have to do is get the cams you want and using a degree wheel and dial gauge work out the lift at each degree (or 5˚) of crankshaft rotation. It's the only way to get a good idea of a cam profile for the maths. Unless you can get the manufacturer to give you them. Unlikely!

I have a couple of books on camshafts but it's fairly in depth so I won't try to explain it but if you could find an online resource that would be your best bet.

I was just going to make the assumption, for now, that the lift was linear from a certain point to fully open, then back down again. Just for the sake of having a fiddle around with the maths.

ok well 20-68 is inlet starts to open or is at 1mm of lift 20˚ before TDC and closes or is 1mm from closing at 68˚ after BDC which is where the 268˚ duration comes from. Exhaust starts to open or is at 1mm of lift 68˚ before BDC and closes etc at 20˚ after TDC. 114˚ is the centre line between the cam lobes.

I think................

Aaah I wondered how the numbers came from.

Here's a graph I knocked up for a piper 285 cam plotted against piston position with a 80mm crank stroke. Interesting to see how far the inlet valve is openas the piston is moving up on the compression stroke.

Just inlet valve

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Inlet and exhaust valves.

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Interesting, I'd heard that one potential problem with the Piper would be blowing the air stright out of the exhaust at high revs.

Here's the equivalent measuremetns for the Schrick Toad...

asymmetric 265/278
lobe centre 112
timing 22-66/70-26
lift 11.2/11.5

I'd be interested to see how they compare.

I think your 180 degrees out between the cam and the crank. Are we talking compression stroke or exhaust stroke.

Here you go...

Not very clear, but I'll work on it.

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ctwg60 wrote:I think your 180 degrees out between the cam and the crank. Are we talking compression stroke or exhaust stroke.

That should be a complete cycle. It didn't look quite right to me either really. I'll fiddle with it.

Toad wrote:

ctwg60 wrote:I think your 180 degrees out between the cam and the crank. Are we talking compression stroke or exhaust stroke.

That should be a complete cycle. It didn't look quite right to me either really. I'll fiddle with it.

The cam rotates twice to every crank revolution. Not sure if that would help your fiddling. Probably teaching you to suck eggs there.