Compression Test

mac · Post by **mac** » Fri Dec 22, 2006 4:30 pm

Does air just flow into the cylinder or would the vaccum caused by the piston falling have any effect?

Mac

robin · Post by **robin** » Fri Dec 22, 2006 6:38 pm

mac wrote:Does air just flow into the cylinder or would the vaccum caused by the piston falling have any effect?
Mac

Those are just two sides of the same coin. The air flows in (or out) to equalise the pressure outside and in; the piston moving creates the pressure differential in the first place.

If you move the piston very slowly at the moment the intake valve closes, the cylinder will be at exactly atmospheric pressure, but will contain only the volume of air that matches the position of the piston in the bore - for example, let's say the intake valve closed at piston bottom dead centre then you would have 450cc in the bore. If it closed at 90BTDC then you would have 225cc in the bore, etc. The standard cams close the intake at 52ABDC - a quick bit of trig says that's equal to a volume of 363cc. Assuming that the 10.5:1 compression ratio means that the combustion chamber has volume 43cc, the dynamic compression ratio would be 8.5:1.

As you start to move the piston quicker, the air isn't so easy to boss about and things move to your advantage.

The quickly falling piston creates a larger pressure differential due to the inertia of the air and so once the air starts to flow, it does so more quickly to fill it and the air will keep moving, even when the pressure is equalised.

To visualise this, imagine a weight sitting on the table with a rubber band tied to it. If you pull the rubber band slowly, it will stretch a little and the weight will start to move; the band will stay at a constant stretch and the weight will keep moving. If instead you yank the band, it will stretch further and will then whip the weight away much more quickly - very likely the weight will overtake the rubber band and smack you in the hand. The air is sort-of-elastic and so behaves similarly.

If the piston is moving quick enough, by the time it reaches BDC and turns about, the cylinder is still at a lower pressure than atmospheric because the air hasn't caught up yet, so even though the volume starts to decrease, the air keeps flowing until the pressures are equalised by the combined effect of the reducing volume and the inflowing air.

Now there will come a point on the upstroke where the pressure is equalised. At this point you would expect the airflow to stop and reverse, but it doesn't because the air has momentum, so you actually need above atmospheric pressure in the bore to stop and reverse the air flow. Just at that point is the ideal point for the intake valve to close.

So for each intake configuration (plenum, throttle opening, cam lift, valve size, etc.), you will reach the maximum cylinder filling (and so maximum torque too, I think) at a certain piston speed. Slower than that and the piston will expell some air & fuel back into the intake before the valve shuts; faster than that and the valve will shut before the air flow stops. Either way you have less air & fuel in the bore to compress and explode.

Now could this explain the discrepency between the 8.5 bar predicted by the static analysis and the 13 bar actually experienced? I don't know - I guess you would need to get a handle on the air speed to decide whether even at cranking RPM you would expect "boost" from the air's momentum.

I still don't understand it!

EDIT:

I do now - just took a look over at wikipedia - marvellous - the pressure increase is more than you would expect from just the change in volume because the temperature of the compressed gas rises as you compress it. You would need to cool the compressed gas back to the starting temperature to get the result I predicted based on just volume change.

The final cylinder pressure at low speed is:

(atmospheric pressure * dynamic compression ratio)^1.25 (this factor depends on engine design 1.2 - 1.3 is typical; it's to do with how much heat is lost to the block, etc.).

Or for our engine with standard cams:

(1 * 8.5)^1.25 = 14.5bar = 210psi.

Cheers,
Robin

mac · Post by **mac** » Fri Dec 22, 2006 7:40 pm

So it's do with the heat involved changing the density????

Mac

Lawrence · Post by **Lawrence** » Fri Dec 22, 2006 8:22 pm

mac wrote:

So it's do with the heat involved changing the density????

Mac

Not really as the answer we're seeking is a pressure so the fact that density is temperature related can be ignored.

Temperature increase has a positive effect on pressure and hence the greater pressure recorded than the compression ratio suggests.

Stu of course knew all this already from the hundreds of engines he's measured and observed 200 psi is good

Had he stated that 13 bar was good then he'd have a much newer test gauge and may not have tested so many engines

mac · Post by **mac** » Fri Dec 22, 2006 8:58 pm

Ah - so it's like when the lid on a pot of boiling water starts to rattle?

Mac

Lawrence · Post by **Lawrence** » Fri Dec 22, 2006 9:43 pm

mac wrote:Ah - so it's like when the lid on a pot of boiling water starts to rattle?

Mac

No that's Head Gasket Failure

tut · Post by **tut** » Fri Dec 22, 2006 10:22 pm

I understand all that, but do not understand how Robin manages to fit so many hours into a day.

tut

robin · Post by **robin** » Sat Dec 23, 2006 5:50 pm

Lawrence wrote: Stu of course knew all this already from the hundreds of engines he's measured and observed 200 psi is good Had he stated that 13 bar was good then he'd have a much newer test gauge and may not have tested so many engines

I never doubted Stu was right with his target figures (in either unit of measurement), it was just that I didn't understand why he was right, and now I know - and lots more besides too - I had never really condsidered the dynamics of the engine filling process before - quite interesting - it made me wonder whether an engine with a variable diameter inlet manifold wouldn't make a reasonable alternative to one with variable valve timing/lift ... it also explains why fitting massive throttle bodies to an engine with standard valves doesn't always give you what you expected!

Cheers,
Robin

Stu160 · Post by **Stu160** » Sat Dec 23, 2006 6:01 pm

Well Robin,most of that went over my head,but I am sure you are spot on.Only reason I know,is because that what I was told,and have measured on my "old " pressure tester

But as this is the youngsters forum,I really should go metric

Stu

offshorematt · Post by **offshorematt** » Fri Dec 29, 2006 2:12 pm

robin wrote: Matt - is this still the won't run fuelling type problem that's been going on for a month or two? Are you around at the start of January? If so I'll come up with a box of bits and stuff and see whether we cannot get it running - how hard can it be

Do you have somewhere dry to keep it/work on it?

Cheers,
Robin

Yup same old problem... car has been tucked up sound in my folk's garage for the last few weeks with barely a visit. I knew once it was out of sight it wouldn't seem quite so important...

I really appreciate the offer of a hand - I'm right on the point of sticking it into a garage and just paying for someone else to troubleshoot it but that feels like admitting defeat

I'll update the other thread this afternoon if I get a chance
Cheers

mckeann · Post by **mckeann** » Fri Dec 29, 2006 2:17 pm

offshorematt wrote: I really appreciate the offer of a hand - I'm right on the point of sticking it into a garage and just paying for someone else to troubleshoot it but that feels like admitting defeat I'll update the other thread this afternoon if I get a chance
Cheers

I know how you fell about giving it to a garage to sort, BUT remember, they are probably going to take just as long as you to sort it and they'll be charging you 45 quid an hour for the priviledge.

Take Robin up on his generous offer, he's quite good at this stuff you know

Stu160 · Post by **Stu160** » Fri Dec 29, 2006 2:34 pm

Any they will just say"eye.........its yur heed gasket mate"

Stu

tut · Post by **tut** » Fri Dec 29, 2006 4:33 pm

Matt

You have AA membership, but I presume not Parts and Labour or you would have used it.

Any reason why you did not take it out, or why you do not take it out now?

tut

ps I have asked this before, but maybe somebody could tell me what I am missing. You have an Elise that is out of warranty, you do a fair mileage in it, you do road runs and possibly track days, yet there are still lots of you out there that do not have the AA parts and labour warranty cover that will cost you £62. For that you get five repairs per year of upto £500 each, and in all my cases so far, paid up for without a quibble. And I have a Honda conversion which I am not even sure that I should be covered for, but it has made no difference.

I would have to get the file out to check, but I think that has been £4,500 of repairs so far. It just seems to me a no brainer, as how many of us have gone several years of ownership without a H/G failure alone?

Stewart · Post by **Stewart** » Fri Dec 29, 2006 6:58 pm

tut wrote: ps I have asked this before, but maybe somebody could tell me what I am missing. You have an Elise that is out of warranty, you do a fair mileage in it, you do road runs and possibly track days, yet there are still lots of you out there that do not have the AA parts and labour warranty cover that will cost you £62. For that you get five repairs per year of upto £500 each, and in all my cases so far, paid up for without a quibble. And I have a Honda conversion which I am not even sure that I should be covered for, but it has made no difference.

I would have to get the file out to check, but I think that has been £4,500 of repairs so far. It just seems to me a no brainer, as how many of us have gone several years of ownership without a H/G failure alone?

Tut
Very valid argument. I have AA Roadside Assistance but no P&L. The reason being that I no longer take the car to a garage to have it serviced and book stamped. I believe that this is a prerequisite for cover? Other than that it appears to be worth the cost.

tut · Post by **tut** » Fri Dec 29, 2006 11:21 pm

Stewart

The conditions are, under 10 years old and 100,000 miles, and it has to meet the service requirements at ANY garage. You just fax your last service invoice to them when you make a claim.

You can use any garage of your choice for the repair, as long as it is VAT registered. All mine have been done by Ricky and John Duthie, they have phoned in the problem with an estimate, and been given the go ahead. Never had an AA engineer around to even check on it.

However you do have to phone in with a breakdown and have an AA van or low loader come out depending on the severity, then have it taken to your garage of choice.

As I said, a no brainer, and if you dont have it, you will kick yourself when you get your next Elise breakdown, as you inevitably will.

tut