CR to cylinder PSI. Relationship?

[Ian Vincent]

So I have just carried out a copmression test on my engine with readings done hot and cold.

Cold
#1 = 170psi
#2 = 175psi
#3 = 160psi
#4 = 170psi

Hot
#1 = 170psi
#2 = 165psi
#3 = 160psi
#4 = 165psi

So if anything my pressures went down slightly when the engine was hot.  I checked the (relatively) low figure on #3 a couple of times and it was a consistent reading within the limits of my gauge which you can only read to about 5psi.  If I compare these numbers with the ones Peter quoted above, mine are bang in the right range for my calculated static CR of 9:1 and an effective CR of 6.04:1, (whatever that means).

I am happy that my figures are pretty consistent across all the cylinders and in line with what one would expect from a fairly standard engine, (albeit with 87mm pistons and a combustion chamber that has been opened up in similar fashion to a TR4a).

Rgds Ian

[Peter Cobbold]

I'll try to explain again. The heat of compression in an engine with geometric comp rat of 10:1 will, in the absence of heat losses ( ie adiabatic) and without late inlet valve closing raise the air temperature and that in turn raises the pressure. The wiiki page shows 10:1 comp rat will generate 25:1 pressure ratio , whcih is 25 x14.7 =  367 psi

This is never seen with a comp gauge because the cranking is so slow that the heat has dissipated into the bore and pitson and head.

How hot would the air become if that heat loss did not occur? Ideal Gas Law gives p1 V1 / T1 = P2 V2 /T2

Using a omline calculator https://www.omnicalculator.com/physics/thermodynamic-processes#isobaric-process

 

T2 is 684 K ( ie absolute scale) which is 684-273 or 411 C (given adiabatic and 10:1 compression )

Of course we know the temperature msut rise, it is the basis of ignition in diesel engines - ensured by their ca 20:1 comp rat.

In a cranked TR engine the temperature rise will dissipate, to well below 411 C , and the repoerted psi will drop, as it must.

It follows that as we do not know the temp of the air at the time the gauge peaked we cannot measure the compression ratio with even rough precision.

Peter

[Peter Cobbold]

1 minute ago, Peter Cobbold said:

I'll try to explain again. The heat of compression in an engine with geometric comp rat of 10:1 will, in the absence of heat losses ( ie adiabatic) and without late inlet valve closing raise the air temperature and that in turn raises the pressure. The wiiki page shows 10:1 comp rat will generate 25:1 pressure ratio , whcih is 25 x14.7 =  367 psi

This is never seen with a comp gauge because the cranking is so slow that the heat has dissipated into the bore and pitson and head.

How hot would the air become if that heat loss did not occur? Ideal Gas Law gives p1 V1 / T1 = P2 V2 /T2

Using a omline calculator https://www.omnicalculator.com/physics/thermodynamic-processes#isobaric-process

 

T2 is 684 K ( ie absolute scale) which is 684-273 or 411 C (given adiabatic and 10:1 compression )

Of course we know the temperature msut rise, it is the basis of ignition in diesel engines - ensured by their ca 20:1 comp rat.

In a cranked TR enginle the temperature rise will dissipate, to well below 411 C , and the repoerted psi will drop, as it must.

It follows that as we do not know the temp of the air at the time the gauge peaked we cannot measure the compression ratio with even rough precision.

Peter

small error: initial temp should be 293 K ( ie 20C).   Might beinteresting to re-run calc with initial temp as water jacket of hot engine, say 80C   ( = 80+273 K)

P

[Lebro]

Ian & Pete.

I would be interested to know the details of your CR calcs, i.e.   piston deck figure to liner top, combustion chamber CC, & estimated (or measured) compressed gasket thickness.  As Ian's setup seems to be very similar to mine, but I get 200 + PSI there must be a good reason why so much difference

Bob.

[Ian Vincent]

49 minutes ago, Lebro said:

Ian & Pete.

I would be interested to know the details of your CR calcs, i.e.   piston deck figure to liner top, combustion chamber CC, & estimated (or measured) compressed gasket thickness.  As Ian's setup seems to be very similar to mine, but I get 200 + PSI there must be a good reason why so much difference

Bob.

The answer is very simple Bob, my head has only ever been skimmed to clean it up (total about 30 thou) and my combustion chambers are all within a gnat's c**k of 63.5ml. (measured with a burette and a piece of glass with a hole drilled in it)

Rgds Ian

Edited May 5, 2021 by Ian Vincent

[BlueTR3A-5EKT]

1 hour ago, Lebro said:

Ian & Pete.

I would be interested to know the details of your CR calcs, i.e.   piston deck figure to liner top, combustion chamber CC, & estimated (or measured) compressed gasket thickness.  As Ian's setup seems to be very similar to mine, but I get 200 + PSI there must be a good reason why so much difference

Bob.

Here are the figures I measured when I built the engine back in 2006.

Regular PowerMax and a steel shim head gasket.  Chambers were 55cc measured with a syringe. 
Compression check gives 180-185 psi.

 

Edited May 5, 2021 by BlueTR3A-5EKT

[Dic Doretti]

I have just checked the compressions of my newly rebuilt engine which clocked up 500 miles on Drive It Day. They were consistent at 180psi which I thought was a bit high so I checked using a second gauge and they dropped to 160psi. So much depends on the quality of your gauge.

Cheers Richard

[Lebro]

Thanks guys.  my readings will remain a mystery then !

Bob.

[ianc]

 In Newsletter 13 (Spring 1973), I published an article on the subject of Compression Ratio for the 4-pot, wet-liner engine.  It was published in the Technicalities booklet (1976) and later in Section P2 of the Technicalities CD. 

Very recently, I have been persuaded to revise and update the article because engines have become larger (up to 89mm now) and there's a greater variety of gaskets available.  I shall be submitting the revised version to TR Action in the near future, and Noel Revlingbone (go on, work it out!) liked it and has put it on his website for the benefit of his customers.

Meanwhile, I took Bob's figures for his engine and checked them against my Excel sheet - I agreed with his calculation to within about 0.1 (I obtained 9.176, but I don't know how far his liners sit above the top of the block).

Ian Cornish

[Lebro]

Ian. These are the liner protrusions I ended up with in thou:

     4½      4½            3½    3½         4½   4½        4     4½

     4         4½            3½    3½         3½   4½        4     3½        front

But I don't actually see how that makes any difference to the CR as it's piston top to liner top gap that matters.

Bob.

 

Edited May 6, 2021 by Lebro

[Peter Cobbold]

The cam will make a difference as the later iv closing occurs the less air is trapped and the lower the psi recorded.

It is nonsens to think that ther is any reproducibilty in determining geometric compreesion ratio by cranking the engine. Heat loss and thence temp fall and pressure drop ruin any precision because they are time-dependent and so affected by cranking speed, and hence battery volta, starter motor, friction is rings and beairngs etc.

Heat of compression is not a minor consideration, it is fundamental to ic engine operation. The only way to eliminate it would be to turn the crank slowly so heat lost to walls and piston goes to near-completion. This would be isothermal compression. The problme here is more air is lost past the rings. And the problem of late ivc closing also arises, but can be calculated if known. However heat is the biggest factor and ring losses can be checked (-/+ oil) and ivc loss calculated. That will be closest to correct.

There is no get-out, far too many variables to even guesstimate geometric compression ratio by spinning the engine on the starter.

Peter

[Ian Vincent]

2 hours ago, Lebro said:

Ian. These are the liner protrusions I ended up with in thou:

     4½      4½            3½    3½         4½   4½        4     4½

     4         4½            3½    3½         3½   4½        4     3½        front

But I don't actually see how that makes any difference to the CR as it's piston top to liner top gap that matters.

Bob.

 

My guess would be that if the thickness of your gasket has been measured by inserting feeler gauges into the gap left between the block and the head this won't necessarily reflect the thickness at the sealing point on top of the cylinder liner.  This will be affected by the amount your liners protrude above the block.

Rgds Ian

[stillp]

13 minutes ago, Peter Cobbold said:

The cam will make a difference as the later iv closing occurs the less air is trapped and the lower the psi recorded.

It is nonsens to think that ther is any reproducibilty in determining geometric compreesion ratio by cranking the engine. Heat loss and thence temp fall and pressure drop ruin any precision because they are time-dependent and so affected by cranking speed, and hence battery volta, starter motor, friction is rings and beairngs etc.

Heat of compression is not a minor consideration, it is fundamental to ic engine operation. The only way to eliminate it would be to turn the crank slowly so heat lost to walls and piston goes to near-completion. This would be isothermal compression. The problme here is more air is lost past the rings. And the problem of late ivc closing also arises, but can be calculated if known. However heat is the biggest factor and ring losses can be checked (-/+ oil) and ivc loss calculated. That will be closest to correct.

There is no get-out, far too many variables to even guesstimate geometric compression ratio by spinning the engine on the starter.

Peter

+1, and also the inaccuracy of the compression tester needs to be considered; I doubt if many are better than + or - 20%.

Pete

[Peter Cobbold]

9 minutes ago, stillp said:

+1, and also the inaccuracy of the compression tester needs to be considered; I doubt if many are better than + or - 20%.

Pete

For the really well-heeled there is an accurate way to record pressures in a running engine !

https://www.kistler.com/files/document/003-280e.pdf?callee=frontend

it's response is fast enough to detect knock too.

Every rr should have one.

Peter

[Peter Cobbold]

21 hours ago, Ian Vincent said:

So I have just carried out a copmression test on my engine with readings done hot and cold.

Cold
#1 = 170psi
#2 = 175psi
#3 = 160psi
#4 = 170psi

Hot
#1 = 170psi
#2 = 165psi
#3 = 160psi
#4 = 165psi

So if anything my pressures went down slightly when the engine was hot.  I checked the (relatively) low figure on #3 a couple of times and it was a consistent reading within the limits of my gauge which you can only read to about 5psi.  If I compare these numbers with the ones Peter quoted above, mine are bang in the right range for my calculated static CR of 9:1 and an effective CR of 6.04:1, (whatever that means).

I am happy that my figures are pretty consistent across all the cylinders and in line with what one would expect from a fairly standard engine, (albeit with 87mm pistons and a combustion chamber that has been opened up in similar fashion to a TR4a).

Rgds Ian

Interesting data, Ian.  The lack of higher readings from hot engine would be expected from  heating and expanding the air on the intake stroke, at atmos prssure. With less gas to compress the temp rise from compression will be less and a lower reading might be expected. However the bore walls etcare hot so the air temp will fall slower, tending to  raise the reading.  When hot the ring gap will close reducing leakage.  I guess the hotter induction won out , marginally over the other factors. Peter

[Lebro]

29 minutes ago, Ian Vincent said:

My guess would be that if the thickness of your gasket has been measured by inserting feeler gauges into the gap left between the block and the head this won't necessarily reflect the thickness at the sealing point on top of the cylinder liner.  This will be affected by the amount your liners protrude above the block.

Rgds Ian

Ian.

I have not measured the gasket actually used, but measured the "fire ring" thickness of the gasket previously removed. the gaskets were the same spec from the same supplier (with in the same year) so I think it's reasonable to assume that the new gasket would deform by the same amount as the previous one, the liner protrusions before & after were only 1 thou different.

Bob.

[ianc]

Bob - if your gasket is, for example, solid copper, it will deform little and will be sitting on top of the liners and its thickness forms part of the calculation. 

If it deforms considerably when the head is torqued down, there may be just a rather thin piece sitting atop the liners, in which case one can consider the head as sitting directly on the top of the liners.

Makes the calculations somewhat more complicated, I'm afraid!

Ian Cornish

[spyder dryver]

For anyone interested here is the compression calculator that I used on my last build. It takes account of pretty much every variable and calculates the "dynamic" compression ratio. 

http://www.not2fast.com/turbo/compression/compression.shtml

 

[Motorsport Mickey]

Now you’ve done it Geoff, so many different imponderables... so few rebuilds to try them out.

Mick Richards

[Peter Cobbold]

Tks,interesting calculator.   IVC 50 deg ATDC dropped dynamic CR one ratio.

[Lebro]

Got 9.3:1 for my details using above calculator.

Bob.