Relationship of Theoretical Static and Dynamic Compression Ratios

[PEW]

I have a normally aspirated Reliant 862 cc car engine (petrol/gasoline, spark ignition), remanufactured from one which (unknown to me at that time) already had a CR of 10.5 to 1.

Because the car I built it for is a competition car (it's called a Liege, for on and off-road trials), I wanted the engine uprated during the rebuild. The motor engineering company who did the work (now closed) skimmed the head by 25 thou and later had to skim the block to get the cylinder deck heights correct.

They never told me what the actual CR of the engine was, but it's obviously higher than 10.5 to 1. Because my engine runs very well, other owners, (including the designer of the car) have asked me about the specifications and in particular what CR it runs.

Short of stripping the engine and measuring it properly, which I'm not inclined to do, all I can do is take the cylinder compression pressure readings with my testing gauge.

The cylinder pressures are all a good 225 psi, engine warm and cranked on the starter at abut 300 rpm, with the plugs out and the throttle fully open. That figure comes up after about 3 compression strokes.

I understand the international standard atmosphere (one bar) is about 14.7 psi. Can this figure be used to calculate or even approximate the CR? If so, my engine has an extremely high compression ratio, which is a lot higher than I expected. I am reluctant to think it was any higher than about 12.5 to 1 but using this formula it would seem to be more like 15 to 1.

Or have I got this all around my neck somehow? Can gauge measured compression pressure be reliably used as a guide to actual compression ratio?

I have been trawling the internet for answers without success and having discussed it with other enthusiasts over time I have heard different opinions.

Thank chaps!

Paul W

 

[oab]

No Paul, The compression ratio is a maths calculation. The gauge pressure is the results of a number of things, cam timing, temp rise from air compression, etc.

Measure with oil poured through the plug hole, with piston on TDC, for compressed volume.

Harvey.

 

[ivymike]

do you know what the valve timings are?

 

[PEW]

Thanks for the replies.

Unfortunately it would be difficult to fill the combustion chambers with oil due to the shallow angle of the spark plug hole although it might be possible by cranking the car over at a 45 or 50 degree angle.

I have no idea of the valve timing, like the CR, I don't think it was measured or changed from standard but it certainly wasn't advised to me. The cam is that which was original to the engine, but it was reground for 15 thou more lift.

What occurred to me was that if there was no inertial ramming (low cranking speed, negligable effect) and there was no appreciable effect from charge heating or leakage, then the compression pressure might be a reasonably good guide to the theoretical / mathematically calculated CR.

Perhaps I should just tell people is what the pressure test gauge reading is. I have been advised that a good, standard engine gives about 185 psi. Unfortunately that issue is clouded because Reliant sold the same engine with a variety of CRs, ranging from 7.5 to 1 upwards, supposedly indicated by the colour the rocker cover was painted. In the case of the 10.5 to 1 engines like mine, a yellow top and an "E" included in the engine number. The heads were apparently externally the same, they just shaved them a bit more as required.

 

[ivymike]

if there was no inertial ramming (low cranking speed, negligable effect) and there was no appreciable effect from charge heating or leakage, then the compression pressure might be a reasonably good guide to the theoretical / mathematically calculated CR

well the trouble with that is that you don't know when compression starts. With no "inertial ramming," you won't start compressing the charge until IVC, which almost certainly happens a while into the compression stroke.

Without knowing when IVC happens, you can estimate the compression ratio from IVC to TDC by assuming that polytropic compression of air is a good approximation of what's going on inside your engine.

p2 = p1*(r.c)^k
k = 1.37

r.c = exp(ln(p2/p1)/k)

using p2 = 225 and p1 = 14.7 gives r.c = 7.33

Now the question is "how much bigger was the cylinder at BDC than it was at IVC?"

 

[patprimmer]

You NEED to pull the head and do the measurements to do it right.

Second best is the way Isaac is trying to lead you.

If you put a dial indicator on the inlet valve, and mark the crank pully or flywheel where intake valve just closes, and you mark TDC, you can measure the dia of the crank pully or flywheel, and the distance from the intake valve closeing point to TDC, do a bit of high school math, and work out about how far the piston is up the bore. To do it exact, you need the rod length. If you don't have the rod length, I would guess it at about 1.8 times stroke.

If the flywheel has a ring gear, you can count teeth to do the measurements, and convert # of teeth to degrees

Regards
pat

 

[PEW]

Hi guys,

I know that the mathematical compression ratio must be measured, but the problem is that I DON'T want to lift the head or pull the engine from the car; as it was really only for the benefit of "rubberneckers" who want their engine to work as well as mine seems to do.

Those measurements for the calculations involving a dial gauge and counting teeth on the flywheel etc wouldn't be possible with the engine in situ, because the car is so small (10 feet long in total) that engine access is extremely limited; I certainly can't get anywhere near the ring gear. I even have to remove both the coolant header tank and the distributor to get at the spark plugs. Having spent over 3 years putting the car together and had it running for only a few months, I really don't want to start taking it apart again just yet, hence my posted question.

I think the sensible answer to tell folks for now is that the CR is known to be somewhat "higher than 10.5 to 1". If the head ever has to come off for maintenance then I will measure it all properly.

Presumably though, any errors caused by late inlet valve closing would cause the pressure test gauge to under read rather than over read? Surely the only way that the gauge could over read is by heating caused by the act of compression, or I must have achieved a volumetric efficiency of greater than 100% which isn't likely!

Thanks again!

Paul W

 

[ivymike]

Presumably though, any errors caused by late inlet valve closing would cause the pressure test gauge to under read rather than over read?

Hence the 7.33:1 (calculated) estimate vs 10.5:1 known minimum.

Do you know what the bore is?

 

[PEW]

Ivymike,

The workshop manual quotes the standard bores as 2.46" / 62.50 mm.

But the liners have been rebored to plus 0.020".

Stroke is 2.72" / 69.09 mm.

4 cylinders give the standard capacity as 848cc, so it's now 862cc. (Every little helps with regard to cubic capacity with an engine this small).

 

[ivymike]

Original CR -> 10.5
Original Bore -> 2.46 in
Original Stroke -> 2.72 in

here's what I calculate:
Original TDC volume -> 22.30 cc
Original BDC volume -> 234.15 cc

TDC volume is increased by bore change
TDC volume is reduced by head shaving
Pistons stay the same
TDC2 = TDC1 * Bore2^2/Bore1^2 - (Bore2^2/4 * pi * .025 in)
TDC2 -> 20.685 cc

BDC2 = BDC1 * Bore2^2/Bore1^2 - (Bore2^2/4 * pi * .025 in)
BDC2 -> 235.995 cc

CR2 -> 11.41:1

So the known geometry changes give you a compression ratio change from 10.5:1 to 11.4:1, and the unknowns will likely change it further.

 

[PEW]

Ivymike,

Thanks for your help on this. The figure of 11.4 to 1 sounds to be more or less what I originally thought it might be. If the block was shaved by a similar amount, I guess it would put the figure somewhere around the 12 or 12.5 to 1 mark.

If I ever find out by measuring it, I'll drag up this thread and post the info.

 

[ivymike]

yeah, shaving 0.25 from the block gives 12.51:1 by my estimate.

 

[PEW]

Thanks again everyone.

For anyone interested in taking a look at a Liege, some pics of my car are on the Liege group pages at:

http://groups.msn.com/liegecars/paulwheatleysliege.msnw?albumlist=2

Paul W

 

[Earnwald]

The theoretical static compression ratio is based on the engine geometry. The dynamic compression ratio would be based on the actual amount of air that enters the cylinder at a given RPM. Is there test equipment that is capable of measuring this air volume and expressing it in say CFMs while the engine is running?
ron