COMPRESSION RATIOS

Can anyone supply me with a formula for calculating compression pressure from bore/stroke/compression ratio for internal combustion engines?

Chas

Bore and stroke make no difference to the relationship between compression ratio and compression pressure. Basically multiply atmospheric pressure of nominal 15psi by the compression ratio to get compression pressure. So 10 to 1 will have a theoretical compression pressure of 150psi and so on.

But in practice compression is affected by the intake valve closing after BDC. The longer the valve stays open on the upstroke the lower the effective compression ratio and pressure.

The intake valve stays open on the upstroke for some degrees to maximise cylinder filling from the momentum of air fuel mix charging down the long intake tract at a high rate of knots. 

Of course if you increase bore or stroke with same size combustion chamber the compression ratio will increase and so will the pressure as per above formula.

Edited By Hopper on 29/08/2020 02:47:07

Edited By Hopper on 29/08/2020 02:50:51

Pi *D^2*h/4 will get you the displacement volume, where Pi = 3.1416. D = diameter of bore, h = stroke and ^2 denotes a square of the number. You will need to know the compressed volume to work out the compression ratio or (theoretical) pressure

Ratio will be combustion volume/ (displacement+ combustion volume). Pick you own units for psi or Bar.

Edited By not done it yet on 29/08/2020 09:29:46

C R = (Clearance Volume + Swept Volume ) / Clearance Volume. Being a ratio, it has no dimensions.

But Compression Pressures will be determined by a variety of factors, such as valve timing, valve sizes, porting, manifolding, cranking speed, and leakage (past piston and rings, and possibly valves )

Don't expect to see a high pressure from a stationary engine with a low compression ratio and "automatic" inlet valves.

Higher cranking speeds may also add to compression pressures because of the adiabatic heating.

Compression ignition engines only work because of adiabatic heating of the cylinder contents coupled with a high compression ratio.

So calculating compression pressures is difficult, to say the least.

The original VW Beetle engine was virtually unburstable, because it was restricted by the inlet manifolding.

As the swept volume was increased, and the valve sizes, it became less reliable. To the point where folk recommended changing the valves at 30 000 miles to prevent them dropping into the cylinders

A highly tuned 4 stroke, because of its more extreme valve timing will show a lower compression pressure than an engine with a more "pedestrian" state of tune, at the same cranking speed.

An engine with poorly seating valves, or leaky rings will show a much higher pressure once those faults have been corrected.

Leakage is a function of pressure and time. An engine with leaky valves may be difficult to start, because at cranking the speeds the time for leakage is greater, compared to when the engine is running. Under those circumstances the engine will perform better (but not as well as it should ) at the higher the speeds, because the time for leakage is less.

You may be able to take a guess, based on multlplying Atmospheric pressure by the compression ratio, but it will be an estimate, not a certainty, because of all the other factors involved

Howard

As Howard says, the compression pressure in use (not calculated) depends on the heating of the mixture by the compression process (less the cooling of the fins, water jacket, etc). But there are two other factors that complicate the issue. First the valve timing, which is not designed to be 'right' at cranking speed (when you measure the pressure with a gauge). It relies on the effects of inertia and resonance in the inlet and exhaust systems, to fill the cylinder properly at running speeds, so the actual running pressure is generally higher than what you see on your gauge. It also depends on how wide the throttle opening is. And these resonances are greatly affected by the combustion itself, so the running pressure depends to some extent on the size of the most recent bang, and this too depends on the throttle opening. Finally, measuring with a gauge in a running engine is not easy, as both the spark and the gauge rely on the same plug-hole.

Books have been written on this topic. Look for Ricardo as a reliable author.

Regards, Tim

I once bought very cheaply a 1300cc Beetle which had dropped its front left exhaust valve. The cooling to the front left cylinder was partially blocked by the oil cooler, so this was the usual culprit. It had done 130,000 miles, but the bottom end (or is it the middle on a Beetle engine) was sound so I rebuilt it with a second hand left cylinder, new exhaust valves, and one new piston. Unfortunately some time later the drive to the oil pump gave up when SWMBO was driving and when it started to lose power she put her foot down to get off the motorway. That didn't do it a lot of good!

Is that 2 stroke or 4 stroke Charles ?

Emgee

Twostroke engines can be much more complex than fourstroke - and working out the compression pressure in advance is almost hopeless. Do you count the stroke as BDC to TDC or from the top of the exhaust port to TDC? And what do you do about the slug of gas that went down the exhaust pipe, was bounced back into the cylinder by sound waves, and did not have time to get out again (or did, depending on the engine revs) as the piston rose?

Cheers, Tim

Hi Tim

You summed up exactly why I asked the question.

If working out swept volume I guess it will be the norm, BDC to TDC.

Emgee

Edited By Emgee on 29/08/2020 22:08:15

Many thanks to all the people who replied. I was sure someone on this forum would know....and voila!

I never realised how many factors came into this issue and it was very interesting to learn about the matter in some depth.

I fell into the usual error of people seeking advice on the forum of not providing enough information. I am a focal point for people seeking information (historical, low technical, identification) on New Imperial motorcycles which were made in Birmingham before the war. In this case it was an enquiry about what the compression pressure should be, measured cold, on a 1931 500cc side valve machine.

Chas

My best guess 120 PSI but open to offers.

Emgee

Would you take 110 psi for it 😀

If you want the pressure, measure the combustion chamber volume (glass sheet with small hole to fill through, from a burette). You can then use that volume and Boyle’s Law to calculate the pressure.

Boyle’s Law means that PV = constant. Where P = pressure and V = volume - all under isothermal conditions.

It will not exceed that theoretical value, ever, but may well be less.🙂

Remember that fuel before the war (WWii) was nowhere near as good (regarding its resistance to pinking etc) as modern stuff. Not only that, low compression engines were favoured for 'ordinary, non-racer' machines as they would slog without needing gear changes. So a ratio of between 6:1 and 7:1 would be a maximum. Unless the head was aluminium, which I bet it wasn't, not on a side valve.

And not-done-it-yet's method is only accurate if the piston is flat-topped, rises exactly to the height of the barrel, and the head is flat across.

Regards, Tim
Ex Norton, ex BSA, ex Hesketh

Redex used to market these for checking compression through the spark plug hole. Most useful on multi cylinder engines to check for burnt valves or broken rings.

Bob D

Charles,

I see you have created a new account - would you like me to email you a new password for the old one?

Send me a PM to confirm.

Neil

And not-done-it-yet's method is only accurate if the piston is flat-topped, rises exactly to the height of the barrel, and the head is flat across.

Quite correct, Tim. Sorry, I overlooked that - likely because I very much doubt that any 500cc side valve engine of that era would have anything but a flat topped piston (not that the engine capacity would make any difference).🙂 Can you supply any examples of any 500cc motorcycle side valve engine of that era with either domed or dished piston(s)? (Mustn't ignore the unlikely option of a multi-cylinder machine.🙂 )

One of my pre-WWll engines (pre-1930 design) has a quoted compression ratio of 4.1:1. That is actually an OHV engine. I might expect some side valve engines to be even lower than that.

Of course, my measurement would not have taken into account the head gasket thickness, either. Not that it would have made a great deal of difference to the result (apart from lowering it, as I said in my post).

The other factor is if the top of the piston comes all the way yo the top of the bore. Or if like some Harley side valve racers it sticks up past the top of the cylinder so it fills part of the combustion chamber space to raise compression while allowing the larger combustion chamber to flow more gas in and out of the cylinder as the piston moved out of the way. Harley got their 750 sidevalves doing 149.9 mph like that -- faster thsn any Manx Norton has ever gone!

But 1930s road bike side valves were more likely 6 to 1 or even much less. 5 to 1 was common on the iron head models or even less. So 5 to 1 would read 75psi on a gauge if all sealing etc was perfect which it never is. Anything over 60 on an old banger like that would be doing well.

Important thing when using a compression pressure gauge is to hold throttle and choke wide open otherwise the restriction causes a false low reading. And kick it over half a dozen times in rapid succession, not just once.

Wow! I had no idea that my simple request would arouse so much correspondence, let alone such interesting stuff. Hopper is right on the money with his comments and I would expect to find a CR of about 5:1 on an engine like this, maybe a bit less.

All the side valve engines I have encountered have flat-top pistons but I'm not familiar with American motorcycles (or any motorcycle made after 1939 for that matter!). My vintage and veteran bikes all get regular airing, or did before covid struck and we were placed on "house arrest" in Victoria, for the duration.

For the duration - now that's a phrase that will resonate with our older UK members!

When I were a lad in the late 40s, I was told that racing engines of that era (Type 159 Alfa Romeos etc ) were high compression at 6:1!.

The 803 cc BMC A series OHV engine which came about in the early 50s was 7.2:1. I think that the 1172 cc sidevalve Ford E93 and 100E engines were 6:1 at that time.

So, for a 1931 side valve road bike, my GUESS would be in the 4 or 5:1 region, so about 60 -70 psi on a wide open throttle and hearty kicks would seem likely. Having taken one or more readings, squirting some oil in through the spark plug hole, may improve sealing slightly, after a while, and improve the reading slightly..

Fuel would probably not have started to be dosed with Tetra Ethyl Lead to improve Octane ratings,in those days, so compression ratios would be limited by detonation.

Howard