In situ measurement of swept volume
In situ measurement of swept volume
(OP)
I have been asked by a colleague who is involved in club-level motor racing, if there is any instrumentation or method of measuring the swept volume of an engine cylinder "in situ", such that it can easily be carried out by a scrutineer, to see if it complies to the regulations without tearing the engine down?
I could only think of using a burette to introduce a benign light oil through a spark-plug hole at TDC until it spills, and then checking the amount added to spill at BDC, however, he wants to be able to check horizontally opposed engines too, so that rules that method out.
Any ideas? Thanks for any suggestions!





RE: In situ measurement of swept volume
Norm
RE: In situ measurement of swept volume
That aside, you could check horizontal engines by tilting them so they are vertical.
RE: In situ measurement of swept volume
Thanks for a good lead Norm, I'd failed to find that one! Unfortunately, its use requires removing a pair of pushrods, which is a step further than the club's scrutineers want to go for fear of upsetting the owners or causing damage (especially disabling valve motion on OHC units)!
hemi,
I agree, this method would only work where the plug hole is below the lowest point of an open valve (unless it's possible to disable the valve, which is not desired as mentioned above). I'd more or less dismissed this idea, because of it's inability to cope with inclined or horizontal cylinders without the use of some kind of rollover fixture for the whole vehicle.
RE: In situ measurement of swept volume
This method assumes minimal blow-by; might help to squirt some thick oil on the cylinder walls beforehand to seal the rings in the same trick people use to check compression leak-down.
RE: In situ measurement of swept volume
Regards
Pat
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RE: In situ measurement of swept volume
Thanks for the additional comments from Pat and TDIMeister.
Pat: I was worried about engine speed and VE when my colleague said that the scrutineers had suggested collecting and measuring the output from the exhaust pipe(s), while the engine was spun over without fuel for a known number of rev's. I told him that it just wasn't that simple but I'd look into it.
The P&G gauge Website says that all plugs are to be removed, so it must only be measured at cranking speed (albeit faster without the plugs), so VE variations will probaly be quite significant, but presumably allowed for in their calibration. The claimed accuracy is one cubic inch per cylinder, but as the range of instruments goes from 500 to 800 cubes, I wonder how well this could be extrapolated for tiddly little European lumps?
TDIMeister: I wasn't sure whether the air injection was to be fitted to a single cylinder so as to administer a known input at the power part of the cycle, or be fitted to all cylinders to "Air Motor" the engine as well. If it was the latter, then I guess that it could be run considerably faster than just cranking on the starter, but at the expense of additional external complexity in the form of a computer controlled valve manifold to deliver correctly timed pulses of air, suitably matched the engine/crank layout.
I really need to find out what volumetric transgressions they're looking for, and that they're not just trying to count "Fairies on a Pin-head" because they think it's easy.
RE: In situ measurement of swept volume
RE: In situ measurement of swept volume
SincoTC: I think the simple answer is that without disabling the valves etc. that it can't be done in any sraightforward way.
RE: In situ measurement of swept volume
I see what you mean, by collecting data of the induction you can determine by the air speed and pressure in the port when the piston is drawing down, when it is stopping and when it starts expelling air back into the intake port as the piston moves up with the valve still open. At constant relatively low speed the error due to elasticity of the air will self correcting as it is opposite at TDC and BDC.
Regards
Pat
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RE: In situ measurement of swept volume
An engine has inertia. At a sufficient RPM of the motored (driven) cylinder, and all the spark plugs pulled out, the engine will keep rotating past BDC.
Many industrial engines are started exactly by a pneumatic blast into the cylinder rather than electrical starters.
RE: In situ measurement of swept volume
And regarding the "complex" air valves, needed for a starter, not to worry; a simple manifold with an air tank & engine driven pump works just fine. An example is the Sukhoi Su-26. There is also a hand pump to service the tank, if needed.
For the old WWII era radials, a shotgun shell,( no pellets, of course ) was inserted into a breech, & fired in a similar manner.
RE: In situ measurement of swept volume
RE: In situ measurement of swept volume
Certainly if air is suddenly applied to the plug hole while the piston is just past TDC on the firing stroke it will spin over very smartly - but, as you would expect, when it comes to the compression stroke it stops and turns in the reverse direction until the piston is at BDC. With the extra pressure in the cylinder (from the airline) the piston will never make it past the compression stroke.
Big industrial and ship diesels etc. have a timed compressed air system to every cylinder for starting.
Some models of the Su-26 have an air starter system - that air was not going into a spark plug hole. It is very possible you could start an engine by kicking it over by air into a plug hole - but how would you get the sparkplug back in after the engine was running?
Cartridge starters were very common to start both piston and jet engines - but the pressure from the cartridge did not go into the engine's cylinder.
The cartridge may have looked like a blank shotgun shell but were purpose-made and usually far bigger than a shotgun shell.
RE: In situ measurement of swept volume
I haven't hashed out the fine details of realising this system, but to my knowledge there isn't something like this that you can go out and buy yet. I should be protecting this idea for a potential patent. :rolleyes: If there are doubters even better for me; I trust the laws of thermodynamics are on my side.
RE: In situ measurement of swept volume
RE: In situ measurement of swept volume
Initially, I referred to a "differential compression test", also known as a "leak down" test. This is an inspection procedure to determine relative health of the recip engine. It is not a starting procedure. It is performed as follows:
And for the purposes of this discussion, I will use an aircraft engine as an example.
1) The spark plugs are removed from the cylinders. ( on aircraft engines, only one plug per cylinder is removed )
2) The piston is brought to TDC, in the subject cylinder.
3) A regulated test Gage set, comprised of two 100 psig calibrated gages in series, separated by a fixed orifice, (.040", .25" in length) is plumbed into the open spark plug hole.
4) And here's important detail; The propeller is restrained from turning.
5) Compressed air is applied to the test rig, and the regulator is adjusted to 80 psig, and the leak rate of the subject cylinder is noted on the second gage ( downstream of the orifice ) thus the name, "differential" compression check
Now, If the piston is not right on TDC, or in the dwell range, the air pressure will drive the piston down it's stroke, turning the propeller with great force. ( I'll leave the calculations to you, assume a 5" bore, and a healthy cylinder, say 75 PSIG ) even through a .040" orifice.
If you read my post I referred to a "poorly executed" compression check, which I will expound on. The technician performing the test, either through poor training, lack of attentiveness, or whatever reason, plumbed the compressed air line directly into the spark plug opening. Thus a full 125 PSI of unregulated air, without the restrictive orifice, was directed to the top of a 5" piston that was just slightly past TDC. The engine motored rapidly until the air was removed, the Magneto impulse couplings firing away. Luckily, the other cylinder's plugs were removed, or it is possible, with a few more "ducks lined up" that the engine would have lit off. Also lucky that someone wasn't killed or seriously injured.
And regarding your assertion that the flywheel effect of a 60 lb propeller would not carry the piston back up the stroke, I leave those calculations to you. But remember, as the piston approaches BDC, the exhaust valve opens, relieving the compressed air somewhat & allowing the crank to coast through.
You are correct in your description of the "shotgun shell" starter system. I stand corrected.
RE: In situ measurement of swept volume
It sounds like that technician (or his assistants) had a very lucky escape as the prop would carry quite a lot of momentum and is not a very "person friendly" device!
However, I'm a little confused when you say that "the engine motored rapidly until the air was removed" and other references to simply blowing reciprocating engines over with a continuous supply of air. I'm with BigVlad in having doubts that it could get past the next compression stroke, as surely it would start out slightly "blown" and then have the incoming air as well as closed valves to overcome. However, as you've seen it and I haven't and a prop is a good flywheel especially without the other plugs in, so I don't doubt that it actually happened!
All the air starters and Cartridge starters that I have seen (Coffman/Breeze), are basically separate devices, similar in concept to an hydraulic swash-plate motor (or latterly, a vane type rotor), where a continuous high pressure medium is converted without valves, into rotary motion to drive the main engine's crankshaft, often through a conventional ring gear. This was considerably ligher and more powerful than an electric starter and their heavy batteries, and with the bonus that the air reservoir could be recharged very quickly, or even by hand pump if necessary. The Cartridge type were fed by a short steel tube from the breech chamber.
Large marine diesels often have compressed air starters where the gas is injected directly into the cylinders, but I think you'll find that every cylinder has it's own accumulator and cam operated valve to give a timed burst to each piston just after TDC on its firing stroke.
I've seen the thread about "Air Motoring" a week or two back, but believe that motor was was of a design that WOULD simply "blow over".
I'm not into gas dynamic theory, but I think TDIMeister should pursue his idea, if it will spin on a single cylinder with a timed pulse (probably with the other plugs removed), after giving it a nudge in the right direction with the starter, and if the P-V diagram will "average out" the affect of the opening valves and give a reasonably accurate indication of swept volume, then it's worth a shot.
I'm of a generation where I would rather try and get the cylinder vertical, disable the valves and burette some incompressible liquid in to measure it. Oh Sh*t; I forgot it's got Dykes pressure-backed rings and I've just filled the sump!! I knew I should have pulled the head and got the calipers out in the first place!
RE: In situ measurement of swept volume
RE: In situ measurement of swept volume
I also wish TDIMeister well with his idea. Just because I don't understand it doesn't mean it doesn't work - it means I don't understand it.
RE: In situ measurement of swept volume
I never understood the tech details around that climactic scene in the 1965 "Flight of the Phoenix" until now. Much more effective than the equivalent in the 2004 remake. But I guess Jimmy Stewart had more experience than Dennis Quaid doing scenes where he was running out of bullets. Amazing the tidbits one picks up on this board . . .
RE: In situ measurement of swept volume
RE: In situ measurement of swept volume
That's right, he flew B-24 Liberator bombers out of the UK during the War, nice luxurious plane by comparison to others of the time, electic nearly everything, including engine starting.
Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK
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RE: In situ measurement of swept volume
RE: In situ measurement of swept volume
I hate to disagree but if you have ever tried to run a leak down test on a motor which is basically what you are describing here it will not work. All of the engines I have tested all stop at BDC and do not continuously spin as you suggest. Have you ever tried and substantiated this on a automotive performance engine?
Larry
Larry Coyle
Managing Partner
Cylinder Head Engineering, LLC
De Soto, KS 66018
RE: In situ measurement of swept volume
Back to the original subject, I think that with appropriate tools it should be possible to simply measure the bore and stroke through the spark plug hole.
RE: In situ measurement of swept volume
Hemi,
I like the plastic idea, but you'd still have to close off the valves at BDC. Also, if a matreial with the necessary properties already existed I'm sure Disney would have made a movie about it. It would be great stuff though!
Compositepro,
Depending on the position and orientation of the plug hole, measuring the stroke could often be achieved relatively easily. However, I feel that measuring the bore that way would be rather like a "ship in the bottle" trick, unless someone out there knows of some clever little ultrasonic or laser device that could do it?
RE: In situ measurement of swept volume
RE: In situ measurement of swept volume
Regards
Pat
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RE: In situ measurement of swept volume
RE: In situ measurement of swept volume
The gas dispalcement type tests are clever but would indeed be made inaccurate by any leaks of the piston rings ect. They would also require everything to be at stable temperatures because any heat transfer to and from the gas would influence the pressures.
I have heard of all kinds of imaginative ways to cheat on such tests like having one cylinder smaller than the others. of course the side effects might be more trouble than they are worth....
RE: In situ measurement of swept volume