Does anyone have any nifty ways of measuring the true volume of an engine's cylinder? - Page 2 - PeachParts Mercedes-Benz Forum

leathermang · #16 01-31-2011, 12:23 PM

Quote:

Originally Posted by W124 E300D

In a practical sense, we are talking about meaningless levels of accuracy here.

All in cc

Nominal standard swept volume is 2,998.20 to 2,998.85

Nominal unswept volume is 149.91 to 149.94

Nominal rebored swept volume is 3037.9

The difference between standard and rebore swept volume is around 1.3%

The range of standard swept volume min/max is 1/50th of 1%

The range of unswept volume is 1/50th of 1%

The actual variations in (air) charge in each cylinder are going to be more affected by things like inlet tract differences due to the design of the valve gear, cylinder head, inlet manifold, air filter, etc.

Diesel engines run "excess air" 99,999% of the time, so the crucial factors here are fuel metering and ACTUAL compression ratio.

ACTUAL compression ratio is going to vary widely from cylinder to cylinder, it is affected by everything from ring sealing through valve sealing to carbon build up and inlet restrictions and exhaust restrictions.

Fuel metering doesn't just mean accuracy of fuel metering, unless that correct volume of metered fuel is correctly atomised then it is the same as having the wrong amount, and in a diesel engine this means your actual cylinder compression, injection timing and injection spray pattern are key.

If you have a nominal 350 psi compression (22:1) and allow 15 psi variation between cylinders (assuming your compression tester is 100% leak free) then you are accepting anything between 335 and 350, a variation of around 4%, which means a variation in compression temperature, which means large variations in cylinder efficiency, torque loading and exhaust gas temperatures and therefore efficiency.

So, don't sweat the minute differences your rebore will make, or the minute variations in unswept volume.

==============================

That is all true....
He knows that, we know that.. he is just dealing with cabin fever... if you see the other thread you will understand that..

leathermang · #17 01-31-2011, 12:26 PM

Quote:

Originally Posted by Cr from Texas

The one time I watched, the engine was on a stand and pretty much assembled. He titrated a cylinder with the piston at bottom and rotated the engine so the spark plug hole was the high point. The titration to full gave him the total displacement of the entire volume (including gaskets). I assume he may have had some concerns about rust if he used water.

That method would be a really good reason to use oil instead of water....LOL
Assuming I guess that you could do it fast enough that none was lost past the rings at the bottom ...

leathermang · #18 01-31-2011, 12:30 PM

Quote:

Originally Posted by layback40

Now Army, dont let all the extra power from the higher compression & capacity get out of hand !!!

Ok.. since we had a some physics stuff in the other thread... and I do know you are kidding.. ( ignore the amount )...
Others should remember that increasing the compression ratio does not increase power...
UNLESS
More fuel is added... and may decrease available power if the heat from compression was already sufficient to completely burn the fuel in the proper time span... you would be talking MORE load ( ... and without added fuel.. less power NET ).....

Stretch · #19 01-31-2011, 12:51 PM

Quote:

Originally Posted by leathermang

Ok.. since we had a some physics stuff in the other thread... and I do know you are kidding.. ( ignore the amount )...
Others should remember that increasing the compression ratio does not increase power...
UNLESS
More fuel is added... and may decrease available power if the heat from compression was already sufficient to completely burn the fuel in the proper time span... you would be talking MORE load ( ... and without added fuel.. less power NET ).....

So we are almost going in a full circle then? Are we going back to setting timing again?

Back to the old thread:-

Timing adjustment methods

Stretch · #20 01-31-2011, 12:54 PM

Quote:

Originally Posted by W124 E300D

In a practical sense, we are talking about meaningless levels of accuracy here.

...

==============================

Old diesel hands like me look for the following, when determining a good diesel engine;

It starts "on the button" from cold.
It settles down to a good idle within 15 seconds.
Cold start rattle/knock disappears within 30 seconds.
Engine doesn't shake and has no leaks or drips.
Engine develops decent (eg blowing relief valve) oil pressure on cold start tick-over with cold oil.

That's it, with #1 being by far the biggest "tell", there isn't a lot wrong with any diesel that cold starts on the button, nothing that can't be easily fixed anyway. Compression, spray patterns, everything will be revealed by the cold start.

If it doesn't do this, then no matter how quick, easy and cheap the repairs required are, treat it as a lump of scrap metal worth 5 bucks tops.

Thank you very much for adding this information - I was being sarcastic about the "benefits" of my re-bore. But none the less I think that your comments are a very worthwhile addition - thanks again.

Stretch · #21 01-31-2011, 01:26 PM

I think I have probably re-invented the wheel - meaning I guess this has been done before. Can anyone identify it?

Objective:- to measure the unswept volume of a piston engine without taking it apart.

It occurred to me that if you could use a large volume - much larger than the swept volume of a cylinder - to hold a compressed gas (I'll start with air) you could use an equation of state to estimate an equivalent volume in the following way.

You close off valve B and fill a volume of known size with compressed air.

You can't use too high a pressure as when you transfer this to the cylinder in the engine, too high a pressure could turn the crank. So fill it say with compressed air to about 15 / 20 PSI.

Close valve A and allow the temperature in the known volume to settle. This is important it must remain constant - the pressure also needs to stay constant too!

Remove a glow plug in the engine and turn the crank until the piston for that cylinder is at TDC. This can be checked by dropping a valve on top of a piston crown and measuring the deflection with a DTI (Described a little bit more here:- Timing marks gone from my OM617 engine). Connect the pipe running from valve B to the glow plug hole.

Very slowly open valve B so that the compressed air in the known volume goes into the cylinder. Keep an eye on the temperature in the known volume to make sure that doesn't change. If you do it too fast the volume will cool (expanding gas).

Measure and record the temperature of the pressure on some sort of recording system (yeah I know this is a big problem for the average DIY home mechanic!).

I would expect to get something that would look like this:-

If the flow of air going into the unswept volume of the cylinder is higher than the flow out of the clylinder due to leakage (then your engine is still quite healthy!) you could then "simply" calculate the true pressure drop within the known volume that is caused by the displacement of gas to the unswept volume of the cylinder. You'd need to assume that the cylinder leaked at a constant rate as recorded in the last part of the plot (see above) and subtract this pressure from the initial pressure drop.

Taking this measured pressure drop you could do some high school mathematics with an equation of state (http://en.wikipedia.org/wiki/Equation_of_state#Peng-Robinson_equation_of_state for example) and calculate an equivalent change in volume for the change in pressure.

I think that should be equal to the unswept in the cylinder.

As leathermang said I do have a serious case of cabin fever!

leathermang · #22 01-31-2011, 02:36 PM

On the other hand.... this is as good a use of winter down time I have ever seen...
When warm weather hits... you should be armed with dozens of techniques and possibly tools for measuring all sorts of physical phenomenon...... whether they need to be measured or not....LOL

leathermang · #23 01-31-2011, 02:43 PM

Army, How do YOU define unswept volume ?... and why are you interested in it ?
I am having trouble visualizing unswept except with regards to that volume INSIDE the precombustion chamber.. which would not be swept but only pressure compressed as the piston drives upwards.... but once the fuel has been ejected... I suspect due to the very small size of the only connections to the bore ( the fuel spray holes ) it is not in any meaningful way ' swept'..
But what difference does the amount of swept area make ?
If it does exist at all.. it is certainly tiny in a diesel engine compared to gas engines due to the high compression ratio....

Stretch · #24 02-01-2011, 03:29 AM

Quote:

Originally Posted by leathermang

Army, How do YOU define unswept volume ?... and why are you interested in it ?
I am having trouble visualizing unswept except with regards to that volume INSIDE the precombustion chamber.. which would not be swept but only pressure compressed as the piston drives upwards.... but once the fuel has been ejected... I suspect due to the very small size of the only connections to the bore ( the fuel spray holes ) it is not in any meaningful way ' swept'..
But what difference does the amount of swept area make ?
If it does exist at all.. it is certainly tiny in a diesel engine compared to gas engines due to the high compression ratio....

Sorry - perhaps I'm not being clear again. "Sweeping" refers to the motion of the piston. So the swept volume is the volume that varies with the motion of the piston. The unswept volume is the bit at the top of the cylinder where the piston does not travel.

I've measured the clearance between the top of the piston and the head on my OM617 to be about 2mm (see OM617 (non turbo) cam profile specs, piston height specs etc). It seems to me that the head gasket, which is 2.2mm in an uncompressed state, does not get compressed very much when you fit the head. I'm surprised - but I also get the feeling that if I had made better measurements of how far the valves are recessed into the head then perhaps I'd be able to have a better idea of this.

Anyway like you say the size of the unswept volume in a diesel engine is tiny. But you've also brought up another interesting problem that has been bothering me - the influence of the pre-chambers.

I'm guessing that the pre-chambers contribute greatly to the OM617's clackety sound. I think they act as resonant chambers. But they also have some effect on the compression in the cylinder (they must do) but I don't think it would be to the extent that one may assume. I don't expect at normal engine speeds there is much "flow" (dodgy term I know) from the cylinder into the pre-chamber. I think that the friction of the gas in those little holes acts more like a sealed surface than an "open" way into the pre-chamber. I have a gut feeling that you get more "flow" (dodgy term again!) of the fuel mixture going into the cylinder than you get gas returning from the cylinder.

It would be interesting to find out if Mercedes considered the pre-chamber volume to be part of the unswept volume or not.

Stretch · #25 02-01-2011, 03:34 AM

Quote:

Originally Posted by leathermang

On the other hand.... this is as good a use of winter down time I have ever seen...
When warm weather hits... you should be armed with dozens of techniques and possibly tools for measuring all sorts of physical phenomenon...... whether they need to be measured or not....LOL

Now we're getting there! If you were to join the mod squad (modification squad) wouldn't you rather know about how your engine worked and what to do with it - rather than relying on fashion and BS?

At this stage I don't know what is worth measuring or not - but I intend to find out!

leathermang · #26 02-01-2011, 09:50 AM

Quote:

Originally Posted by Army

Anyway like you say the size of the unswept volume in a diesel engine is tiny. But you've also brought up another interesting problem that has been bothering me - the influence of the pre-chambers....
I don't expect at normal engine speeds there is much "flow" (dodgy term I know) from the cylinder into the pre-chamber.

... I think that the friction of the gas in those little holes acts more like a sealed surface than an "open" way into the pre-chamber. I have a gut feeling that you get more "flow" (dodgy term again!) of the fuel mixture going into the cylinder than you get gas returning from the cylinder....

I absolutely agree with these concepts... and have been trying to express that concept in the discussion about trying to measure the temperature by using the glow plugs which are IN the precombustion chamber.. ( and isolated by )...
Think about the two relative forces trying to act getting through those tiny radial holes... one pressurized fluid ( almost no compression happening there although fluids can compress close to one percent ... we use the phrase ' non compressibility of fluids loosely when describing brake systems since it has no visible effect )... and the other AIR... easily compressible... so the difference between shooting stuff through those holes.. fluid one direction and air the other... I see almost no air flow given that it is a closed small chamber and the other three strokes of the piston will not be trying to put air into the precombustion chamber at all...Or any way to sweep it out given that it is a closed system with regards to air .... even the other piston upstroke the exhaust valve is open...and air is a liquid in that is flows to the least resistance...

leathermang · #27 02-01-2011, 09:56 AM

Quote:

Originally Posted by Cr from Texas

The one time I watched, the engine was on a stand and pretty much assembled. He titrated a cylinder with the piston at bottom and rotated the engine so the spark plug hole was the high point. The titration to full gave him the total displacement of the entire volume (including gaskets). I assume he may have had some concerns about rust if he used water.

I read this and accepted it as being an accurate way to measure that cubic volume...
Then it occurred to me that with the engine all bolted and sealed up... it would seem like the wrong time to do it....
Usually this fine a measurement of that head dome volume would be FOR the purpose of making all cylinders have the same compression ratio... so differences could be adjusted right then with a grinder or EDM... but to check it with it all buttoned up just seems futile.... LOLOL
And it does introduce that ' leaking from the bottom' concept into an otherwise very accurate way to find the total capacity of the combustion chamber...

Stretch · #28 02-01-2011, 01:04 PM

Yeah I guess it is safer to try to measure these volumes when the engine is in bits - I see nobody has suggested using a balloon; stuffing it into the volume and filling it with oil!

Slightly different subject:-

I was thinking about petrol engines the other day, where lots of old straight six engines (in Europe) used to be modified with the addition of a single carburettor for each cylinder instead of using just one large one for the whole engine. Ford Zodiacs (European model http://en.wikipedia.org/wiki/Ford_Zodiac#Mark_I) were famous for this - you got more power and better economy in return for the nightmare of balancing 6 carburettors!

My OM617 is a little like this in the way that everything is set up for the ideal single situation - perhaps a system that was tuned to each cylinder size would yield benefits?

W124 E300D · #29 02-01-2011, 01:22 PM

Quote:

Originally Posted by Army

I was thinking about petrol engines the other day, where lots of old straight six engines (in Europe) used to be modified with the addition of a single carburettor for each cylinder instead of using just one large one for the whole engine.

I used to make quite a bit of money converting the Z1000 motors to run off a single SU carb.... and then flogging the Z1000 carbs to people tuning mini's... lol

moon161 · #30 02-01-2011, 02:17 PM

20 Years ago, my dad invented a tester for re-perfusion of tissue to measure vascular damage- it would squeeze fluid out of say, your finger and relax the pressure and measure in increase in volume of your finger over time. Resolution was a few CC's or so.

Used the ideal gas law. Pumped up a hi pressure reservoir and used solenoid valves to fill and vent a section of tubing, and measure the pressure change on the vented side of the known volume of tubing. In simplified form:

Tube A with Va is between solenoid valves A and B, the test section is after solenoid A, and a pressure sensor can be between the solenoids or after solenoid B.
The tank would pressurize to P1, solenoid A opens then and the tube
A is at P1. Solenoid B opens, air flows into the test section and the pressure is measured P1V1 = P2 (measured) V2 (this is V1 + unknown volume).