Does anyone have any nifty ways of measuring the true volume of an engine's cylinder?

[Stretch]

Dear All,

I've just discovered something - the cubic capacity of an engine refers to the sum of the swept volume or displacement volume of each cylinder in an engine. So this is simply defined as the area of the bore multiplied by the stroke.

It doesn't include dished surfaces in the head or the piston crowns.

Timing adjustment methods

http://en.wikipedia.org/wiki/Engine_displacement

But how could I measure or calculate the true volume of each cylinder?

Does anyone know? All of the handy engine calculators on the internet assume that you know the volumes of the dished surfaces in the piston crowns and the head to start off with.

With an engine in bits I could fill complicated surfaces with oil and then measure the volume of the oil tipped out of them - but there would be errors due to surface tension (and probably other things). This would also be a pain in the backside.

I have also had trouble measuring by how much head gaskets get compressed when a head is fitted and torqued in place. I know these parts of the engine are only going to contribute small amounts but all the same I'd like to make accurate measurements if I could.

So does anyone have any tips? Has anyone heard of any hair brain ideas that can do this?

[bobodaclown]

valve adjustment will also have to be taken into account. liquids don't compress, hence bending a valve when the cylinder is filled with a fluid (water/coolant/oil) and is rotated.

[Matt L]

If you know the swept volume and the compression ratio, you can compute the unswept volume.

Valve timing affects how much air actually goes through the engine, but that has nothing to do with displacement or compression ratio. These are computed as if the engine were ideal.

[W124 E300D]

Simple trig is good enough for a good enough approximation.

If you want an accurate measure then you need to make up jigs and start weighing displaced light oil of known density / temperature etc etc

3.14 x (radius squared) x stroke is good enough even for engine machine shops.

edit, cos I wrote Pi x d... doh

[whunter]

Originally Posted by Army

Dear All,

I've just discovered something - the cubic capacity of an engine refers to the sum of the swept volume or displacement volume of each cylinder in an engine. So this is simply defined as the area of the bore multiplied by the stroke.

It doesn't include dished surfaces in the head or the piston crowns.

Timing adjustment methods

http://en.wikipedia.org/wiki/Engine_displacement

But how could I measure or calculate the true volume of each cylinder?

Does anyone know? All of the handy engine calculators on the internet assume that you know the volumes of the dished surfaces in the piston crowns and the head to start off with.

With an engine in bits I could fill complicated surfaces with oil and then measure the volume of the oil tipped out of them - but there would be errors due to surface tension (and probably other things). This would also be a pain in the backside.

I have also had trouble measuring by how much head gaskets get compressed when a head is fitted and torqued in place. I know these parts of the engine are only going to contribute small amounts but all the same I'd like to make accurate measurements if I could.

So does anyone have any tips? Has anyone heard of any hair brain ideas that can do this?

Virtual automotive tools/data
Virtual automotive tools/data






.

[leathermang]

You are being ' squishy ' with your wording...and need to tighten it up..
I said in the other thread that ' engine size' is the area swept by the top of the pistion.
That compression is the total combustion chamber volume .. that piston sweep PLUS the amount of space above that ... the max at BDC compared to the total at TDC gives the compression ratio...

but your question about the volume of the head itself... ( assuming some dome ) is that you take it off and turn it upside down.. and titrate water into it... measure the amount of water in the container before dripping into the head... and subtract... ...
This is how equal volumes were produced in race engines.... metal was ground off safe areas until all the head volumes were equal.....

[Stretch]

Originally Posted by whunter

Virtual automotive tools/data
Virtual automotive tools/data






.

Thank you very much a great collection of resources.

[Stretch]

Originally Posted by leathermang

You are being ' squishy ' with your wording...and need to tighten it up..
I said in the other thread that ' engine size' is the area swept by the top of the pistion.
That compression is the total combustion chamber volume .. that piston sweep PLUS the amount of space above that ... the max at BDC compared to the total at TDC gives the compression ratio...

but your question about the volume of the head itself... ( assuming some dome ) is that you take it off and turn it upside down.. and titrate water into it... measure the amount of water in the container before dripping into the head... and subtract... ...
This is how equal volumes were produced in race engines.... metal was ground off safe areas until all the head volumes were equal.....

Squishy I am - squishy I have nearly always been - it is one of many failings. I'll try and do better!

[Stretch]

I've been doing some calculations to see the effect of my engine rebuild on the swept volume and the compression ratio.

My engine has been re-bored and fitted with over sized pistons. The bores are now 91.5mm

I've calculated the clearance volume of a standard engine using data from the FSM and the information given on wikipedia:-

http://en.wikipedia.org/wiki/Compression_ratio

Here is a step by step re-working of the equation (each new line is a step to obtain the final formula)



Vc is the clearance volume, Cr is the compression ratio, and Vs is the swept volume

Using this formula I got a value of about 150 cubic centimeters of clearance volume.

Using the new dimensions of the re-bore (same stroke!) I've now got a 3038cc engine instead of a 2998cc engine.

Assuming that the clearance volume did not change during the engine rebuild I've come up with an estimation of the new compression ratio => I think it is now 21.26:1 instead of a 21:1

So my conclusion? (Considering it was an inconvenient unexpected engine rebuild)

...Well at least I now have a larger engine with a slightly higher compression ratio!

[Cr from Texas]

Originally Posted by leathermang

but your question about the volume of the head itself... ( assuming some dome ) is that you take it off and turn it upside down.. and titrate water into it... measure the amount of water in the container before dripping into the head... and subtract... ...
This is how equal volumes were produced in race engines.... metal was ground off safe areas until all the head volumes were equal.....

I gave my race engine building friend (currently holds a Bonneville record) a buret out of the lab to do this. But he doesn't titrate with water. He uses oil.

[leathermang]

Originally Posted by Cr from Texas

I gave my race engine building friend (currently holds a Bonneville record) a buret out of the lab to do this. But he doesn't titrate with water. He uses oil.

LOL, easier to clean up using water... if your valves seat well enough to hold water... he may be using oil for that reason....or he just likes the smell of it..?

[Cr from Texas]

Originally Posted by leathermang

LOL, easier to clean up using water... if your valves seat well enough to hold water... he may be using oil for that reason....or he just likes the smell of it..?

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.

[layback40]

Originally Posted by Army

Using the new dimensions of the re-bore (same stroke!) I've now got a 3038cc engine instead of a 2998cc engine.

Assuming that the clearance volume did not change during the engine rebuild I've come up with an estimation of the new compression ratio => I think it is now 21.26:1 instead of a 21:1

So my conclusion? (Considering it was an inconvenient unexpected engine rebuild)

...Well at least I now have a larger engine with a slightly higher compression ratio!

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

[Stretch]

Originally Posted by layback40

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

Heady times indeed - I'll see ya at the stop light!

[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.

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

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

1. It starts "on the button" from cold.
2. It settles down to a good idle within 15 seconds.
3. Cold start rattle/knock disappears within 30 seconds.
4. Engine doesn't shake and has no leaks or drips.
5. 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.