Displacement anomalies

[Kultag]

Hi

I made an Excel chart with a lot of cars' data and it showed a lot of anomalies regarding total engine displacements, just one example:

SSC Aero SC/8T:
bore: 99.0 mm,
stroke: 101.6 mm,
number of cylinders: 8.

So displacement should be (99/2)^2 x pi x 101.6 x 8 / 1000 = 6256 ccm instead of 6187. This is just 69 ccm, with some other cars the difference was almost half a liter.

Some other differences with their scale in ccm (without sign):
Aston Martin DB9: 18,
Audi TTS concept: 445,
BMW Nazca C2: 622 (okay this had many version, maybe non-matching data),
Chevrolet Corvette C2: 46,
Isdera Imperator 108i: 568,
Jaguar XJ220 V12 prototype: 56,
Lotec Sirius: 46,
Porsche 911 GT2 S (1996): 73,
Porsche 911 GT2 R (1998): 158,
1965 Shelby Cobra 427: 39.

These are too big for conversion or rounding errors. Input data comes from various (generally reliable) places.

Is it more complex to calculate total displacement? I think the combustion chamber should be ignored here, and this method works precisely with the majority of cars, but there are several major errors in it. Why?

[The_Canuck]

Theres a bit of a gap at the top of the cylinder where combustion takes place that wouldn't be factored into your equations , no?

[ruim20]

Originally Posted by The_Canuck

Theres a bit of a gap at the top of the cylinder where combustion takes place that wouldn't be factored into your equations , no?

Even with almost a liter of difference?

[Matra et Alpine]

WHen you say "reliable" .. are you also cross-checking the data you're finding errors in ?

Esp the gross errors, possibly marketing says 6 litres, engine actually 5700 ??

[johnnynumfiv]

Doesn't combustion chamber size and piston design come into finding exact displacement?

[Quiggs]

Displacement is area swept from TDC to BDC.

[jediali]

combustion chamber shape and piston crown design wont contribute to capacity equation. These differences you spot arent surprising, without looking at the actual schematics of the engine your not going to know for sure 100%.

[acfsambo]

The stroke may be x mm, but the piston may not go all the way down to x mm. and the piston head is concave, and they could have counted the area in the piston out. Yes it could get to half a liter, especially in V8's, V10's and V12's, more so if the bore is bigger.

[Kultag]

I don't think the combustion chamber counts here. I don't think my sources (UCP, SC.net, Wiki, etc.) are unreliable as they all write the same data. I don't think inaccurate displacement data counts here as they are given in a precise ccm value. I don't think concave piston surface counts here as what they subtract at the uppermost position they add at the lowermost. I don't think a cylinder could not move down to the lowest point of its stroke, as that way the crankshaft wouldn't turn. I'm pretty sure the formula is correct, as it works with the wast majority of the cars. If the official displacement is, say, 3421 ccm, this formula will give a result very close to it (within a few ccms, that really is rounding and such, that's okay). Also the sidewall of the cylinder must be cylindrical (sealing).

Any other ideas? Like manufacturers' inaccuracy, to be diplomatic?

[Matra et Alpine]

Kultag, are you sure that the figures aren't getting swept volume and capacity confused ?

Also, what do you mean in questioning the bottom of the stroke ? Bottom of stroke is determined by the crank and the conrod/piston length. It is independent of the actual cylinder length in the block ( other than it can't be more )

[Kultag]

The formula I used is this:

(bore/2)^2 x pi x stroke x numberofcylinders / 1000.

Currently there are 141 cars with which all input data is given. With 95 of them the difference between the "official" ccm and the calculated is 0. 130 of the 141 is within +/- 20 ccm. The rest is out of this 40 ccm range relative to their official ccm. My formula should be okay.

[drakkie]

As posted before by me:

- bore - D (in meter) - diameter of the cilinder
- stroke - S (in meter) - The stroke of the piston is the distance from the Top Dead point (TDP) to the Bottom dead point (BDP).
- Stroke volume - Vs - This is the volume that is moved by the piston. So this is the volume between the TDP and the BDP.

V’s = π / 4 * D2 * S - answer in m3

For an engine with z clinders the total Stroke volume is Vs = V’s * z
Z is the number of cylinders.

- The volume of the combustion chamber or compressionvolume is known as Vc . This is the volume of the combustion chamber when it is in the TDP. This is also called Compression space.

- Compression relation (hope this is the correct translation, please correct me if wrong). For this we have two different indications:
- the nominal compression relation ε . this is the normal indication by the manufacturers.
ε = Volume above the piston in BDP / volume above the piston in TDP
or
ε = V’s + Vc / Vc

See attached drawing number 1

- The true or effective compression relation εeff. This is calculated by using either one of the next two formulas.
Εeff = Volume above the piston at the time of closing inlet valve / volume above piston in TDP.
Or
Εeff = V’seff. + Vc / Vc
See attached drawing number 2

The Thermodynamics of the combustion engine

[Kultag]

Yes, these are okay, I used these. And with most of the cars, they worked. But why they didn't with some of the cars?

(That thing is called compression ratio.)

[digitalcraft]

So if I understand right, the displacement is actually coming out less than what your formula says it should be?

If it were more, the things people have said might make sense.

[Kultag]

Not always less nor more, I left out the signs of the differences in the above list. The 993 GT2 R had 158 ccm less, the Imperator had 568 ccm more. And the problem still is that this formula works perfectly with most of the cars, so it should be correct. But about with 8 % of them, it's not, and I've no idea what the problem might be, how could the displacement of an engine be calculated another way. And why. Or if some of the data given by the manufacturers are wrong, then whats the point in it. Looking it this way, that 8 % is pretty much.