Engine displacement numbers - did they forget something?

[Gewehr98]

Ok, I do come up with some weird questions every now and then, but after rebuilding a few straight sixes and my V-Twin Shovelhead engine earlier, this still bugs me:

Internal combustion engines are described by their cylinder dispacement, ie, 5.0L Ford V-8, 350 Chevy, and so forth.

However - an engine really can't make use of that displacement number, can it?  I'm assuming the number was derived by the volume of each cylinder times the number of cylinders in the block.  Problem is, no inline or V engine of more than 2 cylinders I know of has a point in the firing order where all of the pistons are at bottom dead center - that would be max displacement, which is what I understand they use to describe the engine size.  Due to crankshaft offset during the firing order, the pistons are at various positions in their strokes.  So for my Harley's V-Twin, sure, it can be 85 cubic inches as both pistons reach bottom dead center on a common crankpin.  Opposed twin engines like BMW bike engines probably can get away with it, too.  But a straight 6 or V-8? 

[280plus]

No, you'll never see a point in a 6 or 8 where all the pistons are bdc at the same time.

[280plus]

I just remembered, and anyone feel free to correct me if I'm wrong, but I seem to recall that while the volume derived from the bore area x stroke x # of cylinders does represent engine displacement, the most correct definition of displacement is that volume of air that will be moved (or displaced) by one revolution of the engine.

[Monkeyleg]

Gewehr98, I think you're picking nits.

If you only use pistons that are at BDC for displacement, then it's possible that a manufacturer of a four-cylinder motor could claim more displacement than the manufacturer of a V-8.

[Gewehr98]

I honestly didn't intend to pick nits, it was just one of those odd things that struck me staring at the ceiling in the wee hours of the night.  I was just curious how the displacement numbers were derived, vs. how the internals of a piston engine really work. As I stated before, I believe the 85 cubic inches in my Harley's 45-degree V-Twin are with both pistons at BDC.  That means they could potentially out-displace many smaller four-cylinders, were that the true criteria. 

I suppose they could adopt what the folks at General Motors' EMD division do with the diesel engines on locomotives.  The displacement per cylinder is what they use to identify the engine - hence the 567 and 645 series of V-16 diesel engines from GM, each respectively 567 and 645 cubic inches per cylinder.  But that would be Too Much Information with respect to railroad/locomotive knowledge, and I want Monkeyleg to have a happy holiday season, so I'll stop now. 

Wonder how they figure displacement for Wankel rotary engines? 

[drewtam]

the most correct definition of displacement is that volume of air that will be moved (or displaced) by one revolution of the engine.

As an engineer in the engine business, this is correct (except it should be mass not volume). But its easier said than done; head design and forced induction of several types can make this mass air flow very non linear.

So whats the point of measuring discplacement like we do? In most peoples minds it gives them a good idea of how much power it has and for motorheads it suggests how much volume air flow potential it has (with the right carb, head, cam, and exhaust). Especially back in the days when it was all naturally aspirated.

Drew

[bedlamite]

the most correct definition of displacement is that volume of air that will be moved (or displaced) by one revolution of the engine.

Almost. It's derived from one complete cycle of the engine, which in most cases (exceptions are 2-strokes and rotary) is two complete revolutions.

[drewtam]

Same difference. Ones half of the other. 10kg/s per 2 revs or 5kg/s/rev?

[280plus]

Thanks for the clarifications. First off I wasn't sure if it was air or water that was to be moved. I believe we're talking about mass at atmospheric pressure when displacement is measured? As if you were to turn your motor into a compressor so all down strokes pulled and all up strokes pushed air. Then in one revolution all pistons would have moved through one cycle and the full displacement of the motor would have been uh,,,expelled. If that's the right word for it. 

[Art Eatman]

The flow of the mass of air+fuel has a time function, but the displacement of the engine is "just there".  Call it a convention that's always existed, whether a one-lunger or a W-24.  The sum of the volumes, without regard to where is a piston in its travel.

One thing about CID:  It conveys useful information without complexity.  More cubes = more torque = equals more load capacity.  Even a soccer mom can understand that.

Art

[Brad Johnson]

Also remember that it's not the total displacement of the cylinder, it's the amount of air volume displaced by the cylinder as it transits the cylinder bore.  You can have a cylinder that holds a volume much larger than the actual displacement of the piston during it's stroke.  The displacement of the piston vs the overall displacement of the cylinder is used to calculate static compression ratio.

As others have said, overall displacement is the total volume of air movement during one complete firing cycle of all cylinders.  With an 8 cylinder 4-stroke engine, this means two complete revolutions of the crankshaft.  If each cylinder has a bore/stroke of 4.00/4.00, each piston displaces 50.265 cu inches (823 cc) per firing cycle.  After two complete revolutions of the crankshaft and 8 firing cycles the total air volume displaced is 402.12 cu inches (6590.8 cc or 6.6 L).

Now we move onto the measuring of force (torque) and its application over time (HP)...

Brad