Liters, displacement, etc.

[nahish]

What exactly is displacement? I mean, when used in context, I understand the concept to a degree, but what makes an SL 500's 5.0 liter engine different the SL 55's 5.5 liter engine? Is the displacement part of the engine itself, or is it like the results of power and output and all that good stuff. If someone could explain to me what it is, that'd be cool. If you wanna use my 2.0 liter 160hp Acura RSX as an example to explain off of, that would help it make sense to me. Thanks.

[johnnynumfiv]

It is the measurement of how much volume that the piston displaces when it moves. It is measured by using the formula below. The bore is the width of the cylinder, the stroke is how much it moves up and down.

Displacement = 3.1415 multiplied by ½ bore² multiplied by the stroke multiplied by the number of cylinders


Welcome to UCP.

[h00t_h00t]

The displacement is the volume of air & fuel that all of the cylinders combined can hold.

A 5.5liter engine can hold .5 liters more than a 5.0.

EDIT - Spooky.

[jcp123]

It's something that's part of an engine itself. Displacement refers to the "swept piston volume". That means that when a piston goes from its bottom position in the cylinder to its top position, it displaces that much air (might be better to use water for this, as it does not compress the way air can). Multiply that by how many cylinders you have, and then you have the displacement of the engine.

In other words, when your engine turns over once, it would take in/push out a volume of water equal to two liters.

[KnifeEdge_2K1]

2 engines with different compression ratios and displacement ratings will hold different amount of air/fuel mixture, the one with the higher ratio will produce more power but have less air/fuel per cylinder

[targa]

[repeat=everyone else's answer]blah blah blah[/repeat]

welcome

[Godlaus]

Originally Posted by jcp123

(might be better to use water for this, as it does not compress the way air can.

was about to say; "Then perhaps you can explain why, in forced induction, displacement does not rise?"

"swept piston volume"

Thats TDC to TBC, right?

[Matra et Alpine]

Originally Posted by Godlaus

Thats TDC to TBC, right?

BDC

Top Dead Centre
Bottom Dead Centre

[Godlaus]

IT'S CENTER DAMMMITT!!!!
lol, thanks for the enlightenment.

Is there any formula for something like displacement that takes into effect forced displacement? Such as; displacement of air at 14.7 psi at the headers?

[nahish]

*GASP* It's all starting to make sense to me now! Woo hoo! Thanks, guys, for the explanations and the welcomes to this website. I've actually been viewing this site for a while, but I just created a username so I could talk on the forums.

My RSX is out of the shop in 9 days, and now when I see him, I will have more knowledge of his being!

[BjD]

Originally Posted by Godlaus

IT'S CENTER DAMMMITT!!!!
lol, thanks for the enlightenment.

Is there any formula for something like displacement that takes into effect forced displacement? Such as; displacement of air at 14.7 psi at the headers?

The displacement is the volume of air drawn in, as mentioned. The volume is a physical constant of the engine, it doesnt change when you use forced induction. What does change is the MASS of air drawn in by the engine, it also depends on the air temperature. Thinking about it you could probably use a version of the ideal gas law (ignoring fuel content), PV=mRT IIRC. P is pressure, V is volume (displacement), m is mass, R is a constant and T is temperature. Similar to how early fuel injection ECU's calculated the amount of fuel required, by measuring air temp an pressure they knew the mass flow rate.