I was thinking today about how displacement is calculated. It's pretty simple and all, but I think it's wrong. The way it's measured is by ((bore/2) X pie) X stroke X # of cyl. That gives you the displacement if all the pistons are at the bottom of the cylinder, and that never happens in a engine at one point in time, Also the volume of the head is never counted and I don't think it's necessary to add it in, but if you want to be a real stickler you would have to add 4 times what ever the cylinder head displacement is (usually measured per cylinder). So, then I got to thinking what the actual dynamic displacement is, and I came to the conclusion it should be exactly 1/2 the advertised displacement; because the motor is built so that no matter how fast the engine is moving the time between tdc for each cylinder is the same as the other cylinders. Disregarding of what part of the cycle the cylinder is in; when cylinder 1 is all the way up, cylinder 2 is 1/4 way up, cylinder 3 is 3/4 way up, and cylinder 4 is at the bottom, add all the space up and you get 2 cylinders worth of volume. Which means we actually have a 1.2L displacement engine (unless you count the head volume which is think is about 67cc X 4 = 268 additional cc which would make it 1.47 L). I may be really wrong on this calculation, if I am someone let me know. I know this is probably a really weird post and most people are thinking “no crap I already know this”; but I thought it was cool.

 

If you have this much time on your hands maybe you should come over and wash my car for me.

 

Sorry physics background and insominia make for some strange thoughts. lol

 

I understand. I'm sittinig in the library at school between classes right now trying to relax my brain before going to psychology resurch and disign. I was just reading the joke of the day thread in OT and trying not to **** off anyone by laughing out loud.

 

Nah you are incorrect about the 1/4, 1/2, 3/4 thing. When 2 cylinders are at TDC, 2 are at BDC. If you had a 4 cylinder where one was at TDC and the rest were staggered equally, the engine would run like ****. It would be VERY rough, vibrate a lot. Anyway, displacement is how much air an engine uses all together from all the cylinders. True, a 2.2L CAN be more powerful than a 2.4L, but the displacment gives you a rough idea. Displacement doesn't mean how much air the engine uses PER RPM. It is ALL TOGETHER. It's like putting ice cubes in a glass of water, the solid ice displaces some of the water and the level rises. That's all displacement means, the amount of air/space between TDC and BDC. The head volume has nothing to do with displacement. Displacement is a physical measurement with nothing taken into account for airflow. The 2.5L in my old car was a bigger engine than the 2.4L in the RA, but you can BET our RA uses more air/fuel. For one, it has a MUCH more open exaust (my old car was a 1982 citation). It has a MUCH better fuel/intake system (old car was carbed). MUCH lighter internals.
So don't think anything of displacement. Bore and stroke matter (long stroke/small bore=more torque, short stroke/large bore=higher revs possible at the expense of torque).