I was wondering what exactly one rpm is? I was under the impression that one rpm equals each cylinder firing off and cycling once. I was doing some calulations and I found that this seemed impossible because:
Say at 6000 rpms, that means 100 rotations per second if I am right and that seems way too high. I also figured that it is possible that each time one cylinder fires, it is one rpm. So at 6000 rpms, each cylinder fires off approximately 12 times per second. That also seems terribly high. Can someone explain to me how this works?

Also, how does higher rpms give more power? I don't understand exactly how more power will be made if the thing spins faster. If you dont know what i mean, I can explain it a little better.

 

You might want to try the "How Things Work" webiste. But to start you off:

RPM = revolutions per minute. As applied to the crankshft of a four stroke motor, each piston requires two crank revolutions to go throguh all 4 cycles, which are intake, compression, expansion (or combustion) and exhaust. The plug fires on the expansion stroke. On an 8 cylinder motor, there are four cylinders firing per crank revolution. So, at 6,000rpm each cylinder fires 3,000 times, which is 50 times a second.

Intake stroke = intake valve open, piston moving down the bore and drawing in a fresh fuel/air charge.
Compression stroke = all valves closed, piston moving up the bore and compressing the fuel/air charge.
Expansion stroke: plug fires, fuel/air charge ignites and expands which puses the piston doen the bore. All the valves are closed.
Exhaust stroke: exhaust valve opens and the piston moves up the bore which helps to push the combustion products out of the cyliner.

And round and round she goes.

Rich

 

Alright thanks for clearing that up. So is it correct that a piston will make its full stroke 50 times/second at 6000 rpms? That seems impossibly fast.

Also, I was wondering how more rpms = more power. Can anyone explain that?

 

[QUOTE=Also, I was wondering how more rpms = more power. Can anyone explain that?[/QUOTE]

Again, you need to go to www.howstuffworks.com. They have some very articles on just about anything you would like to ask..

As to the more rpm generating more HP, I'll take a stab at it

It has to do with the way Horsepower is defined. First thing to understand is that horsepower cannot be directly measured. It is computed. I wont go into the origins of the fomula, but suffice to say that HP = (Torque * RPM) / 5252 is the actual formula. On ALL engines (4 stroke, 2 stroke, electric, etc..) if you plot both the measured torque output, and the derived HP output (using the formula) you will notice that TQ = HP @ 5252. Every time. It's a function of the math. Also, by looking at the formula for HP, you can pretty quickly see that you will get a higher HP number by either increasing torque output for any given RPM, or by raising the point at which a certain torque number is produced.

Example:

Engine A makes 300 ft/lb's of torque @ 5000 RPM. It's making 285.60 HP.

ok, lets keep the same torque output and raise the RPM.

Engine B makes 300 ft/lb's of torque @ 6000 RPM. IT'S making 342.72 HP.

The only change was the point at which the torque was produced. Obviously the trick is to maintain high torque at high RPM. Easier said than done...
You can also move the equation around and solve for torque or RPM to get an idea of that you would need to build in order to get a desired HP level.

This TQ to HP relationship is precisely why a diesel makes such tremendous torque but very low HP. They make it all at really low RPM's. Another interesting thing to know about this is that electric motors make 100% of their rated output at basically 0 RPM's As soon as it's turned on, it's making all the torque it ever will.

Don't forget to go here: www.howstuffworks.com and read up on this stuff. It's pretty interesting site for any gear-head..

Dave C.