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View Full Version : Logically, why 8 cyl. instead of 1 big cyl.?



Stallion
01-28-03, 06:39 AM
I was just thinking, what is the advantage to having 8 cylinders than just having an engine with one large cylinder with the same volume of the eight cylinders combined? I can't even come up with a guess. Any ideas?

Thanks! :D

TR

dmkoger
01-28-03, 07:07 AM
Not a mechanical expert by any means, but I would think if you only would, it would have to fire 8 times as fast to produce the same output. Would probably lead to overheating problems, if it could be made to fire that fast for one thing. The same arguement could be made, that instead of having an 8-cyl, why not enlarge 6-cylinder pistons, which is sometimes done (cyylinders are bored). Think of a automatic gun, which tends to get very hot as it fires. Im sure someone has a more scientific explanation, buy just IMHO.

twiget
01-28-03, 07:23 AM
You come up with some pretty good questions TR.:)

Guess I'll take a stab at it. The first reason I can think of for having multipule cylinders is to balance the engine. If you only had one piston in a single cylinder pumping up and down (especially a huge 5.7ltr cylinder), you would have nothing to counter act the pistons inertia, and the engine would go bouncing all over the place.

The second reason would be your redline. Imagine how big a piston on a 5.7ltr single cylinder engine would be. With that much metal (piston, connecting rod, cam shaft, crank...) your max RPM would be so low that the motor would be pretty much useless.

And going back to the balancing thing. Assume you have two cylnders. While piston #1 is igniting the fuel/air mixture and being pushed down (power stroke), piston #2 is being pushed up and forcing the exhaust out (exhaust stroke). Then via inertia piston #2 comes back down pulling in the fuel/air mixture (intake stroke) and piston #1 goes back up (exhaust stroke). Piston #2 then goes back up, compressing the fuel/air mixture and igniting it. The resulting explosion forces piston #2 down (power stroke) and the whole thing starts over again.

A great place for more info on how a 4 stroke engne works is How Stuff Works (http://www.howstuffworks.com/engine.htm)

Hopefully I didnt confuse you anymore. The How Stuff Works web page is really helpfull, and if I am wrong on anything I said, Vig, or someone else is sure to correct me.:)

Jason

Tom73
01-28-03, 08:34 AM
Power strokes is the key. (how often the cylinder is under power rather than doing housekeeping chores :) )

One cylinder = one power stroke for every 2 crankshaft revolutions.

Eight cylinders = two power strokes for every 2 crankshaft revolutions or one power stroke for every 1 crankshaft revolutions.

Sixteen cylinders = four power strokes for every 2 crankshaft revolutions or two power strokes per revolutions.

That is why the GP motorcycles have so many cylinders.

Also there is a balance factor. Have you ever watched a single cylinder motorcycle idle? It is bucking and jumping all over the place.

tom...

Stallion
01-28-03, 08:51 AM
I see what you guys mean. But I guess I always just assumed that all 8 pistons are going down at the same time. Am I wrong? They're not all combusting at the same time and putting force into the crank at the same time? Or are they at different timings?

Thanks! :D

TR

Tom73
01-28-03, 09:01 AM
The power strokes are all different to help ballance out the engine and keep everything smooth. Just take a look at a crankshaft, you will see the throws in all different positions.

tom...

Corvette-Pilot
01-28-03, 09:23 AM
Just my $.02

RPM

Cars with the 12 cylinders have very high RPMs.
I think it is a matter of the mass of the piston.
Many small items vs one huge mass.
The momentum and the crank trying to stop a huge piston at high speed would need huge connecting rods, cranks, ect to withstand the forces. But a small piston can go much faster and not require the same forces to push/pull it.

Same sort of idea with motorcycles that can get 14k RPMs.
Tiny piston going like he!l

I would also guess torque is another issue??

Since I did not spend the night at a Holiday Inn (?) last night, this is just a guess.

JohnZ
01-28-03, 02:55 PM
With a V-8, there's a cylinder firing every 90 degrees of crankshaft rotation - each cylinder fires once for every two revolutions of the crankshaft; with a V-12, there's a cylinder firing every 60 degrees of crankshaft rotation. The closer together those firing impulses are per each revolution, the smoother the engine is.

One of the limitations of an internal combustion engine is piston speed (moving up and down in the cylinder) in feet per minute and its effect on component strength and reliability (connecting rods and wrist pins); the theoretical maximum is 4,000 feet per minute, and things get pretty tricky above 3,000 feet per minute. The more cylinders you have, the fewer feet per minute each piston has to move at peak power.

There aren't many single-cylinder engines (except for small ones like lawn mowers); the only example I can think of right now for an engine with few cylinders and huge displacement is old John Deere tractors, which had a 2-cylinder engine, a HUGE iron flywheel to damp out the firing pulses (one per revolution), and only ran at about 800 rpm.

Engine design is a lot of trade-offs for intended use (power vs. torque), physical size and weight, engine compartment packaging, fuel efficiency, emission controls, durability and reliability requirements, serviceability, part count/complexity, material selection, cost, tooling investment, manufacturing technology available, etc., etc. - nothing simple about it, especially when the investment for a new engine is over a billion dollars (automatic transmissions are even more).
:beer

Stallion
01-28-03, 03:10 PM
Okay, I understand your reasoning. And thanks for the explanation on the timing of the pistons on the crank, too. Didn't know that. :D

Thanks again! :D

TR

warren s
01-28-03, 03:40 PM
Another issue with a giant single piston is the flame front only travels so fast, thats the speed of the exploding air and fuel mixture in the combustion chamber, its one reason why larger engines are less efficiant than smaller ones. Not less powerfull, just producing less power for the same amount of fuel consumed.

jmp
01-28-03, 05:12 PM
Eight cylinders = two power strokes for every 2 crankshaft revolutions or one power stroke for every 1 crankshaft revolutions.

I think I need clarification on 'power stroke'. Let's see here.... 4 cycles per 2 revolutions of the crankshaft: intake, compression, combustion, exhaust; in that order, with each cycle being half a revolution. With a single cylinder, the spark is going off only once every 2 revs, so we're only making power once every 2 revs, hence Tom73 saying:

One cylinder = one power stroke for every 2 crankshaft revolutions.

With 4 cylinders, or 8, or whatever, we want to ignite a cylinder spark everytime a piston completes its 2 revs. For a 4 cylinder, this means 4 sparks every 2 revs and for an 8 cyl it means 8 sparks every two revs. Which fits with

With a V-8, there's a cylinder firing every 90 degrees of crankshaft rotation

Here's where I get lost. What does a 'power stroke' mean? I thought the power stroke was the combustion cycle. But this would mean that there be the same number of power strokes as cylinders. Wouldn't it?

grumpyvette
01-28-03, 05:49 PM
http://www.lcengineering.com/images/procrank.jpg
heres a crankshaft, theres a cylinder firing every 90 degrees and keep in mind the power or cylinder pressure is applied for only about 22 degrees of the 720 degrees in the repeating cycle

http://www.howstuffworks.com/engine.htm

http://www.howstuffworks.com/engine1.htm

http://www.howstuffworks.com/camshaft.htm

http://www.howstuffworks.com/camshaft1.htm

warren s
01-28-03, 05:58 PM
Excellent site!!!

Tom73
01-28-03, 06:29 PM
Originally posted by jmp
Here's where I get lost. What does a 'power stroke' mean? I thought the power stroke was the combustion cycle. But this would mean that there be the same number of power strokes as cylinders. Wouldn't it?
Oooops....my math is off. 1 cyl engine would have a power stroke every other crank rotation; 2 cyl would have one every crank rotation; 4 cy would have 2 every crank rotation; 8 cyl would have 4 every crank rotation; etc, etc. My math was way off.

tom...