I should really buy a book on this stuff...its probably 8th grade reading =P

Anyway, I was just thinking about my V8 which runs off of a distributer. Obviously from the layout of the distributer only 1 cylinder is firing at any given time, hence sequential firing. There is not way that all the pistons are in phase with each other because you couldn't fire all 8 pistons before #1 was back to its compression stroke since it only takes 4 storkes/2 cycles. So in order to fire all 8 pistons without wasting cycles 4 of the 8 pistons are offset by 1/4 a cycle right? Hence after #1 fires, #8 fires when #1 is half way through its power stroke and not when its done with its power stroke. So in a V8, 4 cylinders fire per 1 rev of the crank while in a V4, only 2 cylinders fire per 1 rev of the crank. Im I correct? It seems logical enough =)

 

You guessed it.

In a 4-stroke motor like virtually all cars except Trabants (or chain saws or weed-eaters or dirt bikes, which are 2-strokes), the crank rotates twice for each time a cylinder fires. Look at a lawn mower or something for a simple example. If you start at firing, the crank rotates ½ turn during the power stroke; when the piston reaches the bottom of that, the exhaust valve opens; at the top of the exhaust stroke one full turn after firing, the exhaust closes and the intake opens, so it's in the intake stroke; ½ turn more, 1½ turns total after the last firing, we're at the bottom; the intake valve closes, the piston starts to come back up during the compression stroke, when it gets to the top 2 full turns later, it fires again.

In a V8, 4 cylinders fire each crank revolution, one every 90°.That way for each 2 complete revolutions, all 8 cylinders fire; 2 revolutions = 2 x 360º = 720°; and 8 x 90° = 720°. In a 4 cylinder, 2 fire per rev, in a 6 3 fire per rev, in a 12 6 fire pre rev. Don't ask what happens in a 5-cylinder Mercedes or a 3-cylinder Geo Metro.

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"So many Mustangs, so little time..."
ICON Motorsports

 

if Im right, which I am =P, it doesn't matter if its odd or even # of cylinders. If its a 4 stroke engine, all the cylinders have to fire within 4 strokes or 2 rev. If its a V5 then each cylinder is timed 2rev/5 between each other. Its not even but its probably what happens. I guess then the more cylinders you have the smoother the eingine runs since there are more cylinders firing per rev of the engine.

 

Yes. To a point. There is a fine line of harmonics also. Have you by chance heard or seen a V12 run? They are smooth as glass. However, a V10 is not as smooth as a V8. (Ask Dodge, they'll tell you.) It depends on the number of rod journals on the crankshaft. An even number of rod journals will cause an engine to "run smoother" than one with an odd number.

A Chevy V8 (for instance) has four rod journals on the crank. But a V10 has five. That fifth journal causes the crank to be more difficult to balance because it doesn't have that other "partner" to help cancel out the shock that those pistons put on the crank. Plus it's an odd balance of rotating mass, along with the fact that the placement of the journal isn't 180 deg from the others like a V8. That causes more harmonic vibrations, etc., etc. But a V12 has six rod journals. It's much smoother than a V10 or a V8. That's due to what you said earlier. You know, the more piston thing.

If you've ever noticed a V6 engine has a few braces in the engine compartment that hold the top of the engine in place. They have HUGE rubber bushings in them. That's to help keep the harmonics from entering the passanger compartment and having the driver feel it. But a V8 doesn't have that bad of a problem. The harmonics are a lot less noticable. Again, a V6 has three rod journals on the crank. And the rod journals are 120 degrees apart to try and help. They look more like this from one end

V |
6 / \

instead of this

|
| V8

Just more stuff to confuse you.
AJ



[This message has been edited by AJ_92RS (edited July 31, 2001).]

 

just for future reference, there are v8 engines, and 4 cylinder engines, but no v4 engine. they don't make them. no v5 engines either.inline 5 and 3 work like an inline 4, but with a smaller/larger crankshaft and cam.

 

Yes, no odd numbered motor can be a V-motor !! and they made some 8-cyl motors inline... not only V.

 

Some outboard boat motors are V-4's

 

Am I missing something? A month ago I read an article in Cycle World about a new V5 Honda motorcycle engine. Three cylinders in front, two in the rear, although I do not believe that it is in production at this time...

 

I know of a few of my friends motorcycles that have a V4

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1992 RS 3.1L Bone Stock

"Yeah, it's a V6 SO WHAT!?"

 

Yea, me too! I had a 1994 Honda VF1000-F DOHC V4. It was smooooooooth. Honda still makes V4's....

[This message has been edited by Stuart Moss (edited August 02, 2001).]

 

though I was mostly refering to car engines...motorcyles and boats are different.

 

Right... completely different, only exactly the same.

An internal combustion engine is an internal combustion engine and works like any other, no matter what shape the sheet metal (or fiberglass) wrapped around it is, or how many wheels (if any) the vehicle has. The various configurations like in-line, V, opposed, etc. are strictly governed by the requirements of the device they are intended to be used in.

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"So many Mustangs, so little time..."
ICON Motorsports