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DumDak
Dodge Dakota
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7/17/2003
02:11:56
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 Subject: cylinders
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This is probably a stupid question BUT, if a engine only fires one cylinder at a time,what difference does it make having 2,4,6,8 etc. cylinders? What am I missing?
Dumdak.
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Mr.H
Dodge Dakota
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7/17/2003
09:15:57
|  RE: cylinders
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Well, here's a crack at that question. 6 is better than 4, 8 is better than 6.
Really, with a 4 banger only 4 cylinders fire (power stroke and a compression stroke[not to mention intake and exhaust]) to make one full revolution, but with six you get six power strokes, six compression strokes, and so on. With 8 cylinders you get 8 power,compression,intake,and exhaust strokes, thus having 2 times the internal combustion as a 4. With all that fire and rotating assembly mass, you have what they call torque. HP [actually brake-hp] is measured by the force it would take to stop the rotating mass. Simple really. With all that in mind, you should definitely get a v-8. All that torque combined with fuel injection is exactly why the 4.7 is capable of the best fuel economy of the 3 engines available in our trucks.
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DumDak
Dodge Dakota
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7/17/2003
10:50:20
| RE: cylinders
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Thanks Mr. H.
I think I am following your reply. I was just trying to understand why more horsepower and torque is created by simply adding more cylinders such as the v-6 and 318 which i presume have the same bore and stroke. Thanks
Dumdak
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nate
Dodge Dakota
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7/17/2003
11:02:28
| RE: cylinders
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Not to get too technical, but a four banger fires two cylinders every revolution, a 6cyl fires three and a V-8 fires four. A four stroke combustion cycle takes two revolutions to complete. So, you have either 2,3, or 4 ignition strokes per revolution, which is the difference in power.
On the torque vs. horsepower thing - Probably the most misunderstood thing about rotating power sources. Torque is a static force measurement - think of it as how much you can bench press. It is exactly what it sounds like - the turning force equivalent to putting, say, 300 lbs on the end of a 1 foot lever, 300 lb*ft, or the amount of work to lift 300 lbs one foot off the ground. Horsepower is a rate of force production - think of it as not just how much you can bench, but how fast you can throw it up. It is 100% directly rated to torque by the equation
Torque (in lb-ft) = (HP*5252.11)/(RPM)
or
Horsepower = (Torque * RPM)/(5252.11)
Now, according to this equation, the torque value of an engine should equal its horsepower at 5,252 rpm. Thats why when you look at an engines dyno results, the lines always cross at 5,252 rpm.
hope this helps
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DumDak
Dodge Dakota
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7/18/2003
18:37:41
| RE: cylinders
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Nate,
Thanks for your reply. Do you know of a webpage where I could learn even more about this subject?
SlightlysmarterDAK
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blowndakrt
Dodge Dakota
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7/18/2003
19:11:02
| RE: cylinders
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it's not directly equivelant that more cylinders = more HP... look at the diesel trucks... the successful one's... cummins, Caterpillar, Mack etc... all straight 6's some straight 12's... it depends on the motor configuration... some lend them selves more to torque others to high RPM HP... a F1 motor.. it's 3.0L 10 cylinder... turns 22,000 RPM and makes 900hp but like 85lb/ft of torque... no configuration is better than another is all depends on the application... in a truck used for towing that motor would be useless just as a cat diesel is not necessarilly the best race motor... there are people who hop them up and go fast but it's not the intended purpose.
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nate
Dodge Dakota
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7/19/2003
12:19:29
| RE: cylinders
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blowndakrt -
more cylinders dont always mean more hp. You are right. What they do ALWAYS mean is more torque. How that torque translates into horsepower is governed by the equation I listed above. So if it is a low reving engine, that will never even see 4000 rpm, a massive increase in torque would equate to only a small (but proportional) increase in horsepower.
And, yes - diesels can be made to go very fast. The trick is keeping them in their torque band, which would require pretty tall rear gear and a six speed with relatively high OD1 and very high OD2. But, no truck would ever come with that sort of combination, cause that sort of combo would suck for pulling.
dumbdak -
as far as websites, try a google search. I am a mechanical engineer at georgia tech, so I pull all such information from various machine design classes.
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Mr.H
Dodge Dakota
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7/22/2003
15:45:44
| RE: cylinders
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I thought it was every rotation, but my theory was correct, you're the engineer. I'm just a computer geek.
I'll bet my truck will still be faster than yours though.
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FazDak
GenII
7/22/2003
16:03:18
| RE: cylinders
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The only dumb question is the one not asked.
Try this..it might help.
http://auto.howstuffworks.com/engine1.htm
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