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Old 07-19-2009, 11:07 AM   #1
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4 cylinder vs 6 cylinder

The idea and general knowledge is that a 6 cylinder gets worse mileage. The engine obviously weights more and it probably has a larger displacement water pump, and other things I'm not thinking of right now. But too there usually put in larger vehicles where you need the extra power to move.


But if you where to look at it from the point of view of just the Engine management, and not look at frictional loses or weight of the vehicle. You only so much gas to idle a car. you wouldn't dump in any more fuel then is needed. and if you drove normal wouldn't use more gas then needed. as opposed to a 4 cylinder. lets say you cut a cylinder in a v8. your going to slow down. because your only injecting enough fuel to stay at a speed. so you have to make up for it and accelerate more to get back up to speed. the computer injects as much fuel as needed. in a v6 the pulse width would be less because there's 6 cylinders. in theory you'd think it would ultimately equal the same if didn't included friction or parasitic draw, or weight of the engine.

I'm losing my train of thought. or is it just that v6's are usually put in bigger cars. and it takes more to move a heavier vehicle, period. I guess my point is that if you put a v6 in a smaller car it should do pretty well as far as mileage.
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Old 07-19-2009, 01:08 PM   #2
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I think that for the same displacement and weight of vehicle, the engine with fewer cylinders will generally get better mileage because of reduced frictional losses. Examples - 3 cylinder Honda Insight (original) and 3 cylinder Metro

More cylinders WILL give you more HP for a particular displacement (assuming the same number of valves per cylinder) because more cylinders will allow it to breathe better (more valve area per unit of volume).

Personally, for mpg purposes, I would always choose the smaller engine with the fewest cylinders if the option was available.

If would be interesting if any EPA mileage estimates were higher for a vehicle with more cylinders.
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Old 07-19-2009, 01:12 PM   #3
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You're saying that to move a given car at a given speed, it's going to take the same amount of work and therefore the same amount of fuel would have to be burned. Unfortunately, it doesn't work that way in reality. Look at the EPA ratings of a car available with both a small V6 and a big I4, or a big V6 and a small V8. The smaller engine always has a FE advantage.

That is certainly partially a result of marketing; the market would think the manufacturer is crazy to offer choices of a bigger engine tuned for FE and a smaller engine tuned for power; they always tune the samller engine for FE and the bigger engine for power. However, there are other issues.

I'd be interested to know measurements of engine friction, reciprocating loss, and other types of parasitic engine losses, how they differ between different cylinder counts, configurations (I vs. V), and displacements.
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Old 07-19-2009, 01:15 PM   #4
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Quote:
Originally Posted by theholycow View Post

I'd be interested to know measurements of engine friction, reciprocating loss, and other types of parasitic engine losses, how they differ between different cylinder counts, configurations (I vs. V), and displacements.
Good point- for the same # of cylinders, the V engines would likely have fewer main bearings than the I engines (shorter crank and rods closer together) and thus slightly less friction.
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Old 07-19-2009, 01:32 PM   #5
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A motor with with fewer cylinders of the same displacement and stroke ratio will have less frictional surface area in the rings, the bearings, and internal pumping losses.

Just looking at the contact diameter of the rings for a 8 cyl. vs. a 4 cyl. of the same displacement and stroke ratio, while the pumped volume of the piston is a 1:2 ratio between them, the contact length of the rings is a 1:1.414 ratio. So the ring contact length is actually about 41% more for the 8 cylinder.

Additionally, you have at least two more bearing surfaces per extra piston, and as much as 1/2 the number of extra pistons in extra crank journals. So at a minimum for the typical inline 6 cylinder, there are at least 5 extra bearing surfaces vs. a 4, and most 8s have an extra 9. Then of course there is additional valve train friction, with extra lobes and valve buckets or rockers along with whatever additional bearings they may have.

The main practical reason to use more cylinders it would seem is for smoother operation.
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Old 07-19-2009, 01:43 PM   #6
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Quote:
Originally Posted by Erik View Post
More cylinders WILL give you more HP for a particular displacement (assuming the same number of valves per cylinder) because more cylinders will allow it to breathe better (more valve area per unit of volume).
Valve area is decreased at the same ratio as cylinder area. I.e., a cylinder that is half the area will have valves that are half the area assuming they take up the same percentage of head space. Now if you want to take the discussion into the subject of oversquare vs. undersquare design with the same displacement, that's a seperate issue to which that argument does apply.
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Old 07-19-2009, 01:44 PM   #7
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Quote:
Originally Posted by Snax View Post
Valve area is decreased at the same ratio as cylinder area. I.e., a cylinder that is half the area will have valves that are half the area assuming they take up the same percentage of head space. Now if you want to take the discussion into the subject of oversquare vs. undersquare design with the same displacement, that's a seperate issue to which that argument does apply.
Good point!
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Old 07-19-2009, 10:32 PM   #8
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Quote:
Originally Posted by Snax View Post
A motor with with fewer cylinders of the same displacement and stroke ratio will have less frictional surface area in the rings, the bearings, and internal pumping losses.

Just looking at the contact diameter of the rings for a 8 cyl. vs. a 4 cyl. of the same displacement and stroke ratio, while the pumped volume of the piston is a 1:2 ratio between them, the contact length of the rings is a 1:1.414 ratio. So the ring contact length is actually about 41% more for the 8 cylinder.

Additionally, you have at least two more bearing surfaces per extra piston, and as much as 1/2 the number of extra pistons in extra crank journals. So at a minimum for the typical inline 6 cylinder, there are at least 5 extra bearing surfaces vs. a 4, and most 8s have an extra 9. Then of course there is additional valve train friction, with extra lobes and valve buckets or rockers along with whatever additional bearings they may have.

The main practical reason to use more cylinders it would seem is for smoother operation.
I didn't really think of the frictional loses to this extent, but I agree and it makes sense. on the 2nd part, my understanding was that more cylinders equaled a smoother/less vibrations because there's more power pulses. what that's doing technically I dunno.
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Old 07-19-2009, 11:59 PM   #9
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I recall there's a limit to the practical size of a cylinder in a piston engine, governed by inertia, thermal efficiency etc. Hence when you increase the size of an engine it isn't just scaled-up.
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Old 07-20-2009, 04:20 AM   #10
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Cooling is also a limit in practice. Since heat rejection from the piston is mostly through the rings to the cylinder wall, the larger the cylinder gets the more of a problem this becomes, since the increase in ring contact area is proportional to the bore size whereas the face of the piston is proportional to the bore size squared. If you double the cylinder size, for example, the ring contact area goes up by a factor of two, whereas the face area quadruples. This is the same effect that leads to having multiple engine valves rather than simply making them larger. Active cooling of pistons can get around this, but isn't worth it in small, cheap engines.
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