Random Question about Cylinder Numbers, Displacement and Gas Mileage


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modena_360stradale

modena_360stradale

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One thing I haven't been able to find an answer to is this: would a 2 Litre V12 (think old time Ferrari) have the same gas mileage as, say, a 2 Litre 4 cylinder engine?

Conceivably they would, especially given that their displacement over a period of time 'could' equal out. If one piston going around the crankshaft equals 1 rpm in the 4 cylinder engine, and three cylinders (firing simultaneously or in sequence) make up 1 rpm in the V12, then they would both end up taking in the same volume of air/fuel, and so consequently use the same amount of fuel per mile....right?

Any engineers out there? :t-hands:
 
Interesting question.

It would be kind of hard to figure out since one is an economy engine the other would be more high performance.
It would be easier to look at a 2.3 liter 4 cylinder and a 2.3 liter 6 cylinder.

I think the V12 would use more fuel. :t-hands:
 
Gullwing said:
Interesting question.

It would be kind of hard to figure out since one is an economy engine the other would be more high performance.
It would be easier to look at a 2.3 liter 4 cylinder and a 2.3 liter 6 cylinder.

I think the V12 would use more fuel. :t-hands:
Click to expand...

Yeah, I still have no idea :t-hands:
 
modena_360stradale said:
One thing I haven't been able to find an answer to is this: would a 2 Litre V12 (think old time Ferrari) have the same gas mileage as, say, a 2 Litre 4 cylinder engine?
Click to expand...

I think I can answer your question.

A 2L V12 will be extremely inefficient because of the low volume to surface area of the cylinder, so what you end up is excessive heat loss to the cylinder wall, and the thermal efficiency of the engine will be poor. Also because the cylinder is too small, you can not really inject useful amount of fuel to get useful amount of work done. Also because of the small cylinder, circumference will be too small to place a 4 over head valves above the cylinder. The volumetric efficiency of the engine will be reduced, and the combustion will not be optimised.

The most efficient cylinder size is 500cc, and a lot of high performance car engines have that specific cylinder size, eg the 6.0LV12 from BMW, the 8.L W16 from the Veyron, and a 2.L 4 cylinder engine from various manufacturers.


modena_360stradale said:
Conceivably they would, especially given that their displacement over a period of time 'could' equal out. If one piston going around the crankshaft equals 1 rpm in the 4 cylinder engine, and three cylinders (firing simultaneously or in sequence) make up 1 rpm in the V12, then they would both end up taking in the same volume of air/fuel, and so consequently use the same amount of fuel per mile....right?

Any engineers out there? :t-hands:
Click to expand...

For engine analysis, we analysis each cylinder individually, and not all together, and fuel consumption is measured in kg/s. A 2L V12 will be too inefficient to achieve the same mileage as a 2L 4 cylinder. Feel free to ask me more questions.
 
So what about a 2.3L 4-pot vs 2.3L 6-pot as Gullwing suggested, or 2.0L 4-pot vs 2.0L 5-pot?

I'd love to read your comments!
 
Giannis, the differences in specific volume per cylinder is much less dramatic between 4, 5 and 6 cylinder engines than in the example of a 2.0 litre 4 pot vs a 2.0 litre V12.

These differences can be compensated for in the mechanical construction of the engine (for example by shortening the stroke) but it doesn't change the fact that the generally acknowledged optimum cylinder capacity is 500cc.
 
Sure Martin, but why some most manufacturers choose 4-pot 2.0L engines, while Volvo chooses 5-pot ones? There must be some advantages and dissadvantages.

A friend suggested that the stroke is smaller than the stroke, the engine revs more easily, but is a bit less efficient than an equilevant 4-pot! This is because the pistonheads travel less..
 
Giannis said:
Sure Martin, but why some most manufacturers choose 4-pot 2.0L engines, while Volvo chooses 5-pot ones? There must be some advantages and dissadvantages.

A friend suggested that the stroke is smaller than the stroke, the engine revs more easily, but is a bit less efficient than an equilevant 4-pot! This is because the pistonheads travel less..
Click to expand...
In what way is a 4-pot 2.0L more efficient than the Volvo engine? I am not too familar with volvo engines, but if I remember correctly, they are 2.5L units? There are all sorts of things a car manufacturer considers before determining the power output of their engines, so it is hard to tell the true potential of each engine.

In general, the efficiency of the engine in general is more affected by the actual mechanical design, and component design rather than the number of cylinders. However they are all related to each other in some ways.
 
Monster said:
I think I can answer your question.

A 2L V12 will be extremely inefficient because of the low volume to surface area of the cylinder, so what you end up is excessive heat loss to the cylinder wall, and the thermal efficiency of the engine will be poor. Also because the cylinder is too small, you can not really inject useful amount of fuel to get useful amount of work done. Also because of the small cylinder, circumference will be too small to place a 4 over head valves above the cylinder. The volumetric efficiency of the engine will be reduced, and the combustion will not be optimised.

The most efficient cylinder size is 500cc, and a lot of high performance car engines have that specific cylinder size, eg the 6.0LV12 from BMW, the 8.L W16 from the Veyron, and a 2.L 4 cylinder engine from various manufacturers.


For engine analysis, we analysis each cylinder individually, and not all together, and fuel consumption is measured in kg/s. A 2L V12 will be too inefficient to achieve the same mileage as a 2L 4 cylinder. Feel free to ask me more questions.
Click to expand...

Wow thanks for the great answers (marintbo too) :D

So (essentially) even though two engines have the same displacement, and can theoretically take in the same amount of fuel per RPM, their gas mileage won't necessarily be equal because the total efficiency of the engine may suffer?
:t-cheers:
 
Intersting stuff. Didn't Porsche have a 3.0L 4-cylinder in the 90's era 944 or 968?

Mazda also had a very small, like 2.3L V6 in one of their 90's models too, a hatchback I think. The Mazda Millenia also had a smaller V6, like 2.2 liters or something run on the Miller cycle. Interesting cars they were.


M
 
modena_360stradale said:
One thing I haven't been able to find an answer to is this: would a 2 Litre V12 (think old time Ferrari) have the same gas mileage as, say, a 2 Litre 4 cylinder engine?

Conceivably they would, especially given that their displacement over a period of time 'could' equal out. If one piston going around the crankshaft equals 1 rpm in the 4 cylinder engine, and three cylinders (firing simultaneously or in sequence) make up 1 rpm in the V12, then they would both end up taking in the same volume of air/fuel, and so consequently use the same amount of fuel per mile....right?

Any engineers out there? :t-hands:
Click to expand...

A 2 litre V12 2ill have poor gas mileage wrt a 2 litre 4 cylinder engine. However the 2 litre V12 will smoother and rev better and higher than the 2 litre 4 cylinder.
 

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