Ask A Scientist© Environmental Earth Science Archive

name         Jess
status       student
grade        9-12
location     NY

Question -   We are doing a project on car engine cylinders in physics
class. What are the advantages to having a short, wide cylinder opposed
to a tall, thin one?
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I am not an automotive engineer and the complete answer is probably 
both complicated and a trade off between various factors, but here are some of
the issues that play a role:
1. The displacement volume of the cylinder, V = (pi) R^2 x L where 'R' is 
the cylinder radius and 'L' is the length of
the cylinder. Since the power generated by the engine depends upon the
volume of fuel/air mixture burned, it is clearly more effective to
increase the radius rather than increase the length of the combustion
stroke.
2. If the length of the combustion stroke is too long, a "strength of
materials" problem is introduced --
long piston rods are more likely to warp and so must be larger and heavier.
3. The shorter the stroke the more strokes / sec is possible, that in effect
allows a larger rpm for the engine.
4. The more compact the ignition volume
the easier it is to get uniform combustion of the fuel. Long, thin cylinders
cause the fuel near the ignition source (spark plug) to ignite before
fuel/air mixture further away. This non-uniformity in combustion causes
"pinging" and possibly shock waves in the cylinder (I am not sure of that.).
Hope this helps to give you a starting point.

Vince Calder
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Either extreme presents advantages and disadvantages.  All of which boil
down to the surface area of the piston versus the volume of the chamber.

A long, thin cylinder will give much less push on the piston, (less surface
area to act on), but will push it over a greater distance.

A short wide piston/chamber combination will dramatically increase the
pressure on the piston, but only for a short distance.

Ryan Belscamper
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Engineers would like to have engines with the highest power density and also
the greatest fuel efficiency.  Unfortunately, having both is generally not
possible.

Broadly speaking, engines of the normal 4-stroke poppet valve type are able
to generate high power if they have wide pistons and a short stroke.  To
attain high power, engines must run fast.  The cylinders must be quickly and
fully filled with air and emptied of exhaust.  This requires large valve
area in comparison to the displacement.   The disadvantage is that a very
wide piston and cylinder crown has a large surface area, and a lot of heat
is lost through conduction.

If an engine is turbocharged or supercharged, then a large valve area is not
as important.

Efficient engines tend to have longer strokes so that there is less relative
surface area.  This also helps with emissions (pollution).

Large diesel engines in ships may have cylinders three feet wide and nine
feet long. They are very efficient but very heavy.  That is a 1:3 bore/stroke
ratio.  A racing car may have a 2:1 ratio.

Bob Erck
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Layman's opinion.
For an engine, power and speed are watchwords. In internal combustion
engines, the mini explosion within the cylinder produces power to the drive
shaft and efficiency within the cylinder (the piston goes back for another
run). From what little I know, there are all sorts of combinations. 4
cylinder, 6, 12, I think back in the early 1910- 20s there were cars with
more than 12 cylinders.  As within most things involved in design, there are
trade offs, the bigger cylinder may produce more force, but there will be
more frictional resistance.  This would effect speed. Big/slow,small/fast.
Determining the optimum combination of force and speed is the problem.
Design is starting with a desire, then making a trial, observing the problem,
trying to best solve that problem, and then proceeding to the next level.
Ultimately, it is about money.  The 4 cylinder car would be more economical
when considering fuel consumption, but power and speed is sacrificed.  The
computers now try to solve issues, recycling gases and utilizing fumes etc.,
so you see your question is really part of the next question, what is your
desire?  Are you building a fuel efficient, low speed, small car, or a racing
engine for Indianapolis racing?  Or how about a Hummer?

James Przewoznik
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