Auto Engine and Cylinders
Name: Michael C.
Date: April 2003
Dr Murphy, in his answer on engine terminology, indicates that it would be possible
to have a two cylinder engine with 308 cu in, or a V8 engine with 308 cu in. This brings me to
wonder why we do not see any large 2 or 4 cylinder engines in automobiles. All things being equal (fuel,compression ratio, valves per cylinder,etc) would we expect to have more torque from an engine with more cylinders than one with less cylinders, if the 2 engines have the same displacement?
Would the 8 cylinder engine have some advantages in efficiency over the 2 cylinder engine? Would the 8 cylinder engine produce power more rapidly than the
2 cylinder engine? Most people would equate more cylinders in an automobile engine with greater
power. But is this an accurate assumption? Any help that you can provide to help me understand
this question would be very appreciated.
The total displacement of an engine determines its power. So the larger the TOTAL cylinder
VOLUME the more powerful the internal combustion reciprocating engine. But there are always
compromises. The maximum firing rate of a cylinder is determined by engineering limits necessary
to keep the engine under the hood!! So the fewer the cylinders the greater the vibrations as the
Example: a model airplane engine can fire many more times / sec than a
lawn mower engine, so the shift of the center of mass reciprocates at a much higher rate and
the vibrations of the model airplane engine are much smaller than a lawn mower engine. Another
example, is a dragster funny car. Here too large displacements and few cylinders rule, because
the purpose is to "deliver" the maximum displacement to the wheels as quickly as possible. Since
the dragster only has to stay in
place for a matter of seconds, A smaller number of large volume cylinders is the way to go. On
the other hand, if you want to get a "smooth" ride the engine design tends to dictate more
cylinders, each with a smaller displacement volume. Here too there are engineering limits
because getting, say, 12 cylinders to fire in proper sequence is much more difficult than
getting 6 or 8 cylinders to fire properly.
The number of cylinders is not so critical in regard to torque --the old John Deere tractors only had two cylinders but had considerable torque. Crankshaft lever arm distance and stroke distance have most to do with torque - Big truck/diesel engine are very high in this regard and produce most of their torque and HP at low RPM.
But such a low-RPM two cylinder engine will NOT BE VERY SMOOTH OR BALANCED. More cylinders enhances smoothness for one. It can also reduce weight.
Whether its a motorcycle, car, truck or aircraft you must match the type, size, weight, and shape/vol of the engine to the vehicle and the desired performance level.
I would say in a 4 cycle engine performance does increase with more cylinders....to a point -- 6 vs 8??
Almost all 8s are V8s and that to me is a larger consideration for engine shape and configuration while targeting high HP..
On the other hand all 6s are not V6s - Chevy Trailbrazer uses a straight in-line 6 cylinder block for better balance and smoothness.
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Update: June 2012