Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Auto Engine and Cylinders
Name: Michael C.
Status: Other
Age: 40s
Location: N/A
Country: N/A
Date: April 2003


Question:
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.



Replies:
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 cylinders reciprocate.

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.

Vince Calder


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.

Lou Harnisch



Click here to return to the Engineering Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory