Automobile Engines With a Long Stroke

[california_jobcase]

Couldn’t be too much worse than some of the stuff GM put out- the '61 Pontiac 4, or that split-pin V6 crank. I guess it would have to be a long crank and engine, with a pin for each cylinder. Probably wouldn’t be worth trying, but I find weird engine configurations fun to think about.

[Robot_Arm]

@snfaulkner

I think there would be much difficulty in balancing such a weird crankshaft.

I think you might be able to balance it with an inline-6 if you matched the cylinders that were in sync; say 1 and 6 are oversquare, 3 and 4 are undersquare.

I see other problems, though. At the top of their strokes the pistons would be at different distances from the crank. What would the head look like? Assuming you wanted the same compression ratio in each cylinder, you might need a separate head for each cylinder, or at the very least some odd configuration where the head gasket was not parallel to the axis of the crankshaft.

@california_jobcase

Couldn’t be too much worse than some of the stuff GM put out- the '61 Pontiac 4, or that split-pin V6 crank. I guess it would have to be a long crank and engine, with a pin for each cylinder. Probably wouldn’t be worth trying, but I find weird engine configurations fun to think about.

I’ve heard of some odd ones. Wasn’t there a Honda motorcycle engine some years ago that had oval cylinders? And BRM raced an H-16 engine in Formula 1 back in the '60s.

[DtypeJag]

@Robot_Arm

I think you might be able to balance it with an inline-6 if you matched the cylinders that were in sync; say 1 and 6 are oversquare, 3 and 4 are undersquare.

I see other problems, though. At the top of their strokes the pistons would be at different distances from the crank. What would the head look like? Assuming you wanted the same compression ratio in each cylinder, you might need a separate head for each cylinder, or at the very least some odd configuration where the head gasket was not parallel to the axis of the crankshaft.

I’ve heard of some odd ones. Wasn’t there a Honda motorcycle engine some years ago that had oval cylinders? And BRM raced an H-16 engine in Formula 1 back in the '60s.

By the way in 1967 BRM eventually got 420hp @11,000rpm out of that unusual 3 liter engine. They expected close to 600. It was not a success though it won I think one race.

[SHARKBITEATTACK]

@DtypeJag

If you have 2 single cylinder engines one oversquare and one undersquare but of the the same displacement would they not have the same torque at low rpm? Assuming, of course, everything else being equal.

The longer stroke of the undersquare giving a greater moment at the crank would be offset by the lesser area of the piston given equal pressure in the cylinders, conversely a lesser moment at the crank compensated by the greater area of the piston in the oversquare design. The actual situation is quite complex but in general, contrary to what seems like common sense, in an “on paper” sense they should have equal torque.

I think the general impression that long stroke engines have inherently greater torque comes from practical examples that people are familiar with. These engines are designed for lower rpm operation in the name of longevity namely ring float and inertial loads due to high piston speeds. Cam timing is designed around low speed operation as is the induction system.

Subjectively the lower the rpm the peak torque is reached the greater it feels to the rider or driver. A racing engine that is way oversquare and develops its peak torque near the hp peak will feel less torquey even though of the same peak torque as an undersquare engine.

Yes, what you are describing about bore and stroke has been proven mathematically but as we all know in the real world things work out differently. 2 Cylinders with equal displacement regardless of their bores and strokes should apply an equal amount of force to the crankshaft.
http://craig.backfire.ca/img/torque-bore-stroke.png