Are there any automotive engineers who can explain to me why an inline 4 cylinder vibrates worse than an inline 6? I think it is because all 4 pistons are simultaneously stopped before reversing direction, 2 at the top of the cylinder and 2 at the bottom. On an inline 6 there are always 4 pistons moving when 2 are stopped. I am just not smart enough to present facts that support my idea. I do know that lots of 4 cylinders have counter balance shafts turning at twice crankshaft speed to offset these vibrations. Respond at quickly as you can because I am eating my liver over this.
It is because 6 cylinder motors always have the same amount of piston weight going up and down at any given time. 4 cylinders inline do not. The pistons do not go up at the same speed as they go down due to rod angularity. This condition happens twice per revolution, so no counterweight running at crankshaft speed can offset it, although two running twice at crankshaft speed can.
I am not saying you are wrong about the rod angularity but I am having trouble grasping the concept. All I can think is that at 20 degrees before top center the piston would be traveling the same speed as at 20 degrees after top center. Also the rod would be at the same angle to the centerline of the cylinder in both cases. Is it possible for you to post a diagram that would help me grasp the concept? Thanks for the reply.
I know plenty of four cylinder engines that run smoothly and idle well. I suspect it has more to do with counter-balancing than anything else. I have a SCAT counterbalanced crank in my T's engine with aluminum pistons (of course). It runs very smoothly at pretty much any rpm. But that's only part of the equation. Getting valves, ignition, and consistent fuel delivery are probably just as important.
Late Japan engines with four cylinders in line run so smooth its some times hard to know they are running. The two in my daily drivers with ninety cubic inches have 108 HP------40mpg
Engineering on every thing from homes to cars has come a long way in a hundred years.
Why do the auto manufacturers spend all the money for counter balancing shafts if all that was needed is a counter balanced crank?
This link explains the secondary imbalance which is inherent in every in-line 4 cylinder engine, and how the angular displacement of the con rods means that motions of, and hence forces from, pistons are different in the upper and lower halves of crank rotation. Each piston is below its mid-travel when the crank throw is exactly sideways.
Looks like Tom answered the question with logic. I would like to meet him!
Jim, the answer is they wouldn't spend the money on balance shafts if they didn't need to. Balance shafts, running at twice crank speed as Tom mentioned have become so universal that manufacturers make no mention of them in their promotional literature anymore.
It's referred to as secondary imbalance. When two of the four pistons are at TDC, the other two are of course at BDC. Turn the crank 90 degrees, or half of the 180 degrees needed for a full stroke and all 4 will be at the same distance down in the bore, BUT they are not halfway. They are more than halfway due to the rod angularity Tom mentioned. How much more depends on the length of the rod. That means the inertia or force of the two pistons approaching BDC are not the perfect offset of the two approaching TDC. Again, that happens twice per revolution.
The forces involved in stopping and staring pistons are tremendous. The counterweights on a crankshaft are intended to reduce those forces for the main bearings closest to any particular piston. But counterweight size is a compromise. The counterweights are rotating, but of course the pistons are moving in a straight line. Counterweight forces at a constant rpm will be the same regardless of crank position. No so the forces of stopping and starting the pistons. That is highly variable. So a crank counterweight sized large enough to offset peak piston forces will then impart very high forces at the point where the pistons are transitioning from being accelerated to being slowed down, and stopped and started at TDC and BDC. So the crank counterweight size is a compromise. Also since those counterweights are on the crank they can only deal with primary forces, not the secondary forces Tom and I refer to.
Four cylinder engines do not have any power stroke overlap. Six, eight, and twelve cylinder engines do, and are significantly smoother. Primary and secondary imbalance are deep subjects that for most mortals require some significant study to understand.
Model T engines have all the rod journals in a row so when the pistons are at TDC/BDC all 4 rods are in a row, all 4 pistons are at their dead spots at the same time. There are no offsets on the crank, pistons or rods like other engines use.
Mark, I think what you mean is that a Model T uses a flat plane crankshaft. The centers of all journals lie on a single plane. That is true for 99.99% of all 4 cylinder engines ever made. The first V-8's used flat plane crankshafts and they suffered from the same secondary imbalance issues as 4 cylinder engines, but twice as often and 90 degrees out of phase. At some point Cadillac, IIRC developed the cross plane crankshaft which solved the secondary imbalance issue, at the expense of optimal intake and exhaust airflow. It also makes for a much heavier crankshaft, both effects are at the expense of performance. That is why the latest Shelby GT350 has reverted to a flat plane crankshaft. Dedicated racing V-8's and some exotic performance cars use flat plane crankshafts too.
The use of cross plane crankshafts also necessitated the creation of 2 barrel carburetors. A single carb feeding all 8 intake ports would suffer from some serious mixture distribution problems. In a modern V-8 one of those barrels feeds the middle two cylinders of one bank and the end cylinders of the other bank. The other barrel feeds the rest of the cylinders.
A four cylinder engine could use a cross plane crankshaft but they are more expensive to make.
Got it. How about the cylinders are on the same center line as the crank?
Mark: When clicking on your link I noticed that the 28 Chevy engine has a crank that is offset 3/8 inch to the left. What is the advantage of this?
After reading this thread I think that I should have gone to school instead of letting them burn my desk to get rid of me.
There are lots of reasons for offsetting the crank. Just like any design consideration there are advantages and disadvantages. Offsetting the crank can affect the thrust on the cylinder wall and hence, piston slap. When the crank is offset the piston travels at a different speed on one half of the stroke than the other. In other words it will take longer to go down than to come up, or vice versa, depending on which direction the crank is offset. One way will give a mechanical (leverage) advantage on the power stroke, at the expense of a disadvantage when pumping out the spent exhaust, or vice versa, again depending on which way the crank is offset. Also, an offset crank increases the piston sweep to a value higher than the stroke of the crank therefore increasing the engine displacement.
Offsetting the crank away from the thrust side reduces the angle of the rod on the power stroke resulting in a reduced thrust angle during the power stroke. It makes a "straighter push" on the crank throw. Also, the crank throws are not vertical at the same time the piston changes direction.
I didn't take the classes that would teach me this stuff, got to learn somewhere and this is about as good as gets, lots of people that know a thing or two.
Still learning, so asking the questions.