For example, why would a car manufacturer opt to to use a 2.0L in line six instead of a 2.0L in line four in a vehicle. Are the benefits of including more cylinders in an engine worth the added complexity?
In: Engineering
Straight out one reason to move to an I6 is that that engine layout is inherently very balanced. It’s going to run smoother without the same effort put into the I4 to try to balance it.
Second, an I6 has a piston firing every 120 degrees of crankshaft rotation, while an I4 has a piston firing every 180 degrees. So you get more power strokes per full revolution. Yay!
But say you want it to rev higher to get more horsepower. There’s a mechanical limit on how fast the pistons can go up and down (piston velocity). The higher the rpm of the engine, the faster those pistons are moving. And all that mass of the pistons is being yanked back and forth, which is a lot of stress. Eventually something breaks. For our purposes, let’s say that I4 is already running at the fastest piston velocity it should.
So you have a 2.0l 4 cylinder. Each cylinder is 0.5 liters. This is determined by the diameter of the cylinder and the stroke, or how much the piston goes up and down. If you go to I6, each cylinder is .333 liters, a third smaller.
We go I6 with the same displacement and cylinder diameter as the I4, but with that one-third less displacement per cylinder. This means the stroke of each piston is a third shorter, which means it doesn’t have to go up and down as far. So we get a lower piston velocity on the stroke than we had on the I4, all else being equal. Now we can rev higher and get more horsepower without having to make everything a lot stronger to handle higher piston velocity.
Or we can reduce the cylinder diameter so that the pistons are smaller and thus lighter, and thus easier to build to withstand a higher piston velocity
Or we can reduce both, a somewhat shorter stroke with smaller and lighter pistons.
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