Why are straight six engines so good?
A straight-six is, more or less, the perfect engine configuration. This is due to the fact that by nature, they are perfectly balanced in both primary and secondary mechanical balance.
The primary balance is from the pistons moving up and down. As they move up and down, this creates momentum inside the engine that rocks the engine around. On an inline 6, each piston has a complimentary piston that is the same distance from the center.
For example, piston 1 and 6 (the two on the ends) move in synch. As 1 moves up, 6 moves up. As 1 moves down, 6 moves down. As such, they cancel out each other’s momentum. The same goes for 2 and 5, and 3 and 4.
Secondary balance is the rotation of the engine. As the crankshaft rotates, the arms that the pistons are connected to circle out around the center of the crank. Imagine grabbing a bucket and swinging it around. That bucket will fling you back and forth as it swings.
Now imagine having two buckets, one in each arm, and swinging them opposite of each other. They’ll cancel each other out and you’ll be able to standstill.
This is what inline 6 has. Since there are two sets of 3 cylinders, they create a pair of triangles. Imagine two opposing triangles (much like a 6-sided star). Whenever the crank is throwing its weight in one direction, it has an equal and opposite weight throwing itself in the opposite direction, canceling it out.
What this all means is that the natural vibrations of the engine are almost zero, without ANY effort on the engineer’s part. With other engine configurations, there are LOTS of inherent vibrations that need to be counteracted with weights and balance shafts so that they don’t vibrate like crazy. You don’t have to worry about that with an inline 6. They’re super smooth by nature.
Next, since all of the pistons are in a single line, there can be more bearings in the engine. Many inline 6 engines have 7 bearings on the crankshaft. Most other engines have 3 or 4 (maybe 5) bearings. This keeps everything very stable and makes them last a long time.
Also, their design puts the crankshaft and camshaft very close together. This allows the timing gears to actually TOUCH. This removes the need for a chain or a belt, meaning it’s almost impossible for them to break, or stretch over time, etc. This makes them very robust.
Another perk of an inline engine (I4, I5, I6, I8, etc) is that you can have the intake on one side and the exhaust on the other. This keeps components separated, and reduces heat. With a V configuration, the intake is on top of the engine, sitting in a valley, which is very hot. Inline engines are naturally cooler due to the main heat-generating components being on the side.
Lastly, since they’re a non-compact design, everything is accessible. Both sides of the engine are flat (since the pistons go up and down), so there is usually lots of room to access engine components on the sides. This makes repairs simple.
The downside is that they’re a long engine… This is really about the only reason they’ve been mostly phased out. It’s more difficult to fit them into today’s compact designs. Otherwise, they’re the ideal engine!
from Justin Lloyd, Software Engineer