The reason they don't just fire "one end to the other end" is balance.
The reason the firing order isn't "consecutive" is due to the conventional
way(s) of numbering cylinders.
Straight 4 cyl = 1234
Straight 6 cyl = 123456
Some V8 = Left 1357 Right 2468
Other V8 = Left 5678 Right 1234
Feel free to build you own engine, and make it fire "consecutively" using
normal numbering (have fun as your engine shakes the car to pieces), or
get a felt pen and number your cylinders any way you like. :-)
It's simple. it's not possible to balance the crankshaft in most engines
to do that and still have an engine that fit's into a vehicle. Take a
simple 4 cylinder. The crank throws have to be 90 degrees off from each
other to maintain balance. Then you need to balance out the rocking
coupled frequency from the firing order. If you don't the vibration will
quickly destroy the engine.
On a V engine you have 2 cylinders on each crank throw. So you have
cylinders 1&2, 3&4, 5&6 each sharing a throw. Number one is at TDC but
number two is already past TDC and heading down the bore by the time
number one fires. Impossible to have them fire in sequence.
There are some engines out there that have a sequential firing order,
but the crankshafts have split journals and generally they are not
capable of much power output because of the crank design.
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