Question aboujt crank + rod/piston positioning

[7thGear]

just something that i always had on my mind

I am under the impression that most 4 bangers have the crank constructed in such a way that there are essentialy 3 critical points that the crank/rod/piston assembly reach.

1. 2 pistons on top, 2 at the bottom2. all pistons in the middle 3. 2 pistons on top, 2 at the bottom ( reversed)

same for all the 6 cylinder engines that i have ever seen open ( granded i never took one apart myself, but from general knowledge)

what always bothered me was why not construct a crank that allows for one piston to always be at the top of his cycle

so you would have

one piston on top, one piston 1/3 down, one piston 2/3 down, and one piston all the way at the bottom .

would this arrangment not produce a smoother running engine? i kind of see how having 2 pistons being pushed down at the same time would create more power, but then you have a time slot where no piston is making any power.

???comments?

am i correct to say that my way of thinking about it produces an engine that runs smoother, but at the cost of power? i need to be enlightned because this has been on my mind for a number of years

thx

[911/Q45]

Remember that a 4 stroke engine only fires every other time it's at TDC, so only one of the up pistons will be pushing down.

[7thGear]

so are u saying the way i THINK a four banger should run IS the way it does in most cases?

[911/Q45]

Seems that way to me, but I'm sure those more knowledgable will weigh in.

[forecast]

A cylinder fires once every 720 degrees.

In a four stroke, a cylinder fires every 180 degrees (720/4)

assuming a 1-2-3-4 firing order:

pistion 1 is TDC and firing.

Pistion 2 is BDC and about to compress

Piston 3 is TDC and about to start drawing

Piston 4 is BDC and about exhuasting

In an inline 4 there are four points on the crank.

In a V6 there are just three (for stronger simpler crankdesign) so two pistons share a point.

Imagine 1&2 share and 3&4 and 5&6

However in the V config, the angle of the V determines how the sharing piston compare to head. In an impossible I shape (v completely closed) the sharing pistons hit TDC and BDC at the same time. In a - shape (V opened up flat) they hit TDC abd BDC at opposite times.

Now assume the two heads are seperate engines of inline 3. Time the crank so a cylinder fires every 240 degrees (cylinder 2 then 4 then 6). Now open the V to the point at which the spacing between 1 and 2 is the same as 2 and 3 ... 120 degrees.

So looking at V6 shortblock once could see a pattern such as #4 TDC and #2 #6 at equal heights, while on the other side 1 & 5 are also at equal heights. But this isn't like an inline 4, turn the engine a hair and you'll see 2 and 6 moving in opposite directions.

[Eswift]

the phases in crankshafts are determined by balancing various moments...as you can see, this is why a V6 is almost as bad as a large in-line 4 that requires a balance shaft.

[7thGear]

what about inline 6's like the rb20?, is it still 2/2/2 like a v6, or is it top, 1/5 , 2/5, 3/5, 4/5, and bottom?

[Eswift]

middle two fire at the same time, then the two just outside the inner two, and lastly, the outermost pistons.

thus, a geometric separation of 120 degrees...

for a four stroke engine there will always be one power pulse in each cylinder every two revolutions of the crank. the optimum phase angle for the crank for evenly spaced power pulses is always

720/n, where n is the number of cylinders.

the way you describe woul dbe unstable, resulting in the shaking moment wreaking havoc on the crank and journals.