How many crank revolutions to fire all cylinders?

[RichGT350R]

Looks like you found your answer. Sorry for the confusion! Best Regards!

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[JAJ]

Messing

720 degrees divided by 8 cylinders is 90 degrees. BTW, the cylinder banks are at 90 degrees to each other across the valley.

So when the FPC is "up and down" to the left bank, two pistons are at TDC and two at BDC. Of the two at TDC, one is on a compression stroke and is ready to fire, while the other is at the top of the exhaust stroke. Meanwhile, all the right bank pistons are half-way between TDC and BDC and moving fast. At this TDC moment, the ready-to-fire left-bank cylinder fires.

90 degrees later, the left bank pistons are all half way and moving fast, while two right bank pistons are at TDC, one ready to fire and one at the top of the exhaust stroke. The other two right bank pistons are at BDC. Then the ready to fire cylinder fires exactly 90 degrees after the previous left bank firing described above.

This process repeats every 90 degrees of rotation, and after happening 8 times, all 8 cylinders have fired once and the engine has turned over exactly two revolutions.

In a CPC engine, the same thing happens, but the timing is different because the crank throws are offset 90 degrees instead of 180 degrees. At any TDC moment, two pistons are at TDC, two are at BDC and four are half-way, just like the FPC, except that the CPC makes sure the two at TDC are on different banks where the FPC makes sure they're always on the same bank.

This difference is why the CPC engine runs smoother than a FPC engine. Every 90 degrees, the FPC engine has, on one bank, two pistons stopped at TDC and two stopped at BDC, while on the other bank it has four pistons that are half-way and moving fast. In contrast, every 90 degrees each bank of the CPC engine has one piston stopped at TDC, one stopped at BDC and two moving fast at half-way. It's got a better balance of motions, so it runs smoother, but it has the downside of having two cylinders fire on the same side once every revolution. You trade off highly balanced air movement (ie-power) for highly balanced mechanical movement (ie-comfort).

 

[4sdvenom]

I guess I'm still missing something. How do you get 1 piston at TDC to fire every 90 degrees? When looking at a flat plane crank I see 4 pistons at TDC and 4 pistons at BDC. 2 pistons per bank at TDC to fire and 2 at TDC to finish the exhaust stroke and begin the intake stroke. So if it's rotated 90 degrees from that point all pistons would be halfway in the cylinder 4 going down and 4 coming up! I'm definitely missing how it can physically fire a cylinder at TDC every 90 degrees.

EDIT: Nevermind, my example assumes the left and right cylinder banks are also on the same 180 degree flat plane. I didn't take into account the opposing cylinders are 90 degrees out!
For the record, the post above mine was posted while I was typing up mine lol.
Thanks

 

[Allan]

720 degrees divided by 8 cylinders is 90 degrees. BTW, the cylinder banks are at 90 degrees to each other across the valley.

So when the FPC is "up and down" to the left bank, two pistons are at TDC and two at BDC. Of the two at TDC, one is on a compression stroke and is ready to fire, while the other is at the top of the exhaust stroke. Meanwhile, all the right bank pistons are half-way between TDC and BDC and moving fast. At this TDC moment, the ready-to-fire left-bank cylinder fires.

90 degrees later, the left bank pistons are all half way and moving fast, while two right bank pistons are at TDC, one ready to fire and one at the top of the exhaust stroke. The other two right bank pistons are at BDC. Then the ready to fire cylinder fires exactly 90 degrees after the previous left bank firing described above.

This process repeats every 90 degrees of rotation, and after happening 8 times, all 8 cylinders have fired once and the engine has turned over exactly two revolutions.

In a CPC engine, the same thing happens, but the timing is different because the crank throws are offset 90 degrees instead of 180 degrees. At any TDC moment, two pistons are at TDC, two are at BDC and four are half-way, just like the FPC, except that the CPC makes sure the two at TDC are on different banks where the FPC makes sure they're always on the same bank.

This difference is why the CPC engine runs smoother than a FPC engine. Every 90 degrees, the FPC engine has, on one bank, two pistons stopped at TDC and two stopped at BDC, while on the other bank it has four pistons that are half-way and moving fast. In contrast, every 90 degrees each bank of the CPC engine has one piston stopped at TDC, one stopped at BDC and two moving fast at half-way. It's got a better balance of motions, so it runs smoother, but it has the downside of having two cylinders fire on the same side once every revolution. You trade off highly balanced air movement (ie-power) for highly balanced mechanical movement (ie-comfort).

Perfect explanation. Thanks

 

[oldbmwfan]

Seems like a few folks confused the term "180 degree crank" with the degrees of crankshaft rotation between firing. The 180* crank (flat plane) is descriptive of the relationship of the throws to each other.

By definition, in any 4-stroke engine there will be one detonation per cylinder every 4 strokes, which is one per cylinder every two crank rotations (up-down-up-down for a single piston). So you figure the degrees of crank rotation per cylinder simply by dividing 720 degrees of rotation by the number of cylinders.

In a single-piston, there is 1 combustion every 720 degrees, or two crank rotations. In a twin, it's once per rotation (one every 360*). 240 degrees between combustion events in a 3-cylinder (Triumphs, for example), 180 degrees between combustions in a 4-cylinder, 144 degrees in a 5-cylinder (crazy Audis), 120* in a 6-cylinder, 90* in an 8-cylinder, 72* in a 10-cylinder, and 60* in a 12-cylinder. Notice that every common piston engine configuration divides evenly into 720.

This is one reason why engines with higher cylinder counts run more smoothly - less crankshaft rotation between firing pulses reduces the vibration. There's a whole other discussion on harmonics, which is a major factor in the selection of V-angle - but it tends to correspond to the crank rotation (90 degree angle between banks is ideal for a V8, 72 degrees for a V10, 60 degrees for a V12).

 

[Richie18]

I think the OP was thinking 180 deg / FPC was like a boxer engine.

 

[4sdvenom]

I think the OP was thinking 180 deg / FPC was like a boxer engine.

As was I
All cleared up now!

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