Question re RPM

I think you are sort of mixing two questions together in your mind.

For an individual cylinder of a 4-stroke engine, each cylinder fires every other revolution(regardless of the total number of cylinders) -- so:

individual cylinder at 3000 RPM =
(3000 rev/min) x (1 firing event/2 revs) = 1500 firing events/minute at each cylinder (for both a V8 and a V12)

As far as the TOTAL number of engine firing events occurring for all cylinders, this does scale with number of cylinders -- so:

for a V8 at 3000 RPM = (1500 firing events/min/cylinder) x (8 cylinders) = 12000 firing events/minute total

for a V12 at 3000 RPM = (1500 firing events/min/cylinder) x (12 cylinders) = 18000 firing events/minute total

 

Lets assume its a four stroke engine

The cycle is suck squeeze bang blow for each cylinder

This represents two full rotations of the crankshaft

Each cylinder is connected to the crankshaft via the con rod in such a way as the piston goes up 3000 times per minute and down 3000 times if the crank is rotating at 3000 rpm

So each cylinder fires 1500 times in each minute at 3000 rpm regardless of how many cylinders

If you have the 8 & 12 cylinder engines have the same capacity then the distance travelled for each is a function of the 'stroke' of the engine

If the engines are (for example) 3 litres / 8 cylinder and 4.5 litres / 12 cylinder with the same bore and stroke the distance travelled in each cylinder would be the same

 

Same sort of answer -- at 3000 RPM, each individual piston is making the exact same number of strokes regardless of the total number of pistons; however, the TOTAL number of piston strokes will be more for the V12 just because it has more cylinders.

 

Dave- This animation may help you see what is going on. Only the exhaust valve is shown for simplicity. Count the number of crankshaft revolutions compared to spark plug ignition events.

http://www.animatedengines.com/otto.shtml

At a first principle level, the reason multi-cylinder engines are smoother than those with less cylinders is more power strokes per revolution. For a 12 cylinder there are 12 per 720 degrees or one every 60 degrees. For an 8 cylinder, 8 per 720 for one every 90 degrees.

Multi-cylinder engines also give a better chance of balancing out secondary imbalance forces. On larger four cylinders, for example, a balance shaft may be needed to smooth out those imbalances, while a 90 degree between cylinders V8 or a 60 degree V12 has no secondary imbalances. Ferrari's 180 degree V8 has some minor secondary imbalances, but the advantage of exhaust scavenging outweighs those. The same for Ferrari's 65 degree V12, but there the extra room for the intake tract is the advantage.

Taz
Terry Phillips

 

Yes (for an engine with more than 4 cylinders). On a V8, a different cylinder fires every 90 deg of crankshaft rotation (720 deg/8 = 90 degrees) so the cylinder following the #1 firing occurs when piston #1 is about 1/2 way down. On a V12, the next cylinder following the #1 firing firing occurs even sooner since a different cylinder fires every 60 deg of crankshaft rotation (720 deg/12 = 60 degrees). On a 4 cylinder, the next cylinder following the #1 firing occurs when piston #1 is at the end of its power downstroke (720 deg/4 = 180 degrees).

 

Worth noting that Ferrari V8s have a "flat plane" crankshaft, so do not fire every 90 degrees, but two cylinders fire every 180 degrees.

 

Only "per bank" as with a 4 cylinder. But a flat crank V-8 still fires every 90 degrees, its just swapping from bank to bank between power strokes.

 

Dan- What you are describing is a true boxer engine, but the cylinders have to be arranged at 180 degrees between banks for that to work well. In theory, the two cylinders firing simultaneously on opposite sides of the crankshaft balance out each other. In actuality all the flat four, six, eight and, twelve cylinder street and racing engines in recent memory work like V4s, V6s, V8s and V12s in firing order with VX number of cylinders igniting their charges separately. The last true street boxer engine of which I am aware (someone will undoubtedly find another) was the horizontally opposed BMW boxer twin fitted to their motorcycles.

Taz
Terry Phillips

 

Which is why the TR is referred to as a 180 degree V12.

 

David- So were the three models of the Boxer Berlinetta. Not boxers, but catchy name.

Taz
Terry Phillips

 

Yup.
Have won a few bets on that

 

I admit to not being an expert in these matters, but I thought we had 4 sets of firing per 2 engine turns for a flat-plane crank, for example: Turn 1, bank 1 (cylinder 1 & 3), bank 2 (cylinder 2 & 4) then Turn 2, bank 1 (cylinder 2 & 4), bank 2 (cylinder 1 & 3).

Not sure if its one cylinder from each bank at the same time, but I definitely was under the impression that there were 4 pairs that fired in sequence every 180 degrees???

 

Dan- A flat, 180 degree crankshaft V8 has the same number of firing cylinders per crankshaft revolution as a 90 degreee crankshaft. Interestingly, all early V8s used 180 degree crankshafts (ie Hispano Suiza) until the 90 degree V8 crankshaft was developed around 1915 and first used by Cadillac in 1924.

The 90 degree (crossplane) crankshaft has four throws spaced 90 degrees apart, while the flat plane crankshaft has four throws spaced 180 degrees apart. The advantage of the flat plane crankshaft is that it operates like two four cylinder engines on the same crankshaft and makes exhaust design simpler. The crossplane crankshaft V8, however is smoother, with no secondary imbalances, ideal for a street engine. All current American V8s use a crossplane crankshaft.

Taz
Terry Phillips

 

Very informative!

 

A properly balanced 180 degree crank may only weight 60% as much as a properly balanced 90 degree crankshaft. In addition the intake and exhaust openings are symetrical so that the breathing pulses are symetrical RLRLRLRL while the 90 degree crank has an RLRRLRLL firing interval. Thus you can tune the intake airflow for better power (helmholtz resonators) and get more efficiency from a dual exhaust with headers.

 

I think your getting most of it, but you need to take it apart and see the engine from inside.

The right bank on a 308 is the rear bank. Left is front. Both banks are numbered 1 thru 4, but additionally, the left bank is numbered 5 thru 8. But, #1 on the right bank, and #1 (#5) on the left bank, are numbered from opposite ends. IOW, #1 right bank is on the timing belt end of the engine, and #1 on the left is at the flywheel end. If we stick with 1 thru 4 firing orders on both banks, the firing is duplicitous of an inline 4 cylinder engine. Firing order is then 1,1,3,3,4,4,2,2. Or, if we used a more conventional numbering, keeping 1 thru 4 on one bank, and 5 thru 8 on the other, 1,5,3,7,4,8,2,6 ( note the 1,3,4,2 order). The left bank is firing exactly like the right bank, but 90 degrees later. All 8 cylinders fire within 2 revolutions, 720 degrees, every 90 degrees of rotation, regardless of crankshaft design. In fact all 4 stroke cycle engines, regardless of the number of cylinders, fire all their cylinders within the same 2 revolutions. Just take 720 and divide by the number of cylinders an engine has to find the degree separation between cylinders.

If you can picture how a standard inline 4 operates, add a second bank of cylinders to it and connect the rods onto the throws of the original crankshaft so you have two rods per throw from opposing banks.

 

Very helpful, I understand better now! Thanks.