V12 firing order - 3 firing at once?
Discussion
>hope this is the place for a bit of engine geekery 
<
Was just thinking about the firing order on a V12 (4 stroke) (as you do) and how this compares to a V8? Am I right in thinking that 3 cylinders will always be on the firing part of the cycle per stroke on a V12? Similarly it will be 2 cylinders actively firing per stroke on a V8? So, if it is a similar 5000cc engine, on the V8 it will be effectively 1250cc of 'push' for every stroke and on the V12 it will ALSO be 1250cc swept but happening a third more often in time (although rpm will be the same)?
Brain hurts a bit and apologies if that is nonsense!
< Was just thinking about the firing order on a V12 (4 stroke) (as you do) and how this compares to a V8? Am I right in thinking that 3 cylinders will always be on the firing part of the cycle per stroke on a V12? Similarly it will be 2 cylinders actively firing per stroke on a V8? So, if it is a similar 5000cc engine, on the V8 it will be effectively 1250cc of 'push' for every stroke and on the V12 it will ALSO be 1250cc swept but happening a third more often in time (although rpm will be the same)?
Brain hurts a bit and apologies if that is nonsense!
Ken Figenus said:
>hope this is the place for a bit of engine geekery <
Was just thinking about the firing order on a V12 (4 stroke) (as you do) and how this compares to a V8? Am I right in thinking that 3 cylinders will always be on the firing part of the cycle per stroke on a V12? Similarly it will be 2 cylinders actively firing per stroke on a V8? So, if it is a similar 5000cc engine, on the V8 it will be effectively 1250cc of 'push' for every stroke and on the V12 it will ALSO be 1250cc swept but happening a third more often in time (although rpm will be the same)?
Brain hurts a bit and apologies if that is nonsense!
If you simplify it to old engines with a distributor.< Was just thinking about the firing order on a V12 (4 stroke) (as you do) and how this compares to a V8? Am I right in thinking that 3 cylinders will always be on the firing part of the cycle per stroke on a V12? Similarly it will be 2 cylinders actively firing per stroke on a V8? So, if it is a similar 5000cc engine, on the V8 it will be effectively 1250cc of 'push' for every stroke and on the V12 it will ALSO be 1250cc swept but happening a third more often in time (although rpm will be the same)?
Brain hurts a bit and apologies if that is nonsense!
The dizzy's rotate and fire 1 cylinder at a time on most engines.
So for a normal V8, each cylinder fires only once throughout the 4 stroke cycle.
A flat plane V8 is like 2 x 4 cylinder engines and it will have 2 cylinders firing together each time, once only during each 4 stroke cycle.
Not totally familiar with the Jag V12, but some are treated as 2 x 6cyl engines...so they too may have 2 cylinders firing at a time, once only during each 4 stroke cycle
Generally speaking, most engines have an evenly spaced firing order.
So, over the 2 complete crank rotations that a 4stroke engine must rotate to complete a full cycle, which is 720 degCA, we get:
4 cyl 720/4 = fires every 180 degCA
6 cyl 720/60 = fires every 120 degCA
8 cyl 720/8 = fires ever 90 degCA
10 cyl, 720/10 = fires every 72 deg
12 cycle 720/12 = fires every 60 degCA
However, some multi bank engines have a V angle that is not "symetrical" and hence have unevenly spaced firing angles. Something like a 90deg V10 for example often has a mix of firing angles, because it's v angle (90degC) is not completely divisible by it's basic cylinder firing angle (72degCA)
The common engine architectures are common because these tend to produce the smoothest engines, and the ones where it is easiest to achieve good exhaust scavanging (because it's possible to connect the exhaust primaries in a suitable order) Again, a 90deg V angle V8 can be "dual plane" or "flat plane", with the first being the smoothest, but least powerful and the second being less smooth but more powerful.
So, over the 2 complete crank rotations that a 4stroke engine must rotate to complete a full cycle, which is 720 degCA, we get:
4 cyl 720/4 = fires every 180 degCA
6 cyl 720/60 = fires every 120 degCA
8 cyl 720/8 = fires ever 90 degCA
10 cyl, 720/10 = fires every 72 deg
12 cycle 720/12 = fires every 60 degCA
However, some multi bank engines have a V angle that is not "symetrical" and hence have unevenly spaced firing angles. Something like a 90deg V10 for example often has a mix of firing angles, because it's v angle (90degC) is not completely divisible by it's basic cylinder firing angle (72degCA)
The common engine architectures are common because these tend to produce the smoothest engines, and the ones where it is easiest to achieve good exhaust scavanging (because it's possible to connect the exhaust primaries in a suitable order) Again, a 90deg V angle V8 can be "dual plane" or "flat plane", with the first being the smoothest, but least powerful and the second being less smooth but more powerful.
Penny on its way down..!
I always though all this talk of degrees was the physical angle the cylinders were at. It is however rather an angle of the crank big ends relative to each con rod connection?
So its more firing pulses per 720 degrees of crank rotation on a V12 compared to a V8 an never more than one cylinder firing at once...as per the distributor on my dads rover v8 once!
I always though all this talk of degrees was the physical angle the cylinders were at. It is however rather an angle of the crank big ends relative to each con rod connection?
So its more firing pulses per 720 degrees of crank rotation on a V12 compared to a V8 an never more than one cylinder firing at once...as per the distributor on my dads rover v8 once!
Ken Figenus said:
Penny on its way down..!
I always though all this talk of degrees was the physical angle the cylinders were at. It is however rather an angle of the crank big ends relative to each con rod connection?
So its more firing pulses per 720 degrees of crank rotation on a V12 compared to a V8 an never more than one cylinder firing at once...as per the distributor on my dads rover v8 once!
Angle of cylinders is largely irrelevant.I always though all this talk of degrees was the physical angle the cylinders were at. It is however rather an angle of the crank big ends relative to each con rod connection?
So its more firing pulses per 720 degrees of crank rotation on a V12 compared to a V8 an never more than one cylinder firing at once...as per the distributor on my dads rover v8 once!
The angles he refers to are simply crankshaft rotation and firing strokes relative to this.
720deg, ie 2 full crank rotations is one full cycle of a 4 stroke engine. So all cylinders will have fired once within that period
If you have more cylinders, they will fire at closer intervals.
stevieturbo said:
A flat plane V8 is like 2 x 4 cylinder engines and it will have 2 cylinders firing together each time...
Not so Stevie..."The firing order with a flatplane V8 allows for perfect balance between banks - each subsequent firing cylinder being on alternating left and right banks (there are a number of possible firing orders e.g. 1-6-3-2-7-4-5-8 or 1-4-5-2-7-6-3-8 but all follow a R-L-R-L-R-L-R-L...bank pattern)."
stevieturbo said:
So for a normal V8, each cylinder fires only once throughout the 4 stroke cycle.
For any 4 stroke engine each cylinder only fires once during the 2 crank revolution 4 stroke cycle!stevieturbo said:
A flat plane V8 is like 2 x 4 cylinder engines and it will have 2 cylinders firing together each time, once only during each 4 stroke cycle.
Nope. Each bank has symmetrical 180 degree firing intervals between its pistons as in any straight 4 but the banks are staggered by the V angle, 90 degrees obviously for even firing intervals, so a piston from one bank fires, a piston from the next bank fires 90 degrees later and so on. If the bank angle is not 90 degrees then the firing intervals will not be symmetrical. Now a flat plane flat eight (180 degree V) could potentially have two cylinders firing at the same time. The bank angle is key.stevieturbo said:
Not totally familiar with the Jag V12, but some are treated as 2 x 6cyl engines...so they too may have 2 cylinders firing at a time, once only during each 4 stroke cycle
The V12 is no more complex than the V8 really. With 12 cylinders firing in 720 degrees the average interval is 60 degrees and therefore so must be the bank angle to achieve even firing intervals.The cross plane 90 degree V8 also has symmetrical 90 degree firing intervals but NOT alternating evenly on each bank. The firing order will be RLRRLRLL in a variety of possibilities depending on how the cam is ground. This is what creates the offbeat exhaust burble of the traditional American V8. It's not a firing interval irregularity but an exhaust pulse one.
The key point is that when the V angle is not the same as the average firing interval then the firing intervals must be irregular, UNLESS the shared crank pins are offset ground. I hate to give it to him but Max laid it all out pretty well in his first post. He must be cock-a-hoop at getting something right finally
The V6 engine is the most problematical. With 120 degree average firing intervals the bank angle ideally also needs to be that but then the engine would be too wide. So the bank angle is generally narrower with the crank pins offset ground to bring the firing intervals back to evenly spaced.
Finally, remember that each time a piston hits TDC it can either be on a firing stroke or the end of an exhaust stroke and all that determines this is how the cam is ground. Most straight 4 engines fire in the order 1 3 4 2, but because the cam lobes are ground differently on the Ford Crossflow it fires in the order 1 2 4 3 despite having an identical crank to any other straight 4 engine. For a straight 4 engine these are the only two possibilities but for V engines the number of possible firing orders multiply with the number of cylinders. However the firing intervals will always be a function of the V angle and crankpin offsets.
Edited by Pumaracing on Wednesday 11th February 08:33
stevieturbo said:
I know BMW's old V12....was treated as 2 engines. It even had two dizzy's, two ecu's etc and could run on just one bank should the other side die ( assuming mechanical parts didnt prevent this )
Until recently, Aston V12's were also "two banked", running totally separate Ecu's for each bankPumaracing said:
I hate to give it to him but Max laid it all out pretty well in his first post. He must be cock-a-hoop at getting something right finally
Contrary to popular belief and overwhelming evidence, i am not ALWAYS right! More years ago than i care to remember, on one of my first engine dev jobs, i was given the task of designing a new camshaft for an engine where we wanted more performance. Doing it "properly" i wrote my own dynamic valvetrain model (in Vb!), and developed an awesome profile, that was going to deliver big time.So, we got a few prototype cams made up, built the engines with these super new cams in, and installed one on the testbed.
And, er, we couldn't get the engine started! Backfires, pops, bangs, crank leaping around a bit, but no running.
What had i done wrong? Yup, forgotten this was an "old skool" gear driven cam, and so spun in the OPPOSITE direction to the crank........... ;-)
(face was saved,when after we ground some cams the right way round, the engine went on to make over 50bhp more than it had ever done before ;-)
Talking about camshaft design, 20 or so years ago a well known crankshaft manufacturer of my acquaintance who made replacement cranks for vintage Bentley engines amongst many other things decided they'd have a lash at making the cams too. They had the ideal equipment for roughing out the shafts and lobes and just needed the final grinding outsourced. They had a bunch made up and started selling them and the cam lobes and followers wore out in no time flat. Much head scratching and refunding of customer's money later they finally learned that pushrod cams need a small angle across the lobe because the followers are slightly domed and the lobe angle is needed to ensure contact on the side of the follower to spin it and even up wear. With no lobe angle the cam lobe and follower only contact in the centre. The follower doesn't rotate. They were experts in crank manufacture but didn't know enough about cams to even get one working.
I always say it's not what know you don't know that catches you out. You can research that if you know you don't know it. It's what you don't know you don't know that you never think to check on because you don't even realise it's an issue.
Building perfect engines is an exercise in knowing every damn tiny thing that most people don't even realise are issues.
I always say it's not what know you don't know that catches you out. You can research that if you know you don't know it. It's what you don't know you don't know that you never think to check on because you don't even realise it's an issue.
Building perfect engines is an exercise in knowing every damn tiny thing that most people don't even realise are issues.
Edited by Pumaracing on Friday 13th February 13:03
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