20 replies to this topic

[inox]

As far as I know, Inline 6 and V12 engines contain generally a symmetric crankshaft where crankpins are arranged as follows:

1 0°
2 120°
3 240°
4 240°
5 120°
6 0°

However, I would be interested to know if during the evolution of ICE someone has ever tried out an asymmetric crankpin configuration:

1 0°
2 120°
3 240°
4 0°
5 120
6 240°

I suppose the latter does not provide proper balancing?

[Nathan]

Can't help specifically, but have fun... V-12 Firing Order Display (enginehistory.org)

[Greg Locock]

Firing order would be 1 5 3 4 2 6 or 1 2 3 4 5 6 or 126453 or 156423

 

I think the first or third of those would be the most likely compromise. In general you try and avoid big jumps, as that sets torsional off, but if they are too close together you get charge robbing.

 

+1 for the nerdy site.

Edited by Greg Locock, 31 January 2022 - 03:35.

[inox]

ENGINE BALANCE: Inline 3, 4, 5, 6
https://youtu.be/82rxavW0A3c

Having watched this engine balancing video, I'm pretty sure the asymmetric crankshaft described in my first post does not make any sense and no (proper) engine designer would have ever made such a mistake. It would make naturally balanced inline 6 as unbalanced. The whole point of inline 6 is to have crankpins 1-3 mirrored as crankpins 4-6 to have equally balanced crankshaft at both ends.

[Greg Locock]

"The whole point of inline 6 is to have crankpins 1-3 mirrored as crankpins 4-6 to have equally balanced crankshaft at both ends"

 

I was carefully leaving that particular aspect alone. I think (but haven't checked) that your explanation might be more of a Just So story than have a basis in physics.

[inox]

My wording is far from perfect and I'm not sure if you understood correctly what I meant.

But lets think of an imaginary engine which has only single (loose) bearing in the middle of crankshaft. Now, lets attach 2 pistons with conrods to this crankshaft. In order to keep crankshaft balanced on its single center bearing, pistons must be attached on both sides of the center bearing, equal length away from it and pistons/conrods must be always moving in same direction. Otherwise our crankshaft with loose bearing will flip on its bearing.

Therefore for a perfect balance, inline 6 engine must have crankpins mirrored as follows:

1--|--6 0°
-2-|-5- 120°
--3|4-- 240°

| = crankshaft center bearing


If I got this wrong, please correct me. But I'm quite confident you can't find any other crankshaft arrangement in inline 6 than this symmetrical one.

[inox]

So this asymmetric crankshaft does not make sense from balancing point of view:

1--|4-- 0°
-2-|-5- 120°
--3|--6 240°

[Greg Locock]

Had another think, and your later explanation makes sense, I think you are right, you'll get the rocking couple that plagues I3 engines, since you have basically got two I3s nose to tail. there is a way to work this out but I've forgotten it and all my engine textbooks are 100 km away.

Edited by Greg Locock, 01 February 2022 - 22:48.

[inox]

One could of course wonder what happens in central bearing example (with two pistons), when only one of the cylinder fires?

However, the example considered only balancing the moving masses. (The 2 mirrored crankpins with pistons etc. do not actually balance the engine, but fix the rocking couple problem. All 6 cylinders are required for balancing the masses)

The other thing to consider are forces created by firing the cylinders. I'm not entirely sure how to think of this subject. Obviously cylinders are fired in specific order to obtain best/smoothest possible operation.

But I have some questions:
1) Does firing a cylinder cause significant vibrations (compared to moving masses)?
2) When a fuel explodes, doesn't it cause equal force towards cylinder head and piston, i.e. balancing the forces?
3) When a firing cylinder pushes a piston down with a force, doesn't this force actually affect all of the pistons which are mechanically connected via crankshaft, i.e. not really causing balance issues (at least not in the level that moving masses do)?

[Greg Locock]

The combustion forces are fairly small compared with imbalance forces. You'll measure about 5g rms on an i6 at the redline, on an i4 it is more like 20g, which is mostly second order and multiples due to the L/r effect (obviously second order and mulitples is also firing order which is why I4s are such a delight to work on)..

 

2) yes, but it still exerting a net downward thrust on the piston, so it moves. Yes the forces are internally resolved in the structure of block and bearings and crankshaft, but in doing so they excite resonances. There is a overall net torque at the flywheel which is reacted by he engine mounts.

 

3) it spins the crankshaft that bit harder, so yes the other pistons do move a bit. But you are now in the field of torsional vibrations. That is the crankshaft vibrates in response to the combustion forces, it does not just go round at a constant speed.

[TDIMeister]

2-stroke sixes have different crankpin assignments. Instead of a 3-plane, 120-degree separation, you have 6 planes 60-degrees apart. This is so that no 2 pistons are at TDC and therefore firing simultaneously.

 

V12s can have multiple firing orders but still generally follow the 3-plane crank as Craig already mentioned, generally using splayed crankpins if an unconventional vee-angle is employed to keep the firing sequence equally-spaced (except in race engines). However, flat-12s typically have shared crankpins on opposite banks on 120-deg crankshafts, aka 180-degree vee. Boxer-12s would typically use a 60-degree crank, as a 120-deg one would have 4 pistons at TDC simultaneously with the same problem as described in the top of this post.

 

You may ask - what's the problem with having more than one piston firing simultaneously? It sets up weird harmonics, because you have to think of the crankshaft and crankcase as elastic bodies. When a cylinder fires on one end to having a load at the flywheel or constraint via mounts and a free-end on the other, the crankshaft and crankcase are actually twisting and bending. Having uneven or simultaneous firing events cause different harmonic modes, some of which can be quite damaging (e.g., breaking of crankshafts, premature bearing failure) or at least annoying to the operator in terms of NVH. I believe this is Greg's area of expertise so I'll let him elaborate.

 

EPI has some good articles on this subject - start with here: 

http://www.epi-eng.c...ton_engines.htm

http://www.epi-eng.c..._properties.htm

http://www.enginehis...hortDays/TV.pdf

[Greg Locock]

TBH I've never come across it, but yes that would be murder for TVs, one double bang followed by a no bang at some point would set the whole structure ringing.

[TDIMeister]

Here's a practical application where the firing order is changed to reduce crankshaft stress to reduce incidences of failure at high states of tune.

https://www.dieselwo...he-firing-order

 

This is apparently somewhat common in SBC's.

[TDIMeister]

I haven't seen it stated explicitly as such, but I strongly believe it is also a reason for moving to a flat-plane crank in the C8 Corvette (in addition to better gas exchange tuning). Such a V8 would be 2 inline-4s on each bank mirrored and separated by 90 degrees.

 

If the cylinders were identified as below, using the common I4 firing order of 1-3-4-2 (I can't easily draw the crossover lines but you can visualize them in your head), 

 

1L     4R

2L     3R

3L     2R

4L     1R

 

1L-1R-3L-3R-4L-4R-2L-2R

 

You get a beautifully symmetric firing order, left-bank-right-bank alternation and same-bank cylinder firing event spacing for low crank harmonic stresses and very good gas exchange tuning potential. Unfortunately, like Greg said, the I4 has a secondary imbalance, and the flat-plane V8 makes it even worse (by a 1.4X factor IIRC), and this time manifested as a horizontal shake that is very difficult to design engine mounts for. However, secondary imbalance (and all higher orders) can be eliminated using a harmonic crank mechanism like a Cardano or hypocyloidal mechanism.

 

Example (of an inline-4, perfectly balanced in all orders) below:

 

Craig Laycock not to be confused with Greg Locock

 

In the above example, however, you would have the problem of double simultaneous combustion events already described before. It would be best to realise the above as a double-acting I2, I3 or I5, compromising on primary pitching moment imbalance.

 

Edit to add some other good reading on the subject:

https://www.enginela...ne-crankshafts/

https://www.f1techni...pic.php?t=23764

Edited by TDIMeister, 09 February 2022 - 01:24.

[gruntguru]

Yes, same problem as the Bourke scotch yoke engine.

 

I don't see simultaneous firing as a TV issue though - every firing is two cylinders - could be arranged as two outer cylinders then two inner or one of each. If the engine really was built with Freevalve technology it could run with either sequencing - even switch to the other sequencing at different engine speeds to avoid TVs.

Edited by gruntguru, 09 February 2022 - 23:22.

[TDIMeister]

Yes, same problem as the Bourke scotch yoke engine.

The many claims that the Bourke engine made were indeed problematic and almost all fantasy, But what is the specific problem you speak of Laycock's design which you say it shares with Bourke's?

Edited by TDIMeister, 09 February 2022 - 03:13.

[TDIMeister]

As far as I know, Inline 6 and V12 engines contain generally a symmetric crankshaft where crankpins are arranged as follows:

1 0°
2 120°
3 240°
4 240°
5 120°
6 0°

However, I would be interested to know if during the evolution of ICE someone has ever tried out an asymmetric crankpin configuration:

1 0°
2 120°
3 240°
4 0°
5 120
6 240°

I suppose the latter does not provide proper balancing?

Sorry that I took your thread on a bit of a tangent, but I hope that this will bring it back full circle. The Ford GT employs a flat-plane crank V8 but arranges the crankpins in an UP-DOWN-UP-DOWN manner instead of the traditional UP-DOWN-DOWN-UP I discussed above. This is analogous to your question but in an I6 context. There are two discussion threads of the same topic and original post content in different forums about this:

 

https://www.eng-tips....cfm?qid=401468

https://www.f1techni...pic.php?t=23764

 

There are many theories in the above threads as to the reason why Ford went this way, but none coming from the horse's mouth. The most plausible to me is that the arrangement introduces some primary imbalance intentionally. Most likely it trades off some of the horizontal shake, which gets compounded in large displacement traditional flat-crank V8s due to higher reciprocating mass components and lower rod L/R ratios, and the engineers figure they can live with the overall result. We can't know that this is the reason for sure unless we do a detailed analysis.

 

Ford's approach above (of living with a imbalance tradeoff) does have precedent in the engine world. Inline-5s have more than one crankpin arrangement and firing order in use. The one most commonly employed, 1-2-4-5-3, has a larger secondary couple than primary, but the mode is less objectionable to the vehicle occupant, so most engines with this firing order don't even use any balance shafts at all. GM had a 5-pot in the Colorado with a 1-3-5-4-2 that has a small secondary couple but large primary one, necessitating the use of a balance shaft turning at 1X engine speed.

 

Edit: The most authoritative rationales (from people directly within Ford engineering)come from here:

https://www.edmunds....-voodoo-v8.html

https://www.edmunds....flat-plane.html

 

I have an image of a table in an excerpt from a paper that lists the imbalances of a variety of engine layouts. It shows yet another I5 firing order, 1-5-2-3-4, that swaps the magnitudes of the primary and secondary couple of the 1-2-4-5-3 setup. This one would most likely need a 1X balance shaft as well. I know of no automotive use of this firing order, but it may appear in some industrial engine. Unfortunately I cannot post that image hosted on another forum here, so here's the link to the thread containing that table and a discussion -- with whom else but our longtime resident friend Manolis

https://forums.tdicl...-3#post-2994331

Edited by TDIMeister, 09 February 2022 - 20:03.

[gruntguru]

The many claims that the Bourke engine made were indeed problematic and almost all fantasy, But what is the specific problem you speak of Laycock's design which you say it shares with Bourke's?

 

The one you mentioned in the post immediately previous to mine.

 

 

In the above example, however, you would have the problem of double simultaneous combustion events already described before.

[gruntguru]

. . .. Unfortunately I cannot post that image hosted on another forum here, so here's the link to the thread containing that table and a discussion -- with whom else but our longtime resident friend Manolis

https://forums.tdicl...-3#post-2994331

I haven't seen Manolis at any of his usual haunts for several years. I hope he is OK.

[TDIMeister]

The one you mentioned in the post immediately previous to mine.

Ahh, I see. Well, Laycock's design doesn't have to have double-acting pistons. Then it would be just a well-balanced I4, but I think being able to fit 8 combustion chambers in such a compact package is part of the allure. It can use a cross-plane crank (no more natural primary balance) and solve the problem.

 

I haven't seen Manolis at any of his usual haunts for several years. I hope he is OK.

So do I. I've wanted to make a point to reach out to him, visit Greece, meet him and visit his shop in-person one day.

[Greg Locock]

Maybe he was testing his helicopter.