Connecting Rods - Fork type vs. Side by Side

Harley Davidson aircooled engines still use fork and blade construction; not sure about the liquid cooled (Porsche designed) v-rod engine.

The W engine has way more frontal area than a V, an important consideration in aero engine choice. Length is not so critical in an airframe. Plus, that thing would be a mechanic's worst nightmare. Imagine changing two plugs per cylinder under field conditions, in a hurry to get the plane back in the air! Even the Napier Sabre would be easier to maintain, the plugs were all easy to get at on them.

In the January 2013 issue of Cycle World Kevin Cameron describes BMWs' new clean sheet of paper R1200GS motorcycle engine. In addition to a lot of very state of the art tech it has side by side connecting rods and a balance shaft to damp the piston-inertia forces that twist the engine back and forth around a vertical axis. Obviously BMW could a built knife and fork type connecting rods in this showcase engine. This just reinforces my belief that knife and fork rods are an anachronism.

hrandy said:

In the January 2013 issue of Cycle World Kevin Cameron describes BMWs' new clean sheet of paper R1200GS motorcycle engine. In addition to a lot of very state of the art tech it has side by side connecting rods and a balance shaft to damp the piston-inertia forces that twist the engine back and forth around a vertical axis. Obviously BMW could a built knife and fork type connecting rods in this showcase engine. This just reinforces my belief that knife and fork rods are an anachronism.

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Does it have a single throw crank, or a two throw crank?

Certainly appears to have two crank throws
http://andyw-inuk.smugmug.com/Motor...ms/i-zZt3Gz3/0/L/r1150gs_engine_diagram-L.jpg

This goes along with the description of a boxer engine.

And means that knife and fork rods were never an option.

Im surprised more wasnt done with horizonally opposed air cooled engines. No extra weight from cooling jackets, radiators and pumps, not to mention the weight of the coolant. Counterweights are not going to be much because the opposite piston/rod assembly serves that function. Intake and exhaust plumbing are going to be a bit more involved and streamlining takes a hit, but I would guess that if done right, an air cooled boxer engine is going to be a lot lighter?

dobbie said:

Im surprised more wasnt done with horizonally opposed air cooled engines. No extra weight from cooling jackets, radiators and pumps, not to mention the weight of the coolant. Counterweights are not going to be much because the opposite piston/rod assembly serves that function. Intake and exhaust plumbing are going to be a bit more involved and streamlining takes a hit, but I would guess that if done right, an air cooled boxer engine is going to be a lot lighter?

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A lot lighter than what?

In most instances the engine formed the structural connection between the airframe and the propeller. The engine bore the propeller thrust and side loads, transferring them into the engine mounts. A flat engine may need to be built with thicker gauges to offset the lower stiffness of its layout. At best it would weigh the same as the equivalent V-12.

For streamlining of in-line air cooled engines you need look no further than the de Havilland Albatross to see how well it could be done. The Albatross was powered by 4 de Havilland Gipsy Twelve inverted V-12 air cooled engines.



Some theories thought that burying engines within the wing would produce lower drag than the usual nacelle. A flat engine would have been beneficial for that purpose, though the theory was found to not be true.


Also, horizontally opposed engines with pistons moving in opposite directions (ie with opposed pistons both going in or out) tend to be longer, as they require separate crank throws for each cylinder. In a conventional V-12 you have 6 crank throws, in a boxer 12 you have 12 crank throws. In the boxer you can't use fork and blade rods, since each cylinder has its own throw. As the cylinders aren't directly opposite, they don't balance perfectly, and will require counterbalancing.

The Continental IV-1430 started out as a horizontally opposed engine, but was quickly changed into an inverted Vee. The Lycoming O-1230 was a flat engine. It was soon decided that the engine would not have sufficient power because of its small (relatively) capacity, so that was doubled up to make the H-2470.

I am not sure about the Lycoming and Continental flat 12s, but I believe they both had only 6 throws, so were not boxers. In terms of balance/vibrations you could not do much better than a 6 throw V12 of any angle.

A problem with inline (or flat or V-XX) AIR COOLED engines is that the bore spacing needed to be wider than on liquid cooled engines to allow for the cooling fins. This means a longer engine for the same displacement. You need more fin area in the head/top of cylinder than you do at the bottom which lends itself to tapered cylinders very nicely. Tapered cylinders fit very nicely on radial engines.

A 60 degree V engine has a fair amount of "beam strength". the height of the crankcase+cylinder block+ heads form a structure that is resistant to bending. A separate cylinder engine (air cooled) has to depend on the crankcase alone. If you try to use overhead cam boxes as part of the engine strength members you do gain a bit but then your cam/s and boxes block airflow/fins meaning poorer cooling than a radial.

I knew that air cooled boxer engines require more strength in the their crankcase attachments, and the real bugger is getting good airflow to the rear cylinders while maintaining some sort of streamlined shape for the nacelle. I don't see as much bending forces in a boxer type setup however. The primary and secondary balances are taken care of by the reciprocating engine parts? Does the additional weight of the crankcase completely cancel out the weight savings of a liquid cooled setup and the lesser need for heavy counterweights on the crankshaft?

In terms of balance/vibrations you could not do much better than a 6 throw V12 of any angle.

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I don't see that, at least as far as primary balance. To get that on any 4 stroke engine, its 2 revolutions of the crankshaft to fire all the cylinders. A 60 degree V12 is correct for primary balance 720 degrees divided by 12. Anything wider or narrower in the angle and youre talking balance shafts, split pin crankshafts or something to compensate for it.

dobbie said:

I don't see that, at least as far as primary balance. To get that on any 4 stroke engine, its 2 revolutions of the crankshaft to fire all the cylinders. A 60 degree V12 is correct for primary balance 720 degrees divided by 12. Anything wider or narrower in the angle and youre talking balance shafts, split pin crankshafts or something to compensate for it.

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You are talking equal firing intervals, not mechanical balance.

It is a long time since I knew how to calculate the forces involved, but in terms of balance the V-12 is one of the best, if not the best, around.

Not sure anyone has ever produced a V-12 with balance shafts.

dobbie said:

I knew that air cooled boxer engines require more strength in the their crankcase attachments, and the real bugger is getting good airflow to the rear cylinders while maintaining some sort of streamlined shape for the nacelle. I don't see as much bending forces in a boxer type setup however. The primary and secondary balances are taken care of by the reciprocating engine parts? Does the additional weight of the crankcase completely cancel out the weight savings of a liquid cooled setup and the lesser need for heavy counterweights on the crankshaft?

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The secret to getting cooling to the rear cylinders is the use of pressure cooling. Positively pushing the air through, rather than letting natural airflow do the job.

Weight wise - de Havilland Gypsy Twelve (aka King) - air cooled inverted V-12, 18.3l, 400-500hp. Rolls-Royce Kestrel - liquid cooled upright V12, 21.2l, <1000lb, 600-750hp.

Most BMW 'Boxer' (twin cylinder) engines are single throw cranks, using two normal rods side by side, so there is a little bit of unequal cylinder longitudinal spacing.
The new for 2013 motor has for the modern craze bike-tech for 'mass centralisation' plus slightly smoother running improved performance with reduced vibration and for less of that old BWM traditional torsional twist/kick to one side, has gone for the fork and blade style conrods.

razor1uk said:

Most BMW 'Boxer' (twin cylinder) engines are single throw cranks, using two normal rods side by side, so there is a little bit of unequal cylinder longitudinal spacing.
The new for 2013 motor has for the modern craze bike-tech for 'mass centralisation' plus slightly smoother running improved performance with reduced vibration and for less of that old BWM traditional torsional twist/kick to one side, has gone for the fork and blade style conrods.

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The "torsional twist/kick to one side" is due to the engine's torque and it being mounted longitudinally (crankshaft fore-aft).

I doubt that the crankshaft/rod layout will have much effect on that.

hrandy said:

In the January 2013 issue of Cycle World Kevin Cameron describes BMWs' new clean sheet of paper R1200GS motorcycle engine.

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BMW boxer apart (that, old and new, being true boxers, have two connecting rod on two crank throws) every mororcycle V engine, with the exclusion of HD, nowadays have side-by side connecting rod: Ducati, Moto Guzzi, Suzuki, Honda, ecc... two or more cylinders.

wuzak said:

I am not sure about the Lycoming and Continental flat 12s, but I believe they both had only 6 throws, so were not boxers. In terms of balance/vibrations you could not do much better than a 6 throw V12 of any angle.

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Infact the true boxers are in limited use. The boxer arrangement is necessary to balance only a two cylinder flat engine. When you have four cylinders, or multiple of four, about the same level of balancing can be obtained placing the connecting rod side by side with half of the throws of a boxer.
Those engines are sometimes called "Flat-V" or "V-180°". The so-called "Boxer Ferrari" engine (of the 512BB for example) is really a V-180° engine.

However, a third solution was possible, and was actually used, that with master-and-articulating-rod. It was used by Isotta Fraschini, in both the V12 and W18 engines, and is still used in some particular application (other than in the Asso 1000 / L.180 / CRM W18, still in production as a naval engine).

As well as the Napier Lion



And the Rolls-Royce R

True Wuzak, that the new rod style would have a great affect on the longitudinal 'rocking', perhaps less than 5% reduction, but over the average life of a BWM boxer engine (usually regarded to be well over 100,000 miles if serviced roughly 'to/by the book') this would reduce axial wear upon the 'bottom end' bearings - in single clyinder bike, tend to wear most between the 8 to 5 'O clock (from a left side viewing the anti-clockwise spinning crank - if my memory of Phil Ivings 'Tuning For Speed' book is close to accurate).

@ Dogwalker, there is a good reason why most modern bike V/L normally engines use side by side instead of individual crank throws, most of them are angled to 90 degrees, and so do not need the added weight and width required by the latter style, as they have a better primary balancing than those V's of under 80 degree spacing, which generally require an offset crankpin or individual throws to 'smooth' the balance to an equivilent of 90 degrees.

Although a 90 deg' engine has a greater secondary out of balance phase than those with individual throws and proper individual dynamically balanced weights, they are generally only noticable at part power loadings and in certain rpm ranges, otherwise they are hidden by the power pulses and primary balance rocking forces and can also be partly hidden too with todays EFI - adjusted by miniscule differences in fuelling between front and rear cylinders and ignition timings.
The lower pics with the twin pistoned cranks with master slave rods, are probably for a engine with a V angle of 75 deg' or less, and to save space and some complexity, use the master slave rods to create the effect of an offset crank upon a single crank throw, tricking the engine - but this system in V's works best in lower rpm engines.

Looking at some those master and slave rod set ups, the latter pics remind me of the 30's DKW's split 2-stroke 'singles' - actually twin or more pistoned, but with one (or a dualled/siamese cylinder with common) single combustion chamber, using one piston to act as the piston port inlet and the 2nd one as the piston port exhaust controller. Sometimes they used a 3rd slaved piston to act as piston action super-charger.
There are legends in the biking World, that during '36 or '37, that durring the IOM TT, on a good weather day, you could hear the DKW's across the (inland) sea in Liverpool area when they were racing upon the Mountain!...

Of course since the development of intake systems (rotary valves, disc ports, reed valves) exhaust boosting sytstems ('Walter Kadens'/expansion pipes, YPVS, HVEC/RAD etc) for 2-strokes and improved boost port designs, the split single was relegated to history.
Although theoretically, for 'road' usage a modern split single could have upto say 3/4 more torque than a conventional 'normal' modern 2-stroke, with perhaps only a slight increase in fuel usage compared to the old (DKW) ones.

Dogwalker said:

BMW boxer apart (that, old and new, being true boxers, have two connecting rod on two crank throws) every mororcycle V engine, with the exclusion of HD, nowadays have side-by side connecting rod: Ducati, Moto Guzzi, Suzuki, Honda, ecc... two or more cylinders.


Infact the true boxers are in limited use. The boxer arrangement is necessary to balance only a two cylinder flat engine. When you have four cylinders, or multiple of four, about the same level of balancing can be obtained placing the connecting rod side by side with half of the throws of a boxer.
Those engines are sometimes called "Flat-V" or "V-180°". The so-called "Boxer Ferrari" engine (of the 512BB for example) is really a V-180° engine.

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"

When you have four cylinders, or multiple of four, about the same level of balancing can be obtained placing the connecting rod side by side with half of the throws of a boxer.

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Are there any production flat four cylinder engines with only two throws for the connecting rods? I have never seen any. VW, Porsche, Subaru, Citroen, Honda, Alfa, Lycoming, Continental, Rotax. The flat eight of the Porsche 908 had an eight throw crankshaft as I recall. That was a racing engine, as I know of no flat eight engines in production now or in the past 60 years except for Lycoming and maybe Jabiru aircraft engines and those are rare.