Connecting Rods - Fork type vs. Side by Side

[Piper106]

Hopefully someone with more knowledge then myself can contribute.

The vast majority of all automotive V type engines use two separate connecting rods mounted side by side on a single crank throw, the two cylinders in the V being forward and aft of each other the allow the rods to fit side by side. The majority of V-type liquid cooled aircraft engines used fork and blade connecting rods which allowed the two cylinders on the opposite side of the V to be on a common centerline. Examples being the RR Merlin, Allison V-1710, and the DB 600 series engines. As far as I know, side by side rods (automotive style) were almost unheard of in WWII aircraft engines, and fork and blade rods are equally unknown in modern automotive engines.

I am trying to understand if there are technical advantages to one style (fork and blade vs. side by side) or the other.

As far as I can tell, the difference between the two industries was mostly because "we've always done it that way".

Comments????

Piper106

 

[Shortround6]

With fork and blade rods the cylinders are directly opposite each other. With side by side rods the cylinders have to be slightly offset. The engine winds up being slightly longer (width of one rod?) Not sure if there is a twisting force of the crankshaft journal that might need a slightly heavier crankshaft.

 

[PWR4360-59B]

My ideas. There were pretty much zero V type engines in those days so not much to copy from. Agreed shorter and easier to fit a gear reduction housing too.
More compactness. And likely better structural loading on the case.
The not so good, a clumsy connecting rod design and high bearing loading. The best design for a V aircraft engine is this.

View: https://www.youtube.com/watch?v=t7gtmnOSmvU

 

[wuzak]

The best design for a V aircraft engine is this.

View: https://www.youtube.com/watch?v=t7gtmnOSmvU

What brings you to that conclusion?

 

[GregP]

Pretty much zero Vee-type engines? What about the Hispano and the Liberty from WWI, and others. There were plenty of Vee-type engines around, engguy, you should know that ...

As for the Bugatti, it has side by side rods on one crankshaft. Very neat, but you claim it is an optimum design. You base that claim on what? The animation you provided the link for is good, but what about cooling the heads. The Veyron configuration is a cooling nightmare, with high heat across the entire clinder head area and complex plumbing. Is it a neat engine? Yes! Optimum ... I can't say and neither can yiou without consisderable investigation of other layouts ... and a LOT of engineering backup. Obviously, all aero engine companies disagree with you since there are zero aero engines of this type flying.

Let's be clear. The conventional V-12 engine has more Grand Prix victories than any other configuration, and makes horesepower at least equivalent to the Bugatti. Many make more. It powered about half of the WWII fighters types ... more or less, and is still being produced in quantity. Ferrari loves the V-12 and so does Maseratti. Jaguar still makes one, too. Volkswagen makes a 6.0 liter V-12 that runs great.

The reason for the fork and blade is to keep the case as short as possible for the number of cylinders. It is a VERY good design, but takes more design time, so the automotive people usually decline to do that. Also, for proper lubriaction, you need oil flowing through the crankshaft. Automotive peopel don't do that, either. An automotive crankshaft is usually one piece. An Allison crankshaft has 128 pieces and oil flows through the crankshaft and through the crank throws ... and is forced into the rod bearing races under pressure, keeping things lubricated and cooled.

Last, the aero engine must be reliable!

I think I'll wait until there are a couple of million flying hours on a Bugatti-type aero engine before I buy one. So far the total hours in the air for the Bugatti design are zero.

 

[PWR4360-59B]

Greg P
As you mention fork and blade to keep case short as possible. That is what the VW design does. I think 16 cylinders packed into a v8 size engine is great.
Aero engine companys disagree? For them to engineer and get FAA aproval, the cost is prohibitive, they will keep the old design forever at that rate.
The VW W8 and W12 and W16 design would have been a very desired design back in the day they were trying to increase cylinder density and keep size and weight down. That is why I think its nice. Fork and blade is not a good rod design, its heavy and difficult to manufacture. It increases complexity. The good old side by side works just great, in high performance engines of all sorts. Oh its not a Bugatti engine, it was designed and manufactured by VW.
http://en.wikipedia.org/wiki/W16_engine

 

[kettbo]

the Maybach tank engines for the Tiger and Panther were Fork and Blade.
IIRC, some rod and bearing problems

 

[PWR4360-59B]

kettbo, do you have any links to information on the problems? Maybe you could explain some more.

 

[kettbo]

Engguy,
The problems, seems I remember reading about them while at Ft Know/USA Armor Center in the late 1990s.
Which book, page, etc...sorry.
I believe/recollect not a fault of the design but the vehicle wt and operator technique had to come together. I really want to say that I saw reports of fractured connecting rods, the forked ones.... don't quote me on this

Background:
I grew up next door to a serious gearhead in Southern California. I started learning about engines when I was 14. Very interested in the old technology of WW2 tanks during my life, especially during my 20 years in the Army. I still build the Big Block Chevy at age 52 and drag race during the summer

 

[jerryw]

A couple of the replies to this enquiry claim that fork-and-blade rods produce a more compact installation. But, is this really the case?

In "Aircraft Handbook" by Colvin (1942), the dimensions of the crankshaft and the conrods for the Merlin engine are given in Chapter 3.

From the diagram, it can be seen that the inside diameter of the big end of the blade (or plain) rod is 3.47 in and the width is 0.81 in. This gives a bearing surface area of 8.83 sq ins. which, presumably, is some sort of minimum bearing surface required for the satisfactory operation of the rods under all conditions.

Now, let us assume we are going to convert a Merlin engine from fork-and-blade rods to side-by-side type using the standard Merlin crankshaft. The crankpin diameter is 2.77 in which is less than the 3.47 in provided by the bearing block in the original arrangement, so the blade rods will have to be made wider if the minimum 8.83 sq in of bearing surface is to be maintained. An increase in width to 1.02 in will do the trick.

Thus, two plain rods, side by side will take up 2.04 ins of crank pin space. However, the actual crank pin length of the Merlin crankshaft is 2.32 in, ie the plain rods would require 0.28 ins LESS than the fork-and-blade style. For the whole crankshaft, a SAVING of 1.68 in would result!

 

[GregP]

In the Merlin, every cylinder produces about 125 HP and they wanted the power pulses directly in the center of the rod bearing, not alternating on either side of the bearing. Ergo, fork and blade. The crankshaft length saving you calculate would more than be taken up by extra cylinder offset plus you need a minimum thickness of material from the cylinder to the case egde. With cylinders directly opposing one another, that minimum thickness isn't added to one side and not the other, as it would be if the cylinders were offset.

The end result is a very strong rod setup, the shortest overall case, and a great design. I'm sure they could have made side-by-side rods work, too, but the resulting engine would have been a bit longer and might have vibrations and resonances not seen in a fork and blade rod setup. Not that they could not have cured the resulting vibrations and resonances, but they simply chose another path. Who can really say why.

Think how much better it would have been with fuel injection instead of a carburetor! And maybe with electronic ignition instead of a magneto and distributor. Yes, I know the electronic ignition would be decades away, but they could have done fuel injection.

Oh yeah, in the pic above, the "plain con rod" is a blade and the one on the left is a fork.

 

[Shortround6]

Think how much better it would have been with fuel injection instead of a carburetor! And maybe with electronic ignition instead of a magneto and distributor. Yes, I know the electronic ignition would be decades away, but they could have done fuel injection.

R-R figured that the fuel evaporation in the supercharger cooled the intake charge by 25 degrees C. on the single stage engines.
By the end of the war R-R had changed to an American style carburetor which was much closer to what we now think of as a throttle body fuel injector. It gave better metering, solve the "G" problem, may have helped with intake icing but preserved the charge cooling.

 

[wuzak]

The Merlin bore spacing is 6.075", so making the rods narrower as a set won't be able to reduce any length. By using side by side rods the crankcase would be lengthened by the width of a rod - about 1" longer.

 

[wuzak]

Greg P
As you mention fork and blade to keep case short as possible. That is what the VW design does. I think 16 cylinders packed into a v8 size engine is great.
Aero engine companys disagree? For them to engineer and get FAA aproval, the cost is prohibitive, they will keep the old design forever at that rate.
The VW W8 and W12 and W16 design would have been a very desired design back in the day they were trying to increase cylinder density and keep size and weight down. That is why I think its nice. Fork and blade is not a good rod design, its heavy and difficult to manufacture. It increases complexity. The good old side by side works just great, in high performance engines of all sorts. Oh its not a Bugatti engine, it was designed and manufactured by VW.
W16 engine - Wikipedia, the free encyclopedia

The VW/Bugatti W engines have compromises in their design. They are basically a pari of VR6 type narrow angle vee engines joined together on a common crankshaft. There are 4 conrods coming off each crank throw, and each crankthrow has two parts - half is offset to better balance the engine because of the small angle between the bank pairs. The rods are very skinny to minimise the length of the engine, but even so the bore spacing is quite a bit longer than a V8 with the same bore, or even one with a bigger bore.

So, no the W16 is not more compact than a V8. I doubt it is even as compact as an 8l V8. It is more compact than a V12 or a V16.

The crankshaft is a potential problem fr strength for aviation se, I would think.

http://lh6.ggpht.com/_8MPCKJQzPA8/SygCsrBne5I/AAAAAAAAsTY/3XSaxTAPTSw/Bugatti Veyron Engine[2].jpg

Sorry, couldn't find a pic of the crank.

 

[razor1uk]

I would have to agree, that non blade and fork designs are simpler, easier and do produce out of alignment vibrations and resonances because any cylinders in the opposite bank are offset by that amount; more vibrations/resonance needs more material/metal/design-work to minimise it adding weight and limiting design potential - not something you'd want adding to a damaged A/C's engine at 15,000ft trying to get home on hope, love and cursing, let alone hoping an emeny pilot/squadron finds you...

 

[PWR4360-59B]

Well I guess I have to take back some of what I've said.
I didn't seem to remember another really great engine that also uses the fork and blade arrangement. Its also a GM design and is a very dependable, excelent engine design. EMD Diesel engine that is still being made. Their literature does make note of lower firing pressures of the 2 stroke design. I would like to see more about that.

 

[Trilisser]

The way I see it that side-by-side rods are used in cars because such engines are cheaper to build. Remember: automobile engines=minimum quality for the maximum price vs. aero engines=maximum quality for a maximum price...

 

[Julian_S]

This is an interesting thread. I was looking at some 'course notes' I happen to have for a Paxman Valenta V12 - this is a 4800cid Diesel loco engine with the blade and forked rod arrangement. I think the main idea in the designer's mind is bearing surface area. Effectively you can have double the bearing surface on the crank journal (ie two for the price of one) as the blade rod bearing runs on the outer diameter of the bearing block and effectively has very little work to do (just like the way the articulated rods hook up to the master rod in a radial) If you think about it, each piston (in the pair) effectively has a rod with double the bearing size running on the crank journal than would be the case in the normal automotive 'side by side' arrangement.

Julian.

 

[GregP]

Oh yeah, I clean forgot to mention a technical reason for fork and blade rods ... if you use fork and blade, the torsional vibration will be limited to mostly whole number harmionics plus maybe the 1.5 harmonic and small bit of 2.5.

If you use the side by side rods, you will get all the harmonics, including the most of lower order half harmonics (1.5, 2.5, 3.5, 4.5).

Since safe operation of an aircraft engine is paramount, you definitely want to eliminate or mitigate as many harmonics as you can by design instead of counterweight "fixes."

 

[pressurized]

length was the major consideration for the vee type aircraft engines. fork and blade, master and slave, and articulated rods are all much more expensive than plain ( side by side ) rods.cost is generally not a problem with aircraft engine design, but is why you haven't seen anything but plain rods for decades in automotive engines.