enginehistory.org

[jjuutinen]

Any idea why most V-engines in cars are "fakes", i.e. the conrods of opposite banks do not act on the same crankpin (no fork-and-blade or articulated rods)?

[gryan]

This is a good question (on a favourite topic). I'm not certain I have the full answers to it but my understanding is that side by side rods are cheaper (design & manufacturing costs are lower) and, in addition, automotive rpm levels are higher than in aircraft.

From a technical standpoint they offer a simpler design which should be more rigid than the fork and blade rod type. The big ends may be easier to lubricate.

They offer similar advantages over articulated rods. Articulateds are also harder to balance. At automotive engine rpm levels they would likely introduce interesting vibration problems for the designers and engineers to solve.

Note that engines using the side by side layout in automotive sizes have successfully & reliably operated at speeds in excess of 18,000rpm.

Does any one else have comments about this?

[wallan]

You're forgetting that we are not dealing with two equal things.
The aircraft engine designer must strive for the greatest power-to-weight ratio, and cost isn't a major factor. Using articulated rods makes the engine shorter, stiffer, lighter, and helps with resonance problems. (plus, it gives matched ends on the cylinder banks, allowing easier machining for reduction gearing, etc) The automobile engine designer (mass-market) is driven by cost. In the smaller sizes used in cars, the side-by-side connecting rod, doesn't cause much problems. (for the ideal example, look at the Buick/Rover alloy V-8 engine: Buick were discarding it after 3 years, due to the development of thin-wall cast iron casting being introduced to make engine blocks that were stiffer, stronger, and easier to make. They might have been heavier, but that doesn't much matter in an American car)

[kmccutcheon]

Side-by-side rods are used in automobiles because, compared to fork-and-blade or articulated rods, they are cheap. As gryan points out, they are strong and easy to manufacture. The increase in engine length they cause is of little consequence to autos. The disadvantageous force couples that they introduce seem to not be a factor in auto engines, and would probably not have been a factor in high-powered aircraft engines, had any been built. (The Ford Motor Company built a prototype 1,650 cu in V-12 just before WWII that was to have been competition for the Merlin. It was to have had many production-oriented innovations, including side-by-side connecting rods, integral turbosupercharger, integral aftercooler, fuel injection, and cast crankshaft. When WWII came, the aircraft engine concept became the successful GAA, GAC, GAF, GAN and GAZ series of 8 and 12-cylinder tank engines. Too bad the aircraft version was never developed.)

In an aircraft engine, where weight is at a premium, each bank of side-by-side rods would increase engine length by about one inch over a fork-and-blade rod. In the case of a V-12, imagine the weight of a 6-inch slice! Additionally, the crankpin diameter and crankcheek dimensions must increase in order to handle the increased bending loads in a longer journal – meaning still more weight. I am not aware that fork-and-blade rods are any more or less difficult to lubricate than the side-by-side variety.
_________________
Kimble D. McCutcheon

[jjuutinen]

Sensible or not, I have a built in hatred towards cast crankshafts, side by side conrods or any similar cheap tricks! If Soviets found it sensible to design a short lived (i.e. the life expectation of a tank on the battlefield was shortish) tank engine with DOHC, aluminium head, block and crankcase plus articulated rods, I can see acceptable reason for any cheap tricks.

[wallan]

I take it that you are talking about cast steel crankshafts? The major problem with them is the fact that you can't get the steel hot enough to cast properly, and the designs are wrong. (the moulds would be too expensive, and it doesn't like to flow) S.G, or nodular iron crankshafts are a production engineers dream, as long as the crankshaft is designed, from the beginning, to be made from this material. I think the reason that cast steel crankshafts got/get a bad reputation is that the designers chose the properties they wanted, but tried to copy an already designed item: they should have designed the crank around the properties of the material from the beginning.

By the way, there's a nice statement in Mr. Ludvigsen's V-12 Engine book about conrod selection.

[wallan]

As Mr. Ludvigsen is a member, perhaps he can cut and paste the appropriate paragraph(s).

He did state that the master and slave rod acting on the same crankpin, is the lightest solution.

Cast steel crankshafts: it's never a good idea to try and copy a forged item and use casting. (the casting steel isn't really fluid enough) Plus, a number of the higher alloy steels used for crankshafts are a real pain to work with no matter what you do. (hot-shortness being the most annoying problem)

[szielinski]

There was another thread along these lines and I mentioned that there are two oil films in the fork & blade set-up, which, from what I can gather is better for endurance under load.

[rwahlgren]

Vee engines in automotive applications like Kim mentioned are side by side on the same crankpin. I think it is a proven more durable design than a fork and blade arangement, since rods seem to be an issue with the use of WWII aircraft engines in performance applications.

For a modern design that eliminates the need for a fork and blade to shorten the crankcase-cylinder block, would be the VW W8 and W12 through W16 engines, this is a great idea for shortening a vee engine.

http://www.youtube.com/watch?v=Q34xIzTj8fQ&feature=related

[jwells]

There seems to be an assumption in most of these posts that the fork-and-blade arrangement results in a shorter crankshaft as compared to the side-by-side system. Does it?

The dimensions of the conn-rods and crankshaft for the Merlin engine are given on pp 89-90 in Colvin "Aircraft Handbook" 1942.

The blade rod big end is 0.81" wide and it runs on a bearing that is 3.475" in diameter. This gives a bearing area of 8.83 sq in. which, presumably, is sufficient to cope with the loads imposed on it.

Now let's assume that we are going to run two of these blade rods side-by-side on the normal crankpin which, for the Merlin, is 2.77" in diameter. To do this, the width of the blade rod would have to be increased to exactly 1.0" to give the same amount of bearing area (8.83 sq in.)

Thus two blade rods, side-by-side would take up 2.0" of crank pin length but the length of the Merlin crankpins is 2.325" so, converting to the side-by-side arangement would SAVE 6 X .325" = almost TWO inches!

Which would a designer rather have; a shorter crankshaft or and engine with slightly offset cylinder banks?

[rwahlgren]

Any idea why most V-engines in cars are "fakes", i.e. the conrods of opposite banks do not act on the same crankpin (no fork-and-blade or articulated rods)?

Fakes? A V is a VEE. And is determined by the arrangement of the cylinders. It has nothing to do with the way the rods are attached to the crank pin.
You would never find a fork and blade con rod in an F1 engine reving to
19,000 RPM's.

edit 12-24-09
"opposite bank do not act on the same crankpin"
They do on every V engine in industrial or automotive I have worked on.
Even the offset crankpins used on some V engines are in essence one crankpin.
It is a simpler more durable design, and I would think lighter weight as well.

[csullivan]

I think you'll find that a V-8 engine's opposite cylinder connecting rods act on a common crankpin, but since most V engines of today are 6 cylinder, and many of them were based on the v-8's, in order to have an evenly firing engine, the crankpins of opposing cylinders on those engines are offset. The crankshaft in those engines are known as a "split pin" crankshaft. For even firing a V-8 typically has a bank angle of 45 or 90 degrees. A normal configuration for a V-6 is a 30 or 60 degree bank angle. A 90 degree V-6 needs the "split pin" crankshaft for even firing.

[dpennings]

the last posting is quite old, but I try to reactivate this thread...

Split pin/undivided pins and side by side rods/fork and blade rods are two different topics.

Split pins enables V6 engines with even firing and excellent mass balancing, unfourtunally in conjuction with increased bearing size (=> more friction) for more stability. V-Engines with 8 or twelve cylinders are perfectly balanced, so they don't need that effort. Split pins are not only used on 90° V6 engines but also on 60° engines. A 120° Sixcylinder is often regarded as ideal, but it is not the case. The firing is even, but this configuration has free mass moments of first and second order. A 90° V6 engine without split pins is free of mass moments, but it is odd firing.

I don't know a single V-6 flight engine and I believe there is no split pin in aviation. Even for V6 engines, a 90° V-angle without split pins would be superior to any split pin design.

Bladder and fork arrangement:
This enables not only to eliminate the bank offset, it also gives am even force distribution on the main bearings an streight bulk heads between the cylinders. Both helps to decrease weight. The disadvantage is the complexity (cost!) and the weakend bearing loads because of reduced effective bearing lenght of the fork rod.

I hope there will be someone out there who understands my English...

Dominik