Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules
Cost is a huge driver for most automotive engines, and fork-and-blade or articulated rods will cost more than side-by-side connecting rods.
I don't know about the connecting rods for largish (> 1000 shp continuous) diesel engines, but I believe some use articulated rods.
I wonder (not feeling like doing an analysis; I don't like structures) whether the different loadings on the crankpin is an issue. With either articulated or fork-and-blade rods, there loading tends to go through the midpoint of the crankpin, while for side-by-side rods, there's a moment because the piston loads are reacted near the ends.
Articulated or fork-and-blade: __V__
Side-by-side: --v--^--
(sorry for the ascii "art")
I think you may have conflated crossheads with articulated connecting rods. Articulated rods would not be used on inline engines. Iirc, Brons-Werkspoor used articulated rods on the V12s used in, among other places, the tripartite minesweepers.Indeed, I said as much in my original post. But the question remains, why do modern high performance racing engines where cost isn't an issue, like F1 engines, still use side-by-side rods? Evidently there is some other advantage to them, but what exactly is it?
These aren't the same kind of articulated rods as used in some aviation engines, like e.g. the Mikulins. In aero engines, an articulated rod is like a master-and-slave rod found on a radial engine, in that the other conrod is connected to the first one rather than to the crankshaft.
On large diesel engines, which use articulated rods like you say, the upper part of the conrod is AFAIU fixed to the piston, there is no crankpin where the rod moves, and then the upper conrod is supported by a bearing as it moves up and down, and the joint in the middle of the conrod is where the actual articulation motion happens. So in a way the upper conrod is like an extension of the piston. This is done to reduce piston sidewall loading. These engines are also very long stroked.
Yes, this is the "rocking couple" issue I mentioned in the original post.
I think you may have conflated crossheads with articulated connecting rods.
For the last, I was not referring to engine vibration; I was referring to the stresses within a crankpin.
A google search brought me back here Connecting Rods - Fork type vs. Side by Side
Discussing car and motorcycel practice ignores the huge difference in size and power of the engines. Moto Guzzis Ducatis and BMW twins all had plain conrods side by side. But look at the size of the crank pin and the power output, weight of pistons. The Merlin was supercharged and using exotic fuel and lasted 250 to 1000 hours depending on use, less if WEP was used often. A road going M/C even a sprts type lasted for 10s of thousands of miles.Thanks. I recall seeing that thread earlier when trying to learn about the issue on my own. It's an interesting thread, with nice pictures of various historical conrod designs, as well as some explanation of current practice. However, infuriatingly, it doesn't actually answer my question why the fork-and-blade design disappeared after WWII even in high performance engines where cost isn't a concern.
Discussing car and motorcycel practice ignores the huge difference in size and power of the engines. Moto Guzzis Ducatis and BMW twins all had plain conrods side by side. But look at the size of the crank pin and the power output, weight of pistons. The Merlin was supercharged and using exotic fuel and lasted 250 to 1000 hours depending on use, less if WEP was used often. A road going M/C even a sprts type lasted for 10s of thousands of miles.
I used to be an avid F1 fan. The whole thing started turning sour with the turbo era. They could make an engine that would just make it through qualifying and the race, but frequently they didnt, with a massive number of engines blowing up and sometimes only around 8 finishers. Same with Auto Union, it was bankrolled by the state, the engines only had to last 2 hours or so.I was thinking more of pure racing engines like e.g. used in Formula 1: Formula One engines - Wikipedia
Current F1 engines are maybe a bit boring due to regulations specifying pretty tightly what can be done, but it used to give quite a lot more freedom to the designers. And IIUIC top end teams expected a qualifying engine to last a few laps, there were no restrictions how many engines a team may go through in a season etc. Sure they were all a lot smaller in volume than the Merlin, but at some points they did provide similar levels of power. E.g. the mid-1980'ies turbocharged 1.5L engines produced up to 1500 hp with 5 bar boost at 11krpm (for qualifying only, significantly detuned to last through a race). Crazy! Or after turbochargers were banned, we had the naturally aspirated engines turning at up to 20000 rpm. All with side-by-side conrods (well except some engines that were straight ones and not V).
Or for something more contemporary to WWII aviation engines, the crazy pre-war Auto Union V-16 and V-12 engines: Auto Union racing cars - Wikipedia E.g. the Type D produced about 3x the power/volume than the Merlin. And all with side-by-side conrods (I wasn't able to find a picture of the conrods, but from a top picture of the engine block one can see that the cylinder banks are offset to each other, which I think is enough proof that it uses side-by-side conrods).