Back in the day when ESPN used to show tractor pulling on a regular basis, there would be these heavily modified monsters with as many as six engines, V8s lined up two-by-two in the front. How, precisely, did they lay down their power? Ultimately they had to all drive to a single point where the power would then be distributed through the rear axle to the drive wheels, but I can’t figure out how, or even what the benefit of that would be. How would the transmission work, or if it was dumped directly into the differential how did it not simply overpower it and tear it apart every time?
I don’t know about tractors, but it’s frequently done with over-running clutches in other cases. These are essentially freewheel devices that allow engines to run slower than the main drive shaft. This transfers the load to the other engines which will slow down under the increased load until the slow engine keeps pace. Depending on the configuration, the engines may need to be kept running at nearly the same rate and power production. In a common case in machinery, one motor runs slowly or is unpowered unless the primary motor is taken out for service.
Not sure I understand the question. They would be tied together with a transfer case and clutches. Example You can click on “how it works” on that site.
That example is exactly what I was looking for, assuming that everything similar is just some flavor of that. My problem was that I couldn’t get the angles right in my head. But now, a follow-up: is the power additive, or is there are point of diminishing returns where adding yet more power would make no difference? And other than the novelty of having six engines hanging off the frame, wouldn’t it be easier and cheaper to have one engine of sufficient power doing the work?
There sure are diminishing returns, but where the point is that it’s no longer worth adding more depends on the vehicle and its intended use.
when multiple engines are used, it’s usually because it’s not possible to have a single engine of sufficient power.
Cost could be a larger factor, probably better to get and maintain a bunch of V8’s the a 32 cylinder engine
Unless you use a novel layout it’s going to be hard to make a workable 32-cylinder engine. a V- or X-type engine would be prone to crankshaft torsion throwing piston/valve/ignition synchronization off. A radial engine like old planes would be workable, but in any case you’d pretty much be designing and manufacturing your own engine and I don’t think any of these teams have those kinds of resources.
6 heavily modified racing V8s putting out 700HP each is about 4200HP. No ICE engines with that capacity have been built for 50 years now. A diesel locomotive engine is in that ballpark, but they’re real heavy. And they’re designed to turn slowly.
As I understand modern high-tech tractor pulls, the sled gets harder to pull based on the time since the green light, not the distance traveled. So like a conventional dragster, there’s a real premium on geting as far down-track as quickly as possible.
If the rules permit, it’d be easy enough to use a modern aircraft- or ship-oriented turboshaft engine. They can put out 4000+ HP for weeks at full throttle.
As to diminishing returns, that point comes when one of two things happen:
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the power losses in the transmission from adding one more engine exceed its output. In this case that’s an incremental several hundred HP in transmission losses. Probably not a real big concern.
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Given that all this is homebrew one-off engineering, when the incremental unreliabilty of the extra parts means the decrease in reliability & therefore increase in potential to break down & lose gets bigger than the increase in performance and potential to win.
My wag is the latter is the limiting factor.
If some genius cobbles together an reliable-enough 10-engine powertrain, you’ll quickly see 10-engine monsters take over the sport.
just from my brief reading on the subject they have classifications in tractor pulls just like other racing venues. Turbines are used when allowed and in competitions using ICE engines they limit the number of engines that can be used.
Having multiple engines allows a little more freedom in distributing the weight. The goal is to just get all the weight shifted to the rear wheels at maximum drawbar tension AND to have the weight as far forward as possible, which are somewhat conflicting goals.
Also, with multiple engines you can have fractional spares: A five engine rig might travel with two spares, which would be much cheaper than having a spare for a single large engine. That and high performance cams, pistons, crankshafts, etc. are all off-the-shelf items for a V-8, but you would be bespoke for a train engine.
There is also some ethics involved. There was a fairly vocal outcry a decade or two back over the fact that tractor pullers, boat racers, and pylon racers were burning through all the surplus Allison and Rolls Royce V-12s which was driving up the price to the point of grounding some historic aircraft. At least two significant development projects, The Eagle V-8, and the Pond racer, were undertaken largely to promote conservation of the remaining stocks of V-12s.