45 degree V-twins like those made by Harley Davidson are popular engines for cruiser motorcycles, even though they tend to vibrate a lot. Boxer engines like those from BMW are also popular, although they don't have the same unique exhaust note of a V-twin. So why not combine them?
Take a typical 45 degree V-twin, turn it 90 degrees on its side, and add two more cylinders as a mirror image to the first pair, joining them at the crank. The result is an X-shaped 4 cylinder (see link for picture).
X-shaped engines have been built before, mostly for aircraft but also as a Ford prototype (see link). But these generally had a radial-style crankshaft with a master rod and heavy counterweights.
With my engine the crankshaft should have two throws 180 degrees apart. Each "bank" of two cylinders will share a crank-pin, just as they do in a regular Harley engine. But since each cylinder is lined up with its opposite on the other side, their vibration will largely cancel out as it does in a boxer engine. As with a two-cylinder boxer, there will be some rocking moment imbalances that might require small counterweights on either end of the crankshaft, but it wouldn't vibrate nearly as much as a regular V-twin.
Power delivery wouldn't be as smooth as a regular boxer engine, but for cruisers this might be a selling point, since a rough exhaust note is part of a Harley's appeal. You could play around with the firing order and/or V-angle to make it more smooth or more compact, though. And if the V-angle was narrow enough, you could have a single head per bank, much like a Volkswagen VR6. They could also be "stacked" lengthwise to make an X-8 etc.
Other advantages would include a large frontal area for air-cooling (when mounted longitudinally), with a compact size lengthwise.
Disadvantages might include oil pooling in the lower cylinders.-- discontinuuity, Feb 04 2019 45 degree X-4 illustration https://i.imgur.com/ItRIho9.jpg [discontinuuity, Feb 04 2019] Henry Ford's X-8 prototype engine https://macsmotorci...redible-x-8-engine/with a radial-style crankshaft [discontinuuity, Feb 04 2019] X-engine https://en.wikipedia.org/wiki/X_enginesimilar design with a different crankshaft [discontinuuity, Feb 04 2019] Boxer Engine https://en.wikipedia.org/wiki/Flat_engine [discontinuuity, Feb 04 2019] Bugatti W16 animation https://youtu.be/YxAhPYR05V0 [discontinuuity, Feb 04 2019] Volkswagen VR6 engine explained https://youtu.be/Y0LKWt3TticYoutube video from Engineering Explained [discontinuuity, Feb 05 2019] Isn't this the design of an aeroplane radial engine? Also, I wonder if vibration would be reduced by having no opposing cylinders? - e.g. a ring of 5 or 7 cylinders, rather than 4 or 8?-- hippo, Feb 04 2019 I do not dispute the feasibility, but I can't understand the motivation. You could make a dual V-twin into an X-quad, but cramming it into a cruiser frame wouldn't work, unless you made each cylinder MUCH smaller in stroke. This would make a motorcycle with less capacity and torque with more complexity, very much against the cruiser ethos.
Then, there's access. With a V-twin you have the head and pistons etc. at one end and the nice exposed crank at the other, you can attach your gearbox to that nice free crankcase. With the X-quad you will likely have a very long primary drive to get from central crank to your gearbox, maybe make that a shaft? You can get that access back, with some reorganization. First, build the X- 8, then chop off 4 cylinders. Now you have a V-4 which is kind of like a V-twin, but you can tune for revs/smoothness a little better and it's still easy to attach the gearbox. The V4 is a standard and very baked configuration, however.
A VR4/5/6 would be interesting, and it's been done, a German company called Horex put one together a few years back, but it's power/torque figures aren't exceptional.
Essentially, every engine configuration has been put into a motorcycle. We've ended up with single cylinder thumpers, V-twins of various kinds, 2-6 cylinder boxers, a few oddball triples, and inline 4s/6s. Personally, I'm waiting for a supercharged uniflow-scavenged direct injection 2 stroke.-- bs0u0155, Feb 04 2019 Anzani designed an X-4 just like this, following on from the 60° 3- cyl W layout used in the Bleriot monoplane. It was a 60/120 X. So, Baked, but probably not WKTE.-- 8th of 7, Feb 04 2019 // Isn't this the design of an aeroplane radial engine? //
It's similar, but with two crank pins rather than one. This improves the balance and reduces the need for heavy counterweights on the crankshaft.
//cramming it into a cruiser frame wouldn't work//
My plan was to mount the engine longitudinally like with a BMW boxer engine or a Moto Guzzi V-twin, with the transmission and shaft drive behind the engine.
// A VR4/5/6 would be interesting, and it's been done//
This would be more like the Bugatti W16, but with an 180 degree bank angle. It would be like two VR2s joined together to make a W4.
//Personally, I'm waiting for a supercharged uniflow-scavenged direct injection 2 stroke.//
Two-stroke diesel engines are pretty common in large trucks, trains, ships, etc. but I don't think anyone's put one on a motorcycle. I think it would be perfect for a 50cc moped.
// Anzani designed an X-4 just like this//
I couldn't find anything about an X-4, although Anzani did build a V4 and various radial engines with 3 to 6 cylinders. I'm not positive, but I think that those radials had a different crankshaft design with a single crankpin and a master rod.-- discontinuuity, Feb 04 2019 An efficient geography for the cams, intake, oil drains, sump, and exhaust paths in the block for this are going to be complicated by the presence of cylinder packaging in all the wrong places, and the crankcase ventilation is going to suffer badly on the bottom side.-- RayfordSteele, Feb 04 2019 //An efficient geography for the cams, intake, oil drains, sump, and exhaust paths in the block for this are going to be complicated by the presence of cylinder packaging in all the wrong places...//
There's a couple ways to do the cams/valves:
1. With pushrods like on a Harley engine. You could have two separate cams (one for each bank), or a cam ring like on a radial engine. Or one central camshaft and some weirdly angled pushrods.
2. With overhead cams and belts or chains. The cams would have to be longer for the rear cylinders, unless it had a narrow V-angle like a VR6 engine, in which case you would only need one camshaft per cylinder instead of two.
With a 45 degree V angle the intake and exhaust routing would be similar to that on a Harley V-twin, with the intakes going into the center of the V and the exhaust coming out the outsides/front.
With a narrow-angle V, the intake runners would have to go in one side of the head and the exhaust out the other, much like in the VR6 engine. This would make some of the intake/exhaust runners longer than others in the head, so the intake/exhaust manifolds would a little weird.
Now that I think about it, there's two different designs here:
1. An air-cooled pushrod engine with a 45 degree V-angle and separate heads for each cylinder.
2. A water-cooled DOHC engine with a narrow V-angle and two heads, one for each bank.
//... and the crankcase ventilation is going to suffer badly on the bottom side.//
I'm not sure what you mean. Are you saying that oil would pool in the lower cylinders? That's certainly true, it would probably burn some oil on startup like a radial engine.
To be honest there's not much of an advantage over a four-cylinder boxer engine, except that it would be shorter and the exhaust would sound cool. And you could make it out of existing Harley cylinders and heads.-- discontinuuity, Feb 05 2019 I mean the breather system. PCV. The lower cylinders are going to be all coked-up in no time, so the cal will be wrong and the exhaust won't pass inspection. The PCV system is like your lymphatic system; small and seemingly insignificant but important in weird ways to the engine's health.
PCV on-air separators are usually resident inside the top of the valve covers where there is some clear air. The bottom cylinder cam train will need something like this, too, but where to put it is a bit of a mystery.-- RayfordSteele, Feb 11 2019 You could still have PCV breathers on the upper cylinders, at least enough to create a vacuum in the crankcase. You wouldn't need breathers on the lower heads, but they would need to drain into the sump, maybe with a dry-sump system that would scavenge excess oil out of the rocker covers. If the sump and oiling system was properly designed it wouldn't burn too much oil, although some is unavoidable, at least at start-up-- discontinuuity, Feb 12 2019 //This would make some of the intake/exhaust runners longer than others in the head,//
I read this as a criticism of the VR narrow-angle V design and I thought it smelled fishy. It's only true if you want your intake and exhaust ports on one side and can't be bothered artificially lengthening some of them, something that's done to engines all the time. There's no reason you can't bring the intakes in through the top and have the exhaust exiting on both sides, and in a two- cylinder, there are 3 free sides to choose from. Should be easy.
//There's a couple ways to do the cams/valves: //
Go pushrods. One cam, MUCH more compact, less rotating mass, fewer components. Simpler tuning. The only reason you go OHC is for revs, and if you were doing that you'd buy something Japanese.-- bs0u0155, Feb 12 2019 random, halfbakery