How do you get dual engines to run the same speed?

[rgvkid]

Is there always going to be one that pulls more then the other?

 

[1992suburban]

single linkage.. throttle stop and a tach..

 

[Russ2251]

Is there always going to be one that pulls more then the other?

Yes.

throttle stop and a tach..

I don't quite agree.

 

[modelengineer]

It doesn't really matter if one pulls more than the other, as long as they're both on a live axle.

If the engines aren't on the same axle then I have no idea.

 

[anderkart]

2 identical engines locked in on a live axle would pretty much have to be running at the same rpm. One probably would always be running a little better and pulling more of the load, but you might never notice small differences besides hearing a little diferent exhaust tone between the 2.

 

[1992suburban]

haha... dont know man I was kidding

 

[Russ2251]

haha... dont know man I was kidding

I wasn't. I took it as a serious query.
Only having done this once (recently), I can tell you that it ain't easy.
A tach will only help with only one engine running and the 2nd engine not running and then vice/versa.
Once data is collected for engine #1, then engine #2 can be put into play and data collected on that one.
I've had the best luck by just "eyeballing" linkage and carb butterfly. In other words, when #1 carb is 1/3 open so should #2 carb.
Make sure they both start to open at same time and are both fully open at the same time.
Even a multi-cylinder engine has subtle differences between cylinders and is impossible to avoid. They may be very close...but never identical.
Volumetric efficiency varies from cylinder to cylinder.

 

[mike75925]

easiest way would, i say 'would' , seem to be to blue print both engines with the same parts. same size/weight pistons, cranks, cams, carbs, exhaust, etc. then use a quick reading tach on each and fine tune the throttle boss. i think this may be why they resorted to multi-cylinder engines in the first place.

 

[Russ2251]

seem to be to blue print both engines with the same parts. same size/weight pistons, cranks, cams, carbs, exhaust, etc.

Kind of expensive...
In the "old days", twin kart engines were sold in matched pairs. Clinton 2 cycles were among the most common.

 

[mike75925]

easy almost never equates to cheap (these days). or so we have been lead to believe! what's your take on slipper clutches?

 

[freakboy]

My twin cyl briggs the one cyl always loads up with more fuel.

 

[BriggsTwins]

well freak thats because it only has one carb, and the two cylinders are different, but it makes it run that way! gotta love em, as for the twin engine listen to them each engine speaks its own make them run best separately then see how much a difference there really is. there will be so little you wouldnt normally notice.

 

[frederic]

Even a multi-cylinder engine has subtle differences between cylinders and is impossible to avoid. They may be very close...but never identical.
Volumetric efficiency varies from cylinder to cylinder.

Absolutely true. However, there are obvious differences between cylinders that share a common crankshaft and cylinders that are joined ("timed") by external devices like chains, gears, jackshafts, and so on.

The latter has more "slop" than the former and in turn offers more opportunity for breakage, wear, catastrophe.

Also, timing matters. Assuming we're talking a pair of single-cylinder, 4-stroke engines, you'd want the combustion stroke to occur directly out of phase, like so:

Engine 1: intake, compression, power, exhaust
Engine 2: power, exhaust, intake, compression

This way you minimize the rattling of the external components that hard-link the individual crankshafts together.

It also allows you to use the jackshaft bearings you normally would because only one engine is applying it's 5HP at a time. If they fire in sync, then you have a pulse of 10 HP every four strokes instead of two closer-spaced 5 HP pulses. You'd have to size everything up accordingly of course.

The above assumes you have a commn jackshaft that ties the engines together, then put your clutch/belt/gears between that jackshaft and the rear axle.

Another option is to clutch each motor individually to a common jackshaft (or live axle if you want less parts and the gear you fit work out to your favor). Doing so allows the engines to fire at random intervals to each other. Obviously with the clutches engaged they'd run at the same RPM however nothing stops both engines from being off from each other which will introduce vibration into the clutches/belts/gears/chains/liveaxle/chassis and your *** ;-) Considering how much lawnmower engines vibrate, I think most of you can imagine how much shaking could occur.

The other concern I would have about clutching each engine separately is what happens if one stalls? Most of the smaller kart and minibike clutches do not seem to "bite" as well going in reverse, even though to the naked eye that's not terribly obvious by design. I *think* this would wear the clutch out on the stalling/problematic engine rather quickly.

The other factor is when your clutches are toast, you have two to replace so your cost doubles. Clutches are more expensive than gears/chains to a common jackshaft, right? I've read your posts and seen the dollars you guys spend on clutches!

A third method would be to clutch each engine to a half-axle thus driving only one wheel. The "hard link" between the engines is the ground, and there usually is enough tire slip even on asphalt this might be the most comfortable way to operate a dual-engine kart, though an engine stall condition might lurch the kart to the side that the engine stalled, as the other wheel will be driving very hard while the other one, well, doesn't. If you have clutches that slip nicely when the axle rotates faster than the engine then this artifact will be minimized.

While not applicable to lawnmower engines, many years ago several friends and I cascaded two Buick V6's in series. We machined a two piece adapter for this. The front half bolted where the flywheel would normally be on the front engine, and the other half bolted to where the main pulley would normally be on the rear engine. The two halves then bolt together.

We made it in two pieces because it allowed the bolts that "join" the two crankshafts to be further away from the centerline which in turn allowed us to get a wrench in there.

Both engines were balanced to "stock" and turned 180 degrees out of phase, so they balance each other. The flywheel on the back was balanced to neutral. And yes, the 6" diameter adapter was balanced to neutral within 1/100th of an ounce.

I've taken apart many vertical mower engines and have pondered the idea of making an adapter that replaces the flywheel on top with something similarly shaped but has a set of concentric bolt holes, for a flange that would bolt on and receive the output shaft of another vertical engine sitting right on top.

"vertical stack".

But I've only pondered it, I've never actually dug far enough into it to see how difficult it would be. If I needed more power, I'd simply buy (new or used) a bigger engine.

That's why Kaptain Krunch and others often mention motorcycle and snowmobile engines - much more power and probably less effort than creating a common jackshaft with two sets of aligned gears and chains, and so on.

 

[t0x1k]

What about a 1:1 gear ratio to a 1" jack shaft with a clutch on it?
Of course you need a heavy duty starter to crank both engines over over the jack shaft.

 

[Russ2251]

I don't know why this is, but does indeed "seem" to happen and I cannot prove it.
My twins appear to self synchronize at some point during acceleration.
By self synchronize, I mean that they fire at the same time.
Engines are not linked together in any way other than sharing a live axle. They are basically free to do their own thing until clutches reach the lock-up point.
Can someone explain why this happens?
I have little experience with twins and this is a totally unexpected (but welcome) outcome.

 

[Kaptain Krunch]

What about a 1:1 gear ratio to a 1" jack shaft with a clutch on it?
Of course you need a heavy duty starter to crank both engines over over the jack shaft.

All i can say is if one engine seizes your funked, or even if one stalls you have a chance of braking the chain. If i were to do a twin engine kart, i would go with two engines, a dead axle, and have each engine power its own wheel.

 

[t0x1k]

All i can say is if one engine seizes your funked, or even if one stalls you have a chance of braking the chain. If i were to do a twin engine kart, i would go with two engines, a dead axle, and have each engine power its own wheel.

Well, use #50 chain. Make a custom manifold to use one large carb for both engines.
Sync them so they fire exactly opposite each other.
And the seize issue could be eliminated by using a coupling with a shear pin that connects the jack shaft together so it one seizes, bam, pin breaks and prevents engine damage, just replace jack shaft.

 

[anderkart]

Can someone explain why this happens?

Just thinking out loud here but... how about since their both sharing the work load, one of their clutches simply slips just a little longer until both engines synchronize their powerstroke and pull the load at the same time.

If 2 guys were pushing a really heavy car uphill, I"m thinking the weaker guy would soon discover the pushing was a little easier when he was pushing in step with the stronger guy. Could be your twin-engine kart clutchs are kinda doing the same thing.

 

[anderkart]

All i can say is if one engine seizes your funked, or even if one stalls you have a chance of braking the chain.

But with his dual clutch setup, if one engine stalls or locked up, seems like its clutch would just start slipping/freewheeling as soon as it dropped below its stall speed rpm.

 

[Kaptain Krunch]

i was talking about it in terms of connecting both engines to a jackshaft via chain, then running a clutch from that jackshaft to the axle.