2-stroke runaway

[JWaterstreet]

I've got a question that has been puzzling me for awhile, and figured I'd reach out to the collective wisdom of the group.

Bombardier makes a line of personal watercraft under the Sea Doo brand name. From 1998 to about 2004, they installed a tempermental engine made by Rotax in a line of high performance watercraft. This engine was called the model 947, with 951cc, 2-stroke, oil injected, reed valve induction, variable exhaust pipe tuning via their RAVE technology & water injection. These engines put out 130 HP, nothing major from a racing engine perspective, but pretty healthy for a consumer recreation vehicle. These are direct coupled to a jet pump that provides the motive power for the PWC out the back.

They are notoriously unreliable, many go south with under 100 hours usage, many self destruct with broken piston skirts ending up first in the crankcase, then all over the inside of the hull as the aluminum case shatters.....

But my question has to do with runaway. I rebuilt one and proceeded to get it running "on the hose", i.e. out of the water using a garden hose plumbed into the cooling system. The engine started and ran for a short time, and proceeded to runaway. When this happened, I quickly hit the stop button (which does nothing), and pulled the choke (which killed it) I knew to pull the choke, because this engine is known for runaway, and I had read that pulling the choke was the best way to stop it.

Bombardier specs call for setting the "out of water" idle at 3000 RPM, which results in an idle "in the water" of about 1500 RPM. Mine was set for about 3200 RPM. A well-known tuner of Sea Doo's recommends an out of water idle speed of 2800 RPM to prevent runaway on this engine. I set mine down to 2800 RPM and the runaway problem stopped.

What puzzles me is the mechanism for runaway on this particular engine. 2-strokes are well known for runaway if an extremely lean condition exists, but this particular engine does it when it is properly set up and jetted. I did a plug check at idle, and it showed a fairly rich condition. (plug check at WOT showed a nice tan color)

These engines don't have excessive compression, and are designed to run on 87 octane pump gas. A compression check reveals 135 PSI, which is right on spec for this engine.

This engine is newly rebuilt, so no carbon deposits are present for autoignition. It didn't run long enough for the plugs to get super hot for autoignition. The compression is relatively low for autoignition.

Can anyone present a hypothesis on why this particular engine does this at a slightly high idle setting?

 

[Nemesis4]

Whatever the mechanism of "running away" is it would seem that, as the RPM increases markedly, the engine is making better use of its available fuel and air mixture. I am not surprised that there is some academic interest in the phenomonom - possibly it could be a better way to operate a 2-stroke engine.
Has anybody actually tried opening the throttle and putting the engine under load (like riding the bike or jet-ski) while it is running in this auto-ignition mode? Does the engine stop or continue to run?

 

[patprimmer]

The engine will blow holes clean through the pistons if put under load in a condition lean enough to run as a diesel at idle.

Regards
Pat
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[JWaterstreet]

These engines are direct coupled to an jet pump impeller. When in the water, your "idle" is simply "propulsion at slow speed" rather than an idle condition with no load like an automobile.

They never runaway in the water. Only when out of the water with absolutely no load whatsoever.

This presents another interesting dynamic. I wonder what the A/F ratio looks like when operating at its design intent at idle in the water (roughly 1500 RPM) versus when it is running with no load out of the water (roughly at 3000 RPM)? While butterfly opening stays the same, there should be more vacuum impressed, no? If this results in a shift to a more lean condition, then that would explain rapid plug heating and an autoignition event. However, if it shifts rich or stays the same, then I am still at a loss to explain it.

 

[Dan320]

What happens if you open the throttle during a runaway?
Anyone tried?
If the revs increase, then the ignition may be from exhaust gas. If the engine stops, then it was probably something glowing that was put out by the greater charge flowing through the comb. chamber.
Maybe it's one way for one engine type and the other way for another type.

Dan

 

[turbocohen]

Are you positive the reed plate, case and crank seals are not leaking? Were they replaced?

 

[JWaterstreet]

A pressure test was performed on the engine to ensure the reed plates, case and crank seals were not leaking. Additionally, a WOT plug reading was performed, showing a nice tan color...

 

[LadaTrouble]

To get a runaway to high revs you need enough air to get that fast - what sort of carb?...Round slides are prone to leakage.Crankcase seals,base gaskets,reed block gaskets etc.The ignition source is the plug for sure - but it won't runaway without fuel and air to complete the process.

 

[JWaterstreet]

If you look at my previous posts, you will see that this runaway occurs with very little air; the butterflies are almost closed, and there is no leakage anywhere else. That's the purpose of the pressure test mentioned in my post just before yours; to ensure there are no leaks.

Remember we are talking no load here; all ball and roller bearing so there is little friction loss. Doesn't take much to get the thing moving at a high speed.

 

[patprimmer]

Most friction losses are through the rings, not the bearings.

No air, no power.

No power no rpm.

To much air is getting in somewhere.

Regards
Pat
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[JWaterstreet]

The only air coming in is thru the carbs. Keep in mind that these have a pretty high idle to start with (3,000 RPM), so I was probably misleading by saying the butterflies are almost closed. At idle setting, there is around a 1/16" or so opening.

As I said in the beginning post, this is well documented phenomena with this engine design, not a problem due to leaky crank seals, gaskets, or some other leakage of excess air. Almost all of them will do this if the idle is set greater than or equal to 3,000 RPM.

 

[turbocohen]

As patprimer mentioned they have roller bearings and rings are the major source of friction. Roller bearings are needed for inproved durability when pressurized lube is not in the design and oil is diluted.

Here is where the lightbulb goes bling.. There is work against the case pressure and atmosphere. Bing!

With a carbureted 2 stroke ported for mid to high torque at mid to high rpm, vs ported for torque rise off peak and peak@high rpm as in a chain saw, if you set idle speed at an rpm where the engine operates just below the rpm of a resonant "band" where gas exchange and ve are poor rendering incoming charges highly diluted with the previous cycles exhaust, random hit-n-missfire occurs. When a few less random missfire cycles occur than power strokes then whammo, your engine may momentarily wind up to a higher rpm band where exhaust is more efficiently scavenged and its more power strokes back to back.. none wasted.. The less missfire with better scavenging, the more suction per unit time across the throttle, fewer misfire cycles pumping against atmosphere, more mass flow due to lower map.. all combined = Grenade pucker factor.

The set idle below 2800rpm has something to do with that most likely. Direct injected 2 strokes have eliminated this phenomenon and allows better port timing whereas carburators have to be accomodated via compromise.. yet another reason to ditch ancient carbs.. Who needs buggywhips?

That wasn't nice.

 

[LadaTrouble]

If you have this thing set rich as you say - then the runaway is happening when it gets some extra air and starts to run more efficiently.Set it lean and see if it still runs away....although that could prove fatal.