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?

 

[bcs5274]

i thought ski doo had a rev limiter built into the ign. module??

i know the sled engines do, haven't had much time with the pwc engines

 

[BrianPetersen]

A while back, Honda was playing with a two-stroke engine that used controlled auto-ignition; they called it ARC (Active Radical Combustion). The residual exhaust from the previous stroke has enough heat and chemical reaction going on to ignite the next stroke without needing the spark plug. You're probably experiencing something like that. There is almost no frictional load or pumping losses in a roller-bearing two-stroke (compared to a four-stroke) so it's conceivable that even the slightest amount of fuel delivery would let it run away.

 

[BrianPetersen]

one other small thing; Grand Prix racing two-stroke motorcycle engines don't have idle circuits in the carbs. The rider is responsible for keeping the engine going by blipping the throttle. No runaway that way, but it's not acceptable for something that normal people are expected to use.

 

[JWaterstreet]

bcs5274, they do have rev limiters on the Sea Doo engines, typically set to 7K RPM or so. But a rev limiter has no relevance to what I am speaking about, since they simply cut off ignition. In this situation, the ignition is off and the engine continues to runaway at high RPM. (autoignition situation)

 

[JWaterstreet]

So I just had another lively debate about the cause of the runaway on this particular engine. An additional data point; it seems these engines have a purposely lean idle setting (for emissions), compensated by an accelerator pump to get it off idle. So the slightly off-idle setting that causes these engines to runaway would be assumed to be fairly lean as well.

One person is adamant that the autoignition during runaway is caused by a glowing red spark plug. I don't think so; per my original post, these engines will runaway in as little as 5 seconds after starting, with no load on the engine at all. I don't see how 5 seconds (or 15 seconds for that matter) of a lean run at no load can possibly heat the plug electrode to glowing hot.

Does anyone have any good suggestions how to empirically find out the source of the autoignition, i.e. a way to check if it is glowing plugs or a carryover of the chemical reaction from the previous combustion??

(I thought of perhaps installing surface gap plugs and testing if it ran away...)

 

[patprimmer]

It is certainly possible for the earth straps to glow red hot in a few seconds at a fast idle.

Regards
Pat
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[MikeHalloran]

Like Pat said, the time constant of a ground electrode has to be in milliseconds.

But I'm wondering the 'the hose' is really providing enough water in the right places?



Mike Halloran
Pembroke Pines, FL, USA

 

[JWaterstreet]

Yes; these things are designed to run on a hose, mainly for flushing things out after you ride them. They put a hose connector on them expressly made for the purpose. And the engine must be running during flush, or else you end up with an engine full of water. (due to water injection in the exhaust...)

However, if the ground electrodes on plugs start to glow red after a few seconds, (or milliseconds per Mike) at a no-load idle situation, what temperature do they run under full load conditions? This doesn't seem right to me.

 

[JWaterstreet]

Ok, I just looked up a few things. These engines use NGK plugs. NGK states that the optimal plug temp for both the center and ground electrode is 600 C, and max is 800 C. Anything above 800 C will result in preignition. 600 C is the borderline for when steel begins to glow, 800 is a very dull cherry red.

But it goes on to say that these temps are usually only reached at peak torque loading. In my discussed runaway condition, there is near zero torque.

I have pulled the plugs after one of these runaway episodes. The plugs have a rich (dark) appearance to them; not like the typical white overheated look of a lean condition.

Based on the above, I still have trouble believing that the plugs are the source of the autoignition. However, perhaps in these conditions, what would normally be too low of a temp to induce autoignition, might actually induce it? (say 400 C?)

 

[patprimmer]

Under full load, if it is lean, the pistons will melt about the same time as the electrodes glow red hot. It will then definitely cool off over a reasonable time.

Regards
Pat
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[Junior37c]

Probably the reason that the plug looks rich is because you choked the engine to death when shutting it down.

 

[Nemesis4]

This is a very interesting topic. I would like to make two comments.
Firstly, I have seen several high compression SI 4-stroke engines that would continue to idle very roughly after the ignition was turned off - but, they certainly would not run at high RPM in "compression ignition" mode. Maybe 2-strokes behave differently.
Secondly I have witnessed very odd behaviour from a 4-stroke motorcycle engine with transistorised switching of the primary circuit triggered by a sender unit on the crankshaft. Switching off the ignition would stop the engine - but, disconnecting the battery would not always stop the engine which would continue to idle happily - certainly not in any type of autoignition mode. The workshop manual wiring diagram showed that the ignition switch cut the connection to the battery so it should have had the same effect as disconnecting the battery.
Anyhow the point I am trying to make is that Mr. Waterstreet's engine may well have a similar type of crank-triggered spark - maybe when he turns the ignition off the sparks continue.
It may be tricky to do but possibly Mr. Waterstreet should check to see if the plugs are firing as the engine "runs away'.

 

[patprimmer]

An engine will run with the battery disconnected because the alternator is supplying enough charge to power the ignition.

Regards
Pat
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[Dan320]

I have seen a 2-stroke off road motorcycle engine run away and not stopping even after disconnecting the ignition lead.

Dan

 

[Nemesis4]

I should have mentioned that it was a bench-test engine without the alternator connected - battery power only to the ignition.

 

[Nemesis4]

It also has occurred to me that 2-stroke model aeroplane engines of the "glow plug" variety basically rely on the catalytic properties of the platinum alloy glow plug element. They are started with battery heating of the glow plug but when running the ignition is mainly due to catalytic action of the glow plug on the methanol fuel.
With the 2-stroke jet-ski engine possibly the spark plug electrode (which could possibly be platinum or nickel alloy both of which produce catalyic effects) may be having a similar catalytic effect on the jet-ski engine's fuel.
I also seem to remember reading somewhere that the glow plug catalytic effect only works correctly over about 3,000RPM - which may explain why a lower idle speed stops it "running away".

 

[patprimmer]

If the electrode is glowing red hot, no matter what it is made of, it will ignite the fuel. Platinum and iridium electrodes tend to be very fine, so I expect they heat to red heat faster and easier than std plugs.

Any SI engine that is generating enough heat in the chamber to get the electrodes glowing will continue to run so long as the heat, air and fuel supply is maintained.

If the fuel is methanol, a lean mixture will increase both idle speed and chamber temperature.

If the engine is stopped by choking the airflow, but not the fuel flow, the engine will go rich prior to stalling.

Regards
Pat
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[JWaterstreet]

These engines I am referring to have standard spark plugs, nothing exotic in the conductors. And when you cut power to the engine, the power is cut as well. Others have attempted to pull the plug wires to stop the runaway, to no avail.

Keep in mind that I am still trying to understand the two related dynamics of this situation, an autoignition event, which is where the engine continues to fire in the absence of an ignition spark, and a runaway event where the engine begins running faster and faster until it reaches a very high speed and achieves a stasis of sorts. All of this happening with no change in the amount of air or fuel delivered by the carburetors.

My main hypothesis is that the residual burning (oxidizing) charge from the previous combustion is being carried over to the next charge and igniting it. This event produces more relative power than the previous combustion event (only slightly) which causes the engine to begin increasing speed as well as continuing to run on its own without ignition power. Eventually the process reaches a stasis, where the "charge carryover" cannot effect more power, combined with pumping losses, and the engine reaches an equilibrium speed.

Does anyone have a better hypothesis? (Especially to explain the common two stroke runaway...I have never seen a scientific explanation for the event....)

I am attempting to find surface gap plugs that have the same depth as the stock plugs to at least rule out glowing ground electrodes as the source of the autoignition....

 

[Junior37c]

http://www.sae.org/technical/papers/982481

This paper covers some research into the topic, relating to temperature and chemistry of the plugging charge and the effect it has on autoignition. Fairly interesting stuff if you're into that.