Two stroke water cooled engine cooling issue 1

[dimachorny]

Hello,

I have two stroke water cooled engine and from time to time there are cooling issues with it.

Here some relevant information:
- Configuration - boxer two cylinders about 120 cc total displacement
- The engine max. power - 11 HP
- Working speed - 6500 RPM (constant, 1000 RPM below the max.)
- Continuous power consumption - about 7 HP
- Fuel/oil mixture - 95 Octane, 3% oil by weight
- Liquid coolant - 50/50 antifreeze coolant
- The cooling water cycle is as following: the cooled water from radiator via water pump -> enters the cylinders next to exhaust outlet -> pass through the cylinders -> leaves the cylinders via far nipple from the exhaust outlet -> enters the radiator (see attached picture)
- The CHT measured on the sylinder surface and not below the plug
- The engine has one carburetor common for both cylinders
- There are two independed electronic Ignition Control Units connected to one common pick-up sensor. Each cylinder has a single plug connected to its own ICU.

The temperature issue is: one of the cylinders (the lower one) from time to time has nonstable temperature with oscillations (see attached plot of 1.5 hours of running). It is not measurement issue since beside this the engine sounds different and loose its performance.

The questions are:
1. What may lead to such temperature oscillations only in one cylinder?
2. What is the importance in the way that cooling liquid pass through the cylinder:
- from hot side next to the exhaust outlet to the cool side
- from cool side far from the exhaust outlet and leaves the cylinder next to it

Thank you a lot

Yellow - RPM x100
Gray - Water [°C]
Orange - Left Cylinder [°C]
Blue - Right Cylinder [°C]

 

[SuperSalad]

Is it possible the temperature fluctuations are actually the symptom rather than the cause?

Based on those graphs, if everything is identical mechanically, the first place I would think to look would be the ICU for that cylinder since they each have a unique controller. I don't really know about ICUs so maybe I'm way off base with that thought, but those temperature fluctuations seem too rapid to me to be caused by abnormalities in coolant supply or cooling ability.

The coolants appears to be doing its job since it follows the engine temperatures and normalizes for a period of time. But then something is changing with the conditions.

I am assuming identical coolants routes, and that there was no damage internally during this run.

Andrew H.

http://www.mototribology.com

 

[btrueblood]

Is the water in the coolant loop under pressure? If not, the cylinder temperatures are likely high enough for boiling to occur, giving rise to some unsteady flow conditions as the vapor bubbles form and collapse.

 

[djhurayt]

I'm with MotoLuber the water temp looks like a symptom. But, I'm a little confused by a couple of things in your post:
1) you say the "lower" cylinder has issues, but your graph is left & right cylinders
2) I assume to RPM is of the output shaft of the combined two cylinders, it looks like the RPM goes to zero, and resultantly the cylinder & water temperatures drop and once the RPM picks back up the temperature returns to its normal operating range
3) It looks like the temperature of both cylinders fall with the water temperature fall, but you seem to indicate the issue is with only one cylinder

???

 

[EHudson]

With the single carburetor feeding a common crankcase, the delivery to the cylinders can be slightly different owing to the direction of the crankshaft rotation favoring flow to one of the cylinders. Oil or fuel collecting in the crankcase may be splashed more into one cylinder than the other. This oil and fuel being flung off the crank does affect running quality. (Run it backwards to test?)

How does the exhaust temperature look?

Beyond that, it may be difficult to deliver exactly the same volume of coolant to each cylinder

 

[romke]

That the two engine cylinders do not have exactly the same temperature, as such need not be reason for concern as long as the differences are small. In this case the "problem cylinder" however seems to have a lower temperature on average but higher spikes then the other. That can have several reasons. One possibility is erratic ignition, due to the fact that one sparkplug occasionally behaves in a different way. I would check the ignition system - not only the control unit, but also the high voltage leads and the sparkplug. Another possibility is occasional differences in mixture strength. If the air/fuel ratio varies that will lead to temperature spikes. The third possibility is the cooling fluid itself, as already mentioned. Local overheating may cause localized boiling. Is the cooling system pressure some way or another managed above atmospheric pressure?

You mention the sound also differs, when the temperature runs high. Can you describe the change in the sound produced?

 

[BigClive]

What make of engine is it? What is it being used in?

 

[Tmoose]

"There are two independed electronic Ignition Control Units connected to one common pick-up sensor. Each cylinder has a single plug connected to its own ICU."

I'd swap ICUs between cylinders and go for a "ride."

 

[dimachorny]

Thank you all a lot for the responses.

I'll try to clarify some points.

1. btrueblood, the boiling of the coolant is the root cause I thought about, but can't proof that. The water is not under pressure and I'm not familiar with the techniques of pressuring the water. May you kindly tell more about that?

2. djhurayt, here is the scheme of the engine orientation:

The right cylinder is the lower one. The abnormal behaviour of the temperature as I see it is where frequent oscillations of the right cylinder (blue on the chart) present and reach too high values​

3. EHudson, Unfortunately I have no EGT measuring. Regarding the residual oil/fuel in the crankcase - good point! It seems that exactly the "problematic" cylinder is the lower one and may receive extra-portions of oil/fuel. What kind of problems this may lead to? How I can detect this and treat?

4. romke Thank you for such comprehensive response.

-I would check the ignition system - not only the control unit, but also the high voltage leads and the sparkplug - new plugs. How I can check the leads?​

-If the air/fuel ratio varies that will lead to temperature spikes - I would think this will lead to pretty similar behaviour in both cylinders. Why there is a variance?​

-Local overheating may cause localized boiling. Is the cooling system pressure some way or another managed above atmospheric pressure? - As I mentioned previously, I think this is a root cause but I can't proof this. May be the high temperature of the coolant, circulation speed and resistance of the inlet nipple lead to local pressure drop and cavitation? How I can manage pressure in the system?​

Can you describe the change in the sound produced? - stuttering (if I use the correct word), according to our technician description, I didn't hear this by myself, like the engine goes to cut out​

 

[EHudson]

Crankcases loading up with fuel and oil are nothing new and lead to irregular running. When emissions regulations came around most chain saws went from horizontal cylinders to vertical cylinders in part minimize this fluctuation.

Explore the small check valves used on multi-cylinder outboards to purge pooled crankcase liquids and return them to the intake. These operate under extremely low pressure differentials and use the crankcase pressure pulses to operate them. Run a hose from the valve to the top of the reed block and let the reed action take care of dispersing the liquid back into the engine.

There may be a good spot for the check valve between the lower bearing and respective shaft seal or a low spot in the crankcase. Is one transfer port low and the other high?

The configuration of your engine will never allow thing to be perfect.

Your water temperature is at or near boiling at the temperature probe. It is entirely possible that you have hot spots that exceed the boiling temperature owing to stagnant or low local flow rates. You can drop the inlet temperature or raise the boiling point by increasing the system pressure as others suggest. Automotive radiators have been sealed with pressure regulating caps for years to increase the boiling point along with the coolant pressure.

It’s probably time to fit your engine with EGT probes to help balance the cylinders.