rScotty
Super Member
- Joined
- Apr 21, 2001
- Messages
- 8,289
- Location
- Rural mountains - Colorado
- Tractor
- Kubota M59, JD530, JD310SG. Restoring Yanmar YM165D
We were discussing thermosiphon heating on another forum. After doing some thinking and writing about an overheating problem, some of what we discussed might have some value here as well......enjoy, rScotty
From a post by rScotty:
The more I think about what we've discussed......the more I'm not sure that we know your tractor is really overheating or if it is a warning light malfunction. Is that right? If it has water and isn't spitting steam, do we have any other indication of overheating other than the indicator light?
I'm going to digress for a bit and talk mechanical. Hopefully those who to whom this is old hat will forgive me, but when overheating questions come up I can't resist posting some of the technical bits about why a thermosiphon system works the way it does. After all, the thermosiphon process is one seriously vintage and even downright archaic way to cool anything. My suspicion is that it was originally invented in the days before water pump seals were as reliable as now. Whatever the real reason, thermosiphon cooling was already antique and rather outdated technology when Yanmar resurrected it in their US models of the 70's. Most mechanics who worked with it were of a previous generation and are long since gone. Even back when Yanmars were new to the US, seeing thermosiphon cooling on a commercial machine was an oddity.
It was also a mechanical oddity for Yanmar to combine in one tractor a cooling system so archaic as thermosiphon along with a patented bevel-gear driven front axle that was totally complex, hi-tech, and expensive. That was all part of what made the brand so interesting. Their peculiar powershift tranny was another hi-tech device unique to Yanmar, though I don't believe it was ever combined with the thermosiphon cooling. Anyway, none of those systems were in use by other tractor companies at that time. It was like Yanmar was making an interplanetary spaceship propelled by a steam engine and steered with sails and a rudder. Maybe that was because Yanmar was a private company rather than a corporation at the time, and the owner was known for many curious and unique engineering solutions. The rumors were that tractors were not even Yanmar's main business; that agriculture and aquaculture were basically hobbies of the owner. It would be fascinating to know this stuff. Someone ought to do a biography.....
Getting back to other cool things, one of the nifty things about a thermosiphon system is that it works at a higher temperature than a pump-type circulation cooling system. TS cooling needs to do that to get enough difference between the density of hot engine water versus the cooler water in the radiator to drive the circulation fast enough to do some cooling. We've already discussed cooling fluid and why thermosiphon systems tend to have a larger capacity radiator with a higher heat transfer coefficient. And why that radiator has to be kept very clean both inside and out. It has to stand more pressure as well.
BTW, the higher temperature means higher pressure and that is why a proper radiator cap is important. If everything is designed proportionately, the radiator cap for a pump circulated system should be blowing off steam (and therefore losing temperature) at a temperature below where the thermosiphon system operates most efficiently.
Since a thermosiphon cooling system works at a slightly higher temperature than a pump cooled one it also has a different temperature warning light sender than the water pump models.
Lets compare some common temperature sender types: most of us are familiar with the water temperature senders used on cars today. They are of the temperature-variable resistor type and work by biasing a remote semiconductor relay with a signal current so that it will pass lamp current directly to the warning light. Or - and this is becoming more common - is that the temperature sender passes a signal to a control box containing a computer that then decides logically whether to power the warning light. This variable resistor type of temperature sensor has the advantage of being both inexpensive and analogue in operation, so it can tell the the control box computer by how much the temperature is too hot and how long it took to get that way.
But the old Yanmar cooling systems don't use temperature senders like that. The old Yanmar system is NOT a temperature-sensitive variable resistor. All of Yanmar's old style water temperature senders are a compact and complex on/off switch s driven by heat expansion. This is strictly yes/no, on/off type of digital info. The temperature sender screws into the water fitting at the front of the engine block and inside of that little sender is an expandable pellet that drives a rod that drives a diaphragm spring that pushes a shorting bar across two internal electrical contacts. All that motion working together completes an electrical path to the common chassis "ground" "in order to light the warning light. It is either on or off; no waffling.
For the Yanmar thermosiphon system the overheating warning light sender is supposed to tell the warning light to come on when the water at 248 F and to turn off at 234 F. The error bandwidth is about +- 5 or 6 degrees F. They are darn near bulletproof, but anything mechanical can go wrong, can go wrong...can go wrong...
Confusingly, the Yanmar tractors that have their cooling fluid circulated with a pump use a very similar warning light sender....it might even still be physically interchangeable....., but that one has a different part number and temperature spec. Pump cooled Yanmars tell their warning light to come on at 230 F and turn off at 215 F. Same type construction and the same tolerance ratio.
So It is also possible for someone to put the wrong or aftermarket sender in your tractor. If - as is possible - the sender was from a pump cooled system, the thermosiphon tractor's warning light would come on before the tractor reached normal operating temperature. With a wrong radiator cap - which would also physically fit, by the way - the thermosiphon Yanmar could also spit steam out of the overflow (loudly!) at a temperature below the normal operating temp.
Even if the thermosiphon system is clean, pressurized right, all the right parts, and working perfectly, another downside to any thermosiphon machine is operating on a slope. Thermosiphon Cooling is affected by the horizontal angle between the engine and the radiator, and that is obviously going to change on a slope. In Yanmar's case, I don't recall that they published a spec for operating slope - it would depend on how long and how hard the engine was working, of course.....but eyeballing the system will give you a rough idea of what it can handle. It isn't a whole lot. If you have long hilly acres you are probably going to encounter the limitation. Luckily, Yanmar's are overbuilt and conservatively rated with heavy cooling walls in the engine and radiator. So the temperature is slow to respond to changes in slope, and it can stand some overpressure. The rule for thermosiphons used on slopes is simple: Things are fine as long as it cools adequately. If the slope is so long that the engine overheats....then don't do that.
You can of course replace the warning light with a temperature gauge, the trick there is that the threads are an oddity. Like a lot of Japanese equipment from that era the thread into the cast iron block was originally British standard tapered pipe. There are interesting old tales as to why they made that choice. You can also make up an appropriately threaded and insulated "T" so keep both the warning light and gauge.
Hope you enjoy the ramble.... good luck with your tractors
rScotty
From a post by rScotty:
The more I think about what we've discussed......the more I'm not sure that we know your tractor is really overheating or if it is a warning light malfunction. Is that right? If it has water and isn't spitting steam, do we have any other indication of overheating other than the indicator light?
I'm going to digress for a bit and talk mechanical. Hopefully those who to whom this is old hat will forgive me, but when overheating questions come up I can't resist posting some of the technical bits about why a thermosiphon system works the way it does. After all, the thermosiphon process is one seriously vintage and even downright archaic way to cool anything. My suspicion is that it was originally invented in the days before water pump seals were as reliable as now. Whatever the real reason, thermosiphon cooling was already antique and rather outdated technology when Yanmar resurrected it in their US models of the 70's. Most mechanics who worked with it were of a previous generation and are long since gone. Even back when Yanmars were new to the US, seeing thermosiphon cooling on a commercial machine was an oddity.
It was also a mechanical oddity for Yanmar to combine in one tractor a cooling system so archaic as thermosiphon along with a patented bevel-gear driven front axle that was totally complex, hi-tech, and expensive. That was all part of what made the brand so interesting. Their peculiar powershift tranny was another hi-tech device unique to Yanmar, though I don't believe it was ever combined with the thermosiphon cooling. Anyway, none of those systems were in use by other tractor companies at that time. It was like Yanmar was making an interplanetary spaceship propelled by a steam engine and steered with sails and a rudder. Maybe that was because Yanmar was a private company rather than a corporation at the time, and the owner was known for many curious and unique engineering solutions. The rumors were that tractors were not even Yanmar's main business; that agriculture and aquaculture were basically hobbies of the owner. It would be fascinating to know this stuff. Someone ought to do a biography.....
Getting back to other cool things, one of the nifty things about a thermosiphon system is that it works at a higher temperature than a pump-type circulation cooling system. TS cooling needs to do that to get enough difference between the density of hot engine water versus the cooler water in the radiator to drive the circulation fast enough to do some cooling. We've already discussed cooling fluid and why thermosiphon systems tend to have a larger capacity radiator with a higher heat transfer coefficient. And why that radiator has to be kept very clean both inside and out. It has to stand more pressure as well.
BTW, the higher temperature means higher pressure and that is why a proper radiator cap is important. If everything is designed proportionately, the radiator cap for a pump circulated system should be blowing off steam (and therefore losing temperature) at a temperature below where the thermosiphon system operates most efficiently.
Since a thermosiphon cooling system works at a slightly higher temperature than a pump cooled one it also has a different temperature warning light sender than the water pump models.
Lets compare some common temperature sender types: most of us are familiar with the water temperature senders used on cars today. They are of the temperature-variable resistor type and work by biasing a remote semiconductor relay with a signal current so that it will pass lamp current directly to the warning light. Or - and this is becoming more common - is that the temperature sender passes a signal to a control box containing a computer that then decides logically whether to power the warning light. This variable resistor type of temperature sensor has the advantage of being both inexpensive and analogue in operation, so it can tell the the control box computer by how much the temperature is too hot and how long it took to get that way.
But the old Yanmar cooling systems don't use temperature senders like that. The old Yanmar system is NOT a temperature-sensitive variable resistor. All of Yanmar's old style water temperature senders are a compact and complex on/off switch s driven by heat expansion. This is strictly yes/no, on/off type of digital info. The temperature sender screws into the water fitting at the front of the engine block and inside of that little sender is an expandable pellet that drives a rod that drives a diaphragm spring that pushes a shorting bar across two internal electrical contacts. All that motion working together completes an electrical path to the common chassis "ground" "in order to light the warning light. It is either on or off; no waffling.
For the Yanmar thermosiphon system the overheating warning light sender is supposed to tell the warning light to come on when the water at 248 F and to turn off at 234 F. The error bandwidth is about +- 5 or 6 degrees F. They are darn near bulletproof, but anything mechanical can go wrong, can go wrong...can go wrong...
Confusingly, the Yanmar tractors that have their cooling fluid circulated with a pump use a very similar warning light sender....it might even still be physically interchangeable....., but that one has a different part number and temperature spec. Pump cooled Yanmars tell their warning light to come on at 230 F and turn off at 215 F. Same type construction and the same tolerance ratio.
So It is also possible for someone to put the wrong or aftermarket sender in your tractor. If - as is possible - the sender was from a pump cooled system, the thermosiphon tractor's warning light would come on before the tractor reached normal operating temperature. With a wrong radiator cap - which would also physically fit, by the way - the thermosiphon Yanmar could also spit steam out of the overflow (loudly!) at a temperature below the normal operating temp.
Even if the thermosiphon system is clean, pressurized right, all the right parts, and working perfectly, another downside to any thermosiphon machine is operating on a slope. Thermosiphon Cooling is affected by the horizontal angle between the engine and the radiator, and that is obviously going to change on a slope. In Yanmar's case, I don't recall that they published a spec for operating slope - it would depend on how long and how hard the engine was working, of course.....but eyeballing the system will give you a rough idea of what it can handle. It isn't a whole lot. If you have long hilly acres you are probably going to encounter the limitation. Luckily, Yanmar's are overbuilt and conservatively rated with heavy cooling walls in the engine and radiator. So the temperature is slow to respond to changes in slope, and it can stand some overpressure. The rule for thermosiphons used on slopes is simple: Things are fine as long as it cools adequately. If the slope is so long that the engine overheats....then don't do that.
You can of course replace the warning light with a temperature gauge, the trick there is that the threads are an oddity. Like a lot of Japanese equipment from that era the thread into the cast iron block was originally British standard tapered pipe. There are interesting old tales as to why they made that choice. You can also make up an appropriately threaded and insulated "T" so keep both the warning light and gauge.
Hope you enjoy the ramble.... good luck with your tractors
rScotty
