Tractor Cooling Methods

Engine cooling is a critical part of any engine. Luckily it's a matter that can be solved in a wide variety of ways. And many ways have proven to be practical and effective. These are some of the most popular methods that have been used over the years.

Evaporative cooling is one of the simplest methods that has been used for cooling engines. The basic idea is that a tank of water surrounds the cylinder and absorbs heat. The water will slowly evaporate or boil depending on the load of the engine and thanks to water having a high latent heat of vaporization (it takes a lot of heat to convert water into steam) the steam will carry away a lot of heat with it. This method is used almost exclusively on old stationary engines. Although many of the old stationary engines may run just fine without water for a while the water also prevents hot spots on the cylinder and head. Water must be added multiple times throughout the day to keep the water level above the cylinder. With very limited circulation the only thing that limits the temperature of the engine is the water boiling off at 210 degrees. There is very little that can change this although adding antifreeze can increase it a little bit. This method has mostly been abandoned in favor of more effective and lower maintenance methods.

Air cooling has always been a popular cooling method. The areas of the engine that need cooled are designed with cooling fins and typically have a fan that blows cold air through them. Air cooling has found its way into just about every industry. It is most common in small engines and still found in large engines too. One example is my Agco Allis 6690 which has a 4 cylinder turbo diesel making about 100 horsepower. There are a variety of gas engine using air cooling as well. The Wisconsin engine company made many V4 gas engines used in agricultural and industrial applications with power ratings up to 70 HP that were air cooled. It has also been used in cars and motorcycles commonly. The biggest benefits to air cooling are the simplicity, cost efficiency, compactness, and reliability. The engine can be built with fewer parts and doesn't need coolant that can leak out. And there isn't a need for extra room to mount a radiator and reservoir.  Usually there is a fan mounted to the crankshaft or driven by a belt that blows air into the shrouds around the engine and through the cooling fins. The downfalls of air cooling also lie in its simplicity. Usually the engine has full cooling right away when it is started, this leads to longer warm up times. And the operating temperature isn't well regulated. It can vary significantly depending on ambient temperature and cooling system maintenance. This would have been a major issue for engines running on heavy fuels. The cooling fins also have to be cleaned out occasionally to prevent dirt and debris from blocking air flow or heat transfer. Another big problem is that the engine has less cooling ability at lower rpm. Lugging the engine at low rpm for extended periods can lead to overheating even with a well maintained system. For heavy loads working at high rpm is a must for the life of the engine.

One step above air cooling would be a thermosiphon system. This type of system usually uses water to carry heat away from the engine. The water is circulated by hot water rising and cool water sinking. This said, the lowest point of the system has to be the radiator outlet and the highest point is the radiator inlet. Once the engine warms up, the radiator cools the water and it begins to circulate thanks to the temperature difference between the top and bottom the radiator. This system was very popular on earlier tractors that didn't need powerful cooling systems. I have a lot of tractors with thermosiphon systems including my John Deere L and B and Farmall B and f-20. My Farmall A would also have a thermosiphon if the engine wasn't swapped. Farmall cubs used a thermosiphon until the end of production in 1979. And other tractors continued to use it later than that. The systems I have dealt with are not closed systems, they don't operate under pressure and there's a hose that runs down the side of the radiator for overflow. There are however some systems that are closed and operate under pressure. Temperature control is usually managed by limiting air flow through the radiator if necessary. Gas burning engines are usually happy enough running at any temperature within reason. Tractors that were meant to burn heavy fuels like distillate or kerosene have to be ran hot. These tractors were equipped with radiator curtains in some cases like the unstyled John Deere B, or radiator shutters on tractors like my Farmall B. Distillate tractors typically have a temperature gauge and the operator would manually open or close the shutters to maintain operating temperature. Temp gauges were not as common on gas tractors. Keeping the water level full is critical to thermosiphon systems. The water has to be above the upper radiator inlet or it won't circulate. On open systems that don't have pressure it is a daily maintenance routine to check water levels because the water can evaporate and boil off. In the event of overheating, my experience is that the water will boil and steam will exit the overflow tube and proceed to get sprayed back at the operator by the fan (by this point the steam has cooled off and condensed back into water and doesn't hurt the operator). In unpressurized systems this isn't a big deal to the engine as long as it's not a regular or long lasting scenario since the water will only get to 210 before it begins boiling. A side effect of the high operating temperatures and low boiling point of the water is that sometimes an engine could get small spots of water to boil inside the engine around the cylinder walls and in the head. This led to reduced cooling capacity since the water was no longer in contact with the metal. On my tractors this is most obvious just after shutting the tractor down after a long drive. You can hear the steam bubbles in the engine making noise. Some pressurized systems used a whistle that would blow when the steam started to blow past the pressure valve. As engine performance increased and cooling demands increased thermosiphon systems were mostly phased out in favor of water pumps.

Fully sealed, pressurized, pump based cooling systems have become the most popular form of cooling for modern engines of any substantial size. These systems offer great cooling performance and are easy to use and maintain compared to most other systems. Since they are sealed, water levels don’t have to be checked constantly. And since the pump creates flow there is typically a thermostat that controls the operating temperature. The thermostat would usually create too much restriction in a thermosiphon system. This eliminated the need to manually control coolant temperatures and gave more choices for operating temperature to designers. Adding a water pump also allowed the system to be built cheaper. Cooling systems could deviate from the design restrictions that gravity created and the coolant passages could go whichever direction was convenient. Since water flow or operating temperature could be increased to compensate for a less efficient radiator, manufacturers were able to use steel radiators which were cheaper and easier to get material for during war time. Another benefit to increasing the pressure in the cooling system was that just a 10 psi increase was able to raise the boiling point of the water to 240 degrees. This allowed engines to run hotter without risking boiling near the cylinder walls or in the head. Running the engine hotter could also improve efficiency.