Understanding the Temperature Required for the Engine to Understand the Importance of Thermostat

The thermostat, which is always installed in the cooling path of a water-cooled engine, is an essential component to keep the temperature of the coolant just right. If the purpose of the thermostat is to prevent overheating, the coolant should always pass through the radiator, but why block the passage at low temperatures? If you understand the "temperature" required by the engine, you will understand the importance of the thermostat.

An engine isn't just good when it's cold.

The difference between the in-line type where the thermostat is attached to the engine outlet side and the bottom bypass type where it is attached to the inlet side can be imagined by the installation position. The in-line type is located at the top of the engine and the hose of the thermostat housing is connected to the upper side of the radiator. The bottom bypass type is located at the bottom of the engine and the lower hose of the radiator is connected to the thermostat housing like Yamaha V-MAX.

Because the thermostat housing of V-MAX is located behind the exhaust pipe of the front cylinder, remove the hose and the pipe connected to the case while being careful not to apply unreasonable force. The O-ring at the end of the pipe is often deformed and hardened, so you should replace it with a new one when you restore it. Also, use a non-woven cloth sponge to remove any oozing coolant that may have adhered to the pipe or engine side.

In contrast to air-cooled engines, which can only be cooled when riding, water-cooled engines, which dissipate the heat of the cooling water from the radiator, are considered to be more resistant to overheating. This is why all current model automobiles are water-cooled, although there are still a small number of motorcycles with air-cooled engines.

The cooling path of the water-cooled engine consists of the water jacket inside the engine, the water pump, and the radiator which is often placed at the front side of the engine, but apart from these, the thermostat plays an important role. The thermostat is built into the middle of several radiator hoses, and it is not an exaggeration to say that it holds the key to the water cooling system, although it is an unassuming existence.

A thermostat is used to maintain the proper temperature of something, such as a kotatsu in a Japanese-style room, a fan heater, or an air conditioner, by stopping its operation when it gets too warm and switching it on when it gets cold. A thermostat in the cooling path of a water-cooled engine is there to maintain the temperature of the coolant. Many people tend to misunderstand that the thermostat opens when the water temperature rises = a component to promote cooling, but its function is not limited to that. However, the function of a thermostat is much more than that. If you really think about cooling only, it would be more efficient not to install a thermostat that blocks the pathway.

Herein lies the significance of the thermostat, which is to "maintain the temperature at the proper level". In other words, the cooling path is not just about being cool. One of the reasons why air-cooled engines were converted to water-cooled engines was to bring the engine temperature up to the proper level in a short period of time during cold starts. As owners of air-cooled engines will know, a choke is essential for starting in the cold winter months. By making the gasoline thicker (or squeezing the air) than it would be during normal driving, the choke promotes combustion by sending more gasoline, which is harder to atomize, into the combustion chamber. When the engine starts, combustion heat is transferred to the cylinders and cylinder head, but the lower the temperature, the more heat is lost, and since the temperature does not rise forever, atomization is not stable. It is also difficult for the engine oil to rise in temperature, so it remains in a low temperature and high viscosity state.

On the other hand, in a water-cooled engine, the thermostat prevents the cooling water from flowing to the radiator when the water temperature is low, and circulates only in the water jacket inside the engine (some of it flows in the bypass path). Then, the coolant receives the heat of combustion, and the temperature rises quickly in the closed path, and the atomization of gasoline is accelerated, and the oil temperature rises, and the warm-up is completed in a short time.

Then, when the water temperature in the water jacket has risen sufficiently, the coolant water flows to the radiator for the first time, and after the temperature is lowered by the driving wind, it is returned to the engine for cooling. After that, if the temperature of the cooling water drops below the set temperature of the thermostat, the valve closes and the temperature rises, and if the temperature rises, the valve opens, repeating the operation to maintain the set temperature.

This is the role of the thermostat, and it works to prevent overheating as well as overcooling, and to warm up and retain heat.

There are in-line type and bottom bypass type depending on the installation position.

If the engine is using a long-life coolant correctly, there is no problem, but if the engine is using only tap water, the inside of the housing may be corroded. This V-MAX was properly filled with LLC, so it was fine. It is important to replace the O-ring with a new one and tighten it with the proper torque.

The mechanism of a thermostat, which opens and closes the valve by the change of cooling water temperature, has been almost unchanged for decades. It works on a very analog principle: the valve is opened and closed by a change in the volume of wax, which is solid at room temperature, trapped in a sealed container, and expands as the temperature rises. Although it may seem strange that even the latest super sports models with ultra-high performance have their cooling water temperature controlled by the change between solid and liquid wax, there seems to be nothing better than this in terms of simplicity of structure, reliability, and low cost. In some automobile engines, an electronically controlled thermostat is used for the purpose of strictly controlling the cooling water temperature, but for motorcycles, the mechanical type is the mainstream.

Thermostats are classified into two types according to their installation position in the cooling path, although they have the same purpose of keeping the temperature of cooling water at a proper level. One inline type is mounted between the engine outlet and the radiator. And one more thing.Bottom bypass type, this one is installed on the entrance side from the radiator to the engine. According to the homepage of the radiator equipment manufacturer, the in-line type is mostly used in passenger cars, and the bottom bypass type has long been used in large vehicles. Therefore, the ratio of in-line type thermostats is higher for motorcycles.

Both of them circulate the coolant through the radiator and the water jacket when the thermostat valve is opened, but there is a difference in the movement of the coolant when the valve is closed. The in-line type thermostat is located at the engine outlet side, and the temperature of the cooling water in the radiator decreases rapidly while the valve is closed. Therefore, when the water temperature in the water jacket rises and the valve opens, the coolant water whose temperature has dropped flows into the engine, and a temperature difference occurs.

On the other hand, the bottom bypass type has a thermostat at the entrance side of the engine, so even if the valve is closed, the water jacket and the radiator are in the same path and the temperature does not drop drastically even if the driving wind hits. The advantage of this is that it is difficult to have a big difference between the coolant flowing into the engine and the water temperature in the water jacket. Incidentally, the thermostat of the Yamaha V-MAX introduced here is installed in the lower part of the radiator, in front of the crankcase, and is located at the entrance side of the engine, so it is a bottom bypass type.

The opening temperature of boiling water can be found by a primitive experiment.

To check the function of a thermostat that opens the valve according to the temperature of coolant, you can actually warm it up in water. It is not enough that the thermostat opens when it is placed in hot water, it is important to measure the temperature of the water. Wax liquefies gradually as the temperature rises, so the valve does not suddenly open all the way at a certain temperature like an electric switch, but it opens gradually.

The working principle of the thermostat is that the wax sealed in the tube that fits inside the spring expands as the cooling water temperature rises, and the valve opens by the reaction that pushes out the pin in the tube. It is the same with the auto choke of the moped scooter that the pin in the tube is moved by the temperature and made to work.

The valve of thermostat is always under the force of closing direction by spring. Therefore, when trouble occurs, the valve does not open, that is, cooling water does not flow to the radiator, and it becomes overheat tendency in most cases. However, there is a case that the valve does not close due to physical trouble or foreign object bite and the cooling water temperature does not rise sufficiently, resulting in overcooling. If your motorcycle is equipped with a water temperature gauge or indicator light, check it while driving.

If you want to check the function of the thermostat more directly, you can put the thermostat itself in hot water and see how it works. Thermostats have a fixed operating temperature, and some of them have that temperature engraved on the body. In most cases, the thermostat will gradually open from around 80 degrees Celsius, so you can see how the valve is opening by boiling water while measuring it with a thermometer.

When the wax in the case at the center of thermostat expands, the valve opens while compressing the spring. At this time, you should check if the gasket around the circumference of the valve is hardened or missing as well as the valve opening temperature. If the gasket is deteriorated, the coolant will pass through even if the valve is closed, and the warm-up efficiency may be reduced.

As a result of tuning and customization, there is a part called Rotemp Thermostat which starts to open the valve from lower water temperature than stock as a part for the engine with increased heat generation. This has the merit of suppressing overheating by circulating the coolant water to the radiator from the low water temperature, but on the other hand, it may cause overcooling especially in winter because it starts circulating when the engine is not sufficiently warmed up. If this tendency is noticeable, it may be necessary to take measures such as blocking part of the radiator or returning to the stock thermostat during the winter.

The valve is closed. When soaking in hot water, choose a position for the hook that does not interfere with the spring that contracts or the valve that opens.

When the valve is fully opened, there is a gap like this. Some models have a valve lift setting in addition to the temperature at the beginning of opening and the full opening temperature, so check your motorcycle's service manual to determine the function.