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Water works
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Understanding how water cools an engine helps you diagnose, solve and avoid problems.
Water-cooling is displacing air-cooling in many turf
applications. Increasingly, popular diesels are almost all water-cooled.
It’s difficult for air-cooling to maintain the stable engine temperatures
demanded by modern pollution control standards.
Water-cooling provides a broad “safety buffer” that allows
an engine to work extra hard for brief periods without damage.
Water is the main moving part and working element in the cooling system. In nature, water is found in all three physical phases. It’s a solid (ice) below 0 degrees C (32 degrees F). It’s a gas (steam) above 100 C (212 F). It’s liquid water at all temperatures in between, when under “normal” air pressure of 14.7 pounds per square inch (1 bar in metric terms).
| Next month Why engines need antifreeze |
Like all materials, water expands in volume as it gets hotter. Put a lid on the container, and the expansion produces pressure. Pressure restricts the act of boiling, so the water remains liquid and keeps absorbing heat. With a 15 psi radiator cap in place, the water’s boiling point rises to nearly 125 degrees C (about 250 F). That sets an upper limit. The engine thermostat sets a lower limit on operating temperature. Typical thermostats are rated at 160, 180, 192 and 195 F.
The engine can operate safely between the thermostat’s baseline temperature and the maximum allowed by the radiator cap. That assumes proper lubrication and good overall condition.
Layers of safety
Contributing to the “safety buffer” is water’s ability
to absorb heat. It takes 1 calorie to raise 1 gram of liquid water by
1 C, for a “specific heat” score of 1.000. Almost everything
else has a lower score. The scores for other major cooling system materials
are 0.092 for copper (and brass), 0.107 for iron, 0.217 for aluminum and
0.240 for dry air at normal pressure.
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Water’s heat absorption really kicks into high gear when it is still liquid but reaches boiling temperature. Then it takes 539 calories of “latent heat” to convert that 1 gram of liquid water completely into steam. Latent means “hidden.” It just seems to disappear without raising the temperature.
Adding one more layer of safety are computerized engine controls that track coolant temperature, among other factors, and adjust the engine’s fuel flow and ignition timing to limit pre-ignition damage caused by high engine temperatures.
Given all these factors, you can see why it’s really necessary to let the engine’s water cool down as soon as you see any steam escaping from under the hood or hear the radiator cap popping.
The cool-down process involves one major concept from physics: All physical objects will try to achieve temperature equality with their surroundings. It’s a law of nature. A hot object will send heat energy to cooler objects. This transfer can be by conduction (physical contact) or radiation (heat waves). The bigger the temperature difference, the faster the heat transfer.
The heat is on
The hottest part of an engine is the combustion chamber, where temperatures
run 500 to 850 degrees C (900 to 1,500 F). The hottest single spot is
around the exhaust valve seat.
Combustion chamber heat transfers into the iron or aluminum wall, which
is about 1⁄4-inch thick. The outside of the wall is coated with
water held in the cooling jacket.
The cooler the water, the faster it will accept heat from the wall. Another of water’s properties helps out here — water is second only to liquid mercury in conducting heat energy. So heat quickly spreads away from the wall and into the body of water, and is absorbed quickly.
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But the heat transfer is not immediate. If the water lingers too long against a hot part of the wall, it can turn into a pocket of steam. This pushes water away from that part of the wall, further complicating the problem. And even a small steam bubble can make the radiator cap pop up to relieve the excess pressure.
The engine’s water pump keeps the water flowing, and the designers of engines take extra care to make sure the water jacket passages guide water into all the right places.
Assuming the walls and water are fairly clean, the process of transferring the excess engine heat to the outside air is straightforward. Heated water is pumped to the radiator made of copper alloy (brass) or aluminum alloy. It has a lot of thin tubes covered with fins, creating a huge surface area exposed to air that is cooler than the water. The hotter the water, the more efficiently it dumps heat into the outside air. The faster the fan and shroud guide water through the tubes and fins, the cooler the water is when it returns to the engine block.
It’s a beautiful system, with plenty of excess capacity and self-regulation, thanks to the properties of water.
But water has some really bad properties. Next month we’ll wrap up with a look at why water needs antifreeze to really keep engines cool.
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