Air Cooled vs Water Cooled Engines
1.WATER IS A MUCH MORE EFFICIENT CONDUCTOR OF HEAT:
Let's keep this simple...we need to cool engines because they create heat during the combustion process. The more the heat generated, the more difficult it becomes to get rid of that heat. As you raise compression ratios and run the engine at higher rpm's, at some point with an air-cooled engine, you just can't make the cooling fins any larger, can't fit any more on, and you can't blow enough air over those fins to get rid of the heat faster than it is being produced. This is when the dreaded "piston seizure" usually takes place. At some point it is just not enough to put cooling fins on the engine and hence it's time to go to another way to move the heat away from the cylinder ... water. Water is a whole lot better at conducting heat than air. Have you ever seen a blacksmith drop a red-hot bar into a tub of water to cool it off quickly? Did you ever see him wave it around in the air to cool it off quickly? NO! The water does a much better job of conducting the heat. Yes, water is a better conductor of heat than air. In case of water cooled engines the engine designer loses the fins and puts a watertight jacket around the head and cylinder to contain the water that will carry away the heat. The water is circulated through tubes in a loop from the cylinder to a radiator, where the water is cooled by... AIR! Then the cooled water is returned to the engine to pick up some more heat. This makes for much more efficient cooling with the HOT ENGINE parts loosing temperature much more quickly and efficiently to the water much like the blacksmiths rod.2.Heat Distribution in a water cooled engine:
Water cooled engines run at a constant temperature through out the engine while air cooled engines run at different temperatures. This is likely to lead to hot spots. Unlike popular belief even Air Cooled engines need constant attention particularly in a dusty country like India. If the fins get clogged up with mud and junk the engine will not cool. Needless to say a hot engine will seize.3.Efficiency of the Air Cooled engine depends on the Ambient Air Temperature:
Air cooled engines go through extreme temperature variation because they are entirely dependent upon ambient air temperature for their cooling. The engine can only efficiently dissipate heat in direct relation to the difference between ambient and engine surface temperatures. This is what engineers call delta T. It is obvious that cooler air will cool a hot engine faster than hot air. However, nature does not always give us cool air for this task. As a result the assembly tolerances for air-cooled engines must be built "loose" to compensate for the inevitable expansion of the mechanical parts subjected to high thermal expansion. When they are forced to these high temperatures the oil viscosity diminishes and clearances are closed which can result in bearing or piston failures. Loud air moving fans necessary to cool the engines are a dominant sound. These engine failures are usually heat related. Water-cooled engines are literally bathed in a blanket of thermostatically controlled water so the temperature fluctuations are more controllable and less dramatic. Liquid immersion is a more efficient heat removal method. Radiators are oversized to give a high degree of safety margin. The liquid blanket serves as a sound absorber as well and the fans are significantly quieter.4.Maintenance cost of Air Cooled engines:
Look around yourself. Where do you see a diesel engine? You see them on cars, jeeps, buses, trucks, trains etc. What is the cooling system that you see on these engines? The chances are that in 99% of the cases you will find a water cooled diesel engine under the hood of these vehicles. (A stationary application engine such as the Diesel Genset is a much easier application as compared to a vehicular application.) Where does this lead us? It leads us to economies of scale. Water cooled diesel engines are much more in population and hence the spares for these engines are much cheaper to produce as compared to spares for an Air Cooled engine. In all likelihood you are going to pay much higher OPEX (OPEX- Operating expense) with an air cooled engine than what you will with a water cooled engine.5.Operational problems with Water Cooled engines. Myth or fact?
Until recently it was commonly conceived that water cooled engines require a lot of maintenance like frequent water top ups. They have a larger risk factor, with additional water circuitry being prone to leakages etc. It is mostly a notion promoted by manufacturers of substandard water cooled engines. Most of it is not true for the latest generation engines. This is so because of some of the unique features of the new water cooled engines. Some of these features are listed hereunder. Most engines come equipped with Ethylene Propylene Diene Monomer hoses (EPDM hoses) which is a special material capable of withstanding temperatures up to 250 degree Celsius. It is an extremely durable material ideal for harsh conditions. Built in expansion tanks not only increases the radiator capacity but also eliminates the need for frequent top ups. Most of the times a check after every 250 hours are good enough. Gone are the days that you would need to open the bonnets of your cars and check water in the radiator before starting off for the day. When was the last time you opened the bonnet of your car to check the water level? Modern day diesel engines have immense capability. They are capable of running unmanned for days even months. In the Indian context more than 20000 such cell phone tower sites run unmanned on water cooled engines. Modern day radiators come equipped with pressurized systems that rise the boiling water of temperature this brings the added benefit of NO WATER LOSS from the system. UNITISED SEALS as used on most modern day diesel engines ensure that you dont have leaky engines. So when it comes to choosing between an Air Cooled engine and a water cooled diesel engine make sure that you are selecting the latest technology. Remember that putting water circuits in the engines came when the engines developed the initial engines could only be air cooled. Liquid cooling was advancement in technology. Still don’t believe it then just look around yourself at all the diesel engines in use. 99% of the users cant be wrong.