Cooling Systems in Automobiles & Cars

Cooling Systems in Automobiles & Cars




Cooling Systems in Automobiles & Cars

 



INTRODUCTION


We know that in case of Internal Combustion engines,combustion of air and fuel takes place inside the engine cylinder and hot gases are generated. The temperature of gases will be around 2300-2500°C. This is a very high temperature and may result into burning of oil film between the moving parts and may result into seizing or welding of the same.
So, this temperature must be reduced to about 150-200°C at which the engine will work most efficiently. Too much cooling is also not desirable since it reduces the thermal efficiency. So, the object of cooling system is to keep the engine running at its most efficient operating temperature.
It is to be noted that the engine is quite inefficient when it is cold and hence the cooling system is designed in such a way that it prevents cooling when the engine is warming up and till it attains to maximum efficient operating temperature, then it starts cooling.
It is also to be noted that :
  • About 20-25% of total heat generated is used for producing brake power (useful work).
  •  Cooling system is designed to remove 30-35% of total heat.
  • Remaining heat is lost in friction and carried away by exhaust gases.



What the cooling system does for an engine ?


  1. Although gasoline engines have improved a lot, they are still not very efficient at turning chemical energy into mechanical power .
  2. Most of the energy in the gasoline (perhaps 70%) is converted into heat, and it is the job of the cooling system to take care of that heat. In fact, the cooling system on a car driving down the freeway dissipates enough heat to heat two average-sized houses!
  3. The primary job of the cooling system is to keep the engine from overheating by transferring this heat to the air, but the cooling system also has several other important jobs.
  4. The engine in your car runs best at a fairly high temperature.
  5. When the engine is cold, components wear out faster, and the engine is less efficient and emits more pollution.
  6. So another important job of the cooling system is to allow the engine to heat up as quickly as possible, and then to keep the engine at a constant temperature. 





What is a Cooling System?

A typical 4 cylinder vehicle cruising along the highway at around 50 miles per hour, will produce 4000 controlled explosions per minute inside the engine as the spark plugs ignite the fuel in each cylinder to propel the vehicle down the road. Obviously, these explosions produce an enormous amount of heat and, if not controlled, will destroy an engine in a matter of minutes. Controlling these high temperatures is the job of the cooling system.
The modern cooling system has not changed much from the cooling systems in the model T back in the '20s. Oh sure, it has become infinitely more reliable and efficient at doing it's job, but the basic cooling system still consists of liquid coolant being circulated through the engine, then out to the radiator to be cooled by the air stream coming through the front grill of the vehicle.
Today's cooling system must maintain the engine at a constant temperature whether the outside air temperature is 110 degrees Fahrenheit or 10 below zero. If the engine temperature is too low, fuel economy will suffer and emissions will rise. If the temperature is allowed to get too hot for too long, the engine will self destruct.





How Does a Cooling System Work?

Actually, there are two types of cooling systems found on motor vehicles: Liquid cooled and Air cooled. Air cooled engines are found on a few older cars, like the original Volkswagen Beetle, the Chevrolet Corvair and a few others. Many modern motorcycles still use air cooling, but for the most part, automobiles and trucks use liquid cooled systems and that is what this article will concentrate on.
The cooling system is made up of the passages inside the engine block and heads, a water pump to circulate the coolant, a thermostat to control the temperature of the coolant, a radiator to cool the coolant, a radiator cap to control the pressure in the system, and some plumbing consisting of interconnecting hoses to transfer the coolant from the engine to radiator and also to the car's heater system where hot coolant is used to warm up the vehicle's interior on a cold day. 
A cooling system works by sending a liquid coolant through passages in the engine block and heads. As the coolant flows through these passages, it picks up heat from the engine. The heated fluid then makes its way through a rubber hose to the radiator in the front of the car. As it flows through the thin tubes in the radiator, the hot liquid is cooled by the air stream entering the engine compartment from the grill in front of the car. Once the fluid is cooled, it returns to the engine to absorb more heat. The water pump has the job of keeping the fluid moving through this system of plumbing and hidden passages. 

A thermostat is placed between the engine and the radiator to make sure that the coolant stays above a certain preset temperature. If the coolant temperature falls below this temperature, the thermostat blocks the coolant flow to the radiator, forcing the fluid instead through a bypass directly back to the engine. The coolant will continue to circulate like this until it reaches the design temperature, at which point, the thermostat will open a valve and allow the coolant back through the radiator .




Circulation 

The coolant follows a path that takes it from the water pump, through passages inside the engine block where it collects the heat produced by the cylinders. It then flows up to the cylinder head (or heads in a V type engine) where it collects more heat from the combustion chambers. It then flows out past the thermostat (if the thermostat is opened to allow the fluid to pass), through the upper radiator hose and into the radiator. The coolant flows through the thin flattened tubes that make up the core of the radiator and is cooled by the air flow through the radiator. From there, it flows out of the radiator, through the lower radiator hose and back to the water pump. By this time, the coolant is cooled off and ready to collect more heat from the engine. The capacity of the system is engineered for the type and size of the engine and the work load that it is expected to undergo. Obviously, the cooling system for a larger, more powerful V8 engine in a heavy vehicle will need considerably more capacity then a compact car with a small 4 cylinder engine. On a large vehicle, the radiator is larger with many more tubes for the coolant to flow through. The radiator is also wider and taller to capture more air flow entering the vehicle from the grill in front.




Antifreeze 

The coolant that courses through the engine and associated plumbing must be able to withstand temperatures well below zero without freezing. It must also be able to handle engine temperatures in excess of 250 degrees without boiling. A tall order for any fluid, but that is not all. The fluid must also contain rust inhibiters and a lubricant. The coolant in today's vehicles is a mixture of ethylene glycol (antifreeze) and water. The recommended ratio is fifty-fifty. In other words, one part antifreeze and one part water. This is the minimum recommended for use in automobile engines. Less antifreeze and the boiling point would be too low. In certain climates where the temperatures can go well below zero, it is permissible to have as much as 75% antifreeze and 25% water, but no more than that. Pure antifreeze will not work properly and can cause a boil over .


TYPES OF COOLING SYSTEMS

There are mainly two types of cooling systems :
  1.  Air cooled system .
  2. Water cooled system.

AIR COOLING SYSTEM

Air cooled system is generally used in small engines say up to 15-20 kW and in aero plane engines. In this system fins or extended surfaces are provided on the cylinder walls, cylinder head, etc. Heat generated due to combustion in the engine cylinder will be conducted to the fins and when the air flows over the fins, heat will be dissipated to air .

The amount of heat dissipated to air depends upon :

  • Amount of air flowing through the fins.b) Fin surface area.
  • Thermal conductivity of metal used for fins. 


Advantages of Air Cooled System

  • Radiator/pump is absent hence the system is light. 
  •  In case of water cooling system there are leakages, but in this case there are no leakages. 
  •  Coolant and antifreeze solutions are not required. 
  •  This system can be used in cold climates, where if water is used it may freeze.

Disadvantages of Air Cooled System

  • Comparatively it is less efficient. 
  •  It is used in aero planes and motorcycle engines where the engines are exposed to air directly.

Air cooling 

Cars and trucks using direct air cooling (without an intermediate liquid) were built over a long period from the very beginning and ending with a small and generally unrecognized technical change.


WATER COOLING SYSTEM

In this method, cooling water jackets are provided around the cylinder, cylinder head, valve seats etc. Thewater when circulated through the jackets, it absorbs heat of combustion. This hot water will then be cooling in the radiator partially by a fan and partially by the flow developed by the forward motion of the vehicle. The cooled water is again recirculated through the water jackets.
There are two types of water cooling system : Thermo Siphon System In this system the circulation of water is due to difference in temperature (i.e. difference in densities) of water. So in this system pump is not required but water is circulated because of density difference only.



Pump Circulation System

 In this system circulation of water is obtained by a pump. This pump is driven by means of engine output shaft through V-belts .



Water Jackets

 Cooling water jackets are provided around the cylinder, cylinder head, valve seats and any hot parts which are to be cooled. Heat generated in the engine cylinder, conducted through the cylinder walls to the jackets. The water flowing through the jackets absorbs this heat and gets hot. This hot water will then be cooled in the radiator.

Antifreeze Mixture 

In western countries if the water used in the radiator freezes because of cold climates, then ice formed has more volume and produces  cracks in the cylinder blocks, pipes, and radiator. So, to prevent freezing antifreeze mixtures or solutions are added in the cooling water.
 The ideal antifreeze solutions should have the following properties :
  •  It should dissolve in water easily. 
  •  It should not evaporate. 
  •  It should not deposit any foreign matter in cooling system.
  •  It should not have any harmful effect on any part of cooling system. 
  •  It should be cheap and easily available. It should not corrode the system. No single antifreeze satisfies all the requirements. Normally following are used as antifreeze solutions :

  1.  Methyl, ethyl and isopropyl alcohols. 
  2.  A solution of alcohol and water. 
  3. Ethylene Glycol. 
  4.  A solution of water and Ethylene Glycol. 
  5.  Glycerin along with water, etc. 

Advantages of Water Cooling System

  • Uniform cooling of cylinder, cylinder head and valves. 
  • Specific fuel consumption of engine improves by using water cooling system. 
  • If we employ water cooling system, then engine need not be provided at the front end of moving vehicle. 
  •  Engine is less noisy as compared with air cooled engines, as it has water for damping noise.

Disadvantages of Water Cooling System 

  • It depends upon the supply of water. 
  • The water pump which circulates water absorbs considerable power. 
  • If the water cooling system fails then it will result in severe damage of engine. 
  • The water cooling system is costlier as it has more number of parts. Also it requires more maintenance and care for its parts.  

Engine cooling system 

The cooling system is a key to efficient engine operation. An internal combustion engine only uses one-third of the power produced. One-third heats oil or goes out the exhaust and one-third must be controlled by the water cooling system. 
  1. An engine wears out four times faster if it continually operates at a low temperature. 
  2. A tractor doing the same work will use 3.8 gallons of fuel per hour at 400 and only 2.8 gallons of fuel per hour at 1800. Warm up your engine before putting under load
  3. Too much heat can damage an engine, increase oxidation to the oil, and reduce the effectiveness of the additives in the oil. 
  4. Excessive heat may attack seals, liners, gaskets, and sealants. 
  5. A thin (1/16") layer of calcium carbonate build-up on an engine is equal to 4" of solid cast iron in heat transfer.

Antifreeze 

  1. Antifreeze should be changed every year unless you add chemical inhibitors to reinforce the rust inhibiting ability. 
  2. Diluting antifreeze one-third to one-half with water is usually recommended. More than two-thirds antifreeze is too much. It offers less freezing protection rather than more. 
  3. Distilled or rain water is better than plain water because of the corrosion deposits. 
  4. Ethylene-Glycol antifreeze in the cooling system raises the boiling temperature substantially. This makes for greater heat dissipation. 
  5. Antifreeze is not a waste of money if you consider risk factor alone. It is insurance that makes sense.



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