Air cooled engine and Liquid cooled engine

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Many small engines and some medium engines are air cooled engine.  

This includes most small engine tools and toys such as lawnmowers, chain saws and model planes.  

This will reduce the weight and price of these engines.  

Some motorcycles, cars and planes are equipped with an air cooled engine, which also has the advantage of being lighter. 

Air-cooled motors rely on the flow of air over the outer surface to dissipate the heat needed to prevent the motor from overheating.  

In vehicles such as motorcycles and airplanes, the forward motion of the vehicle provides an air flow over the surface.  

Deflectors and ductwork are often added to direct the flow to a critical point.  

The outer surface of the motor is made of metal with high thermal conductivity and is ribbed to promote maximum heat transfer.  

An automobile engine usually has a fan to increase the air flow and direct it in the desired direction.  

Lawn mowers and chain saws rely on natural convection on ribbed surfaces.  

Some smaller engines have exposed flywheels with surface-mounted air baffles. 

When the engine is running, these deflectors create air movement and increase heat transfer to the finned surface.  

Air cooled engine are more difficult to cool cylinders than liquid-cooled engines.  

You can better control the flow of liquid coolant and direct it to hotspots that require maximum cooling.  

Liquid coolant also has better thermal properties than air (for example, high convection coefficient, specific heat, etc.).  

The cooling requirements are not the same at all points on the engine surface.  

High temperature areas, such as around exhaust valves and manifolds, require more cooling and a larger ribbed surface. 

Cooling the front of an air cooled engine facing the forward motion of the vehicle is often much easier and more efficient than cooling the rear of the engine.  

This can lead to temperature differences and thermal expansion problems.  


  1. Light weight. 
  1. Cost savings. 
  1. No failure of cooling system (water pump, hose, etc.) 
  1. No motor freezing, and 
  1. Faster engine warm-up. 


(1) Low efficiency. 

(2) Noisy, high required airflow, no water jacket to attenuate noise. 

(3) Directional airflow and ribbed surface.  

The standard heat transfer equations for finned surfaces can be used to calculate the heat transfer from these engine surfaces.  


The engine block of the water-cooling motor is surrounded by a water jacket through which the coolant flows.  

This allows for much better control of heat dissipation at the expense of additional weight and the need for a water pump.  

Due to the cost, weight, and complexity of liquid coolant systems, this type of cooling is extremely rare in small and / or cheap engines.  

Few liquid cooled engine use only water as the coolant for their water jacket.  

Schematic of cooling system of liquid cooled engine
Schematic of cooling system of liquid cooled engine

Used as a pure liquid, the freezing point is 0 °C.  

This is unacceptable in the northern winter climate.  

Its boiling point is lower than desired even in pressure cooling systems, and in the absence of additives, it promotes rusting and corrosion of many materials.  

Most of the engines utilize the mixture of water and ethylene glycol.  

This has the advantage of heat transfer of water, but improves some physical properties.  

Ethylene glycol (C2H602), often referred to as antifreeze, acts as a rust inhibitor and lubricant for water pumps, these are two characteristics not found when using only water.  

When it gets added to water, it lowers the freezing temperature and raises the boiling temperature.  

Both are desirable results.  

This applies to mixtures with very low ethylene glycol concentrations up to about 70%.  

Due to the unique temperature-concentration-phase relationship, the freezing temperature rises again at high concentrations.  

The desired heat transfer properties of water are lost even at high concentrations.  

Do not use pure ethylene glycol as engine coolant.  

Ethylene glycol is water soluble and has a boiling point of temperature of 197 °C and a freezing temperature of –11 °C in pure form at atmospheric pressure. 

When ethylene glycol is used as engine cooling water, its concentration with water is usually determined by the coldest weather temperature expected.  

Engine coolant should not freeze.  

When this happens, the radiator of the cooling system will not circulate and the engine will overheat.  

More serious consequences occur when the water in the coolant freezes and expands when it breaks the walls of the water jacket or water pump.  

This will destroy the engine.  

Even in climates where there is no risk of water freezing, some ethylene glycol should be used due to its excellent thermal and lubricating properties.  

In addition to excellent thermal properties, the coolant must meet the following requirements:  

1. Chemically stable under operating conditions.  

2. Non-foaming. 

3. Non-corrosive. 

4. Low toxicity. 

5. Non-flammable. 

6. Low-cost commercial antifreezes meet these requirements.  

Many of them are basically ethylene glycol with a small number of additives.  

A hydrometer is used to measure the concentration of ethylene glycol when mixed with water.  

The specific density of the mixture is determined by the height at which the calibrated hydrometer floats.  

You can use graphs to determine the required concentration.  

Most of these hydrometers are used by gas station employees who are not technically trained.  

For this reason, they are usually not calibrated for concentration, but only for the freezing temperature of the entire mixture of water and ethylene glycol.  

Most commercial antifreezes (Prestone, Zerox, etc.) are essentially ethylene glycol and the same calibrated hydrometer can be used for all of them.  

Some commercially available engine coolants (such as Sierra) use propylene glycol (C4HsO) as the base ingredient.  

These products are claimed to be less contaminated than ethylene glycol if the coolant system leaks or if the coolant ages and is discarded.  

Much less of these products are sold in the United States than those containing ethylene glycol.  

The liquid goes into the engine water jacket, which is usually at the bottom of the engine.  

It flows through the engine block, where it absorbs energy from the hot cylinder walls.  

The flow channel of the water jacket is designed to flow around the outer surface of the cylinder wall and pass through all other surfaces for cooling.  

The flow also passes through other components that may need to be heated or cooled (for example, heating the intake manifold or cooling the oil reservoir).  

The exit is usually at the top of the engine block.  

Enthalpy must be extracted from the coolant flow so that the circuit can be closed and the coolant used again to cool the engine.  

This is done by using a heat exchanger in the flow circuit.  

This is called a radiator.  

Radiator of liquid cooled engine
Radiator of liquid cooled engine

The radiator is a honeycomb heat exchanger, where hot coolant flows from top to bottom and cold air flows from front to back to exchange energy.  

The air flow is generated by the forward motion of the vehicle, which is electrically supported or supported by a fan located behind the radiator driven by the crankshaft of the engine.  

The cooled engine coolant exits the bottom of the radiator and re-enters the engine’s water jacket, creating a closed loop.  

The water pump that drives the flow of the coolant circuit is usually located between the radiator outlet and the engine block inlet.  

This pump is electrically or mechanically driven by a motor.  

Some early cars did not have a water pump and relied on natural convection heat flow loops.  

The air leaving the car’s radiator is also used to cool the engine by passing through the engine compartment and through the outer surface of the engine.  

Due to the focus on the latest aerodynamic shapes and cosmetics of the car, it is much more difficult to send cooling air to the radiator and engine compartments.  

Modern radiator heat exchangers require significantly higher energy output efficiencies. 

Modern engines are designed to run hotter and thus can tolerate a lower cooling air-flow rate. Steady-state temperature of the air within the engine compartment of a modern automobile is on the order of 125°C.  

A thermostat is usually attached to the engine output input of the coolant circuit to prevent the coolant temperature from falling below a certain minimum temperature and to keep the engine running at higher temperatures and efficiencies.  

The thermostat is a thermally actuated Go No-Go valve.  

When the thermostat is cold, the thermostat is closed and no liquid can flow through the main circulation duct.  

When the engine warms up, the thermostat also warms up, and thermal expansion opens the flow path and circulates the coolant.  

The higher the temperature, the larger the channel opening and the larger the resulting coolant flow.  

Therefore, the coolant temperature is fairly precisely controlled by opening and closing the thermostat.  

Thermostats are manufactured for different cooling water temperatures, depending on engine usage and climatic conditions.  

They are usually available in nominal values ​​ranging from cold (140°P) to hot (240°P). Cooling circuits for old cars that operate primarily on atmospheric pressure water. 

This limits the overall cooling water temperature to approximately 180°P, ensuring a safety margin to prevent boiling.  

To increase the operating temperature of the engine for efficiency, it was necessary to increase the cooling water temperature.  

This was done by pressurizing the coolant circuit and adding ethylene glycol to the water.  

As ethylene glycol raised the boiling point of the liquid. The pressurization of the system raises the boiling temperature of the liquid independently.  

The pressure of a normal coolant system is about 200 kPa in absolute pressure.  

It is desirable that the coolant remain largely liquid throughout the flow circuit.  

When boiling, a small amount of liquid turns into a large amount of vapor, making it almost impossible to maintain a stable mass flow rate.  

Ethylene glycol can be used in pressurization systems to achieve high temperatures without massive boiling.  

Local boiling in small hotspots occurs within the engine’s water jacket.  

The hottest spots inside the engine (instantaneous or almost permanent) require the maximum amount of heat dissipation and cooling.  

The phase changes that occur when boiling at these local hotspots absorb large amounts of energy and provide the large cooling required at these points.  

Circulating convection keeps the resulting vapor bubbles away from the hotspot and returns them to the mainstream of coolant.  

Here, the low temperature of the liquid causes it to condense back into the liquid without interrupting the mass flow.  

Hot engine coolant can be used to heat the cabin of a car, if needed, as it leaves the engine block.  

This is done by sending part of the coolant flow to an auxiliary system that supplies the hot side of the heat exchanger from a small liquid to air.  

The outside air or circulating air flows through the other half of the heat exchanger and is heated when directed to the passenger compartment and / or cold windows for defrosting.  

Various manual and automatic controls determine the air and coolant flow rates to achieve the desired heating results. 


Improves engine’s thermal efficiency & prevents overheating.  

A closed system reduces the loss of water due to evaporation.  

Suitable in most climatic conditions with use of the coolant. Water jackets reduce engine noise.


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