Air-cooling is common on smaller internal combustion engines. They may be small but they still generate a lot of heat.
It’s the air that does the work of keeping them cool, so an air-cooling system is usually simple. That’s useful on an engine where weight is important.
And it works best on the engine that’s exposed to a high airflow.
Almost all motorcycles used to be air-cooled but modern motorcycles are larger and more complex, and some are now liquid-cooled.
Some engines use what are called cooling fins. Their design makes the exposed surface area as large as possible, which allows more heat energy to radiate away, and be carried off in convection currents in the air.
More air flows over the fins and more heat is carried away.
For a vehicle moving at speed, airflow over the engine is high.
At low speeds or during idling, heat builds up. Then the engine can use some help.
Air should always be able to flow over the engine effectively.
One way to remove heat is to use a fan, with shrouds and ducts to direct air to the cylinders.
There are many places to mount a fan and many ways to drive it. For instance, in some engines it’s on the flywheel, driven by fan-belts off the crankshaft.
This section examines liquid-cooling systems in gasoline and diesel engines.
In this very basic liquid-cooling system, a coolant is stored in a radiator, and in the engine. As the engine heats up, a natural circulation starts, as coolant rises through the engine block by convection. It passes through the top hose, and into the radiator. Inside the radiator, heat is removed from the coolant as it falls from the top to the bottom. When it reaches the bottom, it returns to the engine through the lower radiator hose.
This process is called thermo-siphon. It was common in older cars which had low-powered engines and high, narrow radiators.
In modern cars, the engines are more powerful, and radiators are low and wide, and a thermo-siphon process couldn’t move the coolant quickly enough.
Instead, a water pump forces it through passages called waterjackets in the engine block. It collects heat by conduction, and becomes hot itself. Heated coolant then returns to the radiator for cooling.
And the cycle is repeated. Heat is removed from the engine, and dispersed. Preventing overheating is one function of the cooling system
It also helps the engine reach its best operating temperature as soon as possible. Every engine has a temperature at which it operates best. Below this temperature, ignition and combustion can be difficult.
Most engine wear occurs during this warm-up period.
And most pollution too.
One function of the thermostat is to shorten the warming-up period.
It operates according to coolant temperature. When coolant is cold, it is closed.
When a cold engine starts, coolant circulates within the engine block and cylinder head and through a coolant bypass to the water pump inlet. It can’t get to the radiator.
As the engine warms up, the coolant trapped in the engine gets hotter and hotter.
This starts to open the thermostat, allowing hot coolant to flow to the radiator.
In a rotary engine, the rotors are cooled by oil from the lubricating system. Each housing, however, is water-cooled, and there are additives in the coolant to protect the housing alloys from corrosion.
The water pump takes coolant from a bottom radiator tank and pushes it through waterjackets in the housings, towards the rear of the engine.
The chamber is surrounded by coolant passages.
Inside the housing is hollow, and it forms a water-jacket.
A pulley on the end of the eccentric shaft drives the water pump and fan.
In the rear housing, the coolant reverses direction and returns to the front of the engine.
It passes a thermostat. As in a piston engine, it is closed when the engine is cold but opens as it warms up.
The rotary engine has an overflow system much like that of a piston engine. As pressure builds up, coolant expands into an overflow tank. As it cools, pressure drops and it returns to the radiator.
A viscous hub links fan speed with engine temperature.
