Air cooling

Every engine wastes much of its fuel as heat, and the metal around the combustion chamber would soften or seize if that heat were not carried away. Air cooling is the simplest answer. The cylinder barrel and head are cast with deep fins that multiply the surface exposed to the air; heat conducts from the chamber wall into the fins and the airstream carries it off. There is no radiator, pump, hoses or coolant to fail or top up, which is why the Hero Splendor, Honda Shine, Bajaj Platina and most Indian commuter bikes under 160 cc use it.

• Combustion heats the cylinder head and bore.
• Heat conducts outward into the fins.
• Air moving past the fins picks up the heat; the faster the bike, the more cooling.
• The engine oil also carries some heat down to the crankcase.

The weakness is obvious at a traffic signal in May: with no airflow the engine temperature climbs, and the design has to leave enough margin to survive it. Scooters solve this with forced air cooling: a fan on the crankshaft blows air through a plastic shroud over the fins, which is why a TVS Jupiter or Honda Activa does not mind crawling in traffic. Riders notice an air-cooled engine's slightly looser mechanical note and, on hot days, a drop in performance and a tendency to run hot. Because the temperature swings more than on a liquid-cooled engine, tolerances are set looser and the tune is conservative; this limits how much power a given displacement can safely make, so sportier bikes move to oil or liquid cooling.