Liquid cooling

Every car and truck engine, and every performance or large-capacity two-wheeler, is liquid-cooled. Passages called water jackets are cast into the cylinder block and head. A belt- or gear-driven pump circulates coolant, water mixed with antifreeze and anti-corrosion additives, through these jackets, where it absorbs heat from the metal. The hot coolant leaves the top of the engine through a thermostat to the radiator, a block of thin tubes and fins at the front of the vehicle. Air flowing through the radiator, helped by a fan when the vehicle is slow or stopped, removes the heat, and the cooled liquid returns through the lower hose to the pump.

• The pump pushes coolant into the block and head.
• Coolant absorbs heat around each cylinder.
• The thermostat opens once the engine is warm; hot coolant goes to the radiator.
• Radiator and fan cool it, and it flows back to the pump.

The thermostat stays shut when the engine is cold so it warms up quickly, then opens to hold a steady temperature. A pressurised cap and an expansion tank let the coolant expand and keep it from boiling. Holding one temperature lets engineers set tight tolerances, run a leaner tune and extract more power from each cc, and it makes the engine quieter because the jacket damps mechanical noise. Riders of a KTM Duke, Yamaha R15, Bajaj Dominar, TVS Apache RR 310 or Royal Enfield Himalayan 450 get consistent performance in a Chennai traffic jam and on an expressway. The cost is weight, price and more parts: a pump, hoses, radiator, fan, thermostat and the coolant itself, which must be topped up and replaced on schedule. A leak or a stuck thermostat can overheat the engine quickly, so the temperature warning should never be ignored.