A water-cooled engine circulates coolant through passages in the block and head to a radiator, where air carries the heat away, with a pump to drive the flow and a thermostat to hold the temperature steady. The flow needed follows from the heat load: to carry 3 kW with the coolant warming by 10 K takes about 4.3 litres a minute, and the water has to be clean and protected from freezing.
Part of Internal Combustion
The circuit of a water-cooled engine, how much coolant it must move, the role of the pump, radiator and thermostat, and the failures of freezing, scale and boiling.
The Circuit Is Block, Radiator And Pump
Coolant picks up heat in the passages around the cylinders and in the head, flows to a radiator, gives up its heat to air passing through, and returns. A pump, usually driven from the crankshaft, forces the flow. A radiator is a set of thin-walled tubes joined by fins, so the water is spread thin and the air surface is large; the corpus describes a similar finned radiator with a fan for cooling producer gas in Radiator Setup, which is the same construction for a different fluid. A thermostat is a valve that stays shut while the engine is cold, so that it warms up quickly, and opens as the coolant reaches its working temperature, so the engine holds a steady level. The working temperature belongs to the engine.
A pump is not essential for a small stationary engine. A thermosiphon circulates water because hot water rises: the corpus article Thermosiphon Systems describes it for a solar heater, with the tank bottom at least 30 cm above the collector top, and How Thermosiphon Works gives about 74 Pa of driving pressure for a 0.5 m rise. That is a small pressure, so a thermosiphon engine circuit needs wide, short, smooth pipes with no sharp bends and a radiator placed above the engine.
What You Need
- A radiator of enough area for the heat load, with a fan if the engine is stationary.
- A water pump or a thermosiphon layout.
- Hoses and clamps that suit hot water; the corpus describes making hose in Hoses and Tubing.
- Clean coolant: soft water such as rainwater, because hard water leaves scale that insulates the passages.
- A thermostat, or a manual shutter, and a temperature indicator.
- A way to drain the system before frost.
Method: Fill And Bleed The System
Step 1: Check the hoses and clamps and drain valves.
Step 2: Fill slowly with clean water, with the engine cold and the highest point open so air can escape.
Step 3: Run the engine and top up as the air comes out. Trapped air blocks circulation and makes hot spots at the head.
Step 4: Watch the temperature gauge as it warms. When it is right, the temperature rises, the thermostat opens, and the reading settles. When it is wrong, it climbs steadily or swings.
Step 5: Check the radiator at both ends. The top should be hot and the bottom noticeably cooler. If the bottom is as hot as the top, the radiator is blocked, undersized or lacks airflow.
Worked Example: How Much Coolant Must Move
Assume the cooling system must carry 3 kW and the coolant may warm by 10 K between inlet and outlet. These are assumptions for the arithmetic. Use the corpus water heat capacity of 4,186 J/kg·K.
- Flow = Q ÷ (c × ΔT) = 3,000 ÷ (4,186 × 10) = 0.0717 kg/s.
- Per minute: 0.0717 × 60 = 4.30 kg, which is about 4.3 litres.
If the allowed rise is only 5 K the flow doubles to 8.6 L/min; if it is 20 K it halves to 2.15 L/min. A small rise means a large flow, a bigger pump and bigger pipes. The corpus Heat Exchanger Design gives the radiator-area arithmetic, in terms of an overall heat-transfer coefficient and a temperature difference, for a different pair of fluids.
Common Mistakes
- Leaving water in the engine through frost. Water expands as it freezes and cracks the block or radiator.
- Using hard or dirty water. Scale and rust insulate the passages and block the radiator.
- Air locks. An air pocket at the head stops the flow there.
- Running without a thermostat and calling it fixed. The engine runs too cold and wears faster, and it takes longer to warm.
- A slipping pump belt. The circulation fails when the engine is working hardest.
Safety
A hot cooling system holds scalding water under pressure
Never open a radiator cap on a hot engine. Coolant above its boiling point at atmospheric pressure flashes to steam when the cap comes off and can scald badly. Let it cool, and open the cap slowly with a cloth. If you use a commercial antifreeze, remember that the common kind is poisonous and sweet-tasting (recalled, flagged); keep it away from children and animals and dispose of it where it will not reach water. Guard the fan and belt.
What’s Next
- Learn next: Clutch Design — connecting the engine to its load
- Combine with: Air Cooling — the alternative that avoids liquid