Cooling and lubrication keep an engine alive: cooling carries off the heat that the roughly 25% efficient cycle cannot turn into work, and oil keeps moving metal surfaces apart and carries some heat away. Losing either destroys an engine in minutes, which is why a rebuilt engine needs a way to see its oil pressure or level and its temperature before it is ever loaded.
Part of Internal Combustion
Why an engine must be cooled and oiled, the two ways of cooling it, what oil does, and the checks that tell you something is failing.
Most Of The Fuel’s Energy Becomes Heat
The carrier takes a petrol engine to be about 25% efficient, as Engine Theory shows. That leaves about three quarters of the fuel’s energy as heat. Some leaves with the exhaust and some must be taken out by the cooling system, and what stays in the metal softens the piston, burns the oil and, if unchecked, seizes the engine. The split between the two routes is not sourced here.
Two cooling systems are used. Air cooling puts fins on the cylinder and head and blows or lets air pass over them; see Air Cooling. Water cooling circulates liquid through passages in the block and head to a radiator; see Water Cooling. Air cooling is simpler and cannot freeze or leak; water cooling holds temperature steadier and suits larger or quieter engines. The airframe version of the problem, getting enough air past an engine at climb power and low speed, is treated in Cooling Systems; this tree covers the engine side.
What Oil Does
Oil forms a film between moving surfaces so that metal does not rub on metal. The corpus describes how a plain bearing runs in boundary, mixed or hydrodynamic lubrication in Lubrication Theory, and how to deliver oil in Lubrication Systems: grease cups for slow shafts, drip and wick oilers for continuous service, ring oilers or pumps for fast ones. An engine has fast, hot, heavily loaded bearings and a hot ring zone, so it uses a pump and a sump.
Oil also cools (it carries heat from the piston underside and bearings), cleans (it carries particles to a filter or the sump) and helps the rings seal. Where petroleum oil is not available, animal fats, plant oils and mineral oils each behave differently; Lubricant Types covers choosing viscosity, and Lubricants and Fuels covers where mineral oil comes from. Plant oils and fats are not made for engine temperatures, so treat them as an emergency measure and change them often (recalled behaviour, flagged).
What You Need
- A thermometer or temperature indicator on the head or coolant.
- A way to see oil level, and pressure if the engine has a pump.
- Clean oil and a filter or settling method, and a schedule for changing it.
- Clear airflow or coolant flow to the engine.
Method: First Run Of A Rebuilt Engine
Step 1: Fill the oil and coolant and check for leaks before starting.
Step 2: Start and watch the oil pressure or oil flow within seconds. If there is none, stop at once.
Step 3: Run at idle and watch temperature. A steady rise to a level and then a flat line is correct. A continuing climb is not.
Step 4: Stop, let it cool, check for leaks and oil level.
Step 5: Load gradually, and keep watching.
Worked Example: The Heat A Litre Of Petrol Leaves Behind
The carrier says petrol holds about 32,000 MJ/m³, which is 32 MJ per litre, and that a litre burned at about 25% efficiency gives roughly 2.2 kWh of shaft work.
- Energy in a litre: 32 MJ ÷ 3.6 MJ/kWh = 8.9 kWh.
- Shaft work at 25%: 8.9 × 0.25 = 2.2 kWh, which agrees with the carrier.
- Heat left over: 8.9 − 2.2 = 6.7 kWh.
So an engine burning a litre an hour makes 2.2 kW of shaft power and has about 6.7 kW of heat to dispose of through the exhaust, the coolant, the oil and the surfaces. That is several times the shaft power. A cooling system sized as though only the shaft power mattered would fail.
Common Mistakes
- Running a rebuilt engine without a gauge. Oil starvation and overheating give no warning until too late.
- Idling hot in still air. Air-cooled engines need airflow; water-cooled engines need the pump and fan.
- Topping up oil with the wrong type. Mixing can thin or gum the oil.
- Leaving old oil in place. It carries abrasive particles and acid.
Safety
Hot oil, hot coolant and a seized engine can all injure
A seized engine can throw a rod or stop suddenly under a belt or a flywheel. Hot oil and coolant scald; do not open a hot radiator cap. Oil-soaked rags can ignite on their own near heat, so keep them in a metal container away from the engine. Run engines outdoors.
What’s Next
- Learn next: Air Cooling — the simpler of the two cooling systems
- Combine with: Water Cooling — the steadier one