Knowledge base
An engine converts about a third of its fuel into work; the rest is heat, and the cooling system exists so that this heat does not stay in the metal. It carries heat from the cylinder walls and heads, from the compressed intake air, from the lubricating oil and sometimes from the fuel, pumps it to a radiator and hands it to the atmosphere. This guide explains the parts of an engine cooling system, what each circuit does and why the radiator is designed to the engine maker's figures and not to a rule of thumb.
Where The Heat Comes From
Combustion heats the cylinder liners, the heads and the exhaust valves; the jacket water that flows around them carries that heat away and keeps the metal within its limits - the jacket water circuit is the oldest and still the largest. The turbocharger compresses the intake air and heats it to 150 °C and more; cooling it in the charge-air cooler makes it denser, which gives the engine its rating and keeps the combustion temperatures and the nitrogen oxides down. The lubricating oil picks up heat from the bearings and the pistons and is cooled in the oil cooler. The fuel, on some engines, returns from the injectors warm and is cooled too.
Taken together, the coolant carries away roughly as much energy as the engine delivers at the shaft: a 1,000 kW engine rejects in the region of 400 to 700 kW to its coolant depending on the engine, the circuits and the rating. The engine data sheet gives the exact figures.
One Circuit Or Two: HT And LT
Small and medium engines use one circuit. The pump pushes coolant through the engine; a thermostat holds the outlet around 85 to 95 °C by sending more or less of the flow to the radiator; the charge-air cooler is either air-to-air in front of the radiator or in the same water circuit. Large engines - from a few hundred kilowatts up - split the cooling into a high-temperature (HT) circuit for the jackets and the first charge-air stage at 90 to 100 °C and a low-temperature (LT) circuit for the second charge-air stage and the oil cooler at 45 to 60 °C, each with its own pump, thermostat and limits.
The split exists because the charge air must end up much colder than the jacket water can make it. The LT circuit's return temperature is therefore close to the ambient on a hot site, which is what makes the LT coil the critical one in a radiator design.
Pumps, Thermostats, Pressure And Coolant
The engine-driven pump has a fixed head, and the engine maker states how much of it may be used outside the engine - the maximum external pressure drop. The thermostats open with temperature and route the coolant to the radiator or back to the pump; a cold engine warms up quickly, a hot one gets the full radiator. The system is pressurised by a cap on the expansion tank, which raises the boiling point and matters at altitude. The coolant is a water-glycol mixture with inhibitors, specified by the engine maker for frost protection, corrosion and cavitation; the glycol percentage changes the heat capacity and the viscosity and is an input of the radiator design.
The Radiator Closes The Loop
Whatever the engine does with its heat internally, it ends in the radiator: an air-cooled heat exchanger with finned tubes and fans, mounted on the generator-set skid with a belt-driven fan or standing remote with electric fans. The radiator is designed to the engine data sheet - heat, flow, temperatures, pressure drop per circuit - at the site ambient and altitude, and an LT/HT radiator carries both circuits in one casing with the LT coil first in the air. TECS designs and builds these radiators for engines of any make; send the engine data sheet and the site conditions and the design comes back as a proposal.
In Short
- Heat sources: cylinder jackets, charge air, lubricating oil, sometimes fuel
- One circuit on small engines; HT and LT circuits on large ones
- HT around 95 °C for the jackets, LT around 50 °C for the charge air and oil
- Pump head, thermostats, pressure cap and the glycol mixture set the limits
- The radiator is designed to the engine data sheet at the site conditions
- TECS builds engine-mounted and remote radiators, single circuit or LT/HT
What To Send With An Enquiry
- Engine make, model and rating
- Cooling data sheet: heat, flow, temperatures and pressure drop limit per circuit
- Coolant mixture prescribed by the engine maker
- Site ambient, altitude and humidity
- Installation: skid, plant room, container, outdoor
- Sound limit and the distance it applies to
Questions We Are Often Asked
Why does a large engine have two circuits?
Because the charge air needs to be cooled to around 50 °C for the engine's rating and emissions, and the jacket water at 95 °C cannot do that. The LT circuit does the cold work, the HT circuit the hot work.
What happens if the coolant runs too hot?
The engine protection reduces the load and then shuts the engine down. Before that the oil thins, the charge air warms and the output falls. A radiator too small for the site shows up this way on the first hot day.
Can the LT and HT circuits share one radiator?
Yes - the usual arrangement is one casing with two separate coils, the LT coil first in the airflow. The coolant of the two circuits never mixes.
Is the radiator part of the engine package?
An engine-mounted radiator comes with the generator set from its maker. A remote radiator is designed separately to the engine data sheet, by a radiator maker such as TECS, for the installation.
How much heat goes to the coolant?
In the region of 40 to 70 % of the shaft output, depending on the engine and the circuits. The engine data sheet gives the figure per circuit at the rating; it is the first input of the radiator design.
Contact Us
Send us the heat duty, the fluid temperatures and flow rates, the ambient conditions and the installation limits. We answer with a design proposal - or write to us on WhatsApp.
