Knowledge base
A turbocharger squeezes more air into an engine than it could breathe on its own, and squeezing air heats it - to 150 °C and more on a modern high-pressure engine. Hot air is thin and makes the combustion hotter, so the air is cooled again before it enters the cylinders: that is charge air cooling, and the cooler that does it is the charge air cooler or intercooler. This guide explains the two ways it is done and why, on a large engine, the charge air sets the toughest duty of the whole cooling system.
Why Charge Air Is Cooled
Cooler air is denser, so the same cylinder takes in more oxygen, burns more fuel and makes more power; cooler air also lowers the combustion temperature, which keeps nitrogen oxide emissions down and the pistons and valves within their limits. Engine makers therefore specify a charge air temperature after the cooler - typically 45 to 60 °C - and rate the engine on it. If the cooler cannot reach that temperature on a hot day, the engine control reduces the output to protect itself.
Air-To-Air Or Air-To-Water
An air-to-air charge air cooler passes the hot charge air through a finned core in front of, or beside, the engine radiator; the radiator fan cools both. It is simple and common on smaller generator sets with mounted radiators, but the charge air has to be piped to the cooler and back, which limits it to compact installations.
An air-to-water charge air cooler sits on the engine and gives the charge-air heat to a coolant circuit, which carries it to a radiator wherever that stands. Large engines use two stages: a high-temperature stage that cools the air with the jacket-water (HT) circuit, and a low-temperature stage on a separate LT circuit that takes it down to the final temperature. The LT circuit often also cools the lubricating oil.
The LT Circuit On A Hot Site
The LT circuit must return its coolant at 40 to 55 °C so that the charge air can reach its target - only a few kelvin above a 45 or 50 °C ambient. That small temperature difference makes the LT coil the largest and most sensitive part of an LT/HT radiator: it gets the coldest air, first in the airflow, and the most surface per kilowatt. When an engine derates on a hot afternoon, the LT circuit is usually the reason.
TECS designs LT/HT radiators with the LT coil sized to the engine maker's charge air limits at the site's design ambient, and air-to-air charge air coolers where the installation calls for them. Send the engine data sheet with the LT circuit data, and the proposal shows what the site needs.
In Short
- Turbocharging heats the intake air to 150 °C and more
- Cooling it raises output and lowers NOx and component temperatures
- Air-to-air coolers for compact sets, air-to-water on large engines
- Large engines cool in two stages: HT and LT circuits
- The LT return temperature is close to ambient on a hot site
- TECS sizes the LT coil to the engine maker's charge air limits
What To Send With An Enquiry
- Engine make, model and the cooling data sheet
- LT circuit: heat, flow, inlet and required outlet temperature
- Whether the oil cooler is on the LT circuit
- Site ambient, altitude and humidity
- Air-to-air or air-to-water arrangement
- Installation and sound limit
Questions We Are Often Asked
What is the difference between an intercooler and a charge air cooler?
None in practice; both names describe the cooler between the turbocharger and the cylinders. "Aftercooler" is also used, strictly for the cooler after the last compressor stage.
What happens if the charge air is too hot?
The engine produces less power, runs hotter and emits more NOx; its control reduces the load to stay within limits. On a generator set this shows as a derating on hot days.
Why does the LT radiator need so much surface?
Because its coolant has to return only a few kelvin above the ambient air; the small temperature difference needs a large surface to move the heat.
Can the LT heat be recovered?
Rarely; at 40-55 °C it is too cold for most heat consumers. It goes to a radiator, while the HT stage heat can be recovered in a CHP plant.
Are air-to-air coolers used on large engines?
Seldom. Large engines have water-cooled charge air coolers on the engine and send the heat to a remote radiator through the LT circuit.
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.
