Heat Rejection

In short. Only about a third of the fuel energy burned in a diesel generator becomes electricity. Roughly a third leaves in the exhaust, a quarter to a third goes to the coolant through the radiator, and 5 to 10 percent radiates from the engine, alternator and exhaust piping into the room. Each path needs its own route out. The manufacturer’s heat-rejection figures for the specific set are the design inputs; a percentage rule is only a sanity check.

The energy balance of a generating set

A diesel engine converts fuel into shaft power with an efficiency of around 38 to 42 percent at its best point, and the alternator passes 92 to 96 percent of that to the terminals. Everything else is heat, leaving by four routes: exhaust gas at 400 to 550 °C, jacket water and charge-air cooler circuits to the radiator, radiation and convection from the block, alternator frame, manifold, turbocharger and exhaust pipe, and the alternator’s own losses into its cooling air. The table shows typical shares for a modern turbocharged set; the data sheet for a specific engine governs.

The arithmetic is worth doing once for every project because it makes the scale obvious: a 400 kW set burns fuel at about 1,050 kW, and 650 kW of heat has to go somewhere.

Reading the manufacturer’s data sheet

Generating-set data sheets list heat rejection to coolant and to the charge-air cooler where it has a separate circuit, exhaust gas flow and temperature, radiated heat to the room, radiator airflow, and the maximum external restriction the fan can tolerate. Three cautions apply. The figures are at rated load and standard conditions; in high ambient the radiator margin shrinks although the engine’s heat does not. Heat to coolant on a two-circuit system must include both circuits. And radiated heat is quoted for a bare set with the exhaust outside the room; every metre of unlagged pipe and every indoor silencer adds to it.

Relating heat to flow and temperature

Q = m × c_p × ΔT

Q is the heat carried away in kilowatts, m is the mass flow of the fluid in kilograms per second, c_p is its specific heat in kilojoules per kilogram kelvin, about 4.18 for water, 3.6 to 3.9 for glycol mixtures and 1.005 for air, and ΔT is the temperature rise of the fluid across the heat source in kelvin. The same relationship sizes the coolant flow, the radiator airflow and the room ventilation; only the fluid and the permitted rise change.

Worked example (illustrative). The 400 kW set rejects 290 kW to coolant. With a 7 K rise across the engine and plain treated water, the coolant flow is 290 ÷ (4.18 × 7), or about 9.9 kilograms per second, which is the duty the water pump must deliver against the radiator’s resistance. The radiator transfers that 290 kW to air; if the air leaves 25 K warmer than it entered, the mass flow is 290 ÷ (1.005 × 25), or 11.5 kilograms per second, about 10.5 cubic metres per second at 40 °C. That airflow is why generator rooms need openings measured in square metres, as worked through in generator-room ventilation.

Cooling arrangements and where each heat path goes

Set-mounted radiator

The standard Indian arrangement. Coolant heat leaves through the radiator air, which must be ducted outside; radiated heat is carried out by the same air as it crosses the room. Its limits are the fan’s static pressure and the radiator’s ambient capability, usually 45 to 55 °C.

Remote radiator

Used where the room cannot pass the radiator airflow or where the set is in a basement. The coolant heat leaves at the remote location, but the radiated heat stays in the room and must be removed by dedicated ventilation fans. Static head on the engine’s water pump seals and pipe friction limit how far and how high the radiator can go; above the manufacturer’s limit a heat exchanger and a secondary circuit are needed.

Heat exchanger and cooling tower

Common for large and medium-voltage sets. Coolant heat goes to a plate or shell-and-tube exchanger and then to tower water. The tower’s capacity depends on wet-bulb temperature, which in Hyderabad’s pre-monsoon weeks is far below the dry-bulb maximum, an advantage over air-cooled radiators in the hottest months.

Containerised and canopied sets

All heat paths pass through a small volume with high airflow resistance from attenuators. Internal recirculation between the discharge plenum and the radiator intake is the dominant failure mode, and the enclosure manufacturer’s thermal data at site ambient is the only reliable basis. See containerised generator systems.

Exhaust heat is part of the room balance

Exhaust gas at 500 °C through a 250 mm pipe loses heat to the room at a rate measured in kilowatts per metre if unlagged. A silencer inside the room is a radiator in its own right. Lagging to 50 mm of mineral wool with cladding cuts the loss by an order of magnitude, protects personnel and keeps the gas hot enough for CPCB IV+ aftertreatment to work. Aftertreatment units themselves run hot and should be counted in the room heat if mounted indoors. The resistance side of the same system is covered in exhaust backpressure.

Keeping the heat path healthy

Heat rejection decays in service. Radiator fins block with cement dust, textile fluff and rice husk; coolant loses its inhibitor and scale forms; fan belts slip; thermostats stick; and a duct seal that opened months ago lets discharge recirculate. A set commissioned with 5 K of margin on a cool day loses it without anyone changing a setting. Coolant temperature trends from the controller, a quarterly fin inspection and an annual coolant test are the cheapest insurance, and cooling and radiator service describes the corrective work. Where a set trips on temperature only under full load or only in summer, the fault is nearly always in the heat path rather than the engine, and root-cause diagnostics starts with the airflow.

Common mistakes

• Designing the room from kVA and a rule of thumb; the heat-rejection data for the specific set can differ from the rule by half as much again.
• Omitting the charge-air cooler circuit from the coolant heat; the radiator is undersized by a fifth on a turbocharged aftercooled engine.
• Running exhaust piping and a silencer unlagged inside the room; the room runs 10 K hotter than the heat balance predicted.
• Placing a remote radiator above the engine’s static head limit; the water pump seal fails within months.

What to send us

Send the generating-set data sheet or nameplate, the cooling arrangement in use or proposed, a sketch of the room or enclosure with the exhaust route marked, the maximum site ambient and, for an existing problem, the coolant temperature trend against load and outside temperature. Photographs of the radiator core, the duct connection and any indoor exhaust piping tell us most of what we need. Send your application details for a new installation, or request a service visit for a set that overheats.

Questions engineers ask

Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.