Diesel exhaust is hot, and there's a lot of it.

Diesel radiators also get pretty hot.

In a typical diesel engine, which one is dissipating more heat?

EDIT: In response to a request for specificity below, I am specifically interested in heat losses on five different gensets I operate. The engines are a 0.7L Kubota D722 (no turbo), two Lister Petter LPW4 engines (also no turbos), a 2.216L Perkins 404D-22 (also no turbo), and a 4.5L John Deere 4045TF151 (turbocharged). These engines are run at 1800 RPM (except for the kubota, which is 3600 RPM), under roughly half of rated continuous load.

That is my specific situation, but I am looking for a general understanding of the matter, not just situation-specific advice.

  • $\begingroup$ They are very nearly equal for typical four-stroke non-turbo diesels under load. A turbo diesel under load should have slightly more radiator loss than exhaust loss. $\endgroup$
    – Phil Sweet
    Apr 20, 2020 at 2:38
  • 1
    $\begingroup$ @PhilSweet That’s a nice answer, but it should be posted as an answer not a comment. $\endgroup$
    – Eric S
    Apr 20, 2020 at 2:55
  • $\begingroup$ This paper seems instructive, but i'm having trouble interpreting it: dieselnet.com/tech/engine_whr.php $\endgroup$ Apr 20, 2020 at 3:19
  • $\begingroup$ You may find useful information if you research Combined Heat and Power (CHP). $\endgroup$
    – Solar Mike
    Apr 20, 2020 at 6:50
  • $\begingroup$ I'm pretty sure this is a duplicate. I also wanted to see if the OP would be more specific about the application. Marine diesels with cooled exhaust manifolds have greater heat exchanger losses and less exhaust heat at the pipes. But it may be the dup I was thinking about was focused on thermal efficiency of the engine. $\endgroup$
    – Phil Sweet
    Apr 20, 2020 at 10:32

1 Answer 1


They are very nearly equal for typical four-stroke non-turbo diesels under load. A turbo diesel under load should have slightly more radiator loss than exhaust loss.

At the bottom is a link to the technical spec sheet for a Cat 3412 powered genset. It's a probably a bit bigger than what you had in mind. It is a turbo with aftercooler (A/C in the doc below). Since it is setup for stationary power, the turbo is big, heavy, slow and efficient, and the aftercooler has a substantial heat rejection.

All industrial engines will have sheets like this. They can be customized by the mfg for unusual operating conditions such as high altitude or arctic conditions. This lets the dealer's installers get everything sized and arranged right on the second try (and they just love to fly to northern Alaska to fix stuff in the winter).

For over-the-road diesels, the heat rejection for internal jacket water and exhaust would be closer together. Radiators are draggy and heavy and you design vehicle engines to minimize them. You can reject more heat from surfaces in a vehicle.

Cat has a nice engineering primer on engine cooling - http://s7d2.scene7.com/is/content/Caterpillar/CM20160713-53120-13199

Vehicles often use air-to-air aftercooling and oil cooling, so the aftercooler and oil cooler heat doesn't end up in the radiator. Most larger engines use jacket water for aftercooling and oil cooling, so you need to add those values to the internal jacket water heat to size the radiator.

Cat 3412 genset technical data sheet (see section titled heat rejection on page 2)

I'll try to dig up a similar sheet for mobile equipment engines later. Or someone else can ;)

  • $\begingroup$ Given the effort in the answer, the OP should be able to find additional info easily. $\endgroup$
    – Solar Mike
    Apr 20, 2020 at 11:44

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