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Heat radiated from an diesel Engine

  1. Oct 29, 2010 #1
    Can any body help me how to calculate the quantity of heat radiated from an diesl engine. any formula based on BHP?

    what are the assumptions that i need to make?

    what are the factors that i need to consider whilem calculating the heat radiated from an diesel engine
    Last edited: Oct 29, 2010
  2. jcsd
  3. Oct 29, 2010 #2
    Do you mean the heat actually radiated from the engine into the engine room, or the total heat lost from the engine via cooling water, up the exhaust stack etc?
  4. Oct 29, 2010 #3


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    Radiated heat is the heat to fuel, minus the heat to power (don't forget your ancillaries), minus the heat to exhaust, minus the heat to coolant, minus the heat to intercooler (if fitted), minus the heat to oil.

    Alternatively you can try and calculate it based on the Stefan Boltzmann Law (sum up all the radiated heats for components with a known surface temperature and area).

    If you want accurate figures, use the first method. You should find it's 3-10% of heat to fuel.
  5. Oct 29, 2010 #4
    You could use engine efficiency if you have a dyno. X amount of energy goes into the engine, Y power comes out to dyno, Z power is used to drive accessories.

    Heat generation = X - Y - Z

    That is a really crude representation, but those values are easier to find.
  6. Oct 29, 2010 #5


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    Cstoos, your method won't give you radiated heat.
  7. Oct 29, 2010 #6
    True, but radiated heat is not an easy task.

    Measuring heat lost through exhaust is not just a measurement of temperature, but requires you to know the mass flow rate, which is not an easy number to find without emissions equipment. The fuel H/C ratio also needs to be tested.

    Heat to water can be measured decently if you know the exact volume, the flow rate, the pump efficiency, the radiator efficiency, and the loss due to radiation from the piping.

    The same thing goes for oil.

    I would say the best bet is to measure directly in some sort of insulated control volume where as many other variables (i.e., exhaust piping, water piping) are eliminated.
  8. Oct 29, 2010 #7


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    Sorry again Cstoos, that's bobbins.

    Heat to exhaust does require an exhaust mass flow rate, but this doesn't require emissions equipment at all. You just need combustion air flow and fuel consumption (both basic measurements). Fuel H/C ratio is also not required, as this is a Diesel engine an assumption for calorific value (say 46MJ/kg if Diesel fuel) you'll easily be within a percent of heat to exhaust. Specific heat capacity for exhaust gas at a known temperature is easy given the AFR as above.

    For heat to water, all you need is the flow rate and temperature difference, and to know the working fluid. The parameters you quoted (volume, pump efficiency, rad efficiency and pipe losses) are irrelevant. The same goes for oil.

    May I ask where you're getting your information from?
    Last edited: Oct 29, 2010
  9. Oct 29, 2010 #8
    Most of what I deal with is http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=185cd2f98508ccae0f7a30e59baed0c7&rgn=div5&view=text&node=40:" [Broken].

    When I was referring the H/C ratio of the fuel I meant that in order to calculate the mass flow rate from the exhaust emissions it is needed. There are standard values that can be used for this, but that changes with the fuel (just because it is a diesel engine doesn't mean it is running on diesel alone). For accuracy, it is best to test.

    I will concede the water and oil factors to you though. It is not quite as easy as that to measure on the engines I work on, so I dismissed it when I probably shouldn't have. I apologize.

    Anyways, every engineer has different methods depending on what they have at their disposal. I use the "carbon in = carbon out" method and have always shied away from the "mass in=mass out" method because of previous experiences, but if you can get it to work, then good.
    Last edited by a moderator: May 5, 2017
  10. Nov 3, 2010 #9
    mr jobrag. i am dealin with genrator room/canopy ventilation. for that i need to consider the radiated heat from a diesel engine surface. so that i can calculate the total air required for a genrator. i the quantity o heat from other sources like alternator. i just need what quantity of heat is liberated from a diesel engine through radiation into generator room
  11. Nov 3, 2010 #10
    The generator part is easy, about 5% of the rated power is the maximum that you’ll get out.
    For the engine I'd start by assuming that all the engine was at the temperature of the cooling water outlet calculate (or estimate) the total area of the engine, think about what the maximum and minimum (summer winter) air temperatures flowing across the engine would be and plug all the numbers into the relevant calculation then multiply the answer by 1.5.
    Alternatively I'd write to the manufacturers and ask "how much heat will your engine radiate at full power?"
  12. Nov 3, 2010 #11


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    The engine will radiate, at worst, around 10% of the amount of energy you're putting in as fuel.

    The alternator will be between 85% and 97% efficient depending on its size (the bigger the better), the rest will be radiated.
  13. Nov 11, 2010 #12
    hello brewnog,

    i have got a formula to calculate radiated heat from alternator,

    heat rejected by alternator to ambient = (1-efficiency) X ENGINE BHP X 42.5

    in this formula i am not able to understand the value "42.5 " stand for ??

    why this is considered any guess???

    is that above mentined formula works fro my consideration (GENSET room ventilation)
  14. Nov 11, 2010 #13
    so yopu are saying,,,, 5 % rated power can be considered as heat radiated from an diesel engine.

    heat rejected by engine (BTU/MIN) = 5% fuel consumption(lt/hr) X spe gravity of diesel X calorific value/3600 X 237.94

    is this formula true??

    is that i need to consider the FAN loss in BHP (in above formula) for a radiator coooled engine???
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