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I Impact of engine thermostat temperature setting on efficiency

  1. Dec 7, 2016 #1
    Will a four cycle, internal combustion engine have greater thermal efficiency with a relatively cool jacket water temperature, say 160 degrees F, or a relatively hot water jacket temperature, say 190 degrees F? These specific numbers are only important in as much as they are commonly available thermostats settings.

    Does the greater temperature difference between hot combustion gasses and the relatively cooler cylinder wall of a system controlled by a 160 degree thermostat drive more heat energy into the cooling system and cause lower thermal efficiency than a system controlled by a 190 degree thermostat? Is any difference likely measureable with commonly available tools and processes.

    This question assumes that that cooling system has sufficient capacity for the application and at the time being considered, the system temperature is controlled by the thermostat.

  2. jcsd
  3. Dec 7, 2016 #2
    For the engine efficiency what matters most is the variations in the thermodynamic properties of the working fluid: the air gas mixture. The entire power cycle happens so fast that the air gas mixture has very little time to exchange heat with the walls and the temperature of the walls has very little effect on the efficiency at least in a direct thermodynamic sense. What the temperature does affect is the strength, wear, and friction of the mechanical parts, and so in that sense the temperature has a very important contribution to the efficiency, but not in a simple obvious thermodynamics way. The temperature can affect how much compression you can handle, or the viscosity of the oil and so how well the rings seal but also how much power is lost to friction etc etc. lots of cascading consequences some of which may show up in the thermodynamic properties of the working fluid (like the compression ratio) but not in a direct obvious way.
  4. Dec 7, 2016 #3
    Thanks, but it is the "thermodynamic sense" that I'm most interested in (and where I need the most help). Without question, one combustion cycle happens too fast to have a material impact on heat transfer to the cylinder wall and cooling system. But, the cycle happens repeatedly and frequently to the point that roughly 1/3 of the heat energy in the fuel ends up in the cooling system and unavailable to do useful work. Maybe it is necessary to consider a more extreme and unrealistic condition to help me understand what is pretty much a theoretical question. What if there are two engines and cooling systems, equal in configuration and weight of air/fuel mixture (fluid) being fed to them. What if they are both stabilized, one is running at a cooling system temperature of 100 degrees F and the other is running at 1,000 degrees F. Which one is running more thermally efficient - more of the fuels heat energy is doing useful work? We also assume that there are no materials issues in this unrealistic situation. These engines are able to run with the same "motoring losses" at these two very different system operating temperatures.

    Thanks again.
  5. Dec 7, 2016 #4
    Yes, but almost all of that is from friction between the moving parts, not direct heating from the hot gasses. (Except the exhaust system which interestingly is not well cooled)
  6. Dec 8, 2016 #5
    I appreciate that there are important and measurable friction losses but I don't think they are that great. According to an SAE paper on friction losses a test with a Honda four cylinder engine quantified the friction losses at 10% of the developed power. And I'll add that the developed power is only 30% (in round numbers) of the the total heat energy of the fuel. A lot of the fuel's heat energy leaves the complete cycle via the cooling system (again in round numbers, 30%).

    I guess I should do a better job of asking my question. It is really just a thermodynamics question. If you have a given engine and it's stabilized operating temperature is controlled by a 190 degree F thermostat, it has some level of thermal efficiency determined by all the resulting operating conditions. If you take that same engine and now control its stabilized operating temperature with a 160 degree F thermostat, does it now operate with greater or less thermal efficiency?

    In the engine controlled by the 160 degree thermostat, does the increased temperature gradient between the hot fluid in the combustion chamber and relatively cool fluid in the cooling jacket cause more the of the fuel's heat energy to be driven into the cooling system? That's all I'm really interested in.

    Thanks in advance to any and all for your time.
  7. Dec 8, 2016 #6
    You need to also consider how much heat energy goes out the tail pipe. I am fairly sure the exchange of heat with the walls over one compression and power stroke is a negligible part of the thermodynamics.
  8. Sep 4, 2017 #7


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    Hi buddy,

    You basically understand the concept. A great way for you to examine this concept is by comparing the extremes of each condition, hot and cold.

    In the first scenario let's assume the surfaces of the combustion chamber are hotter than the combustion event itself during the combustion stroke, in this case the total force from the gases on the combustion chamber surfaces would exceed the total force available from combustion. Because the hotter surfaces of the combustion chamber will further energize the gasses.

    At the other extreme let's assume the surfaces of the combustion chamber are set at absolute zero degrees. In this case combustion would not occur.
  9. Sep 5, 2017 #8
    Thanks for picking up this "dead" question of mine. It didn't seem that I was able to keep responses focused on the essence of my question. Thanks for doing so. The "thought experiment" with extreme conditions is always a helpful tool and that is again true in this case.
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