Quote:
Originally Posted by kevman
but then again, your only considering the heat energy available/escaping with the exhaust gasses.
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I'm neglecting the available heat energy available due to friction losses, and available due to heat transfer of exhaust heat energy into the engine block. Yes, that's true. Then again, my example has been very much simplified in other respects. You'll find that all of the simplifying assumptions I make are placing the example into the absolute best-case scenario, and that real-world considerations only worsen the case for steam generation.
To a layman's perspective, for instance, nothing is added to the discussion by dragging in steam tables or air tables to more accurately model exhaust gas having water sprayed into it. Treating exhaust gas as an ideal gas is the best-case scenario, and more realistic modelling by using tables would only show worse results.
Nor is the discussion helped by dragging heat transfer equations into the surrounding metal of the combustion chamber. For purposes of the example, no heat transfer is assumed because, again, it's the best-case scenario. That it greatly simplifies the example is a beneficial plus. Besides, most engine heating is due to exhaust heat radiating into the engine block. Think about it - there's an entire stroke devoted just to manipulating exhaust gas. During all the time it takes for that stroke to complete, exhaust gases are touching the sides of the cylinder, the combustion chamber roof, the valves, the piston, and the walls of the exhaust port(s). If we posit that we cool off exhaust gas by spraying water into it, then heat transfer into the engine block will be much less than before, so there is much less heat energy available anyway. This observation is borne out by the fact that the two 6-stroke engines mentioned elsewhere in this thread did not need water cooling at all.
The same goes for the actual spraying of water. In my example, I'm assuming the water is atomized into the exhaust gas such that it's perfectly uniformly mixed with the exhaust gas, so as to uniformly raise its temperature as it sucks heat energy out of the exhaust gas. Again, best-case scenario. In the real world, it's kind of tricky to get truly perfectly uniform atomization of water into a combustion chamber filled with compressed exhaust gas at TDC, and water droplets will fall out out onto the metal surfaces as the exhaust is cooled below its ability to vaporize that water.
Heat energy due to friction can be ignored since there's actually very little of it available to do anything, compared the the heat energy directly found in the exhaust gas itself. Warm-up time, remember?
These are the major assumptions I made with my example. Remember, they push the example into an impossible-to-attain ideal condition, for which worse results would have been shown had real-world considerations been taken into account, and for which the example itself would have been needlessly complicated.
Quote:
Originally Posted by kevman
let me ask you this, theoretically speaking, an ideal engine will have no/minimum heat transfer to outside the working medium (pison/valves/cylinder ect) correct? that is why ceramic coated pistons/liners/sleeves ect are benificial as i understand it. which means more heat trapped inside to do work.
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That is exactly why ceramic coating is popular. However, it's also pricey to have it done correctly to engine internals, because you have to ensure that the coating is correctly bonded to the surfaces where exhaust gases would touch. Otherwise, the coating would eventually flake off and cause no amount of mischief to the piston rings. You also can only coat the surfaces that themselves do not experience frictional rubbing. Therefore, the cylinder wall (which is most of the surface area that exhaust heat leaks into) is not coated.