It is pretty straightforward to compute the heat and power generated by the combustion process, given the displacement, RPM and fuel mass flow rate. Similarly, the power exhausted can be computed with knowledge of EGT and conservation of mass. The difference is dissipated as heat, from air cooling of cylinders and oil cooler. I'm looking for a rough ratio between oil cooler and cylinder heat dissipation.
- Conduction to air: 7.2%
- Conduction to oil: 1.6%
- Radiation and misc: 3.4%
The other two areas of loss are exhaust (51.6%) and mechanical (36.2%).
I don't know how "cylinder heat dissipation" would fit into this classification (some combination of air conduction and friction losses?), but looking only at conduction losses gives a ratio of air to oil of 4.5:1.
Typically in an air cooled, reciprocating engine, for the total amount of chemical energy released during combustion, about 40-45% exits the tailpipe as heat in the exhaust gases, 25-30% of that energy is transferred to the propeller through the crankshaft as mechanical energy, 15-20% or so exits the engine from cool air flowing through the cowling over the engine block, and the final 5-10% is removed through the oil cooler.
SOURCE: THE AIRCRAFT ENGINE AND ITS OPERATION - PWA 109702, pp 19 - Pratt & Whitney Aircraft Corporation, 1949.
I knew I had this in my library somewhere.
If we take a look at an airplane eg an SR-22 on takeoff, the IO-540 engine consumes about 25 gal/hr of 100LL and produces 310 BHP, or is outputting 8.3E8 Joules of mechanical energy per hour. With a chemical energy density of 40MJ/kg for 100LL, this means that about 2.6E9 Joules of chemical energy are supplied to the engine every hour, thus giving us around a 31% efficiency in this example, and that’s about what we would expect.