Piston engines often get away with using aluminium as their heat exposed material, while even the early jets required inconel to operate.
Is that because the combustion takes only, for example, half of the time for two stroke engines hence half the temperature?
Still compression ratio of a typical two stroke is twice higher than early jets, while the melting point of aluminium is more than twice lower so it doesn't add up, as if there was another reason for their higher heat durability
Lubrication.
The low wear of piston engines is due to an oil film between piston rings and liner, so both never get into immediate contact. This perfectly thin oil film left on the liner bore by the oil control ring will flash off if temperatures exceed 180° - 200°C, and the piston rings will scuff. This need for lubrication is the weak link which makes improving heat resistance for piston engine materials redundant.
Lubrication in turn requires sufficient cooling which is possible because combustion in reciprocating engines is a discontinuous process with only the end of one and the beginning of the next stroke producing compression and combustion heat while the others allow to cool down the cylinder head and walls.
Contrast this to jet engines. Continous combustion produces much higher thermal loads so any attempt at sealing the compressor and turbine disks against the outer casing is futile. Only the development of heat-resistant alloys has made this possible: Look at how the improving materials allowed increasing compression ratios. Modern turbofan engines reach compression ratios of up to 50:1 while diesels are stuck at around 20:1 and gasoline engines at around 10:1. Aviation pistons with superchargers raise that to levels similar to modern turbofans: The Wright R-3350 turbo-compound engine supercharger increased pressure by 6.5:1 with the pistons compressing air again by 6.85:1 for a total of 45:1. The highest compression ratios were achieved with super- or turbocharged diesels. The Jumo 205D supercharger achieved a compression ratio of 8.85:1 and the engine another 17:1 for a total of 150:1, but in all cases needing intercoolers to keep air temperature low enough and using waste gates or staged chargers to only reach the top compression ratios close to their critical altitude!
Since decades we can read about "adiabatic engines", piston engines using heat-resistant ceramics and doing away with the whole cooling system. Their implementation has so far not happened because the wear between piston and liner has been unacceptable (among a host of other problems, like manufacturing cost and brittleness).
Piston engines do not get as hot ( the metallic components) ; the exhaust gases are hot but not nearly as hot as the first stage of the turbine. Remember the cylinders and pistons are made of aluminum which melts at 1000 +/- F and is pretty useless at 600 F. The first turbine stage gases were up to 1800 F when I last read about them . The high performance turbine blades have small axial holes for cooling air to flow through them. And turbines are always hot while a piston engine only reaches its highest temperatures on one of 4 strokes. The exhaust valves are made of high temperature alloys .
Piston engines are designed to produce mechanical energy, torque, which is used to move a propulsion unit. In aircraft, it is the propeller, in cars, the wheel. There may or may not be a transmission. Heat produced is waste (after expansion to drive the piston) and is removed by a cooling system and exhaust. The enclosed piston is more efficient than a jet at producing mechanical energy per unit of fuel, just as the cannon is more efficient than the recoiless rifle at propelling a shell. With pistons, combustion of fuel is not constant, allowing heat sinks such as air and water flow, to remove the heat before temperatures get too hot. Good conductors of heat, such as aluminum, brass, and chromium steel, work well with these applications, but high fuel burn rates can lead to overheating. Piston engines are RPM limited, and simply cannot produce enough power as ...
Jets. Capable of producing much more thrust by using heat resistant metals and running at much higher RPM. Here the "internal combustion" is replaced with less efficient but lighter jet engines. Jets have an "Ace in the Hole", the ability to maintain thrust at higher altitudes, where greater TAS off-sets lower propulsion efficiency.
This is why the 747 has double the ton miles/per gallon fuel burned efficiency compared with even the mightiest of the piston engined transports. Lower and slower, piston props are better.
