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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

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  • $\begingroup$ The hottest that gasoline gets in an internal combustion engine is approximately 700 C. Meanwhile, jet fuel reaches around 2000 C. Maybe I don’t understand the question, but it seems like a much hotter burning engine would require more heat resistant materials. $\endgroup$ – Aaron Holmes Aug 14 at 1:15
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    $\begingroup$ Most piston aviation engines are 4 stroke. And what do you mean by "front line material"? I believe most cylinder linings and exhaust manifolds are steel. $\endgroup$ – Michael Hall Aug 14 at 1:17
  • $\begingroup$ i don't think theres much difference in combustion temperature, as gasoline and kerosene have similiar adiabatic flame temperature. On top of that combustion in pistons is combustion of stochiometric mixture inlike in jets, and it's also isochoric as opposed to adiabatic as in jet engines, so the peak pressure and therefore temperature are much higher $\endgroup$ – Francis L. Aug 14 at 1:35
  • $\begingroup$ But the duration is much, much less than continuous... $\endgroup$ – Michael Hall Aug 14 at 2:22
  • $\begingroup$ @Francis L.: I'm sorry. I wasn't just guessing about those temperatures. I verified my facts before posting. Jet engines produce fuel burn temperatures that are nearly 3 times higher than piston engines and it is continuous, just as Michael pointed out $\endgroup$ – Aaron Holmes Aug 14 at 2:53
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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).

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  • $\begingroup$ Though we have wandered a bit from "why don't piston engines require as heat resistant materials as jet engines" to "much better to build them with heat resistant materials, and make them turbines", this has been very informative. Would you consider including the turbo compresser in piston engines when comparing with jets? I have read that jets get 1:50 "at the top of their climb, going to cruise" (in much thinner air). How would this compare with a supercharged piston engine? $\endgroup$ – Robert DiGiovanni Aug 15 at 0:00
  • $\begingroup$ @RobertDiGiovanni: Yes, good point. Especially the turbo-compound engines look thermodynamically like jets with a discontinuous combustion process. However, super- or turbocharging rarely went above 1.5:1 (at sea level), on engines which sported 8:1or less initially. Near the engine's critical altitude this could reach 8:1, but total pressure and cylinder wall heating was still restricted. $\endgroup$ – Peter Kämpf Aug 15 at 12:19
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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 .

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    $\begingroup$ I'm not claiming they're as hot as jet combustion products, but piston EGTs are regularly in the 1500+ deg F range. The aluminium portions of the cylinders and pistons certainly remain at much lower temperatures. $\endgroup$ – pericynthion Aug 14 at 7:24
  • $\begingroup$ Yes, exhaust manifolds will be red, about 1400 + F running under load. But pistons , heads don;t get so hot during the limited time of the combustion stroke. $\endgroup$ – blacksmith37 Aug 14 at 19:18
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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.

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    $\begingroup$ Excellent point about cooling... $\endgroup$ – Michael Hall Aug 14 at 17:02
  • $\begingroup$ It is indeed about cooling. But the main difference is about ability to carry the heat out. Pistons can be cooled from outside easily, but the core parts of a turbine engine are surrounded by the hot gas and only cooling medium available for them is the compressed air from before combustors, which is still quite hot from the compression. $\endgroup$ – Jan Hudec Aug 14 at 18:27
  • $\begingroup$ The RPM is not too relevant. Higher RPM does increase the power/weight ratio, and that means needing to take more heat from the same mass of metal, but since the principles are different, the numbers are not directly comparable. $\endgroup$ – Jan Hudec Aug 14 at 18:31
  • $\begingroup$ @Jan Hudec RPM is relevant as far as friction (lubrication) and heat produced in an internal combustion engine. But I would suggest high RPM $electrics$ for future applications. "Core parts of a turbine engine are surrounded by the hot gas and (the) only cooling medium available for them is the compressed air from the combustors" is actually an excellent answer to this question! $\endgroup$ – Robert DiGiovanni Aug 14 at 18:38
  • $\begingroup$ @RobertDiGiovanni, piston engines generally operate at lower RPM in the same power class (race car (F1) engines go to 18,000 or 19,000 RPM these days, but then a PT6, which is in the comparable power class goes to ~45,000 Nc and ~30,000 N1), but they have much more friction surface. In piston engine it is all the piston rings plus the rod bearings which are usually sliding, while in turbine all bearings are roller ones, so there are just the labyrinth seals that come anywhere close to creating friction. But it's still minor issue compared to the combustion heat for both. $\endgroup$ – Jan Hudec Aug 14 at 18:53

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