In contrast to this question (link), I would like to know the opposite - why diesel piston engines aren't used on aircraft with comparable role and configuration to Saab 340, L-410, ATR-42?

There's a very similar question here, but it was mainly focused on small GA aircraft and private owners, while I'm more interested in commuter type piston airplanes.

Here's my thoughts so far after some reading (please note, some of them are just my assumptions and might be wrong).

Pros of diesel engine:

  • Engine itself is significantly cheaper
  • Cheaper to maintain
  • More economical according to this discussion (link)

Cons of diesel engine (vs turbine):

  • Likely they are heavier and more bulkier than turbine of similar power output (link to similar discussion)
  • Diesel engine pylon is less aerodynamic (more drag) due to presence of necessary air inlets for engine cooling
  • Possibly altitude (and thus performance) limited. I'm not really sure about this point though. It's a well-known fact that commuter turboprops can fly at 20-25k feet. I don't have any data for diesel piston engines. I can only assume that diesel engine with a turbo-charger might actually reach similar altitudes and performance.
  • Require more frequent maintenance

Perhaps this is worth asking as a separate question.

How heavy and big would be diesel piston equivalent of PW120 engine? (some ideas can be found in this discussion)

PW120 EASA data for weight, dimension and thrust (link):

  • Length 2130 mm
  • Width 635 mm
  • Dry weight 417.3 kg
  • Take off shaft power 1342 kW
  • Maximum Continuous shaft power 1268 kW
  • 1
    $\begingroup$ "Engine itself is significantly cheaper, Cheaper to maintain" Really? $\endgroup$ – user3528438 Apr 26 '18 at 14:58
  • $\begingroup$ There's several questionable assumptions here @ElectricPilot, the engines are not cheaper, nor cheaper to maintain or more economical. Your discussion links go to wikipedia pages. What are you basing these assumptions on? Don't you think if these were correct we'd be using 3000hp piston engines instead of turbines? $\endgroup$ – GdD Apr 26 '18 at 15:03
  • $\begingroup$ @GdD, links have been corrected now, I must have edited them in the wrong way. $\endgroup$ – Electric Pilot Apr 26 '18 at 15:57

There are many reasons but Ill address your points directly first

Engine itself is significantly cheaper

This is only true for non aviation engines. Even if you were to obtain a type certification for an automotive engine the cost of that would reflect a huge increase in price. This has been tried before and proved so costly the engine maker ended up dropping the FAA type cert due to costs.

Cheaper to maintain

Again this is only true for auto or industrial application engines. Any part carrying approval down to the spark plugs has a large price mark up over their non aviation counterpart (even if the parts are more or less identical). On the whole piston engines tend to also have a shorter TBO than turboshaft engines.

Likely they are heavier and more bulkier than turbine of similar power output

This is perhaps one of the most important cons. In aviation weight is the name of the game. Airlines make money not only on passengers but the carriage of freight in empty baggage space cutting into that cuts into profits.

Piston engines once did rule the skies in that size class, the venerable DC-3 is about as big as what you mention and has long since been phased out of routine use.

For the altitude space in the FL200-FL250 range you also need pressurization which is much harder to drive from a piston engine. A turbine allows you to pull bleed air for pressurization while a piston engine requires a whole separate unit driven by the engine somehow. This adds complexity and again, weight. Over the years there have been some pressurized piston planes but they have fallen out of popularity and use.

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