Why aren’t diesel piston engines used on aircraft like the Saab 340 or L-410?

Question

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

Answer 1

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 little snap rings that hold things on 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.

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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.

Answer 2

Bear with me 1 minute with a brief side note. Someone said that my previous post did not answer the question of Diesel engines vs Turboprops. And while I was writing this to clarify my other post was deleted so this one has to stand alone. Okay now let me answer the question directly so that nobody has to use the info to make their own comparison.

The Diamond DA-62 is a twin engine with each of the 2.0L turbocharged 4 stroke inline 4 Diesel piston engines putting out 180HP (360 combined) maximum and 378 lbs/ft (756 combined) of torque. 3887 engine max rpm, Equipped with a reduction box of 1:1.69 gives a maximum prop speed of 2300 rpm. Rated at 16.6 mpg but the Austrian data doesn't specify weather that's US gallon or the all but abolished Imperial gallon. So I have fuel burn rates here. Each engine is said to burn 9.25 US gallon/hr @ 100% load on the ground, 12.83 US gallons per hour @ 60% load and 12,000 feet operation, 7.4 US gallon @ loiter. The plane is 5 - 7 passenger with 1 pilot. Maximum payload capacity including fuel 1600 lbs. And yes this information is required to answer the question poised. Each engine weighs 410 lbs dry. The manufacturer has listed 1800 hrs average time to overhaul. Manufacturer reccomends Diesel and/or Kerosene (Kerosene is Jet-A. everyone knows they are the same basic fuel) These engines are adapted from Mercedes automotive diesel road engines. Cheap cheap cheap. Solid proven engine design cars have been known to achieve 1,000,000 miles with the original engine.

A similar size turboprop plane is a bit of an oddity but I did locate specifications on a Cessna Denali which is said to have a full of fuel payload of 1100 lbs. It burns 60 GPH but does not specify at what specific engine load nor altitude. It is a single turboprop with 1240 horsepower, weighing about 600 lbs dry. No specs as to the prop shaft speed or torque are provided but this is not a diesel engine so one can safely assume it will spin faster and provide lower torque output. The manufacturer lists 4000-6000 mean hrs between overhaul. They do advertise 20% less fuel burned than the same size competitors engine, I take this to mean another brand of Turboprop engine. Rated for use with jet-a type fuel. These engines are newly developed and very expensive.

The Cessna has a slightly higher maximum speed (285 kt) then the Diamond DA-62 (192 kt maximum cruise speed with an absolute maximum of 205 kt) which justifies a slightly higher fuel burn rate. However not 3 times the fuel consumption as indicated by the fuel burn rate. I would like to see them use a pair of 2.5L, 2.7L, or 3.0L 5 cylinder Diesels, the performance would be closer to that of the single turbine. Especially with use of bigger engines more aggressive props could be fitted to raise the top speed by 80kt. The service ceiling of the turboprop engine is rated higher. Key word being rated. Who actually knows the maximum altitude the little diesels would continue to give adequate, safe, & reliable performance.

Summary Turboprop - <50% higher advertised time to overhaul. - faster and higher rated altitude. Diesel - <60% less total fuel burned plus the ability to burn actual Diesel and Kerosene not just Jet fuel as specified by the engine manufacturer. Diesel engine is cheaper how much though ? Nobody knows. As the turboprop is very new on the market. Two engines of any kind are always safer than 1 in the unlikely event that you lose an engine you can still remain airborne for a period of time. Best land quick.

I hope this is answer enough to the question.

Answer 3

Why aren’t diesel piston engines used on aircraft like the Saab 340 or L-410? There are three reasons: weight, weight and weight. Illustrated in this answer, the Rolls Royce Merlin of 2,000 hp weighs 600 kg, the CT-7 of the Saab weighs less than half of that.

At lower power ratings pistons do have a niche, as mentioned here:

So the smaller the engine, the more advantageous are the circumstances for the piston engine: they scale down much more favourably than the gas turbines. But given enough volume, gas turbines are not inherently wasteful with fuel at all.

The niche for aircraft diesels as started by Thielert is in fuel consumption at lower powered engines, compares very favourably with avgas engines. The Saab 340 falls outside the niche.

Answer 4

Consider the efficiency losses of a piston engine when you compare it to a turbine engine that spins on one axis creating uninterrupted power, with intake, compression, combustion and exhaust occurring simultaneously. The airflow that is effortlessly flowing through the engine is simply injected with fuel which combusts, and expands imparting its energy to turbine blades which turn and drive the prop or reduction gearbox. No components reversing direction, minimal frictional losses. So simple.

On a piston engine, the intake air has to be delivered to/through the cylinder head at specific points in time by going through intake and exhaust valves which are driven by a valve train, complete with the weight of two camshafts (per head) and the chains that drive them. All these valves are housed in a complex and heavy cylinder head.

On the current general aviation aircraft gasoline piston engines (which aren't much more advanced than an air-cooled horizontally-opposed flat 4 on a 1960 VW Beetle), the aluminum head will crack if it is shock-cooled during a descent with a low power setting. However, the Thielert Diesel on a Diamond DA42 TwinStar is water-cooled, which avoids this problem... which reminds me that the cooling system on a jet uses the already abundantly available compressed air created by the engine's compressor section, while a piston engine that uses fluid for cooling needs to enclose the combustion chamber with coolant. Yet more weight.

Once a piston engine finally meters its air and fuel into the combustion chamber it creates power from gas burning and expanding, pushing pistons down into a cylinder, which are connected through connecting rods to a crankshaft that must be attached to an engine block strong enough to take all these loads.

Not only are these pistons reversing direction (which makes it possible to "throw a rod"), but the intake, compression, combustion and exhaust cycles occur sequentially, not simultaneously.

This is more wasted efficiency and frictional losses as the piston slides against the cylinder wall (creating no power on 3 of the 4 strokes) as it wears its piston rings away (all of which must be lubricated), as it charges towards the next mandatory engine tear-down and overhaul after only 2,000 hours of service, due to all of this friction wearing piston rings, cylinder walls, camshafts, valves, water pumps, etc...

All These parts that wear out on a piston engine and limit its lifetime, requiring a mandatory overhaul, don't even exist on a turbine engine.

I don't think we even need to compare performance numbers.

If you are going to fly a single-engine piston airplane over an ocean you better make sure your will is prepared because of piston-engine reliability thanks to all these different parts that could fail unexpectedly. The failure of an oil pump or water pump, alone, will be the end of the engine.

If you are going to fly a single-engine turbine airplane over an ocean it's "just another day at work".

Reliability, maintenance and weight are a few of the reasons that piston-engines haven't been put on airliners or commuter planes for many decades. It's progress.

Piston engines are early 1900s technology... and, compared to a turbine, they lack power above 5,000' altitude unless you add the weight of yet another costly piece of equipment- a turbocharger.

Nobody wants to fly on DC-6s anymore. Not even a diesel version. ;)

Answer 5

One other factor is vibration. Diesels shake a lot, more than gasoline engines due to their high compression ratio, whereas gas turbines, with no reciprocating action, have very little vibration. Aside from making for an uncomfortable ride, that vibration will subject the other components of the aircraft to additional stress, which requires additional reinforcement, which adds additional weight.

Vibration, after all, is a primary cause of metal fatigue.

Plus the nature of component failure in aviation: a vibration related engine mount failure on a big truck isn't a big problem, whereas that same failure on an aircraft can easily end in major disaster.