Does a diesel piston engine's power decrease more slowly with altitude than with an AVGAS engine?

Question

I have a power-altitude curve of the Austro Engine AE-300 provided by manufacturer and I don't know the reason why power decrease lower than other engines. The curve is in the end of the pdf in this link: https://austroengine.at/uploads/pdf/mod_products1/AE300_Technical_Data_Sheet.pdf

Answer 1

Welcome to the community! Because the Austro Engine AE-300 is a diesel engine,(JetA/Kerosene and Diesel are very similar) it is equipped with a turbocharger. Turbochargers compress the engine's intake air to maintain Sea Level air density so that the engine produces full power as you climb into less dense air.

Speaking of car and truck engines, diesel engines are often equipped with a turbocharger to increase the intake air pressure.

On cars and airplanes both, gasoline piston engines are not usually equipped with a turbocharger unless the engine is installed on a high performance car or aircraft where the customer is paying more money to get more performance. In recent years, some cars that are not high performance cars have been equipped with turbochargers to get more (an average level of) performance out of a very small engine.

Turbochargers are very useful for aircraft since they fly up into altitudes where the air becomes less dense, reducing performance.

When flying a non-turbocharged gasoline engine airplane the loss in engine performance becomes quite noticeable when climbing through 5,000'. If it is a hot day with a high density altitude the performance loss will occur at an even lower altitude.

An engine not equipped with a turbocharger is called "naturally aspirated", meaning that it is "breathing" naturally, not with a turbocharger forcing air into it for better performance.

An interesting thing to note about the Altitude Performance Graph you linked to is that they are comparing their AE-300 turbocharger-equipped Diesel engine to the Lycoming IO-360. The Lycoming IO-360 is not turbocharged. The turbocharged IO-360 is called TIO-360 (T= Turbocharged, I= Fuel *Injected, O = Opposed (Horizontally opposed engine design).

The non-turbocharged IO-360 is the equivalent, typical and common General Aviation engine to compare the AE-300 to, in terms of horsepower and performance, but it does not have a turbocharger.
Therefore, when you climb into thinner air the engine gets less oxygen and creates less power, resulting in the altitude performance drop-off in the chart you linked to.

Based upon the linked Austro Engine chart, I think your question is about General Aviation propeller aircraft in which the propeller is driven by a piston engine, not about turboprop engines where the propeller is driven by a jet turbine, is that correct?

Answer 2

It is not because it is a diesel engine, though diesels are more likely to be designed that way.

The engine is so called “flat rated”. That means its power is not limited by simply not being able to burn any more fuel at given piston volume and RPM, but by the peak pressure or temperature.

The controller limits the amount of fuel that can be injected at density altitudes below 9000ft to prevent exceeding those parameters and damaging the engine. Without that the power curve would have the same slope as for the IO360, but the engine would have to be much heavier.

But you don't actually need that power at low altitudes. Drag is the same at the same indicated airspeed, you need the same thrust to maintain your optimal cruise speed at any altitude. But power is thrust times velocity, i.e. true airspeed, so at higher altitude you need more power to maintain that indicated airspeed. So by flat-rating the engine it can allow cruising higher without making it much heavier.

Now diesel engines are more likely to be flat rated.

• Only turbo-charged engines tend to be flat rated. Flat rating basically means limiting the manifold pressure. But the ambient pressure can always get into the engine and the engine always has to withstand that. So the limit is always higher than ambient pressure, which can only occur in turbo-charged engines. There it is also possible to protect the engine from exceeding the manifold pressure limit by adding appropriate waste-gates in the turbo-charger.

• Diesel engines are more likely to be turbo-charged. Diesel engines inject fuel only at the point it should start burning, so they can have high compression ratios, and in fact need high compression ratios to reach the auto-ignition temperature of the fuel just by compression. On the other hand spark-ignition engines have the fuel already mixed in the air, so they must not reach that temperature, which limits their compression ratio. And turbo-charging increases the effective compression ratio, so spark-ignition engines can only be turbo-charged so much as the intercooler can prevent the temperature getting too high, while diesels can be turbocharged as much as the cylinders are built to handle.

So because adding turbocharger increases the power with less weight than making the engine bigger, all diesels are turbo-charged these days and have been for quite a while. And adding a bigger turbo to an engine is relatively small change that allows pushing the maximum power to higher density altitude, so once the engine is turbo-charged, it's the logical next step.

I would add that the big aircraft engines of 40s and 50s were all flat-rated, often to over 15,000 ft. I.e. their power curve had similar shape to what they show for the AE300 even though they were spark-ignited engines.

E.g. the huge Wright Cyclones were limited to 54 inHg manifold pressure for take-off and 49 inHg manifold pressure continuous, but they'd go well above that if you firewalled the throttles at sea level. And they had a second stage turbo-charger that the flight engineer only turned on above around 8,500 ft. At the time there were no electronic engine controllers, so the crew had to pay close attention to the manifold pressure gauge to avoid damaging the engines.

Answer 3

It's all about the air compression!

The graph that you mention shows two diesel/jetfuel geared piston engines, and one avgas direct drive piston engine, the Lycoming IO360. The two diesel engine graphs have a horizontal slope at lower altitudes, which is indeed caused by an intake air compressor.

• D - Direct Drive
• G - Geared (between prop and engine)
• N - Normally Aspirated
• S - Supercharged
• T - Turbocharged
• C - Carbureted
• I - Fuel Injection

The figure above is fig. 4-7a from Torenbeek, Synthesis of Subsonic Airplane Design, and compares different configurations of carbureted avgas pistons engines at constant piston displacement. As can be seen:

• only the normally aspirated avgas piston engine (like the Lycoming O360) has a continuously decreasing performance with altitude;
• the Geared Turbo engine has the best altitude performance, highest power at any altitude.
• the Direct Drive engines are limited by lower RPMs due to the propeller max RPM.

The interesting bit of the graphs is what happens to the right hand side: they all disappear into 0% at 16.5 km altitude. So the higher the initial power at 0 km, the steeper the decline into zero at 16.5 km. And I believe this is basis of the OP question.

• The Lycoming O360 is a 6 l normally aspirated direct-drive engine and develops 180 hp @ 2700 rpm.
• The AE300 is a 2 l turbo-charged geared engine, 168 hp kW @ 2300 rpm prop
• The Thielert/Continental Centurion is very similar to the AE300, with a lower certified power output.

So the question is indeed: why do the diesels not follow the general trend shown in the Torenbeek graph? This is because of the better thermal efficiency of a modern diesel engine with common rail injection:

1. The compression ratio is higher than that of an avgas engine: diesel has no knocking effect at high compression. Avgas engines using high octane fuel (100/130) can use compression ratios of 1:8 to 1:10, the AE300 type engine has a compression ratio of 17:1. So at whatever altitude, the surrounding air pressure is increased more in a diesel engine than in an Avgas engine.
2. The diesels have fuel injection, which results in more efficient combustion. Fuel injection increases power output at any altitude by a few procent.