SMA diesel engines

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Rich
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SMA diesel engines

Post by Rich »

Today we had a team here in the area developing an aircraft they hope to market to the military. It's for aerial surveillance and part of its mission is extensive loitering time (13+ hour missions). It is currently experimental and they've put about 20 hours on it. They are using the SMA SR305-260E engine. Some interesting non-obvious details about this engine:

- Runs at max 100 in. MAP.
- Has a minimum CHT of 212º F. If it gets below that you're likely to get a flameout :shock: . And it's pretty easy to get it below this temperature. This limit has been shown to make it tricky to land, as keeping the temperature up means carrying significantly more power than one wants in this 57-ft/ wingspan plane.
- Most cooling is done by the oil cooler, though with the engine overall being air-cooled the remainder is handled as with a traditional avgas aircraft engine.
- Direct-drive - max 2200 RPM.
- The 230 HP variant (lower boost) of this engine was used in Cessna's attempt - now discontinued - to market diesel-powered 182's. It is still available via STC in certain 182 models.
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Re: SMA diesel engines

Post by TimS »

When Cessna was ending the program on the 182, rumors on multiple sites stated Cessna was having issues with two things.
1. Altitude restarts, especially if engine cooled.
2. Landing power requirements.

I never gave it much credence; but it does match up with what you are posting. So maybe I should have? :D

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Re: SMA diesel engines

Post by chili4way »

Certainly a different compression ignition (CI) engine design strategy from Austro AE4 (or Thielert). SMA adapted a gasoline AVgas design instead of an automotive one. That explains the SMA's 305 cu. in. vs. Austro's 121.5 cu. in.; it runs at about half the RPM due to it being a direct drive engine. I wonder how the design manages the torque peaks that propellers dislike.

The high MAP is typical of CI engines; the Austro generates ~80% power at ~80 in. MAP. The min CHT of 212°F is probably due to the flashpoint of Jet-A being 100°C. Jet-A won't burn below that. (Gasoline's FP is 45°F).

Jet-A's autoignition temperature is 473°F, so the compression stroke has to get the air in the combustion chamber up to this minimum for the CI to take place. This is why restarts are time-limited on CI engines, including the Austro. You cannot let the engine get too cold. It looks like the SMA cylinders are air-cooled and so do not have the same amount of thermal mass as a water-cooled cylinder design. Seems like the design would be subject to shock cooling as well. The Cessna STC likely made use of cowl flaps, something we don't have to manage with Austro-powered engines.

The issue of minimum power to maintain CI is also an operational factor with Austro engines. As pressure altitude increases above a certain point, the engine increases the minimum power level. Pilots that land (and subsequently taxi) at Leadville do so with something like 15% power, even with the power lever fully pulled back. I suppose the air-cooled cylinder design of the SMA could lower the altitude (and perhaps increase the OAT) at which minimum CI power affects how the airplane flies.
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Re: SMA diesel engines

Post by Boatguy »

chili4way wrote: Sun Jun 19, 2022 6:16 pm Jet-A's autoignition temperature is 473°F, so the compression stroke has to get the air in the combustion chamber up to this minimum for the CI to take place. This is why restarts are time-limited on CI engines, including the Austro. You cannot let the engine get too cold.
The Austro uses glow plugs to start in cold weather. I wonder why that wouldn't work at altitude?
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Re: SMA diesel engines

Post by Rich »

Boatguy wrote: Sun Jun 19, 2022 7:34 pm
chili4way wrote: Sun Jun 19, 2022 6:16 pm Jet-A's autoignition temperature is 473°F, so the compression stroke has to get the air in the combustion chamber up to this minimum for the CI to take place. This is why restarts are time-limited on CI engines, including the Austro. You cannot let the engine get too cold.
The Austro uses glow plugs to start in cold weather. I wonder why that wouldn't work at altitude?
It does have glow plug(s) for cold starts, but I'm told it doesn't work in this situation.
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Re: SMA diesel engines

Post by chili4way »

Boatguy wrote: Sun Jun 19, 2022 7:34 pm The Austro uses glow plugs to start in cold weather. I wonder why that wouldn't work at altitude?
Good question. Since it's specifically disabled, there has to be a reason. As an interesting aside, the 4 glow plugs initially consume 100A for the first few seconds of operation before dropping to around 55 A and then dropping further to probably 30A. It's clear the system is not designed for continuous use. That still doesn't answer why the glow plug controller (GPC) is explicitly disabled when not on the ground.

This got me wondering to what extent the spool down of the turbocharger in the event of an engine-out situation at altitude may contribute to the time limit for a windmilling restart of the engine. The cylinder temperature may not be the limitation, and if it's not, the glow plugs won't help. In fact, in normal operation, the GPC doesn't turn on when the coolant temperature is above 20°C. I doubt the coolant would cool from its minimum of 60°C in the specified restart times. The thinking that the engine/cylinder cooldown is the cause may be mistaken. This restart issue may be related to the automatic minimum power settings that appear to be required to keep the turbocharger generating minimum sufficient manifold pressure to sustain CI.
  • Max. demonstrated altitude for immediate restart by windmilling: 15.000ft;
  • Max. demonstrated altitude for restart after 10 min. and ambient air temperature higher than ISA by windmilling: 10.000ft;
  • Max. demonstrated altitude for restart after 5 min. and ambient air temperature between ISA and ISA minus 10 °C by windmilling: 10.000ft;
  • Max. demonstrated altitude for restart after 2 min. and ambient air temperature below ISA minus 10 °C by windmilling: 10.000ft
Stay tuned. I'll see what I can find out. This is something that pertains to all aviation CI engines.
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Re: SMA diesel engines

Post by Boatguy »

I haven't see the glow plugs on the 40NG light up since I departed Canada three years ago (where they did light up in -10˚C).

However, they lit up for every start on the DA62 I was using for training last month.
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Re: SMA diesel engines

Post by chili4way »

Interesting observation; wonder why?
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Re: SMA diesel engines

Post by pbrevard »

Hello Rich. I ran across this old post and it peaked my interest about the SR305-260E. Do you have any detail around who was working with this engine and what there findings were about flameout?
I am assuming this is a liquid vs air cooling issue and that diesel versions of the Diamond fleet don't share the same anomaly.

Thanks for any leads you can provide!
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Rich
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Re: SMA diesel engines

Post by Rich »

I can't recall. They were building a highly specialized surveillance airplane with a working name I also can't recall. They just showed up the one time at one of our regular EAA events.
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