Why do JT-A Diesel pistons have such poor climb performance?

SnoFlyer

SnoFlyer

Filing Flight Plan
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SnoFlyer
Looking at the DA40-NG, for example, you'd think the high-torque of a diesel would be a benefit, but while the efficiency is high, the climb performance is terrible compared to the DA40-XLS. Same with the Archer DX vs the avgas Archer, or comparing the DA42 to an old Baron.

Is this just a design thing - they're putting in under-powered engines because they're targeting flight schools who don't care about hitting 13,000 ft? They got a 300HP diesel in the DA-50 just fine, though that's a $1M airplane, and even that has a similar rate of climb to a non-turbo 182. Is it just characteristic of diesel/JT-A piston engines that they don't climb well, even with a constant-speed prop?

Just curious as I've noticed this when looking at various aircraft specs.
 
Weight to power ratio is probably a big factor.
 
I’d suggest going out and flying a Da40 and a Da40NG rather than comparing the aircraft on paper. It has been about a year since I’ve flown a gas powered Da40 but the overall performance difference between the two seems negligible.

Bear in mind that you’re comparing a diesel with a turbo and slightly less rated power to a gas powered engine with no turbo and a slightly higher sea level power rating. What the diesel lacks at sea level will be quickly overcome with a little altitude.
 
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I'm getting a bit out of my wheelhouse here, but... wouldn't high torque be a benefit to acceleration, but horsepower is what is needed for continuous performance/climb rate?
 
Horsepower is a RPM multiple of Torque. If the DA40-NG is rated at lower HP at the same RPM as the gas motor, the torque must actually be lower than the gas engine - at sea level.
 
kujo806 said:
Horsepower is a RPM multiple of Torque. If the DA40-NG is rated at lower HP at the same RPM as the gas motor, the torque must actually be lower than the gas engine - at sea level.
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Not sure that's correct; my truck is diesel and has 250HP and 440lb/ft of torque; the same size gas motor in that truck is 290 hp, 265 lb-ft.
 
mondtster said:
Bear in mind that you’re comparing a diesel with a turbo and slightly less rated power to a gas powered engine with no turbo and a slightly higher sea level power rating. What the diesel lacks at sea level will be quickly overcome with a little altitude.
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True, the turbo will hold a higher % of that FPM as altitude increases, for sure. But I was surprised how low it was at sea level.
 
SnoFlyer said:
Altitude is my concern - looking at the POH for the NG shows that by 12k the climb rate has dropped to 210ft/min, which is not confidence inspiring for a trip over the west coast mountains...
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I think I'd review the performance charts... even at MGW and 30 degrees C at 12,000 I'm seeing much better performance numbers than you're quoting.
 
mondtster said:
I think I'd review the performance charts... even at MGW and 30 degrees C at 12,000 I'm seeing much better performance numbers than you're quoting.
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Yeah that number was from the wrong chart.. too many windows open.
 
SnoFlyer said:
Not sure that's correct; my truck is diesel and has 250HP and 440lb/ft of torque; the same size gas motor in that truck is 290 hp, 265 lb-ft.
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Because the gas motor revs much higher.....
 
it's just math; horsepower = Torque*RPM

this guy does great non-YouTube selfie-crap videos. If you have a short attention go to 3:15.. although the whole vid is only about 7 minutes

..and if we want to know why diesel has 'more torque', well then, this is actually very well done - makes perfect sense
 
horsepower = torque*rpm/5252

so.. if SnoFlyer's truck is 250HP and 440lb/ft then:

250=440*(3,000/5252)

vs the gas at 290 hp and 265
290=265*(5750/5252)

The diesel spins at 3K rpm vs the gas at almost 5,8K rpm
 
Tantalum said:
horsepower = torque*rpm/5252

so.. if SnoFlyer's truck is 250HP and 440lb/ft then:

250=440*(3,000/5252)

vs the gas at 290 hp and 265
290=265*(5750/5252)

The diesel spins at 3K rpm vs the gas at almost 5,8K rpm
Click to expand...

:yeahthat:
 
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