Nonesense. Anti-gravity.

Get on board, man.

Two things come to mind while I read this stuff:

1. I wonder why all the diesel options seem to have less horsepower than their 4-cycle predecessors, and why nobody seems to be offering MORE horsepower. Seems like there's gotta be a way to bump that 227 horse motor up to about 250 and market it as an upgrade.

2. If Textron's going to be dealing with an engine supplier that doesn't have a lot of name recognition, why don't they make a deal to re-badge it as a Lyc, or even pay a licensing fee to market it as a Cummings. I bet *that* would sell a few :-)

The SMA 305 Diesel was I believe designed by Renault Formula One Engineers.

At the time, Renault was dominating F1. They built incredible engines.

-Pascal

http://www.smaengines.com/spip.php?rubrique41&lang=en

Of course Renault is unknown in aviation...

-Pascal

FIFY

So... if, as you assert, the diesels are able to produce the same amount of thrust for a lesser amount of horsepower, why are they slower in cruise on basically the same airframe?

I'll need some figures / links, etc. I've not seen comparison performance numbers.

The 172 JT-A has a 155 hp Diesel, that's 86% of the power of a spark ignited 172. I see no reason that they should not be able to cruise at the same speed given the same power output. Now, if you bring the Diesel back to 75% of its power, and the Lycoming back to 75% of its then sure, the Diesel will go slower, but HP for HP it should be a wash, and the Diesel should burn less at that setting. In addition, being turbocharged by design, the Diesel should be able to maintain its power as it climbs. My expectation would be that the Diesel should outperform the spark ignited engine even with less HP, and the one report that I did find (AOPA) corroborates that, stating that the 172 JT-A will cruise at "131 knots true airspeed while burning 25 percent less fuel." (vs the Avgas version.)

So... if, as you assert, the diesels are able to produce the same amount of thrust for a lesser amount of horsepower, why are they slower in cruise on basically the same airframe?

OK, you people made me dig, so here's what I found. It's been a LONG time since my last physics class, so anyone else, please feel free to jump in and tell me where I went wrong:

Richard, you may be right about the cowl having more drag. It's necessary to have those scoops for the intercooler and the oil cooler, both due to the Diesel design. No doubt there will be a drag penalty there. That said, Cessna still claims a speed improvement with the Diesel.

Jon:
According to Continental's website, the CD-155 produces a geared 349 lb-ft of torque at the propeller. A backward calculation (Tq=HP*5252/RPM) of the Avgas engine in the current 172 yields a torque of 350 lb-ft at 2700 RPM. So, both engines produce the same torque at the propeller. Now, if the prop had a fixed pitch on the Diesel, it would still lose, because at its 2300 RPM limit, it wouldn't be able to push as much air as the Lycoming at 2700, but by increasing the pitch of the blades, at the same torque, you can achieve the same takeoff acceleration and climb performance, and then transition to a cruise pitch up high where the same torque will pull you just as well (better since you can maintain your 155 HP and 349 lb-ft to a higher altitude).

John

I think you are comparing oranges to lemons.

The diesel Cessna was to build is in the 182 line. Thus compare a 230hp lycoming 100LL motor which at 2700rpm should produce ~445 lb ft of torque to the diesel 230hp at 2200 rpm(taken from Wikipedia as to specs) equals about 549 lb ft for the SMA diesel. I can not find published specs on the power of the SMA diesel at the crank.

I strongly suspect that the diesel may have altitudes where it outruns a NA Lycoming engine; at t/o it should out accelerate the Lycoming. It does have a drag penalty from the scoops, at altitude where the air is thinner that may cost less. Be interesting, if possible, to test a conforming 182T, a conforming 182S (that is the NA version) and the diesel.

RAS

To answer the question - what's wrong....

For that price tag I'm shopping for a used TBM/Meridian or even a MU2/Citation

Dead before it ever got started IMHO. Flight schools who can afford assets in this price range don't let students beat them up....

Peace,
Don

Cessna was at some point also talking about a Skyhawk JT-A. I don't know its current status. I used that because I had some data from it.

Based on the numbers from the Wikipedia article and other sources, torque is higher on the SMA engine at a calculated 542 lb-ft based on 227 HP at 2200 RPM (from the Wikipedia page) compared to the Lycoming TIO-540-AK1A at 235 HP at 2400 RPM which equals about 514 lb-ft. That should make the Diesel a monster in this airplane, with better takeoff, climb, and cruise numbers, draggy cowl or not.

So again I ask for stats on the Skylane JT-A vs the Avgas version showing that it's slower. I can't imagine how it could be.

I will give a mea culpa, I thought I recalled reading a couple reviews which complained about the performance. But at this point all I can find is a Flying article which seems to indicate fine performance.

So... if that's the case, it does seem too bad they're giving up on it. Hopefully they find a way to get their certification issues resolved.

Alright guys... All this talk about torque is BS when looking at an aircraft engine... (I spent nearly 15 years designing a wide range of IC engines and electric motors.)

Horsepower is all that matters - I can increase and decrease torque with a gearbox - not so with power. With a constant speed propeller, the engine will operate at some target speed and at wide open throttle / max fuel, will generate some level of power to the propeller - the prop doesn't know what's turning it (turbine, electric, CI, SI) except for the variation in torque pulses. (variation above and below the average torque). The prop will generate the same amount of thrust with the same input horsepower and RPM - no matter what - it's physics.

Now, everyone always brings up the stupid truck argument on torque... let me try to dispel this one... HP does work - PERIOD. Horsepower is work. Stop there. Stop confusing yourself.

Force across a distance is power. And we are MOVING vehicles across a distance. Power.

Accelerating a truck (or anything) requires horsepower. Holding a truck at 70mph requires horsepower. Go get your physics book and try to prove me wrong - you can't. I've gone through this with many engineers - none have won. Why do they have 2000ft-lbs of torque? Because diesel engines in the 50 to 200hp per cylinder range like to operate between 1500 and 3000 rpm for best efficiency. You get bigger, bring that speed down. You get smaller, that speed goes up. Combustion physics going on here. To accelerate a mass, I need force. (F=M*A). You may say - ahhh - got ya, Force is equivalent to Torque... Yes, but the rate at which you can apply that force is what matters. I can take your watch - yeah, the one on your wrist - and gear it so it can move that semi, but not very fast. In fact, I can gear your watch so that it will put out more torque than that big diesel, but it can't apply the torque at any reasonable rate for accelerating the truck from a stop light or keeping it moving down the highway any faster then the plate tectonics has North America crashing into California.

Why does everyone talk about torque with car engines? Well, when you couple the torque number with a horsepower and RPM, you get an idea of the performance of the engine with respect to RPM. If the peak torque and peak HP happen at very nearly the same point, you better have a lot of gears to keep the engine in that sweet spot. If you've got an engine that puts out peak torque at 1600 rpm and makes peak power at 3200 rpm (my pick-up), you don't need many gears and don't need to shift as often - you have a much broader range of useful power. I can leave my truck in 6th from 25mph, ease down the pedal and hit 113mph - never shifting. Power at RPM is all that matters.

Someone will bring up the jet - it's measured in thrust. Yeah, static thrust. If a jet engine could continue providing that same thrust at Mach 2 or Mach 4 - it's still power. It's got to take a certain mass of air and accelerate it across a distance - power.

Now, lets look at variation in output shaft speed - one engine is outputting 200HP at 2300 rpm, the other putting out 200 HP at 2600 RPM. (Wait, this sounds kind of like running lean of peak and using the same fuel flow at two different rpm's...) Same power, different shaft speeds (more torque at the lower speed.) Fact is, typically the slower turning prop will be slightly (1 to 2%) more efficient - and hence move the airplane faster or provide better takeoff performance or more static thrust or whatever condition you are comparing. This is miniscule though.

Diesels do not suffer from detonation risks - in fact, they are always detonating - more on that later. This is why they require additional structure. The other challenge faced by diesels is that they do run lean - very lean. My pickup runs at about 45 to 50:1 air to fuel cruising down the highway. They start to smoke around 20 to 22:1 and are pouring coal below about 18:1. So, to get the same amount of energy content into a diesel engine requires more air - hence turbocharging and larger displacement engines. (Heavier)

Now, lets talk cooling drag... The 182 (SI) has tons of cooling drag - our GA cowlings are horrible. Radiators can - if the design of the airflow passages are well done - can have very little drag. I would hope that Cessna did their work here. It doesn't look like they have done as much as they could have though - just looking at pics, but I haven't seen inside the cowling. Read up more here: http://ntrs.nasa.gov/archive/nasa/casi. ... 013485.pdf

Now, lets talk continuous performance and altitude performance... I would hope that Cessna had specified that the engine can run at continuous 100% power in this application - and based on the performance numbers, I would guess so. Unfortunately, Lycoming, Cessna, Franklin, and others - in trying to save weight among other goals limits our continuous power at best fuel economy settings due to detonation risks, etc... The diesel is turbocharged - not normalized, but charged. The turbo provides boosted air to feed the engine - and done well, can maintain sea level power for many thousands of feet, subject to a few limits - turbocharger shaft speed, exhaust back pressure / temperature and surge margin on the compressor. Remember, this turbocharger is trying to retain sea level boosted performance at altitude, a well designed turbocharger system should be able to do this into the teens. Automotive applications start reducing power at 6 - 8k, but this is because the turbo is sized for good transient performance (snap the pedal, low turbo lag), not cruise performance. I'll take 227HP at 15k over the current 182 engine at 15k any day.

Why is the power on these engines lower - weight... I would bet that it is the big limiting factor - to get more power you either need more boost or more displacement - both of which require stronger / bigger components. Or you start giving up life. I ran a 6.0 liter engine that put out over 900 HP, but it only ran for a few minutes at full power before we top overhauled (which only took 2 hours.)

Oh yeah, back to detonation... I bet some geek at the FAA certification side is screaming about this engine detonating and that you have to have so much margin from detonation for the FAA to certify it. Can't you guys fix this!!! (I had a Ford Chief tell our team to retard the glow plug timing to reduce NOx emissions - we just looked at each other and

I would put money that the real world performance results in:
Worse takeoff and climb performance until the SI engine derates (loses power wrt altitude) below the turbo diesel.
The turbo diesel cruises faster on less fuel than the SI engine above that altitude.
The diesel needs to carry ~20% less fuel - weight savings (oh yeah, this results in better performance).
(Assuming a lower HP on the diesel than the SI engine)

OK Andrew, enlighten me further:

The original comparison I gave was actually with the Thielert 155 HP and the Lycoming IO360.

The Lycoming makes 180 HP at 2700 RPM, which equates to ~350 LB-Ft of torque.

The Thielert makes 155 hp at 3890 RPM, which when geared down to 2300 results in ~350 Lb-Ft of torque.

If I'm the propeller, do I care which is turning me? I "feel" 350 Lb-Ft of rotational force whether it's from a Lycoming 360 or a Thielert 155. The latter takes a little more time to rotate me, but if the same force is being applied, can I not move just as much air, albeit at a lower RPM?