TDI in an airplane - the way forward

Uberhare

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Sep 3, 2006
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Too many.
When it comes to an airplane engine,I think I'd rather have high RPM horsepower over
torque any day.
Not really....My Lycoming has a redline of 2700rpm! Mind you it's 320cu in with a 5 inch bore. Don't think I've ever seen a torque rating for it anywhere, but it makes 160hp.
 

Pat Dolan

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I really love the 'near certification' or 'very near certification' language used to describe new engine projects in general aviation. Apparently, from looking at the wreckage of chapter 13, chapter 11, and completely dead companies that were very near certification, this terminology is a clear display of the 90-10-90 rule. That is, the design, engineering, manufacturing of prototypes, acceptance, starting of a test program, getting the FAA to promise to work on it, getting the work done, going through any number of test, re-engineer, re-manufacture, test again scenarios, and getting to that 'very near certification' point will take at least 90% of your funds, provided you have a budget measured in cubic $$. Getting past that last 10% hurdle will take the other 90% of your funds (meaning: at least 80% more than you'll ever raise).
Seriously, the herculean effort of trying to get a new engine certified these days explains very well why we aren't seeing any new engines making it to the certificated airplane engine market.
And the fun part is this: even if you manage to get an engine certificated, you still need to get a certificate for each and every airplane model you want to install it into. So if you want to install it into an airplane, and want to change anything about that engine compartment, you'll need to certify it yet again.
I think the GA market is going to evolve into foreign manufactured airframes and experimentals, with the latter taking the largest portion. And companies like Deltahawk, well, there aren't really companies, there's only Deltahawk. The only reason they've survived this far is with sales to the military who don't really need FAA certification to fly a drone. They have expended all the resources of several well-heeled investors, and the only reason they've survived is by selling the company to someone else (the Ruud family if my memory serves). Yes, they have made some very small fortunes, by starting with very large fortunes.
But that is the way it is with airplanes.
I want to build one, someday. But I'll probably be forced to content myself with rebuilding a Corvair engine (which compare very favorably with the similar offerings from lycosaur engines), and if I want to really jazz it up figure out how to do a blown direct injected 2 stroke with liquid cooling. A wankel will be fun to work on in the spare time, if I can figure out how to do a reduction drive that doesn't weigh more than a substantial boat anchor like the Ross unit does.
Or maybe I'll just rescue TDIs, turn them into trucklets, and go feral pig hunting to feed the family.
Cheers!
PH
Yes, what you say is quite true. However, fortunately there ARE a few genuine companies who are actually capable both technically and financially to crack the FAA nut. SMA is the government of France, so they certainly can finance their certification projects, but more to the point, they are the ONLY one thus far to figure out that genav engines are far simpler when direct drive and only make sense when compression ignition. The 305/230 SHOULD have been installed in every airframe that needs that power and can manage the weight, but EADS/Socata/SMA/Renault was and is not very good at understanging the business climate in North America. When they say their 460 is nearing certification, I mean European, with FAA to follow. They have some exceptionally smart features (full mechanical backup, direct drive, air cooling, conventional layout) and there is no question they will get to where they have announced, not sure exactly when.

Austro was a fantastic example of Diamond starting from scratch and getting auto based diesels into certified production in a very short time. Proves it CAN be done. EPS is another good example - clean sheet of paper, have met and exceeded EVERY design goal from the onset, properly financed, not a lot of really deep pockets, but well on the road. I expect to see them selling their engines in 2018 strictly into the certified market (initially all retrofit).

Note how many clean sheet spark ignition designs have been certified recently...I count NONE (the last IIRC was Orenda - also BTW automobile based). If you are going to have a piston engine that makes any sense, it will be a legacy design with some updates (ignition and eventually injection) or it will be a diesel.

What kills things these days are three major factors: #1 is the massive fleet of existing airplanes vs. the shrinking ranks of licensed pilots. We won't have a significant market for new airframes in the certified range for many years or decades. #2 is the ridiculous price of almost everything. The Soloy STC for SMA is a good example. I can remember a time when Joe Lunchbucket could easily afford to slip into the Mom and Pop flight school and order up a shiny new 172 (although I never did figure out WHY one would do that!!!) on a single income and mortgage. Since we discovered real estate with no limits, the vast majority of families in larger urban areas have two people working flat out to pay for "location, location, location", plus our addiction to more "stuff" dramaticaly shrinking the pool of discretionary spending money for airplanes. Finally, #3 is the really big one: lawyers. When Cessna suspended production of light singles in the '80s, more than half of the cost of an airplane was insurance premiums! With an army of ambulance chasers looking to pounce on the weakest member of the herd, airframe and engine manufacturers are simply not smart to be doing business inside of the US borders.

Your comments on gear drives significant. IMHO anyone using a 4 cylinder engine and trying to use a gear drive is nuts. The nature of cyclical variation in torque of such an engine dictates that it be huge and have a phenomenally good drive coupling/damping. Even with the right number of cylinders (i.e. 5 or more for a 4 cycle, 3 or more for two cycle) remember that Lycoming and Continental both had certified, gear reduction 6 bangers back in the day. They are long out of production and overhaul costs are eye watering.

We can only hope that the re-write of FAR 23 will make the climate for aviation business better here than it has been for some time. But without addressing the legal liability lottery, not only aviation but a LOT of other manufacturing jobs and businesses will continue to be sourced offshore.

BUT: I predict that EVERY new genav engine for the foreseeable future will indeed be a diesel. The advantages are just far, far too great to continue wasting time trying to use a truly idiotic fuel and technology to fly higher, further and faster.
 

Pat Dolan

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Location
Martensville, SK
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2003 A4 Variant, 2015 Q7
When it comes to an airplane engine,I think I'd rather have high RPM horsepower over torque any day.
Horsepower is simply an expression of the amount of torque multiplied by RPM to produce power. The shape of the torque curve determines the shape of the power curve, but the limit on RPM is propeller efficiency. The larger the prop diameter, the more efficient it is. The limit on prop RPM becomes the speed of sound. As the tips go into the transonic region, the noise made and efficiency of the system becomes a BIG problem, thus the 6' props out there will all stop at 2700 or less RPM. Gasoline engines (and little automobile diesels) need to turn up a fair bit more RPM to make enough power to get the power/weight ratio needed to drag an airplane off of the ground. Problem is: those gear drives are exceedingly difficult to make reliable and economically viable. Thus, direct drive, and those PRM ranges (2500ish for 200ish HP engines) are duck soup for diesels to run sufficiently high boost to get the torque curve fat enough to make the required power at that speed - without having to be a monstrous displacement.
 

Powder Hound

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Under a Bridge, Crestview, FL, USA
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'00 Golf 4dr White 5sp, '02 Jettachero 5sp, Wife's '03 NB Platinum Gray auto(!)
Well said. Another big plus on the side of compression ignition is the promised demise of 100LL. Hard to fly an airplane that only allows that fuel if that fuel is not available. I think that's the only reason you can still get 100LL anywhere.

I like the idea of a turbocharged direct injected 2 stroke because to me it is probably the easiest way to get a reasonable amount of power (180-200hp range). I've seen people poo-pooing this claiming that 2 strokes are dirty, smoky, inefficient. That those people are sadly uninformed as to the current state of affairs is funny, since the comments came from member(s) of this forum who should know better than to regurgitate old obsolete information.

I think liquid cooling will be the best way to solve the cooling problem due to the waste heat generated by this scheme in the space that heat is generated. If I was able to do this with a converted Corvair, I just don't think the air cooling fins on the OEM cylinders and head are sufficient. And there might be a bonus weight differential since the structure required for the cylinder head on a valveless liquid cooled head (the blown 2 stroke is ported) is much smaller and lighter than any air cooled 4 stroke head.

But, my idea may never see the light of day. As always, massive funds that I don't have are required.

Oh, well. OTOH, a regular 100hp converted Corvair can easily run the smaller Zenith STOL air frames. Fun!

Cheers!

PH
 
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amafrank

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USA
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'06 TDi jetta, '09 TDi jetta wagon, 15 TDi GSw
Also used in British delivery vans for many years and the Chieftain tank both main and its smaller generator engine. The opposed piston seems like a great way to do 2 stroke and having driven a chieftain I'll say that they sound really cool. On the other hand there have been a lot of issues over the years with the details like most engines find. Two cranks with a connecting shaft/reduction create some reliability issues. The need for a blower to feed the air in is also a problem. There is no fail safe if the blower or its drive fail.

Some have said that reduction drives are no big deal and point out the engines used from pre WWII and later. The Merlin, Alison and others of that type-V12, High HP etc all used a reduction drive to keep their huge props in a more efficient speed range while allowing the engine to run at a higher rpm for better efficiency and power. They did work pretty well but if you look into the manuals and specs you find that the redrives had to be overhauled at riduculously short periods. The Merlin wasn't meant to run for 2000 hrs like our ancient tech lycoming/continental flats. A couple hundred hours on a Merlin was an exception rather than a rule. Part of this was the power output demanded from the displacement and part was the redrive.

Radials also used reduction gearing to keep prop speeds down. They normally used planetary gear type reductions so each gear and tooth had reduced loading along with spreading the load on the main gear symmetrically. Still something to consider when looking at running little engines fast with redrives vs direct drive slowpokes.

I love the diesels and would really like to fly one someday. My last Lycoming made 2000+ hours and was still flying well. 160HP O-320. I'd really like to see the torquey diesel with the much better efficiency drop the fuel requirements.

Frank
 

Pat Dolan

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Frank: Blower drives without backup are no different from prop re-drives that also have no backup - except the blower drives are a LOT less loaded and succeptible to harmonics (prop blades make great tuning forks). The Gemini you will note is essentially a 3 cylinder two cycle - that means overlapping torsional inputs, thus the cranks and drive gears can be considerably lighter than those in a 4 cycle 4 cyl - as that configuration suffers a complete torque reversal every 180 degrees making for a VERY heavy crank and HUGE potential to incite very large harmonics. BTW: this is why the prop reduction drive for a 27 litre Merlin is not much bigger than that for a 1.7 litre Thielert.

Oh...I should have added regarding the Gemini engines and backup: the 100HP engine is positive displacement pump (roots I imagine) but the 125HP is turbo compounded - to the extent that one supercharger could be considered and/or configured as backup to failure of the other.
 
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Powder Hound

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The reason they do it that way is to have the supercharger to provide positive boost at very low (read: starting) rpm, then at normal cruising speeds, the turbocharger takes over and allows an efficiency increase.

The Gemini engines sounds good, but I doubt I'll be able to afford it. For some reason, engine companies for aviation want $20k and up, probably a $25k floor by the time I'd be ready for one, and that even for the non-certified market.
 

Pat Dolan

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Location
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Let me tell you a little bit about the Corvair: The engine was originally designed to use the Reynolds 390 aluminum system to make the barrels (same stuff as the CanAm big blocks and the Vega) but the technology wasn't production ready, so they went cast iron. The intent was to compete with Continental 0-200!

Personally< I vastly prefer reduction drives for automotive engines - not just because of the power density, but because of crank loads (propellers need a LOT of torsional and radial as well as a bit of axial load capacity that automotive cranks and cases can't provide).

I had hoped the old Ross drive would do the job, but it seems it is a bit light in the gear department. There are other similar drives (used on rotaries with great success) but have not seen on corvairs. I have built literally hundreds of engines and drives (was once an airboat manufacturer) and to make even a simple timing belt or HTD drive, the weight can get out of hand. Even direct drive VWs, I used a separate torsional damper with a short prop shaft on BIG Timken taper rollers.

http://www.n56ml.com/corvair/donors.html

direct drive build: http://www.hainesengineering.com/rhaines/aircraft/corvair.htm

direct drive guru: https://flycorvair.net/2014/02/02/complete-engines-for-sale/
forget trying to do a poor boy conversion, use the right stuff, pay the price (no where near Rotax or C85 numbers)

and I can endorse that arrangement due to this:m m https://flywithspa.com/corvair-5th-bearing/
 

Doc37

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Jan 31, 2019
Location
Neosho,Mo
TDI
2015 VW Beetle Convertible
Diesel TDI and Reduction drive?

There are four CERTIFIED diesel engines on the market: the 1.7 and 2.0 Centurion from now defunct Thielert (reduction drive problems killed the company - they sold THOUSANDS of engines first), the 2.0 Austro (installed in Da40 and Da42 Diamond single and twin and the SMA 5.0 direct drive that is STCd in a bunch of airplanes (including C182). The sad part is that when Thielert went under, one of their projects was an Audi 4.2 with PRSD to replace big HP 6 bangers in larger light twins (300-400HP class) and was flying in a B50 (Beech Duke). The up side is that TCM has bought the rights to develop SMA technology to a useful stage - and will start at the 200HP range and go on to the 400 end (this is a modular engine that was designed from the start to be direct drive OR reduction drive, 4, 6 and I imagine (as in HOPE) 8 cyl as well.

I am hoping that before I am too old to hold a medical, I can bolt a pair of 400HP diesel into a Ted Smith airframe (either 680/685 Commander or better yet, one of those AND an Aerostar). If anyone goes 500HP or more (easy with gear reduced, 10 litre SMA design) it would be SO SWEET on a D18S with 232 gal tanks (better yet the 266 gal version with all fuel in the wings).

The last thing in the world a Diesel engine needs is a reduction drive, that is backwards thinking. What the VW TDI 1.9 engine needs is an augmented drive of 30%. Diesel engines produce their power at low rpms. The sweet spot on these engines is 1800 to 2000 RPM so what you need is three gears the drive gear 4' an idler gear and a propeller gear of 6"It will take off likea rocket and fly all day this way and get twice the fuel usage of an FAA screwed up engine configuration. You don't need more power you need more options in power transmission, i.e, a varidrive.
What a reduction unit does is slow down the prop so it doesn't go supersonic. The Diesel is and intelligen consideration with the 30% augmentation drive and with a varidrive unit the fun is endless. The major problem is they will fly for so much longer that you may forget to add fuel, And I know this will break your hearts but instead of engine anuals a Bi-Centenial one is overkill.
 

bhtooefr

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Location
Newark, OH
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...you do realize that at 1900 RPM, a stock ALH is only making 56 hp, right?

(And that's assuming it maintains absolute boost pressure with altitude.)

That would be way, way, way too much weight for only 56 hp, I suspect.
 

oilhammer

Certified Volkswagen Nut & Vendor
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outside St Louis, MO
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There are just too many to list....
Isn't it generally accepted that a good rule of thumb for an engine's peak efficiency is smack dab between its peak torque and peak horsepower?

If that is the case, the ALH is happiest right around 2900 RPM, which oddly enough seems to be the best bang for the buck for highway cruising speeds in 5th gear. The turbo is working, but there is no strain or full load fueling.

90hp peaks at 3800

155tq peaks at 1900

3800
-1900
=1900

1900/2 = 950

1900+950=2850
3800-950=2850

So, 2850 is the sweet spot.
 
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bhtooefr

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It'll typically be near the BMEP peak, which is the torque peak. In the ALH's case, it's right at the torque peak.



There are reasons for it to move around, though - BSFC minimum is around 2200ish on the CBEA/CJAA, where the torque peak is at 1750:



In that case, I'm guessing something like swirl patterns or volumetric efficiency being poor, but the turbo just cramming in so much that it has high torque below the optimal operating range of the engine.

Conversely, you've got this:



(ignore the red line and green 230 g/kWh area, those aren't for the engine I'm describing.)

Peak torque is at 3600 RPM, but the peak efficiency range is from 1500 to 3000 RPM or so.

There's a couple factors causing this.

The first is that the engine is a gasoline engine without lean burn, and with high static compression (13:1), but also late intake valve closing, which pushes out excess intake charge and reduces dynamic compression. Late intake valve closing reduces torque (through reducing effective intake displacement), but when combined with "too high" compression, also increases efficiency through a longer effective power stroke. However, it works better at lower RPM - at higher RPM, intake air velocity increases, and the air is actually still coming into the engine after bottom dead center, so late intake valve closing's effects are reduced.

The second is that this engine runs absolutely ridiculous amounts of EGR below 3600 RPM - as much as 21%. This acts to reduce pumping losses at light load even further, but reduces torque too. (And yes, this engine has intake clogging issues. Mine is a later revision that's even more efficient, but pulls 28% EGR! :eek:)

However, in an airplane, you have other considerations. Weight is incredibly critical, because weight directly translates to aerodynamic drag. If your plane needs 90 hp to fly at your desired speed with a single ALH's weight, then you're almost certainly better off running it at 3750 RPM, rather than setting it up for 1900 RPM and then needing two engines (which means more weight, which means more power needed) and flying them each at 50 hp (or more).
 
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Powder Hound

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'00 Golf 4dr White 5sp, '02 Jettachero 5sp, Wife's '03 NB Platinum Gray auto(!)
The best fuel efficiency point for the ALH was at 1900 rpms, according to a map published back in '99 or 2000, IIRC. It was a BSFC (brake specific fuel consumption) map.

Cheers,

PH
 
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