This was written in response to : Dead On Arrival: The Porsche PFM
>>> I'm sure that Alfred Scott would stand to differ about the
technical merits of the PFM:
>>> It's all just opinion, but I'm going to stand with Alfred on this one.
Hi, Bob. I wanted to better clarify my position on this, as I didn't have the time this afternoon.
I read this guy's treatise some time ago, and it made me blow steam out of my ears back then. In fact, I may have even read it when it was in the newsletter, as I used to subscribe. Matters not; it still gets me steamed every time I read it.
First off, the guy starts with a terrible bias. He's obviously against using non-aircraft engines for aircraft, and it shows. This treatise is nothing more than, "See! I told you so!" He starts with a bias against the engine, then proves his point.
I'm not going to hit this point-by-point; I'll just hit the high spots.
The very first "problem" with the engine is that it's different. His opening statements show this: it uses electronic ignition, it uses electronic fuel injection, it uses a fan instead of ram air, it has more moving parts than a comparable Lycoming, it's complicated, the engine is geared, "it looks like it was designed by NASA". He doesn't like:
- torsional dampers
- cooling fans
- twin camshafts
- two alternators and two batteries
- two extra cylinders with valves, guides, etc
(I noted he has no complaints with Lycoming 6-cylinder engines...)
So let's go here. Which of these is "bad"? Which of these parts, if you were designing an advanced engine, would you leave out? Which part of this is NOT an advanced design component? The only problem with this engine seems to be that it doesn't conform to the status quo.
I also note that, even though I don't know this fellow, he would be extremely unlikely to buy a car without all of these features (with the exception of the extra alternator and battery, required by the FAA to obtain certification.) Why is it good for your new car, but bad for aviation?
Because it's different.
It's heavier. Yes, it's heavier compared to the four-cylinder 201J that it complemented, but it's pretty damn near the same weight as the six-cylinder TIO-540 that replaced it.
Again, it's just different.
He hates the fan cooling system. Wow! That's one of the neatest features! Problem was, Mooney didn't get in there and design the aerodynamics package that they should have to take advantage of that air cooling system, and the airplane went slower on more power. Is that the fault of the engine design? I wonder what would have happened had Mooney re-hired Roy LoPresti to do it correctly...
Yes, it's different.
He hates the engine mounts?!? C'Mon! How else are you going to mount an AUTOMOTIVE-based engine that was not designed around a Dynafocal mount?!? And, I love it how he tells us that a Baron-friend of his broke two motor mounts and THAT'S OKAY because there's two more. Yet, no one says instead, "HEY! You're OK with this?!? Why don't you get Lycoming to engineer a motor mount that WON'T BREAK?!?!?"
Well, it's certainly different.
Then there's the PRICE difference. Hoo-boy, stand back for this one. The whole program was actually a coup for Mooney as they got Porsche to pay for the certification of the new, stretched fuselage. For YEARS, Mooney had wanted to get a longer fuselage approved, but they didn't want to shoulder the cost of the certification required. Mooney had been after Lycoming for years to share the costs of the airframe stretch as their customer base wanted a larger, six cylinder engine. Along comes Peter Schutz with a desire to get into aviation powerplants. Mooney jumps ALL OVER this opportunity to get their new stretched fuselage approved with minimal cash outlay and then turns around and certifies it with one of the Lycoming TIO-540's. Mooney seized the opportunity handed to it by Porsche.
So, what then did the PFM buyer get for that $100K large premium over the 4-cylinder airplane besides the prestige of the Porsche label and a really cool interior? They got the most advanced general aviation piston engine available, a much longer and more comfy fuselage, but unfortunately a poorly designed engine cowling that was nothing more than a cobbled-up variant of the one that Mooney was planning to use on the TIO-540-powered airplane to follow. Mooney gets the advertisement for a better airframe, its customers and engine supplier pay the bill for it, then they "fix" the problem the next year with a faster, smoother, comfortable, standard-aviation-issue turbocharged Lycoming 6-cylinder engine. It sure appears Mooney took Porsche to the cleaners for that little number.
Let's face it: in hindsight it sure appears that Mooney had no serious plans for a long-term relationship with this engine supplier, and Porsche went into this with blinders. Because of that, the engine was not given a fair shake in the marketplace, and the reaction among the "it's different" crowd was exactly what that guy on the web page said. Further, a lot of folks saw the word "Porsche" and expected INSTANT success (probably even Porsche themselves). Everyone went in with high expectations, and when those elevated promises weren't met everyone gave up. If, on the other hand, Mooney had committed to a reasonable relationship with Porsche and worked through both the minor technical and major marketing problems, we'd probably have many more aircraft flying around not only with an advanced PFM engine but also with a Toyota engine and possibly even a Honda engine (those guys looked at the fuddy-duddies in the market and said, "SEE YA! We're building personal watercraft instead...!")
Now I'll give you some points that Alfred was not capable of knowing back in 1988. First off, none of these engines ever came even close to the published TBO. Unfortunately, the aircraft were all sold with a guaranteed TBO, and the cost of the overhaul was etched in stone. Porsche got really tired of paying good money after bad to keep the aircraft flying. These are engineering issues that could have easily been resolved (does anyone care to wonder why Porsche has won LeMans for so many years, but rarely ever on the first try?) However, with no future sales, why invest the engineering time? The final straw was the Porsche and Piech families fought for control of Porsche, and the winner canned the aviation engines program before any further development could occur, and its internal champion, Schutz, was canned. It's the same old saw: the marketers got way ahead of actual engineering developments and what engineering could provide, and the customers rejected it.
But technically, the PFM engine, while not perfect, was a significant advance in aircraft engines. It would be a very good solution for our impending fuel crisis with 100LL and it most certainly would have gotten better economy and more power than anything comparable. Our automotive powerplant improvements over the last 25 years has proven this beyond a reasonable doubt: twice the horsepower and three times the fuel efficiency than the "super cars" of 30 years ago.
Instead, we in aviation stick our heads in the sand and demand that engine suppliers give us more and more, yet tell them we will not accept any changes from the status quo. It should be fun to see where we are 30 years from now, if we even have light aircraft flying...
Alfred Scott is/was an aviation Luddite (he's upset that our wings aren't made of wood anymore!!!) He won't be happy unless we're wearing billowing white silk scarves again. He was certainly not capable of leading an intelligent discussion on new aviation powerplant technology, although he writes well.
Of course, we still don't have an advanced powerplant 13 years after he wrote that. We're still using the same engines, ignitions, and carburetors or mechanical fuel injection systems. We're still using powerplant operating manuals written 30 years ago, and we're still using aviation fuel designed 40 years ago that will soon be unavailable. And we're still failing valves at alarming rates on our Lycoming engines, with the manufacturer blaming the pilots.
We just don't use those silk scarves any more.
Sorry for the rant.
I respect your opinions, and I do appreciate the time you've taken to express them, but I am in general disagreement with your theme.
In general, I think that general aviation airplane engines are about as good as their developers and manufacturers can afford to make them. Could they be better? Absolutely. Could they be as powerful, as light, and as efficient as car motors? Maybe. But only if we (as in you and me and other general aviation consumers) spent enough on engines to justify the sort of development money that goes into car motors. That's about $50 million to $150 million.
As for your claims about fuel efficiency, I will defer my opinion until I see these values expressed as horsepower per pound of fuel per hour. And even then, the sfc has to be weighted to account for the differences in powerplant installed weight.
As for the other claims, I am a firm believer that everything on an airplane that has moving parts must be as simple as its function allows. In my own structural and control system designs, I go to great lengths to subtract complexity at every opportunity.
> Which of these is "bad"? Which of these parts, if
> you were designing an advanced engine, would you
> leave out? Which part of this is NOT an advanced
> design component?
Since you have asked a simple, direct question, I will answer it. I will answer it from the basis of my experience as an airplane operator, and occasional mechanic. I will answer from the basis of my experience as a former motorcycle roadracer, builder, tuner, and pit crew. I will answer from my experience as a family auto fleet maintainer (3 Volvo, 1 Subaru, 1 Triumph) and aircraft fleet maintainer (1 O-290-D/G4, formerly 1 Conti A65).
Given about $3- $6 million in hand to develop a new gasoline airplane engine, this is how I'd probably end up answering those questions:
> - torsional dampers
Good question. Those thingies are pretty important if you've got a gear box that has to be protected from torsional harmonics that will cause erratic gear contact, and will brinnell or spall the gear tooth faces. But if I could design an engine that didn't have a gear box on the main drive, and that maximized the torsional stiffness of the main drive, I could save about 24 or 36 parts on the gear box, and another 8 or 12 parts for the torsional damper. To that end, I'd try to keep the crankshaft as short as possible. Sure, that slower turning engine has less volumetric efficiency. However, every part that goes into a reduction system is a potential point of failure, and to that end costs development money to design properly. Not even the nuts and bolts are a given.
> - two extra cylinders with valves, guides, etc
The smoothness of a 6-cylinder powerplant is compelling, and does realize many benefits in reduced wear from vibration. So I'd probably shoot for the shortest opposed 6-banger that I could get 350 cubic inches from, and work hard to keep the crankshaft stiff by using large hollow bearing journals.
> - cooling fans
I'd leave those at home. Fan cooling adds a lot of parts, and adds a whole new critical system. History seems to demonstrate that pitot pressure is more than adequate to meet the air flow demands of an air cooled airplane engine. History also demonstrates that airplane manufacturers do an indifferent job of developing cooling baffling, and that airplane operators do a fair to poor job of maintaining it. For my 350 cube 6, I'd develop a standard cooling shroud system that would include intake plenums and exhaust plenums. The shrouds would be closely-fitted composite parts, that are solidly attached yet easily removable for access. The shrouds would include provisions for an oil cooler, another stepchild system that many of not most airframe manufacturers approach indifferently.
> - twin camshafts
I'd consider twin camshafts, so long as that term might be considered to include one camshaft on each side of the engine. I would explore the possibility of having one head casting for each bank of three cylinders, each of which would include its own camshaft. However, thermal expansion issues would probably make that arrangement difficult to seal against oil leaks. I'd also look closely at valve train designs that would place one camshaft in each case half. This would shorten the valve train a bit, and would slightly complicate the casting and machining of the case halves. However, it would simplify the accessory case a bit - I could just arrange to plug one 'troneto into each camshaft.
I would certainly not spend much time considering designs that feature dual overhead camshafts. Since I've settled on a big slow-turning six, the stiffer valve train that you get from overhead or dual overhead cams is pure wastage. Wrapping it through 12K rpm exiting turn 11 at Sears Point, it would be essential. Loafing along at 2.5K rpm? I'll take the pushrods, please.
And how about those magnetos? A magneto is a great package. It's a coil and an alternator and a timing system and a distribution system all in one. Is it a bad thing that it was invented a long time ago? I don't think so. My magnetos would retain the built-in alternator and spark distribution system that have been standard features since God was a little girl. However, I'd also include an electronic timing system, and perhaps provisions for a manifold pressure sensor. I think that such systems are already in production, just not for airplanes. The thing I wouldn't do is have a huge distributed system that is dependant on other airplane systems. Where I've got a set of complex elements, I'm going to try to keep them all in one box.
> - two alternators and two batteries
Those can stay at home for all I care. I'll provide drive pads for a vacuum pump and a fuel pump, and also a dual-pulley output drive on the back of the accessory case, with rich provisions for the mechanical attachment of accessories. If somebody wants to attach dual alternators to the pulley, great for them. If they want quad-redundant batteries in the airframe, fine. But the engine doesn't want or need them, and as an engine developer it wouldn't be my problem.
And how about the electronic fuel injection? That, I would definitely borrow from the automotive world. However, there would be very little electronics to it. I would contract Bosch to develop and provide a variant of the K-Jetronic mechanical fuel injection system that graces so many mid-80s cars. It is based on mature technology, and has demonstrated good reliability and great effectiveness. One of the neat things is that, even though there are electronic elements to it, K-jet works acceptably well even if all the electronics fail. The one bit of electricity that K-jet demands is for the main fuel pump, which is a constant-displacement pump with a 60 psi output. I'd probably end up with one K-jet system for each side of the engine, each with its on metering kettle and fuel distributor. There'd be one engine driven fuel pump with its own regulator, and one electric backup. There would be a Bosch Lambda sensor for each exhaust system half, but as with traditional K-Jet the system would default to a slightly-rich mixture in the absence of the sensor or its electronics.
However, all of that stuff is pretty much moot. Like you, I'm sitting on the sidelines, waiting to see what the free market economy squeezes out of the current crop of diesel engine developers. Now that'll be interesting to watch.
Thanks, and best regards