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#1
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Engines and Reliability
Things that make you go 'hmm'.
The accepted wisdom is that aviation engines are tough, because they can be run at full rated power for hours on end, and auto engines are fragile, and must not be thrashed or they won't last very long. It was even mentioned in a thread in the last couple of weeks - I don't remember who said it, but they said "operate your car engine like that and it wouldn't last half an hour". I've never really thought about it, but this weekend I learned how to drive the winch at the glider club. After a few launches it got me thinking - this thing about aero engines vs car engines is probably an old wives tale, possibly promulgated so people don't feel so bad about spending so much money on aircraft engine parts when yet another cylinder is cracked. The winch. Basically, the winch is a method of launching gliders. At one end of the runway, you have the glider. At the other end, attached to the best part of a mile of steel cable is the winch. This consists of a take-up drum which reels the cable in, and a power plant and transmission of some sort. Our winch is a 'homebuilt'. The power plant and transmission came straight off a car - a mid-70s Jaguar XJ6. In this era, Jaguar quality was at its worst. Jaguar was part of British Leyland, a nationalised car making monstrosity, beset by problems with trade unions and appaling quality control. The engine is a 4.2 litre inline six with dual overhead camshafts, and dual SU carburettors. The transmission is the standard 3-speed automatic. Winching a glider means you go from idle, rapidly increase the power, then as you see the glider pitch up to about 45-50 degrees nose up, floor it. The glider rockets skywards. Wide-open throttle is held, with the transmission in drive which will select the appropriate speed for the drum. As the glider starts getting towards the top, power is eased back, eventually reaching idle. The cable then comes down on a parachute. A little power is required (especially if there's a crosswind) to bring the chute and cable in and make sure it lands on the runway. The engine is tortured by this idle - wide open - idle cycle maybe 30 times a day when we're operating. Since 4-star leaded fuel (98 octane) which the engine was designed for is no longer available, the engine is now run on regular unleaded. The engine and transmission is nearly 30 years old, it's been in the winch for a few years and it hasn't missed a beat. It starts easily, and runs sweetly despite the abuse it gets - and it was built during Jaguar's worst years. Perhaps auto engines aren't as feeble as people like to make out? -- Dylan Smith, Castletown, Isle of Man Flying: http://www.dylansmith.net Frontier Elite Universe: http://www.alioth.net "Maintain thine airspeed, lest the ground come up and smite thee" |
#2
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"Dylan Smith" wrote in message
... [...] Perhaps auto engines aren't as feeble as people like to make out? I've never seen anyone claim that auto engines were "feeble" or "fragile". What they ARE is complex, and lacking in the necessary redundancy to be operated in an airplane. They are generally too heavy, as they are made stronger by adding material. They also lack an interested party to go to the effort to certify them for aviation use. Even with those impediments, some auto engines are being translated into aviation use. Of course, the aviation-certified ones are significantly different from their auto-based ancestors in some respects, but in many other respects they are very similar. And of course, in the homebuilt arena, many people operate auto engines for extended periods of time quite successfully. Now, all that said, one engine does not a proof or disproof make. Just because your Jaguar engine running your winch has held together for 30 years, that doesn't mean that engines generally do well under that kind of stress. Also, I seriously doubt that the engine has been left unmaintained for its entire lifetime. In fact, I'd guess that someone is taking care of it, and an engine that's being properly maintained can survive quite well. Finally, without a doubt there are *some* auto engines that would never survive in an aviation environment, and which *would* self-destruct in a short period of time if operated for hours at a time at 75-80% power, with extended 100% power climbs. Perhaps your strawman was every bit as flimsy as one might have expected from a strawman. Pete |
#3
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In article , Peter Duniho wrote:
"Dylan Smith" wrote in message ... [...] Perhaps auto engines aren't as feeble as people like to make out? I've never seen anyone claim that auto engines were "feeble" or "fragile". The inference was certainly there. (A recent thread, to paraphrase, one poster wrote "if you operate your car engine at 100% rated power, it would be toast in less than half an hour". That certainly suggests the poster thought auto engines were a bit fragile). What they ARE is complex, and lacking in the necessary redundancy to be operated in an airplane. snip That's not part of the discussion; the discussion is whether auto engines can be operated as 'harshly' as aircraft engines and survive. other respects they are very similar. And of course, in the homebuilt arena, many people operate auto engines for extended periods of time quite successfully. Which also demonstrates that at least those auto engines (I know the opposed Subaru engines are quite popular, as some Ford type, the Mazda rotary and Volkswagen air cooled engines) can be operated at high power outputs for long periods of time. stress. Also, I seriously doubt that the engine has been left unmaintained for its entire lifetime. In fact, I'd guess that someone is taking care of it, and an engine that's being properly maintained can survive quite well. Of course it's being maintained. Again, maintenance was not something I mentioned in the original post. (I strongly suspect our winch engine spent some time sitting in a rotten-out Jag that nobody had driven for a while though, I don't know as I wasn't there when it was put in the winch). Finally, without a doubt there are *some* auto engines that would never survive in an aviation environment, and which *would* self-destruct in a short period of time if operated for hours at a time at 75-80% power, with extended 100% power climbs. I'm not sure there are, certainly in the 100-200hp category. I certainly can't think of an auto engine that's too fragile to be operated at high power for hours on end in that bracket. Certainly not fitted to any modern car. Perhaps your strawman was every bit as flimsy as one might have expected from a strawman. It wasn't a straw man - the thrust of the argument was that maybe it's an old wives tale to generalise that auto engines can't be run at high power for prolonged periods or run in punishing regimes without self-destructing, which seems an oft-repeated Usenet "truism" (which tends to surface in the auto-vs-aero engine debates on r.a.h) -- Dylan Smith, Castletown, Isle of Man Flying: http://www.dylansmith.net Frontier Elite Universe: http://www.alioth.net "Maintain thine airspeed, lest the ground come up and smite thee" |
#4
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Dylan Smith wrote:
The accepted wisdom is that aviation engines are tough, because they can be run at full rated power for hours on end, and auto engines are fragile, and must not be thrashed or they won't last very long. It all depends what "full power" is. I remember a marine diesel (forgot the brand, maybe Volvo), which was available in two variants: Heavy duty and light duty. Both were basically the same engine, only the heavy duty model developed about 40% less power. Of course it could be run at "full power" the whole day long! Now imagine a car with a 360 cubic inch engine that develops a mere 180hp! Stefan |
#5
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Now imagine a car with a 360 cubic inch engine that develops a mere 180hp!
Now imagine a car with a 1200 cubic inch engine that develops a mere 1800hp! At 2200rpm..........., and only the first little bit of boost! ;) Peter |
#6
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It's called "operating parameters"...
An aircraft engine is designed to meet one set of parameters, a land vehicle engine is designed to meet another set, and a marine engine is designed to meet still another set. And you can have additional sets of parameters within these broad categories: economical operation in a passenger automobile, towing capacity in an SUV, sustained high speeds in a NASCAR racer. Obviously, other power train components have to be factored into the equation. You can "sledge hammer" just about any engine into just about any vehicle and "Rube Goldberg" it into operating, but the results will probably not be very satisfactory. The key to long life and satisfactory performance from an engine is to use it within it's design parameters. Aircraft engines are designed for sustained, high RPM operation, automobile engines for a mix of stop and go driving and sustained high speeds. And as long as the engine is operated within it's design parameters you should receive satisfactory service. "Dylan Smith" wrote in message ... Things that make you go 'hmm'. The accepted wisdom is that aviation engines are tough, because they can be run at full rated power for hours on end, and auto engines are fragile, and must not be thrashed or they won't last very long. It was even mentioned in a thread in the last couple of weeks - I don't remember who said it, but they said "operate your car engine like that and it wouldn't last half an hour". I've never really thought about it, but this weekend I learned how to drive the winch at the glider club. After a few launches it got me thinking - this thing about aero engines vs car engines is probably an old wives tale, possibly promulgated so people don't feel so bad about spending so much money on aircraft engine parts when yet another cylinder is cracked. The winch. Basically, the winch is a method of launching gliders. At one end of the runway, you have the glider. At the other end, attached to the best part of a mile of steel cable is the winch. This consists of a take-up drum which reels the cable in, and a power plant and transmission of some sort. Our winch is a 'homebuilt'. The power plant and transmission came straight off a car - a mid-70s Jaguar XJ6. In this era, Jaguar quality was at its worst. Jaguar was part of British Leyland, a nationalised car making monstrosity, beset by problems with trade unions and appaling quality control. The engine is a 4.2 litre inline six with dual overhead camshafts, and dual SU carburettors. The transmission is the standard 3-speed automatic. Winching a glider means you go from idle, rapidly increase the power, then as you see the glider pitch up to about 45-50 degrees nose up, floor it. The glider rockets skywards. Wide-open throttle is held, with the transmission in drive which will select the appropriate speed for the drum. As the glider starts getting towards the top, power is eased back, eventually reaching idle. The cable then comes down on a parachute. A little power is required (especially if there's a crosswind) to bring the chute and cable in and make sure it lands on the runway. The engine is tortured by this idle - wide open - idle cycle maybe 30 times a day when we're operating. Since 4-star leaded fuel (98 octane) which the engine was designed for is no longer available, the engine is now run on regular unleaded. The engine and transmission is nearly 30 years old, it's been in the winch for a few years and it hasn't missed a beat. It starts easily, and runs sweetly despite the abuse it gets - and it was built during Jaguar's worst years. Perhaps auto engines aren't as feeble as people like to make out? -- Dylan Smith, Castletown, Isle of Man Flying: http://www.dylansmith.net Frontier Elite Universe: http://www.alioth.net "Maintain thine airspeed, lest the ground come up and smite thee" |
#7
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Aircraft engines are designed for
sustained, high RPM operation, automobile engines for a mix of stop and go driving and sustained high speeds. And as long as the engine is operated within it's design parameters you should receive satisfactory service. Even my old Ford "tractor motor", a pushrod 250 cubic inch six, purrs along the highway at a designed (and legal in most of Australia) 100kmh (55% cruise) at about 3000rpm in 4th gear, drop it back to a economical 5th gear (look, a variable pitch prop!) and I can crank it out to a whole lot more than that for a good highway run, even without the speed cameras it will do 200kmh (Vne?) and sit on 160 kmh (75% cruise?) in the long roads in the Northern Territory (unrestricted speed limit) even in summer, overtake many another vehicle and plenty of hills that cause it to climb and descend, from full throttle back to idle. Mind you, it has had the odd problem over the years, but most of them have been expected items for motor cars like: water pump, fan belt, spark plug leads, alternator, flat tyres and so on. If it had the same work done to it that most aircraft get, like a 100 hourly service, if it received a proper runup in the morning before heading off to work, (Bloody hell, I'm late again! Better hit the loud pedal!), or had the oil checked more often than once a month if I remember, or when the oil light comes on, whichever occurs first, if it had a fresh batch of pretty green coolant and a fresh radiator hose or two, at least every other year, then it would have run as nicely as any aircraft engine that I've played with. The only reason I'm not going to put one in my homebuilt is that I don't feel like coming in for a forced landing strapped to that dirty great big chunk of steel! Hope this helps, Peter |
#8
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On Tue, 29 Jun 2004 07:32:04 -0000, Dylan Smith
wrote: Perhaps auto engines aren't as feeble as people like to make out? Maybe it's time for me to post the article written by a Detroit auto engineer about exactly what kind of torture auto engines go through during their development and testing. He was in charge of GM's "Premium Engine Program" and wrote this article for "Contact!" magazine about 8 years ago. Begin article: "PERFORMANCE The engine in production form for 1999 develops 215 HP at 5600 RPM and 230 foot pounds of torque at 4400 rpm. As a routine part of an engine development program we tested the engine at full power, maximum RPM. We ran it at 6000 RPM, pulling 215 HP at wide open throttle, for 265 hours. That's a continuous 265 hours of wide open throttle, far worse than autobahn driving, because even on the German Autobahn, you wouldn't be at 6000 RPM. THAT IS A STANDARD DURABILITY TEST. (emphasis mine) We run many engines through this test as a matter of course. Specific development focus is on the crank, pistons, rods, block structure, timing drive wear; we get a lot of full load cycles in a hurry. It isn't necessarily designed to replicate customer driving but to get development answers. Wear and fatigue are accelerated. The test is particularly applicable in proving out dampers and their effectiveness. If the damper is not properly tuned to the engine the crankshaft will inevitably break in that time period. (note, this is evidence you should not discard the stock damper when using the auto engine for aircraft power) A number of other engine tests are utilized. We use a variety of specific tests to accelerate engine wear and to look at fatigue failures. The cyclic endurance test is now called PTED (power train endurance). It closely approximates cyclic durability. The engine is cycled from its torque peak to its horsepower peak, at wide open throttle, then down to idle, then accelerates up to shift points, then back down to the torque peak and then horsepower peak. This test is run for 400 hours. Once again, it's a wide open throttle test for 400 hours. The RPM for this engine, ranged between 4400 and 6000 RPM, back and forth in about a 5 minute cycle. The dyno computer will occasionally bring the engine down to idle, up to 6500 RPM shift points, and then back to the 4400 - 6000 RPM 5 minute cycle. Thermal cycle tests are run to define engine capability under cold weather condition. We run the engine at full throttle at 4000 RPM, bring it down to idle, stop it, switch the coolant valves to drain the hot coolant, pump the chilled coolant from the chiller until the metal temperature stabilizes at 0 degrees F. Frost forms on the outside of the block, as the cold coolant rushes into the engine. When it stabilizes at 0 F, we motor the engine, start it, come to full throttle at 4400 RPM, the valves switch and the coolant temperature starts to climb. It climbs back up to 260 degrees F. It takes 10 -11 minutes to complete one cycle. The engine must pass 600 cycles without any sign of failure. We typically run 1200 cycles and a probe test will run 1600 cycles. That's a (sic) excellent gasket killer test. Head gaskets are the first to fail because of the rapid expansion and contraction. A powertrain endurance test simulates in-vehicle operation. The Ypsilanti plant uses it for testing transmission. We, of course, use it to look at engine performance. The equipment consists of an engine/transmission combination, which sits on a dyno with large steel inertia wheels. The inertia wheels are being driven by the transmission output shaft, just like in a car. They cycle is brutal; the engine is at idle in gear. The engine accelerates wide open to 6200 RPM, upshift occurs, 6200 RPM is reached, upshift occurs to 3rd, 6200 RPM is reached, upshift occurs to 4th, the wheels turn up to 135 MPH depending on the application. The second half of the cycle calls for a closed throttle down to 70 MPH, then wide open throttle with a downshift to 2nd, the engine goes back up to top speed, coasts down so that the transmission selects down to a lower range. The engine is in an overrun condition all the way down to idle; i.e., the engine is being used for braking. That's one cycle. One transmission life cycle is typically 12K - 13K cycles of the above test. We will run an engine through 4 or 5 transmissions. This is a very harsh schedule for the engine, particularly because of the overrun braking. Cylinders and rings suffer the most on this test. We run some idle tests to verify low speed operation. The engine is run at idle for about 2000 hours to make sure of adequate oil flow at idle. We use all those engine tests in addition to fleet tests and extensive vehicle road testing. The customer can be assured that the PV6 engine is a thoroughly tested advanced design that matches or exceeds competing offerings." End of article This is merely typical of auto engine development, all the manufacturers run some variation of this kind of torture. BMW and Mercedes probably beat up their engines even worse. As Peter Duniho pointed out, auto engines have been in use in the homebuilt community for many years now. One guy managed to run his Ford 3.8L engine to 2,000 hours before tearing it down for a rebuild. He found no significant wear in any of the engine components, but he replaced the Gates toothed belt for the PSRU anyway. The engine tear down and rebuild, including the belt, cost him around $1,000 if I'm remembering correctly. Corky Scott |
#9
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The limiting factor in continuous high power engine operation is cooling. If you put an auto engine in PARK, depress the accellerator and run the engine at high RPM, the heat will build up and the engine will self destruct in (I'm guessing) less than an hour. Take that same engine and head down the interstate with the cruise control on and the temperature gauge will stay in the middle of the range and the engine will run as long as the fuel lasts. Guess what happens to your aircraft engine during ground operations? Get it up in the air with the air flowing through the cowling and it will be happy, too. |
#10
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Dylan Smith wrote
The accepted wisdom is that aviation engines are tough, because they can be run at full rated power for hours on end, and auto engines are fragile, and must not be thrashed or they won't last very long. It was even mentioned in a thread in the last couple of weeks - I don't remember who said it, but they said "operate your car engine like that and it wouldn't last half an hour". In reality, that's nonsense and everyone knows it. The standards of testing used by Detroit (never mind the Japanese) are way tougher than anything the FAA ever thought about doing. This has been discussed extensively on rec.aviation groups. Check out this link, or just google it yourself: http://groups.google.com/groups?hl=e...30% 26hl%3Den I've never really thought about it, but this weekend I learned how to drive the winch at the glider club. After a few launches it got me thinking - this thing about aero engines vs car engines is probably an old wives tale, possibly promulgated so people don't feel so bad about spending so much money on aircraft engine parts when yet another cylinder is cracked. That's basically it. The aviation engines we use are obsolete. Sure, they're more reliable and have better power-to-weight ratios than auto engines - as long as you compare them to contemporary auto engines. That's contemporary to the design age - meaning somewhere in the 1950's-1960's. At that point, real aviation went to gas turbines, the designs of aviation piston engines were basically frozen, the engineering talent went away, and no further progress was made. When was the last time you heard of a modern (made in the last 10 years) auto engine that just died without giving weeks (or months, or years) of warning? Not got stalled by a ham-fisted shifter or run out of gas, but actually died? I'm sure it's happened, but it's a huge rarity (whereas in 1955 it wasn't). On the other hand, I don't know ANYONE with more than 2000 hours in piston GA who hasn't had an engine failure. These things eat valves, they crack jugs, they throw rods, their carb floats sink - you name it. The truth is that aviation and auomotive use are very, very different. They have very different duty cycles, cooling requirements, and performance requirements. It really should not be a viable proposition to adapt automotive engines to airplanes - you will wind up with engines that are overdesigned in some areas and underdesigned in others. However, automotive engines have advanced tremendously in the past half century; piston aviation engines have not. Now we even have manufacturers actually using autmotive cores (which are optimized for a completely difference application), dressing them up for aviation, and selling them - that's how Thielert works. It's very much suboptimal - but the existing engines from Lycoming and Continental are such disasters that even this is a viable business plan. Homebuilders have been adapting automotive engines for years. The safety record has been abysmal. Oddly enough, it's never the engine core that fails. It's always fuel systems, ignition systems, reduction drives - all the stuff it takes to make the conversion. In other words, all the stuff that is amateur-designed rather than professionally engineered. That's why I'm not a great fan of auto conversions. It's not that there's anything wrong with the engines - despite being suboptimal for aviation, they're way out in front of the crap Lycoming and Continental are selling, even for aviation applications. The problem is that adapting the engine for aviation use is a major task, and not something to be done by a garage mechanic on the cheap. Michael |
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