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#31
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wrote in message ... In rec.aviation.owning Mike Rapoport wrote: wrote in message ... snip There's lots of ground turbines under 400hp so we know there's a market there; i.e. they must be practical and competive with pistons or they wouldn't sell. A lot of them are used to power natural gas compressors way out in the middle of nowhere and reliability is much more important than fuel efficiency and you have a large suitable fuel supply availible. True. So put it this way, if it were the turbine makers instead of the diesel makers that jumped on this bandwagon, what would be their smallest engine? Given the high initial cost of turbines and the hgiher fuel comsumption, I doubt that turbines would be competitive with gasoline engines given current price differentials between the two fuels. The beauty of a diesel aircraft engine is that it should cost the same as a gas engine, has fewer parts, uses less fuel and lasts longer. The turbine engine is more reliable but costs more and uses more fuel. The lower the hp the less competitive the turbine gets against the diesel. Your first sentence overlooks the fact that turbines are currently competitive at the Caravan level, but I pretty much agree with the rest. I don's see that I've overlooked something relative to the Caravan. The Caravan has a 940hp engine. There is currently no suitable piston engine to power such a large, single engine airplane. It couldn't be anything other than a turbine. OK, let's say I buy into about 400hp as the "up to now" crossover point. Even 400hp is not an economic crossover. It just represents the limit of what is practical in small aircraft turbine engines. The 400hp Allison turbine is really a helicopter engine anyway. The smallest practical application seems to be the around the Meridian/Caravan/TBM 700 size range and these engines are all around 1000hp. The engineers designing airplanes are not totally stupid, if it made sense to install 400hp turbines they would do so. Given the current fuel cost differential, where would you expect that point to move to assuming the engines were available? It depends on how powerful diesels get for aircraft. Under several thousand horsepower the diesel will always be cheaper and more fuel efficient than anything else. There probably isn't an economic crossover point for gasoline engines either unless the fuel price spread is artificially raised even higher than it is now. You have to remember that the HSI and overhaul costs on turbines is much greater than the cost of overhaul on a piston engine. Given that the small turbine is going to consume a lot more fuel and cost more to build and maintain it will never be cheaper. Turbines will be used in applications where cost is a secondary consideration to high power and high reliability. The gas turbine is a mature 60yr old technology, huge improvements in cost or efficiency are somewhat unlikely. For a really efficient turbine see http://www.turbokart.com/about_ge90.htm Jim Pennino Remove -spam-sux to reply. |
#32
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"Mike Rapoport" wrote:
A diesel doesn't cost any more that a piston engine. A STC'd conversion costs more but, in a new airplane the cost should be the same. Right. Some people seem to think that diesels are somehow magic. The basic construction of a diesel and a gasoline engine are almost identical. The only differences I can think of a 1) Higher compression ratio. This could be done with a longer-throw crankshaft, a taller piston, a lower head, or some combination of all three. 2) A fancier (higher-pressure) injector pump. 3) No spark plugs. Which means no ignition system (be it electronic or magnetos). 4) Possibly the addition of some kind of starting assist such as glow plugs. The biggest problem I can see with a diesel is cold-weather operation. I used to have a diesel car (1980's era VW Rabbit). It was a bitch to start in really cold weather. If the glow plugs were in good shape, you were fine down to about 20 F. Once you got down below about 10 F, you probably weren't going to get it started without a preheat. Somewhere down around 15 F, normal diesel fuel starts to gel. These are temperatures commonly experienced aloft even at the altitudes spam cans fly at in the winter in temperate climates. It would be real bad news to get the engine going, only to have the fuel gel up in the tanks when you reached cruising altitude. But, I suppose the Jet-A folks have figured out the right additives to solve that problem. |
#33
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In rec.aviation.owning Roy Smith wrote:
"Mike Rapoport" wrote: A diesel doesn't cost any more that a piston engine. A STC'd conversion costs more but, in a new airplane the cost should be the same. Right. Some people seem to think that diesels are somehow magic. The basic construction of a diesel and a gasoline engine are almost identical. The only differences I can think of a 1) Higher compression ratio. This could be done with a longer-throw crankshaft, a taller piston, a lower head, or some combination of all three. 2) A fancier (higher-pressure) injector pump. 3) No spark plugs. Which means no ignition system (be it electronic or magnetos). 4) Possibly the addition of some kind of starting assist such as glow plugs. The biggest problem I can see with a diesel is cold-weather operation. I used to have a diesel car (1980's era VW Rabbit). It was a bitch to start in really cold weather. If the glow plugs were in good shape, you were fine down to about 20 F. Once you got down below about 10 F, you probably weren't going to get it started without a preheat. Somewhere down around 15 F, normal diesel fuel starts to gel. These are temperatures commonly experienced aloft even at the altitudes spam cans fly at in the winter in temperate climates. It would be real bad news to get the engine going, only to have the fuel gel up in the tanks when you reached cruising altitude. But, I suppose the Jet-A folks have figured out the right additives to solve that problem. Because of the higher compression ratio, a diesel has to be built "beefier" than a gas engine to last as the automakers found out when they tried a direct conversion on their gas engines in the 80's. All the aircraft diesels have a constant speed prop and FADEC. If gelling of Jet-A were a problem, airliners would be falling out of the sky on a regular basis. -- Jim Pennino Remove -spam-sux to reply. |
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#35
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In rec.aviation.owning Roy Smith wrote:
wrote: Because of the higher compression ratio, a diesel has to be built "beefier" than a gas engine to last as the automakers found out when they tried a direct conversion on their gas engines in the 80's. Well, the Rabbit I had was built with exactly the same block, pistons, crank, etc, as the gas version. The basicly just slapped a shallower head on the thing to increase the compression ratio. We drove the car into the ground at about 160k miles. We replaced pretty much all of the accessories (starter, water pump, alternator, radiator, etc) at least once, and the clutch wore out at about 110k, and the body was more rust than steel, and the electrical system was a mess, but the core engine was just fine. The only thing that ever happened to the engine core was a blown head gasket, but that was really my fault. We had chronic overheating problems due to a leak in the cooling system that we didn't fix for a while. Eventually, the gasket said, "OK, if you want to keep abusing me like that, I'm outta here". The debacle I'm talking about was Chevey's (?) attempt to power pickups with a gas engine converted to diesel by basically the same method. You can get away with this if the basic engine is strong to start with and you're not trying to pull too many horses out of it. -- Jim Pennino Remove -spam-sux to reply. |
#36
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#37
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Well, the Rabbit I had was built with exactly the same block, pistons, crank, etc, as the gas version. The basicly just slapped a shallower head on the thing to increase the compression ratio. I could have sworn the Rabbit Diesel had a way different engine, but I could be wrong. The debacle I'm talking about was Chevey's (?) attempt to power pickups with a gas engine converted to diesel by basically the same method. Not pickups AFAIK; station wagons and maybe sedans. This was Roger Smith at his finest. A friend bought one with a dead@55000 mile engine. It was an stock gas block; no where NEAR beefy enough. The blowby was so bad, the engine soiled itself at every seal; he'd get 250 miles to the quart; all leakage. At least it didn't rust! It had a one-of-kind starter and flywheel. The distributor was replaced with a vacuum pump to drive the HVAC door flaps. It had dual batteries, designed wrong. The brakes were run off the PS pump, so when the engine stalled, stop NOW. He put in a gas 350 and drove it for 10 years more. -- A host is a host from coast to & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 |
#38
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In rec.aviation.owning David Lesher wrote:
Well, the Rabbit I had was built with exactly the same block, pistons, crank, etc, as the gas version. The basicly just slapped a shallower head on the thing to increase the compression ratio. I could have sworn the Rabbit Diesel had a way different engine, but I could be wrong. The debacle I'm talking about was Chevey's (?) attempt to power pickups with a gas engine converted to diesel by basically the same method. Not pickups AFAIK; station wagons and maybe sedans. This was Roger Smith at his finest. A friend bought one with a dead@55000 mile engine. It was an stock gas block; no where NEAR beefy enough. The blowby was so bad, the engine soiled itself at every seal; he'd get 250 miles to the quart; all leakage. At least it didn't rust! It had a one-of-kind starter and flywheel. The distributor was replaced with a vacuum pump to drive the HVAC door flaps. It had dual batteries, designed wrong. The brakes were run off the PS pump, so when the engine stalled, stop NOW. He put in a gas 350 and drove it for 10 years more. -- A host is a host from coast to & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 There were pickups. A friend bought one new and had the engine blow at about 40k miles. Thanks to California smog laws, he found his choices were replace it with another new diesel (big bucks) or get an old gas engine and convert it to propane and try to recover some of the investment. This was way before 50k warranties. -- Jim Pennino Remove -spam-sux to reply. |
#39
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#40
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