Hmmmmmmmmmm...

Downeaster

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I went on an overnight trip to fix a jet many years ago. Two mechanics assigned to that station told me a story. One night at 3 AM they went to lunch at a nearby Waffle House. They took the company Ford Econoline van. This was the old one with the six cylinder engine between the seats. At the restaurant were a couple of hot rodders who'd probably come in after the bars closed. They were talking smack. The two airline mechanics joined in and stated that they could beat them with their van and "put up, or shut up!" After the meal, they all stepped out together. The van contained a jet starter. That's a turbine engine big enough to provide air to start the engines of a heavy jet. As they all stepped into the parking lot one of the mechanics engaged the start of the jet starter. So, there was turbine whine and a big fireball out the roof of the van as the turbine lit off. GAME OVER. The two hot rodders immediately declined to engage any further.
:laugh:
 
Idle curiosity: How fuel-efficient are turbines compared to IC engines, horsepower for horsepower?
As I understand it, turbines went into widespread use more for reliability than fuel economy. A jet engine stays on wing far longer than any recip ever did. I believe jet fuel is also a lot cheaper than gasoline. I got into the business well into the jet age. Not many of the recip guys are left.

To the best of my knowledge turbine engines use more fuel than recips. However, big recips are gone and there is no longer investment in that technology (for large aircraft). Turbine engines continue to get bigger, make more thrust and use less fuel. They also stay on wing longer and longer. So, for me, the answer isn't straight forward. Turbines are cheaper to operate, although I admit I don't have the numbers. We're just yacking it up at the bar. Consider DC3's still flying and retrofitted with turbines.
 
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Idle curiosity: How fuel-efficient are turbines compared to IC engines, horsepower for horsepower?
Well I do know that we didn't break the sound barrier with an ICE, can't speak to the actual math though. GT has a far greater HP potential than IC, but I think fuel efficiency varies wildly across the GT world. Most medium sized Navy Vessels have gone they way of Gas Turbine, especially Frigates, Destroyers, and LCS's (littoral combat ships).
 
Back when I was working Health and Safety, my team was responsible for emergency preparedness on a big manufacturing site - one of several gensets we had was turbine powered, and even with plugs and cans, it was LOUD in that room! I couldn't be in there for more than 30 seconds or so without being physically uncomfortable. Thankfully we had infrequent run tests.
 
How are small gas turbines started?
Electric starter or a starter/generator. I don't have any idea how much RPM is required. Big airplane turbines use air start motor with air provided by an APU that was started by a DC electric motor. My knowledge and experience is limited to heavy jets.
 
Idle curiosity: How fuel-efficient are turbines compared to IC engines, horsepower for horsepower?

Now THAT is a good question DE.

I am not an engine expert but as I understand it, the question is fairly complex. Gas turbines are universally poor - fuel economy wise - at sea level but they do very well at high altitudes where:
  • aerodynamic drag is lower (less resistance to the aircraft's motion through the air, so less power is required);
  • there is less oxygen - so less fuel is needed to keep the fire lit.
I am not sure I remember the exact values, but as I recall it, the P&W J75 turbojet engine on a U-2 reconnaissance aircraft (Francis Gary Powers) could produce about 20,000 lbs of thrust at sea level and do maybe 500 mph. They never flew it that fast down low mind you, they used that thrust to achieve an extremely rapid climb to very high altitudes at which the cameras and other sensors could do their job. So, at 70-80,000 ft. the engine was producing only about 4-500 (!!) lbs of thrust - and the plane could do....about 5-600 mph. The numbers may not be quite right - but it was something like that.

Now, the weight of fuel needed to produce 20,000 lbs of thrust was huge, but to make only 4-500 lbs of thrust it was miniscule - so the airplane had an extremely long range as long as it flew waaaayyyy up there. That is also why airliners nearly always fly about 30,000 ft. If they tried to fly at sea level, their range would be cut to a small fraction of what they can achieve at altitude.

The same rules obviously apply to piston engines - which use superchargers (either mechanically driven or turbos) to increase the density of the intake charge - but they produce so much less power than gas turbines in the first place that it isn't much of a contest.

Here is some more interesting math for you:
  • horsepower is defined as doing work at the rate of 33,000 ft-lb/minute or 550 ft-lb/second;
  • so - if you do the math, 1 lb of jet engine thrust is equivalent to one horsepower at about 375 mph (i.e. 375 mph is 550 feet per second);
  • so, a 747 with four engines each producing 50,000 lbs of thrust at 375 mph is producing around 200,000 hp;
  • in contrast, a Boeing B17 with four Wright Cyclone R1820s each producing about 1200 hp, has about 4800 hp on-tap (i.e. about 2% - really) of the power of a 747.
That is why a 747 can take-off and climb faster than ANY WW-2 fighter while weighing nearly 800,000 lbs. and a B17 struggled to get off the ground with a full load at about 65,000 lbs.

There is obviously A LOT more to this than meets the eye, but those are the basis as I understand them.

BTW -That tractor may be a small APU or perhaps a small Lycoming helicopter engine or even a P&WC PT-6 (the smallest versions of which could still churn out around 5-600 HP).

Pete
 
How are small gas turbines started?

Also, given the shaft speed, I'm guessing serious gear reduction is required to use one in a traditional IC application?

Most of the small stationary GT that were used pump and generator application used a regular old automotive starter and run surprisingly low shaft rpms in the 3500-5500 range. Most big stationary GT's for power generation run in the 4000-5000 SRPM range.
 
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