Turbine Engine: No Pistons, no lube, 30% better on gas! 1

[btrueblood]

Wups, that last sentence is wrong, the Voyager completed its flight with an average speed of 115 mph. The GlobalFlyer averaged 342 mph, roughly 2x the speed. The GlobalFlyer was designed for a circumnavigation within a time limit for a solo pilot to remain functional. The speed ratio would suggest that the turbine aircraft would require an 8x higher power output from its engine; that is roughly correct based on engine sizes...but the engines are also sized for takeoff thrust...so there are a lot of variables.

Oh, and its a P&W R-2800 not 28000. Sheesh.

 

[Lou Scannon]

Agreed, aviation is chosen over surface travel for its speed, not economy. But that is a red herring, I think. Within the realm of aviation, economy does matter.
A heavier but more efficient engine offsets its weight penalty by requiring less fuel for a given range. So the net (non-fuel) payload difference depends on the engine weight differential and the required range.

 

[btrueblood]

"But that is a red herring, I think."

Um. Yes, but no. Nobody is operating DC-3 aircraft in the US market, as passenger planes, because they fly so damn slow. You can't pay the pilots, and turn the aircraft around fast enough to book enough passengers per day, and pay the people who sit at the gate and take tickets, etc. etc. to make that aircraft pay for itself. Even though its MPG is pretty darn good, you just can't save enough money on gas to justify the slower flight speed. Or at least, you used to not be able to. A DC-3 works if you are hauling something that 'absolutely positively doesn't have to be there overnight, and you fly from deserted airfields... Or make mutliple short hops (skydiver aircraft).

Within a narrow range of flight speeds, say 500 to 600 mph, fuel efficiency matters, for current airliners. Drop much below 500 mph, and you have a plane that only pays on short runs, where the speed difference is enough for people to justify flying vs. driving, but not enough to justify a jet. Turboprop commuter planes are filling this niche currently. But in places where high-speed rail has become available, short-flight prop planes have seen drops in passenger volume.

"A heavier but more efficient engine offsets its weight penalty by requiring less fuel for a given range."

Well, it can, if the range is long enough and the weight of fuel great enough. Agree with your second statement.

 

[Lou Scannon]

I agree, speed is paramount for mid-range and longer passenger travel, up to the current practical limit of about 600mph.
If diesel engines are going to make inroads, it will begin with freight and possibly short run passenger transport. Like you say, where high-speed surface travel is an option, it will be more difficult for aircraft to compete.

It's too bad that propfans are so noisy. They would allow recip engines to approach 500mph, if I recall correctly the propfan performance envelope.

 

[GregLocock]

"somewhere in there you need D ~ v^2"

No I don't think so. It is already hidden away in there.







Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ivymike]

Greg's methodology isn't missing anything... but it's not a complete picture either. (I'm not going to bother trying to complete it)

In his first equation, he relates total drag (force) to power and velocity

I'm not sure what velocity he assumed (for "cruise") in the first equation.

In his second equation, he relates total drag (force) to lift (force), assuming a glide ratio of 20:1. A 747 at cruise is in the neighborhood of 17:1 (see wikipedia:

http://en.wikipedia.org/wiki/Lift-to-drag_ratio).

The estimate misses off-cruise conditions where L/D may be less favorable, such as takeoff, and depends on having a good cruise velocity as input to the first equation.

 

[GregLocock]

I used 500 mph as in dicer's question.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[btrueblood]

Yup, sorry Greg, it's there. I should've said, it could be made more apparent the drawback to high speed by pointing it out explicitly. I never went back and edited that first line before starting my own calcs.

"It's too bad that propfans are so noisy. They would allow recip engines to approach 500mph, if I recall correctly the propfan performance envelope. "

Well, good turboprops today can (just) reach 500 mph, so yes, a 500 mph turbodiesel is not unlikely...but, yes, props are noisy, and the noise goes up fast as the power level (airframe weight/payload) of the engine increases. The stupid thing is, most airports (at least out here on the left coast) were purposely built away from major urban centers. Then, because commerce moved to be near the airports, housing followed the commerce...and then people who'd built their spanking new home under the takeoff pattern started complaining about the noise. Same thing happened with a local speedway here. I say, f-- 'em, if you're stupid enough to build next to a noise source, live with it. But enough of them complained, and the FAA started regulating noise. If fuel prices continue upwards, they may need to raise that noise limit, at least for cargo aircraft.

 

[globi5]

As far as future passenger aircraft engines are concerned wave rotors:

http://www.grc.nasa.gov/

http://WWW/cdtb/projects/waverotor/index.html

or pulse detonation engines:

http://www.flightglobal.com/article...s-experimental-hybrid-pde-turbine-engine.html

could increase efficiency without increasing engine weight significantly.



Also, a turbofan has less cross sectional at the same thrust than an aircraft with propeller. At least at high speeds this should be an efficiency advantage of turbofans over propellers.

 

[btrueblood]

Hate to be a wet blanket. But, wave rotor research hasn't progressed much since the late '90s.

PDE's hold promise, but are noiser and louder than props; and the weight penalties for valving/containing high pressures so far don't make them very attractive. GE thinks they may adapt them to ground-based aero-derivative turbines in 10 years (basically, it's still a long shot). A company I interviewed with years back, called Adroit Tech. Systems here in Seattle area, was doing quite a bit of research on PDEs, with Tom Bussing as the company president (nice guy, but he wanted cheap young engineers willing to "donate" 80+ hrs/week to the project for very low pay). The company got bought up by P&W, then ... assimilated. Several old friends were digested variously affected by the change. They were probably the farthest along in testing of PDE's but today most of the research on PDE's at P&W is either under deep cover (hah) or just not in existence, no news since 2003 or so.

here is one place you may see PDE's soon:

http://www.pw.utc.com/vgn-ext-templ...toid=3c96b152faafb010VgnVCM1000000881000aRCRD

GE (one of P&W's biggest competitors) is apparently now operating a research program, looking into a valveless PDE... we shall see.

 

[scooterjockey]

a microturbine/compressed air hybrid would be light and have enough power...it would have to be matched with the RMI hypercar(carbon fiber). This car would win the X prize. Could the sound of the engine be isolated?

 

[GregLocock]

"a microturbine/compressed air hybrid would be light and have enough power...it would have to be matched with the RMI hypercar(carbon fiber). "

Great, 2.5 bad ideas in one sentence. The only demonstrated range for an air car is 7 km. carbon fibre is a fairly stupid material to build a real road going car from, in a greenhouse world.

The microturbine may have some merits, so it is only a half bad idea. Work out how to keep it warm enough and get back to me.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[btrueblood]

Greg, I think I know what you mean, "carbon fibre is a fairly stupid material to build a real road going car from, in a greenhouse world", but wouldn't mind seeing you elaborate on that, if you have the time...

 

[btrueblood]

Oh, and apologies to the OP for not pointing to the PDE as an example of the type of engine he is/was talking about (just a different example). The point of all of the detonation-cycle engines is that higher combustion pressures are reached for detonation processes, and since both Otto- (piston) and Brayton-cycle (turbine) engines have pressure ratios in their (ideal) equations of thermal efficiencies, detonations can theoretically drive higher efficiency. The drawback is that detonations are closed-volume processes by nature, and thus there is no continuous detonation burner...so they pulse (and vibrate, and are loud) by nature. Also, detonations by their nature occur at the speed of sound in the combusted (hot) mixture, i.e. you have high-speed flows of hot gas, pulsating, which makes the structures guys cringe...especially when you start talking about ceramics and cermets...

 

[GregLocock]

Carbon fibre is an energy intensive material to manufacture, and is very difficult to recycle in any form other than as filler material. Therefore you are using a lot of energy to create your structure.

The resulting structure has some virtues - it can be light and stiff, but it is not that much lighter or stiffer than other composites. In terms of crash performance it is not very good, as its compressive properties are poor.

The resins themselves are made from oil, and again do not recycle very usefully, and have short and long term environmental problems/costs.

RMI, and the Hypercar project, and UCS, all pushed lightweight composite construction for road cars whilst ignoring the real world costs associated with it. Personally I regard the Hypercar project as one of the most sickening (if succesful) bits of greenwashing and misdirection I have ever seen.

If you look at real world affordable composite cars /that meet crash requirements/, and look at their weight, you will notice that they aren't especially light for their size. Odd that.


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[globi5]

Assuming a car runs 200'000 km on average and a lighter composite car saves only 1 litre per km. That's a saving of 2000 litres of gasoline or roughly 20000 kWh over its lifetime.
Does it really take that much more energy to produce some parts of the car with carbon fiber composites?

As opposed to the convential slow carbon fiber process using epoxy, fiber forge uses thermoplasts (polyamid). Since this reduces process time significantly, it should also reduce costs.

http://www.fiberforge.com/DOWNLOADS/FiberforgeBrochure.pdf

(Btw, you can cheer up, the Hypercar was not succesful - Fiberforge is apparently all what's left).

The compressive properties are poor, but the actual maincell of the car should not be compressed. For example car sections like the roof or the floor can be made out of carbon composites.

Apart from composites: It's at least interesting to note that there are still so few cars with plastic (no composites) hoods, fenders and door panels which could be easily be recycled.

But there is change: Mazda might actually be one of the first car brands which started to make lighter cars instead of continuing to make them heavier. Although using high strength steel instead of composites.

And if carbon fibre is too costly and energy intensive one can also produce composite parts without having to use carbon fibres:

http://www.gizmag.com/lotus-takes-a...with-hemp-based-eco-elise/9625/picture/47569/

 

[patprimmer]

The co-efficient of thermal expansion of most plastics is about 10 times that of steel. That can have some interesting consequences on fasteners and body gaps.

Poor body gaps or panel alignment I imagine does not at all help with aero drag.

I doubt the average life of a car is 200,000Km. I suspect it is more than that if scrapped due to normal wear and tear, but a significant number will be cut well short by accident damage and I would expect that would result in a lower average.

Also, not all panels last the life of the car.

I thought carbon had good compressive strength. It's Aramid fibre that has lousy compressive strength. Carbon is poor in impact and elongation.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[globi5]

Some cars do indeed have plastic body panels (e.g. Saturn).
Apparently the problems related with the different coefficients of thermal expansion can be solved.

Chrysler built a full size car with an aluminium frame and plastic body panels weighing only 1020 kg but at extra costs of $7500. Even with the all-plastic body and sparse frame, the ESX3 passes all government crash tests and will carry a family of five comfortably.

http://www.popularmechanics.com/automotive/motorsports/1268751.html?page=1

And the floor and roof parts are not likely to be exchanged over the lifetime of a car. The maincell of a car must not collapse, therefore overall stiffness and strength in this area is more important than impact strength.
Although impact strength does not seem to be that bad:

http://www.youtube.com/watch?v=imfHBcqzLfI

(scroll to about half the movie)

Carbon actually has a lower coefficient of thermal expansion than steel:

http://www.graphitestore.com/items_list.asp/action/prod/prd_id/96/cat_id/34

http://www.goodfellow.com/csp/active/STATIC/A/Carbon.HTML

The question remains: Can one bring the cost down of carbon fiber parts in a mass production set-up.

Teslamotors uses carbon fiber body parts and writes:

http://www.teslamotors.com/blog4/?p=50

Depending on how it’s processed, for example, a carbon fiber-reinforced plastic part can replace an equivalent steel part using less than 30 percent of the original part’s mass.

Carbon fiber on its own isn’t much use, though. It’s like a very thin fishing line, it is only strong in tension (when you try to break it by pulling it along its length).So, to make a panel that is strong in all directions, carbon fiber is typically woven into cloth (to give it strength in two directions) and then the carbon fiber cloth is encapsulated in plastic. In our case, it is encapsulated in epoxy resin – it has a higher specific strength than the alternatives. The epoxy is strong in compression but relatively weak in tension, so the two materials act together to produce a panel strong in tension and compression.

Carbon fiber parts that you see on some cars, especially aftermarket products, are produced using carbon fiber cloth pre-impregnated with resin (abbreviated to ‘prepreg’) that is heated and pressed against a former in a pressurized oven called an autoclave. The very high temperature and pressure squeeze the air out of the cloth and force the resin to flow around the fiber and create a consolidated molded panel.

This can produce very lightweight and very stiff components, but with a couple of drawbacks. First, the cost of producing the parts is very high because they need a long time to fully cure in the autoclave and the process isn’t cheap. (If you think the Tesla Roadster is expensive now, you should consider how much it would cost if we added several thousand dollars worth of autoclaved carbon panels.) Second, there aren’t many manufacturers with enough autoclave space to produce a whole set of body panels at the rate we need.

An alternative to using an autoclave is to “vac-bag” the parts. This is a similar and less expensive approach that doesn’t use an autoclave but the drawback is that, as the name suggests, you can only apply atmospheric pressure to the parts (by creating a partial vacuum in a bag that surrounds the mold with prepreg carbon loaded onto it). The pressure isn’t high enough to fully consolidate the resin into the fiber so the final panel’s surface isn’t of a consistently high enough level of quality for a car like the Tesla Roadster.

The process we ultimately adopted for our body panels is Resin Transfer Molding (RTM), which uses what’s called a “closed mold.” Two huge blocks of steel are machined and polished so that when they’re nested together there’s a gap between them of less than 2mm representing the shape of the part we want. We lay carbon fiber mat (and some additional material we discuss in more detail below) against the concave surface of the tool, bring the other half of the tool into place to create the cavity, and then inject resin to fill the gap. This technique allows us to control thickness (which keeps weight down), reduce processing time, and maintain a very good level of surface quality. An additional advantage of using a closed-mold tool is that we can vary the thickness of the part in key areas to integrate features that add strength or provide a location for mounting hinges, etc ...

This process however still appears to be more elaborate than what fiber forge presents.

 

[JayMaechtlen]

"The maincell of a car must not collapse"...

Not precisely true.
It can deform, but it has to hold enough shape to protect the occupants.
Nice thing about steel- it can take more than one hit. If you have more than one impact (rollover or multi-car accident) the composite car tends to frag in all directions, leaving nothing to enclose the unfortunate occupants.

Cheers
Jay

Jay Maechtlen

http://home.covad.net/~jmaechtlen/

 

[globi5]

Well, at least since the introduction of carbon composites in motor racing, the number of fatal accidents went down significantly.

http://www.youtube.com/watch?v=a3t6f2YdfB4&NR=1