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

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

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

(OP)
http://gas2.org/2008/06/09/turbine-engine-no-pistons-no-lube-30-better-fuel-economy/

[img]http://gas2.org/files/2008/06/turbine_engine.jpg[/img]

Quote:

    


The DCGT is powered by an innovative new electromagnetic isothermal combustion process that produces complete combustion of fuel-oxidized mixtures in cyclic detonations.

The high-pressure gasses produced by these detonations drive a unique turbine producing shaft horsepower. The engine's unique combustion process allows the DCGT to operate with blower air at low static pressure, negating the necessity of compressing and preheating fuel-oxidizer mixtures prior to combustion. Once the compression of fuel-oxidizer mixtures has been eliminated, the engine both achieves higher thermal efficiency in a simplified mechanical structure, and provides significant advantages over current diesel, gasoline, and gas turbine engines.

I think my Thermodynamics text book needs to be rewritten now.

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

I doubt there's anything going on here that violates laws 0, 1, or 2.  (although it may be claimed that something is)

there's at least one guy over at http://autos.groups.yahoo.com/group/highefficiencyvehicles/  who would enjoy talking about this at length.
 

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

Its hard to believe there is improved fuel effiency when there are flames coming out of the exhaust in the videos.

This engine versus a typical gas turbine could show improved effiecency because it uses a positive displacement air compressor.  The power turbines are closed impellar, don't know how they handle high temp and loads.

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

Jets/turbines highly inefficent.
 

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

"Jets/turbines highly inefficent. "

Um, well, no.  Turbines are generally more efficient than piston engines at fixed power outputs, and can operate for longer durations between overhauls.  That's why you see a lot of newer, large, fixed power installations using gas turbine engines.

But, turbines don't throttle well, i.e. they are relatively inefficient when operating below their design power output level.  Since operation across a wide range of loads/speeds is a necessary attribute for automotive engines, turbines don't compare well to pistons.

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

What if used in a hybrid, especially a series hybrid?

KENAT, probably the least qualified checker you'll ever meet...

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

Might work, Kenat.  Google microturbines, or see these guys

http://www.microturbine.com/

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

Chrysler I think it was went as far as consumer testing a turbine-powered passenger car around the early 60's.  Really neat project.  This concept is supposedly for HD application, but investors would be wise to study the Chrysler experience and determine why the lessons learned there don't apply for any proposals of new applications . . .

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

I've driven (very briefly) a gas turbine car. Rover built several. There was also at least one Indy car that had a GT.

It is just conceivable that a GT hybrid might make sense, but I think that a conventional turbo-diesel is a better bet.

Small GTs just aren't very efficient.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

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

Greg, that conventional wisdom is becoming dated.  Capstone has 30kW microturbines with a published electrical output efficiency of 31%, in single-stage versions (ok, running on natural gas, not kerosene, but still that's a pretty nice figure for something so small).  There's room for better efficiency from their models, from some type of two-spool arrangement or just extra compressor stages.  

Again, the efficiency of a GT drops like a rock when it's throttled, whereas a piston engine can operate over a comparatively wider power range with reasonable efficiency.  A hybrid that took advantage of the throttling ability could perhaps have less battery weight to lug around, giving better vehicle mileage?

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

In a 30kW package size optimized to run on natural gas, 31% thermal efficiency can be be beat by a spark-ignition piston engine, and as you note, it will maintain close to that efficiency over a much broader range of power output. (Natural gas will tolerate a very high compression ratio.)

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

31% - a naturally aspirated piston engine won't beat that by much, it would require turbocharging or supercharging to get there (I could very well be wrong, but I'm assuming standard production tolerances, not blueprinted racing engines).  Such additional machinery would be equivalent to adding spools/stages to the capstone turbine, and is an unfair comparison in my opinion.  What is not unfair is to compare costs, last time I had good numbers (~7 years ago) the microturbines were about 10x the cost of piston engines, with comparable efficiencies.  The only way they make sense is in terms of noise and maintenance requirements.

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

Production motorcycle engines are in that range at their best BSFC point, running on gasoline. Normally aspirated. Natural gas will allow more compression for a little higher efficiency.

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

No, the Prius engine has a best operating point of 37%, and is above 30% for its entire operating range apart from idle.

No turbo.

Automotive diesels can achieve 41% when turbocharged.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

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

Most natural gas engines is the 30kW range aren't turbocharged so the microturbine efficiency numbers would be tough to beat (most MGEs over 100kW ARE turbocharged though).  A natural gas engine less than 100kW is typically used in a wellhead-type gas pumping installation where the pressure and flowrate (read: engine load) vary considerably, so a microturbine would not a be ideal for the application (hence their limited (!?) use here).  The larger gathering/pumping applications have far less variance in the pressure and flowrate so a turbine would win due to efficiency and maintenance (hence their regular use here).  Also, btrueblood stated 31% as the ELECTRICAL efficiency so a generator of 90% efficiency would put it on par with the pistons; which seems reasonable to me for an AC generator.

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

Hm, ok, thanks for that waking up, Greg.  The Prius engine has a bit of "extra tech" on it, in the way of VVT, but it's a fair comparison.

I meant to say that boost would be required to beat the turbine "by a significant amount", my (out of date!) numbers said 30% was about where a typical nat-aspirated gasoline engine can run.

Yes, the capstone nubmer is an electrical output efficiency.  They are known in the GT biz as kind of weird, they never quote the turbine's shaft power/efficiency figures, only system figures.  Part of their story is a very efficient electrical generation control system (I imagine lots of digital circuitry and inverter stuff), so if that were adapted to the Prius engine, perhaps we're still talking close numbers, with the Prius beating it handily by several points.

My point is (was) that only 20 years ago, "small" turbines were in the 1000 kW range, and had efficiencies in the 20's.  They've come a long ways in making smaller versions with good efficiency.  But that rising tide lifts all boats, and it appears piston engines still compete well, and will likely continue to do so, in the "small" sizes needed for typical (land) automotive uses.

Regarding throttling - a GT just can't idle, it must run at near the same mass flow of air, or the turbine stalls.  You can reduce the fuel flow only so far before the burners won't stay lit (ok, with hydrogen fuel I could keep them lit...but let's not go there).  There are some fancy ideas regarding bypass valves and multi-spool designs, but you might as well talk about multi-engines and boosted piston engines then.  

A piston engine, however, can idle at a very small fraction of its best-consumption rpm and mass flow, and so can idle with comparatively low fuel consumption.  And starting/restarting a turbine is a fuel-consumptive, slow process in comparison to piston engines also.  

So, no, I agree with Greg that they won't be seen in large numbers on the roads very soon.

So, let's talk rocket engines instead:  strap a solid-fuel booster onto your favorite chassis, light the fuse...as long as you didn't have any stoplights or turns, you could boost to Mach 3, and coast the rest of the way...:)

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

A graph of the weight of battery required versus the powerpack weight for a microturbine and a Prius type installation would be interesting. As you make the battery bigger the advantage would swing towards the turbine, since the battry could be used to cover for the operating condition where the turbine should not be used.

I'm assuming that the turbine set has a better power to weight ratio than the pseudo-Atkinson engine in the Prius, note hat it also needs a gearbox or traction motor rather larger than the Prius'.





 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

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

But, that battery weighs a lot, and would take the system weight well over the Prius' for a similar output power, which would impact the mileage?

Also, I had another look at their so-called hybrid vehicle "solution", which burns diesel/kerosene.  25% electrical efficiency, at 29 kW output.

Worse, I don't think these meet CARB standards, and the exhaust temperature may be too low to "light off" a catalytic converter:  
NOx Emissions @ 15% O2 < 35 ppmvd
NOx/Electrical Output 0.846 g/bhp-hr (< 2.5 lb/MWh)
Exhaust Gas Temperature 275˚C (530˚F)

And the kicker - that device weighs 1300 lbs (with the genset).

Dunno why this version shows with a recuperator, unless they are adding on a steam turbine to pull some work out of the exhaust heat?

Ah well.  Reality sets in again.

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

Btw, this small XR 50 rotary engine produces 31kW at a relatively low 7800 1/min weighing only 15 kg:
http://www.aixro.de/

If it were combined with a turbo charger it could produce the same amount of power at a lower rpm and thus increase durability and efficiency. However, durability might not be this important in an application where the combustion engine is only needed to extend the range of an electric vehicle (and thus not be operated as frequently and extensively as in a normal car (GM Volt)).
Keep in mind, a piston or rotary engine also has no need for a reduction gear to operate the generator.

A piston or rotary engine with turbo is principally comparable with a gasturbine with a moving combustion chamber allowing higher pressure ratios and higher peak temperatures. Rolls Royce even patented a aircraft gasturbine concept with an integrated rotary engine:
http://www.google.com/patents?id=lJMkAAAAEBAJ&dq=5692372
(Although the drawing does not make much sense, as the airflow produced by the axial flow compressor of the gasturbine is obviously much higher than that of the relatively small rotary engine).

GM produced a Hybrid-Prototype with a series Gasturbine producing 40 kW at a weight of 100 kg for the entire gen-set:
http://www.autoworld.com/news/GMC/Series_Hybrid.htm

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

I didn't look at the link but I can tell you what a recuperator does.  It is a gas to air heat exchanger that returns heat from the exhaust to the combustion air between the compressor and the burner section.  That would help thermal efficiency to some extent.  The difficulty is that the air temperature coming off of the compressor section is already high temperature so the heat transfer is not optimum because of the approach temperatures-but they do recover a portion of the wasted heat.  Those are used often on large industrial gas turbines used in pipeline station compressor drivers.

Simple cycle gas turbines are typically low efficiency because there is so much heat lost in the exhaust.  When that heat can be recovered via a recuperator or a heat recover steam generator (HRSG) and sent to a 'bottoming cycle' steam turbine, the plant thermal efficiencies start approaching the 50% range.

Now enter the fairly new GE LMS-100 (Google works wonders) where with the use of an intercooler and multiple stages of compression, the simple cycle efficiencies are starting to approach the high '50 percentages.  (It has just occurred to me to go ask the LMS-100 designers why they don't have a recuperator too-something else to do.)

Can this concept be downsized?  There may be a Bill Gates building just such a thing in his garage as we debate this.

Because of the point made regarding throttling a gas turbne, the best approach would be to utilize such a device in a hybrid configuration because of the difficulty noted in operating the combustion turbines at reduced speeds.

Now I can just picture some type of miniture LMS-100 configuration type of combustion turbine driving a generator topping off a battery in a Prius.  Wow.  Now where is my patent attorney?

Let's all try to remember this thread in about 5 years IF fuel prices in the USA stay where they are today or rise.  I don't think our imaginations are good enough to conjure up what will be on the scene at that time.

rmw

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

With that Turbosteamer BMW developed, one could build a small combined cycle gasturbine.
http://www.gizmag.com/go/4936/

Unfortunately, downsizing in general increases the surface to volume ratio of any engine/turbine, which again reduces efficiency.

Regardless: Most important is, that the gen-set for the hybrid stays affordable.

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

rmw,

You are correct, but a recup cycle on such a small device doesn't seem like it provides much benefit, at least not compared to a second compressor stage and intercooling (like your GE example).  The real reason for the recup on the Capstone turbine is because they sell the devices primarily for stationary standby electrical generation with cogeneration of hot water or steam heating; the recuperator I think is just along for the ride.  At least, I don't see a preheater HX in the cutaway views.

I stand ready to assist you in giving birth to a multi-stage, intercooled, small GT hybrid.  Will file this in my whacko ideas notebook.

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

rmw, make it a plug in series hybrid and I'm inwinky smile.

I suggest making the 'power generator' modular.  You could have different versions at different performance/price points.

One module could be optimized for natural/biogas, one could be a fuel cell, one could be optimized for whichever bio fuel is most promising etc.  You could bleed the greenies dry based on which fuel source they have locally available, how much they're willing to spend...

KENAT, probably the least qualified checker you'll ever meet...

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

btrueblood (Mechanical),


So then could you name a turbine that is close to 50% efficient?
I can name a recip engine that is.  

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

Ok, name it.  

Rmw already named a turbine with 54% efficiency, although it has a secondary loop running a steam cycle.  The GE LM-100 turbine generates 44% thermal efficiency at the output shaft in simple-cycle mode.  My post compares a combined-cycle turbine to a piston engine running with boost (turbo- or super-charging), i.e. there is additional complexity to create the higher efficiency.  Although, I'd also argue, with a turbine wrapped around a piston engine, is it really a piston engine anymore, or is it a turbine with a piston booster, or...?

Another example is the Solar Turbine Mars 250, in clean turbine (no secondary loops) mode, it has a thermal efficiency of 39%, and can be pushed to around 50% with added steam loops.

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

Though I hope this isn't too much common knowledge to be posting here, the Napier Nomad diesel turbocompound engine from around 1950 was an early demonstration of the kind of thinking being discussed in the posts above.  I wonder if in aviation there is a fuel price point where diesel turbocompounds might compete with or eclipse gas turbines on an overall mission net revenue basis, first on short, then increasingly longer routes, as fuel prices continue increasing.  The major trade-off being operating cost per mile versus capital amortization, considering that a turbojet can fly faster (hence earn gross revenure more quickly) than propeller driven aircraft.
I think also on specialized long range missions, where range and endurance are more critical than speed, a diesel turbocompound might be able to compete, considering the trade-off of fuel efficiency versus powerplant power density (i.e. net weight of fuel plus engines).  It is interesting to consider the effect of auxilliary take-off power (e.g. JATO) on this trade-off.

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

couple tidbits for comparison,

I have an example of a turbodiesel engine that is approx 42% efficient at its best operating point. Not only that, but it is completely practical for day-to-day use in an automotive application and doesn't have problems with varying loads (although obviously the efficiency will drop away from the best-efficiency point). No steam loops, no turbo compounding, no engine stop-start unless I do it myself (i.e. still room for improvement in overall powertrain efficiency, if there is a business case for it). It is a bone stock 1.9 litre Volkswagen TDI that is produced in quantities of many hundreds of thousands per year. The newest versions, just coming onto the market now, meet US EPA Tier 2 bin 5 and maintain approximately the same peak thermal efficiency.

Diesel engines on the scale of large ships exceed 50%, and my understanding is that modern diesels on the scale of those used in on-the-road transport trucks approach 50%.

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

Here's a smaller more efficient one

23 MW  MAN B&W Diesel S80ME-C Mk7 two-stroke marine engine   155 g /kWh 54.4%  

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

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

my understanding is that modern diesels on the scale of those used in on-the-road transport trucks approach 50%.

That would be the exception, assuming we're talking about engines with regulated emissions (I can't think of an example outside of some research projects in either case, but I'm less familiar with the unregulated engines).  43% would be more typical.
 

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

efficiency is easy, just use a super low speed diesel burning hydrogen and oxygen with argon as the inert filler gas. you can condense out the water and reuse the inert argon.(you can't get a better K number than an inert monoatomic gas for huge efficiency gains :))

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

Just back feed a small amount of steam and use the argon for the first time the engine fires.

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

I agree Hemi that is where I was heading with this. Its time to go back to recips in aircraft.

And thanks Greglocock

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

"Its time to go back to recips in aircraft. "

They've never left, at least for light GA planes; they're cheaper enough in initial cost, and light planes tend to log fewer hours, so the higher maintenance costs for piston engines never really overcomes a steeper up-front cost.  

There's been a lot of talk over the years about turbodiesels for aircraft, but there's only ever been one production diesel aircraft engine that I'm aware of...and the bomber it powered got shot down way too often, so the Germans stopped building them fairly early in WW1 (wrong mission for that engine I'd say).  The article on wikipedia for the Continental engine company says their president wants to develop a light diesel to replace their piston engines, remarking that 100-octane avgas is becoming hard to find and expensive, relative to Jet-A.

You may see turboprops more frequently in the future, on freighter airplanes operating out of remote terminals, as they are more efficient, but the prop limits top speed.  They are limited to lighter aircraft for near-city airports, as higher-powered props violate FAA noise limits on takeoffs (though those might change if fuel price pressure continues).  But the greater frequency of required overhauls for piston engines will limit their use compared to turbines for large commercial aircraft.  And the longer MTBF for modern turbines allows planes like the 757, 767, 777, and presumably 787, to be certified for long overwater flights, something the FAA was restricting to 4-engine planes only before the GE90 came along.

Those big, efficient ship engines probably compare well in maintenance frequency (dunno), but weight for a ship engine (all that extra metal for a big crosshead don't come with built-in lightness) is not critical either; I just can't see the one that Greg posted a photo of hanging off a 797 wing...maybe an Airbus though.

The comments on fuel efficiency are still right on all counts, the piston still beats a turbine in most ways.  The Voyager round-the-world flight (30,000 miles plus, nonstop, no refueling) was done with the venerable old Continental opposed 4-cylinder, a mainstay of light aircraft.  But it hasn't been flown again, either...it has a TBO of 1800 hours of operation.  Still, if you want to stay up a long time, you need to use as little fuel as possible, and pistons get you there apparently, see http://en.wikipedia.org/wiki/Flight_endurance_record ...there are no listed records for a turbojet engine, just one for a turboprop...the rest of the (fixed wing) records are piston engines.

...although the time aloft record of 69+ days, the two guys who had a fuel truck chasing their Cessna 172 down roads in the Mojave desert to refuel...that's just nuts :).  Note that they had to quit at about 1500+ hours of engine operating, because they couldn't get enough power out of it to climb away from the truck anymore.

Fun discussion.  I'm done.  You motorheads can go back to muttering about ring friction and I'll leave ya alone.

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

I meant large aircraft. There is not the thing of speed anymore, as the airlines are all slowing down, and the new geared fan jet is for slower flight.
Recip planes can and do approach current and past airliner speeds, just look at some of the speeds flown at the reno airraces.
The question I would like to ask, what are the horse power requirements to keep something like a 757 flying at say 500 mph at 30,000 feet altitude?
 

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

Interesing insights trueblood.  Regarding production diesel aircraft, what about the Junkers Jumo engine that powered the Ju88 bomber in WWII (or is that what you meant, but typoed WW1)?
Do you think, with the money that can be spent developing and manufacturing a commercial aviation engine today, that a lightweight diesel engine might be brought much closer to the MBTF of a turbine engine, assuming a market was there?

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

yes, and probably the mtbf too.
 

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

WAG hp estimate

P=F*v

F=m*g/(L/D)

L/D is about 20 from memory

Gives about 150 hp per ton


 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

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

Greg, somewhere in there you need D ~ v^2.  L/D of 20 is pretty high for a liner I think, but do-able (the B-52 has an L/D of 21.5).

Hemi, You Are Correct Sir!  I was off by one war, dunno where that slip came from, I plead post-vacation fatigue.

For the 757, it is supposed to use two jets with rated thrust of about 40,000 lbf, and cruises at 530 mph.  This gives a power requirement of 62.2e6 ft-lb/sec, or about 113,000 horsepower.

"Do you think, with the money that can be spent developing and manufacturing a commercial aviation engine today, that a lightweight diesel engine might be brought much closer to the MBTF of a turbine engine, assuming a market was there? "

You guys in this automotive forum have a better idea about the likely improvements in MTBF for piston engines.  Somewhere I have read that the MTBF for the boosted versions of the old radials (used as fighter engines in WWII) was halved for every 50% increase in power.  Right now, turbines are pushing 5000 hours TBO, and most mechanics I've talked to say they really don't need any work done at the overhaul, typically, and can go 10000 to 15000 hours before you have to do anything major.  2000 hours is about the limit for NA piston engines before a rebuild is required, historically, but then, my Toyota will probably go 1.5 to 2x that figure.

The power to weight ratio is an interesting number to kick around.  A GE90, putting out rated thrust at 500 mph, generates 120,000 horsepower, and weighs 16k lbm, for a specific power of about 7 hp/lbm.  The P&W R28000 18-cyl. supercharged, air-cooled, water-injection radial developed 2100 hp, and had a PTWR of about 0.9.  Not that engine weight is a huge penalty for an airliner, but every pound you can shave in airframe weight is payload.

It's interesting, I think, to also point out that flying is not a fuel efficient transport method by any measure, except speed.  The best miles per gallon (MPG) is from the distance record flights, i.e. aircraft purpose-designed for best possible range.  The Voyager went 26,000 miles, burning about 6900 lbs of fuel (I had to calculate that number, not sure about it, but it ought to be close).  Using 6 lb/gal (?) I calculate the Voyager got about 23 MPG for its flight.  Not bad, but there are small cars beating that by about 2x today.  The Virgin Atlantic Global Flyer, a turbine-powered Rutan record-setting contraption, flew 25,772 miles and presumably burnt about 18,000 lbs of fuel.  Again using 6 lb/gal (somebody got better numbers for fuel density?) gives a figure of about 8.6 MPG.  Both planes flew at about the same average airspeed, so this is a pretty good comparison of the relative efficiency of the two powerplants, for low-speed, max. range flight.

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

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.
 

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

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.

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

"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.  

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

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.

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

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

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





 

Cheers

Greg Locock

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

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.

 

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

I used 500 mph as in dicer's question.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

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

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.

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

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

http://www.grc.nasa.gov/WWW/cdtb/projects/waverotor/index.html

or pulse detonation engines:

http://www.flightglobal.com/articles/2007/09/14/216816/ge-tests-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.

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

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-templating/v/index.jsp?vgnextrefresh=1&vgnextoid=3c96b152faafb010VgnVCM1000000881000aRCRD

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

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

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?

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

"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

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

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...

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

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...

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

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

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

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-holistic-environmental-approach-with-hemp-based-eco-elise/9625/picture/47569/

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

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

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.

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

"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/

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

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

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

I think the increases in crash survival in F1 and many other forms of motor sport are more to do with lots of details of design, support and equipment than the carbon fibre pod.

Things like:-
HANS devices.
Helmets.
Seat belts.
Fire extinguishers.
Run off areas.
Angles of barriers to direction of travel.
Soft barriers.
Rescue crews.
Fuel cells.
Drivers apparel.
Rigid pod surrounded by energy absorbig items that crumple or break of thus absorbing energy from the impact.


 

Regards

eng-tips, by professional engineers for professional engineers
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RE: Turbine Engine: No Pistons, no lube, 30% better on gas!

That may be true, but I don't remember seeing Aluminum monocoques staying in one piece after hitting a wall at 260 km/h.

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

Gobi

There is a LOT more to the properties of a composite structure than the properties of just one component.

Fibre to resin ratio.
Type of resin.
Chemical types of fibre.
Geometry of fibre.
Surface coatings on fibre.
Fibre orientation.
Yarn structure.
Fabric structure.
Other components that may be used as fillers or laminated layers, like hard shins or lightweight cores or stringers or ribs.

F1 has acces to very high budgets and can afford complex designs and short if highly stressed life of components.

Everyday drivers are best kept relatively simple and cheap and need long life at low stress. Carbon fibre composites have a long way to go to offer value in everyday drivers.

Bottom line.

Refer back to greglocock's post eighth of July
 

Regards

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

That's what I kept on saying:
The cost of carbon fibre is the problem and not the mechanical properties of the material.

Besides, F1 uses epoxy resins, fibre forge uses injection moldable thermoplast, which is obviously less costly but may still be more expensive than steel.

On July 12th I posted lots of facts. I simply prefer facts over opinions.

Btw, Toyota just made a light weight car out of carbon fiber reinforced plastic with a curbweight of only 440 kg, showing that weight can be reduced significantly:
http://www.edmunds.com/insideline/do/News/articleId=122972

Again the costs may be too high, but do you suggest that the lightweight, rigidity and structural safety claim is a scam from Toyota?  

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

Thermoplastics or epoxy typically cost less than 20% of the price for carbon fibre, and the composites typically consist of over 50% fibre, therefore the carbon is a much more significant factor than the resin re costs.

The thermoplastic matrix composites are generally somewhat down on most physicals vs epoxy matrix, but they have a very significant benefit in that they can be thermoformed, but this also severely limits design possibilities as they can only be formed into a simple shape.

One problem with manufacturing thermoplastic composite sheet with fabric reinforcement is wetting the fibre wit the resin to get a good bond. With thermosets, there is time to work the resin in and the air out.

Regards

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

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

Lotus use (or at least used) RTM, it was their vehicle weights I was thinking about. The problem there is that the glass fibre is largely unstressed, the backbone takes a lot of the load.

Some of the problems with designing carbon fibre monocoques that are efficient and crash proof are: surface finish, integration of hardpoints, post yield behaviour, refinement.

The second of those problems is so great that I would be tempted to integrate the suspension arms into the layup, ie use composite hinges, and carbon fibre arms.

In practical terms if you are interested in panel properties (ie out of plane stiffness) then the gain in going from S glass to CF is small.

 

Cheers

Greg Locock

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