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

[rmw]

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

 

[globi5]

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.

 

[btrueblood]

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.

 

[KENAT]

rmw, make it a plug in series hybrid and I'm in;-).

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...

 

[dicer]

btrueblood (Mechanical),


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

 

[btrueblood]

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.

 

[Lou Scannon]

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.

 

[BrianPetersen]

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%.

 

[kenre]

This engine is claimed to exceed 50%

http://people.bath.ac.uk/ccsshb/12cyl/

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[GregLocock]

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

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

 

[ivymike]

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.

 

[carnage1]

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 )

 

[maxc]

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

 

[dicer]

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

And thanks Greglocock

 

[btrueblood]

"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.

 

[dicer]

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?

 

[Lou Scannon]

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?

 

[ivymike]

yes, and probably the mtbf too.

 

[GregLocock]

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

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

 

[btrueblood]

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.