Capstone's site claims that their 30KW turbines are 25% efficient. Standard diesel gensets in this size can offer better than 30% efficiency. I can see the attractiveness of low maintenance, but thats close to 20% difference in fuel economy.

"I can see the attractiveness of low maintenance, but thats close to 20% difference in fuel economy."

coal_burner,

Considering the engine has no lubrication (Foil Bearings) system and no Cooling System no one points out the reduction of petro-based products for this engine. Look at their website, the engine has a built in recuperator via airflow over the core, even with that efficiency enhancer, we all wish the numbers were better. However, this engine like many turbines are technically omnivorous. I would trade a bit of effciency for the ability to burn anything. IMHO omnivorous engines will be the wave of the future, it may come down to can you get the fuel, but then can you burn it. These buses running Propane/CNG/LPG or some Bio-Gas to me is the ultimate set up. Never mind lite rail lines in cities, This is a rapid transit system that is clean and ready to go NOW.

Does each bus use more than one genset? I would be surprised if the average power required were only 30 kW (40 HP).

@ Coal Burner -

a diesel genset probably would be a little more efficient, but as of 2010 it will also need to meet extremely stringent emissions standards. Besides, reciprocating engines generate vibrations, which deters ridership. And at the the end of the day, bus service is evaluated in terms of total cost of ownership per actual passenger-mile (idem for fuel consumption/emissions).

By contrast, gas turbines are very compact and run exceptionally smoothly, though exhaust noise is usually a thorny issue. At small scales, only single-stage units with recuperators are remotely efficient enough because turbine temperatures - and hence the theoretical efficiency - are limited by the available materials.

All simple gas turbines deliver very poor specific fuel consumption in part load, so they should be run near rated power or not at all. Moreover, single-stage designs would be unstable if attached directly to the driveline. For both of these reasons, series hybrid configurations with large energy buffers are really the only way to go for automotive applications.

@ Richard Schumacher -

I expect that some variants have at least two turbines to support extended cruising at freeway speeds and hill climbing. This allows the operator to run zero, one or two turbines near rated power at any given time. As a bonus, you also get redundancy, but that's not a big selling point: single-stage turbines hardly ever fail anyhow.

How expensive are the 30kw turbines?.. I like the idea of one moving part, no maintenance and air cooling.
The sound would be cool :)

RS,

Go to these 2 links:

http://www.microturbine.com/prodsol/solutions/other.asp

http://www.designlineinternational.com/files/273520/HEV%20spec%20sheet.pdf

They have done buses with 2, and they have a bigger Genset as well. This one has the 30kw and also mentions a 60kw, but I am not sure if it is one or the other or both.

Still, why should this turbine bus use 50% less fuel than a standard diesel bus? It sounds too good to be true but assuming that it is true the gain could perhaps come from the fact that the 30% efficiency of the diesel engine is peak efficiency. The average duty cycle efficiency should be much lower. Anybody knows how low it is for a typical city bus application? For comparison, electric motors can run almost constantly at their peak efficiency (which is typically above 90% of their electric power input) independent of how their duty cycle looks like. Correct me if I am wrong I am not an expert in this area.

Nevertheless, with diesel going at $4 a gallon all the commercial vehicle manufactures should be raising to deliver hybrid technology or pure electric drive trains for their future offerings. The economic incentive to use fuel sawing technology is much better for commercial vehicles than for consumer vehicles because the annual fuel cost to vehicle cost is much higher for commercial vehicles. Furthermore, since practically all commercial vehicles are running on diesel right now one of the few ways forward to seriously reduce fuel expenses is to use hybrid or pure electric drive trains in the future.

Capstone's site claims that their 30KW turbines are 25% efficient. Standard diesel gensets in this size can offer better than 30% efficiency.

Small turbodiesels (e.g. VW 1.8 TDI) actually bench-test above 40% peak efficiency, which should be achievable in a series hybrid configuration such as this. Microturbines do have advantages in terms of emissions, vibration and potentially maintenance but this is a huge efficiency gap. I don't see how Capstone can compete, except on "coolness" or in areas where the alternate fuels (e.g. natgas) are much cheaper than diesel.

The Capstone turbines sell for $33333 each ($5,000,000/150). This is a lot more than a 30kW turbo diesel genset which is likely closer to $7000 and perhaps about equally or even more efficient measured by fuel to electric power (about 25%). However, diesel engines are produced in the millions and this turbine is almost handmade and produced in less than 3000 units a year I think. The price of these turbines could improve a lot in mass production.

It would also be interesting to see the economics of a bus with a much larger lithium battery than the 40kWh so that the bus became a PHEV bus running mostly on grid electricity. They say the bus saves 6000 gallons a year or 50%. In other words, a city diesel bus could be spending 12000 gallons a year or $48.000 a year on diesel at $4 per gallon. You can almost get a 100kWh lithium battery for that money. BYD may even be able to do a 100kWh LiFePO4 battery for $30000.

The EcoSaver IV is supposedly rated @ 7mpg diesel, but they are smaller than other NYCTA Buses.

If they would make the power plant combined cycle, they might really have something.

SJC, I agree completely. Turbines really shine when they are used in a combined cycle where the scavenged heat is used to power a second cycle. In fact, I think the heat can also be used to power an air conditioning compressor. In a combined cycle, I think turbines actually exceed the 40% thermal efficiency of a diesel engine. And they weigh a lot less.

I do have to say though: $4 a gallon for diesel means that we are at a point where buying expensive lithium ion battery packs might make sense on a city bus. Right now, diesel fuel costs makes powering a bus cost $1.00 a mile and I'm not even including the maintenance. I am imagining that you could design an electric bus to use 1 KWH per mile and even at a relatively high price of $0.15 per KWH, you would be saving enough in two years to easily buy that lithium ion battery pack and then it's gravy after that.

Am I missing something?

Ralph

All urban buses should have some proportion of electric drive, be it HEV, PHEV, even just regenerative braking.

The benefits in fuel costs and pollution would be considerable.

It probably just comes down to the bus guys getting used to building and purchasing them.

I am sure they are a reasonably conservative bunch, but as long as it happens within 5-10 years, it will be a good job.

The same applies to all stop/start vehicles. Presumably when 3rd party manufacturers get through a generation or two of the electric gear, it will really take off.

Ralph,

It sounds like the hybrid method is in favor here. I would guess that it is a matter of fleet replacement over time. As buses wear out they choose other types.

You are right that the waste heat from the final cycle could be used in absorption cooling of the bus interior. That would get even more out of every BTU used to power and cool the bus.

I think that buses are great platforms for this. They are large and have the room for the hardware. I saw a picture of one of these types with the hatches open. There were two turbines and it still had plenty of room. Buses put on lots of miles and the city air quality is an issue. It would show what could be done.

As someone who has worked in public transit for decades 7 mpg is very good mileage. We are looking at a savings of 14 gallons every 100 miles or close to 30 gallons per day. Transit systems are exempt from fuel taxes. Transit buses spend at least half their working day just idling their engines so an average power of 30 kw is more than enough on many routes. While a diesel may be more efficient under heavy load than a turbine efficiency at idle is a very different thing since a higher percentage of power is used just to keep the engine turning.

There are allready several different brands of hybrid busses. I believe the most popular version is GM/Allison transmission (Allison transmision was spun off ~1 year ago), who got 1700 in one day on top of ~1000 previous orders before then. The hybrid bus was GMs first implementation of the two-mode hybrid that's now on the Tahoe/Yukon and soon the Sierra/Silverado.

City busses are probably get the best utilization out of a hybrid system, second only to probably garbage trucks.

The efficiency of the engine and the real life millage might be little mis sliding.
The true efficiency of vehicle should be measure base on volume of energy that is deliver to the wheels no the efficiency of the engine only. I will call that – engine to the wheel.

The diesel engine today can deliver 40% efficiency, however it is in very limited region of operation.
The bus (or any other vehicle) operate in very wide range of speed. In order to keep the performance in some how reasonable the conversion of speed to torque is necessary, so transmission is introduced. And here is a problem. Any additional mechanical conversion require energy and reduce overall efficiency.
The transmission efficiency vary a lot with speed and load.
For most cars the efficiency that is deliver to the wheel is 5-12% even if your engine is 40% efficient.

The problem with most internal combustion engine is, that there characteristic do not much the cars driving characteristic.
The perfect engine for car will have to have the following:
1.Highest torque at starting point
2.Low load at idle or not idle at all
3.Have relatively flat efficiency over load characteristic and speed
All of that true for electric motor that why it is very good to used them in series hybrid system.
The series hybrid eliminate most of the deficiency of conventional engines, however at the cost and complication (engine generator, batteries /or super capacitors, electric motors = weight and cost).

The combustion turbine definitely have a adge in terms of reliability. That not insignificant in commercial use. The performance of 7mpg is not surprising for me at all, however as somebody mention series hybrid with small diesel will have better overall mileage.
Couple years ego E-traction build Whisper bus that get 14mpg using small diesel gen set with 75hp engine.
http://www.e-traction.com/fuel_consumption_calculator.htm

Are they any other solution that can improve performance of conventional engine. One could be the optimize transmission. Best solution is usage of continuous variable transmission (CVT).
The latest development in CVT become very promising. There are not exactly simple, however the cost is significantly lower then any hybrid.
There was one CVT develop by some Romanian inventor on 1920' that offer simplicity and excellent efficiency. George Constantinesco build car in 1920s that make 100mpg. He use wave and sonic theory and build inertia torque converter.

There other type of engine that offer similar characteristic as electric motor:
1.Compress air engine
2.External combustion engine (mainly steam engine)
Both of the engine offer attractive characteristic for buses and car alike.
In both cases they will not required any transmission (deliver enormous torque and 0rpm)

One of the example is show here:http://peswiki.com/index.php/Directory:EngineAir
That interesting rotary approach.

Example of steam engine could be Cyclone Power Technologies
http://www.cyclonepower.com/cyclone_engine_uses.html

This steam engine deliver excellent efficiency (36%) and do not required any transmission due to high torque (700 to 2310lb/ft depend of the model), and in city drive no idle cycle on stop.

There is a lot of interesting solution, we can just hope that they will be implemented.

SJC,

Lithium ion battery packs are now capable of going over 150,000 miles before needing to be replaced. It seems like the time is ripe for diesel public transit vehicles which travel less than 200 miles in one day to be replaced by lithium ion powered electric buses. Not hybrids -- full electrics. One man on here had an interesting idea of putting induction loops at transfer points and bus depots. While the bus is waiting for the next route to begin, it is getting a quick charge.

The benefits are enormous: less imported oil, no tailpipe emissions and the ability to use more renewables such as wind and solar. At these oil prices, diesel fuel just doesn't really make any sense any more. It's time for a new paradigm.

F big oil.

Ralph

" Transit systems are exempt from fuel taxes.
Transit buses spend at least half their working day just idling "
Yep, I think we all see the problem here. The incentivism is all backwards.

@mki,
very good point about the above claimed Diesel efficiency of almost 40% (never achieved in practical use by any VW driver). Most people like high revs, even the Diesel nerds. Actually, a CVT should be mandatory in every car togehther with a computer always calculating the gear ratio and engine speed to achieve best fuel effiency.

Ralp & T-2:

I fully agree with you.

1) Fossil fuel consumption should not be encourage nor promoted. City buses and other city vehicles should pay all fuel taxes.

2) Electric City buses equipped with quick charge batteries or super caps (EEStor?) or combination units could keep going 24/7 if quick charge automated stations were installed at enroute main regular bus stops + end of the line.

Why isn't that already done in all major cities?

One would think that London, England would already have electric buses + quick charge stations in operation. Wonder what Mayor Linvingston has been waiting for.

@mki

I agree with Michel that you have a truly good point about the wheel efficiency only being 5-12% when using a 40% thermal efficiency diesel engine in driving cycles with many stops and idling as in city buses and consumer vehicles. Your number also fully explains why the ECOSaver IV bus can save 50% fuel compared to an ordinary diesel bus even though it is run by a turbine with 25% electric efficiency. Assuming this bus also has 80% efficiency from electricity to wheel because of loss in the electric motor and loss regarding inversion and power electronics and loss from a simple transmission the end wheel efficiency would be 20% (0.25*0.8). This must be twice as high as the ordinary diesel bus (using twice as much fuel). So that will then have an end wheel efficiency of only 10% even if its diesel engine is possibly 40% in thermal efficiency (your estimate). To conclude, the big loss from 40% to 10% in the traditional bus is caused by a combination of transmission loss, idling and the inability to run the diesel engine at peak efficiency much of the time.

So higher oil prices and dropping prices on lithium batteries is the trick that for pure economic reasons could make the series hybrid the future of most commercial vehicles.

They might get EV buses going. I think it is more likely that we will see transitions. Diesel to CNG to hybrid and maybe to EV. You see articles on here about pilot programs and cities buying a few here and there. They want to make sure what they are buying will fit the needs for the long term. All those buses cost lots of money and no city council wants to make a huge mistake.

As far as the fuel tax for buses goes, I do not think that bus drivers leave their engines running because the company does not have to pay a 30 cent tax on a $3 gallon of fuel. I would suspect that a lot of that idling is done at stop lights and when loading and unloading passengers at the curbs. While it is true that some economic models show waste when something is free or subsidized, I do not think that would be a major factor here.

Re the microturbine's ability 'to burn anything' I think NOx, soot and unburnt VOCs need to be tested under different fuels. You can burn bunker/pyrolysis oil in a low speed stationary diesel and fit a large exhaust cleanup system. You don't have that luxury on a moving vehicle.

Assuming this bus also has 80% efficiency from electricity to wheel because of loss in the electric motor and loss regarding inversion and power electronics and loss from a simple transmission the end wheel efficiency would be 20% (0.25*0.8). This must be twice as high as the ordinary diesel bus

The Capstone CEO said "In recent product demonstrations the ECOSaver IV hybrid buses when equipped with our turbine have seen up to a 100% improvement in fuel economy over a traditional diesel bus...".

Translating "up to" from PR speak into real world data and accounting for the fact that ECOSaver IV buses have additional efficiency features, the actual advantage of the microturbine series drive train over traditional diesel is probably in the 50% range. If microturbine tank-to-wheel efficiency is 20% then traditional diesel would be 13-14%.

But traditional diesel is not the issue. If you can put a 25% efficient microturbine in a series configuration you can do the same with a 40% efficient diesel. With your calculation this gives 32% (0.40*0.8) tank-to-wheel efficiency. That's a 60% increase in MPG over the microturbine. The diesel would also be much cheaper. The microturbine should produce less NOx and soot, but it's hard to see how it can overcome such a large MPG and cost disadvantage.