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Capstone Turbine C30 CNG microturbine certified to 2010 emissions requirements for on-road heavy duty diesel engines for urban buses

Capstone Turbine Corporation has released configurations of the C30 compressed natural gas (CNG) microturbine that meet or exceed emissions standards, including the US Environmental Protection Agency and California Air Resources Board (CARB) 2010 requirements for On-Road Heavy Duty Diesel Engines (HDDE) for Urban Bus.

The C30 CNG microturbine is Capstone’s second engine CARB-certified for automotive applications; the C30 liquid fuel microturbine range extender was certified in June 2010. (Earlier post.) Capstone also has versions of its C30, C65 and C200 products certified to CARB stationary emission standards.

Test emissions from the C30 CNG microturbine measure significantly less than the emissions levels required by CARB and the EPA for 2010 and Subsequent Model Heavy Duty Urban Bus Engines.

C30 CNG emissions
EmissionCNG C30
BS CO 0.11 15.50
BS NMHC 0.04 0.14
BS NOx 0.05 0.20
BS PM 0.002 0.01

The C30 CNG microturbine doesn’t require fuel pretreatment or exhaust aftertreatment to meet these standards, thereby avoiding additional costs, decreased product efficiency and increased vehicle weight.

The C30 CNG microturbine is used in Hybrid Electric Vehicles (HEV) as a range extender for truck and fleet applications such as urban buses, passenger vehicles, trolleys, class 8 heavy-duty tractors and heavy duty trucks. It delivers benefits unattainable with conventional engines for cleaner, more reliable and quieter mass transportation.

Capstone microturbines incorporate lean premix combustion technology, which offers clean burning exhaust emissions operating on gaseous and liquid fuels. To achieve the emissions improvements, Capstone’s team of engineers developed new fuel injection and controls methods that resulted in significantly lower emissions.



No matter how much these cost, they are sure to be a bargain in the eyes of various California bureaucrats.

Henry Gibson

It is surprising how few such turbine have been used in vehicles, but the initial costs have made the capital costs slightly exceed the capital and operating costs of diesel engines. Capstone has never learned how to make a cheap one moving part turbine when the hundreds of parts in a diesel engine are still cheaper to make. ..HG..


HG: Somebody will, sooner or latter, find ways to produce very low cost equivalent micro-turbines. They have the potential to become the ideal e-range extender, specially when powered with NG, SG, CNG or CSG or compressed hydrogen.

Capstone should not give up and create JVs with lower cost producers in China and/or India.

Brian P

What's the brake specific fuel consumption? This has traditionally been the weak point of automotive/truck turbine engines. Turbine engines work well at a utility power generation scale, where you can use multiple stages of regeneration and reheat and recuperation, etc., but they don't scale down well. They don't like starting and stopping, and they don't like running at part load (efficiency drops like a rock). Great for a central power plant. Not so great for a motor vehicle.

If the BSFC is not equal or better than a diesel engine (exhaust aftertreatment or otherwise) or a high-compression spark-ignition engine tailored to run on natural gas, this is a non-starter.


"they don't scale down well." (not true – they scale down really, really badly)

"They don't like starting and stopping" (esp. with foil bearings),

"they don't like running at part load" (efficiency, already very low, drops like a rock).

"Great for [quick reaction backup] at a central power plant," but only WITH, topping and/or bottoming, multiple $tages of regeneration and/or $$ reheat and/or $$ recuperation; all big buck$.


"they don't scale down well" - because nobody really invested into R&D same money as into any other ICE
"They don't like starting stopping and they don't like part load" - Capstone turbines do have air bearings and at power plants they stop and start frequently and working at part load since gas turbines are regarded as peak load generators. You can check gas turbine diagrams.

I can agree on the point that gas turbine as ICE with direct mechanical torque would be bad choice.

I agree that diesel engines for power plant applications are on average more efficient (gas turbines 30% and diesel generators 40% mechanically efficient). But gas turbines are prevailing due to robustness, reliability and maintenance costs. The high price of Capstone turbines shall be attributed to small volumes since ICE systems based on gasoline or diesel engine are way more complicated and sophisticated devices and even weighting many times more. I still believe in gas turbine possibilities as range extender as soon as real "range extender" demand will be visible. For today there in no heavy or light duty automobile on the market which has range extender. Chevy Volt is just plug-in hybrid with bigger motor and bigger battery therefore ICE has duel function - direct torque and genset.

"they don't scale down well" - because nobody really invested into R&D same money as into any other ICE
They don't scale down well because the viscous friction against blades and the leakage around clearances become proportionally much larger as the machine gets smaller.

Future PHEVs range extenders may not have to be very efficient. High performance batteries will do 95+% of the work.


A diesel engine may as well be called a gas turbine with a moving combustion chamber in-between. A piston engine does have more moving parts but it has simpler parts and possibly less parts (with few cylinders) and these parts are exposed to lower average temperatures and the power transmitting parts run at lower speeds.

One option could also be to use a large turbocharger with a high compression ratio (or 2 turbochargers in series) and a reciprocating engine with a low compression ratio (or atkinson-cycled): Essentially a gas turbine with a comparatively small moving combustion chamber and a high BMEP (= high power density). In a range extending application turbo-lag is irrelevant.


If a city bus gets 4 mpg and goes 40,000 miles per year, that is 10,000 gallons of fuel. If they can get it up to 5 mpg that is 8000 gallons of fuel per year. Saving the transit authority $8000 per year on fuel per bus times 100 buses really adds up.

Whether diesel or turbine, you have a LOT of waste heat. Buses and big rigs are obvious candidates for heat recovery. Recuperate the exhaust heat in an organic rankine turbine and make the bus hybrid. You may be able to get that mileage up cost effectively and save lots of fuel.


Small gas turbines are usually designed with centrifugal compressors to avoid the gas leakage problem around blade tips experienced with axial flow designs. This design choice also makes the compressor more compact since fewer stages are required to achieve a given pressure ratio.


How much do the microturbines actually cost? A diesel engine is said to cost $35 per kw so a 100 kw/132 hp engine costs $3500. Next question how does the all up weight of a CNG turbine range extended EV compare to a straight diesel? That is including the weight of the fuel tanks, engines and traction batteries for either system.


A few thoughts on responses:

-Just Capstone PR doing there job to advertise whatever good news they can since there is so little of it.
-micro gas turbine generator makes little sense in this application. Bus market is extremely cost sensitive as their operators have little money of their own to spend and users aren't typically willing to pay a premium for better service. Buses are not weight limited so whatever advantage a gas turbine offers in weight savings is of little value. Any benefit in physical size reduction is moot given the poor energy storage efficiency of CNG and low efficiency of small gas turbines. if somebody wants or their is a need for a clean PHEV bus, they'd be better off using a simple ICE regardless of the fuel.
-@ToppaTom- Well said
-@Engineer-Poet- Right on
-@Harvey D- you can't discount heat engine efficiency if you are concerned with GHG. Except for a handfull of countries who are already invested on nuclear, geothermal, or other non HC fuel source, most countries electrical grid produces as much CO2 by the time it gets to an electric car as does a modern ICE. This can be improved but it will take decades and trillions of dollars so it can not be ignored.
-@SJC-stop start is definitely a great candidate for hybridization, but not so much for an ORC. Need high quality (high temp) heat and stop start is not conducive to this operating environment. Several recent studies have been conducted by NAS and other reputable scientific groups, all of which have shown that the economics don't come close to working for ORC until fuel prices increase by several times.
-@Mannstein- centrifugal compressors suffer tip leakage too. small gas turbines use centrifs to keep them simple and low cost. you can get upwards of 6:1 pressure ratio with good efficiency in a single stage on a centrif which would usually take 4 axial stages. multi-staging a centrif becomes tricky so it isn't often done so if you want to go to higher pressure ratios and use actively cooled turbines, then axial machines are the only way to go.
-@Aussie- Though i agree that $35/kw isn't a bad number for diesel engines, real engine costs are not well represented by such a simple metric. it depends heavily on other factors, such as emissions level, production volumes, BSFC, expected duty cycle, life. As an example, heavy duty engine costs have doubled in developed markets in the past decade due to emissions regulations with negligible improvement in fuel efficiency or durability. At any rate, suffice it to say that we looked at costing out a recuperated gas turbine in this power range for automotive use and came to a figure closer to 2-3X what an ICE costs in volumes 10,000 - 100,000/yr.



I was referring more to a series hybrid and not start/stop. Heat recovery can be a way to increase efficiency, whether an organic rankine or another method.


globi, you just re-invented the Hyperbar engine.


UA: Heat engine efficiency becomes more and more irrelevant as PHEVs range extender when PHEVs are equipped with higher performance batteries. Future PHEVs with 100+ Km e-range will run on electricity from the grid 95+ % of the time and about only 5% of the time (or less) on electricity generated by the on-board genset. The on-board genset or FC would not have to be much larger than 10 Kw to keep the vehicle going at 100 kph on rare longer trips.

Current first generation PHEVs (2010 to 2015) with smaller (20 Km to 50 Km e-range) low performance batteries require higher efficiency gensets. That will not be the case for second generation PHEVs (2015-2020) for reasons stated above.


EP, I was more thinking of a set-up similar to what was used in the JCB Dieselmax speed record vehicle (compression ratio of this diesel engine was only 10.5).
However, I don't think power to weight/volume ratio is really an issue in a bus application. But what if Chrysler needed a small gasoline or CNG range extending engine in a larger car and Fiat would provide its 2 cylinder gasoline engine with a relatively large turbocharger and lower this engines compression ratio accordingly. This would be more sensible than using an expensive gas turbine - as, for instance, was tested in the GM EV-1. Since the turbo-lag is irrelevant and the axis of the turboshaft is not connected to the crank, this system could also be arranged more flexibly. In addition, a heavier and larger IC-engine may be more efficient, but it will require more space and reduce the cars efficiency when the car runs on battery power.


The Russian Yo! has a rotary vane engine with super capacitors and gets 67 mpg. It only weights 1500 pounds and is a series hybrid.

This kind of car may not appeal to everyone, but the PNGV cars all got over 70 mpg with diesel hybrids. Better fuel economy can be achieved, but in our market system it is what the people will buy and use that makes all the difference.


Oh, by the way an EV emits significantly less CO2 per km than a conventional vehicle even with the US-mix and over 50% coal:
Needless to say that oil has to be imported and coal does not and an EV does not have any short-distance inefficiency issues.
Also, a photovoltaic carport produces enough electricity to power an EV. (Whereas a corn field on a roof would not provide lots of ethanol).


I like the corn field on the roof vs carport solar panels.

There is so much clean solar energy available from 100 (M2) panels that the power grid would be mostly idle. Local e-storage plus energy sharing may have a bright future. Both, e-storage units and solar panels price will go down every year making such application practical by 2020.


300 square feet of PV on a roof would go a long way towards helping the grid and charging EVs, but who will pay the $30,000 for the PV? Many people would rather spend that putting in a swimming pool that costs them $1000 per year than putting in a PV system that saves them $1000 per year. Until we can change that mindset we will make little progress.


Back to small turbines.

It seems UA knows more about turbines and ICEs than just about any of us, so we should be careful not mix our dreams and facts.

Yes, heat engine efficiency may be less relevant for PHEV range extenders with higher performance batteries, but COST is relevant and likely excessive;
and if 10 kW is all one needs for a medium sized PHEV, the weight of a low tech, 10 kW ICE would likely be irrelevant.

To cautiously expand on centrifugal compressors;
Multi- centrif staging was used to drive costs down in the AiResearch 331, designed in 1961 (and still successfully selling) and the 109 (ca 1980, higher cost and not produced), are the only ones I know of - there may be more but, as UA says they are not common.
The smaller 331s (~600 hp) cost less than ½ million dollars. !

Compressor efficiency is VERY important because losses add heat to the inlet air which further lowers effective compressor efficiency and increases turbine temperature.

Turbine losses also add heat but it may be partly recovered by later stages or a recuperator or as jet thrust but not so for a low cost, turboshaft engine with a single stage, radial inflow turbine.


28 m2 at 15% PV efficiency = 4.2 kW. 4.2 kW at 17% capacity factor = 6300 kWh = enough to drive over 45,000 km per year with an EV. In Germany complete PV systems are meanwhile sold for €2.2 /W. That's €6600 for a 3 kW system.
But of course people will only install these systems if they can benefit from feed-in tariffs (the US spends a 100 times more on its military than what Germany spends on feed-in tariffs with German jobs and reduction of fuel imports).


globi, if you reduce the compression ratio while maintaining the same peak cylinder pressure, your power goes up... and even the Fiat Twinair is more power than a series PHEV like the Volt needs. Downsizing the engine and using a generator to recover excess turbine power (no wastegate) is probably a better route to higher thermal efficiency. On the other hand, a PHEV used as intended doesn't benefit much because the engine provides little of the total power for the car. Something very simple is in order for them. The ultra-efficient ICEs make more sense in vehicles where they provide most or all of the motive power.

Germany only generated 5.9 billion kWh from solar, wave and tidal sources combined. Nuclear generated 128 billion kWh, total generation was 556 billion kWh (source: EIA). Germany's spending on feed-in tariffs is doing much less to cut fuel imports than uranium is.


We have the equivalent to feed-in tariffs in the Phoenix area and installed PV still makes much less economic sense as you approach the break even point.

And we speend 100s of times less on it than Germany spends on its military.

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