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MicroTurbine Series Hybrid Bus Under Test in New York City

The New York City Transit (NYCT) agency, the largest in the Metropolitan Transportation Authority (MTA) network, is testing a diesel-fueled turbine series hybrid bus from DesignLine International Holdings in New Zealand.

The DesignLine hybrids, a few of which are in service in New Zealand and others of which are under evaluation in cities such as Hong Kong, use a 30kW Capstone MicroTurbine to power an auxiliary power unit (APU) to maintain the approximately 40 kWh battery pack at a medium state of charge. Under normal conditions, according to DesignLine, the buses will return to the depot with -60% SOC remaining within the battery pack.

APU control for the DesignLine system. Click to enlarge.

When the batteries are at a high charge level (>70%), the APU reduces the input to the batteries. At this level, the bus can operate in an electric mode with the APU turned off. At a medium state of charge (40% - 70%), the APU adjusts its output to maintain the average state of charge. Should the SOC drop below 40%, the APU will increase its output to return the battery level back to the required state of charge.

The company predicts that the bus will deliver about 7 mpg in New York. If that is the case, that would be approximately double the fuel economy of New York’s current series hybrid bus fleet, as measured by NREL during a comprehensive evaluation of New York’s Orion series hybrid buses. (Earlier post.)

(A hat-tip to Allen!)



7mpg? Using what fuel? These microturbines are multifuel capable.

Just for comparison purposes, what's the typical fuel economy (in terms of miles per gallon equivalent) of normal diesel buses like? Or that of natural gas buses?


Ack, nevermind. The buses are diesel.

How does the efficiency and emissions of the microturbine change between fuel types however? Some bus systems (OCTA in Orange County, Washington DC, etc) have well-established natural gas infrastructures with which to fuel their fleet. Could this technology be easily adapted to make use of those existing fueling regimens?

Microturbines can easily run on natural gas, and they are cleaner when they do.


Sounds good - for large vehicles like buses. Presumably the larger the better. I wonder could you do it for 40 Ton trucks ? Or is a diesel ICE better.
Is it the stop/start nature of bus driving that makes the turbine / battery combination so good ?

I am sure Volvo et. al. have given it some thought.
Rafael - any comments ?

s dogood

Love to see these buses add a permodrive like hydraulic retrofit. Maybe they could get 12 mpg.


I think this is a perfect example of the battery-buffered series configuration's benefits for three reasons:

1) The turbine is only 30kW (~40 horsepower). Using the batteries to store and discharge the power ("buffering") protects the turbine from having to be much larger to cope with peak power demands. In other words, if the turbine was supplying electricity on demand for the motors, it would have to be as powerful as the motors themselves (several hundred horsepower maybe?)

2) When the bus stops to pick up passengers or at red lights, the turbine continues to recharge the batteries at the optimum efficiency, and takes full advantage of that stationary period.

3) If the bus were operated in a series-parallel operation, the CVT might not be able to handle the torque needed. Also, and more importantly, that configuration is a charge-sustaining operation that relies on charging the battery just like a pure series hybrid, so the electrical energy that works in parallel with the gasoline engine is subject to the same transmission losses as it would be the series hybrid.


So regular buses get only about 2 miles per gallon?!?!
Wow, that is pathetic. I know they're big and all, but I thought with the efficiency of diesel...


what's the typical fuel economy (in terms of miles per gallon equivalent) of normal diesel buses like?

Three to three and a half MPG for a normal transit bus. The national average is 3.6, to be precise.

Depends a lot on terrain, size of vehicle, and how much stopping it does. A Manhattan local isn't going to be like a suburban express jitney.


The comparison laid out in the previous GCC post ( said that the "normal" series hybrid got 3.19mpg, which was "37% higher average fuel economy that the diesel buses".

So I guess their standard diesel bus (presumably of roughly the same weight, chassis type, passenger capacity, etc etc) got around 2.3mpg normally?


On the all-important question of cost, I can't find prices for Capstone's microturbines. However, according to the California energy commission:

"Microturbine capital costs range from $700/kW for larger units to approximately $1,100/kW for smaller ones. These costs include all hardware, associated manuals, software, and initial training. The addition of a heat recovery system adds between $75 - $350/kW."

Without a recuperator ("heat recovery") if these buses use a 40kW unit, that's between $28,000 and $44,000 each (depending on what the commission means by "larger" and "smaller".


So I guess their standard diesel bus (presumably of roughly the same weight, chassis type, passenger capacity, etc etc) got around 2.3mpg normally?

2.5 mpg

Roger Pham

mahonj wrote: "Sounds good - for large vehicles like buses. Presumably the larger the better. I wonder could you do it for 40 Ton trucks ? Or is a diesel ICE better.
Is it the stop/start nature of bus driving that makes the turbine / battery combination so good ?

The thermal efficiency of Capstone turbine generator is only 25% at the electrical terminal output. This, in contrast with the thermal efficiency of over 40% for truck turbo-diesels, makes micro-turbine out of the question for 40-ton trucks.
At the same acquisition cost of ~$25,000 USD, the turbo-diesels is ten times more powerful (335 kW) vs 30 kW for the micro-turbine. 40-ton trucks needs 130-200 kW for steady level cruising @ 60mph, so micro-turbine in the 30-60 kW range is out of the question. The weight of the turbo-diesel (12.5-liter Caterpillar C13) is around 2,300 lbs, vs the overall weight of the 30 kW turbine generator is ~880 lbs (less if waste-heat recovery system not incorporated) .

Turbine generator is tried on buses due to its quietness and low vibration, giving the bus the feel of an electric vehicle. And the continous combustion nature makes it very clean emission-wise, practically no PM nor CO, nor HC, while NOx level is low without requiring expensive post-combustion NOx treatment. Plus, the only maintenance required is an air filter change after 8,000 hrs, and factory overhaul every 40,000 hrs!. The turbine uses air bearing for low friction and zero maintenance requirement. No oiling lubrication system makes it very environmental-friendly. The electrical system is near-maintenance free!

The stop-start nature of city buses makes any hybrid drive train ideal for a vast improvement in energy efficiency.


I saw a small city bus project with two Capstone turbines and hybrid drive described a while back. It seemed like a viable design.


city busses with very short distances between stops could easily be made full-electric with a small battery (like these) : just put a recharging point at every bus-stop at the top of the bus. every stop takes about a minute. Should be just enough to give the power for the next mile.


Agree. Having an engine on a city bus is a waste of space and money.


With the deep cycling of the batteries, not to mention hotel loads, I don't think that a 1 recharge/mile strategy would be very effective. Plus, not all stops have passengers to be loaded or unloaded, and not all municipalities have stops every mile.


If all-electric is the desired goal, electric street cars/trolleys would be a more efficient solution than small battery EV buses.


I'm not so keen on trolleybuses and trams as I think the overhead wires makes the place look unattractive. I'd rather have a 30 mile EV range bus, recharged at the end of each route (say 20 minute charge time if using Altair battery and Aerovironment charger).


Depending on the route, 30 mile range EV buses would be nice, though some buses I have ridden on are run continuously on the same 4-5 mile loop all day long, with only a 10 minute break every few hours and a switch of drivers. This was in a small city, however. So the choice between a a CNG bus, a pure EV bus, or a hybrid bus is really going to depend on the duty cycle. Building effective and clean public transport is going to be a tripartite solution.



The Capstone Micro-Turbine does have a recuperator built into its housing.

Go here for images:


You may be not so keen on trolley buses but I'd not be so keen to be lugging tons of batteries around on a stop/go cycle where weight counts! If efficiency is the ultimate goal then increasing the weight of a stop go vehicle does not seem to be the best possible solution.

Perhaps you should simply ask GM replace all the street cars and trolleys that were ripped out, at their own expense? ;-) /flippant off



The old trolley systems that were retired in the middle of the past century typically suffered from a key defect that people always neglect to mention -- they typically ran in mixed traffic, not in their own rights of way. They were as apt to get struck in street traffic as buses are today, and as apt to collide with people, animals (back in the day) and cars as anything else that is run in mixed traffic. This problem is actually why the first subway tunnels were dug in Boston over a hundred years ago. As trolley vehicles grew bigger, mixed traffic became even more unsafe, and as buses became cheaper and more reliable, they were the better option.

The federal government won't fund a transit project that calls for fixed-guideway vehicles run in mixed traffic (trolleys), though they will fund bus programs or fixed-guideway vehicles in separate rights of way (tramways, subways, etc.). Trams are permitted to have at-grade traffic crossings for intersecting streets, and limited exceptions to the no-mixed-traffic rule are made for older systems like the Boston Green line that are mostly separate but have a few mixed traffic sections. But the point is, don't grow too nostalgic for trolleys that got as mired in traffic (and as liable to create accidents) as buses are today.

Trolley buses were seen as a smoke-free, quiet alternative to buses, but they lack route flexibility, and their overhead cables clutter the landscape, restrict clearances and are expensive to install and maintain. If Boston's fleet is any example, they are also fairly unreliable, because the contact arms frequently disengage from the power lines, and the bus stalls while the driver has to go out and re-attach them. Always an amusing sight. Modern clean-engine buses have eliminated the worst of the noise and smoke downsides.

When a city has both a trolley bus fleet as well as a regular bus fleet -- which it inevitably will if it has any longer-range routes into the suburbs -- the transit operator has to maintain reserves of both categories, as well as spares, maintenance yeards and the like, increasing capital costs to the system. This point also applies to proposals for short-range EV buses. Even if some routes are amenable to such a vehicle, if an operator has to run other routes, it will have to maintain two fleets, two sets of dispatch reserves, two maintenance sections, etc. Maximum uniformity brings economies of scale, which are useful to have.

This point can be applied to manufacturers as well. To the extent that you can reasonably design one vehicle or family of vehicles (i.e. same drivetrain, different length body) to cover the maximum number of market niches or applications, one can spread development and tooling costs over a larger production run, creating an economy of scale.

Basically, if you can build a bus that will work reasonably on both a three-mile, stop-and-go, highly urbanized route as well as a thirty-mile, mostly-highway, suburban express service, you can save a lot on the capital and maintenance side. That's why hybrid approaches are not a bad way to go. Ideally, if you had a limited number of them, you'd schedule them on the most intense stop-and-go type routes on in your territory, but they can be substituted into suburban routes if need be. And if your entire system is heavily skewed towards stop-and-go routes, you'd try to get only hybrid buses (and achieve savings due to uniformity), and live with the fact that they don't yield much benefit on the suburban routes.

On the question of weight, if you can package a small turbine, small fuel tank, batteries and electric motor in the same weight as a full-size diesel engine and large fuel tank, you've lost nothing. And even if they weigh somewhat more, regenerative breaking (imperfectly) recaptures the extra energy spent accelerating the mass once you begin to slow the vehicle.


If a bus averages 12 mph and runs from 6 in the morning to midnight, that's 216 miles of service without interruption.


With regenerative braking and waste heat conversion, the hybrid bus could be the next design after CNG diesel. I favor more small buses crossing the city streets so that you do not have to wait more than 5 minutes for another bus and only one transfer required.

Alex Kovnat

I am curious to know, how many grams per mile of oxides of nitrogen does this series hybrid turbine bus produce, in comparison to various Diesel powered buses (hybrid and non-hybrid)? Also, what is the turbine bus's score with regard to particulate emissions?

This is an important consideration because while the Diesel is the most efficient practical engine we have, mother nature tends to throw a frustrating show-stopper in our faces regarding oxides of nitrogen and particulates.

Best regards to all my fellow participants on GreenCarCongress forums,

Alex Kovnat

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