Turbine-powered series hybrid truck

The concept isn't new, but great that a retro fit is comercially available.

A Volvo concept truck had a large enough battery to allow it to go full EV within housing developments.

 

Hi

There is a fairly simple formula that uses the exhaust temperature and combustion temperature ratio to predict the engine efficiency:

• eff% = 1 - (T_exhaust/T_inlet) ~= 43%
• T_exhaust ~= 900 K
• T_inlet ~= 1600 K

As the turbine extracts work, it cools the gas. The temperature ratio is a measure of how much was removed. Not included are the losses for the compressor.

The next trick to higher, gas turbine efficiency is to recycle as much waste heat as possible to pre-heat the compressor output gas. But that has turned out to be a 'hard problem.' The heat has to pass through a heat-exchanger and that is a materials challenge.

Correction: Microturbine CHP Systems

The higher efficiencies are associated with using the waste heat of the micro turbine for space or other heating. The electrical generation is in the 22-26% range.

Bob Wilson

 

60% is very high for a thermodinamic cycle (Carnot). It can be close in very big equipments, but not in small ones.

 

Yes, bigger ICEs are more thermally efficient, but a turbine is generally more efficient than a piston engine. It is also generally smaller for the power output too. The ability to set the system up for gasoline, diesel, or CNG is another advantage.

I wonder how it deals with emissions. I don't know if the turbine cycle inherently cleaner than a piston, but the exhaust gases are hotter. Jay Leno made a comment about melting a bumper on a Lexus that got way too close to his turbine powered motorcycle at stop. The engine was originally meant for a helicopter, and surely larger than this micro-turbine.

 

The article I found said something about a 'very lean' burn technique. Combustion can be cooled by either excessive rich or well managed, very lean. Very lean is much harder to manage but I suspect modern sensors and controller can handle the technical challenges.

Bob Wilson

 

Wth. I live in the UK and can't access a BBC website because it's only available for people outside the UK? Eh? Why do I have to pay a monthly fee of £13 to the BBC, or risk prison and a criminal record if I don't pay, and yet get excluded

Anyone got a link to the article that I could view?

 

Here's one.
Wrightspeed Combines Gas Turbine And Batteries For Big Fuel Savings - HybridCars.com
It appears to also be a plug in, and this article covers emissions. "There is only one moving part, no cooling system needed, and the emissions are 10-times better than mandated by the California Air Resources Board for this class of vehicle –– and it does it with no after treatment technologies."

The company site: Wrightspeed

 

http://www.cogeneration.org/111011Conf/Presentations/McAvoy.pdf

See page 11, a 1000kw turbine is 35% efficiency max. Some ICEs excel that, example Prius Atkinson.

I can't believe Wrightspeed turbine tech is so different from a CHP system one. And at a fraction of nominal output.

 

Regarding the original post...that MPG should be combined with grid electricity consumption.
It is a new version of the 200MPG Chevy Volt ad.

There is no CS Mode consumption like that when reffered to a heavy vehicle, high drag and a complex energy path system...

 

The thermal efficiency of their turbine isn't published.

The OP article was misleading in not mentioning it was plug in. However, trucks of these size classes don't have published fuel economy ratings. The companies buying them do care about it, but only as a cost. Considering the low mpg the trucks start from, the amount of miles they go in a year, and the higher cost of diesel, even a minor improvement can mean big savings. FedEx ordered 25 systems, and Wrightspeed is saying a 3 to 5 year payback over the replacement cost of the traditional drivetrain.

This may not be like early PHV estimates. It is a pure serial hybrid, and the turbine doesn't just sustain the charge, it can can charge up the pack in 40 minutes. It runs at a single, efficient speed and shuts off once the pack reaches max SOC. The pack is smaller than the Tesla S's 85kWh, and in trucks of those sizes may not have enough of a grid powered EV range to have much much impact on a daily route by itself.

 

Just got a chance to look at this, and it isn't clear what that slide is referencing in terms of efficiency. Our discussion is on thermal efficiency, but this Capstone product flyer is discussing electrical efficiency. http://www.capstoneturbine.com/_docs/Product%20Catalog_ENGLISH_LR.pdf

It isn't at a fraction. The turbine runs at that one speed chosen for efficiency. Turbines don't have all the rattling and shaking that a piston engine produces. So the noise and vibrations are far less of a challenge to handle on the truck. If the ICE on the Volt could run at a point for efficiency, regardless of the occupants' comfort, it would get better CS fuel economy.

In the Route's case, the turbine is powerful enough to charge up the battery while propelling the vehicle. When the battery is fully charged by it and the aggressive regen braking, it shuts off to let the truck go pure EV. It is automatically pulse and gliding. The HybridCars article gave more details.

 

My opinion is fundamented in:
1 - Reducing size drops efficiency (any engine, turbines included)
2 - A complex system like turbine-gen-batt-motor cannot deliver better g/kwh than a direct shaft link

 

1-Yes, smaller engines are less efficient.
2-Complex is how you look at it. It doesn't get less complex than a driveshaft from an ICE to generator. There are energy conversion losses with the mechanical to electrical and back, but not all the electricity made goes into the battery before going to the motors. The only car coming close to direct shaft driven wheels is the Accord hybrid at highway speeds.

The odds are that the HSD is more efficient than this system. The odds are that the gas-electric HSD is incapable of moving a vehicle the mass of the ones the Route is designed for. There are only two(three) HSD models with a tow rating, and they require a third motor on the rear axle to do so. Even then, straight ICE models have higher ratings.

To be successful, the Route only needs to provide better efficiency than the drive train it is replacing. Which is a large diesel(usually) with a traditional type of transmission. Many of the businesses they are targeting also operate these trucks within city routes with multiple stops and starts. We don't know the testing methods that were used for the fuel economy numbers quoted in the articles, but this system will be more efficient than the one replaced.

The 60% originally mentioned was likely just confusion between turbine efficiency and the efficiency of a CHP power plant. I was probably thinking of turbines as more efficient in a power to weight ratio.

Toyota has more efficient piston engines. Their most efficient has a turbocharger, which makes it a turbine/piston hybrid in a way. Toyota has spent a lot money and time to get there. They technologies they and others developed might be applicable to these turbines. Turbines are rarely used in land vehicles. It is mostly in aircraft. Fuel efficiency is important there, but so is keeping everything low weight.

In station applications, like the Capstone cogen units, turbines boost overall efficiency by making use of the exhaust heat. While it would be neat for the use the turbine on the Route to provide heat and cooling to the truck cabin or even cargo box, it probably isn't practical to install with ain a retrofit system.

 

Toyota's new efficient engines, and piston engines for vehicles in generally are designed to a specific fuel. A Prius engine could converted to run on natural gas, or maybe even diesel with some effort, but they will be compromised is some way due to the ICE originally being intended to burn gasoline. NG has an effective octane of 130; the ICE doesn't have the compression ratio to take full advantage of this. So the thermal efficiency will drop. If it was to run on diesel, there is the same efficiency issue, but gas engines of the past that were put into diesel service had reliability issues from not having strong enough blocks.

Then there are the emissions. The diesel would require a whole system to be installed. NG emissions aren't a problem. It will actually be cleaner. The gasoline designed system would be overkill. Which means some ICE efficiency, and maybe savings, can be gained by putting on one meant for NG.

These aren't issues for these turbines. They can burn either of those fuels without changes to the core of the engine without major losses in efficiency. The fuel deliverily system would have be changed, perhaps the control software retuned, but that's a breeze. The piston engine would need to be designed from the ground to get the same efficient out of different fuels. Then the Route turbine is already a couple magnitudes cleaner than CARB standards for these class of trucks without any add on emission controls.

Their mechanical simplicity also makes field conversions easier. A company can install Routes that burn diesel, and later convert to CNG for a much lower cost than on a piston ICE if that fuel becomes more convenient.

 

You simply can't run natural gas or gasoline in a diesel designed engine, as there aren't spark plugs. You can't run diesel is a spark ignited ice because compression isn't high enough.

If you design an spark ignited ice to be flex fuel you can burn natural gas (cng and lng) along with methanol, ethanol, and gasoline. The ice is more expensive though. You likely would do something like mazda's skyactiv but with bigger injectors, or the addition of port injectors for normally aspirated engines. For medium duty trucks though we are talking turbo charged, and if you use variable lift of the valves gasoline would burn in more of a miller cycle than natural gas to all both to run well. What you get in a turbine is possibly lower cost than this turbo flex fuel ice. I don't think you can get a turbine to burn diesel as efficiently as in a diesel truck ice, but you can burn it cleaner needing less pollution control equipment.

The key to a more efficient vehicle like a garbage truck, is efficincy in starting and stopping. Here hydraulic hybrids have been better than phevs, but with battery prices dropping rapidly, using a phev may be able to absorb more of that decelaration energy.

 

My hypothetical was with the Prius engine though. It has a native 13:1 compression ratio. I think the diesel Skyactiv's might be as low 12:1. If feasible in any way, it will still be a major undertaking compared to converting the NG or even alcohol, since you would need to change the valve timing to that of an Otto cycle. Maybe even have to in fancy piston heads like the Skyactiv has.

Being able to burn all those fuels in a piston engine isn't the challenge. Diesel may even be added once HCCI is perfected. The challenge is to have that can burn them all at the same thermal efficiency with minor modification. Take a E85 flex fuel engine. E85 has a higher octane rating than premium gas. A larger compression ratio is needed to take advantage of that and extract the most kinetic energy from the fuel. Most, if not all, flex fuel engines have compression ratios for regular gas. So the ICE loses efficiency while using E85 because of that.

A premium gas and E85 flex fuel might meet the challenge. The Capstone power unit flyer lists the electric efficiencies, and there could be up to a percent difference between fuels for the same size unit. Alcohols and gasoline are pretty similar in properties though. The octane rating is in the high 90's to low 100's. To take advantage of NG's octane of 130 may mean the extra friction for that compression ratio has a more negative impact when running those lower octane fuels.

The class of trucks Wrightspeed is targeting for the Route use air brakes. These can't modulated in as wide a range as hydraulic ones on cars. They are almost on or off in terms of braking application. Trained on such brakes, CDL drivers are probably better at adapting to the harsher regen the Route uses than most of us.

In regards to hydraulic hybrids, I think they will see use along side PHVs. These trucks see long service lives, and their cost means they aren't treated as disposable as a car. We don't know the Route cost, but it is high enough that the truck owner likely won't consider it on a working one, and only ones that need major engine or transmission work. Most hydraulic hybrid work has been on parallel systems, and could affordable enough to install on a truck that is otherwise working fine. Any electric hybrid system that can be installed in such a manner, will likely capture less braking energy due to the battery and motor size compared to the hydraulic one.