Tuesday, November 22, 2011

Piston vs. Gas Turbine Engines, Efficiency, Narrow View

The Narrow View   With a few hours of web searching one can find a number of fascinating and far ranging discussions regarding thermal efficiency, covering everything from the theoretical realm of physics to the mundane world of practical mechanical issues.  

As an example of what is attainable in terms of thermal efficiency, here is a clip of a gas-turbine engine combined with a co-generation steam turbine that can attain a 60% thermal efficiency. 

Unfortunately for those of us designing robotics, large utility power plants, like the one featured above, enjoy an economy of scale that designers of agricultural robots won't have. 

One of the main design constraints for industrial/agricultural robotics is the KISS principle; that is, systems must be field serviceable by workers with a minimum of tools and expertise.  This means that designs for robotic power plants must consist of a minimum of moving parts.  Which means that the added complexities that come with co-generation schemes are off the table as design options.     

Fuel efficiency for an internal combustion engine is generally measured in  Brake-Specific-Fuel-Consumption.  This is a ratio of fuel burned to energy on the engine's output shaft and is given in units of pounds per horsepower-hour, or in units of grams per kilowatt-hour. 

The one aspect that is most important for our discussion is the concept of an efficiency “sweet spot” in the torque versus RPM graph; that is, a region in the torque versus RPM graph where an engine achieves its maximum efficiency [1]. 
BSFC for a Saturn DOHC engine with 20-HP operating curve marked in red.
Source Link

I've looked without success on the web for an equivalent graph for a gas turbine engine.  Based on the information I've come across, the "sweet spot" for a gas turbine would be smaller in area but higher in efficiency than a comparable piston engine; with efficiency dropping off dramatically as one moves away from that "sweet spot" region.    

Again, based on my readings, gas turbines have a higher efficiency over a narrow region in their torque/RPM graph, while piston engines have a "reasonable" efficiency over a broader area in the torque/RPM graph.  

In practical terms, if a robot has a narrow and well defined power requirement, then a gas turbine could be a good choice for its power source.  On the other hand, if a robot's power load is going to vary, then a piston engine will give the greatest efficiency when fuel consumption is averaged over all of the robot's working conditions.  This is one of the reasons that Boston Dynamics' "Big Dog" is piston engine driven.

The Hybrid Engine Concept  The idea is to decouple the engine from the machine it's driving by using an intermediate energy storage reservoir.  The engine drives a generator that charges either a battery or super-capacitor bank.  The robot then pulls its power from there.  This allows the engine to charge up its associated energy storage system while running at its point of maximum thermal efficiency.  It can then throttle back to a fuel saving "idle mode" when a sufficient buffer amount of energy has been stored-up.  This effectively decouples the narrow view from the mid-range view of thermodynamic efficiency.  

[1]. The "sweet spot" for the graph above is centered at a speed of 2500 RPM and a torque of 124.8 Nt-m.  The red 20-HP line represents the average power needed to move the Saturn car under typical driving conditions.  The closed curves represent lines of constant BSFC.  A lower BSFC means less fuel used for the same power output, that is, lower BSFC is equivalent to a higher fuel efficiency.

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