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April 23rd, 2017 Total archive posts: 984
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Range Extenders

I've been looking at range extenders which are conventional fuel-burning motors that can charge the primary battery of an electric vehicle. There are a number of approaches, each with their own advantages.

Recently, turbines have been getting some press as viable range extenders. There are a lot of ways to compare turbines to other powerplants: power per weight, total efficiency, power per physical volume, and so forth. What interested me today was to see the specific power output based on the amount of air ingested by the turbine.

In today's market, it's common for 4-cycle piston motors to produce around 100 horsepower per liter at peak power. But range extenders should be run at their peak efficiency (which is where the torque peak occurs). For a motor like the Honda Fit a dyno plot suggests the peal efficiency is around 5,000 RPM, at which point the motor is producing about 85 horsepower. The engine's 1.5 liter size then results in a specific output of about 57 hp/L.

For turbines, power output is measured in how much horsepower is made compared to the mass of air ingested. For the Honda motor above, at 5,000 RPM the 4-stroke motor ingests about 2.2 cubic feet of air a second, which at 60 degrees F is a measly 0.17 pounds of air. So the motor produces about 500 horsepower for every pound-second of air.

By comparison, high-efficiency turbines produce on the order of 30 horsepower per pound-second of air. Or, to put it another way, a turbine must intake over ten times the volume of air to produce the same power output. This means that on passenger cars equipped with turbine range extenders, you'll expect to see (and hear) a lot more air circulating through the vehicle. Routing, filtering, and exhausting this air will be a major design challenge.

A quick crib: the thermal efficiency of the Otto Cycle engines (like the Honda) is in the neighborhood of 20-25% depending on the engine's compression ratio (typically 10:1). By comparison, Diesel engines can reach 40% due to the much higher compression ratios (as high as 25:1). Turbine engines vary in their efficiency, but generally speaking they are slightly less efficient when scaled to the power output (around 100 hp) of an internal combustion engine, although some claim to have improved upon this substantially.

by Christopher Heiser on April 27 15:42
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