Last time, we looked at the coming revolution in automotive technology: the switch from internal-combustion power to hybrid power and, eventually, to straight electric vehicles. This time, we’ll take a closer look at both the pros and cons of electrics, which hold such huge promise for a cleaner, quieter and more eco-friendly environment.

In order to appreciate how profound this change will be, though, a bit of nuts-and-bolts background is in order. One basic way of seeing how well a machine works is by looking at its thermal efficiency, which is simply the percentage of input energy that’s turned into useful work.

The early steam locomotives of the 1840s — the first motive power that didn’t depend on wind, water or muscle — were about 3 percent efficient. Over the next 100 years, technical improvements managed to nudge that figure up to about 7 percent — a big relative improvement, but none too good in absolute terms. Since a large steam locomotive of the 1940s typically burned about 65,000 pounds of coal per hour, about 60,000 pounds of that coal was effectively wasted.

Editor’s note: This is Part 2 of a two-part series. Read Part 1.

Last time, we looked at the coming revolution in automotive technology: the switch from internal-combustion power to hybrid power and, eventually, to straight electric vehicles. This time, we’ll take a closer look at both the pros and cons of electrics, which hold such huge promise for a cleaner, quieter and more eco-friendly environment.

In order to appreciate how profound this change will be, though, a bit of nuts-and-bolts background is in order. One basic way of seeing how well a machine works is by looking at its thermal efficiency, which is simply the percentage of input energy that’s turned into useful work.

The early steam locomotives of the 1840s — the first motive power that didn’t depend on wind, water or muscle — were about 3 percent efficient. Over the next 100 years, technical improvements managed to nudge that figure up to about 7 percent — a big relative improvement, but none too good in absolute terms.

Since a large steam locomotive of the 1940s typically burned about 65,000 pounds of coal per hour, about 60,000 pounds of that coal was effectively wasted.

In the postwar era, diesel-electric locomotives with thermal efficiencies of around 21 percent arrived on the scene, wiping the wasteful steam engine off the map for good. By comparison, most modern internal-combustion cars are around 26 percent efficient, although their friction-laden mechanical drive lines drag this already modest figure down to about 18-20 percent. In other words, 16 gallons of your 20-gallon gas tank go toward generating heat and nothing else.

The electric car constitutes a quantum leap over this dismal performance. Electric motors are typically around 78 to 90 percent efficient to begin with, and the absence of a mechanical drive line means most of this power actually gets to the wheels instead of being burned up in friction. What’s more, electric cars can use regenerative braking systems that use braking energy to charge their batteries instead of burning it up in heat as today’s cars do.

The electric drive system is also far simpler and, eventually at least, will be much cheaper to build than today’s enormously complex internal-combustion cars.

But the news isn’t all good. While electric vehicles themselves don’t produce emissions, as things currently stand, the electricity they use is far from emissions-free. In the U.S., about two-thirds of our electricity is generated from burning fossil fuels, which leaves a very nasty carbon footprint.

The thermal efficiency of a typical coal-burning generating plant is itself less than 50 percent, and what’s more, transmitting this electricity to the user induces another loss of efficiency, typically around 7 percent. Under these circumstances, plugging in a purportedly "zero-emissions" electric car simply transfers environmental degradation from the vehicle to the generating plant — in effect, these new electric cars are actually burning coal.

The solution is to develop an infrastructure that can recharge vehicles using clean sources of electricity that are locally generated, whether by wind, water or photovoltaic panels. This is the only way an electric car can truly meet its potential as a "zero-emissions vehicle." The challenges are great, but, if history is any indication, our ingenuity is greater.

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