Posted by: jmtoriel | July 7, 2014

It’s the drivetrain, stupid

Efficiency is the ultimate goal to a cleaner and more stable future in transportation and EVs are indeed the best way to go.

I’ve been researching and collecting data regarding plug-in vehicles (and previously biofuels) as a cleaner form of transportation for the past years and am often puzzled as to how “Well-to-Wheel” analyses are calculated without proper comparisons to upstream conventional and unconventional oil sources. Some data is so misconstrued that I often wonder where funding came from and what the motives are to present such unfair comparisons. Alas, the data usually does not account for the superior efficiency of the electric drivetrain. Instead of trying to find a cleaner source of fuel for a inefficient ICE (internal combustion engines), why not focus on the efficiency of the drivetrain itself?

I’d argue that plug-in vehicles are the only real alternative to gas/diesel ICE vehicles presently –for the simple fact that they are both available for purchase, affordable and do not require a new source of fuel (this will only get better with advances in battery technologies and greater leaps in manufacturing). Prices are coming down and more EVs are becoming available to suit more drivers.

While some early well-to-wheel data has emerged over the last few years with the (re)emergence of EV technology (remember that electric cars were on the road before ICEs and even outnumbered them for a few years!) to the mainstream, a more accurate analysis is the Tank-to-wheel measure. In the case of BC, with our exceptionally clean grid (sourced from 93%+ renewable sources), there is no comparison to just how much cleaner plug-in vehicles really and truly are because of how much more efficient the drivetrains are.
“Tank to wheel efficiency” tells us how efficiently engines turn fuel into moving a vehicle, and the “well to wheel efficiency” which adds the energy it took to get that fuel to your gas tank. So if we want to do a straight up comparison, we want to develop similar numbers for EVs.

One of the nice things about electric motors is that they operate efficiently over a wide range of speeds. Not quite wide enough for use in a car, but it’s possible to make very practical designs with two gears, or one like the clever design work GM did in the Chevy Volt.

In an electric car there’s no major transmission system, driveshaft components, and in some designs you don’t even have axles or differentials. A modern electric drivetrain is much simpler than a modern gasoline one, with parts counts that are tens or hundreds of times smaller. In a nutshell — extremely efficient.

In a typical car, the drivetrain eats up about 5 to 6% of the energy from the engine, in an electric design it’s close to 0%.

The motor itself is fantastically efficient, varying between 85 and 95% efficient across the entire range of speeds. But that’s in terms of the electricity being delivered to it from the batteries. That conversion is not direct – the batteries provide DC power but the motor uses AC, so you need to use an “inverter” to change it from one to the other. Modern inverters are about 95% efficient.

In a gasoline car the fuel that’s pumped into your tank is used directly in the engine. That’s not the case in an electric car, where the “fuel” is AC power from your home or building, and the tank is a battery full of DC power. So we have to convert from AC to DC using a charger which is also about 95% efficient.

In short, we start with AC, turn it into DC, back into AC, and motion results.

Technically, fast speed “DC chargers” — of which BC Hydro is currently installing across the province — can remove the AC conversion by charging your battery directly for vehicles that have the plugs (and I highly recommend anyone considering getting an EV to ensure that your vehicle has this).

And finally, we need to consider leakage. When the battery is charged, not all of the power ends up stored, some of it is used up pushing the electrons through the battery. Typical numbers here are about 85 to 90% efficient.

So, a rough estimate of the total round-trip tank to wheel efficiency is:

0.90 (motor and drivetrain) x 0.95 (inverter) x 0.90 (battery) x 0.95 (charger) = 73%

This number jives quite well with the claims of Tesla, which quotes a 75% round-trip efficiency. Tesla and Leaf owners report slightly lower real-world charging numbers, with the charger and battery portions of the cycle on the order of 80 to 85%. If we use those numbers we get:

0.90 (motor and drivetrain) x 0.95 (inverter) x 0.8 (battery and charger) = 68%

This isn’t a huge difference, so we’ll call it 70%.

How does this compare to a conventional ICE car? Quite well in fact. A normal gas powered ICE car has a tank-to-wheel efficiency of 16%.

That’s right, an electric car is over four times as efficient at turning energy into motion. For us efficiency lovers, it’s a non-starter to even compare “fuel-efficient vehicles” with EVs.

Back to well to wheel:
This comparison is not apples to apples because it doesn’t account for where that electricity comes from.

In a perverse world, we could take the engine out of your car, put it in a field somewhere and connect it to a generator, and then ship that power to your electric car over some wires. At that point you’d have the same basic power generation efficiency, but then drop 25% of it in the electric drivetrain. No gain there!

On the macro level, the grid in North America is undergoing a massive switch from coal to natural gas — which will be accelerated further with the latest Obama climate announcement. There’s still a lot of coal out there — here’s looking at you, Alberta. But then there’s also a lot of hydro and some nuclear. RE is growing, but if you’re on this list, you’ll agree that that it is not happening fast enough. Up here in Canuckistan we get over half our power from hydro, so the power mix is considerably cleaner than the US. BC, QC, Manitoba get cleanest grid awards.

NG is burned in large turbines which spin generators. A turbine is about the same overall efficiency as a gas engine running at its peak, turning about 30% of the energy in the fuel into rotating shaft power. The rest, 70% of the energy, is lost as heat. Why anyone claims this to be a “clean” or “efficient” energy source is beyond me — especially considering the rising methane levels and the leaks in its production, storage and transportation. Still, it’s “cleaner” than “clean coal”…

But turbines have one additional trick… in your car that extra heat blows away through your radiator. But in a power plant, it’s captured and heats up water to boil into steam, and then use the steam to drive another turbine. These “combined cycle” generators can be up to 60% efficient. When you factor in things like throttling and load following these numbers go down, but average numbers on the order of 40% are very common, and most modern plants are closer to 50%.

The electrical grid is very efficient. The total losses in the US grid are only about 7%. This number keeps going down as we improve the systems.

So that means the real tank to wheel comparison is:

0.5 (generator) x 0.93 (line losses) x 0.7 (entire car side) = 33%

Now we also have to get that gas to the NG power plant. The drilling and extraction requires energy equivalent to about 9% of the fuel, and shipping it in a pipeline is efficient, accounting for about 1.5% of the energy. It’s hard to nail down leakage, but for the sake of argument, we’ll leave that out. So that means the total cycle end-to-end is:
0.91 (extraction) x 0.985 (shipping) x 0.33 = 29%

Now we have a number that we really can compare to a gasoline car – we’re accounting for everything from the well to the wheel, and that number is around 30%.

For BC’s grid we can assume hydroelectric dams generate at about 90% efficiency so,

0.90 (generation) x 0.93 (line losses) x 0.7 (entire car side) = 60%

So what’s a typical number for well-to-wheel for a conventional car? About 14%.

So, even if we were running primarily on natural gas, if we took the gasoline you put into your ICE car and burned that in a turbine, then sent that power to your electric car, the overall efficiency of the system would double.


The beauty of an “electric economy” is that batteries can be charged up at any time and stored in your vehicle. They’re charging up at night when the reservoirs get a refill and wind is at its best.

The holy grail of energy is storage, which our hydro dams do so very well, but a smart grid will allow the utilities to access the stored energy in our car batteries when they need it most — peak. This is called vehicle to grid technology and it’s getting closer to becoming a reality. Again, minimizing waste and reducing grid demand while benefiting the consumer is an all-around win.

As more and more cleaner sources of energy come into the mix, invariably more efficient and less polluting than existing ones, EVs get better and better while existing ICE car’s efficiency and emissions are fixed at the moment it was built.

Electric cars burn anything or ultimately nothing. No matter what new fuel we invent in the future, your car will burn it, without changing a thing.Even better, encouraging more decentralized production from rooftop PV solar and battery storage will mean less burning altogether!

While we wait for the inevitable conversion to EVs, there are problems we’d like to solve in the shorter term. And the mid-range solution is plug-in hybrids, or PHEVs. This gets us all of the advantages of battery electric vehicles (BEVs) on the vast majority of trips, and gives you a fail-safe option for longer trips. Of course the Model S has a range of 300km+ without a backup ICE already and over 650 Level 2 (240V) public chargers available in BC, so range anxiety will become a thing of the past.

The difference between a PHEV and a fully electric vehicle is that you’re hauling around a back-up ICE engine everywhere you go, even when you don’t need it (which is most likely most of the time). But if you pull that out you need more batteries, so the difference isn’t as much as you might think. PHEVs have less battery, say 1/4 that of a fully electric vehicle, so as long as batteries remain as expensive as they are now, this is a much lower cost option.

Still, I figure if you’re going to reduce your carbon footprint in BC, there is no better way than to tackle the largest contributor — transportation. If you drive, make it electric and the sooner, the better for all of us.

Here’s a really great report on the topic from the Union of Concerned Scientists:


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