With rapid advancements in automotive powertrains it can be hard to keep up with all of the technologies that automotive engineers are using to “electrify” their vehicles, and how they affect emissions and fuel economy. Based on my experiences with the University of Waterloo Alternative Fuels Team (UWAFT), I want to briefly outline various common electrified vehicle architectures, the resulting challenges in measuring their fuel economies, and how the industry is adapting with new standards. Oh, and most importantly, what are the benefits to you?
At two extreme ends of the electrified powertrain spectrum are the internal combustion engine (ICE) vehicle (no electricity), as well as the Battery Electric Vehicle (BEV) (pure electricity). ICE vehicles have lots of range, power, and convenience, but they rely on our limited supply of fossil fuels. BEVs, depending on how the electricity is made, can have great emissions performance and even great power (think Tesla Roadster), but current battery technology cannot provide sufficient range for the occasional long trip, yielding “range anxiety” for most drivers. So, there are a number of powertrains that attempt to bridge the gap and combine the advantages of both technologies.
First, we have the Hybrid Electric Vehicle (HEV), where an on-board battery pack, stop-start gasoline engine, and electric motor(s) with regenerative braking work together to reduce fuel consumption by increasing overall system efficiency. Think Toyota Prius and almost every other hybrid on the road today. Some HEVs can even drive “all-electric”, but only under limited conditions (i.e. low speeds and very short distances). The HEV ultimately gets all its energy from gasoline, it just uses it more efficiently.
Increasing the electrification factor, there is a Plug-in Hybrid Electric Vehicle (PHEV), similar to an HEV except that the battery can be charged from a wall outlet. By using grid energy instead of gasoline we can further displace petroleum consumption (depending, of course, on how the electricity is made). Like the HEV, a PHEV can drive “all-electric” under limited conditions, and turns on the ICE at higher speeds.
Then there is the Extended-Range Electric Vehicle (E-REV). An example you might have heard about is the Chevrolet Volt. In our powertrain spectrum, the E-REV sits between a PHEV and a BEV. It is re-chargeable from the grid like a PHEV, but it can drive all-electric under ALL operating conditions like the BEV. I say it “can” drive all-electric, but there are some very good reasons why you wouldn’t want to do that all the time. The revelation that there is a mechanical connection between the Volt’s engine and its wheels sparked many stories that, unfortunately, haven’t been met with much engineering science to explain the benefits. I will attempt to do so here.
The Volt has a few key powertrain elements: a gasoline engine, an electric generator coupled to the engine, an electric traction motor, a lithium-ion battery pack, and a special transmission to manage power distribution.
Starting with a full charge, the Volt will get up to 160 km/h with just battery power. This is called “charge-depleting mode,” because the battery state-of-charge is depleting as you drive. This is an important distinction if you remember that HEVs and PHEVs cannot drive all-electric under all conditions, so even on short trips you are going to use some gasoline.
Once battery energy is (mostly) depleted, we move to “charge-sustaining mode.” The gasoline engine turns the generator to create electricity, which spins the electric motor. At lower power demands this “double-conversion” of energy is actually more efficient than driving the wheels directly, because the engine is now free to spin at any RPM and operate in its most efficient region. However, as vehicle power demands increase (say, on the highway), the efficient engine power output is similar to the vehicle demands. It no longer makes sense to double-convert, so the transmission sends power from the engine directly to the wheels. This only serves to improve drivetrain efficiency, and in no way invalidates the original claim that it is an “all-electric” when it is charge-depleting. It is this feature that makes it better.
So, how do we compare fuel economies now that we have multiple energy sources on-board? We use what’s called the Utility Factor (UF). The UF is the percentage of your average daily driving distance that is within the vehicle’s all-electric range. So, you might drive 80 miles per day on average. With an all-electric range of 40 miles, 50% of all of your miles are charge-depleting (no gasoline) and 50% of your miles are charge-sustaining (engine is on). If you only drive 40 miles per day, your UF is 100%!
So, we can use the UF to weight fuel economy numbers. If an E-REV gets 5 L/100km in charge-sustaining mode, but uses no fuel in charge-depleting mode, we weight the overall fuel consumption as 50% at 0 L/100km and 50% at 5 L/100km, for a total of 2.5 L/100km. This will vary from person-to-person, but if you look at statistical passenger car driving data, you find that close to half of all vehicle miles could be driven with electricity alone, if everyone drove an E-REV like the Volt. That combination of reduced fuel consumption and extended range cannot be matched by the Toyota Prius nor the Nissan Leaf, although those vehicles have their own benefits.
In conclusion, new powertrains like the Volt attempt to use current technology to give consumers the best of both worlds: high performance, long range, and reduced gasoline use (and consequently, lower daily driving costs). Sure the Volt is more expensive upfront, but it’s the displacement of gasoline that is the key to its appeal. Sure, the Volt mechanically drives the wheels under certain conditions, but only because its engineers figured out how to squeeze every performance benefit they could out of the architecture, the same architecture they promised.
Hopefully this clears the air somewhat surrounding the new powertrains like the Volt, and how the Volt gets an edge on other powertrain technologies like the Prius and the Leaf. Take a look at the Volt website to learn more about the technology.
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