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Comment by ZeroGravitas | Hacker News Reader

Chevy Volt goes a step further and has the gas powered engine hooked up to the wheels. I prefer the approach taken by the BMW i3 where the (optional) gas powered range-extender is purely used to generate electricity, but there's room for both in the transition period and PHEV (plug-in hybrid electric vehicles) seems like it might be an important market segment.

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The Chevy Volt and BMW i3 are identical in the regard of the function of their petrol engine. The Volt is using Chevy's Voltec drivetrain, so there's no way for the engine to put power to the wheels except through charging the battery - the same as the i3.

https://en.wikipedia.org/wiki/Chevrolet_Volt

https://en.wikipedia.org/wiki/GM_Voltec_powertrain

"When the engine is running it may be periodically mechanically linked (by a clutch) to a planetary gear set, and hence the output drive axle, to improve energy efficiency."

Yeah not sure how I skipped that part so effectively. I've since looked through what else I can find about that Voltec drivetrain and it's clear that the planetary gear can engage in such a way that the petrol motor can provide assistance to the electric motor.

In case anyone's interested, here's a fairly good and simple explanation of the 3-clutch system which allows the petrol motor to provide some power to the wheels when in the correct scenario: http://www.youtube.com/watch?v=80E1fOp95rA

Yes, this was the explanation I saw when my Girlfriend purchased a used volt last summer. We explain it to most people that the gas engine is there to act as a generator and charge the battery and act as a generator. Not to directly power the car. She loves her car.

This is not correct, also from the Wikipedia link above, describing the Volt driving modes: "Dual motor extended – The electric motors are used again in dual configuration with increased efficiency at higher speeds. Additionally the gasoline engine contributes propulsion power via the planetary gear. While power is drained from the battery the amount is less than in mode 2 for the same propulsion power, thus extending the range."

Emphasis my own.

The problem with this approach (called a "serial hybrid powertrain") is that it's much less efficient than a parallel hybrid like the Prius. You can see it in the fuel economy figures: the Volt gets much lower highway fuel economy when it's running on gas than the Prius does, and only makes up for it on shorter trips where it takes advantage of its relatively large all-electric range. If you're doing a lot of highway driving, the Prius is a much better choice, both for fuel efficiency and for price.

Using a gas engine just to generate electricity, and then running that through some conversions and battery storage before using it to run drive motors just isn't as efficient as using the ICE to drive the wheels directly. Or maybe GM does a crappy job of it, but the fuel economy figures speak for themselves.

Yes, it's less effecient if you're actually using the engine but the aim would be to not use the engine, and purely use battery storage as much as possible.

There's graphs of average driving distance, the idea would be to get 90+% of those miles (daily commutes etc.) done with grid-supplied electricity, and the gas engine to be a once in a while thing (assuming you even need one, it is after all an optional extra that you need to pay for on the i3).

At that point you can start to save on the engine expenses too (I think the i3 one is repurposed from a motorbike?)

>Yes, it's less effecient if you're actually using the engine but the aim would be to not use the engine, and purely use battery storage as much as possible.

Yes, but a parallel hybrid powertrain like Toyota's shouldn't be any worse here. The Prius also has a plug-in version which can run on battery alone for a certain distance, and it only spins up the engine when needed for more power or because the battery is too low. Of course, the Prius doesn't have the Volt's all-electric range, but it also has a much smaller battery (and lower price).

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