It's the battery, .......

IMHO, there are other big differences, but Volt relies on battery juice only.
Prius is a ICE car with a traction battery, obviously not designed to run EV all the time.

I don't really get to your question, please apologize.

 

It is not about the battery, gas engine, transmission (or lack of), or anything specific.

It is about how synergistic the gas and electric as a whole system perform. In my opinion, Volt doesn't have the synergy that HSD has.

 

As I understand it in not so technical terms,

Prius HSD ICE can directly drive one gearset (I think it's the planetary gear carrier), while MG1 drives the ring gear IIRC, seperately or in combination for maximum efficiency.

One of Volts electric motors drives a gearset, and the gas generator can couple to a gearset over 70 mph and high load conditions, but when the traction battery is at minimum charge, this happens ...

the premium gas powered generator kicks in to supply the electric drive motor(s) (and the battery?).

But, doesn't the Volt gas engine turn an electric generator which supplies charge to the battery and/or electric drive motor which drives the wheels? If right, isn't this quite inefficient?

 

That's the case when it is operating in the series hybrid mode. The ICE won't be clutched directly to the wheels. 100% of the gasoline energy will be converted into electricity. Most of it will power the electric motor. Some of it will recharge the battery.

Let me take a stab at when the Volt's ICE is clutched mechanically to the wheels. First, open the Prius Planetary Gearset simulator for visual reference. Let's relabel it from left to right.

Left = Prius MG2/Wheels = Volt's ICE/MGb (think of this like Honda's IMA. ICE and MG must spin at the same RPM)
Middle = Prius ICE = Volt's wheels
Right = Prius MG1 = Volt's MGa

Since PSD does not change gear ratio, all three have to be in a straight line. For the Prius to accelerate, the left side has to go up. MG2 (bigger motor with 295 lbs torque) is connected to the wheels so it is pretty straight forward. If the ICE (middle) remains at the same RPM, MG1 (right side) has to slow down or turn opposite direction. This is only be possible if the HV battery is capable of supplying all the acceleration power. During hard acceleration, additional power has to come from the ICE. This means the middle will rise up. If both the left and middle go up, the right must go up too (to maintain a straight line). Since the right is the generator, it means it is generating more electricity. What we have is MG2 powering the wheels together with the ICE revving faster while MG1 generating more power (supplying back to MG2). It is beautifully set up.

Let's look at the Volt. To accelerate (raise up the middle), you need to raise the left, the right or both.The left and the right side has to work against each other to raise up the middle. In another word, ICE clutched with MGa (left) has to work <b>against</b> MGb (right) to increase the wheel speed (middle). There is no synergy between gas and electric in this configuration since they are working against each other -- instead of working together like in the Prius.

This is the root fundamental flaw in the Volt's PSD setup. It does not give ICE the flexibility to stay at the optimal RPM or load. Even if the Volt's ICE switches to Akinson cycle, it will not be able to match Prius' MPG.

 

They are playing the angle of full EV capability up to 100 mph, 40 miles range and drives like an EV, not a hybrid.

http://www.motortrend.com/features/...the_chevy_volt_to_see_how_it_ticks/index.html

Only time will tell if the real world efficiency, etc... will be worth the cost.