Volt: 4 modes of operation...

Since Volt has a planetary gearset, just like PHV Prius, the mechanical loss due to it, would be the same.

 

So does the prius. Normal-ev-Eco-power

 

Mode 3 explains why the MPG is so low in CS operation bolow 70 mph because the ICE is running is series hybrid mode which incurs electrical generator losses.

 

I dont know why mode 1 is needed if mode2 exists. Volt only need mode2 and mode4.

 

Hi All,

Thinking about this, I think they should have made two Volts. THe Volt E and the Volt SH. The Volt E would one direct gear from the motor to the axle differential gearset, and be limited to something like 85 mph, with poor efficiency above 70 mph. Or better yet, two motors coupled directly to the two half-shafts, and no differential!

The Volt SH would be what they are going to be selling in a few months. The SH stands for Sports Hybrid...

 

I would like to understand the 'direct' ICE to PSD linkage in mechanical detail. At the moment I gather the ICE has a shaft connected to the generator, and the motor has a shaft to the PSD. How is a mechanical connection made between the generator and the motor ?

 

The generator is connected to the ring gear and the motor to the sun gear. The planet carrier turns the wheels.

 

It is impossible since C2 is disengaged. For both MGs to regen, it has to be in mode2.

It is possible. In CS mode, the generator+ICE can assist starting from 30 mph. Therefore, in mode2 the generator can assist as well.

 

I may just be really ignorant here, but I thought a generator takes mechanical input and puts out electric.

 

It will do that in mode3 when clutch2 and clutch1 are disengaged.

The answer depends on the state of the three clutches.

If clutch2 is engaged, the generator will be connected to the ring gear of PSD.

If clutch2 and clutch3 are engaged, both ICE and generator will be connected to PSD on the same shaft.

 

The torque curve of the traction motor did not seem to meet GM's design requirements for high speed power. This necessitated mode 2 where the generator (MG1 in prius speak) assists the traction motor (MG2 in prius speak). A pure series hybrid was still possible, but would have hurt high speed performance, so they allowed your mode 4 to assist the motor (MG2) in hybrid mode. Then they claimed this software performance enhancement was really to increase efficiency.

The big question is as long as they added this complexity for parallel/series operation why they didn't optimize it more. This would have provided better mileage. Of course the next logical step would to put in the turbo version of the engine from the Cruise and allow a button to provide hybrid acceleration even when the batteries have sufficient power. This likely would have improved mileage when driver dictated or more power and would not have required premium gas.

There are 2 more modes from the gm design. Neutral with no clutches engaged, and neutral with generator with clutch 3 engaged between the motor and generator.

 

The generator is likely not strong enough to resist the traction motor so the psd is locked down making mode one. No need to power the generator (Mg1 in prius speak) while under 70mph in ev mode.

Mode 3 may just be because GM did not design the thing for parallel operation so rpm's and gear ratios are likely not correct at normal speeds. I think it would require another planetary gear set, unless the volt already has gearing between the generator and engine.

 

probably just confusing terms. The engine is connected to the generator with a clutch. In series operation, mode 3, it generates electricity that can be fed to the traction motor and/or batteries. In mode two it acts like a motor and is disconnected from the engine but connected to the psd and draws power from the battery to help the traction motor in high speed operation. In mode 4 it acts like mg1 of the prius and I'm not sure if its a motor or a generator.

If it was a normal series phev as gm originally described it, the generator would only be a generator as in mode 3, and mode 2 and 4 as well as clutches 2 and 3 would not exist.

 

Because at low speeds in CD, mode 1 will be more efficient.

In CS, mode 3 is the 'serial' mode. Here this mode again is more efficient at low speeds. GM needs all the efficiency they can get with all of that extra weight and the crappy ICE they are using.

 

FYI,
Following is the official GM document.
Chevrolet Volt: Engineered for Everyday Life
http://media.gm.com/content/dam/Media/documents/US/Word/101010_volt_launch/Drive_Unit.doc

Ken@Japan

 

That document reveals the gear ratios. Volt has three clutches and it provides 2 gear ratios. I wonder how it will perform while gears are shifting.

 

It really has 1 set of gear ratios, with mode 2 and 4 allowing the ring gear to move with the generator or the generator/motor combo. In fixed gearing (mode 1/2) there is approximately a 7:1 ratio between the traction motor and the wheels. Mode 1/2 transition should be very smooth as the generator and ring gear should engage at 0 rmp. mode 3/4 transition we don't know about as most of the reviews don't talk about it, but I would assume it is also smooth, and the motors can likely adjust speed to have very low rpm difference between the generator and ring gear.

The blurbs on gm_volt seem to suggest the mode 3/4 transition can take place anywhere from 30-70 mph, taking place earlier at lower rpms when there is more load on the engine. The details of the software and power curve of the engine are not really clear.