Non-EV Button?

This is a very bad idea. The electric motor is an integral part of the power plant. When you accelerate to pass other cars or merge onto the freeway, the electric engine provides the initial torque required because the gasoline engine in the Prius is not efficient at providing start up torque. You stated that "mechanial power" is more desireable at freeway speeds than electrical power. Actually, the Prius system uses both the electric motor and the gasoline engine to provide mechanical power to the wheels, so I don't understand your distinction here.

Assuming you could do it, shutting off all electrical regeneration would, in short order, shut down the electric motor by allowing the battery charge to drop below operating levels. The gasoline engine, alone, is not sufficient to provide you with enough power to operate safely on the freeway. There have been accounts of people making it to gas stations on electrical power alone, but this too is a bad idea because the battery needs the regenerative charge from the gasoline engine. They are a pair and you can't split them up.

I don't think you understand the "transmission" in the Prius. There aren't gears in the sense that your old car had, say, five gears. Prius uses a planetary gear system that allows either or both gasoline and electric engines to operate; the computer determines the optimum use of each based on the load being put on the car and the charge in the battery. So, there isn't anything to lock up. When you start up a conventional car, the lower gears and the engine take the stress of moving the two tons of car from a stop to forward motion. This is the hardest strain on the gears and the engine. The Prius uses the electric motor to do that both because the motor is ideal for low rpm/high torque situations and because electric motors are direct drive and don't need gearboxes to translate piston rpms to wheel rpms. The gasoline engine in the Prius is allowed to operate within its optimum rpm range and with a minimum of start and stop stresses imposed on it. That's the inherent beauty of this system: each power plant operates in its optimum efficiency range. Why mess with it?

Toyota got it right and you most likely won't be able to tweak the system for any added hidden efficiencies without voiding your warranty and possibly ruining your car.
Bob

There is no slippage in the Prius because everything is connected by gears that are always engaged. How this can be done is mind-bogglingly elegant. Find a link to Graham's Prius Page for detailed explanations with diagrams and animated gifs.

Due to the nature of the planetary power-split device, a fixed proportion of the torque of the ICE always goes to MG1 to generate electricity, which the computer can send to the battery or to MG2 to help power the wheels. But because of the physics of the device, the faster the car is moving, the less power is shunted via MG1. Thus, at highway speeds, most of the power is going directly from the ICE to the wheels, just as you want it to be. At low speeds, where a conventional car is inefficient and lugs, the Prius ICE runs at efficient RPMS, sending power to MG1 then to MG2 and then to the wheels.

Believe it or not (and the numbers don't lie) the HSD is the most efficient way to drive a gasoline-powered car today. Only the Insight gets higher mileage, and that's because it is so much lighter in weight. If you did tell the car not to charge or draw from the battery at highway speeds, you'd actually reduce its efficiency, because even on the highway, the load-balancing effect improves overall efficiency.

In an attempt to deepen Daniel's understanding of the Prius and bore the rest of you, I'll comment on the following snippet from his well stated post.

Daniel said:

. . . a fixed proportion of the torque of the ICE always goes to MG1 to generate electricity . . .

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Daniel, you need to separate the concepts of torque and power. The generation of electricity requires power, but power depends on the product of torque and angular velocity, both of which are signed numbers. It turns out that when cruising at highway speeds, the torque and angular velocity have opposite signs from the engine to MG1 (i.e. MG1 is spinning backward w.r.t the direction of engine spin). Thus MG1 is providing power to the PSD (it's acting like a motor, not a generator). Since the engine is also providing power to the PSD and there's only one other place for it to go, ALL power from the engine (and from MG1) goes to the shaft attached to MG2 and the wheels. MG2 may choose to absorb some of this power (act as a generator), contribute even more power (act as a motor), or on rare occasion, do nothing.

Once upon a time, Graham called this "heretical mode" since nobody wanted to believe MG1 acted as a motor during cruising. Now that it's generally accepted (proved by the miniscanner), it's mostly refered to as "overdrive" since that's exactly what it is (engine spining slower than output shaft of transmission).

It's an incredibly efficient mode because only a relatively small amount of electric power is involved. I suspect it's more efficient that sending the engine power through the additional gear meshes and other efficiency sappers of a conventional automatic transmission. Just another reason why the Prius transmission is genius!

Daniel said:

If both MG1 and MG2 are acting as motors at highway speed, then how come I still see electricy flowing sometimes to the battery, and sometimes out of it? For there to be battery charging going on, mustn't MG1 be acting as a generator? Or does it go into and out of "heretical mode" somehow as needed?

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Sometimes both MG1 and MG2 act as motors at highway speed. Obviously not during the times that you observe energy flow into the battery. During those times, if you're cruising, MG1 is acting as a motor and MG2 is acting as a generator. Some of the power from MG2 flows back to MG1 to enable it to be a motor, while the rest goes back to recharge the battery. This is actually a pretty common case during highway cruising. The current levels are relatively low compared to what MG2 is capable of generating (for instance during regenerative braking). Something like 5 amps vs. 50 amps. So it's not really going into and out of heretical mode (MG1 acts as a motor all the time during overdrive cruising), but MG2 switches back and forth between motoring and generating. You see the arrows reverse when it does this.

There's a fairly big "dead zone" around the engine speed that would result in MG1 having zero spin. That's why when you ask for more power (to pass, or go uphill for example), the engine speed suddenly jumps much higher (gets much louder). MG1 is switching from a motor spinning backward to a generator being spun forward (where the direction of engine spin is considered forward).

Daniel said:

I think at this point I need to spend some more time on Graham's Prius page.

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If I might suggest clicking "Understanding your Prius", then "The Power Split Device" and considering the red line on the nomograph about 2/3 down the page. It actually works to think of the line as a bar with up to three forces on it. The ICE is providing power, thus pushing up on the red line. This needs to be counterbalanced on both ends since it's steady state and the line isn't moving. The physical drag on the wheels represent a force pushing down on the right side (trying to slow down the spin value on the right side). MG1 also needs to push down. Since it's in the negative spin range, this means trying to spin faster, which means it must act as a motor.

The nomograph is pretty cool since it's drawn so that the relative distances invoke leverage representing the mechanical advantage MG1 has over the ICE.

Motor/generator control is a pretty involved subject. The howstuffworks site can help a lot, as can Graham's site. But conservation of energy makes it fairly easy to understand the strategy. Basically, if the battery is connected to the terminals in a manner that causes electromagnets to pull/push the rotor in the direction it's moving, the m/g acts as a motor and the battery sees a drop in voltage across its terminals causing it to lose energy. This balances the rotational energy being created by the motor. If, instead, the battery is connected such that the electromagnets pull/push the rotor in the direction opposite to it's spin, the m/g acts as a generator and the battery sees an increase in voltage across its terminals causing it to gain energy. This balances the rotational energy being removed by the generator. In both cases "balances" is used loosely. Some energy is always lost to heat/friction/vibration to keep the world's supply of entropy rising.

To go back to the nomograph briefly, if the m/g tries to push it's point on the bar away from zero, it's acting as a motor and requires electrical energy (from the battery or the other m/g). If it tries to push it's point on the bar toward zero, it's acting as a generator of electrical energy. However, near the zero RPM point it's not able to act as a generator because the induced electric fields from the rotor passing the fixed windings are too small. It's also somewhat inefficient as a motor since relatively large amount of energy is lost to heating the windings. So the computers try to stay away from zero if possible.