Question On Hybrid Synergy Drive System

[Aristo]

I was reading up on how the Hybrid Synergy Drive System works on Toyota's website and came across one question:

1) Under heavy acceleration and highway cruising, some gasoline engine power is given to run the generator, which powers the electric motor. So the gasoline engine must do two tasks: turn the front drive wheels and the generator. Won't this process cause the engine to do a lot of work, thereby consuming a lot of gas?

Any advice would be greatly appreciated. Thanks in advance.

[efusco]

I was reading up on how the Hybrid Synergy Drive System works on Toyota's website and came across one question:

1) Under heavy acceleration and highway cruising, some gasoline engine power is given to run the generator, which powers the electric motor. So the gasoline engine must do two tasks: turn the front drive wheels and the generator. Won't this process cause the engine to do a lot of work, thereby consuming a lot of gas?

Any advice would be greatly appreciated. Thanks in advance.

The gasoline engine always does all the work.

The way the system is set up is that the ICE will run within it's most efficient rpm range...when it does so not all the power generated is needed to propel the car...the excess turns the generator which dumps it's power to the battery for later use.

The Toyota site is not intended for a true technical explaination...I'm not sure it even mentions that there are 2, not just 1, Motor/generators in the Prius.

If you really want to understand how this system works Graham's Site is probably the best place on the web. Read and re-read that, and if you still have any questions come on back here for clarification.

[KMO]

The engine isn't really doing two jobs in the example you cite - it's doing just the one job of pushing the car, but the power gets split and flows in two different ways:

72% of the engine's torque goes straight to the wheels.
28% goes into the generator, which then powers the motor, which then turns the wheels.

There's some conversion loss in the torque->electric->torque conversion, but on the other hand the system totally replaces the conventional gearbox, so you save some energy there.

In high-speed cruise, the system is not vastly more efficient than a normal car. You mainly get your efficiency gains from having a smaller engine (which you get away with because you have battery boost for heavy acceleration), and the rest comes from the Prius's better aerodynamics.

The main point of the HSD is flexibility - it can rapidly tailor itself to acceleration, deceleration, low-speed town driving. And it's vastly simpler mechanically than a conventional automatic gearbox.

[Aristo]

Thanks for the response. I did not know that ICE is that inefficient. I will also check out the website so that I can learn more about the HSDS.

[hdrygas]

Thanks for the response. I did not know that ICE is that inefficient. I will also check out the website so that I can learn more about the HSDS.

No the ICE is very efficient because of the Atkins (Miller?) cycle it uses to run. It gets as much efficiency out of a gallon of gas that an internal combustion engine can get while reducing emissions. Having said that internal combustion engines are not the most efficient inventions of man. It will get better and when it does it will have the Prius name on it.

[KMO]

The point about ICE efficiency is that engines are more efficient at particular rev ranges. The combination of the continuously-variable transmission and the ability to "even out" the engine's load by storing excess energy in the battery and giving it back later allows the system to run the ICE more efficiently and cleaner than a normal car can. The engine rpm is effectively independent of the car's speed.

Typically, giving a UK example, after cruising along an A-road at 60mph for a while, if you come across a village or town and have to slow to 30, the car will often have built up a nice store of energy during the cruise, and will be able to proceed the next mile or two at 30 with the engine off, using just the electric motor. The engine then fires up as you accelerate out of the village. The engine thus runs under the more-efficient high-load conditions.

If you're going a longer distance at 30mph through town, the system will cycle between ICE power+charging, and electric-only. While the engine's on it'll be working harder but more efficiently to charge the battery. Overall it works out more efficient than having the motor on continuously.

The actual on-off cycling of the engine is virtually unnoticeable, and can happen extremely rapidly. It may go on and off every few seconds as the load changes. Whenever you start decelerating, the engine will normally cut out instantly, and the regenerative braking starts to charge the battery.

And on top of all that, because the battery+motor serves as a supercharger for heavy acceleration, the engine no longer needs to perform that task alone, so can instead be tuned for optimal performance under moderate load. In particular it uses the Atkinson/Miller cycle rather than the normal Otto cycle - efficiency is gained at the expense of peak power.

[jamarimutt]

When stopping, the ICE will frequently idle for several seconds. To shut it off immediately, shift into B shortly before stopping, and when the car stops shift to N and then to D.

[KMO]

It should only do that once or twice per journey, as far as I know. That's "stage 3" behaviour, as per daniel's nomenclature.

I can't say I'd bother with that sort of mucking around to save a few seconds' fuel. Once you're in stage 4, the engine should stop before you come to a halt, unless it's running to heat the cabin.

I wonder if your attempt to shut the ICE down forcibly might stop it getting into stage 4? The stop-when-stationary is a prerequesite to leave stage 3, for reasons unknown.

[DanMan32]

Another reason for the power split is because motors can produce better torque at low speeds.

From some research I've done, it seems the gen II (04/05) prius could transfer power from MG1 to MG2 without utilizing the battery much. In the 500V side of the booster system is a capacitor, so MG1 could charge the capacitor from the power split from ICE, while MG2 takes from the capacitor to assist in turning the wheels. The synergy is the gradual shifting of power to the wheels from MG2 to directly from the ICE.

[ken1784]

From some research I've done, it seems the gen II (04/05) prius could transfer power from MG1 to MG2 without utilizing the battery much. In the 500V side of the booster system is a capacitor, so MG1 could charge the capacitor from the power split from ICE, while MG2 takes from the capacitor to assist in turning the wheels.

If your theory were correct, the MG2 won't be driven by the power generated by the MG1 without the capacitor.

My theory is the capacitor is used for filtering to reduce the ripple noise.
The MG2 is still be able to be driven by the power generated by the MG1 without the capacitor.

BTW, the original(NHW-10) and the classic(NHW-11) are also equipped with capacitors on the inverter circuits, only difference is there is no such high voltage converter.

Regards,
Ken@Japan