[PeterB]

Why would the ICE be running at all when the battery is totally charged to point of being green after a long descent and I am continuing down a further gentle slope at about 20-30 mph? This just started this week with the warmer weather. Before, the ICE did not come on. I am driving a new Prius that I purchased at the beginning of March so I am still learning about the vehicle. Is the battery so fully charged that the ICE has to come on to cool it or something? Nothing else is on to draw current from the battery.

 

[hyperion]

The MFD is just an interesting thing to behold. It doesn't have anything to do with the logistics of the engineering, So sayeth my Prius rep.

 

[Astrowoman]

Aha, I know the answer to this one! If the battery is in danger of overcharging, the ICE will come on to bleed voltage out of the system. Should only happen if the battery is totally green and you are still regen braking.

However, it does seem there ought to be a way of simply not continuing to charge, oughtn't there?

 

[RSnyder]

Aha, I know the answer to this one! If the battery is in danger of overcharging, the ICE will come on to bleed voltage out of the system. Should only happen if the battery is totally green and you are still regen braking.

However, it does seem there ought to be a way of simply not continuing to charge, oughtn't there?

Well, there are at least two other ways, but both have drawbacks.

1. Use the hydraulic brakes to slow the car. Since it's "brake-by-wire", this can be done even though the driver does not press the brake pedal. But it adds additional complication to a life critical system and engineers hate to do that.

2. Discontinue simulated engine braking. Automotive engineers try to avoid unexpected changes in vehicle behavior as a confused driver can be a dangerous driver.

Also, there's at least one other reason why the engine might have come on, namely to warm it up. The (relatively) cool air rushing past the engine and catalytic converter may have cooled them to near the bottom of the operating temperature range or the cabin heater may have removed too much heat from the engine coolant.

 

[Paulie]

As I tell my friends who are interested in a Prius: It's tuned for emissions, and it will "waste" gas to keep the emissions system operating at peak. The mileage is just a "side effect".

 

[PeterB]

Aha, I know the answer to this one! If the battery is in danger of overcharging, the ICE will come on to bleed voltage out of the system. Should only happen if the battery is totally green and you are still regen braking.

However, it does seem there ought to be a way of simply not continuing to charge, oughtn't there?

Well, there are at least two other ways, but both have drawbacks.

1. Use the hydraulic brakes to slow the car. Since it's "brake-by-wire", this can be done even though the driver does not press the brake pedal. But it adds additional complication to a life critical system and engineers hate to do that.

2. Discontinue simulated engine braking. Automotive engineers try to avoid unexpected changes in vehicle behavior as a confused driver can be a dangerous driver.

Also, there's at least one other reason why the engine might have come on, namely to warm it up. The (relatively) cool air rushing past the engine and catalytic converter may have cooled them to near the bottom of the operating temperature range or the cabin heater may have removed too much heat from the engine coolant.

Unlikely for the last point as with slightly cooler weather, I am now again running on battery without the ICE coming on down the same route (I do it every day). Battery must be overcharging in warmer weather on the long hill down and neeeds engine to bleed off as suggested by astrowoman

By hydraulic brakes do you mean the emergency brake and by discontinuing simulated engine braking do you mean place transmission in B mode?

 

[RSnyder]

By hydraulic brakes do you mean the emergency brake and by discontinuing simulated engine braking do you mean place transmission in B mode?

By "hydraulic brakes", I mean the brakes that consist of brake pads forced by hydraulic pressure against disks in front and drums in back. When they are operating normally (not in emergency mode), an electric motor keeps a pressurized charge of fluid in a canister that the computers can use to activate the brake pads. Currently, the only time they'll do this is if a sensor reports that the driver is pressing the main brake pedal.

By "discontinuing simulated engine braking", I do mean behaving as if the selector is in B mode. Since that is also "simulated engine braking", I didn't quite phrase it correctly. It would have been more correct to say "dump the energy resulting from simulated braking into the engine instead of saving it in the battery".

 

[PeterB]

How do you "brake by wire" without pressing on the brake pedal? Brake pedal (that operates by pressing on it and results in recharging the battery) and emergency brake - is there something else? I understand the B mode which is something else emulating engine braking without charging the battery for mountain descents.

 

[RSnyder]

"Brake by wire" means that computers intervene between your brake pedal and the brake pads. Under normal operating conditions, when you push on the brake pedal, all that happens directly is a computer reads a pressure value. A network of computers then collude to produce the requested braking force, using a combination of the electric motor/generator and applying their own source of hydraulic pressure to the brake pads. Valves prevent the hydraulic pressure generated by the brake pedal from going to the pads directly. In an emergency situation (electric failure, hydraulic pump failure, computer failure, ...) the valves go to their unpowered state which allows direct flow of pressure from the pedal to the brake pads.

This is a slightly simplified view. The details have been discussed in great depth in other threads.