Fan or engine running when full charged reached?

The "B" is engine braking .... just like the big rigs use engine braking to slow down. That's the noise you hear. As far as I know once the bat is filled the computer simply directs the excess charge to ground.

 

"B" and regenerative breaking are different things in the Liftback. Normally when the engine is not running, the valves are open so there are no engine pumping losses which slow down the drivetrain. Going into "B" mode closes the valves, so you do get engine pumping losses, which cause the vehicle to engine brake. Regenerative braking is where the electric motors are functioning as generators, slowing the drivetrain and generating electricity and storing it in the battery.

I recall someone posted that the Plug-in just increases regenerative braking in "B" mode. I'm guessing this is b/c the Plug-in battery can handle more current and is harder to fill. Once it does fill, it may go back to engine braking.

It doesn't direct excess charge to ground when the battery is full; what happens is regen braking stops. This is why it feels like the car starts to free wheel when you are going downhill, with no brake or accelerator being pressed and the battery fills up.

 

you are in the plug in forum/ do you have a plug in or are you talking about you're 08? (gen II forum) the engine has to run to burn off excess energy in both the liftback and the plug in. but you would have to go down a pretty steep hill in the plug in right after a full charge to exceed capacity. there is no 'fan' that i know of to burn off excess capacity.

 

good question. there are people here that know, hopefully they will chime in.

 

I teach at a school on the side of a pretty steep hill. After I leave work (elev 1800 ft) with a full charge, I actually either run the AC on full blast or glide down hill in neutral. Otherwise the engine will turn on and warm up. The engine absorbs the excess energy. The battery will never exceed 86% SoC. I think it goes up to about 85.7 or so.. haven't tried in a while. So I just use up break pads on the way down the hill for a few seconds.. once everything levels out I go back into drive and glide the rest of the way.

I get about 22-25 miles on the EV range coming home fyi.

 

Hi Robert! The excess electrical energy generated by Motor/Generator 1 (MG1) is used to power MG2, which spins the gasoline engine without burning fuel. So the electrical energy is dissipated indirectly by working against the friction and pumping losses of the engine.

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Hi Cheslyn! Are you absolutely sure that the engine is actually burning fuel when this happens, and not just being spun?

In the non-plug-in Prius, the (forced) EV mode is automatically cancelled above a certain fairly low speed. Even with a high battery state of charge, if the engine is cold, the engine warm-up cycle will begin burning fuel when this speed is exceeded. I thought that I had read that the Plug-in has a much higher threshhold (65mph?) where the Internal Combustion Engine (ICE) is activated. Of course the ICE must already start passively spinning at a lower speed, but without burning fuel, and with low drag valve settings.

Logically, I would think that a fully charged plug-in Prius in (forced) EV mode would only passively spin the ICE without fuel consumption until the EV speed limit is reached.

Do you exceed the EV speed threshhold when driving down the hill?

Plug-in experts: Would that trigger the ICE warm-up cycle even though no power is being requested by the gas pedal?

 

It uses a power split device aka planetary gear. See Toyota Prius - Power Split Device.

I think you are confusing "B" mode with the engine starting to dissipate battery charge. People report that their Plug-in MPG drops from 999.9MPG when engine starts to dissipate battery charge, so the engine is burning gas.

 

Hi Fred,

Am I fairly certain the ICE is not burning fuel, no… I only know what the vehicle's iMPG is telling me, and my scanguage2, and both show the MPG at warmup temps and fuel burn around 25 MPG. This brings my MPG for the trip from 999 down into the 100's.

My speeding coming down the hill is around 30 MPH. About 1/2 mile down the hill, you enter a neighborhood and the speed limit is 25 MPH. The maximum EV speed on the PiP is 62 MPH.

 

When my car is fully charged and then I go down one of the many hills in my town, the gas engine definitely turns on to "use" access regen power. The engine definitely uses gas because the average MPG goes down significantly and then the engine stays on even as the road flattens out because once started, it has to warm up. I have been battling this for a while. I wish there was a way to get around it. I am always careful to keep my speed under 62 mph so I know its not kicking on because Im going to fast.

I end up driving like an donkey. I hit my brakes hard to get past regen and use the pads combined with using neutral. If I use neutral the whole way down I end up doing 75-80 in a 55.

 

Don't know what the NY traffic code says, but in California, coasting down a hill in Neutral is a code violation. (How this would ever be enforced is a mystery to me.)

 

Have you tried changing from "D" to "B" in these situations instead of braking hard?

 

That wouldnt help. The ice would still burn gas.

 

You mean you've seen it burn gas in this situation? I find that odd. Traditionally, "B" just closes the engine valves to slow the car down using engine pumping losses and does not burn fuel. I would expect "B" to engine brake instead of regen braking which would avoid generaing excess charge that the engine to burn off.

 

Is that true? I thought the ice turned on in B.

 

In a Liftback, definitely. I'm not sure how it behaves on a Plug-in. Someone posted that it uses more regen braking, but if the battery is full I would expect it to switch to engine braking. I have yet to be in the situation of going downhill with a full battery (much rarer in a Plug-in unless you start fully charged atop a hill), so I don't know first hand.

Ah, found the old thread: Full-battery downhill engine start is a pain! | PriusChat

 

Yes, I can positively say I've seen it burn gas. Aside from the ICE starting when SOC reaches 84%, I believe putting it in 'B' anywhere "near" 84% will start the ICE immediately.

On my way out of my neighborhood, there's a small hill down to a stop sign. When I first got my car, it was a mystery to me why the ICE was starting. After consulting with the experts here and getting Torque up and running and logging, I saw that if I was regenerating while going down that small hill, I would reach the 84% threshold and the ICE would start. I learned to avoid the ICE start by not applying the brakes until I was 2 street lights from the stop sign . This has worked for me for the 6+ months I've owned my car.

More recently, while I was going down the hill (foot off the accelerator), I put it in 'B' and the ICE started immediately. Although I wasnt logging at the time, on that section, I'm usually around 82-83% SOC, but no where near the 84% (using my 2 street light distance, I usually stop at the intersection with around 83-83.5% SOC). So, to me, it seemed like hitting the 'B' even near 84% triggered the ICE to start.