To Drain the Traction Battery or To Not Drain the Traction Battery? That is the question.

I don't think you get it. The most efficient style of driving maximizes ICE use (lightly), not EV use.
The EV power had to come from somewhere and 90%? Will come from gasoline. With energy conversion losses included you are at a loss compared to running the ICE and EV. The main reason for the HV battery is to supply extra power to make up for the weak Atkinson Cycle ICE and to store energy gained from regen braking. This is hybrid 101.

 

You can do whatever you want that the car allows you to and the warranty will cover if it fails before that period.

I got your point and it is not the most efficient path because you have ~15% loss in charging the battery and another ~15% loss in discharging. You are better off not using the battery, if it can be avoided.

Why would you need the battery then? For regen brakes and to use it when ICE is not efficient in very low load conditions.

 

Please, please don't do that!

This is how the Prius hate starts.

I can do 50-60 MPG city driving without blocking the traffic, or taxing the battery or electric motors, so you can do it too.

ICE is for high power needs (like climbing hills). Battery is for nice smooth coasting and starting ICE back on. Doing anything else wastes gas and puts unnecessary wear and tear on the hybrid system.

 

That's really interesting.

But with all this reading, I still have a question. If I decelerate from 60 mph to 30 and go at 30 for a while, should I keep going on the battery or activate the ICE then release the accelerator to keep the speed?

That is if I'm not in a good place for P&G.

 

It depends on how long you plan on driving at that speed and what the SOC is. I would suggest simply dropping to 30mph, apply a bit of throttle for a moment and then lifting off the throttle and reapplying just enough pressure to maintain speed. Then just let the car decide what to do. It sounds complicated but it is quite simple. If you are watching your iMPG then you'll quickly see how the technique works.

If you are traveling a short distance during that 30mph stretch then I would use just EV or glide it out.

 

More to the point, it doesn't help. Going up a hill is an ideal time to get the ICE going at its most efficient point. This allows you store up some energy in both batteries and height.

 

I disagree with that statement. The point of the hybrid electrical system is to capture and use energy that would otherwise be wasted in a typical ICE system, thus improving the efficiency of the engine when compared to those systems. The Prius does this by opportunistically capturing energy from braking and/or the engine when possible.

While the captured energy can be used to give additional power to the drive train, the largest efficiency gains are when the Prius can use that stored energy for propulsion instead of running the ICE in an inefficient manner (such as low speed driving).

So while the energy feeding the battery does indeed come mostly from gasoline, it is energy that would have otherwise been wasted to begin with.

 

Exactly what part are you arguing against?

Not much EV power is captured during efficient braking. Trying to drive using mostly EV becomes less efficient than driving efficiently on the ICE because you will not regenerate enough energy to make up for what you lost by driving on EV for long distances. Thus it is better to use basic hypermiling techniques to reduce EV use except in the case of acceleration and general assist. One can use EV only to accelerate from 0-20mph (approx.) and still be efficient but you have to keep an eye on SOC. Once you drop below the SOC threshold the ICE will need to recharge the battery and in doing so the ICE becomes less efficient during that time.

Keep in mind that the hybrid system is designed to assist the Atkinson Cycle ICE which is known for its lack of power but high efficiency. Without the electric power the car would be a total slug. The recapture of energy from braking is a bonus.

So my point still stands. It is better to drive efficiently and not rely on EV only too much than it is to try and drive mainly on EV. Regen braking doesn't capture much energy if you are driving efficiently.

 

You miss the point that prius computer is already using the battery for propulsion especially at start (either cold start or acceleration from stopped) or even higher speeds if is state of charge is high and less so when the state of charge is low. If you force battery use higher than normal you end up with higher gas consumption, higher temps in hybrid components (chance of failure), and more wear and tear of battery.

 

^ In think there's varying degrees of force:

Push the EV button: that's definitely force. But keeping an eye on the display, keeping the bar graph to the left-of-center a little more, by being careful with pedal pressure? I can't resist that. Especially if it's well to the left, not using much electrical energy.

But once the state of charge approaches half full I'll change strategy, intentionally try to kick the engine on, pulse and glide. And then sometimes I'm battling with the car: it wants to stay in electric mode, even though state of charge is dropping.

I'm really not sure what's the best approach. I do know if you set cruise control, around town, it'll happily run the state-of-charge right down to two bars sometimes, on level roads.

 

You can't "force" the car to do anything the software does not want to do. If you drive it in a manner such that the car selects or allows EV, then EV is a reasonable way to operate under those conditions. Is it the most gas-efficient? Maybe not, but it may be the most environmentally positive which, I believe, is the design goal.

 

Those two things are likely near exactly the same. With the exception of any lifespan issues with the traction battery, the whole of the environmental impact is from gas burning (either directly by running the ICE, or by proxy using battery energy which must be replaced by energy from the ICE (somehow)).

I note that you have a Plug-in, which does change the equation, but this is the Gen III forum, so it is not applicable here.

 

Your initial response, at least to me, came off sounding like the hybrid system itself was unnecessary, i.e. if someone just drove efficiently all the time there would be no need for a hybrid system like in the Prius.

Unless someone has a really odd commute I'm not sure how one could drive in EV only mode too the point where the engine comes on specifically to charge the battery. An "typical" commute really wouldn't provide the opportunities to do so (unless you want a herd of angry gas guzzlers honking at you the whole way). The only time I've had the engine come on to charge the battery was in really bad traffic (creeping along for an extended period of time with no opportunities to get the engine going).

 

Gotcha. I'm definitely not saying that.

Everyone's commute is different and in a lot of posts I have read about people trying to use EV for long distances. It is actually fairly easy to do where I live. I wouldn't be the most popular guy on the block but what Prius drivers are? LOL

 

Corwyn pretty much covered it.

In a non-plug in, the least efficient method is also the most negative to the environment.

 

While I agree with increasing the wear and tear on the battery (though I don't think we need to be too concerned, based on the Taxicab tests), I don't think there is a difference in overall system efficiency unless you drain the battery to the point where the engine has to run to recharge it.

Except for special cases, there will always be more waste energy produced than can be captured by the Prius (otherwise, the traction battery would be empty). To my knowledge, low SOC levels do not adversely effect efficiency until the engine is forced to charge the battery.

Having a low SOC doesn't imply more gas burning later, as there is plenty of waste energy to capture from even when driving as efficiently as possible.

 

If we were to assume that the hybrid system was 100% efficient we wouldn't be worried about low SOC at all (or much anyways). The problem would be easily seen to be high SOC. If the battery is full, the system would have no place for storing energy, and would be forced to waste it as heat (in brakes or engine). As it is, there are efficiency losses inherent in moving energy in and out of the hybrid system, thus it should be avoided when there is some other lower loss option available. A low SOC means that one has taken that efficiency loss (twice) perhaps more than was necessary. The farther the system moves away from it optimal charge level, the more likely it will be that energy will be wasted.

So the question should be, not should one drain the battery, but when does it make sense to use energy from battery.

 

Are you saying that EV operation within the HV mode on long trips changes the equation because of the larger battery?

 

I read it to mean that with a Plug In you are able to replenish the battery without using fossil fuel.

It's sort of like how hypermilers in HEVs don't like to drain SOC at the end of their trip but PHEV owers do. The plug saves us the grief of having to run the ICE to recharge the battery.

 

To work, there is a surface route I take if the freeway is backed up that is several miles of mostly flat terrain, 35MPH. Depending on how many red lights I hit on that stretch, I actually can get the SOC low enough to kick the ICE on. The last couple miles of my commute home will generally get the SOC down to 2-3 pips but not quite low enough to start the ICE.

For the most part, I think this is true. However, when the ICE is running (accelerating or at highway speeds) and the SOC is high, the PSD can use the MGs to help drive the wheels instead of topping up the batteries, so it seems like there could be some reduced efficiency if you get on the highway with a low SOC.

Personally, I go with your school of thought - as long as I'm not getting the SOC low enough that the ICE runs JUST to charge the battery, there's no reason not to use the battery if the computer will allow it. The electricity isn't "free," but it's energy that would have been 100% wasted in a regular car...now at least I can use some of it. I managed to make it through the cold months with 44MPG average (in a v-wagon) which is above average compared to what's being reported on Fuelly. Perhaps I could do even better, but if I can get above average mileage without really having to change my normal driving habits much, that's good enough for me.