Steering paddle for EV mode only.

I agree. I currently own a Gen 2 which I have driven for eleven years. This week while i am in Detroit I am driving a rented Gen 4 Two. In my Gen 2 it is hard to shut off ICE other than coming to a stop or near stop, or coasting downhill at less than 42mph. At other speeds I can get it to idle at 960RPM ( I call it windmilling) but it never completely shut offs ICE above 42mph, even in full downhill coasting.

And the reverse is true also, barely touch the throttle in my Gen 2 and ICE comes on. Starting from a stop sign or red light, even with no one behind me, I can only keep ICE off through painfully slow acceleration. With Gen 4, if someone is behind me ICE will come on, but with no one behind me I can accelerate at a slow but tolerable level and keep ICE from coming on. Not that this is necessarily an efficient way to drive I'm just experimenting to get an idea how the new systems works compared to my Gen 2. I have rented Gen 3 a few times in the past and didn't notice this dramatic of a difference.

 

Having a forced EV switch at highway speeds could risk damaging the HV battery by draining it down too much and safety features to prevent this are already in place in the Prius so even if you switched off the ICE the car would switch it back on again periodically, as it does so now, so what's the point of having a switch. Let the car do it's own thing.

 

I'm disappointed that the Gen4 Prius has such a small HV battery - the Gen 2 NiMH battery was 1300Wh (admittedly only using ~50% of it). The total capacity of the Li-Ion battery in the Gen4 is only 750Wh (I haven't seen how much of that they use). I find that it fills even when going down a small hill and the system goes into engine braking. There are many hills around here and I find it annoying that the system wastes so much energy. Although the overall fuel consumption is significantly better than the Gen2, the consumption for routes that go into the Santa Cruz mountains are slightly worse than the Gen 2. I suspect that is because of the lack of battery capacity.

Toyota first used the Li-Ion battery in a Japan only car back in 2011 - that used 5Ah cells, the Gen4 Prius uses 3.6Ah cells. There is an SAE paper decribing the cells.

I'm sure that Toyota did much simulation of various battery sizes and balanced it with the cost, space and weight but I wish it was 2-3 times larger (say 2kWh).

I once did a calculation for the Gen 2 and adding an extra 100lbs of battery would consume a few Wh per mile more on level ground because of the extra friction from the tires - so it is not the case of more is always better.

kevin

 

Can i respectfully ask how you did there calculations? I'm not asking for proof, I'm just uneducated and would like to know how people calculate these things.

 

As far as I'm aware, I thought the consensus was that the Gen 4 NiMH and Li-ion had similar usable capacity, it's just that the Li-ion had a much wider state of charge range. So, they used lower rated capacity to enable less weight for the same usable capacity, rather than the same weight for more capacity.

 

I agree I have seen that claim but I have not found anything stating the specific range of SOC that is used.

The G4 battery 56 cells of 3.6Ah with nominal 3.7v giving a total of 745Wh. I don't know what the active region is. The SAE article (2016-01-1207) does not indicate the operating range.

The G2 NiMH with 168 cells of 6.5Ah @ 1.2v is ~1300. With just 40% actively used this gives 525Wh of energy after derating.

I have a small hill that I go down shortly after starting in the morning and the ICE will go into engine braking unless I go down the hill slowly. I never saw that with the Gen2 (although there is a rough patch of road that would trigger the Gen2 into stopping regeneration if I wasn't careful but that's another issue!)

To Toyota's credit the engine warms up quickly enough to already be operating in intermittent mode by the time I reach that hill - I could only do that reliably in the Gen2 after I installed an engine temperature spoofing circuit.

kevin

 

We do, by the way, know that the Gen 4 NiMH pack is 201.6 V (168 cells * 1.2 V per the product information, although the cells are in modules of 6) nominal, 6.5 Ah nominal, for a total capacity of 1310.4 Wh - same as your Gen 2. And, AFAIK, it's the same range of SoC - 40 to 80%.

I wonder if Techstream would reveal any info about this, although in my area, it's hard to get the instrument cluster indicating either maximum or minimum state of charge - no long descents to fully fill the battery (although force charging could do it), no long ascents to fully drain it (I guess you could just leave it in neutral and let HVAC bleed off power).

 

Car and Driver published this:
"Some of this weight savings comes from the inherent power density of the Li-ion chemistry, but the new battery also is smaller. Its 0.75-kWh capacity is barely half of the NiMH battery’s 1.31 kWh. (The NiMH pack is pictured here.) This difference is possible because the Li-ion battery can reliably use about 70 percent of its capacity without compromising its life. The older battery can use only about 40 percent, meaning that it is typically not charged above 70 percent of capacity and not discharged below 30 percent. Either way, the usable capacity is a little over 0.5 kWh."

Remember it's a hybrid, not an EV.

I imagine it's a balancing act - to exaggerate - if ½ a tonne of batteries were installed, they'd never charge using Regen, and the ICE might take forever to take it from "empty" to full (plus you'd probably have to tow it in a trailer). TOYOTA have looked at a balance of driving situations we might encounter and picked what they consider the best compromise between weight, reliability, power, driveability and economy.

 

Thanks for the 70% quote - I hadn't seen that or I forgot about it.

I agree that I'm sure that Toyota did a lot of simulation and testing for all sorts of driving scenarios, but I often hit the limits of the battery when going down even small hills (100-200' drop).

Highway 17 to Santa Cruz goes up to 1800'. It worked great in my Spark EV, I could recuperate a couple of kWh coming down there - in the Prius I just have the ICE screaming at 4000RPM :-(

The sort of increase in battery I would like would bring the weight and space up to about the same as the NiMH battery (i.e. 2-3 times - the same as the Ioniq) the system then wouldn't go into engine braking mode as much.

I find it especially troubling when operating using the DRCC as it actively brakes to avoid exceeding the set speed when not following.

I usually turn off DRCC when coming down the hill to avoid that to control things manually (I wonder if it has the same behaviour if I force normal cruise control mode?).

kevin

 

I don't believe normal cruise uses the friction brakes - if it can't get it under control with regen+engine braking, it'll just start accelerating.

In any case, if you want more battery, buy the model with more battery. (There are some mountainous roads where I wish I had it, and then I remember that I have a lot of cargo space where Toyota chose to put the battery on the Prime...)

 

It's a fairly simple calculation.

On flat ground at constant speed the main effect of adding weight is to cause increased tire rolling resistance.

If the tires have a 1% rolling resistance coefficient then adding 100lb (about the weight of the NiMH battery) will add 1lbf of rolling resistance. (Good tires are a bit better than this 1%)

At 60mph (88ft/sec) that means it will take 88 ft-lbs per second of additional work. This is about 120 Joules/sec = 120Watts (Google can do the unit conversion for you - don't you love all this mixing Imperial/US and Metric measurements).

If you do this for 1 hour it will take 120Wh over 60 miles. This is 2Wh/mile.

A Gallon of gasoline contains 33.7kWh of energy - if the system is 33% efficient (the engine is 38-40% efficient at peak but there are other losses) it will produce 11.1kWh. The 120Wh additional drag caused by the battery will therefore require about 1/100 of a gallon of extra fuel per hour.

If you were getting 60mpg normally the engine would be consuming 1G/h. With the extra 100lbs of the battery it will consume 1.01G so instead of the fuel consumption being 60mpg it will be 59.4mpg.

The weight of the battery will have decreased the mpg by 0.6mpg.

kevin

 

Seems very thought out. Would wind resistance make any difference? Given that it's cruising speed on flat terrain, I can understand that. Would any real world considerations affect the difference between the 2 generations? Like stop and go, hills, etc.

 

The assumption is that the added weight does not affect anything else other than the tire rolling resistance. There may be minor changes to wind resistance due to the car sitting lower.

The energy consumption per mile would be constant independent of speed however the effect on fuel consumption will vary with engine load as it will make less effect when the engine is working harder, such as at higher speed with more wind resistance or hill-climbing.

Conversely at low speed in gentle driving conditions it will make a larger effect.

kevin

 

Thanks - I better go find my Senior PHYSICS textbook ... or take your word for it. Yep, the latter.

I loved Physics, but 45yrs later, not having used it in a computational sense, it may be a long hard re-learning. All that more complex with a mix of imperial and metric

 

The extra mass will cause there to be more energy needed to accelerate. Although I'm guessing this also would calculate as relatively low, combined with the fact that only a small fraction of a typical trip is spent accelerating. And anyway a percentage of the additional energy will be reclaimed during regen, since the additional mass will store more kinetic energy resulting in a little more regeneration, which the extra battery capacity will be able to take advantage of. Combiningg all of that with the fact that acceleration is so variable depending on the trip, I guess that's why it's not worth factoring it in?

 

I like your approach on this. I'm going to type out loud my non-math based thoughts on this.

1. I tend to do 30 minutes of stop and go a day. It will dedicatedly affect mpg more than the constant speed rolling resistance. I wonder how much the weight affects it. I can't do the math right now, because it seems too complicated for me. In addition and contrary, on heavy stop and go for long durations, draining the battery is possible since many times it seems more efficient to use the traction battery for creeping than it is to turn on the ice and operate it inefficiently at such low speeds and duration. This is especially true if a bumper to bumper downhill descend is immediately following. In that specific case, it helps to have a larger capacity.

2. if a 0.6mpg increase is found in your scenario due to lower weight, can the larger capacity of the Nimh battery make up for that due to the extra capacity alone? That depends on the driving terrain. I find that the gen 4 computer does very well in normal highway driving to use very little of the battery unless there are large terrain differences or I need to pass. Therefore, if my battery is half charged, I can expect that I won't max out the battery unless there is a large descend coming up, which I can plan ahead for. If I don't top off the gen 4 battery, I won't be able to make up the 0.6mpg with a larger capacity battery. This is not accounting for the differences in the computer's battery handling at different charge levels.

3. This one may be completely false. Some of you more experienced with electricity will have to correct me on this, but it seems like the gen 4 has a higher output than the gen 3. This is only an assumption based on posts of people claiming that the gen 4 can accelerate more than the gen 3 on battery alone. If this isn't just computer programming, could the opposite be true; that the maximum charge rate be higher on the gen 4 than the gen 3 (>100A)? If so, during normal driving with some braking, more energy can be captured on a gen 4 when higher pressure on the brakes are desired/required. Of course, if nobody is behind me, I can probably use minimal and early braking, and planning ahead on the gen 3 to make sure I don't max out the charge rate and use friction brakes.

Having said that, I wonder how much of this gen 3 NiMH carries over to the gen 4 two.

 

There were a number of items espoused for the improvement in economy of Gen 4:

"... thermal efficiency of the 3rd Gen Prius engine is 30% ... 4th Gen is 40 percent." "MG!/MG2 ... improvements in terms of fiction loss, which is on the order of 20%". "... power control unit (PCU) ... size of the PCU is reduced by 33 %, the mass is down by 11% and there is a 20% reduction in parasitic losses, so it is more efficient" "... coefficient of drag for the 2016 Prius is 0.24."

If you dig into the menus for the DRIVE MONITOR, you can see the EV% of every trip, it's often higher than I expect. It's also quite different for different types of commutes. One way to think of that figure is how much the HYBRID system potentially saved you over a conventional vehicle - yes, only roughly, but that's how I explain it to non-PRIUS observers - keeps em happy:

 

It has been noted on here several times to various members that although the ICE turns over on a downhill with full EV battery charge, no fuel it's burned, it's just dumping excess wiggly amps through the electric motor. Yes, you lose some regeneration as waste, but the car is held at it optimal maximum.

Very impressive argument, Kevin although IMHO, you are trying to gild a lily.

 

The Gen2 was quoted as 38.5% thermal efficiency, I've never seen a figure for the Gen3.

kevin

 

I just found this.

“The efficiency will be realized by improving the EGR (exhaust gas recirculation) limit from 21 percent (current model) to 28 percent,” reported Nikkei Technology Online. “The thermal efficiency of 40 percent would be the world’s highest for a mass-produced gasoline engine.”
This boost will make the next generation of Prius engine the most efficient yet for the brand. The first two generations, in comparison, posted 37 percent thermal efficiency. The 1.8-liter engine in the current third generation Prius is at 38.5 percent.

From
2016 Toyota Prius Engine To Boast 40 Percent Thermal Efficiency