Hybrid cruising speed

A silly story that explains M/G 2 and M/G 1 and heretical mode

How the PSD is like a Differential

 

I don't think there's a single "most efficient" mode for the Prius. I shoot for a strategy that maximizes my gasoline MPG and miles/kWh. So far it seems that if there has to be a choice, EV is better used on the highway and HV for city driving. I think it works because there is less acceleration on the highway where you can just maintain a constant speed far longer than you can in city driving. I use HV to accelerate to highway speed or to climb steep hills, but switch to EV to maintain speed on relatively level ground or downhill. I am able to eke out an average of 8.4 mi/kWh and 135 mpg over 2,000 trips of approximately 80 miles with this strategy without access to a charger except at the beginning of the trip, and that's with going down and back up a 2,000-foot elevation difference each trip.

 

How would they enforce that, know what mode you’re in??

 

RapidScreen Roadside Emissions Testing program | Department of Public Health & Environment

 

In a Prius, the engine only ever assists M/G 2, it cannot power the tires alone.

 

This is true. But in that condition, there is no conversion taking place with energy going to or from the battery. So losses are at a minimum.
Here's one of @john1701a's videos where you can see that sweet spot and how elusive it is. I don't think he was trying for max efficiency here. Watch the dancing ball in the top right corner of the Hybrid Assistant part of the screen. The right column shows energy to & from the battery and when the ball is in the circle, that's the sweet spot.

 

That's what I mean about efficiency being dependent on the definition: If it's defined as minimal losses in conversion of fuel - electricity or gas - to motive force, it is unlikely that you will be operating at minimum emissions per mile/km. The reason being that electricity and gasoline have different emissions profiles, and for some reason everyone counts emissions of electrical generation and transmission but does not include emissions of crude oil extraction, refining to gasoline, and transport.

If your goal is minimum emissions for a given trip, the strategy is more complex. On trips that can be completed on EV, one should use EV exclusively, and likewise on trips that exceed the EV range and the opportunity to charge is present. If your trip will necessitate the use of the ICE, minimal emissions can be had by using ICE for strong acceleration and uphill climbs and EV for cruising speed and downhill. It's even more complex than that, but that's probably beyond the scope of this thread.

 

The best I've ever done is 7.2 mi/kWh, and that' going at low (30-40mph) speeds on flat terrain at air temperatures just under the need for A/C with no traffic lights hit...and a slow cruise in N into my already-open garage!

What you're saying is pretty much against what everyone else is saying to do.
HV is efficient wherever you go, of course, it's great. But when I'm on the highway in EV mode, I'm maybe hitting 4 mi/kWh. And EV mode won't get your very far on the highway, unfortunately.
Or is that 8.4 on the downhill portion?

 

It would be fairly difficult to drive 160,000 miles all downhill, don't you think? When it's actually downhill I get infinity mi/kWh, usually negative because I'm in regen.

If you're getting 4 mi/kWh on the highway and it's not uphill you're either driving too fast or slowing down and accelerating repeatedly, both will absolutely destroy your efficiency. The difference between driving 65 and driving 80 isn't 25%, it's like 75%. If I can drive 60-65 behind a truck I can get almost 6 mi/kWh on relatively level highway. I also do not use the cruise control in any kind of traffic because the way it's programmed it's always slamming on the brakes and accelerating. It wastes a ton of energy.

Don't take my word for it. Test it for yourself. Make the same drive using EV as the owner's manual suggests a few times, and then make the same drive a few times using EV only for times when you can keep the HSI indicator below about 33% (which is cruising on relatively level street/highway or downhill). See yellow arrow below:



If you have to push the gas harder than that use HV.

 

Miles per kWh or miles of EV range are not good goals to have in mind, because they are such strong functions of speed, headwind/tailwind, air temp, rain/norain, elevation changes, and many other things. I couldn't care less if I get 10 miles of EV range or 50 miles. What I care about is how many fewer gallons of gas I burn on the same trip, due to choosing where and how to use EV mode vs HV mode vs (yes, even) Charge mode.

My 2017 Prime's EV range is around the same as 0.50 gallons of gas. If I manage to use it at very low speed, or downhill but still using a little power, it increases the number of minutes that the ICE is off. Keeping the ICE spinning has a relatively fixed overhead of "idling loss".

Not every driving strategy is obvious. Accelerating steeply uphill from a stop light, is one example. For a long time, it seemed smart to use ICE in HV mode for that, because it's a fairly high-power scenario, right? But now I generally do it in EV mode until up to 30-40mph, again to get in more EV driving at low speed. What goes up, must come down, so the uphill aspect of it often doesn't matter. It does matter if your charging destination is before "coming back down". But if you're just driving along terrain which looks like a sine wave, you don't need to limit your low-speed EV driving just to level or downhill.

It is trickier than even that, of course. When I switch from EV to ICE, whether in that scenario or any other, I like it to be where I'll be at an efficient throttle setting in HV mode for the ICE. There are many balls to juggle and keep in the air, if you really want to maximize trip mpg.

Caring about EV range or miles per kWh, hurt your chances of doing the very best you can. They are not the goal. The goal is minimizing fuel used, while using up the same amount of available kWh as always.

 

By the way, I don't recall having read many posts by PiPLosAngeles until now, but I've not seen him say anything about driving strategy/technique/results which I have serious disagreement with. Plus, I see many signs that he has really paid attention to his car(s) and has a deep understanding of how to drive a PHEV. That is pretty unusual. Is he the new John1701a of PriusChat? ;-)

Personally, I've long had a good enough bag of Prius tricks, that adapting to the Prius Prime was not a shock to my system, though much has been learned since 2017, and it remains as fun/challenging as I wish. I wish I knew in October 2000, when I got my first Prius, what I know now. Ahhh, the good old days when if you saw another Prius, you'd honk, or maybe even both make a U turn to meet and talk.

Now, the cars are so much more refined, and so much more taken for granted. The current cars are totally superior, though not as far ahead of their time.