Questions about getting the best mileage

My understanding, trying to do the same thing, is that the B function on the shift is for long, downhill grades where one wants to use engine braking versus relying solely on friction brakes. I personally wouldn't use it when getting off the highway, as applying the brakes normally accomplishes the same thing: charging the battery.

I also note that letting up on the gas to drop it into EV mode seems to mess up my Cruise number, but I've also noted that the whole Eco Display thing can be somewhat hard to maximize. There are a LOT of hills here in the Seattle area.

Joined here so that I could learn better techniques!

 

Hey, good to see you here Tony. I had been thinking about sending you this link.
Dan

 

I have never used the "B" mode. I never touch the two buttons between the seats either. For me, the best window to watch is the eco score. Also, a good investment is the ScanGauge or the Ultra Gauge. I have both and prefer the ScanGauge. Much simplier to operate, in my opinion.
dan

 

I quit looking at the eco score button after about the first month. It is so inconsistent. I make perfect stops and it says I suck. I do awful brake check stops and it tells me that I'm great... I just go by the MPG numbers.

 

Welcome aboard tony and adam!

This is a great site for learning to maximize mpg's. The people on this site are really helpful as well. For my contribution on how to get the best mileage (alas I do not own a c yet, however it works no matter what you are driving) I would suggest "driving without brakes" which can be found here. Very handy and easy to implement. Plus you are likely to laugh at the people who pass you only for you to do the same because you didn't stop. Just my 2 cents. Enjoy your c! Make sure to offset my gas guzzling jeep .

 

These are two different things.

EV Mode as activiated by the button and represented by an "EV Mode" indicator elsewhere on the dash is a special restricted mode that holds the car from starting up the ICE (the Internal Combustion Engine) when it normally would, but this comes with many caveats - the battery can't be too low, you can't activate it during the car's warm-up cycle, you can't drive too fast and you can't accelerate too quickly. Really this mode is for very limited scenarios - maybe you're just moving your car in the driveway, or you want to make a stealth arrival or departure from your home. It's not really suitable for any real driving situation and many people never use it at all.

The EV in the green car shape on the other hand is as you have guessed - it simply indicates that the ICE is off.

A couple of people have suggested that using this strategy has seen benefits, but it is not generally seen as the most economical way to drive. All that electric power has to come from somewhere - ultimately it comes from the fuel tank. Any time you convert that power the conversion process is not perfect and bits of energy get lost (as heat). Generally a technique known as "Pulse & Glide" is considered the most economic way to drive a Prius.

The cruise score doesn't seem to like Pulse & Glide either, for what it's worth.

No, not really. D is the place you want to be for most efficient driving. If you keep the blue bar from touching the bottom of the charging section of the HSI (on the ECO Score Display), you'll get the most regenerative braking possible. If you touch the bottom, the friction brakes will come on as well to assist.

Sometimes the car wants to deliberately throw away energy. The car never wants the HV battery to rise above 80% charge (which is represented by 8/8 bars on the battery display) and if you ever get that high - likely when you go down a long hill - it will start throwing away energy by using the electric motor to start turning the ICE around, using it as an air compressor and basically turning energy into heat and throwing it away.

In situations where you know in advance that you are going to be filling the battery up, you can choose to start this disposal of energy earlier in your descent, by selecting B mode. This will allow you to stay on the regen brakes for longer and rely on your friction brakes less.

It's not a commonly held view, but I think there's also an argument for using B mode on any descent that is so steep that you are unable to maintain your desired speed on regen brakes alone... ie. I'd rather throw away energy through the engine than the friction brakes - which really is a similar logic to using lower gears in conventional vehicles.

The key message here is that B mode is not more efficient. It is deliberately inefficient.

No. If you're not on a steep or lengthy descent, I don't think there is any reason to use B mode.

 

It's because that electric power isn't "free". You can't plug a c in to recharge. The only way you get energy into the car is by pouring it into the fuel tank at a petrol station. Sure, you'll get charge going down a hill, but you also had to get up that hill at some point.

If you're trying to keep the car using only the electric motor, the charge will deplete more quickly and the car will have to work at every opportunity to charge up the battery and if it depletes below a certain point, the car won't wait any longer, it'll just turn on the ICE even if you didn't need it for driving.

The logic behind doing a "Pulse" is that you'll have the engine operating at its most efficient and at that point will accelerate the car and - if your HV battery charge is below 60% - it'll probably do a bit of charging too. If your HV battery charge is above 60%, the electric motor will probably actually help your acceleration. If you get up to speed quickly this way, you can also normally spend more time in a "Glide", the holy grail of Prius efficiency. The idea of Glide perfection is not using the ICE and neither charging or discharging the HV battery (and therefore not losing energy in the conversions).

You have to ask yourself - where is the power coming from when the ICE is not running?

 

OK, you are correct that the EV dash light indicates that the engine is not running. EV Mode on the other hand is a suggestion by you that for some reason (not polluting downtown, sneaking home with out the wife hearing the car, reparking in a parking lot, etc.) you want to override the computer's judgement on how to best get good gas mileage.

In most scenarios, it is more fuel efficient to use the engine to propel the car than to use the battery to propel the car and then use the engine to recharge the battery. In some low speed scenarios (up to about 40, I think) there can be scenarios where the engine is so lightly loaded that EV operation may be more efficient, as recharging later will be a higher load for the engine that is more efficient. All power in the car is ultimately from gasoline, so the fewer number of conversions of the energy in gasoline to other types of energy, the better.


I do not have a graph for the Prius c, But this graph shows how high an RPM the Gen 2 Prius was most efficient at. (Lowest grams per kWH) lower engine speeds than that were not as frugal.

B is NEVER more efficient than D, it just keeps the friction brakes cooler, which can help on hills that may otherwise overheat the brakes.

B under the D in the Drive gears? | PriusChat

 

I've sort of come to my own conclusion that minimizing acceleration and deceleration are the most important factors (except when coasting down a steep hill of course... that's free acceleration). If this means you are in EV more often because the car computers don't want to switch to the ICE, so be it. My reasoning is because, even when the ICE turns on, you will keep the RPM of the engine lower in all cases and burn less gas.

 

The overall idea of B (I think) is that it's basically engine breaking. But it's different than in cars without regen braking. In a non-regen car, when you take your foot off the gas, you start to slow down because you're essentially using the engine to brake. When you take your foot off the gas in a Prius, that small amount of engine braking is simulated by a small amount of regen (as if you were pressing the brake lightly), which is why in order to "coast in neutral" you have to press the gas pedal down slightly, so that you're neither accelerating with slight EV (below ~43 mph)/ICE (~43+ mph) because you're pushing the pedal too much nor are you slightly regen braking because you're not pushing the pedal down enough. It's really helpful to look at one of the screens with the bar that shows gas and brake pedal positions (eco score and eco savings?), because that'll show you what's going on when you brake.

So, you want to take advantage of regen braking as much as possible, anticipating upcoming downhill inclines/stop signs/traffic lights and balancing between braking too quickly (little regen, friction brakes used) and not braking quickly enough (speeding, forced to come to a full stop for light that's about to turn green, forced to slam on brakes before light/stop sign/turn). Once the regen section of the brake pedal meter fills up, your "regular" friction brakes will kick in to assist, so you'll be regen braking to max capacity and friction brakes will take care of the rest. If the regen meter is full, shift to B to engine brake, because although you'll lose some of the energy as heat instead of regenerating it, you at least won't wear down your brake pads. Then if that's not enough, your friction brakes are the last resort.

In summary, regen braking (brake pedal 'til brake meter is full) > engine braking (B) > friction braking (pushing brake pedal past brake meter full point)

 

You are absolutely correct. I take advantage of "free charge" by using up the battery both before and after the highway portions of my drive. One more you can do is to use up charge prior to parking for the night. When the ICE must warm up in the morning, it charges your battery at the same time. This should be easy to figure out for a daily commute where you are driving the same route every day.

You mentioned "free charging" while stopped. You can effectively do the same exact thing while driving, get the ICE to turn on and just barely keep it at a low enough level where it won't switch to EV. This would be good use of the ICE up a slight incline in preparation for a long, flat road where you can get it into EV.

 

Some of this about charging the battery is suspect. Always remember that energy transfers promote inefficiency- i.e., gas to battery to kinetic is less efficient than gas to kinetic, or simply maintaining momentum (at low enough speed that air resistance doesn't destroy mpg). My approach is usually, when using EV, to limit power usage (i.e., only using a sliver of power, or just gliding) in order to limit how much the battery is drained, while still obtaining the benefits of gaining distance without burning gas.

 

Converting the energy does result in a certain energy loss. However, it's still more efficient to use the battery when coasting, because the energy loss is less of a problem than the ICE's lower efficiency when running at low speeds.

I do kind of what PriusCinBlack does, except I use up to half of the EV bar - accelerate at 1/2-3/4 of the eco part of the meter to about 5 mph above the speed limit, release the pedal completely, and try to coast using up to 1/2 (sometimes slightly more) of the EV part of the eco meter. Assuming the path ahead is clear and there are no upcoming stops, once I get to 3-5 mph below the speed limit I repeat the above process (accelerate using ICE at 3/4 of eco bar, coast in EV mode). At first I was using all of the available EV power, but I realized that drained my battery quickly and didn't really help my acceleration much. I'll try using only a tiny sliver of the EV mode and see if that changes anything.

My commute does end up with my battery usually at 2-4 bars by the end. My daily commute is 5 miles of highway with 1 and 1.5 miles of city on the ends. I've been getting 60 mpg on that, and 63-77 when I take city streets the whole way (which only takes 2-5 minutes longer). As I said, my battery is usually at 2-4 bars by the end of my commute from the city driving, but it's usually full by the time I get on the highway, due to warm-up, regen braking and acceleration.

When your engine is warming up, you do somewhat get a "free" charge, but it's probably not as much as you think. For part of the warm-up, your battery is being used as much as possible, so mine often drains even more before being fully charged. Even if you park with the battery "full" (6 bars) warm-up will still add another bar to that, so the energy isn't wasted. So, you do get "free' charging, but electricity is still being used during warm-up to avoid the ICE being stressed when cold. Here's a thread about the gen 3 Prius's engine warm-up stages, and I assume they're somewhat similar for the c: Gen3 warming up stages | PriusChat

Acceleration (e.g., onto a highway) is also assisted by the battery, so you'll notice a difference between accelerating with the battery full vs. with it empty, so if you run the battery too low your ICE might have to work harder than the ideal range of efficiency.

 

No, Adam, you're right that the battery gets charged during warmup. That's basically all the "free" ("free" because it has to be done anyway) charge you're going to get without storing kinetic energy (i.e, parking at the top of a hill), though. The rest of charging (by choice) is the ICE using more gas and running faster in order to charge the battery, and it's less efficient to do that than to just engage the EV but not use significant power (gliding). Running the battery down by using extensive EV propulsion is almost never a good idea- you would need to know that a huge decline is coming up immediately to recharge with the brakes for it to increase efficiency.

 

As minkus pointed out though, the battery doesn't necessarily get charged during warm-up. If you don't have your foot on the accelerator it will charge, but if you put your foot down more than a small amount, the car will rely on the electric motor heavily and draw charge from the battery to do so. If your SoC is low when you start up and if you hit a main road quickly you might be surprised to find just how low the SoC gets - and I've once had the car end up seeming to switch into a defensive mode where it was charging the battery and giving me very little power. That was the lowest I've ever seen my SoC go.

Where I can, I really go for pulse & glide during warm up. Especially in a car park... if it's safe I'll give it a kick to the speed I want, then I'll just glide and let the engine do some charging. If it's flat ground and well surfaced, you can often glide for quite some distance.

 

In order of least efficient to more efficient:

1. You can warm-up and charge while stationary
2. You can warm-up and charge (a bit) while crawling on flat surface. foot off the gas.
3. You can warm-up and charge (more) while going downhill. foot off the gas.

note: 1 & 2 can be interchanged depending on the SOC. If SOC is low (50% below), #1 will be more efficient than #2. If SOC is high (55% up), #2 will be more efficient than #1.