Help me understand the real MPG during EV driving

the battery shouldn't run low during pulse and glide

however, given mpg on your screen includes total distance travelled, so that is accurate.

mpg is a misnomer for electric driving, just as miles/kwh would be for a gasser.

 

It works like this... In both a traditional car, and hybrid, you'll use some fuel to increase your speed (increased kinetic energy), or get to the top of a hill (increased potential energy). You then start decelerating (that can include regenerative braking in the hybrid), or going down the other side of the hill. This is where the two cars differ. The traditional car burns off all that energy as heat, either through the friction brakes, or by adiabatic heating (engine braking). That energy is lost forever. Your hybrid, however, converts roughly 80% (that's your 20% conversion loss) of that kinetic or potential energy into electricity, and stores it in the battery.

Now you start accelerating or going up a hill again. The traditional car will have to use fuel again to do that. But the hybrid can use its stored electrical energy to reduce the amount of fuel needed.

The 100MPG instantaneous reading is just the limit of the gauge. You can also set it to 150MPG maximum in the settings. In reality the MPG is infinite when you're using the electric motor only, as zero fuel is being used.

In reality it's a bit more complicated than that. The Prius uses an Atkinson Cycle engine, which is super-economical over a very specific rev range. The computers use the electric motor to try and keep the ICE running at that super-economical speed. When that speed is too fast for your current energy requirements, it uses the spare energy to charge the battery. When you need more energy (e.g. going up a hill or accelerating briskly), then it uses the electric motor to supplement the ICE in an attempt to keep the ICE at that super-economical speed. It can also opt to turn the ICE off entirely if it would be more economical to do so (very low energy requirements, or decelerating, or going downhill). That's when you see the EV light come on.

If you're wondering where that super-economical speed for the ICE is, then look at the screen with the accelerator gauge on it. Press the accelerator until the bar rises to the very top of the light blue area, but not into the red. That's your sweet spot. If you can accelerate from a standstill and hold the accelerator at that position, you'll start getting scores of 95 or above for acceleration. If you're pulsing and gliding, again use that accelerator position for the pulse. During the glide, press the accelerator really gently, so that there is no blue bar visible at all (you're trying to get rid of the tiny amount of regenerative braking the car applies by default). If you don't do this, you're regenerating, and regen wastes 20% of your energy.

 

I think you've got it. You want to be at the top of zone 2 like you thought. Anything below that incurs a greater amount of conversion losses in the long term. Anything higher (red zone) burns through your battery and fuel more quickly because the ICE is not running at its most economical speed and you're using the maximum amount of electrical energy, which means that statistically it's less likely to be available later to optimise ICE speed. If you drop into the lower half of zone 2 then you're likely to be using electrical only and again that means you're just incurring greater conversion losses later as that battery needs to be recharged if it is to be available to optimise ICE speed.

In short, the top of zone 2 means the ICE is producing the most energy for a given amount of fuel, and it's delivering as much of that energy to the wheels as it can (i.e. not incurring unnecessary conversion losses).

All that being said, if you know you're heading mostly downhill for the next couple of miles, you may as well use the battery more. A fully charged battery can't recover any more energy. So you want to avoid situations where your battery is full (0 or 1 bars left empty at the top of the display) as the Prius will start limiting the flow of energy to the battery at that point. There are other situations where you might know better than the computers too.

In the short term, you will get better economy keeping the accelerator anywhere lower than where I'm suggestion, but all you're really doing is using the battery more, which makes you believe you're getting better MPG (and indeed you are for that short time), but in the long term you're just increasing conversion losses and reducing the car's ability to keep the ICE at it's optimal speed. Remember that, as you said earlier, all the energy in the battery came from the ICE via one route or another, and hence it all suffered a 20% conversion loss. So it's generally true that your best plan is to allow the ICE to run at its most economical speed whenever you're accelerating, and avoid regen when gliding, keeping regen only for when you want to actually decelerate the vehicle or control downhill speed, and a avoid the friction brakes (they kick in once the brake pedal pressure moves the accelerator graph to what would be past the bottom of zone 1 on the diagram).

 

One problem with the MPG method of fuel measuring - is that it can cause that problem - in that you could be achieving INFINITY MPG (ie ZERO FUEL use), but a PRIUS hasn't got a way of showing infinity MPG. With litres/100km like most of the world uses, ZERO means NONE, and the bigger the number, the more fuel you use. TOYOTA must have decided that 100 MPG is as good as they show (tho The Professor indicated that can be changed to 150. (I believe Japan uses km/litre, which is similar to MPG concept.)

I've heard your question quite a few times since being on PriusChat. I suspect that there are 3 types (at least) of people who buy PRIUS.

• #1 One just drives it - and gets very good economy;
• #2 Another thinks - "this is an EV" - and tries to sort out in their mind how the ICE fits in with it. And sometimes overthinks it - but probably improves economy a bit;
• #3 The other thinks "this is a very efficient ICE" car - assisted by a battery. And, yes sometimes overthinks it too - and also probably ends up improving economy just the same as #2.

I think it's good that we think about how our cars work. I can still remember my thinking the first time I ever took a driving lesson. I'd devoured information about cars, about how engines, gearboxes, clutches, worm&roller steering mechanisms etc work. And, as I took off in the shopping centre carpark the first time, I could visualise the engine spinning the flywheel, the clutch engaging and the car ... actually moved. I asked my Dad about something after a while - and came to the conclusion that he knew that you put your foot on this pedal and it has this outcome - had no idea what was happening. It's possible to do the same thing with PRIUS (I do) - but, then some people aren't interested, they just drive - and that's fine too - my Dad did it till he was 85 and never thought about what was causing it to work.

 

That's an older Prius. I'd imagine it's a different drive train, different engine with a different sweet spot, different electric motors, different battery, different aerodynamics, different software... And so on. At the very least it's a different representation of engine speed on a different guage so there's no direct comparison.

Also, everything I said is a generalisation. Different journeys and different routes may prefer slightly different throttle positions. Even different weather conditions can make big differences to fuel use. Understanding those intricacies, and how to adapt to them, is the art of hypermiling.

Try different accelerator positions when starting from a stop and see which gives you the best score for acceleration/starting on the hybrid system indicator screen. The score seems to be a function of how much fuel you actually used (directly, or indirectly with battery use) to travel the distance you traveled while accelerating. Just do it safely. My first real test of automatic emergency braking may have been while watching that screen too much.

SM-T813 ?

 

That's the Eco Accelerator Guidance bar, as you discovered.

It takes what I said previously to the next level, taking into account battery level and the energy requirements hinted at by the driver.

When accelerating, put the accelerator level at the very top of that guide (you'll find it's usually where I said, at the top of area 2). It may like to hang around much lower then that if the ICE is off or you've been traveling at a constant speed for a bit and it thinks you're trying to maintain speed, so always remember you're in control and it's advising you based on what it thinks you're trying to do (accelerate or cruise). Give it a nudge of acceleration to make it realise you're wanting to accelerate if needed. When maintaining a speed, keep your accelerator anywhere in the range the guidance bar gives to do that. You'll find the guidance bar slowly drops bit.

Based on how well you do the above, will determine your scores for acceleration and cruising.