I try to keep the ICE in the highest specific fuel consumption range:


My second trick is to use a thermister hack to put the car in the most efficient driving mode as soon as possible. This allows the ICE to auto-stop before the coolant reaches 70C. This is specific to the USA NHW11, 01-03 Prius. I don't know if the USA NHW20 Prius has the same control law.

Bob Wilson

 

Ooh very cool graph Bob - it's even anti-aliased. Someone upgraded their graphing tools ;o) I have to say that's dramatically clearer than the previous one of the same data, cool setup.

I'm not using data readers but one thing I will note is that I don't accellerate sharply at highway speeds - that would likely put the engine beyond it's high-RPM efficient range. For this tank, I accellerate sharply when Pulsing - so, below 42mph.

Your graph suggests the ideal range is 1200rpm to 2700rpm, and above 2000rpm you want to be revving it more for torque than speed - which would suggest keeping the yellow arrow from the electric motor (not from the battery!) below 42 is preferable as you maximize torque.

 

OK, honestly, I am confused now. ..

Question1: So, (1) if the car is driven such that battery power is not consumed while accelerating from 0 - 42 MPH and (2) while driving at speeds > 42 MPH, the engine is ON, and the bars are going TO the battery (in most instances; unless, someone gasses down the pedal for whatever reason) --- then, when is the battery power used? Under what circumstances?

Question 2: if the SoC is high (purple colored battery icon), Prius tries to start from 0 MPH using the battery power under my normal acceleration profile .. should I push the pedal further to kick off the engine so as to get better MPG?

Question 3: Using the battery at high speeds is not efficient, as I read in these discussions. I used to use the battery power for warp-stealth mode. Now, if I am not supposed to do that for the sake of MPG .. then, what is the best way to decelerate from highway speeds to normal city speeds .. for example, while exiting a highway?

Thanks for your answers and the time that you guys take to educate the people on this forum!!

 

Generally speaking, your first goal is to maintain momentum. That's the best way to get great MPG.

For question 1, search this forum for "Stealth Mode" and "Hyper Glide," those are the 2 answers to your question.

When accellerating - which is what this thread is about - you want to maximize the torque of the gas engine when it's on. Below about 10-15mph the gas engine won't turn on because the ~50% fuel efficiency of the battery's power is better than the gas engine can do. Above that it kicks on, and you want to accellerate "briskly" - enough to get as much out of the torque generated by the gas engine as possible. So, you don't want to push it so hard that you require it to rev high and get a little extra torque at the expense of MPG, and not so light that some of the torque at that speed is wasted (and bled off to the battery).

What that looks like on the MFD screen is debated.

Right now, I accellerate with the MFD as follows:
1) Below 10-15mph, the Prius naturally accellerates using electric-only, avoiding the miserable sub-50% efficiencies of the gas engine. I don't mess with the way it works here with any messing around with the pedal.

2) 15-30mph I accellerate "briskly" enough that both red and yellow arrows are going to the wheels, but no arrows in either direction are going to the battery (no second yellow arrow for example). This is maximizing torque from the gas engine at low RPMs.

3) 30-41mph I don't change my foot's angle but the higher RPM of the gas engine cause the MFD to show the yellow arrow to the wheels go away and a green arrow to the battery now shows. Occasionally during the transition, it actually shows both a yellow arrow to the wheels and a green arrow to the battery around 30mph.

 

Once the car is warmed up, try to avoid running the ICE. If you do accelerate, do it moderately and try to keep the engine at or under 2,400 rpm (difficult without instrumentation.) As your engine rpms go higher, the net fuel efficiency falls off. This will not be a 'kick in the pants' acceleration so be mindful of following traffic.

If you don't have to, don't. Just accelerate to your target speed and let the vehicle handle the rest.

I'm not one of the 'warp-stealth' advocates although there appears to be some subtle ways to get on the 'efficient side' of the power curve. Until I get my next hack working, lets leave this for others to address.

As for getting off, I typically 'glide' as much as possible. If the off-ramp slopes up, I'll drop the car into "N" and let gravity do as much as possible. I'll put it back in "D" as an alternative to braking. But coasting is always better, it conditions allow.

As for getting on a highway, I try to hold off on accelerating until the down-slope of an on-ramp. This lets gravity be your friend both for performance and at the pump.

No problem. Many of us started from your questions and with the help of others and experiments, gained a 'clue.' Sad to say, I wish we had a little bit better instrumentation in our cars so everyone could see what the instruments are telling us.

Bob Wilson

 

I agree! I would have loved to see the RPMs of the engine .. in order to keep them low. But I am in general a slow accelerator. So, I think, I am keeping that limit. But I don't want it to be too slow as well, as the engine is not working at its optimum (as I understand from Soopahman's reply). So, basically, I need more instrumentation in my car!

Going into neutral is a good suggestion to come off of a highway .. wonder why I never thought of that !! .. OK, that begs one more question .. what if I need to re-engage the HSD system of the car .. can I just push the lever to 'D', and it will engage itself automatically without problems .. or do I need to come to as standstill and then while the brake pedal is pushed, I need to move the lever to 'D', (as I currently do when I start the car from stationary) ...

 

Thanks Soopahman, It makes sense ... so, use the battery power at 0-15 MPH, where the battery usage is also at the optimum (slow speed), and use the ICE at high speeds (to get the maximum torque from the engine at higher speeds) ...

 

Thank you everyone for the replies ... OK time for more questions ...

1) What is torque .. can you explain it as if you are doing so to a layman ... he/she would understand power/HP from an engine .. but what would torque from an engine mean to him/her?

2) When Prius is going on highway, there is this constant bar from the ICE to the battery, but the battery SOC level does not change .. even though you might have driven on the Hwy for an hour or more .. what is happening to all the power that is being transferred from ICE to battery ... does the arrow suggests that the extra power bleeds of to the battery .. or is it really transferring power to the battery all the time while the arrows are shown on MFD?

Thanks again!

 

Torque is the 'twisty force.' Imagine you and someone else are holding opposite ends of a broom handle. So one of you decides to get the other one to drop their hold. On approach is to try and twist the handle so it slips in their hands. That is torque.

In the USA, we use the units 'foot-pounds' but the rest of the world uses 'Newton-meters.' For one 'foot-pound', take a shaft, like the broom handle, and hang a one pound weight at the end of a one foot rod that is horizontal to the floor. This will be one 'foot-pound' of torque.

Good catch! One of the sad things is you don't see the units from the graphical display.

I have a Graham scanner in my NHW11 and often check the 'battery current' only to discover this is the background, power needed to power the car electronics. This typically is about 500 watts for the headlights, control computers and other creature comforts.

There are some sophisticated flows at highway speed and the display doesn't do them justice. For example, there are times when MG1 is generating power passed on to MG2, which it does by also providing the 'counter torque' for the engine. This power isn't lost because the MG1 'counter torque' actually helps to let the ICE 'push against it' to move the car forward and sent to MG2 for wheel power.

I don't want your head to explode but think of a power path that goes from MG1 to MG2 electrically while the MG1 and ICE torque are balanced in the Power Split Device (PSD). The ICE mechanical power passes on to MG2 using the ring-gear driven chain; at the same time, the MG1 passes the power it generated on to MG2. There the MG1 electrical power and the mechanical ICE power recombine to pass on to the wheels. This combined ICE power is what pushes the car down the highway in normal cruise mode.
Now there are some exceptions. For example, if you need to go up a hill and there isn't enough power from the ICE, the battery can provide the extra power to MG2. Also, when going down a steep enough hill, MG2 will work like a generator-brake, pumping power into the battery. There are reports that in some cases above 42 mph, a little power goes to MG1, which keeps the ICE spinning.

BTW, the sketch is a schematic representation. The actual Power Split Device has a hollow-shaft, sun gear that connects to MG1. The ICE shaft passes through MG1 via a solid shaft and drives the planetary gear carrier on the far-side of the sun gear. Finally, MG2 connects to the ring gear and is also hollow. The ICE shaft finally reaches the transaxle oil pump on the far side of the transaxle case. Without going nuts, this is how everything fits in the small space.

Learning how the Prius transaxle works would give anyone a headache. But it is quite clever and unlike anything folks have really worked with in the past. It took me a while to really understand what is going on. The secret was understanding the invisible force, torque.

Bob Wilson

 

1) Torque measures how much rotational power the engine has. For example, the Prius engine has poor torque at 1000rpm, but good torque at 2000rpm. Horsepower measures how far the engine can take you in how long. A more elaborate explanation.

2) The green and yellow arrows unfortunately do not portray "how much;" that is, if the gas engine is running hard and delivering a lot of energy to the battery, you'll see a green arrow. If the gas engine isn't running hard and delivering a very small amount of energy to the battery... you see a green arrow. They look the same.

Further, if the battery fills up most of one of those bars (which is a lot of electricity by the way), you see no change on the display - it needs to exceed the bar for the next bar to draw.

When you're cruising on the highway you'll mostly see that green arrow, but most of the time very little electricity is going to the battery. The battery bars also don't appear to change because although the battery is slowly being filled, it isn't being filled enough to exceed the 6th bar. If you watch carefully (from the passenger seat...) the gas engine will rev down a bit and that small build up of electricity will assist the gas engine to briefly improve your mileage, then the gas engine becomes the driving force again with small amounts of electricity going to the battery - back and forth like that, constantly as you drive down the highway.

So the short answer is, the MFD is a significant oversimplification, and to really see what's going on requires a lot of rewiring I'm currently too timid to take on... but considering.