Is there a limit to speed coasting in "N"?

As Bob mentioned above, there is a speed limit for MG1 that the ICE needs to be running to prevent overspeed. That is why the ICE will light off when you get above about 46 mph in the Gen III when coasting in EV in D.

When you put the car in N, if the ICE is on it will remain on and if the ICE is off it will remain off. So if you are coasting in N and the ICE is off when you switch to N, then when you first switch back to D - MG1 could possibly be in overspeed before the ICE can get up to speed to prevent overspeed.

So you need to be careful when coasting in N above 46 mph with the ICE off. That is why I stay away from coasting in N.

 

Ordinarily I would agree but when I notices from 42->74 mph, the ICE rpm did not change . . . something is going on and I don't know what it is, yet.

Bob Wilson

 

When decelerating in D you do not need to add any gas, resistance in Motor/Generator1 will keep the engine turning over. In N, you need to add gas to keep the engine idling as the wheels cannot push on it as M/G1 is offering no resistance. With the engine running, N is almost always less fuel efficient than D with no arrows.

N in the Prius is not like N in any other car, assumptions you make about N are almost all wrong for the Prius. Toyota is required to provide an N by US law, you are never required to use it, and I never recommend you do.

(The above is my honest opinion, YMMV. I rarely travel under 37 MPH, so I have little experience in N when the engine is not running))

 

I have no problem with moderating speed down a grade shifting between "N", "B", and "D". Steeper grades are going to need "D" or "B" (*) to moderate the grade. If I gave any other impression, my apologies.

Now there is an interesting relationship between the vehicle drag and grade:

• sine(grade) * weight = motive force :: gravitational force in the forward direction
• 20.234 + 0.01993*v + 0.01874 * v *v = drag force :: roll-down coefficients from EPA web site

We can combine these two formula to give the grade as a function of speed:

• sine(grade) * weight = 20.234 + 0.01993*v + 0.01874 * v * v
  • sine(grade) = (20.234 + 0.01993*v + 0.01874*v*v) / weight
  • grade = arcsine ( (20.234 + 0.01993*v + 0.01874*v*v) / weight )

We can use this to plot terminal velocity vs grade, the terminal velocity, as a function of grade. Knowing the speed and grade, we can calculate the kinetic energy loss. Knowing the difference between the physics speed limit versus the actual speed will give the amount of energy 'saved' in the traction battery. But we can measure this traction battery charge.

It would help to repeat the test with an XGAUGEs for traction battery current and voltage. The product of current and voltage is the watts being pushed or pulled from the traction battery. For good measure, also an XGAUGE for the hotest traction battery temperature. Charging the battery is exothermic, they get warmer when charged. This is why I've always recommended using "B" than "D" to reduce the traction battery heating. But finding the hottest temperature probe will require some testing.

The Prius with OBD scanner is an experimenter's dream. But we need more details to understand the total picture. Using the roll-down coefficients, we can develop a math model. Since my earlier traction battery charge tests were done with the old-style, NHW11 modules, we need to know what happens with the newer modules in the 2010 . . . regression testing.

Testing is the difference between speculation and facts and data. You've done good but we'll need more metrics to understand what is going on precisely. That you ran a test is a big step.

Bob Wilson

 

Bob,

For the Gen III I can tell you that BT2, or battery temp sensor 2, is always the hottest. It is in the middle of the pack and that makes sense. I have monitored all three and 2 is always the hottest. BT2 is the one that controls the battery cooling fan since it is the hottest. Now I just monitor BT2 for battery temps.

 

I know driving mine there is a very soft jolt when speed goes over 76km/hr in EV mode. I take this to be the ICE being allowed to spin to Keep MG1 below the redline. No fuel is used and no spark so there will be no reading. If I just give enough throttle to kill the regen then speed increases like no other car I have driven. I have seen it coast up to 115km/hr on a short downgrade. No fuel is being used according to the display. Therefore if the ICE can spin up at this speed by drag from the transmission is there a brake that keeps the ICE stopped below 76 in EV mode. You certainly feel something changes in the transmission at that speed. Anyone know what it is?

 

Sorry to bring up an older thread but I experience what Greg C said every morning going to work. ICE is up to full temp after a 20 min drive at 100kph or 62mph. Then changes to 80kph then to a 50kph speed zone. So when I switch off the radar cruise at about 75kph ICE fires up momentarily. Also regen is increased.

 

hi mates, yes the jolt is indeed the ICE starting its mandatory spin to keep the MG1 from over-spinning.
If you want to control the car's speed say below 76kph, then either just keep the foot off the throttle or slightly feathering it, or apply the brakes, or put it in B (or both). whatever method is used won't consume any fuel, unless you decide to accelerate quickly again. Below 76kph, the thing that keeps the ICE from spinning is the regular regen process by the MGs.

 

this is interesting, ive always heard that coasting in N doesnt do much noticable increase to MPG. Have you seen better use doing this?

 

Many claim N helps until they attempt to measure it, then admit N hurts MPG in a hybrid Prius.

 

I usually glide using N at slow-moving, stop and go traffic when my HV battery level is nearing 50%. Although it doesn't extend the glides further compared to warp stealth, it uses less power (1.2 amps vs 5 amps) because it frees up the wheels where one can judge when to use it to cover some ground. I engage back to D when i need to slow down the normal way.

 

I find it also more energy-saving to use N in doing the middle part of the acceleration downhill with just gravity pulling the car down before i engage back to D when I reach my personal limit of 57kph compared to just gliding all the way downhill in D. From my butt dyno, going down a slight gradient, I find there's more resistance in D (not to mention more energy used when gliding in D) to accelerate at the same rate than when I'm in N.