Recently on my commute to work, I have been putting my car in neutral when going down a slight grade at about 48 mph with my SOC fairly high. I start off with the car in drive and just feather the gas pedal so as not to be charging or using up the battery (glide mode). The rpms are 0 and the gas consumption is 0, which is what I would expect. Then I put the the car into neutral and notice right away that the car is more free-wheeling, which is great. After about 5 seconds I notice, on my Ultra-Gauge, that the rpms are about 1,000 and the gas consumption is at about .28 gph. Why is the engine spinning and using up gas in neutral, when the car is still only going about 48 mph? Has anyone else noticed the phenomenon?
At any speed over 46 mph in a Gen3 Liftback (different speed for Gen2 and for other models), the ICE must spin to keep MG1 from overspeeding and damaging itself. When you are in N, the MGs are electrically disconnected, so the ICE must burn fuel to spin. Be very cautious about using N downhill. If the ICE is stationary (RPM = 0), the car should be unable to start the ICE while in N, so if gravity speeds you to faster than 46, you are risking damage. It is safest to keep it in D, and lightly press the gas pedal to a minimum-engine-drag position. If you shift to N above this threshold speed, with the ICE still spinning, MG1 is protected. The known risk occurs when shifting below this theshold with ICE off, then rolling up past the threshold. There is some safety margin, so small violations are OK, but we don't know with any certainty how much margin is available.
Neutral in other cars means that the engine is disconnected from the wheels and does not deliver any power or drag to them. With Hybrid Synergy Drive, the engine is always connected to the wheels through a planetary gear set (differentia and "Neutral" is achieved by running the engine and one of the electric motor generators (MG) to achieve a similar effect. I am not sure but IIRC, at low speeds this can be done by the MG alone but at higher speeds, it would over rev the MG so the engine must also run.
I believe the engine is spinning to protect the electric motors. FYI - the car will take care of its own SOC. It is not advised to use neutral as reengaging drive at highway speed could damage one of the electric motors (not a given, but possible)... It is better to use warp stealth mode. Try to feather the gas so it only shows electric useage when going down a slight incline at highway speeds. This should keep the ICE spinning, but cut the injector usage as low as possible.
I experimented with coasting in neutral when we first got a Gen 3 Prius and found that it did not improve my mileage over pulse and glide in drive.
I think it is more correct to say that the engine is spinning because, mathematically, by the gear ratios in the PSD it has to. Go to this link: Toyota Prius - Power Split Device and in the simulation move the slider for MG2 speed up until the line becomes red at which point you can see that the only way to "fix" it is to make the ICE spin. (Note the RPMs in this simulator apply to an older Prius, so ignore that absolute numbers) Mike
