I have a vague (and possibly faulty)recollection that there is an upper speed limit above which the transmission should not be in Neutral while rolling forward. Something about MG1 spinning too fast? Is there actually such a speed? Or maybe that speed limit has to do with rolling backwards? Normally I only shift into Neutral while moving to clean rust off the brakes, and that would be at no more than 35 MPH for just a few seconds. However, I was thinking about trying to calculate the coefficient of drag (from air) and the rolling resistance (from the tires) from MPH vs. time data obtained in a coast down from say 60 MPH in neutral. Wouldn't want to try that if 60 MPH is going to damage the transaxle. Also given the unusual way the transaxle gears are attached to the motor in this car, I'm thinking the data wouldn't be valid until the motor turned off, whereas on a conventional car it doesn't matter what the motor is doing once in neutral. In such an experiment there is going to be some drag in the car from the wheel bearings (probably much lower than from the tires as that is lubricated steel rolling on steel) and from the chain drive and whatever else is spinning in the transaxle. I would assume that the transaxle drag in Neutral is pretty low, since very little power is being moved in either direction, but I have no feel for its magnitude. Anybody have numbers on that? Chain drives are said to only lose 1% to 4% of transmitted power. I don't have any physical intuition on this, since the only experiment I ever did was to spin the front wheels while the car was in neutral and the car jacked up, and I'm pretty sure most of the force I was exerting was going into accelerating the mass of the wheels and axles. Hmm, I guess this drag could be estimated for very low speeds by hand spinning the front tires on the lifted vehicle in neutral and then measuring how it slows down. The unloaded wheel bearing drag could be found by doing the same on the back wheels. Except even a slight amount of brake drag would mess up the experiment.
The speed limit comes from a rev limit on MG1 - there's a maximum speed above which the engine needs to run to counteract that. That's about 42mph. If you shift to neutral above that speed, the engine will be running, and it won't stop because you shift to N, and it will keep running as long as you stay in N. So you won't exceed any rev limits. So, I'd say, no issue for a coast down test from 60mph. Apart from the fact that you will be measuring "engine running" drag. Which presumably must differ a little bit from engine stationary, due to the different motion of the transaxle. The only problem case would be if you were gaining speed while in neutral, so you started off without the engine running, and then exceeded the engine-off limit. I understand newer Priuses will override the electric disconnect briefly to start the engine.in that scenario - don't know about the G2.
Neutral gliding is a well trod path here in America, and in some of our more autocratic states it's actually against the law. (California is one of them, naturally.) I personally do not think that it would hurt a Prius very much at all, but that's a WAG. It's not a very smart thing to do, but I live where it's relatively flat - so it's not something I worry much about other people doing.... For vehicles that DO rely on active transmission coolers, it's one of the thousands of reasons that I tend NOT to buy a car that somebody else has used before me. From the "Googles." Coasting in neutral, or freewheeling, disengages the engine from the wheels, potentially reducing fuel consumption but also diminishing control and safety, especially in emergency situations. Here's a more detailed explanation: What it is: Coasting in neutral, also known as freewheeling, occurs when you drive with the clutch pedal depressed or the gear stick in neutral, disengaging the engine from the wheels. Why some people do it: Some drivers believe coasting saves fuel, but modern cars are designed to cut fuel to the injectors when coasting in gear, making it less fuel-efficient than coasting in neutral. Why it's generally discouraged: Loss of control: You lose engine braking, which can lead to overheating brakes when going downhill and difficulty accelerating out of sticky situations. Reduced safety: You won't have power if you suddenly need it, and shifting to the wrong gear if you need power can be dangerous. Increased brake wear: You rely more on brakes when coasting in neutral, leading to increased wear and potential brake fade. Fuel consumption: While some believe coasting saves fuel, modern cars still require fuel for idling, so coasting in neutral doesn't necessarily save fuel. California law: Coasting in neutral is an infraction under California Vehicle Code Section 21710.
Long ago when we all had really heavy cars and really bad drum brakes that would easily get too hot going down hills was when being in neutral was dangerous. Cars are way safer now, especially with disk brakes and much lighter vehicle weight. As other comments have detailed, it's perfectly safe to mess around with neutral when you got nothing more substantive to do with your life.
I've done several coast down test sessions with my Gen 3 (from speeds above 70 mph) to measure changes in aerodynamic drag. You can read my basic write-up in Julian Edgar's "Vehicle Aerodynamics: Testing, Modification, and Development" (2023; I highly recommend the book if you're really interested in this subject since it contains a wealth of information and was extensively reviewed before publication not just by me but several professional aerodynamicists from Tesla, JLR, Aston Martin, and Porsche) or on my blog: Coast Down Testing Developing a Splitter Optimizing a Tail: Part 5
Probably one of the first topics on the forum about switching to neutral at high speed. Is coasting in neutral a problem? | PriusChat I would not allow the car to accelerate on its own at all, switching to neutral. During emergency braking in neutral mode, the kinetic energy from the car's movement would wear down the brake pads, and I would have to replace them, wearing down my budget.
I was thinking about comparing rolling resistance of tires on cars by comparing coast down tests. The high speed part is just to find the air drag component so that it can be removed from the low speed part. In the link you discuss how many variables there are. I agree completely. The best way to do tire rolling resistance tests is with one of the SAE or ISO approved testing rigs. But the US manufacturers do not release that information, and I don't see any of us ponying up the (tens?) of thousands that an independent lab would charge to do that work per tire tested. For rolling resistance comparisons I'm pretty sure that the biggest variables are going to be road surface and temperature, assuming that wind can be eliminated. Doing all tests in a long straight flat tunnel would be great, but there aren't a lot of those around with such light traffic that it would be practical. In my general vicinity, and this isn't really very close at all, maybe the 2nd street tunnel in Los Angeles at 3:00 AM. Probably not though. Closer to home finding a section of road of any significant length which is flat and level is a huge challenge. In the San Gabriel valley it is all gradient from the mountains down to the lowest parts, modulated perpendicularly by shallow "valleys" from old river and stream drainage patterns. There is a 100m street with no dips near me which has "only" a 1m elevation change, so still a 1% grade. Not a lot of space for a coast down measurements though.
As mentioned, MG1 has a software speed limiter. Under normal conditions, the hybrid control will start the ICE at around 40mph to keep MG1 below around 6500RPM. Shift to neutral and the ICE stays running (but the HV battery won't charge). If you shift to neutral while ICE is off and below 40mph, then the ICE stays off. If you happen to be rolling down a steep hill, then MG1 will go above 6500. I figure you need to hit around 70 to reach 10,000RPM for MG1 (then I'd expect a momentary "non-neutral" event to start the ICE). Posted via the PriusChat mobile app.
