Does it make sense to shift to neutral during driving?

[andyprius]

Originally Posted by dominicsavio

Folks,
I'm wondering on a technicality (to increase fuel efficiency) and need some input:

When driving downhill (or even coasting a modest speeds for that matter), does it make sense to shift to Neutral to take FULL advantage of gravity (or momentum)?

When the car is in Drive, it is still partially engaged and will not run completely free under the effect of gravity (or momentum). However, when you shift to Neutral (while going downhill or even coasting towards a stop), you can go further quicker because of the 'free flow'.

I've tried this with conventional manual shifts and it works just fine.

I'm guessing that for the Prius this needs to be weighed against losing regenrative braking or battery recharching (becase shifting to Neutral disengages the motor/battery)?

Thanks!

I have done this for years with my other cars. But, with the Prius it does not help. I tried this extensively on two half tanks and I actually got worse mileage! As the other posters have said apparently the ICE keeps spinning. However any mode that you are in which does not use the engine is a good mode, whether your running from the regen feedback, the battery or in glide (stealth) The magic normal speeds are 30-40MPH. Generally difficult to maintain under normal driving conditions.

[dominicsavio]

These are all good points. I've tried coasting in Neutral with my previous car (conventional 4 cylinder gasoline engine) and was able to push the fuel efficiency up to 33-40 mpg (depending on cit/hwy/hot-cold temp). The important thing to remember here is that in Neutral, the engine is disengaged from the powertrain (i.e. the car is in a "free-run") so regardless of the speed (i.e. wheels turning), the rpm still stays at a minimum (this was an old car, not sure if this still happens in newer cars).

This experience left me feeling that "Neutral" is the best way to take advantage of momentum/kinetic energy to directly drive up fuel efficiency.

The Q is, in a Prius, does Neutral mean the same as above? I guess not because you still probably run the risk of revving up the engine at high rpm (I wish Prius had an rpm indicator).

Would someone please explain why going downhill on Neutral (in any car) is illegal (or unsafe)? I understand this means that the car is not in 'gear control' but so what - you are still driving with brakes (as you would if the car was in gear)?

[qbee42]

No, in a Prius everything in the drivetrain is connected all of the time. There is no clutch or shiftable gears. Look up HSD if you want the details.

Coasting in N is illegal and considered dangerous because it was and is potentially hard to get back in gear with a manual transmission. In the days before synchromesh gears and good brakes, you could find yourself coasting faster than intended, with no way to slow down. Most of this doesn't apply anymore, and none of it applies to the Prius.

Tom

[JimboK]

Originally Posted by andyprius

As the other posters have said apparently the ICE keeps spinning.

Originally Posted by dominicsavio

These are all good points. I've tried coasting in Neutral with my previous car (conventional 4 cylinder gasoline engine) and was able to push the fuel efficiency up to 33-40 mpg (depending on cit/hwy/hot-cold temp). The important thing to remember here is that in Neutral, the engine is disengaged from the powertrain (i.e. the car is in a "free-run") so regardless of the speed (i.e. wheels turning), the rpm still stays at a minimum (this was an old car, not sure if this still happens in newer cars).

This experience left me feeling that "Neutral" is the best way to take advantage of momentum/kinetic energy to directly drive up fuel efficiency.

The Q is, in a Prius, does Neutral mean the same as above? I guess not because you still probably run the risk of revving up the engine at high rpm (I wish Prius had an rpm indicator).

A key point to using neutral effectively in a glide and, therefore, as a fuel-saving technique, is for the ICE to be off when the shift to neutral is made. As Tom has suggested, the ICE remains in whatever operating mode it is in when the shift is made. So if the shift is made with the ICE on, it remains on in neutral and, of course, is running inefficiently. Andyprius, maybe that explains why your fuel economy worsened.

Then it is important to know that the ICE won't spontaneously shut off until the car has progressed through its warmup stages. But once the warmup process is complete and the shift is made at <40 MPH, the ICE will not be spinning.

There is no risk in over-revving the ICE. If it's on during the shift, it simply runs at idle speed afterwards. The theoretical danger is in over-revving MG1. But that will happen only if the shift is made in an ICE-off condition at <40 MPH before a long or steep downhill that then allows coasting up to ~62 MPH. If the shift is made at 40+, then the ICE is running at the shift (before full warmup) or, as protection for MG1, spinning without fuel flow (after warmup).

Dominic, there are many that share your wish for a tachometer. Some have a ScanGauge to monitor RPM and various other operating parameters.

[ilusnforc]

I never have liked putting automatic transmission cars in Neutral when driving, much less do I like it in the Prius with the CVT. In either case, it seems that when the car is at speed and shifted from Neutral back into Drive again the RPM's rev up like the computer is trying to find out what ratio or gear it's supposed to be in and adjusts it from there. With the engine revving up in the Prius when shifting back to Drive again I would think that it would probably defeat most if not all of any benefit you may have gained by going into Neutral. I personally think you'd be better off (and safer) by just feathering the accelerator to where the MFD shows no power being consumed or regenerated.

I don't completely understand how the Neutral in the Prius works mechanically, but in the case of a regular automatic transmission I would think it causes excessive and unnecessary wear & tear on the tranny. This may or may not be the case with the Prius, but still I've only done it at moderately low speeds just to see how it would react and I didn't like it and will not do it at any higher speeds (~40+). I'm also curious, anyone ever tried Reverse or Park at any speed to see how the car behaves? I'm pretty sure I've tried Reverse before and it just beeps at you, and I don't dare to try park. Just wondering how smart it really is.

[qbee42]

Originally Posted by ilusnforc

I never have liked putting automatic transmission cars in Neutral when driving, much less do I like it in the Prius with the CVT. In either case, it seems that when the car is at speed and shifted from Neutral back into Drive again the RPM's rev up like the computer is trying to find out what ratio or gear it's supposed to be in and adjusts it from there. With the engine revving up in the Prius when shifting back to Drive again I would think that it would probably defeat most if not all of any benefit you may have gained by going into Neutral. I personally think you'd be better off (and safer) by just feathering the accelerator to where the MFD shows no power being consumed or regenerated.

I don't completely understand how the Neutral in the Prius works mechanically, but in the case of a regular automatic transmission I would think it causes excessive and unnecessary wear & tear on the tranny. This may or may not be the case with the Prius, but still I've only done it at moderately low speeds just to see how it would react and I didn't like it and will not do it at any higher speeds (~40+). I'm also curious, anyone ever tried Reverse or Park at any speed to see how the car behaves? I'm pretty sure I've tried Reverse before and it just beeps at you, and I don't dare to try park. Just wondering how smart it really is.

How does N work in the Prius mechanically? It doesn't. Neutral in the Prius, as well as all other gears, are mechanically the same. Nothing changes. All of the same gears remain engaged. The only thing that changes is how the control system runs MG1 and MG2.

Tom

[dominicsavio]

A few clarifying questions:

1) What is MG1 and MG2?
2) Why do people quote "41 mph" as a cut-off speed for ICE activation?
3) When I turn "ON" the car in the morning in my garage (after it has been parked overnight in freezing temperatures), after a few seconds, you can "hear" the car "whirl up" (while it is still parked). I'm guessing this is the ICE firing up? If so, does this happen just to bring the powertrain to an efficient operating temp? Also, I'm assuming gasoline is used during this process (although your MFD would not indicate any arrows because the car is not propelling)? I'm wondering if it is a good idea to let the car warm up for ~10 min before engaging "Drive" (again all this with the aim of maximizing fuel efficiency)

Also, to responde to Tom's post:
"Coasting in N is illegal and considered dangerous because it was and is potentially hard to get back in gear with a manual transmission. In the days before synchromesh gears and good brakes, you could find yourself coasting faster than intended, with no way to slow down. Most of this doesn't apply anymore, and none of it applies to the Prius"

Why would it be hard to shift into (top) gear when coasting downshill in neutral in manual transmission? All this would require is to press the clutch to lock in the gear and rev up the engine. And you could ofcourse slow down using your brakes (just as you would were you propelling downhill under gear). I'm still trying to understand the safety issue here...(I do not have experience with automatic transmission so if this applies only to automatic, I may be missing the point)

[JimboK]

Originally Posted by dominicsavio

1) What is MG1 and MG2?
2) Why do people quote "41 mph" as a cut-off speed for ICE activation?
3) When I turn "ON" the car in the morning in my garage (after it has been parked overnight in freezing temperatures), after a few seconds, you can "hear" the car "whirl up" (while it is still parked). I'm guessing this is the ICE firing up? If so, does this happen just to bring the powertrain to an efficient operating temp? Also, I'm assuming gasoline is used during this process (although your MFD would not indicate any arrows because the car is not propelling)? I'm wondering if it is a good idea to let the car warm up for ~10 min before engaging "Drive" (again all this with the aim of maximizing fuel efficiency)

1) Sorry, we sometimes forget that newbies don't know the vocabulary. MG1 and MG2 are the two motor/generator units. See this thread, including the many good links within it, to learn more about the HSD (Hybrid Synergy Drive) system and power flow.

2) 41 MPH is the quoted threshold, simply, because that's the way the car is designed, for reasons known to few other than Toyota engineers.

3) What you're hearing on startup is indeed the ICE's initial firing, in the first of its warmup stages. It takes several minutes to reach its most efficient operating temperature (157F+), so it's not operating at its most efficient until then. See the link I provided previously in post #14 for a full description of the warmup process.

Do NOT let the car sit and run during warmup if you're interested in efficiency. It's burning fuel needlessly. Better to get in and start driving.

[efusco]

41mph is magical b/c above 41mph the ICE MUST spin (unless you go into an ICE off neutral and coast to a higher speed). Below 41mph the ICE can spin when necessary, but you can also get into stealth mode (ICE off).

Thus, for Neutral to be effective you can must be in an ICE off condition--only achievable under 41mph. But you're correct in that just being under 41mph does not necessarily mean that the ICE is off.

[dominicsavio]

Thanks! This means that when you are cruising at optimal speed (42-50 mph; =speed for maximum fuel efficiency), you are always propelling with ICE "ON". Why would these conditions translate to maximum fuel efficiency? Wouldnt you argue that the optimal speed would be 35-40 when you have periods of ICE "off" (= more fuel efficiency)?

I'm assuming that in the 40-50 range, propulsion is solely by ICE and additional contributions of the battery are nil or insignificant