Does it make sense to shift to neutral during driving?

[JimboK]

Originally Posted by 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

Above 40 MPH is not more efficient. Your logic is correct; better efficiency is obtained at speeds where the ICE routinely shuts down. Actually, one Prius researcher found that the most efficient steady-state speed is 20 MPH.

Though the ICE is mostly what propels the car at 41+, the battery still has a role in certain conditions. It depends on state of charge (SOC). When it's at its optimal level of 6 blue bars (~60%) or below, power comes exclusively from the ICE. However, as SOC gets into 7 or 8 green bars, the car will use some of that power to supplement the ICE. Its goal there is to bring SOC back down to optimal to allow headroom for future regeneration opportunities.

[qbee42]

Originally Posted by dominicsavio

<snip>

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)

This was really an issue with older manual transmissions, which lacked synchronizers. As you point out, it is simply a matter of matching speeds before trying to shift into gear, but apparently someone had trouble with that at some point and caused an accident, causing the usual over reaction and new legislation trying to save people from their own reckless behavior. With early cars you could probably coast faster than the car was capable of driving under power, which would make shifting back into gear impossible. It hasn't been a real issue for as long as I've been around.

Tom

[dominicsavio]

Makes sense - thanks.

BTW, how does "regenerative braking" work - do you happen to have any handy links to this?

I find this concept pretty cool considering that you need friction to brake (i.e. frictional "loss" in energy due to braking is a necessity) yet regenerative braking appears to be a system to "mitigate" this "loss" by capturing/regenerating energy back into the system?

[qbee42]

There are a lot of posts on this site which address regenerative braking. At the conceptual level it is quite simple. When you press on the brake pedal, the Prius runs the motor as a generator which is turned by the front wheels. The electricity from this generator is used to recharge the battery. The harder you press, the more it generates, up to the limit of the generator or battery. Once the car slows down to around eight mph, the generator is going too slowly to generate meaningful power, so the Prius switches to friction brakes.

The Prius will also switch to friction brakes in a panic stop, when ABS is needed, or if the battery becomes too full.

Tom

[cwerdna]

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).

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)?

When Coasting to a Stop, do Not Shift Into Neutral - Feature / Gas Pains: Mileage Myths and Misconceptions - Feature / Features/Classic Cars / High Performance / Hot Lists / Reviews / Car and Driver - Car And Driver and the statements given would disagree with you about benefits of shifting into neutral in a non-hybrid and also addresses the safety aspect.

[dominicsavio]

Cwerdna - thanks for the link. Interesting read. We have to remember two caveats though (1) This applies to newer and mostly automatic cars in whcih engine revs can response to "gear-idling" (= coasting in gear).

You can easily see if this is the case with your (manual) car by looking at the tachometer while coasting with and without gear-engagement.

For my Prius and automatic cars (especially those with CVT), I agree, coasting in neutral does not make sense.

[dominicsavio]

Tom - thanks for outlining the concept. It seems that: the harder you press the brake pedal, the more you charge the motor (via the wheels). How is this 'inverse' relationship possible when the wheels actually spin slower and slower the harder you brake?

[diamondlarry]

Originally Posted by dominicsavio

Tom - thanks for outlining the concept. It seems that: the harder you press the brake pedal, the more you charge the motor (via the wheels). How is this 'inverse' relationship possible when the wheels actually spin slower and slower the harder you brake?

It requires a constant/controlled application of more pressure as your speed decreases.

[qbee42]

Originally Posted by dominicsavio

Tom - thanks for outlining the concept. It seems that: the harder you press the brake pedal, the more you charge the motor (via the wheels). How is this 'inverse' relationship possible when the wheels actually spin slower and slower the harder you brake?

The amount of drag from the motor/generator when it is used as a generator is controlled by the field current. As you press harder on the brake pedal, the electronic brake controller increases the field current, making it harder to turn the wheels. It's like squeezing harder on the brake disk or drum, but it is done with magnetic fields.

You are correct that for a given amount of braking force, the amount of regenerated energy drops off with lower speeds.

Tom

[bwilson4web]

Hi,

Rather than offer data, I'd like to offer methodologies that can be used to add to our knowledge. As my drill instructor used to say, "A grain of observation outweighs a pound of <bolvine scat> anyday."

Originally Posted by 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)? . . .

Let me suggest making your own MPG vs MPH chart similar to these:

The data points came from either my NHW11 or NHW20 data volunteered by others. The protocol is fairly straight forward:

• established a fixed route and baseline the performance - it needs to be reproducible
• using the above, make one change, such as speed, oil, tire pressure, while holding all other variables as constant as possible including temperature

LENGTH OF TEST AND WARM-UP

The engine warms up quickly, within 5 minutes measured by coolant temperature. However, the transaxle takes longer, ~30 minute to plateau and it contributes directly to rolling drag. But after 20 minutes, my observations indicate the MPG has pretty much leveled out. Your MPG display can easily show this warm-up duration by monitoring the 5 minute interval bars holding a constant speed over a flat route with a cold vehicle.

If the temperatures are fairly constant, you can include warm-up data provided the rest of the protocol remains the same. Just make sure you get enough samples (aka., test runs) to average out the differences. One crude but effective technique is to get 5 samples; toss out the highest and lowest; and average the 3 remaining.

DATA AND A MODEL

As a general rule, vehicle drag and 'power on' overhead, dictates how much energy is needed. The 'power on' overhead is both the electrical power needed to keep the vehicle "ON" as well as heat losses that cause the vehicle to run engine to maintain coolant temperature above 60 C.

The vehicle drag comes from two sources:

• rolling drag - normally treated as a fixed overhead regardless of velocity, it includes both the tire and transaxle drag. I've read one paper that suggest tire drag becomes non-linear above 80-85 mph but it is specific to each tire.
• aerodynamic drag - increases as the square of the velocity and linearly as a function of air density, a temperature dependency. A standard day is defined at ~60F (15C) with +/- 5F swing. However, some of my data suggests using 70F +/- 5F (~21C) seems to give more reproducible results.

Once you have a good math model, you can plot the expected MPG and combine it with data points to validate the model:

When warm-up is dealt with and the temperatures are close to a standard day, we get excellent agreement. You'll notice all of the 'above the line' events occur in the summer such as the "NHW20 Marathon." All of the mileage data points came from the Prius display, nothing special was required.

LONGER VS SHORTER TESTS

All of these data came from the Prius MPG display. As such, they don't have the resolution to see finer details such as fuel consumption vs. "N", "D" or "B". For these types of measurement, a SCANGAUGE would be needed and hopefully with the ability to record the data for later analysis.

Before I got my Graham miniscanner, I and others including Hobbit, would install our own instrumentation. Even now, I find somethings are best handled by quality instrumentation. Although the Graham miniscanner provides six data samples every second, this is still too slow for some of the faster events such as engine start and stop.

CONCLUSION

What I'm hoping you consider doing is start treating each drive as potentially another experimental data point. Gather the data and then begin looking at 'single variable' changes. I don't think we know all of the answers and more data is better than less.

Bob Wilson