Does it make sense to shift to neutral during driving?

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.

 

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

 

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

 

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.

 

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

 

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

 

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."

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

 

In hypermiling, WHY wouldn't one use pure electric, you paid for it, why not spend it?

 

Because that electric power is coming right out of your gas tank in a very inefficient way. If you tried to use as much electric power as possible, you WILL get a lower MPG then if you drove like normal. It'll also reduce your battery life by cycling it further and more often.

 

Well, the problem with using pure electric is ... you pay for it. Battery charge has to be replenished, and the source ultimately comes from the ICE. As energy from the ICE converts to electrical energy and then to chemical energy (and then the reverse to actually use that energy), loss of energy occurs at each step of the conversion. To reduce these losses, hypermilers generally try to avoid using the battery as much as possible.

 

OK, I think I have it now, eventually after battery cosumption Prius would again try to charge the HV, BUT this time at a higher rate than IF you had previously used ICE and HV together in order to keep the HV Bat at a nominal SOC. Would that be the general theory?

 

Now, this entertains another situation. I live in Sacto and often return back fron Reno with a drop of about 7500 feet. How do I keep the SOC in the mid zone. Normally a SOC into the GREEN occurring 4 times is not unusual.

 

It's not a theory. Any charge in the traction battery came from burning gas. In the ideal world all of the energy from burning the gas would go to moving the car. Most of it isn't, with engine inefficiencies, heating, friction, and so forth. Converting to electricity and then converting the electricity to chemical energy, just to undo the conversions, is nothing but an additional waste of energy.

The Prius has electric motors and a battery to let it get away with a smaller and more efficient engine, and run that engine in a more efficient manner. There is also a gain from regenerative braking. But once again, in the ideal world you would never brake. So ideally, you avoid braking, avoid running in electric mode, and try to run the engine in its most efficient range. Often it can't be done, but it is a goal.

Tom

 

Ideally, you'd never hit the brakes and just use your engine a tiny bit to give you a "push" to start you off and just let gravity accelerate you all the way back home.... in a perfect world.

But in this case, your forced to charge the battery since 7500 feet is alot of potential energy. Best charge the battery as much as you can and use the energy as efficiently as possible for your trip uphill tomorrow.

 

So we have come right back to a full circle, Whether it's 7500 feet or 50 feet, if the SOC is high, then use it. The only other alternative is to keep charging the battery and then the computer will spin the engine ( or flywheel ) just to discharge that high battery state. " And that is wasted energy"!

 

This should be obvious, and I'm trying my best to not say "Duh!" - oops, I just said it. The real world is not perfect. If it was we wouldn't need the battery and electric motor, and normal cars wouldn't need transmissions. Being able to recover otherwise lost energy is one of the gains from HSD. When the SOC gets high, the Prius will automatically use the extra energy. You don't have to force it, or try to run in electric mode. It will do it for you. Forcing electric mode is a fools game unless you are moving for short distances and don't want the engine to needlessly start. There are also a few special cases where it is optimal to start with a low SOC, such as at the top of a long hill, so coming into that you might want to force EV mode if you can.

Tom