coasting in neutral questions

Coasting with a cvt in neutral at best offers nothing for increased mpg, indeed beecause of the drag on the engine it could easily induce less mpg. When the engine experiences drag it has to ncrease fuel to the injectors to make up for the power requirement.
Neutral in a manual does increase mpg as the drag is much less than it takes to spin that belt in a cvt which is why some manufacturers like 6, 7, 8, 9, and 10 speed transmissions, which have less less drag than a cvt in any rpm. But are expensive as all get out.

 

When it avoids forcing potential energy into the traction battery, the vehicle energy savings dwarfs a slight increase in engine fuel consumption. For example, shifting into "N" will change the ScanGauge II reported engine fuel consumption from ~1,400 miles per gallon to ~250 miles per gallon. But at these levels, we're talking fumes. In contrast, the energy lost in the charge-discharge cycle is an order of magnitude higher.

We've learned the engine control laws will shutdown the injectors provided the car is in "D". However, it is the total energy flow that dictates the vehicle mileage, not just the current engine fuel consumption.

Our Prius CVT does not use a belt. It is a computer controlled, dual-motor, power split device. Shifting into "N" eliminates any engine drag outside of what is already in the engine even at idle. If the engine is not spinning, there is no engine drag.

Bob Wilson

 

Shifting into N seems to prevent the car from being able to switch to stealth EV mode. Make sure to let up on the pedal and wait for the car to switch to EV before putting it in N otherwise the ICE will continue to run the entire distance you are coasting (correct me if I am wrong).

That being said, I experimented using N for a bit here and there and eventually found that it was better to just train myself to leave it in D and zero out the power bar. It takes a little practice while working on your coasting, like a tank or two, and afterwards you'll wonder why you ever bothered to use N.

 

You look for energy flow from the wheels to the traction battery. At that point the engine will be off if it can go off. There are speeds above which the engine has to run. For internal gear speeds, the faster the car is going, the faster the engine will idle but the fuel consumption I remember remains close to the idle, 0.20 gal/hr. Still memories are not data.

I'll see if I can record the data sometime this weekend. Metrics always trump memories.

Bob Wilson

 

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Yes, that's correct. Being in N prevents the engine from turning on if it's off, off if it's on, and prevents MG2 from providing assist or regen.

Bob - based on the simulator at Toyota Prius - Power Split Device , I would expect the minimum idle speed to be 1000RPM up to 66mph, and increasing linearly at higher speeds. But experimental confirmation could be interesting.

 

i've tried it a few times. once on the interstate going about 65 down a bug hill and then again going 35 down a large hill, i personally get a very uneasy feeling shifting into N and then re-engaging it back into D while still moving and from what I am reading isn't giving me a large enough bonus to perform this risky technique. Not to mention as others pointed out it may be illegal in some states

 

This web page is concerned with a similar issue (neutral at a red light) but lists many states where coasting in neutral is illegal. (it is not saving fuel in a Prius even where legal, since nothing is disengaged when in N)

united states - Is it illegal in any U.S. jurisdiction to be in neutral at a red light? - Skeptics Stack Exchange

 

I can't imagine it would be illegal, or manual transmissions wouldn't be legal... oh wait... maybe that's why they are dinosauresque.

 

I repeat my post above: I failed my first driving test long, long, long time ago with manual transmission car because I stopped in N at red lights in CA. The judge said it's illegal because it's not safe, you can't drive the car right away in case of an emergency.

 

I don't doubt that a long ago test examiner failed you for this specific reason, but still have doubt that it is actually codified into law. Lacking a legal reference, it seems more likely that this was how that particular examiner was taught to drive. My driver ed teacher didn't require the same.

I was taught to operate manual transmissions on farm equipment, and was required to put them into neutral and release the clutch pedal when standing still for similar times. It was claimed to be something to prevent wearing out the throw-out bearing, though it now seems that modern cars are more robust on this than that old equipment. And once when dad violated his own 'neutral' rule, he nearly crushed my leg as I worked behind the machine and he slowly and absentmindedly let go of the clutch.

Leaving the car in gear is no assurance of being able to move the car right away in an emergency. It must also be placed in 1st or 2nd gear, not left in high gear. Was he checking for that? I don't downshift to those lowest gears while approaching stoplights, it puts needless strain on the parts.

 

When I researched "neutral" traffic laws, it seems they date from the days of mechanical brakes and trucks. The problem being a truck descending a steep grade could overhead the brakes and engine braking reduced the risks. Curiously, there is a reason why "B" makes sense when descending tall hills for our hybrids.

Charging our NiMH batteries is exothermic, generates heat. Descending tall hills in "B" reduces the charge and my measurements show it helps to keep the traction battery a little cooler. Combine that with climbing on just engine power (i.e., moderate speed and monitoring the energy flows) further helps to keep the traction battery cool.

Bob Wilson

 

Additionally, once these trucks suffered serious brake fade and got going too fast, it was nearly impossible to re-engage the unsynchronized transmission from Neutral to a useful gear. Modern cars generally don't have this problem.

 

To ansewer the orginal post I would like to share my expeince with my 2013 Prius C. I have a scan gauge type device to monitor many things while hypermiling. I have expermented with this concept of coasting in D or shifting into N and letting my foot off the throtle. No matter the speed I do this in, I see nearly identical MPG estimates. The most important observation Ihave made is the distance to which I am able to coast. When I coast in D the Prius C slows down more quickly; as if there is slight resistance on the car. When I shift into N I can coast at the same speed with little resistance and therefore stay in "glide" mode much longer. In low speeds I can glide almost double the distance in N. Has anyone else with a Prius C observed this behavior. I feel this is an important technique to be explored for the P&G as it will increase overal MPG since you are able to stay in glide for much longer distances. Also I would like to add; I have a long hill on my commute each day and the speed limit is 45 mph. I intentionaly stay below 45 mph so I do not activate the ICE. If the ICE comes on at 46 mph then my MPG drops from say 3K MPG to about 250 MPG. A significant difference I think to further more discussion on the matter. Just my 2 cents but let the depate continue

 

As long as the shift selector is in "D," the car has the ability to put a load on MG2 or MG1. Going into "N" prevents any power flow to or from MG1/MG2 so less drag and longer ballistics coasting.

As for fuel consumption, let's assuming the low values are accurate and that is a 1 mile hill:

• 1/3000 = 0.0003 gallons
• 0.0003 gallons = 0.043 ounces
• 1/250 = 0.0040 gallons
• 0.004 gallons = 0.512 ounces

Now I have a 1.8L Prius and the no-load, idle fuel consumption is ~0.20 gal/hr.

Bob Wilson

 

• 1/3000 = 0.0003 galons
• 0.0003 gallons = 0.043 ounces
• 1/0.004 gallon = 0.512 ounces

Now I have a 1.8L Prius and the no-load, idle fuel consumption is ~0.20 gal/hr.


Bob Wilson[/quote]

So the savings from gliding with the ICE off is not that significant??? Do you think it is safe to glide in N while in speeds below 40 MPH? The only reason I suggest this is because I have been experimenting with this type of gliding and I have been able to increase my gas savings by 10 MPG

 

It is better to coast in "N" with the ICE off. When shifting into "N" and below 42-46 mph on a flat or ascending grade, I'll cycle to "D" and back to "N" to let the engine stop. But above the hybrid threshold speed, I don't see a big difference other than as you've noticed, coasting in "N" is always lower drag and minimizes the kinetic and potential energy loss.

Bob Wilson

 

I find N coasting helpful in one particular section of my daily commute. An undulating stretch of road of about a 2-3 miles with a general downhill profile. In my hypermiling efforts, I find that for that particular section, D and warp stealth uses more energy (about 3% state of charge ) for a marginally faster travel time. If I use N-all the way and just let gravity do it's work, I only use 1% SOC for that stretch. The 2% SOC that I saved proves useful for the coming uphill section.

 

I will never have a 1000 mile tank, and I enjoyed watching the evolution of yours. However, I disagree with your position on battery management. The efficiency on the gas engine is around 25-28%, with a maximum laboratory efficiency of 38%. The efficiency of the charge-discharge-motor system is easily twice that. The general assumption made here is that the battery energy is somehow related to the energy costs of the gasoline burned in the engine. To the extent that the engine runs to charge the battery, this is at least partly true. However, when the battery is charged through the regeneration system, the cost is very low, if not zero. Shifting to N bypasses all of the regen systems and you lose this efficiency. Using the battery to extend the glide and to provide short periods of power for smaller uphill segments will allow you to maximize the mpgs. I live in a fairly mpg-unfriendly part of the country, with a lot of up and down and slow speed curves. Using this strategy, I have been able to get 60+ mpgs from my tanks, and I hope to get a 700 mile tank before the weather turns cold.

 

It really all depends on the situation one is in. Generally, if the road perfectly flat to slightly downhill where the momentum carried is enough to coast the car in N at steady-to-slightly diminishing speed (e.g. 30mph) , this will be the most efficient given that you cover the same distance at the same amount of time with half the energy used.

The energy consumption reading in amps (BTA) in my SGII when coasting in N at 1-31 mph is 0.9 - 1.7. If I engage it in D and go stealth (no arrow/ mid line gliding), the reading is usually 3.0 to 5.0 and up depending on the speed as the car tries to simulate a glide still using a bit of energy (twice actually)... and if one lets go of the accelerator, the car slows down and you lose momentum (unless you really have to slow down). So in this instance, N-gliding will be more efficient than using the traction battery to glide the same distance at the same average speed.

It's a balancing act and using both techniques to conserve momentum/energy (N-glide) AND capture energy when needed will eventually lead to better mpgs than if using just one gliding technique (D-glide).

Being also in a generally mpg-unfriendly area, the elevation change I deal with everyday is 524 feet (sometimes tackling it twice going 1 way) with traffic to net me only 19mph at best. If you have SGII, try to monitor BTA and compare the energy used. Pick a part of your commute where it goes slightly downhill for a while where you can glide in N safely (i hope it's not illegal in your state) up to 37mph. Be wary of the speed where the ICE needs to turn and not over rev the MG and shift to D when needed. My personal limit is 34mph before I engage D to control my speed through regen. If you observe SOC at the pre-defined starting point and end-point, you'll notice you've saved a few % SOCs using N than if you're just in D and gliding conventionally. This will mean a lot if you started in a low SOC and need to go uphill on the next segment which means using the ICE more sooner rather than later.

 

No problem as our Prius has more than enough built-in instrumentation to support testing different protocols. It won't be the first time a beautiful theory has crashed into a hard reality. <grins>

What speed range?

In my case, I used a cruise control speed of 26-27 mph knowing it would achieve 99.9 MPG. What I did not realize until the marathon drive was that the minimum engine power when it ran in "D" provided more power than needed for the vehicle, even when climbing a small grade. The excess power is banked in the traction battery. At 26-27 mph, there is no need for regenerative energy to sustain the traction battery charge. But this is not as important as 'doing the experiment.'

Let me suggest trying the two different protocols in an A-B-A test scenario. This is how I did my early tests in 2005 with our 2003 Prius. Now I've got access to an area, "West End Loop," that I can use for testing different driving protocols. The only problem is having instrumentation to handle the very high MPGs to detect a difference. Last night, four loops on cruise control and it had already set the vehicle MPG at 99.9.

I have a ScanGauge II that has 'trip' function. I'll have to read up and do some testing but this may provide a way to test different driving protocols. I know Ken@Japan was not happy with the ScanGauge II accuracy at low speeds. This may be due to the faux 0.02 gal/hr the Prius reports when the engine is not spinning.

If that doesn't work, I may be able to use the miniVCI to record usable metrics but there is a another approach.

I can reset a trip meter and put 10 miles at say 60 mph on it to preload it with a non-trivial fuel burn. Then proceed to the "West End Loop" and do 10 miles of laps using any given protocol. This pre-loads the vehicle trip meter with enough distance that with the entrance values and exit values, we can calculate the car reported, fuel burn and distance in the loop. It is a math trick that adds a value to a function to stay out of forbidden areas and take it out again after doing the math.

Bob Wilson

ps. Another Prius analysis of coasting in "N" vs "D": coasting in neutral questions | Page 2 | PriusChat