Highway Driving in the Prius II

This is my attempt to give back to the pc community. Do with it what you will.

Happy driving.

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Highway Driving in the Prius II, a summary of highway driving techniques for the novice driver by a novice driver.

The purpose of this write-up is to give an overview of various highway driving techniques written in terms that the novice driver can understand. Also, this write-up will give a foundation on how a novice driver can learn to get better fuel economy than cruise control by applying these techniques in a step by step build-up process.

Why is cruise control not the most efficient way to drive a Prius on the highway? The computer can't see the terrain ahead and adjust for inclines before the car has reached them, resulting in high-inefficient-rpm to maintain speed. Cruise control doesn't take advantage of downhill segments by maintaining one speed instead of allowing the car to speed up.

The discouraging part about attempting to use the techniques the advanced drivers talk about is that you might not see immediate results. The pulse and glide below 40 mph in the Prius is almost error proof, if you are able to get into and hold the no arrow glide it is rare to not see immediate results. At highway speeds with higher wind resistance and varying terrain, applying the techniques in the wrong way or in the wrong terrain can produce results lower than cruise control while averaging lower speeds. Also, some of the advanced drivers are driving with advanced setups (tire pressure and low viscosity oil), which can account for some of a large difference between your initial results and the results posted by advanced drivers on their best days in perfect conditions.

The infinite combination of gears that the Prius transmission offers in the Hybrid Synergy Drive allows the driver to use the gas pedal to get the car into many different gears at a given speed. Most Prius drivers have experienced an increase in current fuel economy when they back off the gas and still are able to maintain the same speed. By lifting up on the pedal, the driver told the computer to change into a higher gear and lower RPM into a more efficient state. These drivers have already found one of the states of highway driving.

The premise of attempting for higher mileage highway driving in the Prius is to switch between up to three states based on your terrain and traffic conditions:
Acceleration State: An RPM and gear combination which increases speed
Cruising State: An RPM and gear combination which maintains speed
Efficiency State: A state which uses less, little, or no fuel for the given speed

To go for high highway mileage, the Prius driver needs to learn what each of these states are in given speed range and how to get in and out of them efficiently.

The stock Prius instrumentation does not include RPM or gear settings, but a device such as a scan gauge will allow you to monitor settings which translate into a map to the three states. I have found that the combination of the three settings, RPM / IGN (ignition timing) / TPS (throttle position), give a map to whether you are in an acceleration, cruising, or efficiency state.

It is my belief that if you practice these following highway driving techniques and apply them in the situations that best suit them, you can achieve fuel efficiency 10-20% better than cruise control for your car's setup.

A. Driving at one RPM, or Driving with Load
B. Keep RPM between 1500-1800 (FireEngineer method)
C. ICE-on in sweet spot range + Warp Stealth when power not needed (Hobbit method)
D. Super Highway Mode for speeds between 47 and 55 mph (xcel / dan on cleanmpg).


A. Driving at one RPM, also called driving with load

The theory of this method is that you are eliminating the bad parts of cruise control. With no efficiency or acceleration state, the driver stays in cruising state at all times.

Pick an RPM that will maintain a desired speed on flat roads. Hold this rpm at all times, slowing as you climb hills and picking up speed as you descend back to your flat terrain desired speed.

Advantages over cruise control: No aggressive high-rpm state when climbing to maintain speed, on downhills takes advantage of pitch to speed up to whatever speed the downhill section takes you to.


B. Keep RPM between 1500-1800 at all times.

This method simulates being in an overdrive or high gear (5th gear in a stick shift) state the majority of the time, with RPM at the lowest level which maintains speed and momentum. Start with an RPM that maintains a desired speed on flat ground and vary slightly vary RPM based on terrain. When approaching an incline, increase RPM to downshift and/or pre-accelerate before it begins and hold that RPM on the way up. On the back side of inclines, return to cruising RPM or go slightly lower to take advantage of the downhill to make up for some of the extra fuel you burned on the way up.

To apply this method effectively, the driver must learn how to maintain momentum over hills and through declines by varying the rpm within the range without high RPM times outside of the range.

Advantages over A: Can maintain a higher average speed without a significant drop of fuel economy, can do better by staying in a lower average rpm over the period of the drive at the same average speed.


C. Gas-engine-on in sweet spot range + Warp Stealth (gas engine spinning via battery power using no gas, see pulse and glide + warp stealth write-up on cleanmpg.com ) when power not needed

The theory of this method is to be either accelerating or holding speed in the most efficient gas-engine-on range (see hobbit's sweet spot write-up for an extremely technical write-up of the sweet spot range theory) or be in a no fuel use state and switch between those states based on the terrain. Basically, If your going to burn fuel, get all you can out of it, or don't be using fuel.

Acceleration and Cruising State: RPM in 1700-2300 range
Efficiency State: Warp Stealth, HV battery spins ICE with no gas use

This method is difficult to learn and apply correctly, frequent switching between a gas-engine-on state and a no fuel state requires experience on how and when to apply. It is difficult to hold the efficiency state for a long period of time as the pedal position which allows this state has a much lower window than gliding below 40, and it takes practice to hit the desired RPM on the re-start of the gas engine from the efficiency state.

Advantages over A&B: Can achieve a true high speed pulse and glide under the right conditions and application.

Best place to try this: Practice switching to efficiency state on highway exit ramps or on highway downhill sections and hold the no fuel state then resume normal driving.


D. Super Highway Mode

The theory of this method is to hold and maintain a near idle state as both the cruising and efficiency state with an acceleration state at the low end of the gas-engine-on efficiency range (1600-1700 rpm).

At a speed between 50 to 55 mph, remove pressure from the accelerator until rpm is at one of four states:
~1340 (Also scan gauge settings: IGN 15 + TPS 19)
~1280 (IGN 14 + TPS 18) - try for this one in initial attempts
~1220 (IGN 14 + TPS 17)
~1180 (IGN 14-16 / TPS 16)

Watch the current fuel economy jump up to 75+, you'll start to slowly bleed off speed. Hold this state as long as you can as you slowly lose speed, then transition to rpm = 1600 and IGN = 18, slowly and patiently accelerate back to a higher speed and repeat.

The lower the RPM the higher the fuel economy, but, the lower the rpm the faster you slow down.

This method is harder than B and much easier than C. Anticipating terrain to maintain momentum is key, even more so than method B. If you don't switch out of the idle state to the acceleration state before you get to an incline, you'll see your speed drop like a rock and you'll need to take RPMs to 1800-2000+ to get back up to speed, throwing away all of your patience.

Best place for a first try: About a mile from a highway exit point on flat or declining terrain, remove pressure on the gas pedal into the efficiency state and hold until the exit point. The fuel consumption display should be 75-90+ during this time.



13 Step plan to go from a cruise controller to a higher highway miles per galloner.

It is assumed that you will practice these techniques on generally flat terrain, if you have long inclines to deal with drive as you normally would on them and monitor what happens when not on the inclines. You must practice these in no traffic situations following all applicable driving laws considering safety first.

I have tried these methods at speeds up to 63 miles per hour flat land cruising state.

1. Buy a scan gauge or other monitoring device, this device should pay for itself over time.

2. Drive a highway segment in cruise control, watching RPM / IGN / TPS and the five minute consumption screen bars to get a baseline of how your car performs in the given conditions and your setup, remember the RPM / IGN / TPS setting that cruise control uses to maintain your desired speed on flat ground (it will jump around, but around a set of values).

Try Method A.

3. Drive the same highway the next time you are on it, use method A with the rpm from step 2.

4. Keep driving this way until you consistently see better fuel economy than cruise control in step 2 for the time you are in it.

Now try method B.

5. Drive with cruising state RPM from step 3 (I assume it is in between 1500 and 1800), Pre-accelerate or pre-downshift to a RPM level 200 RPM above your cruising RPM when approaching inclines and hold when you are going up the incline. Go 100-200 RPM below or go back to cruising RPM on the backside of inclines.

6. Repeat until you are satisfied that you are comfortable with this method and seeing positive results.

I suggest waiting to try the warp stealth and super highway efficiency states until you are comfortable with how to efficiently maintain momentum in your normal drive over average terrain. This is so your results aren't influenced by these methods.

7. Try warp stealth on highway exit ramps 1/4 mile farther out than you would normally let off the gas and warp stealth glide to where you need to brake.

8. Try super highway mode efficiency state about 1 mile from your exit point and hold until the point where you transition to warp stealth in step 7.

Once you are comfortable with the efficiency state, highway exit in steps 7 and 8:

9. Try these efficiency states on the backsides of inclines, transitioning back to your cruising RPM at the bottom.

Once you are comfortable transitioning from either of the efficiency states back to cruising state:

10. Try super highway mode on generally declining or flat terrain, start at low-mid 50s mph, lock in and hold as long as conditions permit. You will slowly bleed off speed as you aren't applying enough power to sustain your speed. When you need more speed or see an incline coming, transition to 1600 RPM and slowly regain speed and repeat within whatever range of speed suits your terrain. In optimal terrain applied correctly you can get 70+ 5 minute bars.

11. Try high speed pulse and glide, rpm of 1800-1900 and warp stealth efficiency state from your normal cruising speed to 5 mph below or a range of 5 mph around it.

12. Figure out where on your commute to apply all of these techniques and see your mileage increase!

13. Don't sue me because something happened, you are trying this at your own risk.


Some final thoughts:

The RPM values I quote are for how my car is setup on the terrain I drive in. Your values and ranges may vary, use the suggestions as a starting point.

Please do not drive unnecessary miles to try this.

Don't compare results to advanced drivers, compare them to your own baseline results. I am not posting numbers or screen shots because they aren't needed as I know I'm doing much better than cruise control and am improving everyday. You can do better if you work at it, with patience and desire and a few thousand miles of practicing these techniques they will become second nature and you'll be able to instruct others and tell me where this guide needs improvement.

I spent a significant amount of time practicing and sacrificing tank averages over the past four months to teach myself how to drive the car better at highway speed. Also, I have spent several hours writing this report, please respect that when critiquing this effort.

 

<div class='quotetop'>QUOTE(Ichabod @ Oct 12 2007, 01:32 PM) [snapback]524731[/snapback]</div>

This seems to be the one place where you can't get away without having a scan gauge if you want to try to do better. P&g below 40 you can monitor current fuel numbers, I tried to see if I could do it with the stock instrumentation, but kept "putting past the hole" when trying to transition between states.


<div class='quotetop'>QUOTE(usbseawolf2000 @ Oct 12 2007, 01:47 PM) [snapback]524742[/snapback]</div>

Next time you're in a distance draft in low 50s mph like in the first part of the video, try to get in the ign 15 / tps 19 state, you can get pulled along nicely with 80+ mpg.

<div class='quotetop'>QUOTE(bwilson4web @ Oct 12 2007, 07:57 PM) [snapback]524925[/snapback]</div>

My conclusions are that the driver needs to know all of the methods and when to apply each of them if they want to see a significant gain over "just driving it". Cruise control goes through the states that I list from all the methods, the advanced drivers have learned where these states are and when to get into them and hold them, as opposed to cruise control just passing through them or holding them briefly.

If you drive cross country on unfamiliar roads an average driver might not see a large enough of an increase to make it seem like the extra work is worth it, an average driver applying these techniques in their daily commutes could get significantly better than just driving normally.

The intent of the post was to summarize the theory of what the advanced drivers are trying to do so that people could try them out in friendly places so they could see results within the first few tries. I'm confident that I could put up my results in my commute against the experts and come close to what they do, and I think that anyone can if they put in the time.