A comparison of highway driving methods for the Toyota NHW20 Prius

After much procrastination on my part, I submit for PC members' reading pleasure the following paper describing an attempt at using the scientific method to compare two different highway driving methods. Many thanks to PC members Bob Wilson, Hobbit, and Catgic for their thoughtful review and critique of the draft.

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A comparison of highway driving methods for the Toyota NHW20 Prius:​

Electronic cruise control vs. RPM-guided pedal control​

Introduction

The question of how best to drive the Toyota Prius at highway speeds for maximum fuel economy (FE) is one of substantial discussion. The consensus appears to be that electronic cruise control (ECC) is best on level terrain. For hills the optimal method is less clear. Some (1,2,3) suggest using ECC in hilly terrain too, whereas others (4,5) have maintained that manual control of the accelerator, holding RPM within a certain “sweet spot” range of efficiency, produces better FE. Obvious disadvantages of the latter method are the need for a tachometer – with which the Prius is not factory-equipped – and the need to continuously monitor RPM and make pedal adjustments. Nonetheless, some FE-conscious drivers might choose this method if a significant FE benefit can be demonstrated.

Though most of my driving is off-highway commuting, I take occasional out of town highway trips. Most are in moderately hilly terrain typical of the central Virginia Piedmont. Experience on those trips suggests that the RPM-guided pedal control (which I will call RGPC here) method may achieve better FE than ECC. But I have not done a controlled test on the same highway at the same average speed in the same or similar weather conditions.

I sought to perform such a test. I hypothesized that a carefully performed driving test using the RGPC method at highway speeds would yield FE superior to that obtained with ECC in a 2005 Prius.


Methods

The test vehicle is a 2005 Toyota NHW20 Prius with the following characteristics, modifications, and accessories:

• Approximately 26,000 miles
• OEM Goodyear Integrity tires inflated to approximately 50 PSI on the front and 48 PSI on the rear
• EV switch, installed as described here
• Engine block heater
• CAN-View
• Window tinting
• Toyota brand front-end vinyl bra
• Toyota brand mud guards, front and rear
• Body side mouldings
• Approximate combined weight of driver and cargo 315 pounds

The vehicle was fully warmed and at operating temperature before testing began.

The test route is part of VA288, a limited access highway in the suburbs of Richmond, VA. The route was chosen because of its gently rolling but long hills, relatively light traffic, and convenience. Speed limit is 65 MPH. The surface is concrete.

The test was between points near the highway’s interchanges with VA10 and US250, approximately 25 miles one way. The start and end points for each segment were a median crossover on the south end and a bridge over a creek on the north end. Both start and end points are at an approximate elevation of 200’. The route peaks at approximately 380’, and its low point is approximately 170’. Details of the test route are shown here.

Weather during the test was pleasant: sunny, relative humidity in the mid 40s, temperatures in the mid to upper 70s, and winds calm or nearly so. A laptop computer with mobile broadband was on board to allow periodic checks of local weather stations at Weather Underground during the test. Significant changes in weather, especially wind and temperature, would require adjustment of test data, and would be done with the Prius MPG Simulator.

Cabin climate control was run on automatic mode as needed for comfort. Initial temperature setting was 78, with adjustments to as low as 76.

CAN-View was used to monitor RPM, trip data, and fuel mileage. CAN-View’s fuel mileage calculation had previously been calibrated after a fuel fill-up, with its reading compared to the calculation from the factory multi-function display (MFD) after a recent highway trip of approximately 200 miles. CAN-View’s calculation was judged to be within 1% of that shown on the MFD.

The RGPC segments were completed first. A speed of 65 MPH was achieved before crossing the start point. Speed was then maintained between 55 MPH and 65 MPH and RPM between approximately 1400 and 2400 as displayed by CAN-View, staying as close as possible to a presumed ideal of approximately 1700 RPM when terrain allowed. On downhills where speed seemed likely to exceed the maximum despite RPM held at 1400+, “warp stealth” (WS) was applied. (The link describes WS in depth, but in brief, it is a term often used to describe a Prius driving technique where the go-pedal is completely released at speeds above 41 MPH, allowing the internal combustion engine, or ICE, to completely cut off, followed by immediate slight pedal pressure to release regenerative drag. The effect is similar to coasting with the car in neutral.) On uphills, progressively more pedal pressure was applied to keep speed at or above 55 MPH. If 2400 RPM was insufficient to maintain 55 MPH, then more accelerator pedal pressure would be applied; 55 MPH was deemed the lowest safe speed for the test.

Average speed was calculated at the end of each RGPC leg. Those averages were used as the basis for the ECC test speed settings.

The ECC segments were performed immediately after the RGPC test. They began similarly to the RGPC tests, with test speed attained before crossing the starting point. From there, ECC was kept activated with a constant speed for the complete segment.

One round trip test run was completed with each method.

At the conclusion of each segment, the following data were observed from CAN-View:

• Total distance
• Total time
• Average speed
• Average MPG

Data values were spoken into a voice recorder for later transcription into a spreadsheet.


Results

Here are results in distances, times, speeds, and FE:

Method, Segment, Distance (miles), Time, Avg. Speed (MPH), MPG

RGPC, Northbound, 24.8, 0:24:41, 60.3, 55
RGPC, Southbound, 24.8, 0:24:36, 60.5, 61
RGPC, Total/Average, 49.6, 0:24:39, 60.4, 58

ECC, Northbound, 24.8, 0:24:53, 59.8, 57
ECC, Southbound, 24.8, 0:24:48, 60.0, 61
ECC, Total/Average, 49.6, 0:24:51, 59.9, 59
​

As one can see, there was very little difference in results.

At all times during every segment, the hills’ gentle inclines allowed speed to be sustained between 55 and 65 using the RGPC technique as described above.

There was little fluctuation in weather conditions. Temperature ranged from 76F-78F, and humidity remained in the mid-40s. Wind, whose potential influence was considered to be greatest among weather variables, remained light and variable, with velocity ranging from calm to 3 MPH. Because of the variability of direction, no reliable adjustment in data could be made.


Discussion

The results were somewhat surprising and differed from my expectation. The difference between the two methods can probably be considered insignificant. Given that the RGPC method requires a tachometer and close attention to RPM and pedal position, most drivers likely would choose just to relax and use ECC on a highway like this.

My previous highway driving has been on other routes with presumably steeper hills. On those routes, the RGPC technique employed here often requires either progressive slowing on uphills to below the low speed target, or engine speed considerably higher than 2400 to maintain the target speed. On long downhills, extended periods of WS can be achieved. Downhills even occasionally result in acceleration to potentially unsafe speeds or those well over the speed limit. With practice I have learned to mitigate that tendency somewhat by allowing more speed to decay, with lower RPM toward the end of uphills if safe, or releasing the pedal and beginning warp stealth sooner on downhills. If needed to avoid excess speed, I occasionally release the pedal completely to allow regenerative coasting. Using ECC on these routes, with its aggressiveness in maintaining speed, often causes spikes in engine speed to well over 3000 RPM.

On this test, the RGPC method was able to maintain speed and RPM within their target zones, and opportunities for WS were infrequent. On uphills I never had to choose between an inefficiently high RPM or decelerating below the target speed. On downhills, I usually maintained ICE-on conditions with RPM within its target without using warp stealth.

During uphills on the ECC phase of the test, RPM rarely exceeded 2400. On two occasions it briefly climbed to ~2900. Though above the target, it still was below another RPM efficiency dropoff at 3000 RPM that has been described. On the other hand, it was interesting to note that RPM frequently fell below 1400 on level terrain or slight downhills, occasionally as low as 1200. It has been suggested that the ICE “loafs” inefficiently at these RPM ranges. This loafing, however, seemed not to adversely affect the results in this comparison with a method designed to avoid it.

All of this suggests that this route, even with its hills, allowed relatively efficient operation with ECC, comparable to RGPC.

I have not compared on a topographical map the elevation changes on this route to those on my more typical highway trips. But my findings suggest that the changes are gentler, making it difficult to extrapolate the results either to my typical trips or to those of others on routes with substantial hills. Nonetheless, those whose highway driving is on routes similar to the test route may find these results useful. Even in my own driving, I likely will use ECC more often in the future on segments whose terrain is similar or more level than this test route.

On segments with steeper hills, however, the RGPC method may still be superior. Hobbit has demonstrated that keeping the ICE within its efficient range in hilly terrain produces MPG results well into the 60s in warm weather. RPM may provide a clue as to when to change techniques: RPM values consistently above 2400 with ECC, and especially above 3000, may be a useful signal to turn off ECC and begin manual pedal control. Along with moderation of RPM swings, that would likely increase the opportunity for zero-consumption warp stealth. This is an area for further study.

The Prius is not equipped with a factory tachometer, so drivers using RGPC require an add-on device. In my case it was CAN-View. Others have used a ScanGauge or an inexpensive analog tachometer. For those without the money, desire, or ability to add instrumentation, I have previously proposed a rule of thumb for keeping RPM below its top efficiency limit: Keep the instantaneous MPG (iMPG) as shown on the MFD to a value greater than half the vehicle speed. I first proposed the rule based mainly on my observations below 40 MPH, where it appeared to apply across a broad range of speeds. As a secondary observation on this test I monitored iMPG during several accelerations, both during RGPC and ECC. At no time did I see iMPG drop to less than half the vehicle speed. I could have missed such occurrences; if so, they were rare. Therefore, the rule seems applicable to highway speeds as well as slower ones, and may be of help to those without a tachometer. If during ECC travel the iMPG stays above that threshold, the engine is likely staying below its upper efficiency threshold. On steeper hills, the rule could be applied to prevent inefficiently high engine speeds. In those cases, iMPG is more likely to drop below half the vehicle speed, in which case a change to manual pedal control may produce better fuel economy. This too might be an area for further study and validation.

A substantially different highway driving technique dubbed “Super Highway Mode” (SHM) has recently been proposed. SHM involves a deliberate attempt to keep highway power demands continuously low, effectively keeping RPM below the “loafing” threshold of 1400 or so. Some suggest that maintaining ignition timing (measurable with an aftermarket device like ScanGauge or CAN-View) in a relatively low and specific range is the basis for efficient SHM, although these ignition readings correlate closely with RPM; the latter may be monitored more easily, depending on the vehicle’s instrumentation. My initial anecdotal experience suggests that SHM seems best suited for lightly traveled non-freeway routes on level terrain. Speed will slowly but invariably decay even on level roads, and those who have used it report a maximum speed of 55-60 MPG, with speed decay to as low as 40 MPH – impractical and perhaps even illegal on interstate highways.

Regardless, modification of the RGPC test protocol to use different RPM targets, different speeds, or different methods (such as SHM) might have yielded different results.

Though wind was light in this test, it still could have influenced results. A 3 MPH wind could cause instantaneous fuel economy at 60 MPH to fluctuate by as much as 6 MPG, according to the MPG Simulator. Because of the wind’s variability in direction, however, wind-related adjustments to results could not be made reliably. A day with perfectly calm winds throughout the test would be ideal though difficult to find. Multiple test runs using each method might have minimized the effect of wind shifts on results, and would have added scientific rigor to the test (and possibly yielded different results), irrespective of weather.


Conclusion

Using ECC on a 2005 Toyota Prius achieved fuel mileage results comparable to RGPC in this test on a central Virginia highway with long but modest hills. ECC is easy to use, whereas the RGPC method potentially needs add-on instrumentation, and requires continuous attention to vehicle speed, engine speed, and accelerator pedal position. Therefore, ECC appears to be the superior method on this route and others with similar characteristics, though more controlled testing would allow conclusive judgments to be made. Modifications of the RGPC technique or testing protocol might yield different results.

 

Thanks to all for the comments!

I posted a link to this on CleanMPG, where some additional discussion has ensued, FYI.

Seawolf, I'm familiar with that chart and considered including it as a linked reference, but I figured it might be a little technical for some -- like me!

Evan, the page with the route map has two links on the left side labeled Elevation, small or large. Click either of those to see an elevation profile below the map. I agree with you, BTW, that RGPC probably is superior with steeper hills. But I don't have the data to prove it ... yet.

 

Re: Warp Stealth vs. Super Highway mode for Dummies

OK, I'll see if I can get it to a "See Spot run" level.

Warp stealth is induced the same way as a low speed glide: release the accelerator completely and then feather it back down a tiny bit. The difference between a low speed glide and WS is that in the former, all arrows disappear from the MFD (implying no current flow at all, which is actually not the case, but we'll save that for another day -- we're keeping it simple here). With the latter, the MFD shows energy flow out of the battery. WS creates a freewheeling effect similar to a conventional car running in neutral, though there is no fuel flow to the ICE. It is used for downhill segments > 40 MPH where momentum is enough to maintain or gain speed. Though not part of the WS technique itself, many that use WS often alternate it with flat or uphill segments that attempt to maintain ICE RPM in an efficient range (what I've described in my paper). The rationale for the whole package: Either the ICE runs in its most efficient range or it doesn't run at all.

SHM involves keeping the pedal mostly steady and in a position where the ICE runs nearly continuously at a very low RPM. Hobbit (and others) have suggested that the low end of ICE efficiency is in the range of 1400-1700 RPM, depending on vehicle speed. The ICE supposedly "loafs" below that. But SHM RPM is lower, generally <1300. Even though it supposedly is running inefficiently at those speeds, there is still enough power to maintain speed or allow an ever-so-gradual speed decay, depending on terrain. In my own experience, SHM probably is not for interstate driving. When the vehicle approaches interstate speeds, the power needed to overcome the increased wind resistance is not provided at those low RPMs. Nor is it suited for hilly terrain; the low RPMs won't supply the power needed to climb even modest hills. SHM seems most appropriate for non-interstate rural roads on mostly level terrain with light to moderate traffic.

Neither aims to increase regeneration. In fact, a basic principle of maximizing mileage out of the Prius is to avoid regeneration whenever possible. Instead, use kinetic energy to your advantage. (That is what WS does.) Energy actually is wasted through regeneration (though, of course, not as much as braking in a conventional car wastes).

 

On behalf of this dummy, you're most welcome.

Yes, if you use WS on a long enough downhill, eventually it will deplete the battery enough where the ICE insists on lighting to recharge it. But the only time I've seen that is either coming down a mountain or when I've begun WS with a low battery to begin with. When you reach the bottom and begin climbing the next one, the ICE will do its magic and gradually begin topping it off again.

Again, avoidance of regeneration is best. Regeneration involves conversion of energy forms, with a little energy lost at each step: kinetic energy converted to electrical energy converted to stored chemical energy in the battery, then a reversal of the process to use that energy again. Best is to use kinetic energy to your full advantage by reducing as much mechanical drag -- and regeneration -- as possible. Feathering the pedal slightly accomplishes this. If that becomes unsafe or illegal (from excessive speed), then next best is regenerative coasting with the go-pedal fully released, followed by regenerative braking -- light to moderate brake pressure at 7+ MPH. Below that speed or with hard braking, the friction brakes apply and most kinetic energy is lost to heat.

It may seem counterintuitive, but those with the best "hypermiling" results in the Prius generally achieve them by staying out of the battery as much as possible -- again to avoid the conversion losses.

As for the hill that approaches your house, I would glide (<40 MPH) or WS (>40) down the hill if safe and legal, developing as much momentum as possible to help carry me up the other side.