Ever since buying my Prius, I've tried a number of techniques to maximize its fuel efficiency when running the engine. While "pulse and glide" is now a well-understood technique of using the engine efficiently or not at all for high MPG at low speeds, many drivers experience a significant drop in fuel economy when they have to travel above 41 MPH. This is partly because the drivetrain wants to have the engine turning when the car is at 42 MPH or above, and partly due to the ease with which the system can fall into inefficent, low-demand modes in which the engine is still burning fuel for relatively little motive power produced. But it doesn't have to be like that, because the Prius actually can be a very efficient system over *all* of its operating speeds. Sometimes it just takes a little encouragement from the driver. This article offers some improved guidelines for best engine utilization at highway speeds and in the mysterious "mid-speed" ranges. It also presents supporting rationale for a conceptual "sweet-spot" indicator, to give a driver immediate visual feedback on whether the engine is being used under solid torque load or not. The shortest possible "executive summary", which doesn't tell the entire story and may still not even be entirely right, is this: We're ignoring the hybrid battery in this discussion. Recommendations when burning fuel at highway speeds [~60 MPH] are: keep instantaneous MPG between 35 and 75 MPG, and/or keep RPM between 1400 and 2200, and/or stay in the "good" region of a "sweet-spot meter". Competent "warp stealth" holding ability and other subtle aspects of HSD operation is assumed. Making best use of this information requires some knowledge of what is going on overall in the drivetrain, so the reader will want to make some time to digest this and the subsidiary information carefully and get some on-road practice. For a good review of the driveline, particularly of "heretical mode" and complete with cool animations, see Graham Davies' seminal writeup. Adding appropriate instrumentation to the car also helps.It's a concept ...
There is a strong personal-choice caveat to this, too: strictly utilized, the sweet-spot range and methodology as presented can leave the car with the acceleration and climbing power-to-weight ratio of a loaded semi, so a self-imposed limit on the high end of that range may not be for everyone. You are free to push harder than that when needed, at the expense of a few MPG. We will readily note that once those loaded semis do get up to speed, they fly right along! It is assumed that a hybrid driver interested in fuel economy is satisified with maximum speeds between 60 and 65 MPH, beyond which the effects of air resistance rapidly climb. But for most terrain, staying within these suggested ranges of performance works well and returns surprisingly good fuel economy compared to typical Prius drivers' results. In these higher speed ranges, pulse-n-glide becomes more like "run under load and warp-stealth glide when appropriate", with more of a tendency toward steady-state running as speeds increase. So, what does "under load" mean? This is really the central principle, and most of this discussion is about the throttle control necessary to achieve it. In general it means running the engine close to its maximum *torque* output across a broad range of RPM. It is well-known to the "hypermilers" that running through the gears quickly and driving in the highest one that still allows the engine to run correctly at lowest RPM without dangerously "lugging" gives the most efficient transfer of the engine's output to the road -- versus letting it "float" at high RPM with relatively little shaft load. In general engines are more efficient when loaded, and everything beyond that, for any engine, is a matter of optimal RPM and how that relates to cylinder geometry, intake/exhaust design, etc. The torque and efficiency curves for the Prius in particular are very broad and forgiving, as shown in this pair of diagrams. And before we get too far along in this and risk misleading the reader somewhat, take note:
Update: Subsequent studies indicate that the low end of this range, below 5.8 milliseconds or so, also falls off the peak efficiency curve somewhat at true highway speeds. See this later article for more details on power output and how to really optimize highway fuel economy, and for what is said below here apply the caveat that it may be too delicate an approach in many cases.
The Prius has no step gears, of course -- its output ratio comes from an computer-defined torque balance between the two electric motors across the planetary gearset, and the computer meanwhile gets to handle the throttle opening as it pleases. It presents a different way of thinking about engine control, and can be problematic for drivers to get used to. In my own experience, it seemed as though I was stuck running in higher RPM ranges at highway speeds at the expense of good MPG, and couldn't quite figure out optimum strategies -- if it's better to try and pulse the engine hard and then "warp stealth" to glide, or just try to bring everything to a steady state and hold it there. I already knew the cruise-control is *way* too aggressive about holding an exact speed, and that while some owners are perfectly happy to just let the cruise-control do its thing on highways and deliver mid-fifties MPG at best, there had to be a better way. Interestingly, it has often been through talking with Honda Insight owners that I've learned the best ideas about high-mileage driving technique. With the Insight's traditional throttle linkage and transmission, drivers get much more manual control of their gearing and throttle settings, which when properly played can easily return 100 MPG for them. They also have the advantage of lean-burn, which does require a lighter load on the engine. But then trying to plow those ideas back into how the Prius operates has been a bit of a struggle since it seemed like I had much less control over the throttle and powertrain output ratio. It seemed impossible to duplicate the high-torque, low-RPM "almost lugging" methodology of running through the gears quickly while accelerating and get into the highest gear as soon as possible. It always seemed like if I wanted to go a reasonable speed on the highways, RPM would automatically climb into areas of more fuel consumption. So the big question became, "how do I get the Prius into the equivalent of high gear earlier in the game?" I started occasional discussions about this in the Prius technical forums, which were somewhat inconclusive but allowed me to bring my own observations forward to see if anyone else had ideas or followups. Shortly after getting aboard the "hypermiler" hangout at cleanmpg.com I posed the questions again and some of the condensed observations and thinking to date, but even the discussion there was still rather inconclusive. There were several half- formed theories about the Prius, but nobody seemed to have the answer yet. That exchange is still worth reading, because at that point in time I didn't know but was SO CLOSE to finally getting it: http://www.cleanmpg.com/forums/showthread.php?t=915 The answer had been sitting right there in front of my face the entire time. The very first instrument I ever played with in this car, in fact, was my old vacuum gauge left over from previous projects, and after driving around a little with it I was *completely* mystified as to what the Prius throttle was doing. A vacuum gauge is a good "how hard is the engine working" indicator, since a larger throttle opening resists engine suction less and lets it pull more air/fuel mixture through the cylinders. Traditionally, a vacuum gauge tracks the inverse of the accelerator pedal position fairly linearly, dropping to zero at or near full throttle opening. But with the Prius, the throttle is controlled electronically and completely decoupled from the pedal. While I didn't always understand exactly what it was showing me about the Prius, the vacuum gauge has been part of my extra-instrumentation panel since the very beginning and has been useful when trying out yet another theory about efficient highway travel. At a minimum, I knew that when the vacuum began to rise away from 5 inches of mercury, that the engine was likely to be "loafing" -- still burning gas, but inefficiently and not providing any appreciable power, just heat. But with new factors like the Atkinson-cycle engine and computer-controlled throttle, I couldn't be sure about the possible causes of what I was seeing in the gauge. Before the trip out to Hybridfest 2006 in Wisconsin and back, I had also begun playing with monitoring fuel injector pulse time and duty-cycle percentage. This started with simply watching it on an oscilloscope and eyeballing some timing, and then once I understood what that was doing I started playing around with some circuits to integrate pulse times and display proportions as a voltage. Through a bit of dumb luck with some components stuck into a prototype circuit, I seemed to have given myself a meter that would start reading just as low vacuum was reached, and crest at 10 volts as vacuum headed a little lower yet above 2300 RPM or so on the highway. A conceptual 0 - 100% display of what I believed would be an efficient range, in fact. With all this sitting on the dash as the typical breadboard bristling with parts, I set off for another 2500-mile learning experience. And the running MPG during that trip was respectable -- north of 60 the whole way, at speeds between 60 and 65 MPH. "Sixty-three at sixty-three", I started calling it. Nicely above the EPA numbers, especially the highway rating for the Prius -- and I began believing that it was due to letting a little needle waving back and forth govern what my foot was doing. Maybe it wasn't perfect, and there were still other techniques I could be doing to get even better, but this lashup was doing more for my running MPG than anything prior. But on the way back from Hybridfest, the real epiphany came to me. It was an observation that lasted only about a quarter-second, but which completely confirmed all of this new thinking. I was on some secondary highways in Pennsylvania, and had just come down a fairly large hill that had run the battery up to its displayed "full" at 80% SOC. In that state, the hybrid system often tends to get a little confused and try to do things to bleed off some of the charge -- such as by starting and stopping the engine a few times, and various other thrashing that is a bit less smooth than the usual control scenario. After the downhill, I was coming up a gentle rise toward a red light, and trying to use the injection meter and vacuum gauge to stay just barely "under load" while still letting my speed gradually fall. During the next run/stop transition, I suddenly heard a very brief but aggressive "rev-up" from the engine -- like you would hear upon stabbing the clutch pedal on a manual transmission while cranking uphill in high gear. The motors, just for an instant, had evidently "let go" of the engine's output, and then just as quickly recovered and rebalanced the torque. But that blip ran the engine RPM up so fast that I realized that my vacuum gauge had never lied to me -- when it read low, the engine was indeed running in a high-torque mode, even if I couldn't really feel it pushing the car. I thought about it a little more, and realized that even though the engine was running barely above idle -- 1100 RPM or so -- the throttle was significantly open, shaft torque was high, and RPM was being kept down by the "heretical mode" electrically-routed overdrive feeding straight into the road load. *There* was my "high gear" answer. It was already happening -- I didn't have to do anthing except make sure I kept the system demand down near the low end of my "sweet spot", but not let it drop down past there. Toyota's throttle-control strategy took care of the rest. It's bloody brilliant. As I played around with this concept more on secondary highways and surface streets, I found that carefully holding the low-RPM "high gear" scenario without yielding to the temptation to push harder can indeed get the car going respectably fast, it just takes longer -- as one would expect from doing a similar thing with a manual gearbox. It requires a little patience, or that MPG-eating RPM starts sneaking up again. If a lower road speed is required, the driver must alternate this mode with some engine-off periods by going to electric-only or warp-stealth state -- which only serves to deliver even better MPG!. This switching can usually be matched to upcoming terrain with a little forethought. While Toyota's control strategy does in general try to hop over the "efficiency pit" near the low end, it often enough manages to fall in, requiring driver intervention. As I worked my way homeward, the MPG average kept climbing higher than I'd ever seen it go during *highway* travel, and after I got home I wrote up a preliminary report. So, to review: as non-intuitive as it seems, more right foot is equivalent to downshifting. "High gear" in a Prius is achieved simply by applying *less* right foot, within certain limits. The next question is how to determine those limits. Conceivably, it can be done very easily if such a "sweet spot meter" makes it past prototype stage and becomes an easy add-on kit for the Prius and possibly some other cars too. But exactly where that range [or ranges!] exists and how to display it is still under discussion and research. It can be done in a minimally instrumented Prius by watching a tach -- keeping RPM between 1400 and 2300 at highway speeds. If you have a vacuum gauge, the observed range generally starts as vacuum drops to 5 in-Hg or lower as demand increases, and tops out before it sinks lower to 2 or 3 while crossing 2300 RPM. Torque is also kept high at lower RPM at lower speeds, but it appears that efficiency starts to fall off below 1400 RPM regardless as the engine isn't producing as much net *power*. To some extent, it can be approximated in an uninstrumented Prius at nominal highway speed above 55 MPH or so, by keeping the instantaneous MPG bargraph between 35 and 75 MPG. The downside to this is that the entire scale of what's optimal shifts confusingly at lower vehicle speeds, leaving a certain amount of guesswork. This is why a simple add-on meter unit would be cool. However, using any such sweet-spot range is merely one technique out of an entire suite of high-MPG tools. It is definitely necessary to understand the other major running modes such as warp stealth too, to avoid burning any fuel during the times you aren't applying meaningful power to the wheels. That is possibly the most important one to learn about since it gives the equivalent of electric-only "glide" in the higher speed ranges. The bottom line is that efficient running can happen equally well above or below the magic 42 MPH, subject to the other factors that creep in such as air resistance and outside temperature. One thing to note is that between 42 and about 55, such as on secondary highways, even a minimal-power efficient running mode can get the car going too fast for local conditions, requiring backing off into a zero-consumption mode like warp stealth until power is needed again. And secondary highways tend to have more extreme hills and speed changes and traffic lights, bringing all the high-MPG driver's skills and gauge-watching into wild manic-depressive play one after the other. Nonetheless, proper use, overall smoothness, and attention to state changes *does* solve that puzzling mid-speed problem, returning 70 or 80 MPG segments on surface roads in many cases. On the interstates, it's generally a lot easier but one must still try to track hills to some extent and decide just how much "rollercoaster" constant-load driving can be allowed. So, let's do a little analysis of what's going on here, using my own instrumentation as a guide. First, here's the key to what's what on this panel:
and in this case the "multifunction voltmeter" is connected to the prototype circuit sitting in front of it, displaying a theoretical "sweet spot" from 0 to 100% at 10V or higher. [Meter goes to 15V] On a slight downhill, we're in warp stealth:
As the state was entered, the manifold vacuum backed off from over 20 in-Hg down to 15 or less, indicating that the VVTi valve timing had been fully retarded. With a throttle setting of very slightly above idle level, net airflow through the engine is almost zero and minimal "engine drag" pumping loss is presented. No injection is happening at the LED indicator. About ten amps are drawn from the battery to keep the engine turning and very slightly pushing the car. The tach erroneously reads 0 RPM, but that's simply because its input comes from the IGF spark feedback line -- which isn't sparking since there's nothing to burn. Over on the screen you can't see, instantaneous MPG is reading "infinite". Once back on the flat, we need to bring on some engine power to maintain speed:
50% or less sweet-spot range [SSR] is often enough to carry us along. If a downhill is immediately followed by an appreciable uphill, then it is likely that 100% SSR is needed immediately to keep the most momentum up while attacking the climb. Vacuum has dropped to its normal highway 5 in-Hg, injector pulse time is running about 6 ms, battery current is back to essentially zero. At this point sweet-spot range is entirely affected by engine RPM, from 10% around 1400 RPM up to 90 or 100% at 2200 RPM. This is accompanied by injector pulse length between 5.5 and 6.5 ms -- since the throttle remains pretty much at the same setting through that entire range with only a slight increase to allow for higher airflow, injector timing doesn't vary a lot either. Screen probably reads about 50 - 60 MPG at this point. I had the circuit tweaked so it was pretty close to reading 50 MPG at 50% SSR, but that varies a bit with vehicle speed. The important thing to note here is that over a large range of right-foot demand, injection and vacuum and throttle all vary VERY LITTLE, while the HSD simply controls engine RPM by sliding the electronic "ratio" between MG1 and MG2 up and down. Too low, and we get into the "loafing" scenario which is death on MPG; too high, and we get into more of a "power" regime which also guzzles fuel. Here's what happens if we push harder than 100% sweet-spot on a rise:
Note that the vacuum has dropped a little *lower*, to 2 or 3 in-Hg. There is a very definite high-side cutoff point, in fact, which I still don't entirely understand -- nearest theory at this time is that in the described sweet-spot range, the valve timing runs fully *advanced* for best compression and volumetric efficiency, i.e. range 4 in the valve diagrams below, and begins retarding into range 5 as RPM crosses up over 2300 or so. As that happens, there's a rise in RPM vs. power output that other people have also observed, which appears to lead to somewhat lower *road* efficiency. Injector time can widen out to as much as 7 or 8 ms along with the other changes. UPDATE: the above theory about valve timing has been debunked. It's simply done with throttle control. If we push RPM well up over 3000, something completely different happens that I've referred to as "WFO mode", a common play on "WOT" or wide open throttle. Under serious high-power demand, it's pretty clear that the engine goes into either an open-loop or enrichment-driven mode to deliver that. Evidence of this is LOUD intake noise, larger throttle openings, vacuum at *zero*, and scooting briskly down the road with MPG completely in the toilet. What's more interesting is that if demand is brought down very slowly, it can *stay* in that mode while returning down to 2000 RPM or below, with vacuum still at zero and injector timing hunting all over the place but remaining fairly long. This is a completely useless state for high mileage, but one should be aware of its existence simply because if the WFO mode is entered, the way out of it is to back completely off the accelerator and gently reapply. The aggressive must-hold-speed behavior of the cruise control can easily enter WFO mode up a hill and then fail to get back into a more efficient mode for a while thereafter, thus killing MPG for no reason. Not that someone driving for efficiency optimization would be using the CC in the first place... Valve timing charts, from Toyota:
These two diagrams are from the same section of the New Car Features documents, and point out some very interesting relationships. For vehicle speed ranges between 30 and about 60 MPH, the slight rise and plateau of the vacuum [or lower MAP, if you think of it that way] appears to be a result of intake valve advancement, running in range 4 -- fairly high load, low RPM. Put another way, a higher compression ratio returns better thermal efficiency, but we've still got the Atkinson-cycle difference between compression and expansion which helps optimize that even more. It's a balance. Pushing harder brings vacuum slightly lower around a very specific engine RPM whose value depends on vehicle speed, and it turns out that over most of the useful speed range that point is about where MG1's backward rotation [in heretical mode, being driven by electricity from MG2] comes to zero and begins rotating forward as a generator again. In general, MG1's response to varying driver demand is to drift freely back and forth between backward and forward rotation -- it never locks, and we rarely if ever actually feel the reversals. But we need more engine RPM to compensate for getting through the zero point and entering the forward-spinning scenario, so we see engine demand change fairly quickly right where that zero-crossing is. This begins to become less true at higher vehicle speeds, where higher demand is likely to drift out of the "overdrive" scenario -- say, to sustain speeds much over 70 mph. The whole "sweet spot" thing probably falls apart at the high end in those operating ranges. More research is needed into this and what the valve timing is really doing. It is clear that Toyota's engineers had a bunch of interesting high-efficiency thinking going on when designing the control strategies, but it apparently remains up to us, the consumers, to ferret out how to really utilize it. Some time later I went out and tested the high-side scenario in a reasonably well-controlled A/B/A/B set of 55-mile loops with an MFD reset between each. The variable was either staying strictly within the higher-vacuum range or allowing slight [but not extreme] excursions above the high-side RPM limit on uphills. Ambient temps were a fairly brutal 85 - 93 F, but I stuck it out for the four or so hours this took, kept my speeds about the same over each run to eliminate wind-resistance as best as possible, and logged the average MPG over my runs: 63.4 constrained 62.3 allow 2400+ 64.6 constrained 63.7 allow 2400+ While the differences seem minor, it seems that entering range 5 too often -- if that's what I was doing in the first place -- is a bit of a mileage hit. Other prior observations about higher RPM ranges seem to support this, well enough that I don't believe it's just due to some incremental delta in air resistance. If it develops that a particular operational range that best optimizes fuel economy in highway travel can be "metered", it must still only serve as a guideline. So far the working theory appears to return good results for most terrain and at speeds that are most recommended for improved highway mileage. The major human acceptance problem here is that not allowing excursions into the above-noted higher-power mode delivers about a maximum speed of 73 MPH on the flat, and you'll be joining the trucks in the "slow vehicle" lane up the mountains. So it's an area that requires best judgement and tradeoff from the driver. If such a meter were to become sufficiently researched and packaged to become an add-on product for the car, I could still see some people installing one, thinking it might magically gain better miles per gallon numbers, and then completely ignoring what it tells them, sort of like those "upshift" idiot lights that suggest economical use. There's no magic here. The light bulb, as always, has to *want* to change._H* 060808, 070506