The Tao of HSI

Getting the most from the 3rd-generation Prius dash

The owner's manual for the new 2010 model Prius doesn't say a whole lot about the Hybrid System Indicator display option, but after a bit of research it is evident that the HSI brings together, in a somewhat qualitative way, the essence of several other instruments I've had the personal luxury of being able to observe for the last four years in my '04 with its add-ons. Now everyone with a 2010+ Prius can enjoy riding some of those delicate little threshold states that obtain the best results from the system, simply by monitoring one display area.

This article explores the mapping between the HSI and what we can see with various aftermarket gauges. So far it is based on relatively brief experience with the 2010 and is intended as a community-built living document, so in this not-100%-finished stage any and all substantiated corrections are heartily encouraged to help build a reliable knowledge-base. Many questions from the earlier versions of this have been answered, and there are only one or two bits remaining to figure out which at this point aren't burning issues.

The pictures are sort of an assembled hack-job and may not reflect precise visual reality, but are enough to describe the intended content.

Besides some shortish driving runs I had done in some forum-friends' new cars over the summer, a couple of the sales staff at Ira Toyota were very helpful in setting up some additional test time in one of the new 2010 Priuses on their lot to help me verify some of the remaining unknown parameters and finish the article. In particular, Samuel Mistiva kindly took a zero-revenue hour out of his day to go with me on a trial run, including a stretch up my favorite local "test track" -- a long, straight piece of I-95 between Danvers and Newburyport which the Ira dealership is right next to one end of.

If you're in the North-Boston burbs and want to test-drive a Prius in as close to flat-steady-state as the New England roads allow, check Ira out.

This is basically the battery-power or EV area. To some extent, together with the "charge" or regeneration area to the left, this indicates hybrid battery current applied to electrically pushing the car -- but only when the engine is OFF. Both areas showing no level therefore signifies close to zero battery current in either direction, as seen in a gentle glide state or with the car in Neutral or sitting still. It does not show accessory or A/C loads also derived from the big battery, only driving load.

However, the relationship between the battery-power bar and go-pedal angle is not linear and varies quite a bit depending on battery state of charge. What this area more properly indicates is the threshold of electric-only demand before the engine will start, and when the SOC is lower the EV area will fill toward the HSI center line much more rapidly for a given pedal depression.

Thus, the system may be kept in electric drive by not exceeding something around here, staying substantially shy of the center line as a very slight tremor of one's foot may momentarily edge it over the line. Depending on SOC this may or may not provide a useful level of power for pushing the car, but may help when stealthing around parking lots and such.

In general one does not want to do this too much, though, as heavy battery use will run down the SOC and require making it up later. If it seems too sensitive to pedal push, it's best to let the engine light and start recharging because the system needs it! This also comes with the caveat that if the engine is already running, the power indication will show similarly but the engine will be "loafing" and consuming fuel for little useful output. Common long-held Prius knowledge is that it's better to run the engine efficiently to get going and then use minimal battery to keep going, rather than requesting a heavy electric-only demand and recharging later.

So, what is efficient engine running, which happens when we push into this upper area? It means under high torque load, but not excessive RPM. This is somewhat counterintuitive since many drivers have come to expect higher overall power by pushing a "throttle pedal" more and having the engine rev up a bit and maybe even drop a gear in an automatic transmission to help accelerate. But the Prius system has always tried to keep its output torque high and RPM down, thus keeping its operating curve high on the "efficiency mound" of the BSFC chart.

It doesn't always get this right, however, often due to hysteresis and nonlinearities in the system, but an attentive driver can help it avoid the inefficient engine-run regions. Remember the "sweet spot" meter concept? Well, here it is, straight from Toyota. As the HSI reaches up into the area I've shaded green here, engine load as reported by the ECM rises to about 80% and pushes toward 85% by the top end of that range. This means the throttle is controlled so that the engine is steadily producing most of its available torque, over a fairly wide changing range of RPM between roughly 1200 and 2100 as the pedal pressure is varied and speed changes.

Constant high torque, and variable RPM to obtain the desired overall *power*. That's always been the key to it, known to every efficiency-aware driver whether they're in a hybrid or not. Curves for the really low-RPM ranges are something you won't see on most BSFC graphs, either, as they tend to show data collected from wider throttle openings than we usually see in the real-life vehicle.

So here is about the *lowest* we want to go on the HSI when the engine is running, to avoid backing off the electronic throttle too far and losing our output torque. This may be suitable for steady-state cruise at certain highway speeds, but minor factors like local speed limit and wind direction may dictate whether this can be sustained long-term or not. The engine is likely to continue running at reduced output, uselessly, as the HSI descends toward or below the centerline and won't shut down until driver demand reduces significantly farther. That's the hysteresis effect, but the easy workaround to help the system decide is to get off the pedal momentarily and gently reapply pressure but not to exceed the EV area limit. Using one's foot to force the jump between loaded engine-run state and gentle electric thrust is the typical pulse-n-glide routine, but now we have a better indication of how low we can keep the pulses and still be efficient. As always, working engine-run times smoothly into upcoming road elevation changes generally gives the best results.

Going about 30% up the "engine half" of the HSI, maybe hovering just under the right edge of the ECO logo, yields the traditional gentle acceleration scenario in which instantaneous MPG happens to roughly equal MPH -- one of the suggested guidelines favored by some second-gen drivers for minimal-power but efficient takeoffs. If surrounding traffic allows the leisure to stay around this level during acceleration or pulses, it is likely to produce some very satisfactory high MPG numbers. If you're watching RPM during this, you'll see that just like in the second-gen system the whole scale of "loaded" RPM tends to shift upward as vehicle speed increases. In fact it doesn't feel like the overall control strategy for the synergy-drive has changed a whole lot between the second and third generations.

Some owners report a "stickiness" around the center line, where further pedal depression doesn't advance the HSI into the right half quite as soon as one would expect. Another nonlinearity, which I've observed as well. Unfortunately, it's likely the engine is already running by the time the high area starts filling in so it's probably best to move straight into the high-efficiency mid-region when we want the engine to light anyway, but then keep the level within that fairly low as conditions permit. I conjecture that the discontinuity is a side effect of the hybrid controller doing a whole different set of math when calling for engine power to figure out what proportion of driver demand to display in the HSI.

For slightly brisker acceleration or gentle uphills, we want to bring the engine RPM up a bit but not ridiculous. Note that this notion of "brisk" will still seem like anything but brisk to your typical gear-head driver, but if traffic and conditions allow deliberate limiting of our takeoff output to here or less, then it will still keep us well-planted on the "mound". It will also eventually bring the car up to quite a high speed -- at least one owner playing around with the top of this range reported chickening out somewhere over 90 MPH, and my own brief test peaked around 80 and the powertrain felt like it had plenty left to go beyond that if I didn't back off. Holding at about the two-thirds mark in the HSI engine area seems to maintain a nominal highway speed of around 65 MPH, closely matching my own second-gen observation at around 1900 - 2000 RPM, six-ish millisecond injection time, and 15 or so kilowatts of output according to the Scangauge.

Gentle acceleration out of intersections is generally much safer anyway, allowing more margin to avoid light-runners and the like. What happens on the "shoulders" of a trip segment really matters -- even if it's a long steady-state highway stretch, starting in a fairly leisurely fashion with the instantaneous MPG kept high, and then a nice long zero-fuel glide off at the end all has a profound effect on the overall trip MPG.

The new owner's manual does hint toward this thinking:
So here we have the "back off and reapply" method more or less recommended to force engine shutdown, but Toyota suggests using quite a bit more battery to keep going than may be prudent for best efficiency. Running the EV side as high as shown here will pull the SOC down fairly quickly. And if this is done at higher non-EV speeds, the engine is likely to keep running in an inefficient way.

Ken1784, a longtime Japanese participant at Priuschat, suggests a slightly more moderate range for gliding with gentle electric use.

For the steeper hillclimbs or highway on-ramps, we may occasionally need to shovel on some serious coal. But if "serious" can be constrained to just where the "PWR" indicator touches the rightmost tip, we're still running fairly efficiently relative to power request. This brings the engine to about 3000 RPM and reported load closer to 90%, and it has been shown in the previous 1.5 liter system that the Atkinson cycle efficiency begins to fall off quite a bit when pushed harder than that. Note that when the "power bar" fills up the engine is nowhere near cranking out its real maximum yet, and perhaps this is trying to tell us something.

Now, here's the best part. We now have an easy way to find and maintain warp stealth at speeds above 42 or 45 MPH, without adding a special current meter. It looks like this -- just a tiny tickle into the EV region. But one has to back off briefly past the null-point and slightly into regen to coerce the system into that mode in the first place, and then reapply a tiny bit of foot until the first little "triangle" of the EV range fills in. Lock it there and enjoy the zero-consumption ride.

Increase across the EV region will be *very* rapid in response to any more foot pressure, so for long warp-stealth runs it may help to select "ECO button" mode to obtain a pedal response curve with finer control over what's going on. The engine may re-light well *before* the center line in warp stealth, sometimes giving a perceptible little lurch as it starts burning fuel again. The threshold varies depending on SOC and sometimes WS may be just about impossible to maintain anywhere above the null point, just like in the previous model system. More experimentation is needed while having an injection monitor available. A Scangauge can show the generic engine RPM parameter, usually holding down at 980 or thereabouts in warp-stealth, but it is quite slow to update and thus difficult to use in eyeballing exact cutover points.

Obviously, this is also what an engine-off glide with very gentle electric assistance looks like and will probably deliver better overall results in a pulse-n-glide scenario than Toyota's suggested method of heavier battery use.

The "energy" screen is completely useless for warp stealth, as it shows regeneration the entire time. In fact the energy screen is pretty useless for anything, as it tells the same "lies by omission" we're already used to.

And at some point, we have to slow down!

No feet on the pedals yields typical "fake engine drag" regeneration, just a little flow into the battery. Applying just enough go-pedal to make this vanish is our typical zero-current glide situation; at speeds low enough for engine-off operation one can also just switch to Neutral and relax one's leg muscles.

When using the brake pedal to slow down, avoiding the maximum regen level keeps the system from exceeding the charge current that the battery can accept and stays almost completely off the physical brakes. A long and evenly-applied braking event keeps the currents lower and steady with the least energy loss, so the earlier one can plan allows for the least peak regen current or hydraulic assistance needed heading into the stop point. Slowing events of a quarter-mile or more using light regen can be commonplace on open country roads with good visibility ahead, and yield a healthy amount of usable extra battery charge to get going again. This is in fact the major benefit of any hybrid system -- the ability to absorb significant stopping energy and give it back on the next go, and now the HSI can help a driver get the most from that aspect as well without exceeding limits that were generally hidden in the previous model cars.

As with the second-gen, the "B" shift position should be avoided unless descending long steep downhills where simply throwing away excess energy becomes needed for safety. It can also be used briefly to try and get back a little more regen if harder braking over bumps causes the hybrid system to disable brake-pedal integration momentarily -- that "braking sag" transition feeling is still an issue in the third-gen Prius.

Note that none of this endorses unduly rapt attention to the HSI in preference to eyes on the road and the task of highly predictive driving. Be safe out there! A brief glance should be all that's required to set or check a desired condition, and then the right foot can remain locked where it is to hold that while the driver waits for the results. Experienced Prius pilots are well-versed in doing this already. Unfortunately Toyota still forces the driver to look toward the middle of the car rather than placing such important realtime info directly in front and almost on the forward sightline, but it's close enough and the HSI display reacts fast enough to changes that it's much better than trying to track things only through the second-gen MFD.

It is also thoughtful that the new display allows the driver to see the instantaneous MPG, average MPG, *and* the hybrid battery state of charge all in one place without any inconvenient screen-switching. Pressing the "ECO" mode button by the shifter provides instant access to the HSI in the display, for those who would rather not have to punch the steering-wheel DISP button some variable number of times. However, the ECO mode may not be suitable for most normal driving as its output response curve is almost annoyingly "slow-bottom", but toggling back to normal linear response only takes one more push.

Whoever designed the way that DISP button works to access displays and navigate menus needs to try a completely different career, because that is one of the most embarrassing user-interface failures of the third-gen car.

It is no mistake that the button-selectable "power modes" have not really been discussed so far. Other than EV mode which allows higher demand before the engine is brought in to help, the mode buttons are largely irrelevant and only change how high the HSI goes for a given amount of pedal depression. Toyota illustrates this quite simply in the New Car Features document:
noting that by "opening" they really mean "driver demand", since the pedal doesn't actually open anything.

That's all that the buttons do, period, other than to slightly change the heating and air-conditioning operation in ECO mode. And the response curves change very slowly in the software, e.g. if you lock your foot at some mid-range position and jump from ECO to PWR mode, the HSI will climb a bit over the next two or three seconds. Way more attention than is warranted has already been given to these buttons, and it's really just confusing people and causing all kinds of disinformation to be propagated about which mode anyone "should" be in. Pick your preferred response curve, that's it, and read the HSI for the real story on what you're doing. Again, for finer control around the bottom end such as warp-stealth operation the ECO mode may help, but it still requires driver skill to hold the HSI at the right place.

One mildly interesting note about the buttons is that the three of them are wired to completely different computers in the car:

  • EV   is wired to the main hybrid ECU   [now called the "power control ECU"]
  • ECO   is fed to the air-conditioner controller
  • PWR   is an input to the engine ECM
So the hybrid ECU which actually reads the go-pedal has to obtain the fact that ECO or PWR were pressed as data sent over one of the several networks in the car.

Go figure.

Additional references

Dan's early 2010 review
CleanMPG preview
Long Priuschat thread about the HSI
Further discussion of the "ICE line" and engine states

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