While driving today, I did some more careful observations of the horsepower generated by the ICE. What was revealed to me is that the number of horsepower you consume when you max out the ECO Bar changes depending on speed. I don't recall this being documented before. When you are less than 60 mph, maxing out the ECO Bar (100%) is 25 hp. When you are at 60 mph or greater, maxing out the ECO Bar (100%) is 30 hp. Then at 68 mph, when you max out the eco bar, you are using 40 hp. So it seems the HSI Bar and Eco Light is a sliding scale. Just to note, I repeated this several times. SOC was pegged at 60%, battery amps were near zero. I turned off A/C on some of the trials, but this does not affect it. Observations were made using Scangauage and the following gauges: HPR X-Gauge SOC X-Gauge AMP X-Gauge RPM This is an update to my previous post (that had the wrong conclusion but lead me to observe the actual behavior): http://priuschat.com/forums/gen-iii...i-bar-position-power-according-scangauge.html
So basically the HSI is a power demand bar and not a power output bar. (I think someone may have mentioned this before in passing). That seems reasonable.
I don't think it really represents power demand either. Somehow someone at Toyota decided that this behavior was better overall than having a HSI that was pegged to actual power output. Still, it's completely arbitrary to have the same indicator represent 25 up to 40 hp in the span of 8 mph.
You are assuming that it is supposed to represent horsepower. It certainly isn't 'completely arbitrary' for the gas gauge to vary greatly over the span of 8 mph. It would be nice if Toyota told us what it represents (if there even is one thing it represents; it could just be a custom function of a number of inputs).
From what I'm reading here for few weeks now I feel that HSI represents something between power demand and total power output from the system (to the wheels - excluding ICE power going by MG1 to battery, powering internal devices like AC, lights, EPS and so on) to the wheels. If you take the car and drive it at certain speeds (ie. 10 mph, 20, 30...) it needs exact amount of power to keep it running at exact speed (assuming no road friction change, same wind power and direction, same car weight). The faster car goes, the more power it needs. Then - increasing speed, at certain acceleration, needs more power if accelerating from 20 to 30 than from 10 to 20 mph. I think that Toyota took this math, put it on the graph, making "ECO zone" - maintaining certain speed, accelerating mildly, with acceleration margin lower, the faster car goes. (Someone near German highways may check what is the highest speed when ECO lamp is still on. I think it is limited nearby 140 kmh / 87 - 90 mph). So the ECO lamp will be on if you maintain certain speed or accelerate mild to moderate. If on uphill or agains strong wind you will feel that ECO lamp goes off faster, than while driving downhill or with wind direction. So I think that HSI indicator is a graph power demand/output scaled from 0 to 100% (incl. PWR, ecxl. CHG), where 0% is no power demand at all (while not regen also), and 100% is full power output possible at certain speed (ICE / MG2 combined). Then depending of the speed MG2/Eco range is "longer" when on lower speeds, where electrical engine takes bigger part in total power output. Right part of ECO/Hybrid range is "longer" when driving faster, where more ICE power is used in keeping speed. And at all... PWR part I think is also dependend on the speed. On lower speeds it is harder to go to PWR range, than in high speed - high acceleration is harder to the car while driving faster than in low to moderate speeds. I'm making those assumptions on what I felt driving Toyota Verso with 1.8 Petrol Engine and CVT transmition (it also has power demand indicator with ECO lamp) and on gear shift indicator on my Auris D-4D (on manual transmission it shows whether to change gear up or down depending on car speed, engine power and power demand calculated from accelerator pedal). Let me know if all what I wrote here makes sense or it is totally off the Prius... I can't check it on myself until Nov 30, when my should arrive...
I made a similar observation last weekend: At 20 mph or so, the upper end of ECO bar was around 20 hp. At 60 mph or so, the upper end of ECO bar was around 30 hp. I think it is a speed-scaled power demand bar. The speed-scaling keeps the rate of acceleration modest and makes the bar useful in both low and high speed driving. If it was not speed-scaled, it would need more horizontal distance to remain relevant across the speed range.
If the road surface is flat (neutral gravitational load) then at a lower speed, there should be less of a load on the transmission when the Prius is travelling at a lower speed than at a higher speed ( the power plant must rotate the transmission longer for a fixed distance travelled at a slower speed than at a higher speed ). Thus at a lessor speed , less horsepower should be required. So is there a MPG vs horsepower chart in the making?
