Executive summary: the third-generation Prius has a higher-speed "glide" mode that uses no fuel and allows much longer runs of coasting, but it's very subtle and hard to find. It *is* a major key to higher mileage in that tricky "mid-speed" range from 42 - 55 or so mph on secondary highways. Here's the how and why. Most Prius drivers are delightedly familiar with "glide" or "stealth" mode, where the car moves solely under electric power without the engine running. Electric traction and the ability to bring the engine in and out of play based on go-pedal demand is one of the major hallmarks of such a hybrid system, and contributes to its ability to use very little fuel at lower speeds with a foot technique called "pulse & glide". But alas, this game seems to cease at 42 miles per hour, when the engine comes on and stays on as long as the car is going faster than that. For most people, mileage suffers somewhat since even at idle or lower power demand, the engine is consuming *some* fuel. The concept of a good "glide" seems to be inaccessible at those higher speeds. The reason for running or at least turning the engine above 41 mph is to avoid having MG1, the smaller motor-generator, turn at ridiculously high speeds backwards to make up for the engine sitting still. While Toyota improved the top-speed RPM rating for this part from 6500 to 10,000 in the 3rd-gen Prius, they don't necessarily want to *run* it at those speeds routinely. The easy answer is to let the engine spin, which sort of rebalances all the component speeds in the planetary transmission in a more sensible way for those road speeds. However, just because an engine is spinning doesn't necessarily mean that it is using fuel. The concept of "fuel-cut" during coasting has been around for a while in all cars -- when going downhill with no throttle, the fuel injectors can simply remain closed and thus not even dribble a little bit of the precious commodity into the engine. No reason to; it doesn't affect how the car runs when the engine is being pulled along by the wheels. Fuel injection immediately re-commences once the throttle starts opening again, or speed drops below a certain level, etc. to make sure the engine remains running properly. [Some cars do produce a slight lurch when that happens.] The Prius does fuel-cut at higher speeds too, but several other useful things also happen when you take your foot off the accelerator: Engine RPM drops to around 950, maintained by the motors VVTi valve timing is retarded as far as possible Regenerative "fake drag" comes into play, charging the battery and of course fuel injection ceases entirely. This is the typical coasting/slowing mode, which feels just like it would in a conventional car. But once we're in this state, we can take better advantage of these low-consumption conditions. By feathering the accelerator back on just a *tiny* bit, a driver can eliminate the fake drag and actually bring the battery in, just a smidge, to help keep spinning the "dead engine" and push the car a little. Fuel-cut is maintained, and the retarded intake cam timing reduces the amount of air sucked in around the throttle flap and through the engine to almost zero. This is a very low-resistance state for the engine to be in, and it only takes a kilowatt or two to keep all that merrily turning. Since the regenerative drag is reduced or eliminated, the car will coast along a lot farther -- still losing speed on the flat, but fairly slowly, and one can easily maintain speed or accelerate going downhill. It's really the higher-speed equivalent of "glide" with perhaps a little more brought in from the battery to compensate for the spinning dead weight of the engine. If held long enough, the HV battery charge will begin dropping just like it would during gentle EV running at lower speed. How do you know when you're in this state? Having good instrumentation like a battery-current readout helps, but you can even tell from the "energy" display. It looks just like electric-only drive -- battery to motor to wheels, no engine. At any speed over 41 mph! This is why it's called "warp stealth" -- stealth really has nothing to do with it, since the engine does spin and at those speeds the car is making a lot of tire noise, but the only power source being used at that point is electric. Unfortunately the display updates sort of slowly, so if you slip out of this mode by pushing a little too hard you may not be able to tell for another second or two. The best seat-of-the-pants way to reach and hold this mode is to back completely off for 2 seconds, and then feather very slowly back on until the drag *just* goes away and you see the electric-only arrows. There isn't any "deadband" scenario here -- it's much easier to hop onto the "step" that draws a little battery current. Push a little farther, and the engine comes back on and in fact runs fairly *inefficiently* at a low power demand. If you reach the state and back off again, you'll see the green regen arrows reappear -- occasional small bursts of that can be used to help make sure you're holding just above that spot and you can probably feel the small bit of drag. Once a driver gets used to holding the accelerator unmoving *right there*, very long, free, high-speed and zero-consumption coasts can be enjoyed. And the numbers will speak for themselves [or at the very least, say something grumpy about the effects of air resistance]. A possible downside is that it only works in shift position "D", so the driver *must* maintain the accelerator position. Moving to "N" simply causes the engine to idle, consuming more fuel. That's about the same RPM as in warp stealth, in fact. If it's going to turn slowly anyway, why burn gas and generate more waste heat doing it? A battery-current meter makes all this much easier to see. Removing your foot from the accelerator brings in about 20 charging amps of regenerative drag, and then feathering back on quickly reduces that to zero and then crosses over to 10 or so of discharge and that's exactly when you're in warp stealth. The really interesting thing is what happens with a little more accelerator. Over a certain small range of increased demand, nothing changes! If the battery is at its nominal 60% SOC, current drawn from it to maintain warp stealth stays constant for a while, and then past a certain demand point suddenly the engine re-lights and battery current goes back to its normal slight wandering high and low of zero. There is actually a little "plateau" in demand response, and to make it even more interesting that plateau's width varies with battery SOC too. With the caveat that your instrumentation may reverse the sign of battery current, have a look at this little chart:
which illustrates the "breakover thresholds" as sudden drops to zero, but really indicates the demand point beyond which the engine goes back to running and battery-management returns to whatever it would have ordinarily been doing. There is hysteresis in the system, so to get onto the plateau again you have to back the accelerator demand completely off and slowly try again. This has two notable implications: If you've just warp-stealthed a long way on a gentle downhill, it will become harder and harder to hold the state because the threshold to start the engine running again becomes tighter as the battery SOC drops. At three or four bars or so, you won't be able to go any higher than zero battery amps before the system kicks back into running and charging. If you've just regenned up to 7 or 8 green bars down a mountain and now there's a long flat part, you can likely warp-stealth the whole way because a high SOC allows you to draw much *more* current out of the battery to push the car. For a while, at least, until SOC starts dropping back toward 60%. Then the plateau begins narrowing as above. The fact that the response plateau exists at all and the fact that battery SOC affects it convinces me that it is a designed-in mode. The cruise control can occasionally find it by simulating the same complete backoff and gentle re-application of accelerator, but if the cruise falls off the plateau and the engine starts running again it has no clue what happened. It's just luck of the terrain. You as the driver who doesn't mind a little coasting speed variance have much better control over how long you choose to warp stealth -- until you run low on battery, or the right terrain, or that annoying traffic light up ahead changes and gives everyone the "sixty to zero surprise"... There is no observable dependency on vehicle speed, other than falling back to normal EV stealth below 42 mph. Warp stealth works anywhere above that. By the time you're finished with the long-mountain-descent-into-flat exercise, the engine may have lost so much heat from not burning any fuel that once it re-lights, it may insist on running a bit more until it gets back up over 70 deg C. It really tries to stay around 80C. Of course, poking your EV button can rein in that behavior until you're really ready to use it. Oh, and you don't even have to be in full stage 4 for this to work. So if your route immediately takes you onto mid-speed secondary highways, you can still play some great high-mileage games. One great use of warp stealth is approaching highway exits -- timing the jump onto the plateau just right can yield a graceful half-mile or more of zero consumption, through warp stealth and then true stealth, all the way to the stop at the end of the ramp. The data for determining all this came from various instruments I've added to my Prius over the past year or so: battery current meter, center-null injection monitor [pickoff and LED from #1 injection coil lead] vacuum gauge coolant-temp gauge tachometer [but useless when ignition ceases; the 950 figure came from other sources and monitoring the NEO crank-speed output..] I do not yet know if this applies exactly as set forth to the second-gen "classic" Prius; someone needs to help me out here.Reference:
http://techno-fandom.org/~hobbit/cars/five-stages.txt
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(from Priuschat) -- The Five Stages of Prius Hybrid Operation
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