Not quite. But I think I’m finally figuring out how to drive my wife’s Prius Prime.
Above is the gas mileage on my wife’s Prius Prime, after a round trip from Vienna VA to Harper’s Ferry WV. This is all after resetting the odometer once the battery was depleted. So it’s straight-up gas mileage.
This trip contained a short section of high-speed driving, but was mostly hilly primary and secondary roads in western Virginia and West Virginia. And I think I finally understand how I’m getting such great mileage.
The Prius Prime loves hilly roads. It is an excellent car for a particular style of pulse-and-glide driving.
Potential energy pulse-and-glide
Pulse-and-glide is a hypermiling technique, that is, a method of driving a vehicle to get maximum gas mileage. It is always — and incorrectly, as I explain below — characterized as repeated cycles of accelerating and coasting. In the most efficient case, the “pulse” portion is run at the gas engine’s most efficient load, and the “glide” portion is run with the gas engine shut off.
It is well-established that pulse-and-glide can deliver substantially better gas mileage than driving at a steady speed (e.g., reference).
It’s also generally acknowledged that pulse-and-glide is a stupidly dangerous way to drive on the highway. It involves alternately speeding past traffic, followed by being an obstacle to traffic.
But accelerating and coasting is not the only way to do pulse-and-glide.
At root, pulse-and-glide alternately stores and releases energy. During the pulse, you run the gas engine loaded for peak efficiency, and store that energy in the form of kinetic energy (speed). In the glide, you release that kinetic energy to overcome friction, with the gas engine shut down.
Done correctly, the gas engine only runs at its most efficient loading. This is why the gas mileage from pulse-and-glide will typically be substantially better than the mileage from running at constant speed. At constant speed, virtually every vehicle on the road today will be inefficiently lightly loaded. With pulse-and-glide, you can achieve the same average speed, with less gas, because you only run the gas engine efficiently.
But you can do the exactly same thing using potential energy, not kinetic energy. That is, as height, not speed. Virtually all discussion of pulse-and-glide ignores this. But as far as the car is concerned, there’s no difference between coasting on the flat with the gas engine off, and coasting down a hill with the gas engine off.
It’s not as if this is some cosmic concept. It’s not even new. Back in the 1970s energy crises, plenty of folks with manual transmissions got in the habit of taking the car out of gear on the downhills. And coasting. (Even though that is, strictly speaking, totally illegal.)
The heavy-duty electrical portion of the Prius Prime means that you can do this safely, that is, at more-or-less constant speed. (And with the engine instantly engaged with the drive train, in case you need it.) On the uphills, you load the gas engine heavily, building a bit of speed and charging the traction battery. Near the top of the hill, you briefly lift your foot off the gas, which snaps the car into EV (electrical) mode and shuts off the gas engine. (One of American’s champion hypermilers termed this maneuver the “forced auto-stop”.) You complete the rest of the cycle with a light touch on the gas pedal, keeping the car in EV mode — gas engine off — at more-or-less constant speed.
Repeat on the next uphill.
To some degree, the Prius will do this on its own. The only art here is to encourage the car to keep the gas engine shut down over an efficiently large portion of each uphill/downhill cycle.
The idea is almost exactly the same as classical pulse-and-glide. You try to use the engine only with an efficiently large load on it. Otherwise, you shut it down.
This has a twist, in that the generation/storage/use of electricity involves losses. So there is an efficiency penalty for using the electrical portion of the car to maintain constant speed. But I think that more than pays for itself by allowing you do get some pulse-and-glide efficiency without being a pain in the ass for all the other drivers on the road. Something you can’t do with classic pulse-and-glide.
The results are nice, but not exactly staggering. This car will routinely get 65 MPG anyway. All this tells me is that, on the right terrain, it’s not hugely difficult to stretch the Prius Prime gas mileage out to 80 MPG, more-or-less. Which means that for every 100 miles, this saves you about a quart of gasoline. Nice, but not exactly a solution for global warming.
