I wouldn't know Depending on the hill length and incline I might go as fast up as I do down or on the flat. Actually, I routinely build up my fastest speeds just before the hill. Rolling hills are fantastic.
I've been impressed with how well my Prius is designed to be fuel efficient regardless of how I drive it. I generally think most people over think it, or try to outsmart the software and HSD. That being said, I vote FOR applying "Pulse and Glide" when possible. Seems to me The Prius is designed to maintain momentum once gained, as efficiently as possible. So in my opinion, it's kind of counter intuitive in comparison to a regular vehicle. My observation is The Prius seems to do best, if I accelerate like I'm in a Police Chase, but then reach my speed and level out and then drive like I'm 94 years old and trying NOT to spill my colostomy bag. But in general? I'm not disciplined enough to consistently maintain a structured driving approach. I basically drive The Prius with an eye towards getting from point A to point B...safely. Without even thinking about it, I'm often impressed with all the work The Prius seems to be doing for me automatically.
You'd be wrong about that. Every body style has a speed where it generates maximum efficiency. (boats are the same way) At that speed, the aerodynamics are working at their best. The faster you travel at a given RPM or engine load, the better the MPG. So the most efficient speed is where all those things come together. For most cars that's in the 40-50 MPH range depending on drag coefficient, gearing, etc.
Right speeds, wrong reasons. Friction looses per speed *always* increase with higher speeds, but up to a point are more than offset by increasing ICE efficiency.
Wayne Gerdes, CleanMPG founder, has a few tutorials. Discussing hill climbing he says to stay right and keep the speed down. If there's a special truck lane, all the better.
He mentions once in this tutorial to set speed with the HSD (the bar graph), not the speedo. For me that kinda goes out the window with REALLY steep hills, say climbing a ski hill. Then I just take my lumps. At least a couple of times a year my wife and our younger son put a gun to my head and take me up there. ;(
Modern emission controlled cars should run the same mixture ratio at all elevations, no difference there. But the reduced air drag from the thinner atmosphere is a big help. Your old non-hybrid also benefited from less Otto-cycle pumping loss. No, he is very close to right about that, as shown by Bob Wilson's Prius MPG vs MPH graphs. Other sources show the Prius's best MPG is in the general vicinity of 15 mph. Car aerodynamics work best at zero speed, and get monotonically worse as speed increases. Maximum efficiency speed is a function of the powertrain. Traditional nonhybrids have rotten engine efficiencies at low speed, so generally peak near the lowest usable speed of the highest gear. I know that my manual transmission nonhybrids peak well under 50 mph, probably even under 40. The Prius HSD does not suffer the same constraints, so can produce better mpg at much lower speeds.
Basically you're burning fuel to create friction. Most of the friction is from air-resistance, therefore the slower you move, the less friction. The other factors such as hill-climbing, drive-train friction and heat-loss aren't as speed-dependant.
I drove through North Carolina before on I-85 and I went through similar experiences; I would hate hearing the engine rev so high on the uphill climbs when the car was still had less than 5,000 miles. With that said, I would be just as content with 42 MPG as I am right now with 52. And really, I do expect the MPG number to drop after I exceed 10,000 miles. The only time I ever get people complaining is when I drive on local roads after high school students are released from school; there are high school kids who constantly honk at me and flash their high beams on two-lane roads, thinking that they have reasonable opportunities to pass ... and this is when I drive 40-42 on 35 mph speed limit zones.
Greg, I suspect hill climbing efficiency falls off at higher speeds not so much due to aerodynamics as to the engine being above it's efficient rpm/load range. Kind of like if you're doing an uphill on a bicycle, you shift down, take it at little more than a walking pace.
So a couple of things... At low speeds air resistance is NOT the dominant term in the loss equation. Even with the lowest Crr tires, rolling losses win up until at least the mid 30s. It is true that the ICE in the Prius is allowed to run more efficiently than a typical car, but that does come at a cost. This is also why it is foolhardy to try and do EV mode a whole bunch without having one or more good reasons to do so. Round trip on the battery charge, when taking the efficiency of both the battery and electronics is not particularly high. At low speeds, that is just a loss and the incremental cost of driving a bit faster is going to be less than what is gained by putting more and more directly into the wheels. By far the things that make the biggest difference in my mileage are 1) minimize how often you drive from cold, 2) minimize the use of brake pads, 3) keeping the little [ECO] light lit while underway, and 4) keeping to 60 mph or below. I'd say these are in descending order, but that is mainly because I don't ever drive above 60 unless my passengers are getting antsy. I just drive the speed limit in cases where that happens and the speed limit is higher.
Since it's a variable-speed transmission, I'm assuming that the engine is always near it's most efficient range of speed.
Load is balanced by the power-split-device, sending "excess" power from the engine to the generator for the sake of keeping RPM in an efficient state. The resulting electricity can be used right away, stored for use later, or both.
That's one way to look at it, but the amount of power needed is determined by the accelerator, which you push in response to your desired speed, and there's not usually much excess. Power is divided between the wheels and the generator based on the requirements of both, determined by the computer. The generator is also a motor, which is used to control the amount of power which is divided between the engine, the motor/generators and the wheels. The only scenario where you might have "excess" power, is when you are regenerating downhill, and the battery is up to capacity. At that point, the engine provides some braking, but if that's not enough, you might burn some brake pad material.
Nope, it routinely happens climbing hills too. Get yourself a gauge to watch the SOC. You'll see it go up.
If the battery can take more power, I'm sure some of the power from the engine goes to charging it, but that doesn't seem like it's shunting unused power. It makes sense that the engine is putting out just enough power to climb the hill at the speed you want to travel.
