Every time a Prius stops, some of that energy gets stored in the battery. When it starts up again, some of that stored energy is used to help the car accelerate. Plus at city speeds the car can run purely on battery power. (My husband also found a trick that with just a little pressure on the pedal there will be no energy going into or out of the engine–the thing can coast for a surprisingly long distance on a flat road like that. Doesn’t work above about 30-35mph though). Not to mention that the gas motor shuts off when the car stops, which is one area that a conventional car will always be wasting gas.
Conversely, on the highway the gas engine never really gets a chance to shut off, because of the higher speed and the lack of regeneration. The battery gets almost no charging from highway use, while it gets a lot in the city. So while the battery helps a lot in the city, on highways you’re just using a (fairly efficient) gas motor.
We’ve probably been getting roughly the same mileage in both cases, usually around 45mpg–on one trip to my MILs, we got 40mpg on the way there, and 50mpg on the way back, all highway.
On reread, I probably said way too much. The point is that the Prius can go for significant stretches in urban areas without any energy input from the gasoline engine, while on the highway it needs constant input. It’s not just the regenerative braking recycling energy that’s already been used, but also the lack of a real idle (it’s actually rather creepy coming to a stop and feeling the gas engine shut off)
In extremely hilly terrain would it be possible that the OP’s friend were doing alot of downhill braking triggering the charge up during a low fuel consumption phase of downhill coasting?
Another way to tweak mileage is to start the trip with full batteries, and then to drive 100 % electric for as long as you can. You return home with an empty battery that needs recharging, but you have cosmetically “saved” extra on gas.
>Every time a Prius stops, some of that energy gets stored in the battery…
>Conversely, on the highway the gas engine never really gets a chance to shut off…
Yes, I understand. My point was that this energy storage and recovery could not possibly be more than 100% efficient, so having to repeatedly start and stop could not possibly increase the gas mileage. It would have to reduce it, though the goals of the hybrid design would include making this reduction smaller.
Of course, the times the car is running purely on battery power, when the gas mileage could be calculated as infinite, are a pathological case. They are misleading unless gas mileage is calculated in a more sophisticated way, such as between periods when the vehicle has been parked and the batteries have been returned to full charge. You could, for instance, drive the car around a little while without the gas engine running at all, and then park it, and let the engine run to recharge the batteries. During just that parked time the car gets zero mileage, or consumes an infinite amount of gallons per mile.
Odd isn’t it. But Toyota claims better mileage in the city than on the highway. 48 city, 45 highway. Everything I have heard is that a hybrid is not always the best choice if you do most of your driving on the highway.
Which makes sense, since the hybrid design really takes advantage of urban traffic flow, but on the highway it’s not much different from any other car.
I don’t really consider 45 vs 48 mpg to be a significant enough difference to say that the mileage is better in the city. But we have been getting about that mileage both in the city and on the highway, so if nothing else they’re the same. And our averages have been lining up well compared to when we gas up the car.
The car very rarely will run the gas engine if not actually moving. You could perhaps game this by pressing the gas pedal (not about to try it myself), but 99% of the time if the car is stationary, the only power draw will be from the battery. Which, unless you have AC running, is a negligible amount.
I’m skeptical of the situation presented in the OP as well.
Another Prius anecdote: the best mileage I’ve gotten in mine was when I commuted on a road with a 35 mph speed limit and very few traffic lights. I maintained a fairly constant speed of about 40 mph for seven or eight miles, twice a day. Got an average of 48-49 mpg, and the average was higher when I did less highway or other city driving on a particular tank of gas. For example, when I was pregnant and wasn’t feeling well for many weeks, I pretty much only went to work and back, and my average mpg went up.
Now, as a stay at home mom, I just do your average city/highway driving and I get in the high 30s/low 40s.
The Prius would get some battery energy from regenerative braking in this case, but it couldn’t have a net gain in energy from this- the amount of energy gained from regenerative braking would necessarily be less than the kinetic energy that the car had at the higher speeds. In an all-downhill trip, your scenario could work, but this is a round-trip. There’s no free lunch to be had here, just a slightly cheaper one.
If it’s true, perhaps altitude had something to do with it? I can think of a few effects high altitude would have:
Fewer pumping losses due to the smaller difference between outside air pressure and interior pressure at part throttle.
Running leaner due to less air. The engine would be capable of making less horsepower at altitude than at sea level, unless it’s turbocharged. In this case, less power is good.
Lower air resistance. The air’s thinner, so the vehicle won’t use as much energy pushing through it.
These reasons are why airplanes tend to get more fuel efficient as they fly higher, to a certain limit.