22.2 MPG going Uphill 4.7 Miles

You posted this as a joke right? Of course non-hybrids suffer huge losses when going uphill. Unpowered cars suffer more but show me another car the size of a Prius that can average 50mpg with a 50ft. to 1,200ft. elevation change commute.

 

I'm not really able to answer your specific question as to the % drop you would expect from other vehicles, but I can assure you that it takes a lot of energy to lift all that weight. Note however that much of that energy has now been "stored" as gravitational potential energy which will be converted back to kinetic energy when you descend the hill. In other words, when you go back down hill, you will see MUCH better MPG, and the average of the round trip (uphill and back downhill) will not be nearly as bad as the 50% you are mentioning here. This is only a problem if you intend to always drive uphill without ever driving back down (highly unlikely given the confines of the atmosphere).

 

I suspect that since the Prius has such a low cd, the fuel mileage decrease expressed as a percentage due to the additional work climbing a hill would be higher. In other words, when you are climbing a hill, a larger percentage of the energy output would be used to move the car up rather than overcome drag.

 

If you ride a bicycle at 15 miles per hour over flat ground, do you consume the same amount of energy to continue at 15 miles per hour up a steep hill? And do you consume the same amount of energy to bicycle downhill at 15 miles per hour?

 

I wish... Maybe I would ride my bike more

 

My Jeep Cherokee XJ got 4.2 mpg going uphill. It was a 4 cyl. 2.5L

 

Also keep in mind that MPG is a ratio. You have to be careful about comparing ratios and using numbers like 50% drop in MPG. It doesn't always have the effect that you think it does.

Example:

Assuming your Prius normally gets 44.4 MPG:
4.7 miles at 44.4 MPG = 0.10585 gal.
4.7 miles at 22.2 MPG (50% drop?)= 0.2117 gal.
Extra fuel used to travel uphill = 0.10585 gal.

Assuming some other vehicle normally gets 30 MPG:
4.7 miles at 30 MPG = 0.1567 gal.

If it used the same extra 0.10585 gal to climb the hill:
4.7 miles using 0.26255 gal. = 17.9 MPG.

So even though the "other vehicle" only dropped by 40%, it still used just as much extra fuel (0.10585 gal.) as the hypothetical Pruis.

The worse the starting MPG, the less the percentage has to drop to use an equivalent amount of extra fuel. Imagine a vehicle that "normally" gets 20 MPG? If it was any worse than 13.79 MPG climbing the hill then it would be using more extra fuel than the Prius for the purposes of climbing the hill. That is only a drop of 31%.

It gets even worse if you consider the total amount of fuel used rather than the "extra" amount used to climb the hill as compared to flat driving. If that hypothetical vehicle mentioned with a normal 30 MPG drops more than 26% then it is using more fuel to climb that hill than the Prius. And the hypothetical vehicle with the normal 20 MPG? It can't do better than the Prius even if it's MPG doesn't drop at all.

 

It all averages out. I get 46-48mpg driving 48miles from 50ft. to 1,200ft.. On the return trip I get 60+mpg. It averages out to 50-55mpg depending on conditions.

The last 12miles of my trip goes from 250ft. To 1,200ft. and my instant mpg drops down to 25-30mpg and it hurts overall mpg but on the trip down I easily get 80+mpg so it averages out.

 

YES

I have a friend whose Range Rover drops to single digit fuel economy going up hills.

If you think 22 mpg is bad in a Prius, try the same test in another car and be prepared to be more shocked.

 

Every day to work I have a 600 ft altitude change. On the way to work (down) I average about 75-90 MPG. On the way back I often see instantaneous stretches of 20MPG or lower. By the time I get home it is about average 63 MPG or so (63.5 +\- 3.4 average and standard deviation since Feb). The fuel usage is around 0.27-0.3 gallons round trip.

 

Please post MPG #'s for the trip home. Then let's calculate your commute's average MPG!

 

F8L hit it right on the nail. In addition, you will get much better mileage going down because the ICE is not running. Non-hybrid (save a very few), the ICE runs even going down hill. In addition, the ICE runs a richer mixture when idling.

I drive over a 7500 foot elevation frequently. Since gas is cheaper on the other side of the grade, I always fill up immediately before climbing the grade. The mileage will show in the low 20's by the time I reach the summit ... but over 50 mpg by the time I reach the flat on the home side.

The Prius is a remarkable machine .... don't try to out think it.

 

Two factors will make non-hybrids drop a lower percentage on hills --
(1) they are more wasteful on the flats, so the extra energy needed to lift them up the hill is a smaller percentage of the total;
(2) when cruising on the flats with typical gearing, their Otto-cycle engines are operating far from peak efficiency. It actually improves when climbing hills, if they can do so without downshifting (though not nearly enough to provide all the needed lift). Prius' Atkinson-cycle engine is already getting this efficiency improvement - and much more - during flat cruising, leaving essentially no extra improvement to be gained while climbing.

The end result is that Prius uses less extra fuel to climb, but because of its tiny fuel use on the flats combined with the nonlinear nature of the MPG scale used in the U.S., the difference looks bigger. If we used the European liters/100km scale, the results would be more clear and no longer scary.

My rule of thumb is that a typically loaded GenIII needs an extra gallon of fuel for each 10,000 feet of climb, in addition to the fuel needed for a flat road in otherwise identical conditions and speeds. An empty car with a lightweight driver needs less, a fully loaded car needs more.

If you normally get 50 mpg on flat roads, then to climb this hill you need:
4.7 miles / 50 mpg = 0.0940 gallons for flat road;
(1250 feet - 50 feet) / (10,000 feet/gal) = 0.1200 gallons for elevation change;
Total fuel = 0.0940 + 0.1200 = 0.2140 gallons;
Climbing MPG = 4.7 miles / 0.2140 gallons = 21.96 mpg, nearly identical to the 22.2 mpg you observed.

This grade is nearly 5%. If the engine is already warmed, you should be able to coast down this with no fuel, at infinite mpg, for a total round trip of 44 MPG. And maybe use regeneration to fill the traction battery up to 8 bars, if traffic speed allows.

 

Time for physics back of the envelope.

It takes 5.5MJ of energy to lift the car+passenger(s) 1200' (1500kg * 365m * 10 m/s/s). This is on top of the other energy for wind, rolling resistance, etc. E10 (American gas) has a volumetric energy density of 33.2 MJ/L. Lets call the ICE efficiency 33%. Then, the ICE produces about 11 MJ of mechanical work per liter of fuel. 5.5MJ/11MJ/L = 0.5 liters of fuel to lift the car 1200'. 1 liter = 0.264 gallons, so 0.5 liters = 0.132 gallons. Pretty close to what fuzzy estimated. Your FE may have been made better, actually, by the fact that you would have used some of your battery energy during the climb.

As an aside, that 4.5 MJ per 1000 feet translates to 1.25 kWh per thousand feet. Something to keep in mind if you have a plug-in Prius.

Climbs have a huge, unavoidable impact on MPG. 10,000 feet per gallon is a very good approximation. The trick is, on the downhill trips, being able to coast to the bottom using little or no fuel and store a bit of that energy in the battery as well.

 

High School physics will help here:

The additional energy (in joules) required to raise the car (and contents) 'h' meters is m*g*h, where m is mass in kg and g = 9.8.

Addendum: Oh, I see others have shown the calcs

If Physics is not your thing, then how about asking "is the energy spent going up hill recouped on the way down?" Zero the mpg meter before the ascent, and check it at the end of the descent. Try to start the ascent at the same speed you will be going at the end of descent so that differences in kinetic energy are not in play. For similar reasons, try to start and end the trip with the battery at the same level.

 

The best round-trip mileage you could possibly get would be 44 mpg, assuming absolutely no fuel is burned on the return drive. So the 22 number seems a bit low tow to me.

 

OP said that his usual MPG is in the mid 40s