Most EV miles on one charge?

Already done.

 

Not EV, but I did do the drive from Reno to Sacramento on I-80. It was an hour of downhill driving with occasional flat spots. I let the car coast up the few inclines that I encountered. I made it over 45 miles without the ICE kicking in. Eventually the hybrid battery hit the bottom of it's SOC. The entire energy use display showed 100MPG bars.

That was in a 2002 Prius.

 

I'm sorry I missed it. I skimmed this thread and didn't see any posts of that nature from you. Could you please link to the one with the equations? Thanks.

 

You are ignoring the variations in engine thermal efficiency over different routes.

 

What would make the thermal efficiency change when driving over a hill versus driving in the same conditions on level ground?

 

Assuming the driver is trying to maintain same speed(there are different ways of driving the same route), they will depress the accelerator more going up the hill. That will open up the throttle more, which decreases pumping losses. Fuel economy drops, but more energy from the gasoline is going to moving the car. While fuel use going uphill increases, the down allows for less. Coasting can all that is needed then, which reduces fuel use to that of engine idling. Unless the conditions allow fuel cut off, then it is zero fuel use.

On a level road, the throttle position isn't going to change. It will stay the same with the speed. With most cars, the power needed to maintain steady cruising is below the engine speed of peak efficiency.

 

This thread is about EV range, so gasoline efficiency isn't germane. It's still an interesting discussion, and I'd be interested to see data showing the Prius's gasoline engine being so much more efficient under load that it is enough to counter the extra energy required to climb a hill vs. travel on level ground.

My experience has been that fuel efficiency declines dramatically with even slight inclines. In HV mode I can easily get 80+ mpg traveling on a level highway, even in the PiP which doesn't allow any kind of EV locomotion over 62 mph. To match that efficiency I'd have to be able to get somewhere near 40 mpg going uphill.

 

Engine thermal efficiency isn't a single fixed number, but varies considerably depending on where it is operating on its BSFC chart (fuel to energy conversion efficiency vs RPM and torque). Flats and hill climbs require different engine power levels, so the engine operating point shifts.

Your contested claim wasn't limited to EV mode, but was independent of propulsion mode.

So now you are ignoring the descent half of the round trip, narrowing the discussion to the climb phase only. That is a very different discussion, and not one I'm interested in.

 

The discussion had already branched out to hybrids. Going further beyond isn't a big step. The peak efficiency of most engines are around the 70% load point. That is a far higher load than needed for maintaining speed on flat surface streets. A hybrid could run the engine nearer to that efficient point at those times by putting excess output from the engine into the battery, then going electric when the battery is full. That is an auto pulse and coast. Do a manual pulse and coast in which you avoid the conversion losses of putting energy into and out of the battery, and you'll do better than steady speed. Gentle rolling hills encourage pulse and coast driving by making it possible while maintaining a constant speed.

Even electric motors have efficiency curves. Part of the reason Tesla with AWD can maintain high efficiency is that the front and rear motors have different efficiency at different speeds.

 

I "claimed" the energy required to move a mass a given distance is independent of the energy source. That is a fundamental fact of physics. It will always take n Newtons of force to move y kilograms of mass a given distance under the same conditions.

Now that we've moved the goalposts from EV range to gasoline consumption, it has not been established that the efficiency of the Prius's ICE under load is sufficiently increased over its efficiency at smaller loads to counteract the additional energy needed to ascend a hill vs traveling the same distance while not ascending a hill.

If you want to achieve 80 mpg overall by coasting for 1/2 of the distance, you're going to have to do better than 40 mpg on the other half. That's all I said, and it's a mathematical fact because hopefully nobody is here to dispute that 40 mpg / 0.5 = 80 mpg.

 

So now you are (indirectly) denying that friction is a variable that can be changed or improved. Among other things, such as air drag being reduced at higher altitude.

I refuse to set aside the fact that the prime mover's efficiency at converting fuel to mechanical energy is also variable, and can be played with.

That first goalpost had already been set aside.

Another very important point, for those continuing to miss it: hills need not be symmetric, and around here, most are not. The uphill and downhill legs need to be treated separately, and it is just fine -- usually even better -- if the downhill side is longer and less steep than the uphill side.

Note that I have not been putting any restrictions on how steep the uphill side is, because I don't believe that is where most people lose the bulk of their mountain fuel economy. As long as propulsion system is running near peak fuel-to-mechanical-energy conversion efficiency, it doesn't matter all that much. It is only the downhill glide side that needs to be quite slope limited to avoid energy-wasting braking (including less than ideal regeneration). It is the downhill side where mpg usually gets tanked, often forced by the terrain, but sometimes inadvertently by inefficient driving methods.

Go back to the illustration in Post #32, a 5 mile hill, 3.8% slope, (750+1464) = 2214 Wh to climb a 5 mile segment. At 50 mph, this is a power rate of 22 kW. But note that the Gen3 Prius BSFC chart shows the peak efficiency zone going up past 30 kW, and near-peak efficiency to past 50 kW. A little re-figuring indicates that this car could efficiently climb a 6% slope, and not do poorly on significantly steeper slopes too.

But as previously described, efficient descents (mechanically gliding instead of electrochemically regenerating), are limited to roughly 2% and shallower. So the descent is allowed to be several times longer than the ascent. E.g. if one achieved X mpg on that 5 mile 3.8% climb, than glided down a 1.9% slope for 10 miles, the round trip (in elevation -- or maybe an alternate route down the same side of the mountain?) MPG could potentially be as high as 3X mpg. Or for a 6% climb and 2% descent, 4X mpg.

Now somebody should go measure X. Ideally, for both for 3.8% climbs, and for climbs half that slope.

 

I'm not denying anything. Your constant mischaracterizations are fundamentally dishonest. It makes having a civil discussion very difficult. Are we talking about flying Priuses now?

 

The round-trip EV-range limit for the Prius Prime is 43 miles. It's virtually impossible to exceed it even if you crawl on a flat road with no traffic lights.

Of course, going downhill, the maximum range is infinite, and you charge the traction battery, too.

 

The Challenge is, Pahrump to Kettlemen City, surface temps are 105 to 114 with over 5000 feet of elevation travel . You can use any route you want. Must leave on a Sunday at 11am or end of a holiday weekend over the summer months through august. Points are calculated by distance divided by hours traveled + average trip fuel mileage no more than 10 hours of travel time allowed. I'd really like to see someone break much past the 100 points on this trip. It's also a fun trip with stuff to see.

 

Et tu?

I have been pointing out necessary if unintended consequences of certain broad statements, and incorrect assumptions behind others.

For other readers, to summarize my position in this thread, I have been challenging a claim that driving in mountains or hilly terrain necessarily or inherently requires a greater amount of fuel or energy (after descent to the starting elevation) than does driving the same distance on level roads. That the hilly route must "use far more energy" than the flat route. That having to "give it more 'gas' to go up a hill" means it must be less efficient for the full round trip.

While this is a very common result, and is very often unavoidable, it isn't an inherent requirement of mountains and hills. There are such roads where a careful driver can get nearly the same MPG on the mountain/hilly route as on a flat valley route. Or even exactly the same MPG. Or even slightly better MPG. This ought to also apply to kWh/mile measures in EV mode, though since some of the most useful gasoline engine quirks don't apply to EVs, their circumstances to achieve this are narrower.

A hallmark of these equally-efficient mountain routes is a descent slope that needs little to no braking, even regenerative braking. I.e. a generally shallow enough slope to safely and legally glide down it without braking. Steep segments are allowed if short enough to not accumulate excessive speed before the rollout or uphill beyond. Slopes too shallow to glide at desired speeds still qualify, by firing up the engine part-time to maintain speed in a pulse-and-glide pattern.

For steeper descents where safety requires significant braking, fuhgettaboutit. You won't be matching your flat lowland MPG, not even with the best regenerative braking currently available. The best you can do is to minimize the loss.

Many people focus on the reduced MPG of uphill segments and declare that this kills the deal. No, this is a simple consequence of the First Law, climbing really does require extra energy. But this extra energy is stored by gravity and released later when descending. The 'conversions' between kinetic and gravitational potential energies are about the most efficient we have, much much closer to ideal than any common form of electric or battery regeneration. Pendulums are good examples of how well this can work, performing a full round trip 'conversion' every stroke.

Gasoline engines, typically quite inefficient all around, commonly have a best thermal conversion efficiency at an operating point not corresponding to ordinary highway cruising, but instead to acceleration or hill climbing. Today's more efficient vehicles are tuned and geared to bring these points much closer together than was the norm in the past. But a gap usually remains, and careful drivers can make use of it.

Other factors also come into play. Climbing up the mountain, air pressure and density fall, reducing air drag (normally the largest mechanical loss at highway speed) and saving energy. The thinner air also reduces certain operating inefficiencies inherent to gasoline engines (though not EVs). Even boosting apparent tire pressure -- not the absolute pressure in the closed container, only the gauge pressure difference between interior and ambient outside -- though a good portion of this will disappear in the time it takes the tires to cool to the reduced temperatures also normally found at high elevations. Higher gauge pressure means reduced rolling resistance, though this is a very small benefit compared to reduced air drag.

The boundary between shallow-enough and too-steep descents would typically be in the vicinity of 2% at common mountain road speeds, to possibly around 3% at expressway speeds. Practical climbing slopes should go significantly steeper without breaking the bank.

Don't give up on high MPG merely because a route has mountains or hills. While some are bad, some others are just fine, equivalent to the flats. Take up the challenge.

 

Hi ,
I hope you don't mind to share your setting on EV which could help you got up to 39 miles driving? Thanks.

 

Not sure what you mean settings. I got 42 miles the other day. After a 11 mile trip to drop off little one, I had a 31 mile trip to work..12 miles on interstate, about 12 on a highway at 55mph. Ac on. Rolled in with 0.1 mile range left

 

You certainly can't get 42 miles round-trip doing 55 mph, not to mention high A/C load. So, this is obviously going downhill. As a matter of fact, you can get more than infinite miles going downhill, in other words recharge the battery. Report the range in the other direction, which is probably no more than 15–20 miles.

You can get 42 miles round-trip, which is the maximum attainable round-trip range, but only if you stay around 30 mph and with minimal A/C load.

 

Largely flat road, not like I was going down a mountain side for 15 miles. It's Northwest Indiana, corn country. Maybe some hills but nothing obviously down hill more than up. Range going home was around 34, 31 to pick up the little one on ev with wind 11% left. but I didn't take the same route home. I stayed on interstate more cause I was in a bit more of a time crunch on way home. Either way I by no means was doing 30mph in either direction.

 

You don't need a mountain or a hill—even a small elevation difference of a few hundred feet make can make a ten-mile difference in the EV range. That's why you need to report the round-trip EV range. You are saying that your round-trip EV range was 36–38 miles, which is still too high for 55 mph driving but more reasonable. The true number is probably 30–35 miles.