Is my hill climb theory sound?

[user removed]

The best strategy I have seen in hills. This is assuming the hills do not exceed the grade threshold where you can not coast downhill without exceeding the speed limit to the point where you are risking a ticket, or seriously aggravating the drivers around you.

It's much like a roller coaster which can negotiate several hills on inertia alone.

Powering uphill you have the opportunity to use your engines best BSFC to add inertia to the vehicle. Highest gear that allows 80% of max load gives you the most power for the least energy, at about twice the typical average efficiency.

While that consumes more energy than just driving normally on flat ground, it more than makes up for the additional consumption by increasing your energy stored in the mass of the vehicle as you increase its altitude.

Then go engine off on the downhill coast and you recover that stored energy as speed maintained without fuel consumed.

The best tactic is to determine the speed you need to crest the hill, to reach the bottom with your desired highest rate of speed (without risk) and to let that peak speed carry you uphill as far as practical. Then you apply your maximum efficiency pulse to repeat the process reaching the peak of the next hill at the ideal speed to reach your maximum speed at the bottom of hill #2.

When the hills are the perfect grade, one which allows you to stay close to the same average speed at any point, either going uphill, or downhill, then you do not lose energy to aerodynamics at higher peak speeds at an exponential rate.

Hills slightly less steep than the ideal grade require some speed variation below your average, while hills above the ideal grade require higher speeds at the bottom.

Another thing you can do when the hills are greater than the ideal grade is to use accessory loads to lower your peak downhill speed. Things like AC (if its really hot) and alternator (if you can control its cycling) can be used to reduce your peak downhill speed to an acceptable level. While that is less efficient that not using them, its more efficient than having to change your strategy and get a ticket or ticking off everyone around you.

regards
Mech

[fidalgoman]

Ah, theory vs reality. Sometimes the same, sometimes not. I have a little puzzle. An airplane in Alaska is picking up mail from a small remote village and needs to fly from the city to the village and back in the same day. The airplane cruises at 150 MPH and burns 10 gallons per hour. The next city is 300 miles away. If there was no wind the trip would be two hours each way and burn twenty gallons each way. For a total flight time of four hours and 30 gallons burned for an average 15 MPG. If for example there was this day a fifty MPH headwind that remained steady the whole day, then what would the stats be? It's similar to our hill if we try to maintain some speed limit. Try and figure it out. It's not hard and the answer is revealing.

Back to our car going up and down a hill. The internal engine drag is proportional to the square of the RPM (but there is a lower efficient limit) and the computer will enrich the mixture above a certain throttle setting (and at idle to run smoothe) to help cool the engine internally. So it isn't a simple intuitive call as far as the uphill portion of the hill is concerned. You could plot a graph if you knew the exact data and exact changing conditions. I think however to get the max MPG up hill you will need to know in general the best operating range of your engine as far as throttle position versus RPM versus Speed Versus current gear ratio. Going down hill you will probably peak the unit unless the grade is gradual. IMHO that boils down to using general principles of high mileage and learning to peak your instantaneous MPG via a ScanGage, etc.

[redpoint5]

Originally Posted by Bicycle Bob

However, I also disagree with DWL, as long as one stays away from throttle openings that go to a rich mixture for power, abandoning economy. Bike racers work hard going uphill, but rest when they would only be fighting the air.
Perhaps someone with a scan gauge will do a comparison between total consumption going over a hill using DWL vs constant speed to the same average speed.

Appreciate the thoughtful response Bob. Intuitively I figured going up a hill quickly on a bicycle is efficient. I would imagine a racer who dominates the uphill portion of the race will likely win. What is DWL though? I hope to get a splitter for my OBD2 port and then pick up a scan gauge soon. This will certainly go on my list of things to test.

Originally Posted by CapriRacer

But aerodynamic losses at slow speeds are pretty small - which probably why people are arguing the point (the principle doesn't manifest itself clearly). I think you'll find that it becomes easier to understand if you think about going DOWN the same hill. Clearly, using the brake to limit the speed ALWAYS results in energy loss.

I tried to isolate the concept by ignoring certain factors such as wind resistance. How one goes down a hill seems mostly with respect to how to most efficiently go up a hill. For the purposes of my concept, I imagine a hill of infinite height. The question then is how do we get up the hill the furthest with the least amount of spent fuel.

Obviously this is a complex question, as many sources of drag must be taken into consideration.

[Weather Spotter]

buy a scangage off Ben (fire sale before he moves), you need to move fast but should get a good deal.

[Bicycle Bob]

DWL = Drive With Load, which translates to "keep the throttle opening as constant as possible and let the speed vary." However, wind resistance goes up as the square of speed, so for the least energy use at a given average speed, you keep the speed constant. Of course, if you know you'll have to brake on the way down a hill if you top it at your best speed, it pays to back off a bit so you won't have to brake as much.
A rowboat has a slight variation in speed as the rower moves back and forth, and the IOC banned the sliding rigger that allows a more constant speed.

[Appletank]

(searching about hill climbing on google again)

Hey Redpoint, I think I agree with you that a spherical car hill climbing in a vacuum would ideally be using maximum power to go up an infinite hill to minimize the effects of gravity. And vice versa, engine off on an infinite downhill because gravity can do all the work for you.

I believe the conflicts with IRL are: assuming you never have to go downhill, assuming you're starting off stationary, and other inefficiencies from the engine working too hard and going too fast.

If you have a decent speed before you hit the hill, and the hill isn't too high, then you functionally don't have to change throttle as momentum alone can bring you over the hill. Losing speed/DWL is thus ok because fuel/kinetic energy is being converted into potential energy, you reduce air drag, and the energy is repaid on the downhill gravity assist. Thus, you end up getting back to your initial speed, even without additional gas if you have the space. Using too much power puts the engine into too inefficient an operating zone to come out ahead of minimizing gravity's effects, such as running at 5K RPM instead of 3K.

I think your scenario is most applicable if one finds themself at a stop sign at the bottom of a hill. Going over a mountain, for example. Cars are all inefficient in the first couple of gears; you want to spend as little time possible stuck in them, even more so if you're having to fight gravity on top of it. Accelerate up until: aero drag starts becoming dominant, which leads to the engine power required being too inefficient to balance out the effect of fighting gravity, air, and friction to keep accelerating, or a downhill coming up.

In which case, you try to lock the throttle into its efficient operating zone, aka DWL, until you reach the crest of the hill. Here you can let momentum take you the rest of the way over, after which gravity takes over.

TLDR: I think High power up a hill is mostly applicable from the speeds 0 - 30 mph, depending on % grade, in which case DWL/steady state/gliding may be more appropriate.