Hill climb: 80% Load or "slow speed bleed"
 

Here's something I find contradictory and hard to fathom... Climbing a hill, I find the most fuel efficient method is a gradual speed bleed. Yet we are also told that the most efficient way to accelerate is at ~80% load. These two seem contradictory- my own findings are that if I accelerate up a hill it is less fuel efficient. Can anyone explain?

This is indeed a conundrum I face daily.

What I have found is if you can stay in overdrive, do so. This will of course entail a speed bleed, otherwise you will downshift and it's all lost.

Accelerating I would think take somewhat less power than going uphill because you are not fighting to go uphill. I can push my car up to probably 3 mph with leg power on a flat surface, but I can't budge it on even the slightest incline.

You have to look at the whole trip, not just the momentary mileage while climbing the hill. Watch a trip average mpg gauge, if you have one. Try the methods and see which one costs you the least.

I can say that in the cars I've tested, 80% load and low rpm works best. It means slight acceleration, so I do a puuuuuuuuulse and mini-glide.

If the hill is the perfect grade then you can maintain 80% load in top gear.

If the grade is less, maintain 80% and increase your speed.

If the grade is more bleed your speed and maintain 80% load.

You have to consider the load and the amount of time it takes to reach the peak of the hill. Least time in top gear while maintaining 80% load is your objective.

If you can't maintain 80% load in top gear, then try the next highest gear with 80%. The 80% is your best efficiency but the longer it takes you to climb the grade the greater time spent at lower mileage.

regards
Mech

There is a hill I commute up and over in my automatic car about one hundred times per year. It takes about 30 seconds to travel up, so it is quite long. My current tactic is to accelerate before the bottom very hard (about 80-90% throttle) to reach the highest gear (which kicks in at about 70km/h). By the time I've hit the slope of the hill I'm at about 90km/h and thereafter gradually lessen the pressure I place on the throttle to avoid it shifting down. If maintain that 80-90% throttle, the car still loses speed up the hill and it drops a gear to try compensate. Half the time, I am able to avoid it shifting down, and the other half the time, once it has fallen below about 65km/h and 20% throttle, it shifts).

Surely easing off the throttle (load) whilst travelling up the hill and losing speed, rather than maintaining 80%, in order to prevent it shifting down, is better?

The slow speed bleed it what works best for me. Then again, I have an automatic, so I have little choice - kick the load up that high and the auto grabs a gear or two.

Accelerating up a hill from a dead stop is the biggest conundrum I face.

Do you have a device to monitor the load?

I have carefully studied how my 1998 Toyota Camry behaves when ascending a hill, and it won't downshift until 90%+ load is reached or RPM drops too low. It may drop out of closed loop mode on a very long and sustained 85+% load. I keep the load right at 80% on sustained hill climbs both to keep it in top gear, and keep it in closed loop mode. If the hill is very steep, then the car may downshift once RPM drops too low. It doesn't take much of a hill for the relatively tall-geared, heavy, and modestly powered Camry to loose enough speed that it drops down a gear.

The answer to the question is not either 80% load or slow speed bleed, but both. Sustain 80% load, and if the hill is too steep, you will slowly bleed speed.

Notice that as RPM drops, and without moving the throttle position, the engine load will increase. As RPM drops, smaller throttle openings are required to maintain the same engine load. That means you will need to constantly adjust the throttle in response to the engine RPM increasing or decreasing. Less throttle for lower RPM, more throttle for higher RPM; same 80-85% target load.

Ah, this is where I went wrong. I always erroneously assumed load correlated with throttle position. This is what I attempt to do anyway; I maintain the maximum load the automatic transmission will allow before it shifts down, which is just another way of stating this thread's newfound consensus, at least for automatics.

To be honest, I had taken it for granted that an auto and DWL were so mutually exclusive, that I never tried. Yes, I believe that my UG will calculate load. I'll have to experiment with this a little bit.

Around 85% throttle, most engine control computers transition from closed loop, stoichiometric, to open-loop, much richer.