I seem to recall the Continental gasoline engines used in the trucks were in the 450 to 500 cubic inch range, @meanjoe. I worked as a mechanics helper at a dairy and learned to spot trailers and one thing led to another and when a truck broke down somewhere I was the most expendable person to take a “spotter” tractor to bring in a trailer. And a common problem was dollies sinking in hot asphalt parking lots which was always assigned to me.
One of the biggest problems I see with using engine power downhill so you can coast uphill is that you have very high peak speeds combined with a low average speed. The power needed to overcome air resistance increases eight fold for every doubling of speed and the energy lost to air resistance per mile quadruples with every doubling of speed.
Using the engine for the climbs not only puts the engine in its efficiency sweet spot, i.e. it’s not so under loaded that it is inefficient, but also gives you the lowest peak speed for a given average speed and thus the least energy lost to wind resistance.
On a bicycle, anything you do to go uphill as fast as possible will increase your average speed much more than high peak speeds on the downhill part will. Yes, get a running start on steep hills, but don’t overdo it, better to save your energy so you can give the next climb everything you have than it is to expend all your energy setting a speed record on the preceding downhill.
The power needed to overcome air resistance increases eight fold for every doubling of
speed and the energy lost to air resistance per mile quadruples with every doubling of speed.
True, but on a downhill, much of that “power” comes for free from gravity.
We each make our judgement on this question based on personal experience and my experience with hybrids is insignificantly minimal while my experience with under powered buses, trucks, farm tractors and bicycles is somewhat more extensive. Northern Alabama and eastern Tennessee highways long ago added a third lane designated for trucks ascending hills and on a recent trip across the Montana, Idaho and Washington I found a third “slow” lane was added on long ascents on the Interstate. I noticed that truckers familiar with the road would use the race down one hill to better climb the next when no sharp curves would force them to brake before ascending. I was amazed that heavily loaded tractor trailers with 24 wheels(single trailer) were reaching 85+ mph on 6% downhill grades and able to ascend the next hill at speeds of 60+ mph. Certainly cutting the elapsed time was critical to the drivers but I can only imagine the fuel needed to operate that much gross weight up several miles of steep grade would be significant. And, whether the truck ascends the hill at 60 mph after taking a plunge down the preceding hill or clawed its way up at 15 mph the engine would be operating at near wide open throttle. The fuel savings would seem to be significantly less at 60…
True, but on a downhill, much of that "power" comes for free from gravity.
Gravity is not a source of “free” energy, it returns stored energy. The people skiing downhill are not really being powered by gravity, they are ultimately being powered by the chair lift that takes them back to the top.
Gravity is not a source of “free” energy, it returns stored energy.
Yes that’s definitely true.
However, the context of the above discussion assumed the vehicle was already at the top of the hill and the “power” needed to double one’s speed downhill.
But we are not considering a single event, @B.L.E. The issue is what would be the most economical technique to operate over mountainous terrain. Which reminds me of an old truckers legend. Supposedly an independent trucker in Georgia couldn’t afford a more powerful truck to get him through the mountains profitably but his friend had a well stocked junk yard and a well equipped shop so he mounted a Ford V-8 under the trailer and exchanged the front trailer axle with a tractor axle and joined the two through a clutch. With the throttle locked at wide open, operating a cable would engage the clutch to spin the engine over and start it and add power to the trailer axle adding enough power to get over the hill at speed. When cresting the hill the ignition to the rear engine could be cut and the clutch disengaged. It made a great tale and several truckers swore it was true. There were a few occasions that I have wished that a V-8 were behind me. When you watch the tach drop on a long grade and know that it might be necessary to double clutch to get the rear case into low range. And, if the shift is missed it means coming to a stop, then smoking the clutch to get moving again at about 3 mph with the tachometer in the red.
The idea about using gravity kid of depends on where you live. Here in the east, it works great. The mountain grades are either shallow and long or short and steep, but in either case, you don’t build up that much speed on the down hill and what you do build up will help get you up the other side.
Now in the western states, its a completely different story. The grades are too long and steep although you can build up a little speed as you approach the bottom of one grade to help get a running start up the other side.
The flaw in this theory is when your a 17 year old kid with only a years driving experience and your dad lets you take a segment of the driving, pulling a single axle, 17’ Airstream (actually an early 50s model Silver Streak, only one year of single axle then went to double axle) that has a tendency to whip badly and you think you are at the bottom of the grade, but you’re not and the trailer brakes stop working as you approach 100 mph. It was the longest three miles of my life, and the longest I ever held my breath. It was also the whitest I ever saw my dad turn and the last time he let me drive his car or pull that trailer.
@RodKnox: I was in a similar situation, and was sorely tempted.
I was transporting some guy’s beat-up 24’ box truck (very used former U-Haul), stuffed to the gills, plus about 7000# in the form of a 3/4 ton truck on a dolly.
Climbing the hill between Alomogordo and Las Cruces, it became apparent the truck was unhappy: making about 25mph up the hill, it would make it about 1/2 mile at a time, then pull over due to overheat.
Since the 4x4 had a transfer case, we didn’t drop the driveshaft. I thought about using the pickup as a “pusher” to get us up the hill; if I had a spare driver, I probably would’ve. (Unfortunately, boss man was the only other driver, in his own (newer) truck.)
The fastest way to drive through mountains is to drive fast up the mountains, and drive fast down the mountains. If you’re trying to save gas, you only accelerate going downhill, and you let the vehicle lose speed going uphill. See the difference yet?
I really depends on the vehicle. On an overpowered or very light vehicle, you pretty much want to do the opposite, accelerate or hold a steady speed uphill and just let the vehicle coast downhill keeping the speed more or less steady and avoiding high peak speeds that cause a lot of energy to be lost to air resistance.
That pretty much has been my hill driving strategy on my Kawasaki Ninja 300 motorcycle, resulting in 75 to 79 mpg.
Here’s a link to the Ninja 300 page of Fuelly.com so you can compare my gas mileage to what most people get on this model of motorcycle.
http://www.fuelly.com/motorcycle/kawasaki/ninja%20300
I’d bet, even on a Ninja 300, you’d get better fuel economy if you let your speed drop going uphill, and accelerated back up to speed going downhill. I’m not recommending you actually do that though. It would be dangerous. At 75 MPG, there is little reason to try to eek out a little more fuel economy. However, the bike is still going to use more fuel holding a constant speed going uphill than it would if you allowed it to decelerate, and I wouldn’t want to try coasting downhill on or in anything that is less aerodynamic and light than your Ninja.
I love riding my 750s through the mountains. On a cruiser with a windshield and other gear, wind resistance keeps you from going downhill too fast, and a throttle lock on a cruiser works just about as well as cruise control works in a car. Sometimes I wish I could install a throttle lock on my car instead of cruise control. I bet I would save more fuel with a throttle lock than with cruise control.
You can attempt to rationalize your thoughts at the best mileage all you want. But for some reason I will continue to trust the mileage calculations that I have personally obtained. Sorry but the scientific method trumps your feelings nine ways to Sunday.
One final note, when you are traveling down a mountain with 10 miles averaging 7% grade, braking isn’t optional. It is what keeps you from becoming a grease spot. So I brake, I stay alive, I don’t endanger others by racing up behind them (when braking becomes less effective) and I still manage to obtain superior MPG numbers.
So far, you’ve made exactly 5 posts on this site. You signed up specifically to comment on a fairly obscure thread, and have done your best to trash-talk long-standing members with your five total posts. Oh, and you’re factually wrong.
So, what’s the deal? What about this thread of little merit inspired you to sign up and do a “drive-by?” Was a family member killed by a hypermiler or something? Or do you just derive some thrill from being rude to people you don’t know?
Up 'till an hour ago, I thought you’d departed for parts unknown. I’d just like to point out that doing so is still a viable option…
Per the OP’s original question, I can imagine that how a particular car’s ignition system retards the spark when the engine is loaded might give better mpg on the return trip. If going up the 1% grade allows a 30 degree advance but going up the 6% grade pulls it back to a 5 degree advance, this might make enough difference to favor the return trip. I’d have to see a controlled test for the final answer.