No matter how careful I am, it's hard for me to get lower than 130% of the fuel consumption reported by the manufacturer. Perhaps I can do 120% of the reported consumption at 90 km/h, but I can hardly get lower than 140% on the highway (130 km/h) and in city traffic.
Actually, it depends. Modern cars do better in rolling hills than on perfectly flat land, since they can cut fuel entirely on the downhill, and the climbs don't require that much extra fuel over just maintaining speed.
> Modern cars do better in rolling hills than on perfectly flat land, since they can cut fuel entirely on the downhill
This is physically impossible. Even if the car was perfectly efficient at overcoming the force of gravity on the uphill portions (which it is not), you cannot do better than flat terrain (unless by hilly you mean 'net downhill').
The StackExchange post that he linked lists several cases where it's possible. I was skeptical too - I was a physics major in college, I know all about potential vs. kinetic energy and rolling resistance. But engines are not perfectly efficient: you're going to lose some energy to internal friction within the engine, and to combustion efficiency, and to transmission losses, and to a number of other effects which vary based on speed, RPM, and torque. Add in an engine computer that's trying to minimize these effects based on inputs, and actual fuel efficiency will vary significantly from the simple "chemical energy in = kinetic energy out" model.
I've also observed the effect that the OP mentioned: in my Honda Fit, I get a fairly consistent ~45 MPG on 101 (completely flat highway terrain), ~40 MPG on 280 (rolling hills, highway), and ~43 MPG on 84 (across the Santa Cruz Mountains; ascends 3000 ft. in a series of tight switchbacks and then descends 3000 ft. in a wavy line). My working hypothesis is that the descent on 84 is consistent enough that the engine computer can idle the engine for all of it, burning no fuel - this is consistent with the car being nearly silent on the downhill, and of the instantaneous mileage display being stuck at its max of 80 MPG. Probably also helps that speeds are much lower, so there's less air resistance.
Anyway, the point is that cars are complex enough that the simple models you learn in Physics 101 don't really hold.
Engines are most efficient at WOT. Hilly ascents at WOT will burn the fuel more efficiently (even though you're using more). Add a descent in which the computer stops providing fuel and it's possible that you could get a better mileage.
It's known that you can get better fuel economy by oscillating speed on flat ground - using WOT to gain speed, coasting back down, opening up WOT again. This is because the engine is most efficient under load and the average consumption works out better. Adding in the hills would average this effect.
Generally not wide open throttle - see the efficiency maps I linked to in another reply. Generally peak is at somewhere around 1/2 to 2/3rds maximum output.
Wide open throttle and percentage of maximum output are two different concepts. An engine operating at partial throttle has higher pumping losses (pulling air past the partially closed throttle plates).
An engine operating at WOT but still producing a fraction of its maximum output is certainly possible (and generally more efficient than an engine producing the same output at a partial throttle setting). A diesel engine is almost always in this configuration (metering fuel but generally not restricting air with a throttle plate) and a gas engine at low RPM but WOT is also in this state.
If tuned for efficiency in that config, it can be very efficient and is how many piston airplane engines are regularly operated where fuel consumption is an important part of range (and operating economics).
I think you should reconsider "physically impossible". Sure, in some idealized situations, the statement you're responding to would be wrong, grievously wrong.
But the actual situation has two novel features: (1) the engine uses up power just by being on; (2) you must maintain a minimum speed in order not to block traffic.
If you put these constraints in, you can see that it's possible for a pulsed-acceleration strategy to win. You put in a pulse of acceleration to get up the hill, turn the engine off, and coast down, maintaining enough speed to keep up with traffic.
Now sure, without the minimum-speed constraint, you'd probably be better off flat, and possibly using a pulsed-acceleration strategy.
I haven't done the math, but I think it would be reckless to say it's "physically impossible" for all values of static energy use and slopes.
Take a look at the stack exchange answer the commenter above linked to.
I read years ago that pulsing was the most efficient strategy. Engines have very different efficiencies at different rpms and power settings. The idea with pulsing is to try to operate it at its most efficient rpm and throttle setting, which of course rarely corresponds to the speed over flat ground you'd like to be travelling at. Hence pulsing.
Yep, and this is why moderate hills are better. It's essentially constant speed pulse-and-glide. Just building and releasing potential energy rather than kinetic.
Not at all, in many vehicles. 6th gear in my car is quite long (Turns 1950RPM at 75MPH) and I have no problem maintaining speed on highway grades.
Even if you do downshift that doesn't necessarily mean you're being less efficient either. Being a bit higher up the rev range may actually put you in a better spot on the efficiency curve for a given power output.
ICUs are more efficient if you open the throttle more (up to a point...). So it could be possible that the extra potential energy you store going uphill at better efficiency makes a difference.
I recently purchased prius which has much better mileage than my previous car so I'm monitoring each of my trip.
I actually observed this and it is true in my case; I do get better mileage when choosing a route through a canyon than when using regular freeway.
I suspect though that the real reason is that it basically affects how I'm driving. When I ride in a canyon I can't accelerate much because there are constant turns, while on straight freeway I most likely will accelerate.
This also would explain why most of the time I get better mileage in traffic (the EV is used most of the time).
I cannot imagine a cyclist imagining this to be true!
Obviously aerodynamics play a part, however the cyclist's 'engine is cut' on the downhill and on big cross country roads no brakes need be used on the descents. When the 'engine' is on it goes at a certain not-quite-racing amount whether up hill or on the flat. But place a hill in the way of A to B and vastly more 'fuel' is used than A to B 'going the flat way'.
Consider a different visualization: what if instead of a hill, you were crossing a valley? Starting at the top of the hill, you coast down and make it most of the way up the other side. Are you certain this will take more energy than pedaling the whole way across the flat?
What if you can start by coasting 90% of the distance with zero expenditure of energy? What if it was 95%? Now assume that your chain is extremely rusty (low efficiency drivetrain). Would it be possible that the energy to go the short distance up the remaining hill (after coasting downhill and halfway up the other side) might be less than overcoming the friction of pedaling the whole way?
I'm not certain that they occur on a bicycle in the real world (perhaps you need a custom crafted rusty chain that has essentially constant high losses regardless of pedaling speed), but I think there are cases where the variable losses due to friction might make the hilly case more efficient than the flat.
(Yes, I realize that chain friction is not the dominant factor in the real world. It's a stylized example.)
The best terrain I've ever ridden a bike on was a well-paved path of little resistance over forrested dunes. You start on top of a dune, pedal a little on the way down (not a lot of effort) and arrive on the top of the next dune with nearly zero speed. Perfect coasting and a cool breeze most of the time.
Even a rusty chain is ridiculously efficient - in the high 90%'s. An ICE and drive chain is pitiful in comparison, so let's substitute the chain for a missing rear axle nut so that, when pedalling, the wheel pulls to one side and creates tangible frictional losses.
I grew up in a very hilly area plus I have done a few continental rides where hills can be graded nicely and go on for 10 - 20 miles with little change of gradient. The hope to coast up the other side of the valley never really works out as well as one might hope, with a road that goes down a big hill you are invariably pedalling on the lower reaches of the down hill part, even if you were doing a good 30+ mph at the top. This can be due to the valley heating during the day giving a prevailing headwind due to air expanding.
Meanwhile, in the steep hills I grew up in, you can get a measured 40+ mph, conservative guessing (I would be boasting if I mentioned top measured speeds as they can be quite incredible but 40 mph is the town limits speed and it is cool to exceed it on a bike). Clearly this speed scrubs off with aerodynamics not being favourable. So you really do not blissfully roll up the other side, roller-coaster style.
On a daily basis I go over an arched bridge that does have a hill leading in to it. So you can coast down the hill, up over the arch and on down the other side. The fun is to not pedal over the bridge, to be in 'roller-coaster' mode. This feels good, and easy, but there is a psychological factor going on here. One's brain is distracted by the 'roller coaster' (or traffic) and not really noticing how much of a slog things are.
Essentially the only way things could work out anywhere near how you imagine is if there is no atmosphere. The cost of speed is aero and you just do not get the free ride up the other side of the valley, period.
If it was quicker/more efficient to go up and down hills we wouldn't have land speed records be they by bicycle or rocket car set on dead-flat courses, would we?
With the sibling reply regarding undulating paths, that is part 'mind trick' as it is mentally stimulating going up and down like that. It is just an illusion...
However, I can in part rescue your line of thinking on this - muscle groups... I actually like hills and you can use your arms, legs and plenty of muscles in between to 'conquer' a hill. On the flat you are more likely to just spin (unless sprinting). Maybe if you factor in the baseline power your body needs for pumping blood around, maybe that could make things 'possible'.
But the more I think about it - no... Look at motorways and trucks. Trucks are more efficient than cars even if they have lots of wheels and no obvious aerodynamic considerations. Lorry drivers do not choose the undulating 'A road next to the motorway when one is available. They stay on the motorway doing their most efficient speed (as that is how to make a profit). Hills that any car or lorry could go up are cut through with motorways, to make things as flat as possible.
> If it was quicker/more efficient to go up and down hills we wouldn't have land speed records be they by bicycle or rocket car set on dead-flat courses, would we?
That's a total non-sequitur. Land speed record attempts happen in flat places to keep them from turning into AIR speed record attempts.
Nope. If your vehicle goes over a hump back bridge then maybe, but not if you go down one side of a valley and up the other side, obviously with an inverted wing shaped profile to the vehicle?
Did you read the linked stackoverflow discussion? I thought it made a convincing case that greater efficiency was possible across a valley given a sufficiently-aerodynamic low-rolling-resistance vehicle with an engine of non-constant efficiency. Would your intuition also say that analysis is wrong? Is it?
I suggest you enroll in a physics course, this is physically impossible for the average case of net-zero displacement at a near-constant speed (i.e. roundtrips).
There are scenarios where it's true, but there are a lot of factors involved (drag, power/efficiency curves of the engine, etc..., net-negative displacement, etc...)
My Subaru Outback has been a nice surprise in this area.
The combined highway/city mileage on the sticker was 24 mpg, and I've been getting 27 or 28 mpg for the entire 2 years I've owned it. For a while I was commuting between Denver and Boulder (about 45 miles each way) five days a week and taking it up to the mountains for hiking or skiing every weekend, so it's been over a wide variety of terrain.
I'm using the numbers on the in-car, computer, though, so maybe I should double check...
Car computers are notoriously inaccurate. I have observed errors up to 12% in vehicles from multiple manufacturers. Car computers can be helpful in a relative sense to tell you whether one style of driving is more or less efficient than another but they're basically worthless for absolute calculations. The only reliable way to accurately calculate fuel economy (in mpg units) is to divide the number of miles since you last refueled by the number of gallons to fill the tank.
