Gas mileage and a vehicle's efficiency

Factual Questions
1

Hello,
I have always wondered about my driving and how much my car uses gasoline, especially in this “climate change” age. With today’s engineering and aerodynamics my assumption is that all cars perform the same. I have read that cars are at their most efficient with gasoline in a straight line with no incline or decline at 48 MPH. This also excludes an 18 wheeler in front of you at the same speed. Is this true or is it a falsehood?
And with the introduction of electronic vehicles is this “fact” still true in which you will drain the battery most efficiently at the same rate of speed?

Thank you…

2

This isn’t true at all.

3

Heck, I can change my car’s performance with the push of a button. “Eco” mode, standard mode, “Sport” mode…

Why would anyone assume that a Jaguar would perform the same as a Kia?

4

There are a couple of opposing factors at play here:

  1. the amount of mechanical energy pissed away per mile of vehicle travel. This is the rolling resistance of the tires and the aerodynamic drag of the whole vehicle, with the latter being most of it. Aero drag scales with the square of vehicle speed: a car traveling at 80 MPH has about four times the aero drag of a car traveling at 40 MPH, and so will require about four times as much energy per mile.

You may however have noticed that your fuel economy at 40 MPH isn’t four times as good as it is at 80 MPH. And that’s because…

  1. An engine is less efficient at light load (this is particularly true of gasoline engines, which have higher peak combustion temperatures than diesels (and so lose more of their combustion energy to the cylinder walls through heat transfer before it can be converted to mechanical energy) and also have to suck air past a mostly-closed throttle plate (which takes a surprising amount of effort). This is why automatic transmission quickly shift up and up and up to the highest gear the engine will tolerate: to get RPMs down and (more importantly) to get load up.

So at 80 MPH your engine is operating with decent efficiency (cuz there’s substantial load on it), but you need to burn a lot of gas per mile to make up for the high aero drag losses. At the opposite extreme (10 MPH), there’s very little drag to contend with, but most of the engine’s fuel consumption goes toward simply keeping itself running, and you will get really crappy MPG. The middle ground, typically somewhere between 35 and 55 MPH, is where those two factors cross over and the typical passenger car finds its best fuel economy.

This is an odd assumption, given the wide variety of passenger car configurations (and especially since the MPG figures for such vehicles are public information). It ought to be quickly obvious that a Honda Fit and a Cadillac Escalade, with their very different aerodynamic properties and engine displacements, are going to give very different results.

I don’t have numbers handy, but my suspicion is that an induction motor paired with a VFD has good efficiency at all but the very lightest of loads, which would mean that the best vehicle range comes at a much lower speed (where aero drag is lower) than for a dinosaur-powered car.

An interesting aside:
some time ago in one of XKCD’s “what if” scenarios, Munroe pointed out that miles per gallon, if inverted to become gallons per mile, basically represents units of volume divided by length - which is simply area. In effect, gallons per mile represents the cross-sectional area of a stream of gasoline used by you car as it moves down the road. Example:

30 miles per gallon = 0.033333333 gallons of fuel per mile traveled

0.0333333333 gallons per mile = 0.004456019 cubic feet of fuel per mile traveled

0.004456019 cubic feet per mile = 8.43943E-07 cubic feet of fuel per foot traveled

8.43943E-07 cubic feet of fuel per foot traveled = 8.43943E-07 square feet

8.43943E-07 square feet = 0.000122 square inches

That’s a circle 0.0124 inches in diameter. So if your car gets 30 MPG, you can imagine a stream of gasoline 0.0125 inches in diameter stretching the length of your trip, and that’s how much your car will use during that trip.

5

In competitions for the most miles travelled on a gallon of fuel, the technique is to stay in the highest gear possible at the lowest speed that the engine will still run. This is not good news for the engine or the transmission, but extraordinary MPG’s can be achieved.

6

Machine Elf is spot on. The lack of a throttle is one reason diesels tend to have such good real-world fuel economy compared to a gas engine of equivalent size or power. puttering around at low load with a gas engine has the throttle plate mostly closed, and the power the engine needs to use to pull air in through it (called “pumping losses”) puts a real hurt on the engine’s efficiency.

IIRC a gasoline engine is running at peak efficiency when operated at wide-open-throttle (WOT) at the RPM where peak torque output occurs. needless to say, practically no car on the road is being operated like that at any given moment.

7

And the other big factor in fuel consumption is braking. With ordinary brakes, every time you stop, you throw away all of the kinetic energy you had by turning it into useless heat, and then when you start up again, you need to spend that same amount of energy to get back up to speed. This is a major reason why most cars get better overall efficiency on the highway than on city streets: On city streets, you often have to stop for stop signs and traffic lights and throw away energy.

With electric (or hybrid) cars, though, you can use regenerative braking, which lets you turn most of that kinetic energy back into electricity that you can store in the battery. So frequent stops aren’t a big deal, which is one of the reasons why hybrids outclass pure-gasoline cars so much in city driving.

8

I thought competitors generally used the “pulse and glide” technique, i.e. accelerating at full throttle and then coasting in neutral or engine off? The efficiency of the engine itself is highest at full throttle (as already explained above).

9

This ties in with Chronos’s comment about regenerative braking (bear with me here…).

Regen braking is enabled by the ability to store energy in the battery, but that storage capability also allows you to adjust the engine’s power output as you see fit. AIUI, part of the fuel economy of the Toyota Prius (and probably other hybrids) is that it can run the engine at elevated load (for efficient operation), and take the excess engine output and stuff it into the battery for later use. So highway operation, instead of having the engine operating at steady load, might have it operating on a cycle of high load (with the excess power going to the battery) followed by light load (with makeup power coming from the battery). The engine may be inefficient during the light-load phase, but the overall average of that duty cycle is a higher efficiency than you would achieve with steady operation.

In the fuel economy competition strategy you describe, the storage “battery” is the mass of the vehicle itself, storing kinetic energy: rather than running the engine at a steady part-load, you run it at high load for a time, store the excess energy (that hasn’t already been consumed by drag) in the mass of the vehicle, and then shut the engine down while the mass of the vehicle drives it forward. Ideally the engine is very small so that it takes a decent amount of time to accelerate the vehicle from its minimum coasting speed to its maximum driven speed.

ISTM you could achieve something similar on the road:

Contrary to this, I expect you could see better fuel economy on an idealized cycle of upslopes and downslopes: you cruise uphill at modest speed with high engine load, storing the excess energy in the increased gravitational potential of the car, and then take your foot off of the gas for the downslope, with said downslope of a magnitude that maintains a modest speed without requiring any use of the brakes. The engine isn’t wasting a lot of fuel as it idles during that descent, and the improvement in efficiency during the climb (maybe the engine gets 20% efficiency instead of the 5% seen during a level cruise) more than makes up for it.

10

A good primer on the techniques used by “hypermilers” to get the best fuel economy out of any given car:

11

The obvious exception is hybrid cars, which try to run the engine at the most efficient output, or not at all. When the engine running but its full output isn’t needed, it’s still run at the full output, and the “leftover” power is used to power the generator. (Not all hybrid cars do this, but the more efficient ones do.)

12

With my wife’s Camry hybrid - it micromanages energy so we get pretty much highway gas mileage in city driving. A lot of this has to do with regenerative braking. It also uses the electric motor and battery as an assist for acceleration - electric motors put out power through a much wider range of speeds, so the gas engine is not burning a lot of fuel trying to accelerate inefficiently at low speeds. (It also uses a continuously variable transmission, I think - making the engine even more efficient). Finally, because the gas engine is mostly used for cruising (and charging), rather than having to do the heavy lifting of accelerating all by itself, it is smaller - 1600cc rather than the 2.4 to 3 litre typical for a decent-sized car. And so, with a smaller engine running at full power, it is also more efficient when needed.

13

the Camry Hybrid used a 2.4 liter engine up until 2012 when it was bumped to 2.5 liters.

14

Here’s a good example of a Brake-Specific-Fuel-Consumption (BSFC) plot that illustrates this point well. For best engine efficiency, you generally want to operate as close to the BSFC “bullseye” as possible.

In the linked (1.9L Saturn engine) plot, assuming, for example, due to aerodynamic drag and rolling resistance, it only requires a constant 20hp of engine output (blue line, right side of the plot) to maintain the current speed selected. Tracing this line back to its point of maximum efficiency (where it intersects the pink line, and gets the closest to the red bullseye in the middle) is right around an engine output of 1400 RPM. Which means that a rather high gear (and pretty open throttle - about 65% of max) needs to be selected to stay as close as possible to this region.

15

You may be able to find efficiency curves for your car online. Many of the newer add-on computers generate them. They are popular for enthusiasts to post in car forums, at least for cars where mileage is one reason they are purchased.

I have the chart for my Scion xB, a very boxy vehicle. The mileage peaks at around 35 mph then begins dropping. It is still better then the other cars on this particular graph. I average 37 mpg in the summer. Taking trips does not help at all, the speed is too high.

In my case, anything over 60 actually lowers my overall average. I have taken a few short trips on back roads (50-55 speed limit) and made it into the 40 mpg range.

Dennis

16

Many hybrids like the Camry use an Atkinson cycle engine. That variety was developed back in the late 1800’s but revived for it’s efficiency (gets more out of the heat generated but this is a better explanation - http://blog.caranddriver.com/what-is-the-atkinson-combustion-cycle-and-what-are-its-benefits/ )

17

Could you please elaborate on this ? I am used to thinking that higher hydrocarbons (more carbon atoms) burn hotter. I would have guessed the diesel adiabatic flame temperature will be higher ? Both are burning at stoichiometric air, right ? Is the lower temperature of gasoline flame due to EGR ?

18

Isn’t diesel mostly hexane, and gasoline mostly octane? That would indeed make gasoline a higher hydrocarbon.

19

no, both are not running at stoich. Gasoline engines have to, due to their reliance on spark ignition; gas engines are homogeneous-charge spark-ignition engines. if the mixture is too rich or too lean, the spark can’t reliably ignite it.

diesels are stratified-charge compression-ignition engines; they rely on the heat of the compressed air charge to initiate burning of the fuel immediately as it’s injected. they do not rely on a stoichiometric air:fuel ratio to ensure ignition reliability. diesels almost always run an overall lean air:fuel ratio; as high as 100:1 at idle. even a heavily loaded engine belching black clouds is running lean; the soot formation is from the uneven mixing of air and fuel. also, the high compression ratios/chamber temps of diesel engines means they generate more oxides of nitrogen (NOx) and thus run way more EGR than gas engines. In fact, modern diesels have cooled EGR systems and have had to incorporate throttle plates in the intake; not to govern engine speed, but to create intake manifold vacuum to increase the amount of EGR pulled through the restrictive cooler(s).

20

Hexane is C6H14.
Diesel is composed mostly of hydrocarbons with 11 to 18 carbons. Pumps used to advertise cetane content- C16H34 (they may still- I just haven’t driven a diesel since '98.)