Gas mileage and a vehicle's efficiency

Factual Questions
21

Did you mean to say electric vehicles? If so, then the answer is absolutely not. The most efficient speed for an EV is basically zero.

ICE (internal combustion engine) vehicles tend to get better fuel economy on the highway and worse in the city. For EVs (electric vehicles), the reverse is true. The slower an EV goes, the more efficient it is. You get basically twice the fuel economy when you cut the speed in half.

The major factors for fuel consumption of an ICE vehicle are rolling resistance, wind resistance, energy lost to braking, and waste heat from the engine. The amount of waste heat depends on how long you leave the engine running, regardless of how fast the car is moving. Every Joule of heat off the engine is a Joule of gasoline that could have been used to push the car but wasn’t. Total energy lost to waste heat is essentially proportional to time spent with the engine running. Wind resistance is a force proportional to the square of the vehicle’s speed. At low speeds, you decrease wind resistance but you increase running time to reach your destination. At high speeds, you decrease running time but increase wind resistance. Somewhere in the middle is a good compromise of the most efficient speed for that ICE vehicle. It may be around 40 MPH or it might be 70 MPH.

But with EV’s, waste heat (which is proportional to the square of the current) is a tiny fraction of that from an ICE. The major factor of wind resistance dwarfs heat loss by at least an order of magnitude. This means that, unlike the ICE, there is no fuel savings in getting to your destination quickly. You save fuel by going as slow as possible, like below 5 MPH.

Consider my Mitsubishi i-Miev. The EPA range estimate is 62 miles. When I drive 60 MPH on the freeway, my range is about 45 miles. But when I drive 30 MPH, I can go 90 miles. Cut the speed in half and you double the fuel economy. I am confident that, if I lowered my speed to 15 MPH, I could go 180 miles. But that would take 12 hours and I have never had the patience to waste 12 hours testing this hypothesis. I have read articles about people who have done it. Read any article about someone setting a record for distance in an EV and you’ll find they did it at a really low speed. Here is a forum post by an i-Miev driver who went 123 miles on a charge by keeping the speed below 25 MPH.

In the movie Driving Miss Daisy, the title character says “The slower you drive, the more gas you save.” This is patently untrue for ICE vehicles, but it is very much true for EVs.

22

Are you confirming or negating Machine Elf 's statement ?

23

Sorry, bolding in the quote was mine. Edit window passed before I remembered.

24

I just bought a new car.
It has a cool but maddening feature - a display of instantaneous MPG and trip-average MPG. A week with the car showed that the big MPG killer in city driving is not braking, but sitting at traffic lights. I can watch the trip MPG creep down while the instantaneous MPG is at zero, waiting for the light to change. So, cars that do start-stop can get substantially better MPG in city driving that those that don’t.

25

Neither. Two different things.

26

Here are my real life numbers with the same car.

4.57 liters per 100km on urban & ex urban at max speed of 90Kph
5.23 liters per 100km on urban & ex urban at max speed of 100Kph

5.59 liters per 100km on Motorway at 120Kph
When I take to the motorway, it’s usually for a round trip of 600km in one day.

27

Yes, of cos, you spend 0.001 gallon and drove 0miles, meaning the computer calculates that at this given speed you’ll need an infinite amount of fuel to travel just 1 single mile. However, your average fuel calculation will then take this into account again once you’re moving again.

Or let me put it this way, when you have a full tank and let your car run idle outside, it will eventually run out of fuel, even tho, it didn’t travel any distance.
In diggers, tractors and other work machinery this is usually called work hours, since let’s say a a digger, that has worked hard for 10 years 20 hours per day may only have traveled 10 miles in that time.

28

It’s…complicated.

Real-world diesel engines do have higher compression ratio, which means the temperature at the start of combustion is going to be higher, with the result that in the immediate vicinity of injected droplets of fuel, the diesel engine will indeed develop higher temperatures; combined with the availability of excess oxygen nearby, this does indeed make for the creation of a lot of NOx.

However, once you move away from any given plume of injected/burning/burnt fuel there’s a whole bunch of diluent air/EGR that’s not so hot. During/after the combustion event, the turbulent motion of all of this stuff quickly mixes the hot cloud of burnt fuel with the cool envelope of air/EGR, resulting in a lower overall temperature than would be seen in a gasoline engine operating at comparable load, thereby lowering the amount of heat that gets lost to the combustion chamber walls during the expansion stroke - leaving more of it available for conversion to mechanical work.

There does seem to be a general trend of higher adiabatic flame temp with increasing number of carbon atoms, but I think the trend scales mostly with the hydrogen-to-carbon ratio. on this list, for example, methane is about 1960 C, whereas kerosene and fuel oil are all around 2100 C. for straight-chain hydrocarbons the H/C ratio is just (2N+2)/N, where N is the number of carbon atoms. For N=8, the ratio is 2.25, and for N=16, the ratio is 2.125 - not a big difference. So yes, the adiabatic flame temp for diesel would be expected to be higher than for octane, but not by much - and as noted earlier, diesel engines always run far lean of stoichiometric, whereas gasoline spark-ignited engines generally run at or close to stoich.

idling can only lower MPG, but have you noticed how much your trip MPG suffers when you accelerate away after the light turns green? That acceleration - a necessary consequence of the braking you did to stop at the light - is the real MPG killer. A city driving trip with a bunch of 2-minute stops for traffic lights will certainly give you worse fuel economy than the same trip where all those lights are replaced by stop signs, but the difference will be pretty small.

Many years ago there was a study that sought to identify the operating parameters with the biggest effect on overall trip fuel economy. In the end, they identified “microtrip duration,” i.e. the amount of time elapsed between accelerating away from a dead stop and then coming to a dead stop again (a trip, for which the fuel economy is being calculated, is composed of multiple microtrips). Short microtrip duration is caused by:

-short distances between stop-start events
-hard acceleration at the start
-high peak speed during the microtrip
-hard braking at the end (necessitated by a high peak speed)

So imagine you’re driving ten miles in the city. The traffic lights are all conspiring against you, so you have to stop every block. You’re in a bad mood, so every time the light turns green, you floor the accelerator trying to make the next light, but you never do; you always have to slam on the brakes after one block. Short microtrip duration means shitty MPG.

Imagine a different scenario in which the lights are better behaved, so that you can go three blocks at a time before a red light makes you stop. Plus you’re in a better mood, so you accelerate more gently when the light turns green, and you choose a more modest top speed. Your microtrip duration is now considerably longer, and you enjoy better fuel economy.

29

Spurred by this thread, I came across an interesting article: https://blog.automatic.com/the-cost-of-speeding-save-a-little-time-spend-a-lot-of-money-5e8129899fec

This website is associated with an app and device that plugs into your vehicles data port. Apparently (if my quick skim is correct), the app gathers data about fuel mileage and speed and relays it to the big database in the sky.

If you scroll down about 3/4 of the way on the page, there is a graph showing MPG by speed for several different vehicles. Most indicate peak efficiency around 45 to 50 MPH, but I’m perplexed about the hump in MPG for the Honda Civic at about 65 to 70 MPH. It has one peak at 50 MPH, after which the MPG drops, but then the MPG goes up again at 65 MPH or so. Is this some sort of data mistake, or is there a rational explanation for the two-hump behavior. The only thing I can think of is going into a super-overdrive gear which drops the RPM, but I would expect that to happen at a much lower speed. The BMW 328i shows a less pronounced second hump.

I’m going to have to dig around in this website to see what else they have.

30

The text below the graph says:

There are a number of variations of the VTEC system. I’m speculating here, but it may be that for high-speed cruising, they select a cam profile that limits air intake in a way that allows the throttle plate to be open wider, reducing pumping losses (e.g. by keeping the intake valve open later into the compression stroke). Whatever the cause is, that plot is interesting because it shows that the Civic’s best MPG is right around 70 MPH, rather than the classic 40-50 MPH for most vehicles.