In theory can an engine have greater fuel efficiency at higher RPM?


#1

Hello everyone, Rick here.

Vehicle in question is an 04 silverado 5.3l.

If you are going down the road, level ground, steady wind, 50mph and shift from 4th gear to 3rd gear and keep the speed the same the instant MPG readout will dip down a bit for a few seconds, then go back to just about where it was in 4th gear.

Now I know those instant MPG meters are not 100 percent accurate and I also know there are many variables. It did get me thinking though, Is it theoretically possible to get the same or better MPG with the engine in a lower gear?

I know it makes no sense since manufacturers put taller gearing in vehicles for MPG benefits and also added OD transmissions and they didn’t do this for any reason other to save fuel. But are there some situation that you could have the same fuel consumption with higher RPM? I would think it would take more fuel to spin the engine faster, however there are so many factors I get confused.


#2

In general, no. Going from 4th to 3rd increases rpms and closes the throttle, increasing pumping losses. Like you said, that’s why tall gears get better mpgs: reduced pumping losses.


#3

Why would you even do that?


#4

Your engine is operating at its peak efficiency during full throttle operation. That is because of the low pumping loss, and more importantly, the increased density of the mixture during combustion.

But peak engine efficiency (the ratio of power developed to fuel consumption), does not mean peak MPG. MPG is much lower during full throttle.

Can your vehicle get better MPG when you downshift from 4th to 3rd? Perhaps, but there are too many variables, including, increased engine friction in 3rd, slightly increased pumping losses (though that would be minor for this comparison), and most important, the manufacturer’s programming for timing and fuel mixtures at those different rpms.


#5

As mentioned before, low engine speed requires a larger throttle opening to achieve the required power level for the speed. While this lowers pumping loss, it increases heat loss. With a low engine speed, there’s more time for the burnt gas to stay inside the cylinder and transfer its heat to the cooling system. Heat transferred to the cooling is not going to help with propulsion.

For most engines, there’s a point between 1 to 2 thousand rpm where decrease in pumping loss is negated by increase in heat loss. Go below that point and it is better to down shift. This is why you shouldn’t try to accelerate from 25 to 50 in top gear if you drive a manual.


#6

I have an analytical mind, or ADD, or both!

I have had several vehicles with econometers installed. I always have found them interesting and I do different things when I drive to occupy my mind.

Do you know what happens if you are driving down the road and turn the key to the crank position? I do, the TCC Unlocks for some reason, engine continues to run perfect. When you let go of the key and it returns to run position, the abs, airbag light and whatnot come on and blink just like when you start the car.

I have run down the road in each gear up to almost redline and held it there steady to watch the econometer and compare MPG.

One of the most entertaining vehicles I ever drove is my old prius. It had several econometers, doodads, and heehaws, it was fun. But now that Trump is president its back to sucking gas at an alarming rate in my silverado and roaring my engine from stoplight to stoplight. I dont drink but I even filled the bed with discarded beer cans and marlboro red packs so I look the part.


#7

Interesting. I have played around when accelerating from say 35 to 55mph, sometimes I give the most throttle I can before the TCC unlocks thereby “lugging” the engine as much as possible, or sometimes just letting the tcc unlock and letting the revs get up there a bit and it seems I get better MPG with letting the tcc unlock.

Im not trying to reinvent the wheel, I just find it interesting.


#8

I’m still driving my 42 mpg Yaris and my 70 mpg Kawasaki Ninja 300 and will be regardless of who is in the White House. A sudden sharp rise in gas prices and people will be ruing the day they bought those gas hogs.
My observation, those who drive gas hogs and turn down the AC real low so they can sleep under quilts in the middle of August in their huge suburban mansions are just as likely to be Democrats as Republicans.


#9

Well I went with the theory if you can’t beat then join them. I have had far less problems driving my pickup vs the prius.

You could be going 10 over the speed limit in the prius and have some yoke right on your bumper. Far less of that in a pickup.

So now I just go with the flow. It’s easier. I’ll drive a 12mpg pickup truck until gas goes up.


#10

Not necessarily full throttle, more likely 75% or thereabouts. At full throttle, many automotive engines use a extra rich fuel mixture for maximum power, extra cooling, and to prevent detonation.
Be being programmed to go to a rich fuel air mix during those brief full power bursts, the compression ratios can be higher allowing for higher efficiency during part throttle cruise where the engine spends most of its time.

In this chart, this particular engine has a lower specific fuel consumption at half throttle up to about 3500 rpm, above that, the half throttle specific fuel consumption is higher than full throttle, as the pumping losses increase with higher rpms.


#11

I agree that an engine can be more efficient at higher RPMs, but that does not translate into higher MPG. At higher speeds, the major factors in losses are: first, wind resistance, and second, friction.

wind resistance depends on the square of the velocity. So if you double your speed, the wind resistance goes up by a factor of four. Which means the MPG goes down by a large factor, all else being equal.


#12

I have personally never driven or even ridden in a Prius so I wouldn’t know. Where I live, they are just another car, nobody notices them anymore. You gotta drive a Tesla to get peoples attention here.

Maybe if you put a gun rack and a NRA sticker on the rear window of your Prius they’ll back off.


#13

Around here, even a Tesla won’t get many stares

What I do notice is that around here, an extremely large percentage of Prius drivers are very aggressive. They change lanes without looking or signaling, they cut you off, forcing you to slam on the brakes, tailgating, honking horns at you, etc.

Exactly the opposite of what some people might expect

Perhaps the rationale is that they can drive aggressively, yet still get very good fuel economy . . . ?

Maybe they feel they’re getting the best of both worlds?


#14

Around here prius drivers tend to be older folks. Driven reasonably. The worst drivers around here by FAR are the ones with diesel pickups. Not bone stock ones, they usually have bigger tires, an aftermarket exhaust, and a tuner that allows them to blow raw diesel out of the exhaust.

They drive like morons almost all the time. And those trucks are everywhere here.

Some of the most courteous drivers are in diesel pickups as well, but they are always stock and usually driven by an older person, sometimes pulling a camper. They may not be local though.

Worst car drivers seem to be in older clapped out gm cars. Usually an older grand prix, or grand am, in horrible shape and you can almost assume they don’t have insurance.


#15

We got them too. Some have a huge stack going straight up out of the bed filling the sky with black smoke while they drive like jackasses.
I’m just waiting for the ultimate in ridiculous exhaust stacks, I’m imagining a steam locomotive stack coming out of the top of the hood, one of those spark arrester stacks used on wood burning locomotives in the old west.


#16

When I said “full throttle”, I was only restating what we learned back in my internal combustion engines class.

.


#17

Around here, a lot of young people drive Priuses. And they do NOT drive reasonably.

Yup, that’s the Prius drivers around here


#18

Yes, engine lugging is in issue in higher gears if you aren’t going fast enough, going uphill, or into a headwind. There are times you can tell you you are not pushing as hard on the gas in a lower gear. This tells me it is a more optimum engine speed for the conditions. Yes, I can climb a hill in 5th gear without losing speed but you can tell the engine is running more at its optimum in 4th gear. The engine spins at a faster speed and you aren’t as into the gas as if you were in 5th gear. I guess this is my foot “throttle position sensor”.

As for the Prius, I am used to the stereotype. Usually it has an NPR sticker and whatever liberal politician that is running or has been elected at that time. Bernie Sanders comes to mind. I once saw one with all the NRA, gun, and don’t tread on me stickers on every window and panel. That wasn’t what I was expecting but I got a good laugh out of it.

Like everyone else, I hate the coal rolling trucks. They have to program them to waste fuel in order to do this, otherwise they would never run that rich. As for the worst drivers, it is anyone out during the day when most people are at work. This is when all the unemployed druggie people and such finally wake up, get out of bed, and come out. You can usually count on seeing someone getting hauled out of Wal-Mart in handcuffs at 2:30PM. I swear that is like the scariest time to be out on the roads or anywhere for that matter. This is the Jerry Springer crowd! Just look at what is on TV at this time.


#19

Late evening is when all the people who can’t get to the next red light fast enough are on the road where I live. The nightclub and bar traffic.


#20

And that’s why you’ll typically get better mpgs in 5th. More ‘into the gas’ means the throttle is more open, means less pumping losses.