What causes my car to burn the most fuel?

General Questions
#1

Say i am driving along in top gear and have to reduce speed. I haven’t slowed down too much so i can stay in top gear if i want but will need give it more throttle to prevent the car from stalling. Alternatively i could drop a cog and therefore not have to give it as much throttle.

But which is best for the car, both in terms of engine load and fuel economy? I mean, one makes for lower revs but opening the throttle more fires more fuel into the mix yes? On the other hand i can have more revs but less throttle so less fuel being pushed through? Also, i would normally associate higher revs with more engine wear but in the ‘stay in top gear’ scenario the engine sounds like its struggling a bit so does this cause more engine wear than changing down?

Higher revs = more fuel being burned or more throttle = more fuel? Which is it? Am i being really stupid here?

#2

More throttle = more fuel

Generally speaking, higher revs = more engine wear, but if you are getting the engine close to stalling the mechanical stresses of uneven firing and jerking will be worse than the stresses you get just from shifting into a higher gear.

Trying to accelerate in too low of a gear can also cause excessive stress on the engine. It’s like when you are pedalling a bicycle and you are in too low of a gear, it’s awful darn hard to push your leg down. In a car engine, the piston is essentially your leg. If you try and accelerate in too low of a gear, you’re putting excessive strain on the pistons, rings, seals, etc.

#3

stay behind big trucks. That cuts the fuel consumption a bit as well.

#4

For this to be at all effective you would have to be frighteningly close, and unable to stop in time if the truck slams on the brakes. Far enough back to be safe and you are just being buffetted by their turbulence which, if anything, is going to make your fuel economy worse not better.

#5

Let’s get real specific here!
What kind of vehicle?
What engine package on that vehicle?
Makes a big difference if you’ve got a Dodge Ram with a Cummins turbodiesel…
Exactly how low are you talking about taking your RPMs if you “coast down in top gear” ?
If you’re talking under 600, then you’re definitely need to gear down.
If you’re talking about getting your RPMs down to no lower than 1500, you’re probably not hurting much of anything.
Somewhere inbetween, you’ll probably want to set your downshifting threshhold.
Finally, when you say “give it more throttle to prevent the car from stalling” how much gas are you talking? Going from 24% to 33% throttle? Up to 50%? Flooring it?

In regards to your question about whether or not RPM or throttle controls gas usage, the answer is both.
Full throttle won’t make your engine burn as much gas at 2000 RPM as it will at 5000 RPM.
In fact, if you hold your throttle steady as you accelerate and watch a diagnostic scanner, you’ll notice that the “gallons per hour” you’re passing into the engine will go up as you stay in the same gear but increase your RPM. I can hold my Sable at a given throttle setting and it’ll burn anywhere from 1.8 to 2.1 GPH depending on RPM, and that’s all in a narrow band below 2600 RPM at highway speeds.
I’d love to see some mechanicaldoper input here…

#6

Tee off of a manifold vacuum line and install a vacuum gauge. Run in the gear that produces the highest relative vacuum. For any given condition, the engine will be most efficient at the RPMs where manifold vacuum is greatest.

To put it roughly, think of the intake tract and throttle plates as being a restriction to the total power of an engine. When the engine is at its most efficient RPM (and sending more of the air and fuel’s potential energy down the driveshaft as kinetic energy), then the intake will have to put more of a restriction on the air entering the engine than it does at other RPMs. Otherwise, the engine will produce even more power and the car will accelerate more. I could be wrong, but this is what I’ve heard over the years and it does seem to make sense.

Most of the BFSC curves* that I’ve seen show that production car engines lose efficiency as RPMs increase, but I expect that there are exceptions out there.
*Brake Specific Fuel Consumption. Admittedly, I’ve only seen a few.

#7

This is NOT true. Very high vacuum levels produce high pumping losses. This is the primary reason that large displacement SI (gasoline, not diesel) engines are not effecient at light loads, unless they have extreamly tall gearing to lower the manifold vacuum.

That said, very low vacuum levels produce poor vaporization, and in computer controlled engines will shift the control algorithm to “performance” settings, both of which will also hurt economy.

There is a fuel economy “sweet spot” at around 70 - 80% maximum available torque where gasoline still vaporizes well, yet pumping losses are not huge. That corrisponds to around 8-12" Hg on a vacuum gauge.

Properly functioning EGR systems will greatly distort vacuum readings. In so doing they reduce pumping losses. EGR is one of the few emission reduction techniques that doesn’t hurt performance, and also saves gas. It’s a little tricky, and the valve tends to wear out, so it is loosing favor…pity that is.

#8

Actually it isn’t losing favor, we just do it a different way now. With the advent of variable valve timing on both camshafts we can vary the overlap of the cams as needed. During part throttle cruise (when EGR is needed) we can set the cams to overlap and get internal EGR to lower NOX emissions without having external plumbing, valves and controllers. :cool:
Also as far as acceleration goes, EGR should not be present during acceleration. Combustion temps are not high enough to create NOX and therefore no EGR is needed.

#9

Isn’t the time when you’re producing the highest b.m.e.p. the time when the combustion temperatures peak? I know that you’re richer during acceleration, but you’re also way on up the pressure and temperature curve too.

I have hundreds of graphs of engine temperature versus motoring conditions; they are at home, so I’m going off of memory here.

#10

As it has been explained to me, EGR is needed at cruise as the computer is set to run lean (14.7:1 or higher). The leaner mixture cause the temps to spike above 2500F. Which is the point where NOX is formed.
Looking at a 4 gas chart NOX is almost not there if the mixtue is richer than 14.7:1. At 14.7:1 it spikes and stays high as the mixture goes leaner.
During acceleration he computer adds some fuel above the 14.7:1 for power, hence little to no NOX during this time.

#11

Kevbo, I stand corrected. Thanks for the follow up.