How do throttle position, RPM's, and gas consumption correlate?

Something I've been thinking about while trying to climb hills with the 1.6. Seems to me that engine RPM's does not directly correlate to the amount of gas consumed, but I'm not sure that the throttle position does either.

In other words, climbing a hill in 2nd gear with half throttle vs 4th gear with full throttle, same speed...which uses more gas?

That's a very complex model that I don't think you can really simply that far. The system is going to have more loses at higher RPM, but the engine might be far more efficient wound up.

Depends on where you get most the power and the gear ratio, I suspect.

Gasoline engines are more efficient at larger throttle openings, due to reduced pumping losses through the throttle plate. Also, higher RPMs result in more frictional losses inside the engine. OTOH, wide open throttle often puts the car into an open-loop mode and makes it run richer. You're also going to get better fuel efficiency when running at the VE peak.

So the answer is "it depends". :)

Gross oversimplification that will be corrected I'm sure, but generally the engine is most efficient with a wide throttle opening and a relatively low RPM.

There are two main ways you waste power: internal friction and pumping losses (how much energy it takes to move air into and out of the engine). Higher RPM introduces more losses from friction, obviously, because it's moving a lot more.

The more complicate part is the pumping losses. A small throttle opening means there's maybe 1 PSI less pressure on the intake side of the throttle plate. The pressure drop across the throttle is sort of like the opposite of a turbocharger and you wind up with more pumping losses than if the throttle plate were wide open. Basically, imagine how tiring it would be to do all your breathing through a drinking straw.

Some of this changes when you start getting into engines without throttles, but for a traditional gas engine, you want to run near WOT and a low RPM.

It takes X amount of fuel to to move Y pounds up a given hill. The rest is in the efficiency of the engine and the car at a given RPM.

OSULemon wrote: Something I've been thinking about while trying to climb hills with the 1.6. Seems to me that engine RPM's does not directly correlate to the amount of gas consumed, but I'm not sure that the throttle position does either. In other words, climbing a hill in 2nd gear with half throttle vs 4th gear with full throttle, same speed...which uses more gas?

IF:

Both are at MBT spark Both are running stoich fuel neither are at a point where combustion sucks

Then the lower speed will be better.

BUT: Lots of engines are knock limited, especially at low RPM, so it's possible that the losses due to the higher speed and pumping losses will be less than the losses due to spark not running at optimum.

AND: At wot, many cars need to run rich to prevent more detonation and keep components cool. That tends to make problems due to spark even worse.

AND: Engines stop making WOT power/torque very well below engine speeds somewhere near 1300rpm (depends on the engine), so that can also be added to the above effects.

One must remember, this is a power game- what POWER output gives me the most efficient combustion/ best brake spec fuel consumption. Both situations, the wheels see the same power, and the engine makes about the same amount of power- the torque will be drastically different.

So, yea, it depends.

Also, EFI engines tend to jump to open loop whenever you drop below 10" of vacuum or so, so that comes into play as well. Just mounting a vac gauge to train yourself not to do that will usually give you a few mpg.

http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption

The answer is it depends.

Bryce

Kenny_McCormic wrote: That's a very complex model that I don't think you can really simply that far. The system is going to have more loses at higher RPM, but the engine might be far more efficient wound up.

Exactly. You'd need to chart the horsepower you're using against a BSFC chart for your engine. This isn't generally something available to the public, although I've seen them for some diesels.

You need a three dimensional BSFC curve. You could make one with enough dyno time. It really depends a lot, and the number of variables is enormous.

Two empirical examples:

98 Civic was more economical at 80 mph than at 70 mph on rt 81 because (I assume) it made more power at that RPM for the larger throttle opening that resulted from maintaining speed on big Pa hills.

84 pickup that would get huge economy gains by reducing speed on the highway essentially as much as you could stand.

Experiment and report back.

I found that a carbureted RX-7 got best fuel economy at 5500rpm and whatever speed that equates to with a .825 overdrive, 4.78 gears, and weenie little stock size tires. Indicated MPG was over 40 but actual was closer to 33.

I usually get best MPG by treating the throttle like it's a switch. I shocked a friend by claiming 31mpg with the Quantum... 25 is more common. Just bolt the throttle to the floor and coast as necessary...

I forgot, one more. Hammering the throttle immediately (like Knurled mentioned above) to somewhere around 1/3 and keeping it there until the desired speed has been reached is worth about 2 mpg -improvement- on the Caravan over creeping up to the speed.

Yah, WOT in the Quantum is like 1/3rd throttle in a 3-liter Caravan

Accelerate firmly to speed and stay there. One of my tricks with the RX-7 was that you could feel where the secondaries would open and I'd use that little firm spot as acceleration control.

Knurled wrote: Yah, WOT in the Quantum is like 1/3rd throttle in a 3-liter Caravan Accelerate firmly to speed and stay there. One of my tricks with the RX-7 was that you could feel where the secondaries would open and I'd use that little firm spot as acceleration control.

My MX6 has that as well... Though mine also seems to be the only one i've ever driven that had that "catch" in the gas pedal.

Maybe that's the secret to my MPG in that car. I've found that anything past that catch = boost.

Let's not forget the role played by aeros as well. The drag created by a boxy profile can drastically increase fuel consumption when compared to a more slippery shape and like anything aeros have their sweet spot.