BimmerFest BMW Forum banner

1 - 20 of 24 Posts

·
Grinning from ear to ear
Joined
·
2,029 Posts
Discussion Starter · #1 ·
Ok this is another gas mileage question - but this time more theoretical.
I was always under the impression that the higher your engine RPMS were the more gas you were burning. However the instant fuel guage seems to be more dependent on how far down my gas pedal is. Here's an example: I can be cruising along at 4500 RPMs. If I keep my foot steady, barely pressed, the RPMs will stay at about 4500, but the instant fuel monitor hovers around 30 mpg. However if I'm at 1200 RPMS, and step on the gas even remotely hard, the instant mpg reads <12 mpg.

So is the monitor accurate? Which is worse, cruising at high RPMS, or keeping the engine at low rpms, and then stepping on the gas to speed up?
 

·
My TARDIS is a BMW
Joined
·
5,352 Posts
In order to increase the velocity of the car, you need the engine to make more power, which is accomplished by pushing the throttle to give the engine more gas, thus lower mileage. However, when just crusing, you only need enough power to keep the car moving at a constant velocity, which means you need less gas, so higher mileage. The exact RPM range that you are in will be determined by which gear the car is in, so getting on the gas at a lower RPM in 2nd will eat more gas than keeping steady at a higher RPM in 4th, for example. If you want all the math/physics behind it, I'm sure others can provide the gruesome details.
 

·
Grinning from ear to ear
Joined
·
2,029 Posts
Discussion Starter · #3 ·
operknockity said:
In order to increase the velocity of the car, you need the engine to make more power, which is accomplished by pushing the throttle to give the engine more gas, thus lower mileage. However, when just crusing, you only need enough power to keep the car moving at a constant velocity, which means you need less gas, so higher mileage. The exact RPM range that you are in will be determined by which gear the car is in, so getting on the gas at a lower RPM in 2nd will eat more gas than keeping steady at a higher RPM in 4th, for example. If you want all the math/physics behind it, I'm sure others can provide the gruesome details.
So if the engine is holding steady at 4000 RPMs, each individual firing of the cyliners is using less gas, than a firing while the engine is at 1000 RPMs and you gun it? So the car can actually modify either A) the air/fuel ratio or B) the amount of combined air/fuel injected into the cylinder based on speed, and amount of throttle applied?

I was thinking that X ml of gas were used on every firing. That means 4000 firings per minute would burn more gas than 1000 firings per minute regardless of acceleration.
 

·
Family Man User
Joined
·
5,540 Posts
How about, which uses the most gas: steady speed at 2500 rpm in 5th, steady speed at 3500 rpm in 5th, or steady speed at 2500 rpm in 6th?
 

·
Registered
Joined
·
2,437 Posts
icemanjs4 said:
So if the engine is holding steady at 4000 RPMs, each individual firing of the cyliners is using less gas, than a firing while the engine is at 1000 RPMs and you gun it? So the car can actually modify either A) the air/fuel ratio or B) the amount of combined air/fuel injected into the cylinder based on speed, and amount of throttle applied?
Yes: fuel injectors.

If you listen to your car going up a hill at a steady 3500 rpm versus coasting down a hill at 3500 rpm, it'll sound different because the former involves more fuel and more combustion, whereas the latter is mostly the transmission pushing the engine. As operknockity said, the exact rpm of your engine is really a function of the speed of your wheels and transmission. But you or the car's computer can control the fuel flow.

With fuel consumption, there are multiple curves involved; it's not just about getting rpms down. If your rpms are too low, you can dump more gas into the engine and get low output because it's out of the power band. If you try riding a bike up a hill in your highest gear, your legs will tire quickly from burning fuel to exert a lot of force at low frequency (low rpm). If you bike up the same hill in your lowest gear, your legs will tire quickly from burning fuel to exert less force but at a high frequency (high rpm).
 

·
My TARDIS is a BMW
Joined
·
5,352 Posts
swchang said:
How about, which uses the most gas: steady speed at 2500 rpm in 5th, steady speed at 3500 rpm in 5th, or steady speed at 2500 rpm in 6th?
Not sure about that question. I'm thinking that the 2500 RPM or 3500 RPM in 5th would probably use about the same amount of gas.

As for which is better between either of the RPMs in 5th vs 2500 RPM in 6th, I don't know... What I can say is that the higher the gear, the less the engine has to work to keep the car at a steady speed. That is why in many manual tranny cars, the next to last gear is a 1 to 1 gear (eg. 1 rev of the engine gets you one rev of the wheels) whereas the top gear is an "overdrive" gear (eg. 1 rev of the engine gets you more than 1 rev of the wheels). The top gear is meant to give you better mileage when cruising at speed (but with a really large drop in torque!)

I know that in my '02 with only the 5 speed, that 5th is 1 to 1, while I believe the later 6 speeds still have 5th as 1 to 1 and 6th is overdrive. (And my '86 Celica had a 5 spd with 5th being overdrive).
 

·
Registered
Joined
·
2,437 Posts
operknockity said:
What I can say is that the higher the gear, the less the engine has to work to keep the car at a steady speed.
I don't know the exact answer either, but this is what I get working backward:

At a given speed X on level ground, there is an exact horsepower requirement Y to maintain that speed. This horsepower Y is enough to overcome all combined forms of friction from the tires, aerodynamic drag, resistance in the engine and drivetrain, and so on. You can roughly find the hp requirement at speed X by doing some deceleration measurements.

Since you're at speed X, that means you have a choice of rpm Z based on the gear you select. You then take that rpm Z, multiply it by the gearing you selected, and see what horsepower value you get from the engine's power profile (dyno chart).

Now here's the fuzzy part: assuming the gear you selected gets you >= Y hp, you then need a fuel consumption profile to figure out how much fuel would be consumed at that rpm to get that much hp out, since dyno charts show you power output at WOT. This is where I get lost. :dunno:
 

·
Keeping it surreal
Joined
·
43,276 Posts
I think you guys are reading way too much into this....IMHO, the mpg indicator is nothing more than a glorified vacuum gague, measuring the amount of intake manifold pressure at any given rpm/throttle opening, i.e., WOT at low rpm= zero vacuum & poor mileage, while almost-closed throttle= high manfold vacuum, and higher mileage.

Regards,
Bob
 

·
Registered
Joined
·
243 Posts
An engine is most efficient when running at the RPM corresponding to its torque peak at WOT.

This does not mean that you will get the best fuel economy here since WOT delivers maximium power, but I would guess that selecting the gear that keeps the RPMs near the torque peak would give the best mileage.

Steve D
 

·
Registered
Joined
·
2 Posts
Fast Bob is correct. The MPG guage is nothing more than a fancy vacuum guage. Low manifold vacuum = high MPG. High manifold vacuum = low MPG.
 

·
Registered
Joined
·
212 Posts
Don't think of fuel flow as being dependent on engine speed, but rather on engine change-of-speed. To a positive change of engine speed, more fuel-flow is required. For a negative change in engine speed, it could almost starve itself of fuel.
 

·
Grinning from ear to ear
Joined
·
2,029 Posts
Discussion Starter · #12 ·
SProboscis said:
Fast Bob is correct. The MPG guage is nothing more than a fancy vacuum guage. Low manifold vacuum = high MPG. High manifold vacuum = low MPG.
Well that's partly what I was getting at in my original post. It seems that the guage is nothing more than a rough heuristic for fuel efficiency. Rather than rely on it to see how well I'm doing, I'd rather adapt my driving style to a way that I know will give me better efficiency. I find this dillemma all the time cruising at 40-45mph. I could either be in 3rd gear at a higher rpm, or 4th gear at a lower RPM. My instinct has told me to stay in 4th (higher gearing = better gas mileage). However as my speed drops in 4th, since I'm at such low rpms, it seems the engine has to work harder to speed back up to my cruising level. In 3rd gear, it's much easier to control my speed accurately (albeit at higher rpms).

According to what I'm reading - it actually seems the 3rd gear approach would be more efficient? Is this right?
 

·
Registered
Joined
·
1,441 Posts
SProboscis said:
Fast Bob is correct. The MPG guage is nothing more than a fancy vacuum guage. Low manifold vacuum = high MPG. High manifold vacuum = low MPG.
The fuel consumption gauge is more simple than a vacuum guage - the fuel flow at the injectors is all that is displayed with some damping in the gauge needle, so it is actually quite accurate as it uses an engine management signal that is accurate. The instrument cluster is all electronic in an E46.

There is a point in any engine's power curve that is the optimum for fuel efficiency at a given speed, the specific fuel consumption of an engine (fuel consumed per unit of power output, i.e. litres per KW or gallons per BHP) is actually best at close to the maximum power output at WOT, but if you don't need the full power output, the specific fuel consumption increases as the throttle is closed.

The most efficient engine speed for the power required in a steady cruise for a given road speed is a point at which the rate of increase of specific fuel consumption is best balanced against the rate of decrease in engine power output. This point is typically a low engine speed, but not the lowest that the engine could pull at without "lugging".
 

·
I hate door dings
Joined
·
1,322 Posts
Steve D said:
This does not mean that you will get the best fuel economy here since WOT delivers maximium power, but I would guess that selecting the gear that keeps the RPMs near the torque peak would give the best mileage.

Steve D
If this is the case then we should always drive at about 5000rpm. It is fun and at the same time save gas!
 

·
King of Rear Clunks
Joined
·
12,903 Posts
jsc said:
The fuel consumption gauge is more simple than a vacuum guage - the fuel flow at the injectors is all that is displayed with some damping in the gauge needle, so it is actually quite accurate as it uses an engine management signal that is accurate. The instrument cluster is all electronic in an E46.
There is DEFINITELY speed involved in what's showing on the gauge. Look at the way it behaves when coming to a stop. It swings to the right, then below a certain threshold, drops all the way to the left. This would NOT happen if it were either manifold vacuum only or injector pulse width only.

If the gauge works properly on forced-induction cars, it's obviously not manifold vacuum.
 

·
Registered
Joined
·
514 Posts
Remember that as engine RPMs incrase, so does friction and pumping losses. The engine is really just a large air pump.

It takes more energy to intake and exhaust the gasses at higher RPM than at lower RPM, since you're moving more gasses. There are many other parasitic losses that will increase with RPM, such as driving the oil pump, cams, water pump, etc.

I believe this is why you might get better fuel economy cruising in top gear at a given RPM/MPH combination rather than cruising in a lower gear at the same MPH but higher RPM combination.
 

·
Registered
Joined
·
43 Posts
swchang said:
How about, which uses the most gas: steady speed at 2500 rpm in 5th, steady speed at 3500 rpm in 5th, or steady speed at 2500 rpm in 6th?
Easy,
steady 3500 in 5th will burn more gas than steady 2500 in 5th because you will be travelling physically faster at 3500rpm relative to earth ground and you will have greater drag coefficiency both from the road and the air - you will be breaking through more air resistance in the same amount of time, which equates to more gas needed to keep up that faster speed. But you will get to your destination earlier.... So, same gear comparisons are easy - more RPMs = more gas. There is also the minute amount of loss due to mechanical friction throughout engine, transmission, etc.

Now 3500 in 5th vs 2500 in 6th, assuming you are travelling at the same speed in both, lets say 70mph for example, then since your air drag and road drag are equal in either gear, you only have the mechanical coef. loss of spinning things at a faster pace - what ever that may be.

It is all about time vs distance.

Also, the instant fuel consumption gage is VERY accurate because it works with the manifold vacuum pressure and not directly tied to gas pedal position. The greater negative pressure in the manifold (created by the suction of the pistons), the more fuel will get sucked in when the injector opens up, thus the corralation of manifold pressure to gas being drawn in is very accurate. As for the number of the actual gage, I don't know, but the movement of the needle is relative to the amount of fuel being sucked into the intake manifold - whether the fuel gets burned or not, that is another science, but then, once it has left the gas tank, it is wasted whether it was burned or not... ;-)
 

·
Registered
Joined
·
1,441 Posts
Kaz said:
There is DEFINITELY speed involved in what's showing on the gauge. Look at the way it behaves when coming to a stop. It swings to the right, then below a certain threshold, drops all the way to the left. This would NOT happen if it were either manifold vacuum only or injector pulse width only.

If the gauge works properly on forced-induction cars, it's obviously not manifold vacuum.
The final motion of the needle as you come to a stop is consistent with the fuel consumption at the injectors relative to distance, when you are still moving with your foot on the throttle you have a quantifiable consumption rate. When you shut the throttle and fuel is shut off to the injectors, you have a 0 l /100km reading (infinite mpg), when you come to a halt, you have an infinite l/100km reading (0 mpg), so the needle moves from full-scale on the left at zero throttle, to full scale on the right as you come to rest. When you have come fully to a stop, the logic seems to then bring the needle back to 0 l /100km, as the distance travelled is no longer resolvable.

As speed is the differential of distance relative to time, it is logical to take the injector consumption over a fixed time interval (for example 1 second), multiply it by speed and drive the fuel consumtion gauge with a signal proportinal to that value.
 

·
John Firestone
Joined
·
2,913 Posts
The last time I checked, BMW's fuel consumption gauges were electronic not pneumatic, and combined the injection time, engine speed, and road speed to calculate and display the fuel consumption. The U.S. mpg gauge is difficult to read because it is non-linear and sparsely marked. It is non-linear because the movement sweeps out liters per 100km, a scaled inverse of miles per gallon.
 
1 - 20 of 24 Posts
Top