Fuel consumption: dependent on RPM or Throttle?

[Guest Anonymous]

Quite simple. Manual transmission achieves better gas milage becuase you can shift into high gear which lowers RPM. Yet what if you are going 25mph in 5th and you have the pedal floored in order to accelerate? At that moment RPMs are low but the butterfly is wide open? On carbed cars this might mean more fuel spent, but isnt fuel injectors keyed to RPM, or are they?

Just trying to delve deeper into the mystry of fuel economy, as gas is expensive and i am poor.

Just wondering if there is a point where your gear is so low you are wasting more gas to stay in it than you are saving due to low engine load. Perhaps someone can enlighten me? I can only postulate....

[Guest greimann]

There are several generalities about fuel consumption that we can touch on, but be aware there are always exceptions.

 

1. All else being equal, a manual trans car will get better fuel economy than an automatic because of the direct coupling of the engine to the transmission via the clutch. Automatic transmissions have a fluid coupling in the torque convertor that allow slippage that wastes engine power. Lock up torque convertors solve some of this, but they usually only lock in high gear.

 

2. Fuel injection systems are tied to RPM, MAP, throttle position, engine temperature, air tempreature, and sometimes air flow. They are more efficient than carburetors because they deliver a more precise mixture for a given operating condition and atomize the fuel better for more complete burn. Carburetors meter fuel based on air flow through the venturi and are tuned to deliver fuel, that on average, is the correct ratio.

 

3. An engine is most efficient with a given volume of fuel at its torque peak. Operating the engine at RPM ranges that are way off the torque peak is going to reduce efficiency. It is possible to operate at too low an RPM that will waste more fuel than shifting down a gear and raising RPM that brings the motor closer to the torque peak. A good example is that I know from analyzing the fuel usage from the data acquisition on my Holley TBI, that I consume more fuel at 60 MPH in 5th gear, than in 4th gear. In 4th at 60 mph, my motor is turning about 3000 RPM, very close to the 3500 RPM torque peak. In 5th, the RPM drops to 1800 RPM, way off the torque peak, and fuel consumption goes up.

 

This is not true for all motors. Some stock motors that are very torquy at low RPM, like LT1's, are very efficient when runnning at 1200 rpm. That is the sole reason behind the .5 overdrive on a T56 transmission. Any "built" motor that raises the torque peak to a higher RPM, cannnot use such a low OD gear.

 

This doesn't begin to cover all of the other factors that influence fuel consumption, but hopefully it will shed a little light.

[johnc]

Both...

 

Greater throttle openings reduce pumping losses and increase engine efficiency. Less rpm reduces operating friction and increases engine efficiency. That's the basis for BMW's eta series of 6 cylinder engines in the 1980s. But, if you run a CVT transmission, then you can tune the engine for even greater efficiency in a much narrower rpm band.

[Mudge]

Originally posted by theoompaloompa:

Yet what if you are going 25mph in 5th and you have the pedal floored in order to accelerate?

Bigtime strain on the engine, and you'd be in a non optimal gear so where you are in the powerband would not be pretty, in other words you'd be in a highly inneficient area of the RPM band to be going WOT. This means poor MPG also.

 

Like said above, RPM is friction to a point as well, peak fuel comsumption versus RPM occurs guess when, peak torque output. Increase the RPM and you increase fuel consumption, but likewise running too low RPM is not only risky for proper oiling, but you will put the engine out of its efficiency range if your trying to cruise at 500 RPM with tons of throttle.