Speed / RPM / Fuel Economy

[CR94]

Originally Posted By: Kestas
The auto industry designs the drivetrain to optimize efficiency at 70 mph.
That might technically be roughly true, in terms of fuel energy consumed per unit mechanical work done by the engine, but it's certainly not normally true in terms of fuel consumed per distance traveled.

 

[Dave9]

Originally Posted By: Kestas
The auto industry designs the drivetrain to optimize efficiency at 70 mph.


Have any supporting evidence? All the data I've seen from owner MPG vs speed contradicts this. The "average" speed at which the "average" US automobile gets best MPG seems to be pretty near 50 MPH, a little higher for more modern vehicles with 6+ speed transmissions and less aero drag but not up around 70 MPH.

This has been extensively studied, which source would you like?
https://www.google.com/search?q=best+speed+for+fuel+economy

 

[supton]

I do believe efficiency is not mpg. Efficiency is power out divided by power in. Wind drag goes up by the square of speed. So I could see an engine getting more efficient at higher engine speeds--but ultimately using more gas in order to cover the distance. Meanwhile, plodding along at a slower speed might have a higher percentage of waste heat--yet burn less gas per mile traveled.

 

[KrisZ]

Every other indystry, be it aerospace, construction, transport, basically where equipment is nothing more than a tool to accomplish a certain task, uses specific fuel consumption (SFC) to compare the efficiency. SFC is based on a load, which is usually around 80% of the total engine output.

Unfortunately in personal and light duty transportation it is hard to guess the average load and there is also the emotional aspect. Hence engines are way overpowered for their average load. Therefore they have to be geared for low RPM operation. It gets fuel economy bump despite the engine not operating in its optimal range.

 

[Kestas]

As supton suggested, I didn't say you get the best mpg at 70 mph, but rather the best efficiency of the drivetrain -- most notably the gearing, engine valving, etc.... unless you have a CVT.

 

[sleddriver]

Originally Posted By: CR94
Aerodynamic drag is proportional to the square of speed, therefore the energy required to overcome it for a particular distance is too. That also means power required to overcome aero drag is proportionate to the cube of the speed.
THIS^^^^^
True & brutal when you want to go fast.
Ditto when you also want to go BIG & Tall (Loaded Surburban flying down the freeway at 75 MPH...)

 

[meep]

I've paid a lot of interest to the mitigation of pumping losses and enjoy the newer strategies which keep revs down but throttle way open outside of the powerband. Our CRV seems to do this and the overall experience is not intrusive and yet the real world mpg is excellent for its form. Other designs such as multiair, valvetronic, etc., reduce pumping losses by removing the pressure variations on both sides of the piston by removing the throttle plate (if I understand that right?).

But here's the recent question I've had. In a modern turbo, such as the EB in my truck, the output side of the turbo still presses up against a throttle plate, which equates to some level of exhaust resistance on the other side. Wouldn't it work better if the turbo were downstream of the throttle, so under low-load conditions it was spinning in a vacuum where it would pose less of an exhaust restriction? Obviously there could be a throttle feel issue here requiring another level of mitigation, but it seems like a lot of gains could be made there. Yet, the real world mileage of for this setup has been a significant advantage as-is for me, so they've done something else to mitigate pumping loss. How did they do it?

-m