[Shotimes] towing and fuel economy

[van Oss]

You and Porter have explained the physics.  I grasp the physics.  I think my 
question is more specifically about engine mechanics.  Let me break it down.

-- Cruising at 40 MPH in 3rd gear, without trailer, the tach reads 2500 RPM. 
Same speed and gear with trailer, the RPM should be the same, right? 
Because in the same gear the whole drivetrain is turning at the same rates, 
right?

-- I grasp that with the trailer the engine is "working harder" and using 
more fuel.  With trailer, it takes more force to push the piston down.  The 
injectors squirt longer, yes.  The computer, aiming for stoich, keeps the AF 
ratio as constant as possible, doesn't it?  So here is the main thing I 
don't grasp:  If the RPM is the same, then valves are opening and closing at 
the same rates.  How does more air pass through if the valvetrain is at the 
same RPM?

Maybe I'm the slowest guy on the list.  Sorry.

VO


----- Original Message ----- 
It really boils down to simple physics.  It takes X amount of energy to move
a mass Y distance.  If you double the mass and all else being equal, it will
take twice as much energy to do the job.  In your case, your engine is
converting the btu's in the gasoline to move the mass.  Doubling the mass
requires doubling the fuel requirement.  Of course the frictional drags
don't double, so the fuel consumed isn't entirely double the amount.
Robert Bruce