Two vehicles, both low on gas. A thought experiment, maybe?

This is something I think about from time to time:

Many years ago, I had a friend compete in an overland rally.  Along the way he and his buddy both found both of their vehicles low on fuel with no refueling stations in sight.  The two of them decided that the best way to proceed was to lash the two vehicles together and tow one with the other until it ran out of fuel and then swap.  (this was all hard packed, but unimproved, roads)

And that had me scratching my head...

Is it better to operate one engine at a higher demand? (one car tows both)

Or operate two engines at a lower demand? (both keep driving normally, no one tows)

Thoughts?

At least with my 454 powered 3/4 ton pickup, towing a car and trailer and all (so much more load than just lashing two cars together) only impacted the fuel economy by about 30%-40%. So it was noticeably worse, but still much better than driving two vehicles separately.

dculberson (Forum Supporter) said:

At least with my 454 powered 3/4 ton pickup, towing a car and trailer and all (so much more load than just lashing two cars together) only impacted the fuel economy by about 30%-40%. So it was noticeably worse, but still much better than driving two vehicles separately.

Damn.  yeah, that makes simple sense.  Your fuel economy doesnt increase 100% when you flat tow...
 

It also depends on the engine and how knock limited it is.

For sure, most engines will have peak efficiency at high relative output- but that's also at lower engine speed for the best of the best.  It's still generally better at higher load than lower load, though- especially if the engine is not knock limited.  One would assume that that race cars are running the best fuel that they can find, so assuming knock isn't a problem- "slowly" pulling two cars will be more efficient than the two separately going on their own.  That's how trains work.

"Slowly" is that way because it's relative, too- no idea what that actual speed is until you experiment.  As long as the engine is running stoich and not rich, the pair will remain efficient compared to each on their own.

The goal would be high enough speed and load to not run rich or compensate too much for knock.  

With the van I get 12-13mpg empty vs. 9-10 towing a car on an open trailer.

With carburetors the theory was low rpm more thottle, but not doubling the load.

Don't know if this works for fuel injection.

  They would get farther by draing the gas from one car and add it to the other.

iceracer said:

With carburetors the theory was low rpm more thottle, but not doubling the load.

Don't know if this works for fuel injection.

 

  They would get farther by draing the gas from one car and add it to the other.

And abandoning one car.

Above around 35mph, wind resistance starts to become a major factor in fuel consumption, so towing a car essentially removes the wind resistance for the second car.

if you can draft behind a truck, it really makes a significant difference in mpg, the closer you get the better..it was tested by mythbusters

https://www.autoblog.com/2007/10/28/mythbusters-drafting-10-feet-behind-a-big-rig-will-improve-mile/

In general though, running two engines will consume more fuel than one engine because there is more friction involved, like how a v8 gets worse mileage than a I4 even though the v8 is loaded much less than the I4.  I've often been tempted to build a car with two small engines so one can be totally disconnected when not needed.

Streetwiseguy said:

iceracer said:

With carburetors the theory was low rpm more thottle, but not doubling the load.

Don't know if this works for fuel injection.

 

  They would get farther by draing the gas from one car and add it to the other.

And abandoning one car.

But then they would have to go back for it.

Hungary Bill (Forum Supporter) said:

Is it better to operate one engine at a higher demand?

Yes.  BSFC is going much better at higher demand (within reason)

Probably not the same but we have a gas motorhome and get the same fuel mileage if we are pulling the car on a dolly or not.