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Moving toward a transportation system that fuels healthy people and a healthy planet.

MAX Update No. 28: MPG Heats Up

I've been on the road for a couple weeks, laptop by my side, weighing the suggestions about pumping up MAX's performance (see Update No. 26 and the numerous comments attached to it).

One that especially sparked my interest was the idea of relocating the turbocharger to the back of the car — the commenter explained the idea in detail and it makes sense. The comment, from Glen2Gs, ended with this:

“… The "Still Born" Top Gear USA television program took a VW Rabbit Diesel (AKA The Sipster) and were able to "tweek" it to 84mpg with a 0-60 time of 7 SECONDS!  

May be time to send the Kubota ... packing.” 

For those unfamiliar with the Sipster project, they took an '81 Volkswagen Rabbit and put in a power plant from an '03 Volkswagen Jetta TDI (diesel). The word “put” is a bit of an oversimplification and waaay out of my skill set, but that's the basic idea. They backed up the engine swap with aerodynamic modifications and got the results quoted above. It's a good story and worth the read, and it got new folks interested in the subject. All in all, I think it's pretty cool.

But before I toss the Kubota and trade MAX in for a Rabbit, we're not really talking apples and apples here. For example, they used different cardboard-and-duct-tape aerodynamic modifications for the performance runs and the economy runs. But the X Prize Foundation isn't going to let us change MAX’s body depending on which task we're facing, so that's apples and oranges at best.

Maybe not even apples and oranges, since they were “hypermiling” — drafting, coasting, and generally making life rough for the surrounding traffic— for their economy run. So we're comparing driving techniques as much as we're comparing cars, so maybe it's apples and donuts.

But still, 84 mpg is pretty spectacular. But then I read how they measured the mileage and said, “Whoa, that's how the hucksters did it back when they were selling high mileage refrigerator magnets to tie on your fuel line.”

In brief, one morning they filled the tank all the way up to the gas cap (diesel cap?), drove 70 miles, and filled it to the gas cap again. The pump stopped at 0.833 gallons, less than a gallon by a pint and a third. Pretty simple, so what could be wrong with that?

What's wrong is that as the day heats up, the fuel heats up expands, filling the tank from the inside. A factor in this case is the TDI system re-circulates fuel through the fuel pump (which heats the fuel), into the engine compartment (which heats it further), and back to the tank. Diesel expands at roughly 0.05 percent per degree Farenheit and...

Well, I haven't a clue what the difference was between starting and ending fuel temperatures in Top Gear's mileage test. But neither do they, so I'll guess a number that makes the math easy: 25 degrees. That would give an expansion factor of 1.25 percent; times 10 gallons (the capacity of the Rabbit tank) is 12.5 percent of a gallon, or one pint. That’s not a factor to ignore when the line between success and failure is a pint and a third (as was in their case, where they were shooting for 70 mpg). Now we're comparing apples and honey bears.

By the way, this fuel expansion thing is not just theoretical. When Sharon Wescott and I won Escape from Berkeley (see Update No. 14), supporters had brought us extra fuel at the finish line. So we filled our under-the-hood tank to the top before we headed out of town. The fuel was veggie oil — I don't know its expansion rate, but it was greater than our fuel consumption rate. In Vegas traffic, we couldn't burn it as fast as it expanded — 15 minutes down the road we stopped to see if we'd sprung a leak, but no, oil was overflowing from the top of the tank. Imagine what fabulous gas mileage we could claim if we used the fill-drive-refill mileage measurement technique.


Browse previous MAX Updates.
Read the introductory MAX article, Here Comes the 100-mpg Car.
Visit the Kinetic Vehicles website for more technical details on MAX.