Building a 100mpg consumer vehicle

[drmiller100]

http://ecomodder.com/forum/tool-aero-rolling-resistance.php?Weight=1000&WeightUnits=lbs&CRR=.00 8&Cd=.25&FrontalArea=12&FrontalAreaUnits=ft^2&Fuel Wh=33557&IceEfficiency=.22&DrivetrainEfficiency=.9 5&ParasiticOverhead=0&rho=1.22&FromToStep=5-200-5

[some_other_dave]

Originally Posted by drmiller100

60 inches wide front fenders, 2 feet tall.

cockpit is 42 inches tall by 35 inches wide.

The windshield is behind the fenders. Does that mean my frontal area is 10 square feet, or 10 plus 5?

Depends on how much overlap there is.

If you took a picture from the front, and colored in everything that was the car (fenders, cabin, wheels, tires, etc) the total of the 2-D colored area would be the frontal area.

-soD

[Frank Lee]

With those dimensions it's physically impossible to have frontal area of 10 sq ft.

[drmiller100]

ok.
60x24 for the nose = 1440
20x33 for the cockpit above the nose = 660. minus probably 60 because the top corners are rounded.

call it 2000 square inches - / 144 - = 13.8 square feet.

can I make it as clean as a Prius or Insight? I sure hope so.

[Ken Fry]

Originally Posted by prepperdad

Where can I buy such a car? The fuel economy sounds absolutely incredible.

I have two slots left for 2012. See Think Big. Drive Small. for details. My phone number is on the site, so feel free to call anytime.

[drmiller100]

Originally Posted by Ken Fry

Hi Doug,

The Zing! is driveable. I
Ken

so I am confused. Is the Zing! a single seater or 2 seater?

[Ken Fry]

Originally Posted by drmiller100

ok.
60x24 for the nose = 1440
20x33 for the cockpit above the nose = 660. minus probably 60 because the top corners are rounded.

call it 2000 square inches - / 144 - = 13.8 square feet.

can I make it as clean as a Prius or Insight? I sure hope so.

If you have 38" headroom, and 5" ground clearance, and a 4" thick seat, you'd have 47" height with the seatback vertical. A Corvette is 48" which may be a reasonable minimum for being seen, especially with the narrow cabin. The Lamborghini Countach was only 42," as is the T-rex. I've towed the Zing! POC, and see just a tiny bit of the roof out the back window of my Accord. The POC is 44", and the production version is 49."

It sounds like you are saying that the top of the fender is 24" off the ground. A 24" diameter is a smallish tire. You will probably want several inches for suspension travel.

There is no standard for how a manufacture must measure frontal area. I prefer to use the actual dimensions from a cad drawing, including wheels, etc. In your case that would reduce the area a little, because we wouldn't be measuring the air under the car. We'd have to count the projection of the rear wheel.

Your 13.8 figure is probably pretty good, if you're OK with the height.

[Ken Fry]

Originally Posted by drmiller100

so I am confused. Is the Zing! a single seater or 2 seater?

The Zing! POC seats 1; the Zing! production version seats 2.

[Ken Fry]

Originally Posted by drmiller100

http://ecomodder.com/forum/tool-aero-rolling-resistance.php?Weight=1000&WeightUnits=lbs&CRR=.00 8&Cd=.25&FrontalArea=12&FrontalAreaUnits=ft^2&Fuel Wh=33557&IceEfficiency=.22&DrivetrainEfficiency=.9 5&ParasiticOverhead=0&rho=1.22&FromToStep=5-200-5

Hi Doug,

The calculator you linked to is useful for certain very limited purposes. As you probably know, the urban cycle has an average speed of 20 mph. I plugged in numbers for a Prius (plus driver) using its peak fuel efficiency of 38%. At 20 mph the Prius gets 196 mpg, according to the calculator. We know the Prius really gets 50 mpg on the urban cycle, so the calculator would appear to be off by a factor of 4.

the calculator

The "problem" is that the calculator is applying one efficiency number to all speeds. If you look at 85 mph (on the perfectly flat road represented by the calculator) then the number is close to 50 MPG, meaning that at that speed the engine might be operating at a point near peak efficiency. But we would have no way of knowing that, because we didn't look up the efficiency at the power required for 85 mph. (Because we are working backwards, we can conclude that the Prius engine must be close to 38% efficient at 85 mph, where it is producing 27 hp. So at some speed, the calculator produces a number that is close. At all other speeds, it produces a number that is incorrect -- any where from wrong to incredibly wrong. And we have no way of knowing where it is producing a correct number be cause we can't feed in the right BSFC for each load condition.

To use the calculator, you'd have to plug in actual efficiency figures (from a BSFC chart) that correspond to the engine efficiency at each speed. (This would be much easier to do with another spread sheet made for the purpose.
That other spread sheet, or series of spread sheets, could assess the power required at each five seconds on the UDDS, lookup the relevant efficiency at that power point, and use that in doing the math.

For many cars, you can come close to a correct combined figure if you calculate the fuel efficiency while going up a 2.5% grade at 45 mph. You calculate the hp required (more or less as as the linked calculator does, but incorporating grade) then find the efficiency at that hp level and then do the math to find mpg.

Optimizing for running near peak efficiency is why a Prius gets twice the mileage of an Accord. You can't just plug in a single efficiency value for either and get a good mpg number.

Of course, you can skip the math, and just go ahead and build it. It will probably get pretty good mileage.

Regards, Ken

[Ken Fry]

Originally Posted by drmiller100

If I get close, but can't get it all the way, I've thought about using V-tec to hold two cylinder's exhaust valves open, and shut off two fuel injectors. Idea would be I would shut off two cylinders and lose the pumping losses at part throttle.

I've got another couple of ideas to minimize pumping losses and relieve the penalties of a large cc engine.

Sounds like fun. You'd probably get fewer pumping losses with both valves closed, rather than pulling in exhaust through the open exhaust valve.