Drive it Forever
Q. I have a question about your answer in a recent column on HHO. You are correct, energy can neither be created nor destroyed. And, yes, the law does apply to the production of HHO utilizing the electrical generator in the car. The HHO generator puts a load on the generator, which in turn puts a load on the engine, which then consumes more fuel to support the additional load.
But if I were to use solar panels (12V/25W) solely to provide the necessary energy to create the HHO, which is then combusted in the engine, then there is no additional load on the engine. Using this configuration, then, yes, you will see a significant increase in fuel economy. Correct?
A. Sigh. "A significant increase in fuel economy?" No. Let's do the math. You've postulated a 25-watt solar cell. Park this in the sun for 23 hours per day, stipulating that we live on the equator. The cells are not oriented directly to the sun full-time, so we'll stipulate that the cell's output is 25x12 watt-hours x 0.5, roughly 150 watt-hours/day. That's enough energy into the battery to provide roughly 10 amps, at 12.6 volts for one hour, to run our HHO generator.
Suppose we simply put that electricity into the battery pack of a hybrid car instead of generating HHO. The Chevrolet Volt has a projected range of 40 miles, which should deplete the 16-kWh battery pack by 8,000 watt-hours. So our solar cells will take 53 days to recharge the battery pack enough to drive 40 miles. That's a little more than a mile per day.
Now let's use that same electricity to make HHO. Some of the electricity turns into heat in the gas generator, some is lost as heat in the wires to the generator, but we'll simply ignore those losses.
If our gas generator uses 10 amps to electrolyze water into hydrogen and oxygen, that's roughly 120 watts, for one hour, until we've consumed the electricity added to the battery by solar cells. One horsepower is 746 watts, so we're pumping roughly one-sixth of a horsepower into the intake manifold in the form of HHO.
It takes five to 10 horsepower to make a car run 60 miles per hour. Multiply the one-sixth horsepower by the efficiency of the internal-combustion engine burning gasoline or diesel fuel - I'll stipulate a generous 30 percent for that - and you get roughly 0.5 percent. Consequently the HHO is contributing a potential one-half of 1 percent to the vehicle's fuel economy, and probably less.
Remember, we're talking about a 100-plus-mpg Chevrolet Volt that doesn't exist yet. Adding an HHO generator to your 14-mpg pickup will have so little effect that it couldn't even be measured.
Q. I own an Audi 2001 2.7T Quattro, and I had four Michelin Plus tires on all four wheels and a brand-new spare, a Michelin Pilot, that I used on the passenger-side rear wheel when I had a flat. Since the Michelin Plus are discontinued, can I buy one Pilot and put it on the other rear wheel or, because the car is a four-wheel-drive, will I have to buy four new tires?
You must match at least both tires on the same axle, regardless of whether the car is two-wheel-drive or four-wheel-drive.
What really matters on four-wheel-drive is that the tires have the same diameter. Specifically, the revolutions per mile must be the same. If they're not, the center differential will slip constantly.
How important is this? Putting two new tires on the rear, which is where they always should go, and leaving tires with 50-percent tread wear on the front is asking for trouble. That's why I like to rotate tires regularly, even though some car manufacturers don't recommend rotating tires on four-wheel-drive vehicles.
The Michelin dealer can give you a specific number for the revs/mile for those tires from his product data. Then you can measure the tread depth for all of those tires and pick the two that are closest.
Mike Allen is a senior editor for Popular Mechanics magazine. Questions should be sent via e-mail to driveit@nytimes.com.
