SANTA MONICA, California — Before we reached here, we crested the Rockies after a delay for snow and ice. We basked in their jagged, snow-capped majesty.

Our clean diesel Audi TDIs have easily made the climb with the incredible tug of torque that diesel produces, aided by the advantage of turbo-charging.

Up at about 12,000 feet and higher, the air gets very thin. Regular engines gasp; turbo pulls in extra oxygen for smooth running.

But as we descend toward the vast flats, we push on — easy to do when you're behind the wheel of a luxurious Audi, its diesel humming, clean, quiet and efficient.

Diesel has carried us through this grueling ride and you have got to wonder why the world loves diesel while it seems America hardly knows its name.

We are on the second half of the four-leg Audi Mileage Marathon, pushing ever Westward, sipping ultra-clean diesel fuel. Audi has provided us with the cars and the lodging on this road trip that began with some 180 journalists. 

Into the second week, many of the journalists have dropped out along the way. I am sticking with it, waiting to see that Santa Monica pier.

But first there are forbidding and beautiful lands to cross — once you are on them.

The Great Sand Dunes National Monument: Fine brown grains blow down from the Rockies and pile high in southern Colorado.

We are driving four models of Audi — A3, A4, Q5, and the hulking Q7. They continue to pile up impressive mileage figures — high 20s to more than 50 miles per gallon, even as we have crossed one more line of demarcation that helped define America's identity as settlers came to the East Coast and headed West.

There are parched parcels between here and sprawling, gawdy Las Vegas and beautiful stops that spring up along the way — Sedona, the sheer-faced majesty of El Capitan at Yosemite National Park, Mammoth Lake, Big Sur, the Pacific Coast at Monterey.

It is these flats, however, on the way to that brilliance, that give us time to ponder the diesel, to hear foreign companions' take on America and clean diesel's place in it.

Why are diesels so efficient? Because they do not operate the same way as gasoline engines, which suck in fuel and air, then compress it, and then ignite this cooler air using spark plugs.

Diesels take in the air, compress it first — which makes the air itself hot enough to enhance ignition. Then it injects the diesel fuel. The result is a more powerful and efficient burn.

The problem for the earlier diesels was not only poor exhaust systems, but the fuel itself. Like gasoline — and home heating fuel — it is derived from crude oil.

But diesel is lower on the fuel chain because even as gasoline is separated in the refining process, it remains heavy with hydrocarbon compounds and sulfur. Without getting too complex, a way to burn off "dirtier" diesel had to be engineered. And that is where Audi's — and other companies' — technology comes in. It's not just about burn, but also about exhaust.

To that end, turbo-charging and enhanced catalytic conversion, along with cleaner fuel and urea injection (a tank filled whenever the oil is changed), make for a cleaner, more economical ride.

And that's what we are getting out here on the big flats, through what must have been an unexpected majesty of the West for those pioneers who stumbled across the Grand Canyon, the Great Salt Lake, the High Sierras, and the crash of Pacific waves on the western edge.

How economical has this journey been?  Even at more than two-and-a-half tons, the Q7, capable of hauling seven people in luxury, averaged 28 miles per gallon coast-to-coast.

The Q5, a "downsized" Q7, got 32. The A4: 36 miles per gallon. And the A3: 45 miles per gallon.

From Central Park to the Santa Monica pier, no saddle sores, a quiet if attention-grabbing marathon, and, perhaps, a question for our new president, Barack Obama:

You promote hybrids and fuel efficiency, but why not cut the high taxes on diesel fuel — aimed at the big diesel trucks that ceaselessly roam our highways — delivering the goods we need but also raising their costs by the very existence of these taxes?

It works in Europe and other countries. It’s clean and efficient.

When it comes to diesel, I would quote the president and say: “Yes, we can.”

(This is the second of a two-part series on diesel.) For the first part of the series, click here. 

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