WASHINGTON — I should have set aside a day to celebrate its being, the goodness and specialness of it. But it was too easy to take it for granted, to expect that it would always be there fulfilling my will.

It’s gone now. Volkswagen took it back to lend to other automotive journalists, many of whom probably will write about it strictly in terms of nuts and bolts, as if those were the things that mattered most.

The 2010 Volkswagen Jetta TDi sedan is far more than the sum of its parts. It is proof that economy and efficiency need not be in conflict with fun-to-drive. It demonstrates beyond a reasonable doubt that one good drive system to move a car or truck makes more sense than the use of two to do the same thing.

The heart and soul of the Jetta TDi is its turbocharged diesel engine. It is a 2-liter four-cylinder device that delivers a maximum of 140 horsepower and 236 foot-pounds of torque. (Engineers prefer the construction “pounds-feet torque.” But theirs is a devotion to technical accuracy that loses meaning in translation to daily language.)

“Horsepower,” an English concept derived from the power exerted by a horse in pulling a load, is a technical measure of an engine’s ability to do the same thing. Thus, the traditional thinking in the automobile industry has favored horsepower — the more, the better.

But power and efficiency are not the same. “Efficiency” speaks to management of effort. Power better managed requires less power, which requires less fuel to do identical work.

In a car or truck, “work” speaks to “torque,” the measure of an engine’s ability to create twisting force around an axis, to turn drive wheels. An engine that requires relatively less power to create the same, or relatively more, torque is a more efficient engine.

A turbocharged engine recirculates exhaust gases to drive an impeller that pulls more fresh air into air-fuel combustion chambers, commonly called cylinders. The ability of cylinders to hold an air-fuel mixture speaks to their “volume,” which is expressed in liters in commonly used metric measurements.

Generally, more air and fuel means a bigger bang. But turbo-charging more efficiently combines air and fuel, thereby creating a better bang — more power without an appreciably increased expenditure of fuel.

Therein resides the genius of the 2010 Volkswagen Jetta TDi sedan. It combines turbocharging technology with ultra-low-sulfur diesel fuel to deliver 30 miles per gallon in the city and 42 on the highway, and to do it with muscle and chutzpah. It proves that not all cars need 200 horsepower or more to be considered worth driving.

I drove a Jetta TDi sedan all over the great state of Virginia, using its highways and byways in perfect harmony with substantially more powerful vehicles. Empirical observation indicates that I made better time than many of them because I seldom stopped for fuel, which means I often caught up with and passed cars that had zoomed past me on tollbooth-monitored highways.

When necessary, finding ultra-low-sulfur diesel, the fuel required for the Jetta TDi, was easy. The fuel has been widely available in the United States since 2006, when the federal government required 80 percent of the nation’s highway truck fleet to begin using cleaner diesel.

Sulfur resides in crude oil, from which diesel is refined. It is the stuff that manifests itself as thick black smoke when traditional diesel fuels, which have 500 parts of sulfur per million, are burned. Ultra-low-sulfur diesel has been refined so that its sulfur content is 15 parts per million. It can be found at many urban gas pumps, and it’s nearly always available at highway fueling facilities.

Diesel engines are 30 percent more efficient than their gasoline counterparts. But, alas, that competence is not rewarded at the pump. Recent price checks showed diesel fuel selling at an average of $2.96 per gallon in the United States, compared with an average of $2.74 per gallon for gasoline.

Also, diesel technology costs more. The base gasoline version of the Volkswagen Jetta, for example, starts at $17,735. The base version of the diesel-powered Jetta TDi is priced at $22,830.

The bottom line

The diesel technology is worth its price premium. You tend to get better mileage using one drive system, instead of the two (fossil fuel and electric) used in hybrid vehicles. Diesel fuels and engines have been improved to greatly reduce soot and other particulate emissions. Reducing fuel consumption while boosting torque and overall road performance, as evidenced in the Jetta TDi, gives a whole new meaning to “fun to drive.”

Ride, acceleration and handling: The Jetta TDi has more of a sports feel, which means a harder ride than many of its midsize sedan rivals. Acceleration builds slowly. Most of the torque in this one is at the high end, where it counts. Handling is very good.

Head-turning quotient: It’s a work of middle-class propriety striving for stateliness in lieu of elegance, bereft of anything approaching passion in exterior design. But like a good marriage, its interior is put together well.

Capacities: There are seats for five people. Luggage capacity is 16 cubic feet. The fuel tank holds 14.5 gallons (ultra-low-sulfur diesel fuel, available at many service stations, is required).

Safety: Standard equipment includes front and rear disc brakes (ventilated front, solid rear), four-wheel anti-lock brakes, emergency braking assistance, electronic stability and traction control, and side and head air bags.

2010 Volkswagen Jetta TDi

Base price: $23,930

As tested: $27,969

Type: Front-engine, front-wheel-drive compact/midsize family sedan or wagon (the Jetta SportWagen), offered with gasoline or diesel engines.

Engine: As tested, a turbocharged 2-liter, 16-valve in-line four-cylinder diesel engine delivering a maximum of 140 horsepower and 236 foot-pounds of torque. It is mated to a six-speed transmission that can be operated automatically or manually.

Mileage: Real-world mileage was spot-on with the federal government’s rating of 42 mpg highway and 20 mpg city, running with two people, light luggage and use of the air conditioner.