Die-hard hybrid fans would like to see the technology used in all its many varieties: full-hybrids, mild-hybrid, micro-hybrids, plug-in hybrids, and biofuel hybrids. But we should be careful about slapping the word “hybrid” on too much sheet metal. All hybridizations don’t pass the three-part test of feasibility, appeal and cost. In Europe, where diesels represent half of the car market, the idea of diesel-hybrids periodically gets paraded out—mostly at car shows in the form of a concept vehicle—as the latest silver-bullet-du-jour.

The Citroën C-Métisse diesel hybrid was unveiled last year at the 2006 Paris Motor Show. It was obviously nothing more than eye-candy at the auto show, but less exotic diesel-hybrid concepts (see Citroen C4, Peugot 307, Ford Reflex, Ford C-Max minivan, and a Mercedes Benz S class diesel hybrid) continue to be dangled. The vision of a fuel-saving diesel-hybrid double whammy is too alluring for environmentalists to resist. TreeHugger.com, the excellent environmental blog, claimed that diesel-hybrids are “realistic and attainable prospects” and a “happy sight.” It makes sense, right? If hybrids save fuel, and diesel vehicles save fuel, then automakers could theoretically combine the two technologies to produce super-fuel-saving diesel hybrid passenger cars. In fact, the hybrid-diesel combo has been employed in city transit buses, military vehicles, garbage trucks, and delivery trucks for years. How hard could it be to downsize the approach to passenger cars?

Greenies were further encouraged when Japan Today reported in November 2006 that Toyota was planning to commercialize a diesel hybrid subcompact car as early as 2010. Should we temper our optimism when the sources for this story refused to be named? Let’s dig further into the diesel hybrid archive.

Rewind: Cost Matters

In March 2005, Wired Magazine ran an article entitled “Diesel Hybrids on the Fast Track.” John Gartner wrote that the regular gasoline-electric hybrids—you know, the ones that are actually being driven by the hundreds of thousands across America—are the current champions of fuel economy, but warned that the Prius and Civic Hybrid “may soon get lapped” by diesel hybrids.

The Wired article included details about GM’s Opel Astral Diesel Hybrid concept, promising 59 miles per gallon, and the Dodge Ram hybrid pickups, which DaimlerChrysler was going to release as a diesel hybrid with the goal of getting “better than 30 miles per gallon,” according to company spokesman Cole Quinell. Mr. Quinell also said that DamilerChrysler’s diesel hybrids, based on technology developed with General Motors, would be available in late 2007 or early 2008. The clock is ticking, and little if any news has come out about these vehicles.

In the final lines of the 16-paragraph Wired article—most of which casts the diesel hybrids as imminent prospects—Gartner gets to the bottom line: “Integrating both hybrid and diesel technology could add up to $8,000 to the price of the vehicle.” Six months prior to the Wired article, it turns out that Reuters reported the same sticking point for diesel hybrids: cost. “The main problem is that diesel hybrid cars cost too much to produce—thousands of dollars more than petrol-electric hybrids like Toyota Motor Corp’s Prius,” according to Reuters.

Robert Peugeot, vice president for innovation and quality at PSA/Peugeot-Citroën admitted to Automotive News in January 2007 that the company’s "challenge is to move from prototypes to an affordable car." He said the current estimate of a marketable diesel hybrid for $5,000 extra is "clearly too much." Andrew Fulbrook, powertrain analyst at CSM Worldwide, added, "I can’t see a point in the next five to six years where [diesel] hybrid systems will become a commodity."

Double the Cost, Not the Benefit

That same cost-payback criticism was waged against gasoline hybrids, but Toyota has achieved economies of scale with battery technology and has been offering incentives on Priuses, Camry Hybrids, and Highlander Hybrids. The result: Prius sells like hot cakes and adds to Toyota’s bottom line. GM also deserves credit for introducing the award-winning Aura as a mild-hybrid, which after taxes, comes within a few dollars of the conventional Aura’s cost. These vehicles are on track toward mainstream market acceptance, while combining diesel and hybrid technologies may always double the costs (and market risks) without yielding twice the fuel-saving benefits. The costs have to come much more in line before a carmaker will put such a vehicle into production.

Automakers know they have to respond to increased pressure for reduced emissions.  This is particularly true in Europe where diesels are popular and represent a significant environmental challenge. Carmakers and green-leaning car buyers are thirsty for solutions. But let’s get real. There have been—and will continue to be—winners and losers in the hybrid technology race.