When it comes to traversing

great distances at highway speeds, the diesel engine's higher compression ratios and lean-burn combustion provide an efficiency that no gas engine can currently match—at least not without a major assist from an expensive hybrid system. Over the diesel's operating range, the average thermodynamic efficiency—how much work the engine produces from the fuel—is in the mid 30 percent range, at least 15 percent better than a gas engine. Not even close, right?

The reality is that this lead is shrinking. As emissions regulations stiffen, diesels are slowly losing their edge; the same pricey after-treatment systems that scrub diesel exhausts also happen to crimp efficiency. Meanwhile, gas engines continue to improve.

"There is certainly a convergence in efficiency levels between gasoline and diesel engines," says Uwe Grebe, GM's director of global advanced engineering. "While diesels will always maintain a slight advantage, the gap will nearly close in as little as 10 years."

Over the past decade, once-exotic efficiency-enhancing hardware such as variable camshaft timing, direct fuel injection and turbochargers have become commonplace on spark-ignited engines. Certainly, these technologies aren't new, but incremental improvements in electronics and materials have pulled them into the mainstream. And there's more on the way, like lean-burn combustion and homogeneous charge compression ignition (HCCI), a gas-combustion technology that blurs the line between gas and diesel engine cycles. Ricardo is working on a turbocharged engine that uses E85, a lofty compression ratio and high boost levels to achieve diesel-like efficiency. Rod Beazley, Ricardo's VP of spark-ignited engines, boasts that "our ethanol-boosted concept engine achieves thermal efficiency in the low 40 percent range."

But don't expect the diesel engine to lie down and play dead. "We'll continue to see incremental improvements in diesel efficiency," says Marc Trahan, Audi's North American director of quality and technology. "It won't be as large as going from sequential port fuel injection to direct injection, but there are still more gains to be made." Trahan says these smaller gains will come from hardware such as variable valve timing and independent cylinder combustion control, as well as improved after-treatment systems.

Moreover, there are other factors in play. As GM's Grebe points out, diesel fuel contains about 14 percent more energy by volume than gasoline. This gives compression-ignition engines a significant edge in fuel economy, as opposed to thermal efficiency. Of course, everything will change if and when spark-ignited engines switch to more energy-dense fuels. This race is far from over.

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