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"  When it comes to traversing great distances at highway speeds, the diesel engine&amp;#039;s higher compression ratios and lean-burn combustion provide an efficiency that no gas engine can currently match\u2014at least not without a major assist from an expensive hybrid system. Over the diesel&amp;#039;s operating range, the average thermodynamic efficiency\u2014how much work the engine produces from the fuel\u2014is 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.  &amp;quot;There is certainly a convergence in efficiency levels between gasoline and diesel engines,&amp;quot; says Uwe Grebe, GM&amp;#039;s director of global advanced engineering. &amp;quot;While diesels will always maintain a slight advantage, the gap will nearly close in as little as 10 years.&amp;quot; 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&amp;#039;t new, but incremental improvements in electronics and materials have pulled them into the mainstream. And there&amp;#039;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&amp;#039;s VP of spark-ignited engines, boasts that &amp;quot;our ethanol-boosted concept engine achieves thermal efficiency in the low 40 percent range.&amp;quot; But don&amp;#039;t expect the diesel engine to lie down and play dead. &amp;quot;We&amp;#039;ll continue to see incremental improvements in diesel efficiency,&amp;quot; says Marc Trahan, Audi&amp;#039;s North American director of quality and technology. &amp;quot;It won&amp;#039;t be as large as going from sequential port fuel injection to direct injection, but there are still more gains to be made.&amp;quot; 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&amp;#039;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. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/cars/a6274/are-gas-engines-now-more-efficient-than-diesel/>