Are Diesels Cleaner than Gasoline Engines?

The conventional tradeoff between spark-ignited gasoline engines and compression-ignition diesel engines is that diesels are significantly more efficient—on the order of 30%. One reason for this is that diesel fuel is more energy dense (by volume). And more energy density translates into better fuel economy. Despite the added efficiency benefit, diesels are also more expensive due to:

  • Combustion: Diesel combustion results in greater pressures, requiring sturdier construction (which also contributes to the engines’ generally superior longevity).
  • Advanced Technologies: Diesel engines have employed more advanced technologies to improve efficiency and emissions, including high pressure direct fuel injection and turbochargers.
  • Aftertreatment: Diesel engines often require more expensive aftertreatment to meet tailpipe limits as compared to gasoline engines.

This historical tradeoff, however, is beginning to change. Advanced gasoline engines are deploying some of the same technologies—direct fuel injection and turbochargers, for example—that diesel engines have long used. While this is increasing the efficiency of gasoline, it’s also increasing the cost at a much faster rate. In addition, a recent report shows the price of advanced gasoline engines is likely to increase even more due to another reason: the particulate matter emissions of these advanced gasoline engines consist of vastly finer particles than diesel engines.
A study, conducted by the German TUEV Nord in November 2013 and published by the Brussels-based environmental lobby group Transport & Environment, says that new direct-injection gasoline (GDI) engines emit 1,000 times more particles than their predecessors and 10 times more than modern diesel engines.
Air pollution in Europe is estimated to contribute to over 400,000 deaths and cause €330 to €940 billion in damage to the economy annually. In fact, 90% of the population is exposed to levels of particulate pollution that represent a health risk and one third lives in areas above permitted limits. According to the study, new GDI-powered vehicles could take over the lead in particulate pollution among combustion engine-powered vehicles while modern diesels have become more environmentally sustainable.
None of the three GDI-powered compact vehicles in the study—from Ford, Hyundai and Renault—could pass the Euro 6c requirements that will be introduced in 2017, an emissions standard that today is already met by modern diesel engines.
Vehicles are the most important cause of the urban pollution crisis in Europe, says Greg Archer, clean vehicles program manager at Transport & Environment. A shift towards GDI engines offers an improvement in fuel economy but runs the risk of worsening the particle pollution, resulting in illness and even death.
Since 2004, Achates Power has focused on revolutionizing the diesel engine by making it cleaner and significantly more fuel efficient than today’s conventional engines—putting Achates Power in the lead when it comes to developing a sustainable technology that will ensure automakers meet current, and future, efficiency and emissions standards.

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