There are valid engineering reasons why the diesel engine has not been the powerplant of choice for passenger cars in North America. Diesels have a very high compression ratio, a necessity of compression ignition, which means they must be built quite robustly to deal with the cylinder pressures, and that translates to added weight. They have narrow engine speed ranges and power bands, which is why they work well as constant-speed, stationary oil-field well-pump engines or power generators, for example, and why big-rig trucks often have 12-, 15- or 18-speed transmissions. Typically, they have always been noisy, with that familiar diesel clatter at idle a result of the compression-ignition combustion process. Their exhausts have reputations for giving off visible, black soot that's composed of tiny particulate matter and looks bad, smells worse and chokes anyone standing nearby. They have traditionally been hard to start, particularly in cold weather.
Those issues have been secondary concerns in applications where heavy work needs to be conducted with relentless reliability and optimum efficiency -- large trucks, heavy equipment, big boats, ocean-going ships, electrical generators and so forth. In such cases, their operators readily accept the diesel's shortcomings and benefit from its rugged durability, known dependability and its capability to get tough jobs done with proven economy of operation.
A growing demand for increased vehicle fuel efficiency could soon result in an equally-growing demand for more diesel passenger cars, but car buyers continue to dislike many of the diesel's traditional drawbacks. As a result, extensive development has resulted in modern automotive diesels that are essentially transparent to the occupants. The clatter is gone, power bands are widening, exhausts are cleaner, soot is no longer visible, there's no more bad smell and they start and run just like a passenger-car engine should.
But one issue specific to the diesel remains. Compared to a spark-ignition gasoline engine, a diesel engine must be more stoutly constructed and it requires a higher-pressure fuel delivery system that can deal with more extreme operating conditions. This means an engine that's heavier and more expensive to manufacture, and it's unlikely technology will shift change those requirements much to tilt the balance more in diesel's favor. If you can make a diesel lighter and less expensive, you can probably apply that same technology to a gasoline engine and make it lighter and less expensive as well. So if operating economy and reduced fuel consumption is a reason to buy a diesel car, then that economy of use must be balanced against what likely will always be a price premium at time of purchase.
Another thorny issue is emissions. Cleaning up certain diesel emissions is an involved and expensive process. It includes the use of ultra-low-sulfur diesel fuel, which is currently expensive, and some form of filtration or trap to deal with the particulate matter -- the soot. Currently, several manufacturers have introduced diesel after-treatment systems that result in emissions that are no worse than those from a current gasoline car, but those systems are relatively expensive and complicated.
And, as with the gasoline engine, the diesel will always be tied to all the environmental, international, political, societal, financial and other issues of petroleum, even though diesels of the future might not operate on petroleum-based fuel. Perhaps most important, for the foreseeable future diesels will bear the baggage of the past -- that they are dirty, noisy and smelly -- even when none of those things is true.
That's half the story. Here's the other:
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