VW will be dumping its line of TDI diesels by 2007 and switching to a common-rail design for all of its oil-burners, with annual production of the new engines totaling over 2 million units. This announcement can probably be taken as evidence that mechanical diesel injection has reached its practical limits, despite the relatively high level of refinement that VW has achieved using the technology. Indeed, there will probably be a number of faithful TDI owners who see no reason to give up the system that has brought respectable power and refinement to passenger-car diesels.

Current TDI products sold here in the states uses a four-port radial injection pump, driven off the timing belt, to achieve the 20,000 PSI or so needed for directly injecting fuel into the cylinder, while VW diesel engines sold elsewhere are available with an innovative system that uses one pump assembly per cylinder. Both systems rely on complex electromechanical and electrohydraulic means to control the volume and timing of fuel delivery. It?s expensive, somewhat finicky (a diesel mechanic in a good mood is indeed quite rare), and doesn?t offer the exacting control over the injection sequence that?s required to meet upcoming emissions targets and customer expectations.

Common-rail injection systems operate much like standard gasoline port injection systems, with a single pump pressuring one or more fuel rails (depending on the physical layout of the engine, there?s usually one rail per cylinder bank), and electrically-actuated injectors providing precise control over fuel delivery. The main difference is the pressures involved - 20,000 PSI and beyond, compared to 40-55 PSI for a typical port injection system. Obviously, it takes one heck of an electromechanical injector to open and close at such pressures, but the technology has come a long ways in recent years, and such systems are quickly obsolescing mechanical injection pumps just as EFI replaced carburetors two decades ago. This system is far less complex from a mechanical standpoint, and things such as multi-pulse fuel delivery become easy to implement.

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