TDi vs Common Rail Diesel - a Commonsense Guide

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TDi vs Common Rail

Without going into too much detail, diesel engines don't have spark plugs - they use really high cylinder pressure to generate ignition.  As pressure on a gas increases, so does its temperature.  By putting massive pressure on diesel fuel in an engine cylinder, ignition temperature can be reached without the need for a spark from a spark plug.  However, more recently, engineers have discovered that additional efficiencies can be obtained by also pumping the fuel into the cylinder under great pressure.

This is where new technology has lead to diesel engines being able to go from Melbourne to Brisbane on a single tank of diesel in a passenger car.  The two technologies are TDi (Volkswagen) and Common rail (Fiat).  While both technologies (TDi 2005 version) yeild similar fuel efficiencies, they achieve additional cylinder pressure in different ways.

Common rail electronically pumps the diesel into the cylinder under enormous pressure through a series of pipes.  TDi has seperate mechanical pumps attached to the top of each cylinder with no pipes.  The latest common rail engines have pump pressure which is so high that special engineering has to go into injector nozzle design and manufacture.

The argument is that, while diesels have been renowned for having less moving parts than petrol engines, there are now more things to go wrong.  Volkswagen argues that seperate pumps means no pressure is lost within a complex system of pipes, pressure can be increased as technology evolves, and there is less to go wrong and fewer breakdowns.  Fiat argues that common rail boasts marginally better fuel economy in the lab, however, on the road, the results were unable to be differentiated until the most recent designs.

On 25th January 2006, Volkswagen announced it will be dumping its line of TDI diesels [‘pump duse’] 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.

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