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How Mercedes-Benz BLUETEC Works

A technical tour of Mercedes' new super-clean diesel

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2007 Mercedes-Benz E320 BLUETEC engine cutaway

2007 Mercedes-Benz E320 BLUETEC engine cutaway

Photo © Mercedes-Benz
BLUETEC is the brand name Mercedes is applying to their new "clean" diesel cars. Let's take a technical tour of the BLUETEC system from engine to tailpipe.

NEW 3.0 LITER ENGINE

The heart of Mercedes diesel cars like the E320 BLUETEC is a new 3.0 liter V6 turbodiesel engine. The engine has four valves per cylinder and each fuel injector is located at the center of the top of the combustion chamber, in the same location where most four-valve gasoline engines locate the spark plug, for optimum fuel burn. A chain-driven balance shaft inside the engine smoothes out vibration.

COMMON-RAIL INJECTION

Whereas older diesel engines had a mechanical pump that fed each cylinder individually, the BLUETEC's injectors are fed by a central fuel rail that is supplied with fuel at extremely high pressure (approx. 23,000 psi).

PIEZO INJECTORS

Diesel combustion is achieved compressing air to raise its temperature and then injecting fuel. The fuel burns and expands, pushing the piston down. Traditional injectors used a mechanical or magnetic valve. The Mercedes engine's individual injectors use piezo-ceramic elements whose crystalline structure changes shape as electric current is applied. The piezo injectors can divide the injection cycle into as many as five separate injection events, each specially timed to maximize combustion efficiency. This not only improves economy and lowers emissions, but it also reduces noise.

EXHAUST TREATMENT

The BLUETEC system has a number of components that "scrub" the exhaust before it is released into the atmosphere. Two variants of the BLUETEC system exist: the NAC+SCR system and the AdBlue system. NAC+SCR is used on the "45 state" version of the E320; AdBlue cars will be introduced in the 2008 model year and will be sold in all 50 states.

NAC+SCR

Exhaust leaves the engine and passes through a Diesel Oxidation Catalyst (DOC), which reduces carbon monoxide and unburned hydrocarbons in the exhaust. Next is the NOx Absorber Catalyst, or NAC, which removes and traps oxides of nitrogen (NOx is one of the chief elements in diesel pollution). During periods of lean operation (low fuel-to-air ratio) NOx is stored; under richer operating conditions (which can be created by manipulating the fuel injection) the NAC undergoes a regeneration process and releases ammonia into the exhaust. The ammonia is stored downstream in the Selective Catalytic Reduction (SCR) catalyst which uses it to further reduce NOx.

In between the NAC and SCR catalysts is a particulate filter that traps particulate emissions (soot). As the particulate filter becomes full, the engine computer manipulates the fuel injection process to raise the exhaust gas temperature, which in turn burns off the particulates.

ADBLUE

The AdBlue system houses the DOC and particulate filter in a single housing. In addition to the NAC catalyst, ammonia is supplied by injecting a fluid called AdBlue into the exhaust upstream of the SCR catalyst. The addition of AdBlue fluid enables the SCR catalyst to reduce NOx emissions to a level even lower than the NAC-SCR system. AdBlue is carried in an on-board tank which can be replenished when the car is serviced; a gallon of AdBlue fluid lasts approximately 2,400 miles.

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