Mercedes-Benz USA (MBUSA) is introducing a limited number of its E320 BLUETEC diesel sedans in California. The sedan is now available through a special two-year/24,000-mile lease, and is lease-priced on a par with its gasoline sibling, the E350.
At its initial US introduction, the E320 BLUETEC slightly exceeded the CA LEV II, EPA Tier 2 Bin 5 emissions threshold, and so was not put on sale nationwide. (Earlier post.) To meet the stricter standards of California and the other LEV II states, Mercedes-Benz made some modifications to the aftertreatment system, including changes in the control software. Although the catalyst material used in the NOx catalyst is the same, the unit is slightly different, according to Professor Dr. Herbert Kohler, Vice President, Group Research & Advanced Engineering Vehicle and Powertrain, Mercedes-Benz Cars and Chief Environmental Officer for Daimler AG.
|Aftertreatment system for the E320 BLUETEC. Click to enlarge.|
The E320 BLUETEC features a new-generation 3.0-liter V6 diesel engine. The new 72-degree V6 diesel features four valves per cylinder, dual overhead camshafts and centrally located piezo-electric fuel injectors. Designed to spray diesel fuel directly into the center of the combustion chamber, each injector is positioned in the aluminum cylinder head about where a spark plug might be found on a four-valve gasoline engine. This layout ensures even dispersion of fuel as its flame front spreads concentrically across the combustion chamber.
Although many non-turbo diesel engines have compression ratios higher than 20:1, Mercedes engineers found that, in conjunction with the BLUETEC engine's exhaust-driven turbocharger, the engine is most efficient with a compression ratio of 16.5:1. The turbocharger features Variable Nozzle Turbine (VNT) technology which is integrated with the electronic engine management system. To vary the combustion air volume quickly, VNT adjusts the guide vanes electrically as load and engine speed conditions change.
Diesel engines usually operate without any throttle in the intake system, so that fuel delivery alone controls engine load and speed. However, Mercedes engineers found that, at light load, throttling one of the two diesel intake ports on each cylinder creates air turbulence that helps optimize the combustion process and further reduce fuel consumption and exhaust emissions. The BLUETEC engine makes use of electronic intake port deactivation that’s automatically managed by the electronic control unit.
The quick piezo-electric injection system enables the application of a few pilot injections in rapid succession just milliseconds before the main injection to pre-heat the combustion chamber, thereby making pressure and temperature increases more gradual. Smoothing out combustion pressure and temperature spikes softens the usual diesel noise and reduces oxides of nitrogen formation as well.
In addition to using pilot injection and exhaust gas recirculation to minimize oxides of nitrogen, the E320 BLUETEC engine features an advanced-technology NOxstorage catalyst that temporarily stores oxides of nitrogen. Regeneration pulses release a form of nitrogen oxide that reacts with exhaust gas to become nitrogen. Control of these regeneration events was one of the modifications made to bring the E320 into compliance with LEV II.
The 208 hp, 400 lb-ft E320 BLUETEC carries a fuel economy rating of 35 mpg.
The California lease program will function as a bridge as MBUSA prepares to begin phasing in 50-state BLUETEC vehicles nationwide starting in 2008. The first step will be the extension in January 2008 of the special lease program to other states which have adopted the California emissions standards and so which do not currently allow sales of new diesel automobiles.
Later in 2008, Mercedes will introduce another BLUETEC system on its M-, R- and GL-Class vehicles. This system utilizes AdBlue injection, a process that adds precisely measured quantities of a urea-based solution into the exhaust stream which enhances long-term emissions performance sufficient to meet the Tier 2 Bin 5 standards.
MBUSA will offer 50-state diesels across its SUV model lines (M, GL and R-Class) late next year and will continue to add to its alternative powertrain offerings, with BLUETEC mild hybrids, two-mode gasoline hybrids, and more BLUETEC models announced to be delivered over the next few years. (Earlier post.)