2007 Mercedes Benz E320 BlueTec

Phillipe Crowe

From the outside, the new Mercedes-Benz E320 BlueTec looks like any other E-Class sedans. In fact, only the "BlueTec" badge on the trunk lid and an added warning light inside (indicating glow plug operation) identifies it as something special. It's not until you look under the hood that one finds a new technology that could change people's perceptions of diesels forever.

Mercedes' new BlueTec system combines several technologies in an effort to reduce diesel exhaust emissions. It's built on the foundations of an advanced V6 engine that includes four valves per cylinder, centrally located Piezo-electric injectors, the latest CDI direct injection, a turbocharger with a variable nozzle turbine and an exhaust gas recirculator.

FARTHER, FASTER, CLEANER

Like any diesel, the E320 BlueTec gets improved fuel mileage and increased range over its gasoline-powered counterparts, but more important, it does it without the usual tradeoffs associated with diesel ownership.

Start the BlueTec on a cold morning and you still hear a muted version of that characteristic diesel engine sound. However, once the car's warmed up, most people would be hard-pressed to finger it as an oil burner.

Under acceleration, the E320 BlueTec feels faster than the gas-engined E350 six-cylinder, a result of its substantial 388 lb-ft of torque (as compared to 258 in the petrol models) - that pulling power is only three lb-ft short of the E550 V8! And trust us, with all this power available, the BlueTec E-Class is truly fun to drive.

FOUR-VALVE V6 WITH CENTRAL INJECTORS

The E's new 72-degree 3.0-litre V6 features four valves per cylinder, dual overhead camshafts and, as mentioned, centrally located Piezo-electric fuel injectors. Designed to spray diesel fuel directly into the center of the combustion chamber, each injector is positioned about where a spark plug would be found on a four-valve gasoline engine. The layout ensures even dispersion of fuel and a burn that spreads concentrically across the combustion chamber.

Based on an aluminum block with cast-in steel cylinders, the BlueTec V6 has an 83 mm bore and 92 mm stroke, a ratio that engineers refer to as "under-square." While most Mercedes-Benz gasoline engines are over-square (the bore is larger than the stroke), many diesels have a relatively long stroke, in part to produce the high compression needed to make the intake air hot enough for ignition.

To neutralize vibration that's inherent to V6s, a balance shaft, located between the cylinder banks, counter-rotates at the same speed as the crankshaft. Twin overhead camshafts in each cylinder bank operate the intake and exhaust valves, with the intake cams driven by a double chain that also drives the balance shaft. A gear on each intake cam, in turn, drives the exhaust camshafts.

VNT TURBOCHARGING

Although many non-turbo diesel engines have compression ratios higher than 20:1, Mercedes found that, with the BlueTec's exhaust-driven turbocharger, the engine is more efficient with a lower compression ratio of 16.5:1. The turbo uses a VNT (Variable Nozzle Turbine), which works with the engine's electronics to open and close its guide vanes as loads and speeds change.

AIR-TO-AIR INTERCOOLER

To keep the turbocharged air as dense as possible, it passes through an air-to-air intercooler -- essentially a radiator that cools the drawn-in air. The intercooler lowers the air temperature by up to 180 degrees F, ensuring a higher air volume and more power.

ELECTRONIC THROTTLE CONTROL

Diesel engines usually operate without any throttle control on their intake system -- the fuel delivery alone controls its load and speed. However, M-B engineers found that, at light loads, throttling one intake port on each cylinder creates air turbulence that helps optimize combustion, reducing fuel consumption and exhaust emissions.

As a result, the BlueTec engine uses electronic intake port deactivation, automatically managed by a sophisticated electronic controller -- the same unit that determines everything from fuel injection quantity and timing, to the vane angle of that VNT turbocharger and the quick-start glow-plug operation. This versatile microprocessor also networks with the seven-speed automatic transmission and the ESP stability control systems.

COMMON-RAIL DIRECT INJECTION

The E320 BlueTec uses the latest version of the Mercedes' CDI system, which keeps the fuel injectors supplied with approximately 23,220 pounds per square inch of fuel pressure through a common-rail fuel loop. A fuel pump, driven by the engine, keeps consistent pressure along the rail, while the amount of fuel used is determined by how long the electronic injectors stay open. A major benefit of the CDI system is its ability to maintain fuel pressure when the injectors are open.

Gone are the days of dealing with a mechanical injection pump's gradual fuel pressure build-up for each cylinder, not to mention its pressure drop-off during each cycle - both distinct disadvantages, especially at higher engine speeds.

In spite of the high pressure it generates, the engine-driven fuel pump requires less than 20 percent of the power needed to turn a mechanical unit. Plus, while mechanical pumps build pressure only for each injector pulse, the CDI system isn't affected by torque spikes, putting less stress on the pump drive.

PIEZO INJECTORS USE MAGIC CRYSTALS

Electronic injectors of any sort were considered technically impossible for diesels until the mid-1990s, since they must remain reliable when subjected to approximately 23,000 psi of fuel pressure. For the first time in a Mercedes diesel, the BlueTec V6 is fitted with Piezo injectors that work faster and more precisely than the previous magnetic solenoid valves, making up to five injections possible with each piston stroke.

The new injectors use a ceramic element with a crystalline structure that changes shape within 0.1 milliseconds when an electric current is applied. As a result, the Piezo injectors are lighter and twice as fast as conventional electronic solenoid valves.

PILOT INJECTION

Historically, the diesel combustion process has involved faster pressure increases and, as a result, more noise than a gasoline engine. With the newfound speed and precision of electronic injection, M-B engineers found that igniting a small quantity of fuel a few times in rapid succession -- just milliseconds before the main injection -- pre-heats the combustion chamber, making pressure and temperature increases more gradual. Smoothing out combustion pressure and temperature spikes softens the usual diesel noises and reduces nitrogen oxide formation. Pilot injection isn't practical with the slow reaction times of mechanical injectors, but with the lightning-quick Piezo design, five injections per stroke can had, instead of the previous three.

BLUETEC EXHAUST EMISSIONS

Because they usually operate un-throttled, with lots of excess air, diesel engines inherently produce less carbon monoxide and carbon dioxide than a gasoline engine. However, because of their high combustion pressure and temperatures, diesels usually make more nitrogen oxides, which contributes to smog.

Emissions, primarily in the form of soot, are kept extremely low in the BlueTec engine by those high-tech injectors and two after-treatment devices in the exhaust system. An oxidation catalyst encourages after-burning of any leftover hydrocarbons, while a particulate filter traps microscopic particles headed downstream. It's automatically cleaned periodically too, via what Mercedes liken to an oven-cleaning cycle.

NOX STORAGE CATALYST

In addition to using pilot injection and exhaust gas recirculation to minimize nitrogen oxides, emissions are further reduced by a third device in the exhaust stream -- a catalyst that temporarily stores NOx. It releases a different form of nitrogen oxide that reacts with the exhaust gas to become harmless nitrogen.

With the availability of low-sulfur diesel fuel in North America, the future of diesel-powered cars seems bright. While most people are probably already aware that diesels get significantly better fuel mileage than gasoline-powered cars, they are just now realizing the traditional trade-offs of choosing a diesel model are nearly gone. Not only are they now quieter and faster, but, in the case of Mercedes' E320 BlueTec, the exhaust is so scrubbed that even a white handkerchief held over the tailpipe stays clean.

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