Mercedes-Benz Debuts Direct-Injection Engines

A number of 2012 Mercedes-Benz models are powered by new engines that feature direct fuel injection. The new four-cylinder and V6 get better fuel economy while producing more pulling power – an impressive feat, considering that increasing either power or fuel economy usually decreases the other.

201 Horsepower 1.8-Liter Four-Cylinder

The new-generation SLK roadster, the new C-Class coupe and the redesigned C-Class sedan are all available with a 1.8-liter in-line four-cylinder engine (201 hp, 229 lb.-ft. of torque) that's fitted with direct fuel injection and a turbocharger. Designed with the latest four-valve-per-cylinder and double-overhead camshaft technology, the new turbo four also has two internal balance shafts to ensure it's as smooth as a six-cylinder.

302 Horsepower 3.5-liter V6

The new-generation M-Class and R-Class sport utilities, the C- and E-Class lines and the new SLK roadster can all be equipped with a 3.5-liter V6 that produces 302 horsepower and 273 lb.-ft. of torque. Fitted with multi-spark ignition as well as direct injection, the engine is assembled in a new block with 60-degree cylinder angles, so that a balance shaft isn't needed for smooth operation.

The previous V6 had 90-degree cylinder angles so it could be built in the same modular plant as the company's V8s. Like its predecessor, the new engine has aluminum cylinder heads, pistons and cylinder block (with cast-in Silitec cylinders), as well as a crankshaft, connecting roads and valves made of special forged steel.

Some Direct Injection Background

While most conventional engines still use indirect port fuel injection, the world's first gasoline engine with direct injection was the legendary 1954 Mercedes-Benz 300SL Gullwing. Since then, the company has pioneered direct injection for several high-efficiency European models, including the 2006 CLS350 CGI, powered by the world's first gasoline engine with piezo-electric direct injection and spray-guided combustion. This advanced engine achieved 10 percent better fuel economy versus the normal V6 with port injection.

The New Direct Injection System

The fuel system on the new four-cylinder and V6 engines represents the third generation of modern Mercedes-Benz gasoline direct injection systems. First used on Mercedes-Benz diesels, the electronic fuel injectors spray gasoline directly into the combustion chambers.

In addition, the new V6 makes use of industry-leading electronics technology that features a piezo-ceramic crystalline element in each fuel injector. The piezo crystal changes shape instantly when electrical current is applied, which makes it possible to design very sensitive and precise injection systems, including the ability to program several small injections with each piston stroke. This is especially impressive, considering that engines idle around 20 strokes per second and at high speeds, run at about 200 strokes every second.

The first injection is sprayed into the combustion chamber as the piston is descending on the intake stroke. Depending on speed, load and temperature conditions, another injection or two takes place during the compression up-stroke before ignition, forming a stratified mixture.

A fourth injection can stabilize combustion if it's needed. Among other things, this advanced spray-guided combustion process demonstrates the potential of the internal combustion engine for continued development and refinement. System pressure of the electronic direct injection is similar to mechanical injection systems – up to 2,840 psi.

Streamlined Cam Chain Drive

A new cam chain drive system on the V6 allows the hydraulic camshaft adjusters to be more than a half inch smaller in height and width. At the same time, they are now 35 percent faster, and can vary valve timing through a wider range of 40 crankshaft degrees. The crankshaft drives an intermediate shaft, and in turn, the intermediate shaft drives two short chains – one for each cylinder bank – that loop around the intake and exhaust camshaft drive sprockets.

The new chain drive results in less tension and lower chain dynamics, for even lower friction and less noise. A fourth chain drives a new variable vane-type oil pump in the bottom of the engine. At low engine speed and load, the oil pump only generates about 28 psi (or two bar) of oil pressure, and nozzles that spray cooling oil on the pistons are off. As engine speed and load increases, oil pressure goes up, and the oil spray nozzles are turned on. In this way, less energy is used when less cooling and lubrication is needed.

Multi-Spark Works with Multi-Squirt

Working together with direct injection, a rapid multi-spark ignition system on the V6 begins combustion with the first spark, but has the capability to recharge and deliver up to three more sparks within a single millisecond, creating a gas plasma with more expansion than conventional ignition.

The time lapse between sparks is adjustable, so combustion duration can actually be controlled, resulting in two percent better fuel economy, and a total of four percent improvement in combination with direct fuel injection.

Three-Phase Low-Load Cooling System

Even the cooling system is significantly refined in the new engine, beginning with a two-stage flow circuit through the cylinder head. This improved coolant flow results in better heat dissipation, despite lower coolant pressure, so that the water pump uses less engine power.

A three-phase cooling system helps the engine warm up very quickly. When the engine is first started, no coolant circulates. Then, as the engine warms up, coolant begins to circulate within the engine, but not through the radiator. Only when the coolant temperature reaches 221 degrees Fahrenheit (or 189 degrees F. under high load), coolant also circulates through the radiator. Coolant circulation through the heating system for the car's interior is controlled separately.

Low-Friction Silicon-Aluminum Cylinder Sleeves

Mercedes-Benz was the first automaker to use innovative cast-in silicon-aluminum cylinder sleeves with a low-friction surface that allows piston-ring spring tension to be reduced by 50 percent. The efficiency payoff for low internal friction means both fuel savings and increased power.

This sleeve technology is also designed to provide exceptional block stiffness while minimizing weight. The sleeves are more than a pound lighter than conventional iron sleeves, resulting in very light components.

The Building Blocks

Assembling the new Mercedes-Benz V6 engine underscores its efficient design. An exceptionally stiff forged-steel crankshaft is bolted into a pressure-cast aluminum block, and an aluminum oil pan also contributes to block rigidity. Aluminum pistons are mounted on the connecting rods, then slid into the silicon-aluminum sleeves, which are an integral cast-in part of the block.

Connecting rods must be made in two pieces for assembly on the crankshaft, and Mercedes engines use hollow, forged steel rods that initially are made in one piece, then hydraulically “cracked,” instead of being machine-cut and reground. The irregular fracture provides a very strong, durable fit, even at high engine speeds, and shortens the production process since re-grinding isn't necessary.

The two cylinder heads are bolted onto the block, and twin camshafts are installed in each head, with cam drive chains looping around the intake and exhaust camshaft sprockets. Double-wall exhaust piping is used to keep the exhaust air as hot as possible leading down to a catalytic converter below each cylinder bank.