In the old days, engines used a carburetor to mix air and fuel together. Compared to fuel injection, a carburetor is a pretty primitive device. It's a lot like a can of spray paint or a perfume atomizer. In an effort to better control exhaust emissions, port fuel injection came into favor. Indirect or port fuel injection uses an injector to spray fuel upstream from the intake valve and combustion chamber. Injection provided way to supply the air/fuel mixture more equally to each cylinder. This resulted in lower emissions, better throttle response and more power.
The First Direct Injection Gasoline Engine in a Production Car
In the diesel world, a different approach was used. Rather than port injection, diesels used direct fuel injection. Since diesels have such high compression, they don't need a spark plug. The fuel is sprayed directly into the combustion chamber, where the compression is so high that the fuel simply ignites. Although much more efficient, the complexity and costs prohibited this technology from being used in gasoline engines.
The world's first gasoline engine with direct injection was the 1954 Mercedes-Benz 300SL Gullwing. Adapted from Mercedes-Benz' Formula One powerplant of the time, the original Gullwing's engine used direct injection and produce more power.
Since then, Mercedes-Benz has used direct injection to improve fuel economy on several high-efficiency European models. The 2006 CLS350 CGI is a good example. It was powered by the world's first gasoline engine with piezo-electric direct injection and spray-guided combustion. This technology improved fuel economy by 10 percent over the same engine with port injection.
Mercedes-Benz' Third-Generation Direct Injection Systems
The fuel injection system in the new Mercedes-Benz bi-turbo V8 is the third generation of modern Mercedes-Benz gasoline direct injection systems. The new V8 features piezo-electric fuel injectors that spray gasoline directly into the combustion chambers. This industry-leading technology was first used on modern Mercedes-Benz diesels.
The new engines feature two state-of-the art turbochargers. One turbo is used on each cylinder bank. This gives the new engine tremendous flexibility in terms of power and fuel economy. The exhaust-driven turbos are lubricated and cooled by a special pressure-fed oil system. The intake charge is cooled by a liquid to air intercooler to increase efficiency by reducing intake air temperatures before they are ingested by the engine.
These new bi-turbo V8s have better fuel economy, more power and cleaner exhaust emissions than the previous generation V8s. The Mercedes- Benz powertrain engineers have once again demonstrated why they lead the world in engine development and refinement.
The New 4.6 Liter V8
The 2011 Mercedes-Benz CL550 marks the debut of the new V8 engine. This new engine can get up to 10 percent better fuel economy while producing more than 30 percent more torque. This is an astounding feat, as most engines usually produce increases in either one area or the other - not both. Internally known as the M278, the new V8 features direct fuel injection, twin turbochargers and multi-spark ignition.
Based on the 5.5-liter predecessor, the new 4.6-liter V8 engine has 20 percent smaller displacement but generates 429 horsepower and 516 lb.-ft. of torque. This is a 12 percent in horsepower and a 32 percent increase in torque than the outgoing engine. The results are sub-five second zero to sixty time.
Like the previous 5.5 liter V8, the new engine features aluminum cylinder heads, pistons and cylinder block with cast-in Silitec cylinders. The crankshaft and rods are forged steel units. The new V8 has a 10.5-to-1 compression ratio. This is relatively high for a turbocharged engine, so the pistons crowns are four millimeters thicker to handle the higher combustion pressures. The connecting rods are two millimeters shorter than the previous units. This allows the existing block dimensions to be retained. The pistons are cooled via oil spray jets in the back of the main bearing webs.
The smaller displacement of the new engine is more than offset by the increase in power supplied by its dual turbochargers. Each Garrett turbocharger is welded directly to the exhaust manifold. Aft of each turbocharger is a catalytic converter. The close proximity of the cats allows them to quickly come to temperature. The turbos themselves don't have blow-off valves. This must be handled differently than most turbocharged applications.
On a turbocharged engine, the compressor side of the turbo builds boost, sending pressurized air through the throttle body and intake manifold. When the intake valves open, air is forced into the combustion chambers. This is opposed to being sucked in, as is done on a non-boosted application. Pressurizing the air is how a turbo produces more power.
Eventually, the accelerator pedal is lifted. At that point, the throttle body closes and the exhaust pressure drops dramatically. However, what happens to the boosted air that has left the turbo but not passed through the throttle body? Typically, it's released by the blow-off valve. If not, the pressurized air typically slows down the turbocharger. This is called compressor surge. Compressor surge strains turbocharger components and reduces power until the turbo has spooled up again.
Since the M278 turbochargers don't have blow-off valves, they probably have an internal valve that recirculates the compressed air back into the turbo, just before the compressor. This allows compressor speed to stay high and doesn't strain the turbine, shaft or compressor. One benefit to eliminating the blow-off valves is it saves space.
The wastegate is computer-controlled and vacuum-operated, with maximum boost set at 0.9 bar (12.9 PSI). The wastegate on these turbos is designed to allow the allows the turbos to free-wheel during deceleration.
The engine has a Behr liquid to air intercooler nestled in the vee of the engine. As a result of being pressurized by the turbochargers, the intake air is heated. The intercooler is used to cool it back down. The benefit of colder intake air is increased power, due to the air being denser. A special radiator in the nose of the car with a dedicated coolant circuit is used by the intercooler to help ensure consistent engine power.
Dual Electric Fuel Pumps
A low-pressure electric pump in the gas tank delivers fuel at 84 psi (six bar) to a high- pressure pump that supplies the piezo injectors. Fuel in the high-pressure rail varies between 1,420 and 2,556 PSI (100-180 bar) based on demand. To maximize fuel economy, this variable fuel pressure allows the electrical load of the high-pressure pump to be reduced whenever possible.
Incredibly Fast Injectors
The injectors have a piezo-ceramic crystalline element that changes shape when an electric current is applied. This allows the injectors to open in 0.1 milliseconds. These incredibly fast injectors make it possible to design very precise injection systems. This new injection system has the ability to perform up to five injections with each piston stroke. This is especially impressive, considering that engines idle around 20 strokes per second and at high speeds, runs 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 stroke, prior to ignition. This forms a stratified mixture. If needed, a fourth and fifth injection can then be us stabilize combustion.
Working in conjunction with direct injection, a multi-spark ignition is used. Combustion with the first spark, but the system has the ability to deliver an additional four sparks within a single millisecond. This creates a gas plasma, with more expansion than conventional ignition.
The time between sparks is adjustable, allowing the system to control combustion duration, resulting in two percent better fuel economy. Direct injection provides another two percent improvement in fuel economy, creating a total of four.
The variable vane oil pump is driven by a new chain drive. At low engine speeds and under low load conditions, the oil pump generates about 28 psi (or two bar) of oil pressure. Under these conditions, the nozzles that spray cooling oil on undersides of the pistons are turned off. As engine speed and load increases, oil pressure goes up, and the piston oil spray nozzles open. This allows the energy required to drive the pump to be used more efficiently.
Borrowing a page from racing-type dry-sump lubrication systems, a second stage of the oil pump is designed to scavenge oil from the turbochargers. This system helps keep oil out of the intake and exhaust passages of the turbochargers, further reducing exhaust emissions as well as increasing oil flow through the turbos.
When the engine is cold, oil flows through an oil-coolant heat exchanger that uses engine coolant to help the oil heat up quickly. When the engine is warm, an oil thermostat redirects the oil through an external oil cooler.
Upgraded Cooling System
The cooling system on these new engines has been refined. The cylinder heads have a two-stage flow circuit. This coolant flow produces better heat dissipation with lower coolant pressure, so the water pump requires less engine power.
A three-phase cooling system is used to help the engine warm up quicker. When the engine is initially started, no coolant circulates. As the engine warms up, coolant begins to circulate through the engine, but does not pass through the radiator.
When the coolant temperature reaches 221 degrees Fahrenheit (or 189 degrees F. under high load), coolant is then allowed to circulates through the radiator. Coolant circulation through the heating system for the car's interior is controlled separately.
Mercedes-Benz was the first automaker to use cast-in silicon-aluminum cylinder sleeves with a low-friction surface that allows piston-ring spring tension to be reduced by 50 percent. Lowering internal friction resulted in both fuel savings and increased power. This sleeve technology also provided exceptional block stiffness while minimizing weight. The sleeves are more than a pound lighter than conventional iron sleeves, resulting in very light components.
Redesigned Valve Timing Adjusters
Computer-controlled, electro- magnetic camshaft adjusters are mounted on the ends of the intake and exhaust camshafts. These adjusters use four pivoting actuators to vary valve timing. The new system is 35 percent faster, and with a wider range of 40 crankshaft degrees, yet the adjusters are more than a half inch smaller in height and width.
New Cam Chain Drive
The smaller valve timing adjusters are made possible by a new cam chain drive system. In the new system, a crankshaft chain drives an intermediate shaft above the crank. The intermediate shaft drives two short chains, one for each cylinder bank. The short chains loop around the intake and exhaust camshaft drive sprockets. A fourth chain is used to drive the oil pump. The new chain drive system has less tension and lower chain dynamics. This causes less friction and reduces noise.
The Bottom End
The M278 has a forged steel crankshaft with eight counterweights and the standard five main bearings. The crank is bolted to a pressure cast aluminum block using a rigid iron bedplate rather than separate bearing caps. This design was initially used by Mercedes-Benz Motorsports. With a bedplate design, the bottom is secure. It eliminates cap walk and the main bearings should wear like an engine with a low redline. Hollow, forged steel connecting rods are bolted to the crankshaft. To be bolted to the crank, connecting rods must be two pieces. Continuing in the tradition of the M113, the M278 connecting rods are forged as a single piece, and then hydraulically cracked. This manufacturing process results in rods that are stronger than those that are machine cut and then 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.
Advanced Materials and Design
The aluminum pistons are coated with an iron particle reinforced resin. The pistons slip into silicon-aluminum sleeves, which are an integral cast-in part of the block. Two cylinder heads are bolted onto the block. Each head has dual camshafts that control four valves per cylinder via low-friction, no-maintenance cam followers.
Double-wall exhaust piping keeps exhaust air as hot as possible. The exhaust passes through the turbos, and down to a catalytic converter below each cylinder bank. Exhaust flows into a front muffler on each side, then to a single center muffler, which divides into left and right rear mufflers. Including the catalysts, there are seven separate exhaust chambers in the new biturbo engines.
The new dual turbo V8s use a Bosch MED 17.3.3 computer to integrate and manage the fuel injection, ignition, valve timing, turbo boost pressure and the variable oil supply. Networking with other systems in the car, the engine management computer has more than 30,000 different parameters stored in its memory, and is able to perform up to 260 million operations per second.
The ignition system has eight individual ignition coils and ignition amplifiers, ensuring a strong spark under all speed and load conditions. This arrangement also helps reduce the electronic load on the engine control unit.
Welded to the double wall exhaust manifold of each bank of cylinders, are catalytic converters that feature two ceramic elements in each housing. The front element is coated with palladium, while the rear has a bi-metallic coating of rhodium and palladium. This design improves catalyst efficiency while reducing the number of converters on each side.
One oxygen sensor is before each converter, and a second one in each converter, between the two ceramic elements. The oxygen sensors provide immediate feedback to the engine computer, so the fuel-air mixture remains balanced under all conditions. This system is designed to keep exhaust emissions low, fuel economy high, and prevent damaging the catalytic converters by overheating them. In particular, this system benefits fuel economy at full throttle, because the fuel-air mixture can be leaner than in engines without it.
5.5 Liter AMG Engine
AMG introduced its own 5.5-liter version of the new 4.6-liter Mercedes-Benz dual turbo V8. The AMG version also produces more power and better fuel economy than the engine it replaces. In the 2011 S- and CL-Class, the new AMG twin-turbo V8 produces 536 horsepower and 590 lb.-ft. of torque. This is about three percent more horsepower and 12 percent more torque than the outgoing 6.3-liter engine.
The AMG engine has more boost (1.0 bar or about 14 PSI), slightly lower compression (10.0:1) and nearly a liter of added displacement than the standard 4.6 liter biturbo. To increase displacement, AMG increased both the cylinder bore and crankshaft stroke.
AMG Pulsation Holes
The AMG cylinder block has longitudinally drilled pulsation holes between each main bearing support. These are used to help equalize air pressure changes under each piston has they move up and down. This reduction in pumping load helps increase power.
The ECO Start-Stop System
AMG models with the new V8 come with an ECO stop-start system. Whenever the car stops in "D" or "N" with the brake pedal pressed, the engine is automatically turned off to save fuel. When the driver touches the accelerator pedal, the engine computer decides which piston is in the best position for first ignition, and the direct fuel injection and multi-spark systems work with a starter motor to restart the engine almost instantly. In the future, this new technology may be used to start engines without using a conventional starter motor.
AMG Performance Package
AMG models with the new biturbo engine are available with the optional P30 Performance Package. The P30 Performance Package features higher turbo boost and recalibrated engine electronics. This produces an additional 27 horsepower (total of 563) and 74 lb.-ft of torque (total of 664 lb-ft.). In addition, the electronically limited top speed is increased to 186 mph. A carbon fiber engine is included in the P30 option.