New 2011 Mercedes-Benz Biturbo Direct Injection V8 Engines
INTRODUCTION
To mix fuel and air together for combustion, the first gasoline engines used the carburetor, a relatively primitive device not unlike a spray can or perfume atomizer. In the 1960s, the need to reduce exhaust emissions gave birth to indirect or port fuel injection for many gasoline engines. Upstream from the intake valve and combustion chamber, injectors sprayed fuel into each intake port to provide a more equal fuel-air mixture for each cylinder, which not only resulted in lower emissions, but also better throttle response and more power.
The First Direct Injection Gasoline Engine
In the parallel universe of diesel engines, direct fuel injection has been essentially a requirement. Spraying diesel fuel directly into the combustion chamber causes it to ignite without a spark plug if compression is high enough. In a gasoline engine, using direct injection has always been possible, but for years, its cost and complexity kept the auto industry from using this technology.
The world’s first gasoline engine with direct injection was the iconic 1954 Mercedes-Benz 300SL Gullwing. Adapted from the company’s legendary Formula One powerplant of the 1950s, the original Gullwing’s engine used direct injection to produce more power.
However, since then, Mercedes-Benz has pioneered direct injection to improve fuel economy on several high-efficiency European models. For example, the 2006 CLS350 CGI was powered by the world’s first gasoline engine with piezo-electric direct injection and spray-guided combustion. This advanced V6 engine achieved 10 percent better fuel economy versus the normal V6 with port injection. With fuel prices of more than $5 a gallon in Europe, this engine was an excellent value.
Mercedes-Benz Third-Generation Direct Injection Systems
The fuel system in the new Mercedes-Benz bi-turbo V8 represents the third generation of modern Mercedes-Benz gasoline direct injection systems. The new V8 features industry-leading technology first used on modern Mercedes-Benz diesels — piezo-electric fuel injectors that spray gasoline directly into the combustion chambers.
Twin Turbos
Two state-of-the art turbocharger units — one for each cylinder bank — give the new engine tremendous flexibility in terms of power and fuel economy. Not only are the exhaust-driven turbos lubricated and cooled by a special pressure-fed oil system, but a water-cooled intercooler increases turbo efficiency by cooling down intake air before it enters the engine.
With better fuel economy, more power and even cleaner exhaust emissions, the new Mercedes- Benz bi-turbo V8 engine dramatically demonstrates that there’s still exciting potential for the internal combustion engine to be further developed and refined.
4.6-LITER ENGINE More Than 30 Percent Increased Power
The 2011 Mercedes-Benz CL550 marks the debut of the new V8 engine, which can get up to 10 percent better fuel economy while producing more than 30 percent more torque — an astounding feat, considering that increasing either power or fuel economy usually decreases the other. Known within the company as the M278, the new V8 features direct fuel injection, twin turbochargers and multi-spark ignition.
Based on its highly successful 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 — 12 percent more horsepower and 32 percent more torque than the engine it replaces. This power produces 0-to- 60-mph acceleration times of less than five seconds.
Like its predecessor, the new engine features aluminum cylinder heads, pistons and cylinder block with cast-in Silitec cylinders, as well as a forged steel crankshaft, connecting roads and valves. The new V8 has a 10.5-to-1 compression ratio — relatively high for a turbocharged engine. However, pistons crowns are four millimeters thicker to handle the higher combustion pressures, while connecting rods are two millimeters shorter to allow existing block dimensions to be retained. Small oil spray jets in the back of the main bearing webs spray cooling oil on the underside of each piston.
Intelligent Turbocharging
The new engine’s smaller displacement is more than offset by the power boost of its twin turbochargers — one for each bank of cylinders. Each Garrett turbocharger is welded directly to the exhaust manifold, which saves space while helping the catalytic converter just below the turbocharger to heat up quickly. Eliminating the usual blow-off valves also saves space, and to ensure good throttle response without blow-off valves, all the intake air ductwork is extremely short.
The exhaust-driven turbochargers force intake air into the engine at a pressure of up to 12.9 pounds per square inch (or 0.9 bar) above normal atmospheric pressure. Maximum turbo boost pressure is limited by a computer-controlled vacuum-operated wastegate valve, which among other features, allows the turbos to free-wheel during deceleration and further increase gas mileage.
Water-Cooled Intercooler
The engine’s compact layout includes an air-water intercooler nestled in the “V” of the engine — where the intake manifold is located on a conventional V8 engine. As intake air is pressurized by the turbochargers, it heats up as a result. A Behr intercooler then cools it down, further increasing its density and helping to boost power. A special radiator in the nose of the car with a dedicated coolant circuit for the intercooler helps ensure consistent engine power under all temperature and operating conditions.
Two Electric Fuel Pumps
A relatively 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 is varied between 1,420 and 2,556 psi (100-180 bar) on a demand basis. To maximize fuel economy, this variable fuel pressure allows the electrical load of the high-pressure pump to be reduced whenever possible.
Piezo Crystals Make Lightening Fast Injectors
Featuring a piezo-ceramic crystalline element that changes shape when electrical current is applied, piezo injectors with a blazingly fast opening time of 0.1 milliseconds make it possible to design very sensitive and precise injection systems, including the ability to program 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, 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 and fifth injection can stabilize combustion if it’s needed.
Multi-Spark Works with Multi-Squirt
Working together with direct injection, a rapid multi-spark ignition system begins combustion with the first spark, but has the capability to recharge and deliver up to four 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.
New-Design Oil Pumps
A variable vane oil pump in the bottom of the engine is driven by a new chain drive. 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 undersides of the pistons are off. As engine speed and load increases, oil pressure goes up, and the oil spray nozzles open. In this way, less energy is used when less cooling and lubrication is needed.
Borrowing a page from racing-type dry-sump lubrication systems, a second stage of the oil pump is designed to scavenge or suction oil out of the turbocharger units. This system helps keep oil out of the turbo’s intake and exhaust passages, further reducing exhaust emissions as well as increasing oil flow through the turbos.
When the engine is cold, oil flows through a special oil-coolant heat exchanger that uses engine coolant to help the oil heat up quickly. When the engine is warm, an oil thermostat circulates the oil through an external oil cooler.
Three-Phase Cooling System
Even the cooling system is refined in the new engine, beginning with a two-stage flow circuit through the cylinder head. This coolant flow results in better heat dissipation, despite lower coolant pressure, so 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. 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 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.
Streamlined Valve Timing Adjusters
Mounted on the ends of the intake and exhaust camshafts, computer-controlled, electro- magnetic camshaft adjusters use four pivoting actuators to vary valve timing. The latest system is now 35 percent faster, and with a wider range of 40 crankshaft degrees, yet are more than a half inch smaller in height and width.
New Cam Chain Drive
These smaller valve timing adjusters are made possible by a new cam chain drive system, in which a crankshaft chain drives an intermediate shaft above the crank. In turn, the intermediate shaft drives two short chains — one for each cylinder bank — that loop around the intake and exhaust camshaft drive sprockets. A fourth chain drives the oil pump. The new chain drive system results in less tension and lower chain dynamics, for even lower friction and less noise.
The Building Blocks
Assembling the Mercedes-Benz V8 engine underscores its efficient design. An exceptionally stiff forged-steel crankshaft with eight counterweights spins in five main bearings. It’s bolted into a pressure-diecast aluminum block with a one-piece cast-iron bearing cover (instead of separate bearing caps), and an aluminum oil pan also contributes to block rigidity.
Cracked Connecting Rods
Each crankshaft connecting rod journal carries two connecting rods, and each rod must be made in two pieces for assembly on the crankshaft. Mercedes engines use hollow, forged steel rods that 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.
Advanced Materials and Design
Iron-coated for lower friction, 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. The two cylinder heads are bolted onto the block, and twin camshafts are installed in each head, with new cam drive chains looping around the intake and exhaust camshaft sprockets. The camshafts open and close four valves in each cylinder via low-friction, no-maintenance cam followers.
Double-wall exhaust piping keeps exhaust air as hot as possible leading through the turbo units and down to a catalytic converter below each cylinder bank. Exhaust flows into a front muffler on each side, then into a single center muffler, which divides into left and right rear mufflers. Including the catalysts, there are seven separate chambers in the exhaust system of the new biturbo engines.
Networked Engine Management
The new biturbo V8 makes use of 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.
Eight individual ignition coils and ignition amplifiers ensure a strong spark under all speed and load conditions. This arrangement also helps reduce the electronic load on the engine control unit.
Two-Stage Catalytic Converters
Welded to a double-wall exhaust manifold for each bank of cylinders, the catalytic converters feature two different ceramic elements in each housing. The front element is coated with palladium, while the rear one has a bi-metallic coating of rhodium and palladium. This design improves catalyst efficiency while eliminating a separate housing on each side.
An exhaust sensor is located before each converter housing, and a diagnostic sensor is located in each converter between the two ceramic elements. These sensors provide immediate feedback to the engine computer, so the fuel-air mixture remains balanced under all conditions. This layout keeps exhaust emissions low, fuel economy high, and prevents any possible overheating damage to the catalytic converters. In particular, it benefits fuel economy at full throttle, because the fuel-air mixture can be leaner than in engines without this system.
5.5-LITER AMG ENGINE
AMG is introducing its own 5.5-liter version of the new 4.6-liter Mercedes-Benz biturbo V8 that also produces more power and better fuel economy simultaneously. Replacing AMG’s normally aspirated 6.3-liter V8, the smaller engine features twin turbochargers, direct fuel injection, and multi-spark ignition.
In comparison to its predecessor, reduced displacement is more than offset by the power boost of its twin turbochargers — one for each bank of cylinders. In the 2011 S- and CL-Class, the new AMG twin-turbo V8 produces 536 horsepower and 590 lb.-ft. of torque — about three percent more horsepower and 12 percent more torque than the outgoing 6.3-liter engine.
Compared to the new 4.6-liter biturbo engine, the AMG powerplant makes use of higher turbocharger boost (up to 1.0 bar or about 14 psi above atmospheric pressure), slightly lower compression (10.0 to 1 as opposed to 10.5 to 1) and nearly a liter of added displacement. AMG increased both the cylinder bore and crankshaft stroke by nearly 1⁄4 inch to gain the additional displacement.
AMG Pulsation Holes
The AMG cylinder block also features longitudinally drilled pulsation holes between each main bearing support that help equalize air pressure changes under each piston has they move up and down. Reducing this pumping load is yet another refinement that helps increase power.
The ECO Start-Stop System
AMG models with the new V8 come with an innovative ECO stop-start system. Whenever the car stops in “D” or “N” with the brake pedal depressed, the engine is automatically turned off to save fuel. As soon as 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 re-start 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 an optional P30 Performance Package that features higher turbo boost and recalibrated engine electronics for an extra 27 horsepower (total of 563) and 74 lb.-ft of torque (total of 664 lb-ft.), as well as a higher electronically limited top speed of 186 mph. A carbon fiber engine cover provides a visual cue to the additional power on tap.
