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The Mercedes-Benz 4.3 Liter V8
The Mercedes-Benz CLK430 is powered by a 4.3 liter V8. At Mercedes-Benz, the internal designation for this engine is the
113.943. This "M113" design is used in three engines; the 4.3, the 5.0, and the 5.5 liter, and is used in both
normally aspirated and supercharged forms. The M113 design has a number of interesting features, but the part that
stands out is the three-valve cylinder head. This three-valve head has two intake valves and one exhaust valve. From a
technical standpoint, the big question is, "why a three-valve head?" While developing the M113, Mercedes-Benz
was already using a four-valve engine design called the M119. Since multi-valve technology wasn't new, what drove
the engineers at Mercedes Benz to design an engine with fewer valves?
One reason may have been the price. The M119 appears to be more expensive to manufacture. The M119 uses dual overhead cams to control the four valves per cylinder. The cost of the additional cams, cam drives, the additional valves, valve springs, valve guides, rocker arms, valve seats, valve caps, and the machining of the heads must have cost more that the M113 design. Going the other direction, to a five valve design, seems like a logical move. Ferrari uses a five-valve head and they seem to be happy with it. Then again, if price was the problem, a five-valve design is even more expensive than a four-valve design. But hold on a minute, we're talking about Mercedes-Benz here. These guys are known for spending whatever's necessary to get results. Before reaching any conclusions, lets look at what we got for our less expensive design.
On paper, the M113 design looks impressive enough. Each head has a single hollow camshaft driven by double roller chains and rubber coated sprockets. This low mass valve train has dual rocker shafts in each head. The rocker shafts hold low friction, roller-tipped, aluminum rocker arms, and the system is maintenance free, as the rocker arms house small hydraulic lifters. Unfortunately, that still doesn't tell us if it's an improvement over the M119 design.
When comparing engine design, one measure of success is the power output. The 4.2 liter M119 made 275 horsepower, and 295 pound feet of torque. The 4.3 liter has the same peak numbers, but the 4.3 liter makes this peak power over a wider rev range. This is most obvious when comparing a 1997 E420 against the 1998 E430. Aside from the engine, those two are the same car. According to Mercedes-Benz, the zero to sixty times decreased from 6.7 to 6.4 with the M113 design. Initially, one might think the extra 100cc of engine made the difference, but the fuel economy increased as well. According to Mercedes-Benz, the M113 design gets 13% better fuel economy.
So the three-valve M113 has better fuel economy, more power, and it costs less to manufacture. This sounds almost too good to be true - but wait, there's more.
The engineers at Mercedes-Benz were able to pull another rabbit out of the hat - fewer pollutants. As much as 40% less under certain conditions. If you're wondering how, it's because of heat transfer.
When an exhaust valve is open, heat is transferred from the combustion chamber to the valve. The surface area of the single exhaust valve is about 30% less than that of the comparable four-valve engine. This means less heat is absorbed by a single valve, than two. In addition to the lower mass, the large single exhaust valve has a sodium filled stem. This also helps keep the valve cool. The less heat absorbed by the exhaust valve, the hotter exhaust stream is. Hot exhaust streams result in better cold start emissions. To help retain exhaust heat, the exhaust manifold is a seamless double-wall design. The manifold is fabricated using a high-pressure liquid-forming technology. The results of this type construction are a more durable lighter weight part.
Mercedes-Benz says there's no loss in performance with the three valve design in comparison to the four-valve design. That's because in an efficient, well designed engine, the size of the exhaust valve needs to be a bit smaller that the intake valve area. This is because the residuals from the combustion process take up less space than the incoming air fuel mixture. This is especially true for a modern clean burning engine.
Another benefit of a three-valve configuration is there's more room in the head. This didn't escape the engineers at Mercedes-Benz, as they used this as an opportunity for another performance enhancement - dual plugs. Using dual spark plugs in a single cylinder creates two flame fronts. The dual flame fronts increases combustion efficiency, allowing more of the fuel to be burned. The resulting combustion forces produce a better piston ring seal and less friction between the piston and cylinder.
The dual plug design isn't new (race cars builders discovered this decades ago), but the way in which it's employed is. Mercedes-Benz refers to their ignition as an "offset-phase twin-plug" system. According to the Mercedes Benz press kit:
'the dual ignition system activates the two spark plugs in each cylinder one after the other in quick succession (rather than simultaneously), with varied "stagger," depending on engine load and speed. This phase-shifted twin-plug sequence (as well as the basic ignition timing) changes after each combustion cycle for extremely precise control of the combustion process. This also enables combustion chamber pressures to be closely controlled to minimize combustion noise without sacrificing efficiency.'
This new ignition system allows the engine to utilize an extremely lean air/fuel ratio and retard the ignition timing by
5-19 degrees during the warm up cycle.
Looking at all the benefits shows that the three-valve head used in the M113 design wasn't simply a cheaper alternative. It appears that cost may have been the least of the benefits derived from this upgrade. The three-valve, dual plug head design was so well received, that Mercedes-Benz incorporated it into their redesigned V6 and V12 engines. Aside from the increased power, better fuel economy, and reduced emissions, the new engines are as much as 25% lighter than the design they replace.
Although the three-valve head is an interesting feature, there's much more to the M113 design than just the head. The rest of the engine has a number of innovations as well. For example, this design utilizes the world's first cast-in cylinder sleeves made of silicon aluminum. This low-friction surface allows the piston ring tension to be reduced by 50%. Using these low-friction cylinders, single overhead cams, the three-valve head configuration, and a number of other friction reduction techniques has resulted in a lessening of the engine's internal drag by 45% in comparison to the previous power plants. This increased efficiency has resulted in both a more powerful and more efficient engine.
The cylinder sleeves were also designed to increase block rigidity, and are a big part of decreasing the weight of the block. As an example, the bare aluminum block of the new 3.2 liter M112 V6 weighs a mere 57 pounds. That's over 50% lighter than the cast iron block that it replaces. In addition to the lightweight aluminum block and cylinder heads, the valve covers and intake manifold are both made of magnesium, which is lighter than aluminum.
Aside from being lightweight, the intake manifold on normally aspirated M113 V8 has the longest intake runners in the
industry. Under hood space was saved by casting the manifold so that the long intake passages spiral around to each
cylinder. Internally, the intake manifold has eight valves built into it. These valves are closed below 3,700 RPM,
directing the intake air across the long intake path. That's because when an intake stream follows the long route,
pressure waves develop that result in an improvement in low and mid-range torque. Unfortunately, this long path becomes
a detriment at higher RPMs. In the M113, above 3,700 RPM the valves open and the intake air follows a much shorter
route. This shorter route dramatically increases high speed efficiency, resulting in a substantial increase in
horsepower. These ECU controlled valves create an environment in which the engine can breathe at maximum efficiency
throughout the rev range.
At the heart of the engine is a forged steel crankshaft, which is bolted to a pressure cast aluminum block. The crankshaft has the standard V8 five main bearings. The two end caps are four bolt mains, while the center three have six bolt caps. Hollow, forged steel connecting rods are bolted to the crankshaft. To be bolted to the crank, connecting rods must be two pieces. The M113 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 cut and then reground. Aluminum pistons top off the rods, while a lightweight aluminum oil pan buttons up the bottom end.
For the 2002 model year, the M113 engine is managed by a Motronic ME 2.8 Electronic Control Unit (ECU). This ECU not only controls the valves in the intake manifold, electronic throttle control (drive by wire), the complete fuel injection system, and the phase-shifted dual ignition, but the ECU is also networked via a CAN data bus with other micro-computers for the automatic transmission, traction control, and the Electronic Stability Program (ESP). This allows data to be continuously exchanged, ensuring good fuel economy, performance, low emissions, and handling safety.
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© Marcus B. Fitzhugh 2005
Mercedes-Benz Specifications