Mercedes-Benz M279 engine (2012-on)

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Introduction

The Mercedes-Benz M279 was a twin-turbocharged V12 petrol engine that was introduced in September 2012 for the Mercedes-Benz R231 SL 65 AMG. Key features of the M279 engine included its:

• Full-aluminium crankcase with pulsation bores;
• Forged crankshaft, connecting rods and pistons;
• One-piece chain drive;
• Roller-type cam followers actuating two intake valves and one exhaust valve per cylinder;
• Two turbochargers with air/water intercooling and wastegate valve;
• Intake manifold injection;
• Second-generation “Controlled Efficiency’ start/stop function; and,
• Low-temperature circuit with expansion reservoir.

Based on the M275 V12 engine which it replaced, the M279 engine differed in that it had new turbochargers with larger spiral cross-sections, flow-optimised wastegate ducts, a start/stop function, new ignition coils (dual ignition) with multi-spark ignition and a new engine control unit. Similarly, peak torque for the M279 engine was electronically limited to 1000 Nm to avoid transmission damage.
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Crankcase

The M279 engine had a die-cast aluminium crankcase 82.6 mm bores – spaced at 90 mm intervals – and a 93.0 mm stroke for a capacity of 5980 cc. Within the cylinder bores, the M279 engine had Silitec (aluminium-silicon) cylinder liners. Furthermore, the crankcase had a bedplate design with individual bolt-on main bearing caps made of cast steel.

The crankcase of the M279 engine was ventilated via a centrifugal oil separator that was driven by the left camshaft. Vapours from the crankcase which contained oil mist would enter the centrifuge and rotate so that the oil particles would separate from the gases and drip back into the crankcase. The cleaned air would then flow into the pressure regulator and – depending on the operating state of the engine – would be fed directly into the charge air manifold or into the intake line of the left turbocharger.

For cooling, the M279 crankcase had three inter-bore bridges between the cylinders: two feed bores that supplied coolant to the bridge and a drain bore. Coolant would flow out of the bridge via the bore in the cylinder head where there was one feed bore for each inter-bore bridge.

Crankshaft, connecting rods and pistons

The M279 engine had:

• A forged crankshaft made of ‘high-quality alloy steel’ which had precision-turned counterweights;
• Cracked and forged connecting rods that were produced from a high-strength steel alloy; and,
• Forged pistons that were made from a high-grade aluminium alloy and had an electroplated iron (‘FerroTec’) coating.

Cylinder head

The M279 engine had an aluminium cylinder head with a single overhead camshaft (SOHC) per cylinder bank. The forged camshafts were made from internal high-pressure formed (IHPF) steel tube with joined cams. The camshafts were driven by a one-piece, twin roller chain.

For each cylinder, the M279 engine had two intake valves and a single hollow-stemmed exhaust valve that was filled with sodium for heat dissipation. The valves were actuated by roller-type cam followers with hydraulic valve clearance compensation. The valve springs were identical on the intake and exhaust sides, and specially designed to cope with the higher exhaust back pressure prevalent in turbocharged engines.

Like the M275 engine which preceded it, the M279 engine did not have camshaft adjustment. For the M275 engine, Mercedes-Benz considered that the engine’s torque output rendered camshaft adjustment ‘superfluous’.

Turbocharging

Each of the cylinder banks of the M279 engine had a liquid-cooled turbocharger. The turbochargers were made of cast steel and the turbine housings were integrated in the exhaust manifolds. The turbochargers drew in fresh air via the air filters at the compressor intakes and directed it via compressor outlets into the charge air pipes upstream of the charge air coolers.

To cool the turbochargers, coolant would flow to each of the turbocharger bearing housings via a different connection in the cylinder crankcase and pass through the bearing housings from bottom to top. The cooling streams would then be fed back into the main cooling circuit at the right cylinder head. The flow of coolant through the turbochargers was assisted by the pressure differential in the water jacket – this is an example of the thermosiphon principle whereby the difference in density between warm and cold water causes the warm (i.e. lighter) water to rise upwards.

For each turbocharger, the turbine wheel and compressor impeller were supplied with oil via the engine’s oil circuit. The lubricating oil was taken from the main oil duct of the cylinder crankcase and would flow through the bearing housing in the opposite direction to the coolant. The lubricating oil would then be is extracted individually for each turbocharger from the engine oil pump.

Charge air coolers

The M279 engine had one charge air cooler for each cylinder bank and both charge air coolers were connected to the low-temperature cooling circuit which included a low-temperature cooler, expansion reservoir and charge air cooler circulation pump. The heated charge air would transfer its heat to the coolant which flowed through the charge air cooler, acting as an air/water heat exchanger. The coolant, in turn, would transfer the absorbed heat to the ambient air in the low-temperature cooler. Controlled by the ECU according to charge air temperature, coolant was pumped through the circuit by the on-demand electric circulation pump. If the charge air temperature exceeded 45 degrees Celsius, the charge air cooler circulation pump would switch on; when the charge air temperature dropped below 35 degrees Celsius, the pump would switched off.

Boost pressure control

To regulate boost pressure, the exhaust streams that drove the turbine wheels could be diverted through bypasses by opening wastegate flaps; these flaps were opened by the vacuum cells via linkages. For the M279 engine, the turbochargers would start to produce boost pressure at approximately 1000 rpm, while maximum boost pressure of 1.5 bar occurred at approximately 2300 rpm. Maximum boost pressure, however, was only allowed at charge air temperatures below 65 degrees Celsius.

Fuel supply and injection

For the M279 engine, fuel pressure varied from approximately 3.0 to 5.2 bar (relative), depending on the fuel requirement. At engine start, however, fuel pressure was raised to 6.0 bar.

The M279 engine had conventional port injection whereby the fuel injectors would spray a calculated quantity of atomised fuel into the intake port of each cylinder. Each injection valve was actuated sequentially by ground pulses of appropriate duration directly by the ME-SFI control unit.

Ignition

Compared to its M275 predecessor, the M279 engine had new dedicated ignition coils (dual ignition) with multi-spark ignition process. Similarly, however, the M279 engine had two spark plugs per cylinder. In the standard single-spark ignition mode, the ignition coil would charge to a target primary current and, at firing point, the charging current would be shut off to produce a spark.

In multi-spark mode, however, the coil would not be completely discharged and the secondary current – which was dependent on the charge level of the coil – would be measured in the coil. If the secondary current dropped below a threshold, the coil’s electronic control would re-open the power amplifier to allow charging current to flow, while the level of the primary current would also monitored. When the primary current threshold was reached, the power amplifier would close the primary circuit and high voltage would be generated again, causing another spark to be produced. Subsequent sparks were also produced in the same manner.

Start/stop function

Unlike its M275 predecessor, the M279 engine had a second-generation ‘Controlled Efficiency’ start/stop function. As such, the engine could shut down when the vehicle was stationary in traffic to reduce fuel consumption. When the driver lifted their foot from the brake, the engine would re-start. Furthermore, the M279 engine had a variable output alternator which would increase its output when the vehicle was coasting or braking to re-charge battery.

Oil supply and cooling

The M279 engine had a two-stage oil gear pump that was driven by the crankshaft via a single bush chain. The pump had two suction levels for the returning oil from the turbochargers.

The M279 engine had an oil cooler (an oil/air heat exchanger) – positioned behind the front bumper – which enabled warm air to flow through an opening in the underfloor panelling directly into the atmosphere. Furthermore, an additional engine oil cooler with fan was installed in the right front wheel arch.

Exhaust treatment

To reduce exhaust gas emissions, the M279 engine had near-engine mounted firewall catalytic converters. Through oxidation, carbon monoxide would be converted into carbon dioxide, and hydrocarbons into water and carbon dioxide. By chemical reduction, the nitrogen oxides would be converted into nitrogen and carbon dioxide.

The M279 engine also had secondary air injection for faster warm-up of the firewall catalytic converters. The injected air was provided by an electric air pump – actuated by the ME-SFI control unit – via the secondary air injection relay. When the air pump switchover valves were actuated, they allowed the vacuum from the variable intake manifold to flow to the air shut-off valves. These valves would then open and the air from the electric air pump would be injected into the exhaust ports of the cylinder heads. The injected air would react with the exhaust gases in the exhaust ports and firewall catalytic converters so that carbon monoxide and hydrocarbons would be oxidised. This after-burning increased the temperature of the exhaust gases for faster warm-up of the catalytic converters.
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