Under the microscope: Direct injection technology – 3rd generation spray-guided direct injection
Two new operating modes for extended lean-burn operation
The latest-generation piezo injection technology makes multiple injection possible in even the tiniest quantities. As a result, Mercedes-Benz engineers have been able to extend the lean-burn operation of the new generation of V-engines across an even wider range of the characteristic map while also providing two additional operating modes:
“Homogeneous stratified combustion” (HOS): this is a combination of homogeneous lean-burn and classic stratified combustion. With the engine unthrottled (throttle valve open), the first injection is sprayed into the intake stroke, forming a homogeneous basic mixture. Actual “stratified” injection takes place during the compression stroke before ignition and is a single or double injection depending on the characteristic map.
“Homogeneous split” (HSP): in this homogeneous combustion process, more than 95 percent of the fuel is singly or multiply injected, followed by a very small “ignition” injection to stabilise combustion. This is used when combustion conditions are difficult.
In order to ensure that the lean mixture is also ignited reliably at all times, the third-generation direct injection system also features “rapid multi-spark ignition” (MSI). Following the first spark discharge and a brief combustion period, the coil immediately above the corresponding spark plug is recharged rapidly and a further spark is discharged.
The MSI system enables up to four sparks to be discharged in rapid succession within one millisecond, creating a plasma with a larger spatial expansion than conventional ignition. The extension of the lean-burn operating phases means that a suitably light touch on the accelerator and a consistent driving style now make it possible to attain extremely favourable fuel consumption figures at speeds up to about 150 km/h.
A close look at the new direct injection engines
Lower fuel consumption, greater smoothness
Both the new V6 and the new V8 from Mercedes-Benz have aluminium crankcases, pistons and cylinder heads. The crankshaft, connecting rods and valves are of special forged steel. Through thoroughgoing lightweight design and intensive fine-tuning of details, engine friction also was reduced compared with the previous engine, by 28 percent.
Less power is required to drive the engine accessories. The Mercedes engineers achieved this through a number of measures. They include an optimised water pump with second-generation thermal management, a demand-controlled oil pump, a volume-controlled high-pressure fuel pump and an intelligent alternator management system.
Cylinder head with new camshaft adjuster
In addition, new variable, hydraulic vane-type camshaft adjusters were developed for the intake and exhaust sides. These now have a large adjustment range of 40 degrees with reference to the crankshaft and are 35 percent faster than before.
The extreme compactness of the camshaft adjusters was achieved by the new, two-stage chain drive. This drives two short secondary chains – one per cylinder bank – via a primary chain and an intermediate gear.
All three chains can be individually adjusted via a chain tensioner. This results in low tensioning forces and low chain dynamics, ensuring consistent timing and outstanding acoustic properties, with friction reduced even further. In short, the new chain drive is compact and ensures low-noise operation.
The coolant ducting in the cylinder head is also completely new. Rigorous reduction of hydraulic pressure losses and detailed improvements to the cooling system as compared with previous systems have made it possible to reduce the operating energy input of the water pump despite an increased engine output and cooling requirements.
It was possible to cut component weights by the concerted replacement of aluminium and steel by plastics, for example in the thermostat, belt pulley, impeller, heater valve and hydraulic lines.
Depending on the load on the components, the flow of coolant is regulated to various target temperatures between 80 and 105 degrees Celsius. New is the extended temperature range in which heat primarily is supplied to the interior. It means that during the warm-up phase of the engine the temperature of the vehicle interior now reaches the selected setting more quickly at low ambient temperatures.