Lean-burn operation under partial load and emission control with storage catalytic converter
Mercedes-Benz has a long tradition in the field of direct petrol injection. As early as the mid-fifties, the Stuttgart manufacturer unveiled this technology in the 300 SL, the legendary "Gullwing". This classic sports car was the first series-produced automobile to receive a four-stroke engine with direct injection -- a sensational development that significantly boosted the six-cylinder powerplant's output.
Nowadays further development of this particular injection technology focuses on two different aspects: fuel consumption and exhaust emissions. Major advances in the fields of emission control and engine management have resulted in a solution to a familiar technical problem: producing an engine that is sufficiently lean-burning to provide the desired fuel consumption whilst using catalytic converters that usually require a "stoichiometric" air/fuel mixture (14.6 : 1). The solution in question is a newly developed NOx storage catalytic converter which absorbs the nitrogen oxides during the lean-burn cycle and then re-releases them in brief regeneration phases, so that they react with other constituents of the exhaust gas to form harmless nitrogen.
Thanks to this technology, which Mercedes-Benz combines with exhaust gas recirculation and secondary air injection in the new direct-injection petrol engine, the CLK 200 CGI is able to comfortably comply with the future EU-4 emissions standard. Up to 35 percent of the exhaust gases can be recirculated, depending on the engine operating characteristics, thus bringing about a substantial reduction in nitrogen oxide emissions. Another new innovation comes in the form of the "linear" oxygen sensor which becomes active immediately when the engine is started from cold and supplies information about the exhaust gas composition. This information is then processed by the engine computer, for the purpose of warm-up control among other things. It enables the catalytic converter to reach operating temperature in considerably less time. In addition, a newly developed, electronically networked NOx sensor with digital control is used, to ensure optimum control of the NOx storage catalytic converter.
Direct injection at a pressure of up to 120 bar
In the direct-injection system, the air and fuel are not mixed until they reach the combustion chambers. With the help of an injector, the fuel is injected into the cylinders at an angle of 42 degrees and, depending on the engine operating characteristics, at a pressure of between 50 and 120 bar. Here the fuel droplets and the air particles form a mixture which flows to the spark plugs via specially shaped recesses in the pistons. By way of comparison, the fuel pressure in a four-cylinder engine incorporating conventional injection technology is approximately 3.8 bar.
The CGI engine has two separate intake ports with flow characteristics that have been developed using sophisticated processes. These ports ensure optimum swirl in the mixture, thus making combustion fast and as complete as possible. In each case, one of these intake ports also has an adjustable swirl flap. The high-pressure fuel pump is driven by the intake camshaft. A pressure regulator controlled by the engine computer regulates the pressure in the fuel line (rail) which is directly connected to the injectors. The regulator receives signals from a pressure sensor.
Under partial load, at a mean pressure of four to five bar and at low engine speeds, the four-cylinder engine is "lean-burning". In other words, with the throttle wider open and with a high level of excess air. Engineers talk about "stratified charging": the fuel is injected into the air compressed by the piston at a relatively late stage. Outside of this range, the direct-injection engine is operated with the usual air/fuel mixture for engines with catalytic converters (lambda 1).
Aluminium cylinder head with variable valve timing
Made from high-strength aluminium alloy, the cylinder head in both versions of the new Mercedes engine has 16 valves and double overhead camshafts. Unlike in the previous four-cylinder engine in the CLK-Class, both camshafts are adjustable and have variable valve timing. A newly developed vane-type adjuster with integrated control valve ensures optimum valve timing at all times. This has a positive effect on torque characteristics under full load as well as on fuel consumption and exhaust emissions. The valves are controlled by means of cam followers with roller mechanism and have hydraulic valve lifters.
Highly efficient, low-noise compressor
The Mercedes developers have reengineered the compressor – which ensures exemplary bottom-end torque characteristics in both engine versions – by reducing the clearance between the rotors and the supercharger housing and by using a new coating for the rotors. The result is an even higher rate of air flow and even better efficiency than in the mechanical supercharger used in previous Mercedes four-cylinder engines. Special wide-band silencers are installed on the intake side and pressure side.
After being compressed, the air exits the compressor and flows through an intercooler, thus reaching the optimum temperature and density for the combustion process. The standard version and the CGI version of the new engine have different plastic intake pipe arrangements: in the direct-injection version, they separate into two lines per cylinder before actually entering the cylinder head. In each case, one of these pipes has an electrically adjustable swirl flap which further promotes air swirl.
Electronic control unit on the air cleaner housing
The round air cleaner attached to the intake pipe has the advantage of being able to provide a relatively large filter area in the smallest of spaces. In addition, the electronic control unit – equipped with two powerful processors – is located on the side of the air cleaner housing. The two processors share the complex tasks: one is responsible for the real-time processes, such as calculation of the ignition and injection pulses, whilst the other deals with time-synchronised tasks, such as lambda control or determining the ignition timing and the injection quantity. The control unit is integrated in the electronic network of the new CLK-Class via CAN databus (Controller Area Network) and swaps data with other microcomputers. The hot-film air flow sensor, incorporating reengineered functions, is positioned directly behind the air cleaner housing.
As is the case in all state-of-the-art petrol engines from Mercedes-Benz, the new four-cylinder generation uses electronic monitoring for components that simply have to function correctly in order to ensure low emissions. By way of example, this on-board diagnostic system permanently monitors catalytic converter efficiency and the ignition system as well as checking the purge valve electrics and the oxygen sensor. The "Check Engine" display on the CLK-Class instrument cluster lights up if a fault occurs in one of these systems. Meanwhile, details of any malfunctions are stored so that service engineers can detect the problem immediately and remedy the fault.
Refinement similar to a six-cylinder engine thanks to newly developed counter-balancer
Alongside exemplary power delivery, low fuel consumption and low exhaust emissions, the new Mercedes four-cylinder engine has a further advantage, namely refinement and acoustics similar to a six-cylinder engine. This is primarily down to the newly developed Lanchester balancer which consists of two forged shafts supported in multiple bearings. The shafts rotate in opposite directions at twice the crankshaft speed and, in so doing, counter-balance the inertia forces caused by the motion of the pistons, for example, which tend to cause irritating vibration.
The size, weight and arrangement of the counter-balance shafts have been perfectly harmonised to neutralise the vibratory forces in the four-cylinder engine. This technology thus substantially reduces noise emissions and vibrations and vastly enhances ride comfort.
The aluminium housing that contains the bearing-mounted balancer shafts is located in the oil sump and bolted to the crankcase from below. The drive is provided by a newly developed silent chain with low masses. The crankcase also contains the engine oil pump which is driven by one of the two shafts by means of a gear pair.
The crankcase is made from diecast aluminium (previous engine: cast iron) and is a major component in the lightweight design concept at the heart of the new four-cylinder engine. To put this into figures, the engine only weighs around 167 kilograms, making it some 18 kilograms or almost ten percent lighter than the preceding model.
