New Mercedes-Benz Four-Cylinder Engines

Mercedes-Benz four-cylinder engines in general have always been criticized for their lackluster character and sound. And while they may deliver good performance, the current supercharged fours are far from the last word in refinement.

All that will change this summer, when a new range of state-of-the-art all-alloy, supercharged, dohc four-valve fours comes on stream. The new engine tips the scales at just over 35 lb (16 kg), and is almost 10% lighter than its predecessor, despite the addition of many new features.

Claiming a quantum leap in torque characteristics, economy, emissions and refinement, the new engines feature the TWINPULSE system, combining various technologies such as an intercooled supercharger, four valves per cylinder, variable valve timing, a Lanchester balancer for smoothness and an adaptive drive system.

Small Is Beautiful
Increased efficiency from the combination of these new technologies is the reason for these high outputs. The 1.8-liter capacity was chosen for the lower frictional losses and improved thermodynamics inherent with smaller pistons and rings. This contributes significantly to the improved fuel economy, which ranges from an average of 10% for the supercharged conventional engines to 19% for the supercharged CGI direct-gas-injection version.

Supercharging and variable valve timing make up for the loss of capacity, with the supercharger alone worth a 13% boost in specific output over a normally aspirated motor of the same capacity. The TWINPULSE effect refers to higher performance coupled with lower fuel consumption.

Refinement
Making the four-cylinder engine as smooth as a six was a major development goal. The Lanchester balancer uses two counter-rotating shafts in the sump, driven from the crankshaft by a newly developed low-mass silent chain. They rotate at twice crankshaft speed to neutralize the four-cylinder engine's inherent vibrations.

The Numbers Game
BMW and Mercedes-Benz continue to confuse the issue, badging their cars in a way that bears no relationship to the actual capacities of their motors. Initially at least, the new engine is a 1.8 liter-only supercharged and intercooled all-alloy unit made in three states of tune, with power outputs ranging from 143 bhp to 192 bhp. The C-Class was the first to get the new motors last summer, the bootlid script reading C180 Kompressor, C200 Kompressor and C230 Kompressor.

Other models that used the previous petrol fours--the C-Class Sport Coupe, new E-Class and the new CLK--were re-engined as production ramped up for the 2003 model year range.

The Fourth Element
At the end of last year, the new engine range was augmented by a fourth derivative, the 170-bhp 200 CGI. This was the first-ever production supercharged direct-injection gas engine, CGI being the acronym for (Stratified) Charged Gasoline Injection, a highly efficient lean-burn technology. It was also the first production engine to combine this direct-injection technology with a Lanchester balancer shaft and a supercharger.

Despite developing more power than the outgoing (163 bhp) 200 Kompressor motor, the 200 CGI offers a 19% improvement in fuel economy. Thanks to a beefy torque curve offering 184.5 lb-ft (250Nm) from 3000 to 4500 rpm, with over 75% of this is on tap from just 1500 rpm, driving is more effortless. Supercharging and direct-injection are not new to Mercedes-Benz engineers. The mighty SSK road racers of the 1930s were supercharged, and the 300SL Gullwing of 1955 was the world's first production car with direct fuel-injection. In 1955, no one really considered fuel economy or emissions. Today, however, they are big issues, and the Mercedes-Benz answer is to use modern advances in electronics and material sciences to solve problems with proven engineering concepts.

The objective was to make an engine that is sufficiently lean burning to provide good fuel economy, while using catalytic converters that normally require an air/fuel mixture of 14.6:1.

The solution was found in a radical new design of catalytic converter, which absorbs and stores nitrogen oxides (NOx) during the lean-burn cycle. The system re-releases them in brief regeneration phases, and they react with other exhaust gas constituents to form harmless nitrogen. These new catalysts also have a very fast warm-up time.

In the CGI engine, air and fuel are not mixed until they reach the combustion chambers. Two separate intake ports with carefully tailored flow characteristics encourage optimum swirl, making combustion as fast and complete as possible. A high-pressure fuel pump driven off the inlet camshaft injects fuel into each cylinder at an angle of 42 degrees and a pressure between 50 and 120 bar, dependent upon operating requirements. The engine ECU controls the fuel pressure regulator. Once in the combustion chamber, the mixture is guided toward the spark plug by specially shaped pockets in the pistons. These create a swirl motion to optimize the flame path and thereby the mixture burn for best combustion characteristics.

Thanks to an adjustable swirl flap in one of the intake ports, the engine is lean-burn under part load. Under other parameters, it operates with the normal air/fuel mixture for engines with catalytic converters.

The supercharger was re-designed to enhance low-end torque. Finer tolerances reduces the clearance between the rotors and the housing, while a new coating technology reduces frictional losses. This results in better internal airflow and higher efficiency.

Dynamics
Apart from being smooth and free-revving, the new engine is also smart. The driver's wishes are relayed to the engine via a drive-by-wire throttle, and the ECU monitors driving style, tailoring throttle response accordingly. If you are driving hard, response is sharpened up, and if you are obviously not in a hurry, it adapts itself to a more leisurely style.

The reading of driver intent is such that even if the actual throttle opening is the same, a sporty driving style will result in the throttle valve being opened further for snappier response and vice versa.