The Mercedes-Benz M121 engine was a 1.9 liter

single overhead camshaft An overhead camshaft (OHC) engine is a piston engine in which the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combusti ...

inline four-cylinder engine introduced by Mercedes in 1955 and used in various model lines during the 1950s and 1960s. Rated at at 5500 rpm in its base version, it replaced the 1.8 liter M136 introduced in 1935, offering improved performance over the M136's side camshaft design. The engine's first application was in the W121 Mercedes 190SL luxury roadster in May of 1955, in a high-performance M121 BII version. Applications included mid-size sedans, such as the 190, the 190SL roadster, and trucks such as the

Unimog The Unimog (pronunciation in American English: ''YOU-nuh-mog''; British English: ''YOU-knee-mog''; German: , ) is a Daimler Truck line of multi-purpose, highly offroad capable AWD vehicles produced since 1948. Utilizing engine-driven power tak ...

and L319 models. The M121 series was in production until 1967, when the M115 engine took its place.

History

The M121 engine was developed just after the end of World War II. It was built in the Sindelfingen Works factory, which assembled the Mercedes Ponton Model series to which the engine belonged to. The M121 engine made its debut in 1955. It was the first generation of 4-cylinder engines from Mercedes Benz. The M121 replaced the less efficient M136 1.8-litre engine, a pre World War II engine that was introduced 2 years before in 1953. The M121 benefited from innovations and technology from Mercedes' larger engines such as the M186 3.0-litre and M180 2.2 liter engines. One of these innovations was a new single over head camshaft design which allowed for more power and efficiency. It was developed by a team led by Hans Scherenberg. Within this team, Karl-Heinz Goschel was one of the key engineers who developed the engine design.

M121 variants

M121

The M121 engine featured a single chain driven overhead camshaft layout with staggered valves operated by long and short

rocker arm A rocker arm is a valvetrain component that typically transfers the motion of a pushrod in an overhead valve engine, overhead valve internal combustion engine to the corresponding intake/exhaust poppet valve, valve. Rocker arms in automobiles are ...

s. The cylinder block was made from cast iron. The cylinder bore diameter for the M121 was . The three ring crankshaft was forged. The stroke of the engine was . This gave the M121 a displacement of . The engine had a compression of 8.5:1 and a single exhaust. Through this configuration, the M121 BII engine had an initial rating of at 5500 RPM and of torque at 4000 rpm. After testing of the final vehicle, the power rating was lowered to . The

crankshaft A crankshaft is a mechanical component used in a reciprocating engine, piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating Shaft (mechanical engineering), shaft containing one or more crankpins, ...

was initially carried by three

main bearing A main bearing is a bearing in a piston engine which holds the crankshaft in place and allows it to rotate within the engine block. The number of main bearings per engine varies between engines, often in accordance with the forces produced b ...

s, raised to five in the M121 BIX variant.

M121 BI

The M121 BI was developed for the new W110 190c introduced in 1961. It had an increase of , yet retained the same fuel consumption ( on premium gasoline). Top speed increased to from . Both performance increases were achieved despite the new model's significantly heavier bodywork required to meet new safety regulations.

M121 BII

The first version of the M121 to reach the market, the M121 BII was put into serial production in May of 1955 for use in the new W121 Mercedes 190SL roadster. Fitted with dual carburetors, it produced at 5700 rpm. For the 190SL,

acceleration In mechanics, acceleration is the Rate (mathematics), rate of change of the velocity of an object with respect to time. Acceleration is one of several components of kinematics, the study of motion. Accelerations are Euclidean vector, vector ...

to took 14.5 seconds, and the top speed was . This made the M121 BII one of the faster engines on the road in its day. The fuel economy of the M121 BII was .

M121 BIX

The final version of the M121 engine was the 2.0 L M121BIX, produced from 1965 to 1968 for the new W110 Mercedes Benz 200. A increase in bore from enlarged displacement to . Compression was increased to 9.0:1, the crankshaft went from three to five bearings, and a second Solex 38PDSJ carburetor was added. Combined, power was increased to at 5200 rpm and of torque was produced at 3600 rpm.

Applications

The M121 engine was first employed in the 1956 Mercedes Benz W121 180 "Ponton". The subsequent 190 and 200 models featured an updated version which offered greater performance through variations in compression ratios and improved carburetor systems. Its use continued in the W110 "Fintail" 190c and 200. The M121 engine has also been used in trucks such as the Mercedes-Benz L319 introduced in 1967. 1967. Some Unimog off-road models also used the M121.

References

Winning Foursomes
{{Webarchive, url=https://web.archive.org/web/20110111063151/http://www.emercedesbenz.com/Mar08/25_001082_Winning_Foursomes_A_Look_At_Mercedes_Four_Cylinder_Engines.html , date=2011-01-11 , ''eMercedesBenz'', March 25, 2008. Retrieved on 11-22-2010. M121 Straight-four engines Gasoline engines by model