Daimler is downsizing its diesels, says John Kendall
The arrival of the new Mercedes-Benz B-Class brings with it a new diesel engine, or to be more precise, an extensively modified range based on an existing Mercedes diesel unit. The engine in question is Mercedes’ OM 651 four-cylinder, which with a 2.2-litre displacement powers everything from a C-Class to an S-Class, as well as the Vito and Sprinter vans. The engine has been given a comprehensive re-working to fit the B-Class.
Firstly, the 2.2-litre engine was designed solely for rear-wheel-drive, with longitudinal installation, so to fit the front wheel drive B-Class involved extensive re-packaging. Then, in keeping with the spirit of the age, it has been downsized to 1.8-litres, while maintaining similar power and torque to the outgoing B-Class two-litre diesel.
Preparing the engine for mounting sideways involved re-positioning the variable geometry turbocharger and modifying the crankshaft belt drives. Since the engine uses gear-driven camshafts, these remain unaffected by the changes. The engine was designed for transverse mounting from the outset and the cylinder spacing of 94mm ensured that the cast iron block was short enough for use in Mercedes’ front-wheel-drive models. The smaller engine capacity of 1796cc, compared with 2143cc for the 2.2-litre OM 651, was produced by shortening the stroke from 99mm to 83mm.
Although four-cylinder engines enjoy perfect primary balance because the outer pair of pistons rise and fall together, 180 degrees out of phase with the inner pair, the additional forces introduced by combustion upset the secondary balance. This is smoothed in the new engine by using a pair of crankshaft-driven Lanchester counter balancer shafts, set low in the cylinder block. There’s nothing new in this – many other petrol and diesel engines use the same principal. On the downside, the shafts introduce additional friction, so the ‘comfort’ factor is offset by a small loss in efficiency. Mercedes has addressed this by running the shafts in roller, instead of shell bearings, which offer less resistance to motion. Other efficiency improving measures include oil and water pumps that only work when needed. The oil pump generates lower pressure too, also helping to reduce energy use. Peak injection pressure has been raised to 1,800 bar. Mercedes has also used a two-piece water jacket in the cylinder head, helping improve coolant flow and eliminate hotspots. This in turn enables the engine to run with higher combustion pressures – up to 200 bar.
The cylinder head design and the ‘on demand’ oil and water pumps have been adopted from Mercedes’ 3.0-litre diesel V6, which was modified with similar measures earlier this year. Another technology adopted from the larger diesel is the finer honing of the cylinder bores. The smoother finish reduces friction, thus reducing fuel consumption.
As a result, the 1.8 litre engine generates either 107bhp (B 180 CDI) or 134bhp (B 200 CDI) between 3,200 and 4,600rpm, compared with 107 or 138bhp from the outgoing B-Class two-litre diesel. Similarly, torque output equals that from the 2.0-litre engine – 184lb ft and 221lb ft in the 1,400 to 3,000rpm range.
With the help of standard engine start-stop technology, Mercedes’ engineers have managed to get the combined fuel consumption down to 64.2mpg from 54.3mpg for both models with corresponding CO2 emissions of 115g/km and 116g/km compared with 136g/km in the previous model. Sounds like a real recipe for success!