OM654

Mercedes-Benz presented the OM 654 in February 2016, five months after the Volkswagen emissions scandal broke, and called the diesel indispensable in the same breath. It replaced the OM 651, the most-produced engine in the company's history, and did so with the first all-aluminum four-cylinder diesel Stuttgart had built: 168.4 kg against 202.8, single-stage turbocharging instead of two, and steel pistons in an aluminum crankcase — a combination that sounds wrong until the block warms, expands around pistons that barely move, and closes up its own clearances. NANOSLIDE-coated bores and stepped combustion bowls did the rest, for 13 per cent less fuel than before.

The other half of the design was packaging. Every element of the exhaust aftertreatment was mounted on the engine rather than the car, and the interfaces to the vehicle were standardized across model series, so one unit could be installed longitudinally or transversely, driving the front, rear or all four wheels. That is how a W213 E-Class engine reached these compacts: the transverse OM 654q — q for quer — arrived in 2018 in the W177 A-Class and spread through the CLA, GLA and GLB. Certified to Euro 6d, it met the Stage 2 real-driving-emissions limits before they came into force in 2020.