Over the last two decades, engine research was mainly focused on reducing fuel consumption in view of compliance with more stringent homologation cycles and customer expectations. As it is well known, the objective of overall engine efficiency optimization can be achieved only through the improvement of each element of the efficiency chain, of which mechanical constitutes one of the two key pillars (together with thermodynamics). In this framework, the friction reduction for each mechanical subsystem has been one of the most important topics of modern Diesel engine development.
The present paper analyzes the crankshaft potential as contributor to the mechanical efficiency improvement, by investigating the synergistic impact of crankshaft design itself and oil viscosity characteristics (including new ultra-low-viscosity formulations already discussed by the author in [1]).
For this purpose, a combination of theoretical and experimental tools were used to design an extremely lightweight crankshaft and to evaluate the effects of main and conrod bearings dimensioning, clearances and oil viscosity, considering not only the impact from a friction perspective but also from a structural and lubrication stand point.
Experiments were conducted at both component and engine test bench level on a prototype light-duty 1.5ℓ 3-cylinder Diesel engine fitted with such an innovative crankshaft, characterized by a friction level typical of a gasoline engine.
The main results indicate that a reduction of bearings dimensions and an increase of its clearances combined with a reduction of oil viscosity are all contributing to friction improvement, but also to a reduction of the Minimum Oil Film Thickness requirements. The above-mentioned crankshaft was then validated from a durability and functional stand point confirming friction and bearings dimensioning levels comparable to a gasoline crankshaft, at sustainable cost increase while keeping the Diesel-expected durability.