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Design and Development of Ultra-Low Friction High-Power Density Diesel Engine for the Indian Market
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on April 14, 2020 by SAE International in United States
Diesel engines are known for their excellent low-end torque, drivability, performance and fuel economy. The ever-increasing customer demands push the diesel engines continuously to deliver higher torque and power. However, the requirement of higher power and torque puts a great challenge on the mechanical friction which can greatly influence the vehicle level fuel economy in a negative way. This paper explains the methodology to design a high power-density diesel engine capable of 180 bar peak firing pressure yet achieving the lowest level of mechanical friction. The base engine architecture consists of 8mm crank-offset which is an optimized value to have the lowest piston side forces. The honing specification is changed from a standard plateau honing to an improved helical slide honing with optimized Rz, Rpk and Rvk values. The cumulative tangential force of the piston rings are reduced to an extreme value of 24 N. A rectangular special coated top ring and a low-friction architecture oil ring is used to reduce the friction without increasing the blow-by and oil consumption. A special low-friction coating is applied on the piston skirt in addition to the optimized skirt profile to have reduced contact pressure. Piston pin is coated with diamond-like carbon (DLC) coating to have the lowest friction. Main bearing and crank pin diameter and width are optimized to have the lowest friction yet maintaining the minimum torsional and bending stiffness requirements of the crankshaft. Main bearing shells with partial oil groove is used to reduce the oil leakage rates to reduce the oil pump size. Oil pump is chain driven to reduce the operating speed and rotor diameter and inertia. Water pump efficiency is improved by adapting a closed-vane curved impeller and low-friction bearing. Chain guides are designed with polyamide 46 (PA46) material to reduce the friction; low friction coating on the chain links are used for further friction reduction. A low-friction single-plunger fuel injection pump (FIP) is used in place of a 3 plunger pump. The static and dynamic tension of the belt system is reduced by 30% by using a over-running alternator decoupler (OAD). Oil with special additives are used to reduce the cold-friction yet maintaining the required dynamic viscosity at high temperatures. The mechanical friction is quantified as frictional mean effective pressure (FMEP) calculated from the motoring torque of the engine. A strip-down method is used to evaluate the overall engine friction as well as the individual system contribution. The results reveal that the overall engine friction is measured to be <0.7 bar at a motoring speed of 2000 rpm at 90degC coolant and oil temperature. Cylinder system is the top contributor to the overall engine friction with ~30% contribution. Further, the engine level friction is further reduced to <0.6 bar by adapting a variable displacement oil pump, switchable piston cooling jets and a switchable coolant pump. Hence, the engine can be easily developed to meet the future emission and fuel economy norms. The paper explains the details of all the design optimizations done in each system to achieve the lowest levels of mechanical friction.