Numerical and Experimental Investigation of Oil Jet Cooled Piston

Thermal loading of diesel engine pistons has increased dramatically in recent years due to applications of various advanced technologies to meet low emission and high power requirements. Control of piston temperatures by cooling of pistons has become one of the determining factors in a successful engine design. The pistons are cooled by oil jets fired at the underside from the crankcase. Any undesirable piston temperature rise may lead to engine seizure because of piston warping. However, if the temperature at the underside of the piston, where oil jet strikes the piston, is above the boiling point of the oil being used, it may contribute to the mist generation. This mist significantly contribute to the non-tail pipe emissions in the form of unburnt hydrocarbons (UBHC's), which has unfortunately not been looked into so seriously, as the current stress of all the automobile manufacturers is on meeting the tail pipe emission legislative limits.

A numerical model has been developed using finite elements method for studying the oil jet cooling of pistons. Using the numerical modeling, heat transfer coefficient (h) at the underside of the piston is predicted. This predicted value of heat transfer coefficient significantly helps in selecting right oil type, oil jet velocity, oil jet diameter and distance of the nozzle from the underside of the piston. It also helps predict whether the selected grade of oil will contribute to mist generation. Experimental validation of the numerical modeling was carried out on a flat plate. Problem of mist generation was also investigated on a flat plate using high speed camera.

NOMENCLATURE

C_p

Specific heat of the oil, J/kgK

d

nozzle diameter, m

D

diameter of the disk, m

h

local heat transfer coefficient (W/m²K) at the bottom surface of the disk

k_jet

thermal conductivity of the oil jet (W/mK)

k_r

thermal conductivity of the material in r direction

k_z

thermal conductivity of the material in z direction

Nu

local Nusselt number = hD / k_jet

Nu₀

Stagnation point Nusselt number

Pr

Prandtl number of the oil =

r

distance from the left hand side of piston.

Re

jet Reynolds number based on the nozzle diameter =

v

v_{jet(absolute)} - v_piston = relative jet velocity (averaged over a cycle), m/s

w

weight function

z

distance from the underside of the piston.

z_o

vertical distance of the disk from the nozzle exit, m (Figure 3)

γ

Kinematic viscosity of the oil, m²/s

μ

Dynamic viscosity of oil, kg/ms

Ψ_j(r,z)

shape function