Estimation of Engine Frictional Power

P M V SubbaraoProfessor

Mechanical Engineering Department

Understand and Analyze All means of Power Draining…

Lubrication Regimes in An Engine

Different automotive components rely upon different modes of lubrication to achieve acceptable performance, and each may

experience more than one regime of lubrication during a single cycle.

Lubrication regimes in the engine

• The key operating tribological parameter in a powertrain system is the lubricant film thickness separating the interacting component surfaces.

• It is defined as the relative magnitude of the lubricant film compared to the combined surface roughness of the two surfaces, the so-called film thickness ratio, or parameter, .

Where h is the film thickness and is the root mean square(rms) surface roughness.

Challenges in Implementing Optimal Lubrication

Anatomy of Piston Assembly

Piston assembly

• The heart of the reciprocating internal combustion engine is the piston assembly, forming a critical linkage in transforming the energy generated by combustion of the fuel and air mixture into useful kinetic energy.

• The piston assembly includes the ring pack, which is essentially a series of metallic rings, the primary role of which is to maintain an effective gas seal between the combustion chamber and the crankcase.

• The rings of the piston assembly, which form a labyrinth seal, achieve this function by closely conforming to their grooves in the piston and to the cylinder wall.

• A secondary role of the piston ring is to transfer heat from the piston into the cylinder wall, and thence into the coolant, and to limit the amount of oil that is transported from the crankcase to the combustion chamber.

Piston Assembly And Piston Ring Function From An Internal Combustion Engine.

Geometry of the lubricated pair piston rings-cylinder wall

The mixed lubrication regime for the ring:

The total load acting in the ring in radial direction due to the sum of the hydrodynamic lubrication (FH) force and the solid-to-solidinteractions (FC)along the asperities of the surfaces.

CHR FFF

The Total Friction Force

The total friction force Ff, acting on the ring in tangential direction, is the sum of the friction force between the asperities and the shear force about the HDL regime:

Na : the number of asperities in contact, ACi : the area of contact of a single asperity i; τCi : the shear stress at the asperity contact i; AH : the contact area of the ring in HDL regime and τH : the shear stress of this component.

The Friction Coefficient

The friction coefficient f can be calculated by the following equation:

Both the components, FC and FH, show a strongly dependence on the minimum nominal distance between the ring and the cylinder wall.

Parameters of Piston Assembly for Optimal Design

• Gas pressure variations

• Ring groove clearance

• Groove flank profile

• Ring flank groove interaction

• Piston motion

• Elastic and thermal deformation effects

• Non-axisymmetric considerations including non-circular bores

• Ring and cylinder liner wear

• Surface topography influences

• Ring and piston mechanical design

• Crevice volume

• Cavitation considerations

Friction Force

pf Sf

Friction force is directly proportional to piston velocity

1

sin

cossin

60 22

R

LNS p

1

sin

cossin

60 22

R

LNf f

where is a coefficient of friction that takes into account the global frictional losses

Piston Assembly Friction

The cyclic integral of Friction work

2

221

sin

cossin

60

R

LNSfP fp

dPW frictioncycle,

strokestroke

frictioncyclemep V

dP

V

WF

,

Frictional Loss Vs Geometry & Speed of Engine

At a given engine speed, a longer stroke increases engine friction and increases stress on the crankshaft.

Effect of Cylinder Geometry on Frictional Losses

• Engine friction is affected by the stroke-to-bore ratio because of two competing effects:

• Crankshaft bearing friction and power-cylinder friction.

• As the stroke-to-bore ratio decreases, the bearing friction increases because the larger piston area transfers larger forces to the crankshaft bearings.

• However, the corresponding shorter stroke results in decreased power-cylinder friction originating at the ring/cylinder interface.

Piston Force Balance

Piston Side Thrust Load