ENGINE FRICTION:

 

INTRODUCTION

Friction generally refers to forces acting between surfaces in relative motion. In engines, frictional losses mainly occur due to sliding and rotating parts. Typically, engine friction, broadly speaking, is calculated as the disparity between indicated power(ip) and brake power (bp). Engine friction is commonly quantified in terms of frictional power (fp). Frictional loss is primarily attributed to the following mechanical losses:
(i) direct frictional losses
(ii) pumping losses
(iii) power loss to operate the components for charging and scavenging
(iv) power loss to operate other auxiliary components

A well-designed engine should not permit total frictional losses to exceed 30% of the energy input in reciprocating engines. It should be the objective of a competent designer to minimize friction and wear of the parts experiencing relative motion. This is accomplished through proper lubrication. In this section, the various losses associated with friction are listed.

1. Direct Frictional Losses:

This refers to the energy absorbed due to the relative motion of various bearing surfaces such as piston rings, primary bearings, camshaft bearings, etc.

2. Pumping Loss:

For four-stroke engines, a significant amount of energy is expended during intake and exhaust processes. Pumping loss is the overall power expended by the engine (piston) on the working medium (gases) during intake and exhaust strokes. In the case of two-stroke engines, this is minimal since the incoming fresh mixture is utilized to purge the exhaust gases.

3. Power Loss to Drive Components for Charging and Scavenging:

Some four-stroke engines employ superchargers or turbochargers to supply intake charge at higher pressure. Supercharged engines utilize engine power to operate the compressor, while turbocharged engines utilize exhaust gases to drive the turbine. These mechanisms decrease engine output, considered as negative frictional loss. In two-stroke engines with scavenging pumps, engine power propels the pump.

4. Power Loss to Drive the Ancillaries:

A significant portion of the generated power output is consumed to operate ancillary components such as the coolant pump, lubricating oil pump, fuel pump, cooling fan, alternator, etc. This is viewed as a loss since the presence of each of these elements diminishes the net output of the engine.

MECHANICAL EFFICIENCY of Friction:

The mechanical losses can be written in terms of mean effective pressure that is frictional torque divided by engine displacement volume per unit time. Mechanical efficiency indicates how good an engine is,in converting the indicated power to useful power. The value of mechanical efficiency varies widely with the design and operating conditions.

 

MECHANICAL FRICTION

As previously noted, friction loss arises in the bearing surfaces of the engine components due to their relative motion. Mechanical friction in the engine may be categorized into six classes, which are elaborated upon in the subsequent sections.

1. Fluid-film or Hydrodynamic Friction:

Hydrodynamic friction is linked to the phenomenon wherein a full film of lubricant exists between the two bearing surfaces. In this scenario, the frictional force solely relies on the lubricant viscosity. This form of friction constitutes the primary mechanical friction loss in the engine.

2. Partial-film Friction:

When rubbing (metal) surfaces are not sufficiently lubricated, there is a contact between the rubbing surfaces in some regions. During normal engine operation there is almost no metallic contact except between the compression (top) piston ring and cylinder walls. This is mainly at the end of each stroke where the piston velocity is nearly zero. During starting of the engine, the journal bearings operate in partial-film friction. Thus, partial-film friction contributes very little to total engine friction and hence, it may be neglected.

3. Rolling Friction:

Rolling friction occurs between surfaces in rolling motion, such as ball and roller bearings and tappet rollers. These bearings have a friction coefficient independent of load and speed, caused by local rubbing and continuous roller climbing. During engine startup, journal bearings have higher friction due to high oil viscosity and partial friction. Rolling friction is minimal compared to total friction.

4 Dry Friction:

Even when an engine is not operated for a long time there is little possibility for direct metal to metal contact. Always some lubricant exists between the rubbing surfaces even after long periods of disuse. One can take the dry friction to be non-existent and hence, this can be safely neglected while considering engine friction.

5. Journal Bearing Friction:

Journal bearings consist of a cylindrical shaft rotating against a cylindrical surface called the bearing, with partial bearings covering less than the full circumference. Research efforts have extensively studied their performance under different operating conditions, considering both continuous and oscillatory rotary motions. Engine journal bearings face varying loads over time, yet they adhere to the same fundamental principles as conventional bearings, albeit with potentially differing coefficients of friction.

 6.Friction due to Piston Motion:

Friction due to the motion of piston can be divided into
(i) viscous friction due to piston
(ii) non-viscous friction due to piston ring
The non-viscous piston ring friction can be further subdivided into
(i) friction due to ring tension
(ii) friction due to gas pressure behind the ring

TOTAL ENGINE FRICTION

The disparity between I.P. and B.P. is referred to as overall engine friction loss. This encompasses:

1. Direct frictional losses
2. Pumping loss
3. Blowby losse
4. Valve throttling losses
5. Combustion chamber pump loss
6. Power loss to operate the auxiliary systems.

1. Direct frictional losses:

This includes bearing losses (main bearing, camshaft bearing), piston and cylinder friction loss, etc. Frictional losses are relatively higher in reciprocating I.C. engines.

2. Pumping loss:

In four-stroke cycle engines, a significant amount of power is consumed during intake and exhaust processes. Pumping loss is insignificant in two-stroke cycle engines since the incoming fresh mixture is utilized for scavenging the exhaust gases and charging the cylinder.

3. Blowby losses:

These losses occur due to the leakage of combustion products past the piston from the cylinder into the crankcase.
- These losses depend on the inlet pressure and compression ratio.
- These losses increase directly with compression ratio but decrease with an increase in engine speed.

4. Valve throttling losses:

Reducing the exhaust valve size smaller than the inlet valve can result in inadequate exhaust flow and increased exhaust pumping loss. Inadequate attention to valve size, timing, and flow coefficients can lead to significant losses as engine speed increases. Inlet throttling loss is caused by restrictions from components like the air cleaner, carburetor venturi, throttle valve, intake manifold, and intake valve, resulting in pressure loss. Similarly, pressure loss is necessary for exhausting combustion products.

5. Combustion chamber pump loss:

This type of loss is caused by the pumping work required to pump gases into and out of the pre-combustion chamber. Its exact value depends on the orifice size (connecting the pre-combustion chamber and the main chamber) and the speed. The higher the speed, the greater the loss, and the smaller the orifice size, the greater the loss.

6. Power loss to operate the auxiliary systems:

Some power is needed to operate auxiliary systems such as the water pump, oil pump, fuel pump, cooling fan, and generator. This is also considered a loss since a portion of the engine power developed is used for these purposes.

FACTORS AFFECTING MECHANICAL FRICTION:

Several factors influence engine friction. In this section, the impact of some of these factors on mechanical friction is discussed.

1 Engine Design:

The design parameters that influence friction losses are:

1. Stroke-bore Ratio: A lower stroke-bore ratio may tend to slightly decrease the friction mean effective pressure (fmep). This is mainly due to less frictional area in the case of a lower stroke-to-bore ratio.
2. Effect of Engine Size: Larger engines have more frictional surfaces, hence, lubrication requirements are greater in such engines.
3. Piston Rings: Reducing the number of piston rings and reducing the contacting surface of the ring with the cylinder wall reduces friction. Light ring pressure also decreases friction.
4. Compression Ratio: The friction mean effective pressure increases with an increase in compression ratio. However, mechanical efficiency either remains the same or may improve slightly due to the increase in the indicated mean effective pressure (imep).
5. Journal Bearings: Reducing journal diameter/diametrical clearance ratio in journal bearings decreases the fmep. Short pistons with reduced mass along the gudgeon pin axis will minimize inertia loads, thereby reducing friction loss.

2 Engine Speed:

Friction increases rapidly with increasing speed. At higher speeds, mechanical efficiency starts deteriorating considerably. This is one of the reasons for restricting engine speeds.

3 Engine Load:

Increasing the load raises the maximum pressure in the cylinder, leading to a slight increase in friction. This load increase also raises the temperature inside the cylinder and of the lubricating oil, which decreases oil viscosity, reducing friction. In gasoline engines, opening the throttle to supply more fuel decreases throttling losses, potentially lowering frictional losses. However, in diesel engines, frictional losses due to load remain relatively constant since there is no throttling effect.

4 Cooling Water Temperature:

An increase in cooling water temperature slightly reduces engine friction by decreasing oil viscosity. Friction losses are high during starting since the temperature of water and oil are low and viscosity is high.

5 Oil Viscosity:

Viscosity and friction loss are directly proportional to each other. Viscosity can be reduced by increasing the temperature of the oil. However, beyond a certain value of oil temperature, failure of the local oil film may occur, resulting in partial fluid film friction or even metal-to-metal contact, which is very harmful to the engine.