Internal Combustion Engines

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Ellen Kennedy
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1 Friction & Lubrication Lecture 1 1

2 Outline In this lecture we will discuss the following: Engine friction losses. Piston arrangement losses. Measurement of friction losses. Engine lubrication systems. Lubrication oils and their characteristics. 2

3 Friction Friction occurs when one surface moves against another due to the contact between the high points on each surface. The idea behind lubrication is to keep the surfaces hydraulically separate through adding an oil film in the space between them. This applies to parallel moving surfaces and to shaft & race (bearings) arrangements. The position and thickness of the film is in the order of a few µm. 3

4 Friction Losses In an I.C. engine, work lost through friction can be given by the following equation: W f = W i W b Where, W f : Friction work W i : Net Indicated work W b : Brake work And is usually indicated using the mechanical efficiency of the engine: η mech = W b /W i * 100 % 4

5 Friction Losses These friction losses usually take no more than 10% of the net indicated work at WOT conditions (η mech = 90%). While they increase to 100% at idle conditions (η mech = 0%). Work lost through friction ends up dissipated as heat transferred to the engine oil and to the coolant. 5

6 Friction Losses Measurement The most common experimental method used to determine friction losses is using the motor mode on an electrical dynamometer. The power used to motor the engine at a given speed by the dynamometer (without combustion) is approximately equivalent to the power consumed by friction losses during normal running conditions at the same speed. In order to make sure that conditions are similar between both cases (especially with regard to the temperature) the motor test is done directly after a normal engine test is done. 6

7 Friction Losses Friction losses in an engine can be roughly attributed to the following sources: Piston arrangement : 50% Valve arrangement : 25% Crankshaft : 10% Auxiliaries : 15% Auxiliaries (accessories) include: Continuous demand: fuel pump, oil pump, water pump, fan, supercharger. Partial demand: A/C, power steering pump, brake pump. 7

8 Friction Losses 8

9 Friction Losses Friction losses resulting from hydraulic shear in the piston arrangement lubricant are proportional to the engine speed: F f = π B L µ ( U/ y) Where: F f : Friction force B : Bore L : Length of piston s skirt µ : Dynamic viscosity ( U/ y) : velocity differential between the two surfaces It can be seen why the average piston speed is limited as a design parameter in engine design. 9

10 Piston Arrangement Losses It is also desired to use lighter and shorter pistons to reduce the friction area. Friction forces on the piston are greatest near the TDC and BDC due to the additional boundary friction. At TDC and BDC, the piston momentarily stops to change direction. This squeezes out the oil film from between the piston and the cylinder walls. As the piston starts to move in the opposite direction, very little lubricant is available and metalto-metal contact occurs. 10

11 Piston Arrangement Losses This causes the friction forces to significantly increase. Increasing the compression ratio by one can increase these losses by up to 20%. This also increases the losses on the crankshaft bearings due to the larger bearings required. In addition, increasing the compression ratio usually requires additional piston rings. 11

12 Piston Arrangement Losses Piston rings are another major source for piston arrangement friction losses. The trend is to reduce the thickness of the piston rings in order to reduce these losses (in the range of 1 mm) and to reduce their number (2 compression rings and one oil ring). Oil rings do not usually experience the same level of pressure difference and friction losses. Adding one compression ring can result in a 20% increase of the piston arrangement losses. 12

13 Engine Lubrication Systems Most modern engines rely on pressurised lubrication systems. They consist of an oil pump (often electrical) and a circulation network passing through the engine block, valve seats, connecting rods etc. In addition to this, oil is sprayed on the cylinder walls and the back of the piston crown. Older engines relied on splash systems, where the crankcase is used as the oil sump and oil is splashed due to the effect of the crankshaft movement. 13

14 Lubricants In addition to lubrication, engine oil is meant to act as a coolant, a way to remove contaminants and to slow corrosion. Lubricating oils are required to: Adhere to surfaces to form continuous lubricating film. Operate properly over a wide range of temperatures. Withstand high temperatures without breaking down. Resist oxidation and have a long working life. Free from toxic elements and compounds. 14

15 Lubricants Lubricating oils are mainly made up of heavy hydrocarbon compounds derived from crude oil in addition to a combination of additives. These additives include: Anti-oxidation agents. Anti-wear agents. Anti-rust agents. Anti-foaming agents. Detergents. 15

16 Lubricants Lubricating oils are named and rated depending on their viscosity. The most common system is the SAE system (Society of Automotive Engineers): SAE 10, SAE 20, SAE 30, SAE 40, SAE 50 etc. At a given temperature, the higher the number of the oil the higher its viscosity. It must be noticed that the viscosity is a function of temperature. An increase of 50 C can result in a decrease by an order of magnitude in the viscosity. 16

17 Lubricants Oils with lower numbers are useful in cold weather conditions since they are easier to pump (lower viscosity). On the other hand, oils with higher numbers are useful for normal and high power conditions since heat significantly reduces the viscosity. 17

18 Lubricants Multi-grade oils offer a solution to this variation in requirements, where their dependency on the temperature is reduced through adding polymers to the oil. This in effect offers an oil with the characteristics of a low SAE number at cold temperatures and an oil with the characteristics of a high SAE number at high temperatures. 18

19 Lubricants For example: SAE 20W-50 has a viscosity equivalent to that of SAE 20 oil when cold and a viscosity equivalent to that of SAE 50 when hot. 19

Engine Friction and Lubrication Internal Combustion Engine

Engine Friction and Lubrication 2103471 Internal Combustion Engine Friction Friction refers to the forces acting between mechanical components due to their relative motion and to forces on and by fluids