Introduction. 1/3 Heat to Useful Work 1/3 Heat to cooling System 1/3 Heat to exhaust system

Transcription

1 COOLING SYSTEM

2 Introduction I.C Engines at best can transform about % of the chemical energy in to the fuel in to mechanical energy. About 35 % of the heat generated is lost to the cooling medium remainder being dissipated through exhaust and lubricating oil. 1/3 Heat to Useful Work 1/3 Heat to cooling System 1/3 Heat to exhaust system

3 Introduction The Purpose of Cooling System Prevent Overheating Excess Heat generated in engine Peak temperatures exceed melting point of metal Regulate the most efficient Temperature Regulate Temperature Allow engine to warm up in cool weather Maintain engine in optimum range.

4 Introduction Consequence of engine running too hot: Pre-ignition Detonation/Knock All result in very high pressure rise and possible damage to engine Heat Fatigue of components Burnt Pistons Burnt Valves Failure of lubrication system Oil breakdown Oil film at cylinder destroyed at 200C Scoring of piston & sleeves Warping & fracture of components

5 Introduction Consequence of engine running too cold: Unnecessary Wear Poor Fuel Economy Incomplete Combustion Lower coolant Temperature More energy transferred out of cylinder Energy from is wasted & not available for work (Power) Promotes corrosive conditions in engine Water of combustion reacts with sulfur oxides in exhaust Forms acids Allows water & sludge to accumulate in crankcase Over time, fuel diluting the oil will accumulate Normally lighter volatile fuel will evaporate as temp. rises

6 Introduction Critical engine components that need to be maintained at optimal designed temperature Combustion chamber walls Cylinder wall Cylinder head Piston Exhaust valve Spark plug Gasoline / Diesel injector Engine lubricants

7 Cooling system Cooling System components Function Centrifugal pump Cooling fluid Radiator Fan Cooling fluid circulation Heat transfer Heat exchange with the ambient Air through the radiator at low vehicle speed Thermostat Engine temperature stabilization

8 Cooling system Cooling System components Function Expansion tank Fluid expansion and gas release Filler pressure cap Passenger compartment radiator Lubricant radiator EGR cooling radiator (Diesel) Cooling circuit pressure Passenger compartment heating Engine lubricant cooling Exhaust gas cooling

9 Cooling system Heat transfer fluid Etilene glycol mixture in water (30 60% concentration) Water High specific heat Low viscosity High heat of vaporization High thermal capacity Low pressure drop Low gas formation Constant characteristics vs time and temperature Etilene glycol Low freezing point

10 Cooling system Ethylene glycol freezing point vs concentration in water EG Weight Percent (%) Freezing Point (deg F) Freezing Point (deg C)

11 Cooling system where: Q Q u Q w Q g General equation for engine thermal balance Q = Q + Q + Q + f u w g Q i ( kj h) = heat introduced into the engine through the fuel combustion f = work equivalent heat at the engine shaft = heat realeased to the engine cooling system = heat rejected to the exhaust gases Q i = lost heat for radiance

12 Introduction Variation of Gas Temperature Piston Temperature Distribution

13 Introduction Heat Transfer As a result of combustion, high temperature are produced, inside the engine cylinder Considerable heat flow from the gases to the surrounding metal walls Shearing of the oil film Heat transfer from gases to the cylinder walls may occur predominantly by Convection & Radiation Heat transfer through the cylinder wall occurs only by conduction The temperature profiles across the cylinder barrel wall For water cooled engine Air cooled engine

14 Introduction Cylinder wall temp profile

15 Parameters Affecting Engine Heat Transfer Engine heat transfer depends upon many parameters, unless the effect of these parameters is known, the design of a proper cooling system will be difficult. Fuel-Air Ratio A change in fuel-air ratio will change the temperature of the cylinder gases and affect the flame speed. Spark Advance More or less spark advance from the optimum value will result in increased heat rejection to the cooling system Pre-ignition and knocking Engine output Engines which are designed for high M.E.P or high piston speeds, heat rejection will be less Cylinder wall temperature

16 Characteristics of an efficient cooling system The two main characteristics desired of an efficient cooling system It should capable of removing about 30 % of the heat generated in the combustion chamber while maintaining the optimum temperature of the engine under all operating conditions of the engine It should remove heat at a faster rate when engine is hot. However during starting of the engine the cooling should be minimum, so that the working parts of the engine reach their operating temperatures in a short time

17 Types of cooling System There are two types of system in general Liquid or indirect cooling system Air or direct cooling system Liquid cooling system Mainly water is used and made to circulate through the cooling jackets provided around the cylinder, cylinder head, valve ports and seats where it extracts most of the heat The heat transferred from the cylinder wall and other parts by convection and conduction The heat from liquid in turn is transferred to air. Hence it is called the indirect cooling system

18 Methods of water-cooling System Water cooling can be carried out by any one of the following five methods Direct or non return system Thermosyphon system Forced circulation cooling system Evaporation cooling system Pressure cooling system

19 Direct or Non return System This system is useful for large installations where plenty of water is available. The water from a storage tank is directly supplied through an inlet valve to the engine cooling water jacket The hot water is not cooled for reuse but simply discharged.

20 Thermosyphon System Heat is supplied to the fluid in the tank A Because of relatively lower density, the hot fluid travels up Its place being taken up by comparatively cold fluid from the Tank B through the pipe p 2 The hot fluid flows through the pipe P 1 to the tank B where it gets cooled The fluid circulates through the system in the form of convection current

21 Thermosyphon System Tank A represents the cylinder jackets Tank-B- represents a radiator and water acts as the circulating fluid. The advantages are Its simplicity Automatic circulation of cooling water The man limitation of the system is its inability to meet the requirement of large flow rate of water, particularly for high output engines.

22 Forced circulation Cooling System This system is added in a large number of Automobiles. Here the flow of water from radiators to water jackets is by convection assisted by a pump

23 Forced circulation Cooling System Water or coolant is circulated with through jackets around the parts of the engine to be cooled is kept in motion by a centrifugal pump which is driven by the engine The water is passed through the radiator where it is cooled by the forward motion of the vehicle A thermostat is used to control the water temperature required for cooling This system consists of 4 components Radiator Fan water pump thermostat

24 Basic Schematic Layout Cab heat exchanger Heater Control Thermostat Engine Block & Cylinder head Water Jacket Water Pump Fan Radiator

25 Parts of Engine Cooling System

26 Cooling System Construction Radiator Cap Top Hose Radiator Thermostat Fan Engine Block & Water Jacket Water Pump

27 Cold Engine When an engine is cold, the thermostat is cold. Coolant flow is through the bypass hose and the water jackets. This allows the engine to warm up evenly.

28 Warm Engine The thermostat opens when the engine warms up. This allows coolant to circulate through the radiator and the water jackets.

29 Water jackets Defined as the open space within in the cylinder block and cylinder head where coolant flows Water jackets are designed to allow coolant flow to the right spots flow so that maximum cooling can be obtained

30 Water Pump The purpose Is to circulate the water through the cooling system Located on the front part of the engine In most of the vehicles it is driven by a belt is attached to the crankshaft As the crankshaft turns the fan belt turns

31 Cooling system The water pump provides circulation of the engine coolant (antifreeze) through the cooling system: it pushes the coolant through the passages (water jackets) in the engine cylinder block and cylinder head and then out into the radiator. The hot coolant passes through the radiator where it cools down and then returns back to the engine. Centrifugal pump is the most used:it is a rotodynamic pump that uses a rotating impeller to increase the pressure and flow rate of a fluid. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward or axially into a diffuser or volute chamber, from where it exits into the downstream piping system. A water pump is usually driven by the engine through the driving belt and only sometimes by a timing belt. A water pump consists of the housing with the shaft rotating on the bearing pressed inside. At the outer side there is a pulley mounted on the shaft. At the inner side there is a seal to keep the coolant from leaking out and the impeller. 31

32 Cooling system Main design characteristics Impeller diameter ( = mm) Impeller height (h = mm) Paddles number and design (z = 5-10) Axial and radial impeller clearance Drive ratio n pump τ = = n engine 32

33 Thermostat One of the most important parts of the cooling system Purpose Is to keep the engine coolant at most efficient temperature The thermostat is used to bring the coolant temperature up to operating as quickly as possible It is designed to sense the temperature of the coolant

34 Thermostat

35 Cooling system Target - In internal combustion engines a thermostat is used to maintain the engine at its optimum operating temperature by regulating the flow of coolant to the external air cooled radiator. It must balance the heat rejected from the engine to the coolant and the heat rejected from the radiator to the ambient in any operating vehicle mode. This type of thermostat operates mechanically: it makes use of a wax pellet inside a sealed chamber. The wax is solid at low temperatures but as the engine heats up the wax melts and expands. The sealed chamber has an expansion provision that operates a rod which opens a valve when the operating temperature is exceeded. The operating temperature is fixed, but is determined by the specific composition of the wax, so thermostats of this type are available to maintain different temperatures, typically in the range of 70 to 90 C. Modern engines run hot, that is, over 80 C, in order to run more efficiently and to reduce the emission of pollutants. Most thermostats have a small bypass hole to vent any gas that might get into the system, e.g., air introduced during coolant replacement, which also allows a small flow of coolant past the thermostat when it is closed. This bypass flow ensures that the thermostat experiences the temperature change in the coolant as the engine heats up; without it a stagnant region of coolant around the thermostat could shield it from temperature changes in the coolant adjacent to the combustion chambers and cylinder bores. 35 Wax thermostatic elements permit the transforming of thermal energy into mechanical energy. Their working principle is based on the large increase in the thermal expansion of waxes when they pass from the solid to the liquid state

36 Radiator Purpose Is to allow fresh air to reduce the temperature of the coolant As the coolant passes through the tubes air is forced around the tube This causes a transfer of heat from the hot coolant to the cooler air. This is called Heat exchanged In this case, heat is exchanged from the liquid coolant to air. This is called a liquid-to air heat exchanger

37 Radiator Coolant flows through the air fins The fins and tubes are cooled core

38 Radiator Parts

39 Radiator Hose Purpose Transport coolant from engine to radiator and back

40 Radiator pressure cap Pressure caps are designed to Increase the pressure on the cooing system Reduce cavitations Protecting the radiator hose Prevent or reduce surging It is important to put pressure on the cooling system. Radiator pressure caps are typically near 15 psi As the pressure increases the boiling point of the coolant also increases (about 3 degrees for each 1 psi increase

41 Pressure Cap

42 Pressure cap

43 Pressure cap

44 Expansion tank

45 Expansion tank

46 Fan

47 Electric Fan Relay

48 Coolant Temperature Indicator

49 Coolant Temperature Indicator

50 Types of Coolant

51 Coolant

52 Coolant

53 Coolant

54 Anti-freeze IAT: inorganic additive technology OAT: organic acid technology HOAT: hybrid organic acid technology Extended and conventional anti-freezes cannot be mixed EG: ethylene glycol PG: propylene glycol

55 Anti-freeze Maximum concentration: 67% anti-freeze Minimum concentration: 50% for corrosion prevention Pure anti-freeze has higher viscosity and does not flow well Does not transfer heat well