Unit 2 lubrication, cooling and fuels

Reference text booksInternal combustion engines –V Ganeshan

Unit 2Lubrication and

Cooling

GITAM SCHOOL OF TECHNOLOGY

COOLING SYSTEM FOR IC ENGINES• An automobile's cooling system is the collection of parts and substances (coolants) that work together

to maintain the engine's temperature at optimal levels. Comprising many different components such as water pump, coolant, a thermostat etc. the system enables smooth and efficient functioning of the engine at the same time protecting it from damage

• The following are the two main characteristics desired of an efficient cooling system

1. It should be capable of removing about 30% of heat generated in the combustion chamber while maintain the optimum temp of the engine under all operating conditions of engine.

2. 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 engine reach their operating temperature in short time.


NECESSITY OF COOLING

The cooling system is provided in the IC engine for the following reasons:• The temperature of the burning gases in the engine cylinder reaches up to 1500

to 2000°C, which is above the melting point of the material of the cylinder body and head of the engine. (Platinum melts at 1750 °C, iron at 1530°C and aluminum at 657°C.) Therefore, if the heat is not dissipated, it would result in the failure of the cylinder material.

• Due to very high temperatures, the film of the lubricating oil will get oxidized, thus producing carbon deposits on the surface. This will result in piston seizure.

• Due to overheating, large temperature differences may lead to a distortion of the engine components due to the thermal stresses set up. This makes it necessary for, the temperature variation to be kept to a minimum.

• Higher temperatures also lower the volumetric efficiency of the engine


EFFECTS OF OVERCOOLING

(a) Increased cylinder wear.

(b) Dilution of oil due to poor vaporization of petrol

(c) Greater formation of sludge.

(d) Oil does not thin out properly and fluid friction losses are increased.

(e) Engine does not achieve full power.

(f) Burnt gases, which leak past piston, condense in the crankcase to form corrosive acids in oil.

(g) Lower thermal efficiency i.e. more consumption of fuel


TYPE OF COOLING SYSTEM

• In order to cool the engine a cooling medium is required. This can be either air or a liquid accordingly there are two type of systems in general use for cooling the IC engine. They are

• Liquid or indirect cooling system

• Air or direct cooling system


AIR COOLING• In this type of cooling system, the heat, which is conducted to the outer parts

of the engine, is radiated and conducted away by the stream of air, which is obtained from the atmosphere. In order to have efficient cooling by means of air, providing fins around the cylinder and cylinder head increases the contact area. The fins are metallic ridges, which are formed during the casting of the cylinder and cylinder head

• The amount of heat carried off by the air-cooling depends upon the following factors:

(i) The total area of the fin surfaces,

(ii) The velocity and amount of the cooling air and

(iii) The temperature of the fins and of the cooling air.

TYPES

1. Natural flow type

2. Forced Convection type


ADVANTAGES OF AIR COOLED ENGINES1. Its design of air-cooled engine is simple.

2. It is lighter in weight than water-cooled engines due to the absence of water jackets, radiator, circulating pump and the weight of the cooling water.

3. It is cheaper to manufacture.

4. It needs less care and maintenance.

5. This system of cooling is particularly advantageous where there are extreme climatic conditions in the arctic or where there is scarcity of water as in deserts.

6. No risk of damage from frost, such as cracking of cylinder jackets or radiator water tubes.


LIMITATIONS OF AIR COOLED ENGINES• Can be applied only to small and medium sized engines.

• Cooling is not uniform.

• Higher working temperature compared to water-cooling.

• Produce more aerodynamic noise.

• Specific fuel consumption is slightly higher.

• Lower maximum allowable compression ratios.

• The fan, if used absorbs as much as 5% of the power developed by the engine.


LIQUID COOLED SYSTEM

• In this system mainly water is used and made to circulate through the jackets provided around the cylinder, cylinder-head, valve ports and seats where it extracts most of the heat.

• It consists of a long flat, thin-walled tube with an opening, facing the water pump outlet and a number of small openings along its length that directs the water against the exhaust valves. The fits in the water jacket and can be removed from the front end of the block.

• The heat is transferred from the cylinder walls and other parts by convection and conduction. The liquid becomes heated in its passage through the jackets and is in turn cooled by means of an air-cooled radiator system. The heat from liquid in turn is transferred to air. Hence it is called the indirect cooling system.

TYPES OF WATER COOLED SYSTEMS

•Direct or non-return system•Thermo siphon system•Forced circulation cooling system•Evaporative cooling system•Pressure cooling system


THERMO-SIPHON SYSTEM• This system works on the principle that hot water being

lighter rises up and the cold water being heavier goes down. In this system the radiator is placed at a higher level than the engine for the easy flow of water towards the engine. Heat is conducted to the water jackets from where it is taken away due to convection by the circulating water. As the water jacket becomes hot, it rises to the top of the radiator. Cold water from the radiator takes the place of the rising hot water and in this way a circulation of water is set up m the system. This helps in keeping the engine at working temperature.

Disadvantages of Thermo-Siphon System1 Rate of circulation is too slow.2. Circulation commences only when there is a marked

difference in temperature.


FORCED OR PUMP SYSTEM

• This system is similar in construction to the thermo-siphon system except that it makes use of a centrifugal pump to circulate the water throughout the water jackets and radiator

• The water flows from the lower portion of the radiator to the water jacket of the engine through the centrifugal pump. After the circulation water comes back to the radiator, it loses its heat by the process of radiation.

Advantages

• Cooling is ensured in all conditions

Disadvantages

• Cooling is not temperature dependent

• Cooling stops as soon as the engine is stopped, which is undesirable


THERMOSTAT COOLING


THERMOSTAT

• Placed between the cylinder head and top radiator hose. • Regulates engine coolant temperature • The temperature that the thermostat opens is called thermostat

rating. (85-900 C most common) • Solid Expansion design – wax pellet expands as temp increases, valve

begins opening at rating & is completely open within 10 degrees • If it fails in open position, engine runs cold resulting in poor mileage

and high wear & tear. If it fails closed, creates temperature in the engine well beyond normal limits. Many types of damage may occur. (Can be checked by placing in the boiling water)


PRESSURIZED COOLING• In pressure cooling system moderate pressure, say up to 2 bar,

are commonly used. As shown in fig a cap is fitted with two valves which are loaded by a compression spring and a vacuum valve.

• When the coolant is cold both valves are shut but as the engine warm up the coolant temperature rises until it reaches a certain preset value corresponding to the desired pressure when the safety valve open. But if the coolant temperature falls during the engine operation the valve will close again until the temperature rises to equivalent pressure value. When the engine is switched off and the coolant cool down vacuum begin to form in the cooling system but when the internal pressure fall below atmosphere the vacuum valve is opened by the higher outside pressure and the cooling system then attains atmosphere pressure

ADVANTAGES1. Can take overload very easily2. The placement of radiator is not critical3. Corrosion is lo because of the pressurized

coolant4. No loss of water due to boiling or evaporation


EVAPORATIVE COOLING

• This is predominately used in stationary engine. In this the engine will be cooled because of the evaporation the water in the cylinder jackets into the steam. Here the advantage is taken from the high latent heat of vaporizing of the water by allowing evaporating in the cylinder jackets. If the steam is formed at a pressure above atmospheric the temperature will be above the normal permissible temperature.


COMPONENTS OF WATER COOLING SYSTEMWater cooling system mainly consists of :

(a) Radiator,

(b) Thermostat valve,

(c) Water pump,

(d) Fan,

(e) Water Jackets,

(f) Antifreeze mixtures.


RADIATOR• It mainly consists of an upper tank and lower tank and between

them is a core. The upper tank is connected to the water outlets from the engines jackets by a hose pipe and the lover tank is connect to the jacket inlet through water pump by means of hose pipes.

• There are 2-types of cores :

(a) Tubular

(b) Cellular

(c) Gilled tube radiator:

• When the water is flowing down through the radiator core, it is cooled partially by the fan which blows air and partially by the air flow developed by the forward motion of the vehicle.

• As shown through water passages and air passages, wafer and air will be flowing for cooling purpose.

• It is to be noted that radiators are generally made out of copper and brass and their joints are made by soldering.


WATER PUMP• It is used to pump the circulating water. Impeller type pump will be mounted at

the front end.

• Pump consists of an impeller mounted on a shaft and enclosed in the pump casing.

• The pump casing has inlet and outlet openings. The pump is driven by means of engine output shaft only through belts. When it is driven water will be pumped.

FAN•It is driven by the engine output shaft through same belt that drives the pump. It isprovided behind the radiator and it blows air over the radiator for cooling purpose.

Cooling water jackets are provided around the cylinder, cylinder head, valve seatsand any hot parts which are to be cooled. Heat generated in the engine cylinder,conducted through the cylinder walls to the jackets. The water flowing through the jackets absorbs this heat and gets hot. This hot water will then be cooled in the radiator

WATER JACKETS


ANTIFREEZE MIXTURE• In western countries if the water used in the radiator freezes because of cold climates, then ice formed has more volume and produces

cracks in the cylinder blocks, pipes, and radiator. So, to prevent freezing antifreeze mixtures or solutions are added in the cooling water.

The ideal antifreeze solutions should have the following properties :

(a) It should dissolve in water easily.

(b) It should not evaporate.

(c) It should not deposit any foreign matter in cooling system.

(d) It should not have any harmful effect on any part of cooling system.

(e) It should be cheap and easily available.

(f) It should not corrode the system.

No single antifreeze satisfies all the requirements. Normally following are used as antifreeze solutions :

(a) Methyl, ethyl and isopropyl alcohols.

(b) A solution of alcohol and water.

(c) Ethylene Glycol.

(d) A solution of water and Ethylene Glycol.

(e) Glycerin along with water, etc.

LUBRICATION


PURPOSE OF LUBRICATION

• Lubrication produces the following effects: (a) Reducing friction effect (b) Cooling effect (c) Sealing effectand (d) Cleaning effect.

(a) Reducing frictional effect: The primary purpose of the lubrication is to reduce friction and wear between two rubbing surfaces. Two rubbing surfaces always produce friction. The continuous friction produce heat which causes wearing of parts and loss of power. In order to avoid friction, the contact of two sliding surfaces must be reduced as far a possible. This can be done by proper lubrication only. Lubrication forms an oil film between two moving surfaces. Lubrication also reduces noise produced by the movement of two metal surfaces over each other.

(b) Cooling effect: The heat, generated by piston, cylinder, and bearings is removed by lubrication to a great extent. Lubrication creates cooling effect on the engine parts.

(c) Sealing effect: The lubricant enters into the gap between the cylinder liner, piston and piston rings. Thus, it prevents leakage of gases from the engine cylinder.

(d) Cleaning effect: Lubrication keeps the engine clean by removing dirt or carbon from inside of the engine along with the oil.

Engineering

Unit 2 lubrication, cooling and fuels