Aim
To study and prepare report on the constructional details working principles and operation of the following automotive engine systems and sub systems
a) Multi cylinder; diesel and petrol engines
b) Engine cooling and lubricating system
Theory
Labeled diagram, construction details, working principle and operation of the engine systems and sub systems
Methods of cooling
Various methods used for cooling of automobile engines are
1 Air cooling
2 Water cooling
Air cooling
The basic principle involved in this method is to have current of air flowing continuously over the heated metal surface from where the heat is to be removed. The heat dissipated depends upon following factors;
a) Surface area of metal into contact with air
b) Mass flow rate of air
c) Temperature difference between the heated surface and air
d) Conductivity of metal
Advantages
1 Air cooled engines are lighter because of the absence of the radiator, the cooling jackets and the coolant
2 They can be operated in extreme climates where the water may freeze.
3 In certain areas where there is scarcity of cooling water, the air cooled engine is an advantage.
4 Maintenance is easier because the problem of leakage is not there.
5 Air cooled engines get warmed up earlier than the water cooled engine
Disadvantages
1 It is not easy to maintain even cooling all around the cylinder, so that the distortion of the cylinders take place. This defect has been remedied sometimes by using fins parallel to the cylinder axis. This is also helpful where a number of cylinders in a row are to be cooled. However, this increases the overall engine length.
2 As the coefficient of heat transfer for air is less than that for water, there is less efficient cooling in this case and as a result the highest useful compression ratio is lesser in the case of air cooled engines than in the water cooled ones.
3 The fan used is very bulky and absorbs a considerable portion of the engine power (about 5%) to drive it.
4 Air cooled engines are more noisy, because of the absence of cooling water which acts as sound insulator.
5 Some engine components may become inaccessible easily due to the guiding baffles and cooling which makes the maintenance difficult.
Water cooling
In water cooling system, the cooling medium used is water. In this, the engine cylinders are surrounded by water jackets through which the cooling water flows. Heat flows from the cylinder walls into water which goes to the radiator where it loses its heat to the air. Usually some antifreeze is added to the cooling water, due to which it is often referred to as coolant. Both these terms have been used in this chapter, often meaning same unless the context requires otherwise.
Water cooling systems are of two types;
1 Thermosyphon system
1 Pump circulation system
Advantages
1 As the circulation of coolant is maintained by natural convention only, the cooling is rather slow. Therefore, to have adequate cooling, the capacity of the system has to be large.
2 Due to the quantity of coolant being large, it takes, more time for the engine to reach the operating temperature.
3 Radiator header tank must be located higher than the top of the cylinder coolant jackets, which is no more possible with the modern body styles.
4 Certain minimum level of coolant water must be maintained in the system. If the coolant falls below that level, continuity of flow would break and the system would consequently fail.
Systems of engine lubrication
The various systems adopted for the lubrication of automobile engine are
1 Petroil system
Splash system
2 Pressure system
3 Dry sump system
Petroil system
This is used generally for small two stroke engines, e.g. in majority of scooter and motor cycle engines. It is the simplest of all types of engine lubrication systems. Certain amount of the lubricating oil is mixed with the petrol itself, the usual ratio being 2% to 3% of oil. If it is less, there is danger of oil starvation or insufficient lubrication causing damage to the engine; if however, it is more, there will be excessive carbon deposits in the cylinder head and the engine will also give dark smoke.
When the petrol mixture enters the crankcase, due to high temperatures there, the petrol component vaporizes leaving a thin film of lubricating oil on the crankcase, cylinder walls, crankshaft and bearings.
The main requirements of lubricating oil for two stroke engine are that it should readily mix with petrol and burn without leaving much residue.
Splash system
This was employed for the engines of early motor cycles. It is one of the cheapest methods of engine lubrication. A scoop is made in the lowest part of the connecting rod and the oil is stored in the oil trough (fig.) it being pumped there from the crankcase oil sump. When the engine runs the scoop causes the oil to splash on the cylinder walls each time it passes through its BDC position. This affects the lubrication of engine walls, gudgeon pin, main crankshaft bearings, big end bearings etc.
Pressure system
This system is used almost universally in modern car engines. The splash system though cheaper, is not suitable for automobile engines because of the absence of positive lubrication. In the pressure system (fig ) an oil pump takes the oil from the wet sump through a strainer and delivers it through a filter to the main oil gallery at a pressure of 200 to 400 kPa. The oil pressure is controlled by means of a pressure relief valve, situated in the filter unit or the pump housing.
Dry sump system
This system is employed in some racing car engines for situations where the vehicle has to be operated at very steep angles, for example, sports cars, jeeps etc. if ordinary pressure system of lubrication is used in such cases, the situations may arise when there is no oil at the place where oil pump is installed. To avoid such instances, dry sump system is used (fig )wherein two pumps, instead of one, are used. The scavenge pump A is installed in the crankcase portion which is the lowest. It pumps oil to a separate reservoir B, from where the pressure pump C pumps the oil through filter D, due to the cylinder bearings; a full pressure system of lubrication is employed. The oil pressure is maintained at 400-50 kPa for the main and big end bearings while about 5- – 100 kpa pressure is used for timing gears and cam shaft bearings etc.
Reference Books
1) Automotive Mechanics – Crouse / Anglin
2) Automobile Engineering – Dr Kirpal Singh
Viva Questions
1 What is the necessity for cooling an engine?
2 Name various method of engine cooling?
3 Name the main components of water cooling system?
4 What is the function of radiator in cooling system?
5 What is the necessity of engine cooling?
6 State different S.A.E. viscosity rating of oil?
7 What is the multigrade oil?
8 Which solid lubricant is most commonly used in automobiles?
9 What is viscidity?
10 Give an example of synthetic lubricant?