Mechinopedia For Mechanical Engineers

In this System, oil is stored in the crank case. A small scoop is attached with the big end of connecting rod. When the crank is rotated, the scoop dips in the oil and splashes the oil. The oil is splashed on cylinder wall, connecting rod ends and valve mechanisms. This method is used in some motorcycles and single cylinder stationery engines. Greater care should be taken that the oil in the crank case is filled up to the desired mark. There will be insufficient lubrication when the oil level is low.  Disadvantages :  * It is not efficient , if the bearing loads are heavy. * It is very difficult to introduce oil in the minute gaps between the       sliding surfaces. 

 In this method, oil is supplied to the parts to be lubricated by means of gravity. This system uses a drop feed oiler. It consists of  a cup and needle valve arrangement. The needle valve is operated by means of screw. The valve is raised to increase the flow of oil and lowered to decrease the oil flow. This system is used for lubricating external moving parts such as bearings, cross head , crank pins of simple steam engine. 

 The various methods adopted for lubrication Of  I.C Engines are : i) Petrol lubrication or mist lubrication system, ii) Wet sump system, and  iii) Dry sump system.  I) Petrol or Mist Lubrication System :    It is simplest of all types of lubrication. This method is used in light vehicles such as motor cycles and scooters. About 3 to 6 % of lubricating oil is mixed with petrol in fuel tank. Here, there is no separate sump and pump. The oil mixing with petrol acts as lubricant.  The disadvantage of  this method is, if the engine remains idle for the long time, the oil will get separated and cause clogging of fuel passage in carburetor there by resulting in starting trouble. If the quantity of mixing oil is less, lubricating is insufficient which causes damage to the engine. If the quantity of oil is more, it will release more carbon deposits in the cylinder. So, the engine will give dark smoke. II) Wet Sump System :    In this method, the lubrication oil is stored in the oil sump. From

 The Lubricant used in I.C Engines should have some properties for the successful performance of the engine. The properties required for a good lubricant are listed below :  i) Viscosity :      Viscosity is defined as the measure of fluid resistance to flow. Viscosity of the lubricant decreases as temperature increases and vice versa. This property is very important property of  lubricant because it determines how efficiently oil film separates the moving surfaces from each other and prevents them rubbing. If high viscosity ( i.e , Too Thick ) oil is used, it will lead to power loss, higher operating temperature and excessive wear. If low viscosity oil is used , it cannot lubricate properly and leads to rapid wear of  moving parts. ii) Oiliness :      It is the property of an oil to spread and attach itself firmly to the bearing surfaces. In general, high oiliness is required for better lubrication. iii) Fire Point :     The Fire point is the lowest temperature at which the fuel burns

 a) Internal Surface of cylinder walls.  b) Crank Shaft bearings  c) Crank Pin  d) Cam Shaft   e) Cam Shaft bearing  f) Valve Mechanism  g) Piston rings   h) Piston pin or gudgeon pin  i) Timing gears   j) Big end and small end of the connecting rod bearing.

 In an IC Engine, moving parts rub against each other causing frictional force. Due to the frictional force,   heat is generated and the engine parts wear easily. Power is also lost due to friction. To reduce the power   loss and also wear and tear of the moving parts, a foreign substance called lubricant is introduced in   between rubbing surfaces. The lubricant keeps the mating surfaces apart. Lubricant may be solid (graphite) , or semi-solid (grease) or liquids (oil) . The liquid lubricant is generally used is mineral oil. This is obtained by refining petroleum. Grease is also used to lubricate certain parts of the engine. PURPOSES OF LUBRICATION (OR) FUNCTIONS OF LUBRICATION : a) It reduces friction between moving parts. b) It reduces wear and tear of  the moving parts. c) It minimizes power loss due to the friction. d) It provides cooling effect - During Circulation, It carries that from the hot moving parts and delivers  it   to the surrounding through crankcase. e) It provides

 i) In S.I Engine, the knocking occurs at the end of the combustion whereas in C.I Engine knocking occurs at the very beginning of combustion. ii) The detonation in the S.I Engine is of  a homogenous charge causing very high rate of pressure rise and  very high maximum pressure. In the C.I Engine, The fuel and air are heterogenous and      hence the rate of pressure rise is normally lower than that in the         detonating part of the charge in S.I Engine. iii) In the C.I Engine, The fuel is injected into the cylinder only at the  end of  the compression stroke, there is no question of pre-ignition as in S.I Engine. iv) In the C.I Engine, severe audible knock is always present during knocking as compared to S.I Engine.     

If the delay period is long , a large amount of fuel will be injected and accumulated in combustion chamber. The auto Ignition of this large amount of fuel may cause high rate of pressure rise. This high pressure rise causes heavy vibration of the engine and creates lot of noise.  This Phenomenon of Combustion causing heavy pressure rise during uncontrolled combustion is called "Diesel Knock" . Diesel Knock generally starts at the very beginning of the combustion process due to sudden auto ignition of large amount of fuel whereas the knocking in the S.I engine generally starts at the end of combustion period. The following method can be adopted to prevent diesel knock. * By reducing the delay period by doping e. g adding 1% of ethyl nitrate or any nitrate so as to accelerate the combustion. * By increasing the turbulence of the compressed air injected. It promotes homogenous mixture by strapping the fuel from the spray. * By arranging the fuel injector in such a way to inject

Pre - Ignition is defined as the phenomenon of ignition of the charge before the ignition spark occurs. This type of  ignition is caused when some parts of the combustion space e.g. spark plug , exhaust valve, carbon particles in the combustion chamber are over heated under certain operating conditions. When pre-ignition occurs , it is the equivalent of an advanced spark and may cause detonation. Conversely, when detonation is severe and long continued, it may heat up the spark plug points or carbon particles.   Pre - Ignition may cause high rates of pressure rise due to multiple ignition points and advanced timing together with irregular detonation. Pre - Ignition leads to increase in pressure before the piston the reaches TDC and hence the piston movement is opposed and the power output is reduced. In Heavy - duty engine, excessive heating due to the pre ignition may cause piston and cylinder damage. In multi cylinder engines pre - ignition may develop in one or more cylinders, th

 If  the temperature of air - fuel mixture is raised high enough, the mixture will self-ignite without the need of a sparkplug. This phenomenon is called as self-ignition or Auto Ignition. The temperature above which self-ignition occurs called the self - ignition temperature. If the temperature of the unburnt mixture exceeds the self - ignition temperature during the ignition delay period, auto - ignition occurs at various locations in the cylinder. This will generate pressure pulses. These pressure pulses can cause damage to the engine and to the engine and quite often are in the audible frequency range. This phenomenon is often called knocking or detonation.  EFFECTS OF KNOCKING :  *  The impact on the engine components and structures may cause          failure and creates undesirable noise which is always                          objectionable. *  The lack of control of combustion process leads to pre ignition          and   local over heating. Therefore,  piston  may be damaged 

FACTORS AFFECTING KNOCKING IN S.I ENGINE :  The various engine variables affecting detonation can be classified under four factors, namely, the temperature factors, density factors, time factors and composition factors. a) Temperature Factors :     Increasing the temperature of  the  unburnt mixture by any factor in design or operation will increase the possibility of the knock in the S.I Engine. The temperature of the unburnt mixture is increased  by  the  following factors :  * Raising the compression ratio.   * Supercharging   * Raising the inlet air temperature.  * Raising the coolant Temperature.  * Increasing the load.  * Raising the temperature of the cylinder and combustion chamber walls.  * Advancing the spark timing.  b) Density Factors :     Any factors which increase the density of  the charge lend to increase in knocking by providing excess energy release. The following method will increase the possibility of Knock on S.I Engine.  * Increasing the compression ratio.  * Ope

 The air supplied per kg fuel is determined when the volumetric analysis of  dry flue gases and percentage of carbon by mass in fuel are known.  Mass of air supplied , Ma = ( 1 / 33 ) * ( CN / (C1 + C2) ) Where, C = Percentage of  Carbon in Fuel.               N = Percentage of  Nitrogen.              C1= Percentage of  CO.              C2 = Percentage of  CO2. Nitrogen does not make any effect in combustion reaction. It means, the nitrogen remains same as nitrogen after combustion.  

 STOCHIOMETRIC AIR FUEL RATIO :  The Amount of air required to burn 1 kg of fuel for making complete combustion is known as air - fuel     ratio. It is also known as Stochiometric or Theoretical air - fuel ratio.  STOCHIOMETRIC  AIR - FUEL RATIO :        = Amount of air required for complete combustion /  Amount of  fuel used. ACTUAL AIR- FUEL RATIO : Practically , Air is required to make complete combustion of fuel is slightly more than the theoretical air . So, the excess air is supplied to obtain complete combustion. Therefore, the actual air-fuel ratio is slightly higher than the theoretical air -fuel ratio of stochiometric   air -fuel ratio. Then, the percentage excess air can be calculated as Percentage Excess Air = (Actual A/F ratio - Stochiometric A/F ratio)                                          / ( Stochiometric A/F ratio )  Note : Conversion of  volumetric analysis to mass analysis is done by multiplying its molecular weight. Mass analysis = Volumetric analysis * Molecular

FUELS :  Fuel is a substance from which heat energy is produced by sufficient quantity of air while burning.  TYPES OF FUELS :  Fuels are mainly classified into following types, 1. Solid Fuels. 2. Liquid Fuels. 3. Gaseous Fuels. I) SOLID FUELS : Wood, peat and coal, sugarcane crushing, municipal waste etc., are solid fuel . When the fuel burns, fixed carbon , hydrogen , Sulphur , oxygen , and nitrogen will release. These constituents are determined by either proximate or ultimate analysis. II) LIQUID FUELS : Generally, carbon and hydrogen are the basic combustible constituents in all liquid fuels. Mainly, Liquid fuels are obtained from petroleum which is a mixture of various hydrocarbons such as paraffin's , olefins , naphthene , and aromatics. These liquid fuels are mainly used in power generation.  Advantages : 1. It has higher calorific value.  2. It requires lesser space in use.  3. It keeps cleanliness surroundings. 4. It eliminates wear and tear of grate. 5. It is easy to con

Calorific Value (CV)  of a fuel is the thermal energy released per unit quantity of the fuel when the fuel is burned completely and the products of combustion are cooled back to the initial temperature of the combustible mixture.  The amount of heat released in one kg of fuel in one hour is called as calorific value. It is denoted by CV. It's unit is kJ / hr. Generally, gaseous fuel has two calorific values such as higher and lower calorific values. 

The complete record of heat supplied and heat rejected during a certain time by an I.C Engine is entered in a tabular form known as heat balance sheet. All the heat energy supplied to an engine can not be converted into useful work. Some of  the heat energy may be lost by means of some sources. The source of  heat losses are given below,  i)   Heat rejected to the cooling water. ii)  Heat Carried away by exhaust gases. iii) Unaccounted heat losses by means of radiation , incomplete combustion and errors in observations etc.  The Following Valves should be calculated for tabulating heat balance sheet. i)  Heat Supplied by the Fuel (Qs) :       Q s = m  f * CV kJ / hr  ii) Heat absorbed in B.P. Produced ( Qip) :      Brake Power Q bp = 2 (3.14) N T kJ / hr  (N in rev / hour )                              Q bp = 2 (3.14) N (  W - S ) kJ / hr  iii) Heat Rejected to the cooling water (Qw) :        Q w = m w C w ( T2 - T1 ) kJ / hr       Where,  mw = Mass of  the cooling water circulat

  The Efficiency of the engine is defined as the ratio of the work done by the engine to the energy supplied to an engine. The various efficiencies of an I.C Engines are given below, a) Mechanical Efficiency ( N mech ) :  It is the ratio of the brake power to the Indicator power . N mech = Brake Power / Indicated Power  = BP / IP  b) Indicated Thermal Efficiency ( N it ) :  It is the ratio of  indicated power to the heat supplied to an engine.  N it = Indicated Power /  Heat Supplied  Heat Supplied per sec = ( m f * CV / 3600 ) in Kw  where m f    = Mass of fuel consumed per hour            CV = Calorific Value of fuel kJ / kg .            N it = IP / ( mf * CV / 3600 )  c) Brake Thermal Efficiency ( N bt ) :  It is the ratio of Brake power to the heat supplied to an engine. N bt = Brake Power / Heat Supplied  N bt = BP / ( m  f * CV / 3600 )   d) Relative Efficiency ( N r ) : It is also known as efficiency ratio. It is the ratio of the indicated thermal efficiency to the air s

 The commonly used method of measuring the consumption of air is known as Orifice Chamber Method. It consists of an airtight chamber fitted with a orifice of  known coefficient of discharge. The size of the chamber of is generally 500 to 600 times of the swept volume in single cylinder engine. This is used for making steady flow of air through orifice. The pressure difference causing the flow through the orifice is measured with the help of manometer. This measurement is further utilized for air consumption calculations. 

 There are two ways of expressing fuel consumption  (i) By Volume  (ii) By Weight  I) Volumetric Type Flow meter :      Two glass vessels of  100CC and 200CC capacity are connected in between the engine and main fuel tank through two way and three way cocks. When one is supplying the fuel to the engine, the other is being filled. By using stopwatch, the time for the consumption of 100 or 200cc , fuel is measured.  II) Gravimetric Flow meter :    The efficiency of an engine is generally related to the kg of fuel consumed per unit time. Weight of the fuel consumed is directly measured by this method. It consists of a small glass tube which is attached to the main fuel tank. When the fuel rate is to be measured, the valve is closed so that fuel is consumed from the fuel tube. The time taken for a known value of fuel consumption can be measured.  Then the fuel consumption rate can be calculated by , Fuel consumption rate, kg / hr = (Fuel consumed * Specific gravity of fuel ) / (1000 * t) 

Friction power of  an engine may be defined as the difference between the indicated power and brake power. Two kinds of losses occur in the engine. They are pumping losses and friction losses. The pumping losses occur during suction and exhaust stroke. The friction loss is due to the friction between the piston and cylinder walls , piston rings and cylinder walls, and between the crankshaft and cam shaft and their bearings etc. The friction power can be determined by using,  Friction power = Indicated power - Brake Power                  F.P    = I.P    -   B.P  The friction power of an I.C Engine can be determined by the following methods :  i) Willan's line method       In Willan's line method , the gross fuel consumption is plotted against B.P and the line so obtained is extended backward to zero fuel consumption. The negative intercept on the B.P. axis gives the value of the friction power (F.P.). This negative work represents the combined loss due to mechanical friction,