Working of 4 stroke engine , Air standard cycles , Thermochemistry , Engine performance parameters , IC engine , GATE & ESE

 Working of 4 stroke engine 

Spark plug 

• For sparking high voltage is required to overcome the air resistance present in the spark gap.
• Theoretically platinum is the best suitable material but practically spark plug should be made up of nickel alloy because its thermal coefficient of expansion tends to zero. 
  
  
  
  
  
• After power stroke as the exhaust valve gets opened the major exhaust gases will escape out of the cylinder at BDC only (heat rejection at constant volume) ( figure 4 ).
• After heat rejection the remaining exhaust gas will be thrown out of the cylinder as the piston moves from BDC till TDC (exhaust stroke). 
  

Power stroke

Engine material selection 

• Piston rings : it should be made up of silicon cast iron or  silicon steel so that it will be smooth as brittle. Brittle because if it is ductile then after some usage time it will  deform and create a space from where AFM leaks out. 
  
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Note : gudgeon pin 

• It is subjected to fatigue loading conditions and it should be made up of ductile material by forging. 

Note : connecting rod and crank rod is also made up of forging but especially by drop forging. 

Air standard cycle 

Air standard assumption 

• Air is only working fluid. 
• Perfect gas
• Ideal gas equation. 
• CP ,CV and specific heat should be constant. 
• Heat addition: the internal combustion is modelled as heat addition into the system from an external source. 
• Heat rejection : the exhaust process in which majority of exhaust gas escape out of the cylinder is modelled as heat rejection. 
• Process ratio
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• Efficiency of otto cycle
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• Maximum work done condition (Otto cycle) : T1 and T3 is considered to be the constant because T1 is assumed to be depend upon the atmospheric condition which is not going to be affected by any process parameter and T3 is restricted due to the temperature resistance of any material .

Mean effective pressure 

• It is an imaginary pressure which will provide the same work done as the actual cycle for the same change in volume . 
  

Question : an engine working on air standard Otto cycle having bore of 10cm and crank radius of 7.5 cm. The clearance volume of the engine is 196.3 cm^3. Assuming the ratio of specific heat as 1.4 and specific heat at constant volume is 0.718 kilo joule per kg k. The ambient condition of the air is 1 bar pressure and 27 degree celsius temperature. The heat added into the system is 1800 kilo joule per kg of air then determine :clearance ratio ,air standard efficiency, Network done,Maximum pressure and maximum temperature, Mean effective pressure in bar, Temperature and pressure after expansion.

 Thermochemistry 

• It is the branch of science in which there is a study of transformation in chemical energy of the fuel into its thermal form due to the oxidation reaction of combustion. 
• Note: the total amount of time required for the chemical reaction or time from the beginning of spark into beginning of ignition is termed as chemical delay. 
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Combustion reaction 

Perfect combustion 

• When the combustion of the fuel mass in such a manner that there is no fuel remaining in any form no there is any oxygen left. 

Question: propane (C3H8)is burnt in an oxygen atmosphere with 10%  deficit oxygen (O2) with respect to the perfect combustion condition. Assuming no hydrocarbon in the exhaust then determine the volumetric percentage of carbon monoxide in the exhaust? 

Ques:tion octane (C8H18) is burned in an air atmosphere with 2.5 moles less available of the bulk air molecules than the perfect combustion state. Determine the molar percentage of carbon dioxide and carbon monoxide in the product. Also determine the mass percentage of water vapour. 

LEAN

PERFECT

RICH

SUPER RICH

Note : exhaust gas analysis (lean AFM) 

ORSAT analyser 

• It is a device which is used to absorb the exhaust gases from an IC engine and gives the volumetric percentage analysis of dry air exhaust. 
  

Question : in order to burn 1 kg of ch4 completely oxygen in kg?

Equivalence ratio 

• It is defined as the ratio of fuel air ratio in actual combustion condition with respect to the theoretical combustion condition. 
  

Basic concept of diesel engine

• In diesel engine only air is entered into the cylinder during the suction stroke and due to the very high compression ratio the temperature inside the cylinder after compression becomes more than the self ignition temperature. 
• Hence as the fuel enters the cylinder it starts its ignition and during that time period piston displaces and it is assumed as heat addition at constant pressure. 

Diesel fuel injector 

• Fuel is injected into the cylinder by the use of heavy pressure difference and as the opening is rough the fuel mass is a split into the smallest possible size called atomization. 
• After atomization the surface area increases and due to which fuel state changes from liquid form to its vapour called vaporization. 
• After vaporization the fuel mass is enforced uniformly mix up with the air molecule to create a air fuel mixture called uniform distribution. 

Note: 

• The total time required from the beginning of the injection of the fuel to the beginning ignition of the fuel is termed as ignition delay in CI engine. 
• Whereas the total time from the beginning of the sparking to the beginning of ignition is termed as ignition delay or chemical delay in SI engine. 
• The total time required for the atomization ,vaporization and uniform distribution of air and fuel mixture is termed as physical delay. 
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• Diesel cycle
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• Ratio theory
  
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Question : an engine working on diesel combustion cycle having inlet air condition as 1 bar pressure and 27 degree celsius temperature. Assuming the ratio of specific heat as 1.4 and the ratio of cylinder volume as 1/17. The pressure after the isentropic expansion is 4.5 bar and specific heat at constant pressure as 1.005 kilo joule per kg k. Determine air standard efficiency, Cutoff ratio, Percentage of stroke at which cutoff occur, Maximum temperature of the cycle, Heat addition in kilo joule per kg, net work done in kilo joule per kg ,mean effective pressure in bar. 

Dual combustion cycle 

Efficiency of dual cycle 

Explosion ratio 

Note : 

• Heat added at constant volume should be less then heat at constant pressure because already the pressure inside a cylinder is high. If heat addition at constant volume increases then it will increase the T3 and that may cause the cylinder explosion. 
  
  
  

Energy transformation in IC engine 

Part 1 

Heat added 

• The total amount of thermal energy or power available from the fuel due to the combustion of 1 kg of mass is termed as calorific value. 
  
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Indicated power 

• Due to the expansion of the air pressure and volume changes which is found from the PV diagram indicated diagram is termed as indicated power. 
• It is the input energy or power available at the piston of reciprocating motion. 
  

Brake power 

• The total amount of power on energy available at the end of engine shaft of rotary motion which is found by using brake mechanism. 
  

Brake power 

Note : energy conservation 

Thermal efficiency 

Engine performance parameters 

Relative efficiency

Note:

Question :the pressure at 25% of the stroke is 1.4 bar whereas the pressure at 75% of the stroke is 4.2 bar ,on the compression curve following PV^Gama = constant. Assuming the ratio of specific heat is equal to 1.4 then determine compression ratio,Air standard efficiency of the Otto cycle, If the relative efficiency is 50% and engine develops the brake power of 25 KW then determine the rate of fuel consumption in KG per hour if calorific value is 42 megajoule per kg .

Question :following data refers  for the four stroke for cylinder petrol engine having bore of 11 cm and stroke length of 30cm. The engine speed is 2250 RPM. The total brake power load on the engine is 30 kg at the drum radius of 0.6 m. The total frictional losses in an engine is 12.5 KW and the rate of fuel consumption in the engine is 10.5 kg per hour. The calorific value of the fuel is 45 mega joule per kg. The ratio of the specific heat is 1.4 and the ratio of clearance volume and swept volume is equal to 0.1. determine brake power,Mechanical efficiency,Brake thermal efficiency,Brake mean effective pressure ,air standard efficiency, Relative efficiency. 

Question :in a standard Otto cycle the compression ratio is 10. The condition at the beginning of isentropic compression process is 100 kilo Pascal and 27 degree celsius temperature. The heat at constant volume process is 1500 kilo joule per kg while 700 kilo joule per kg is the heat rejected during the other constant volume process in the cycle. The specific gas constant for the air is 0.287 kilo joule per kg k. Determine the mean effective pressure for the cycle in kilopascal.

Actual suction process

• After heat rejection the piston displaces from bdc to tdc to thrown out the remaining exhaust gases out of the cylinder.
• At very near to the end of exhaust stroke , exhaust stroke valve will get close down , hence for the remaining gases exhaust stroke it compresses the exhaust gas.
• At the beginning of suction stroke there will be no entry of the fresh air fuel mixture as the pressure inside the chamber is slightly above the Patm.
• Hence some pistion displacement is wasted during the suction stroke until the pressure inside the cylinder reaches to Patm , then the actual suction begins .

Volumetric efficiency

• It is the ratio of the actual volume of air that entered into the cylinder during suction stroke to the theoritical swept volume.
  

Question : a six cylinder four stroke gas engine operates with 10x12 (bore and stroke) cm is tested at 4800 rpm , the drum brake radius is of 55 cm and during a 10 minute test it measures the total brake load of 45 kg, the engine consuming total of 5 kg of  the fuel of calorific value 45  mj/kg , the air intake rate in the engine which is measured by the help of sharp edge orifice is 10 kg /minute at the atmospheric condition of 1 bar pressure and 30 degree celcius  , the specific gas constant for the air is 0.287 kj /kg , assuming the ratio of specific heat as 1.4 and the ratio of cylinder volume as 0.1.

the frictional losses in the engine is 25 kw , determine : brake power , mean effective pressure , air fuel ratio , brake thermal efficiency , relative efficiency , volumetric efficiencey , mechanical efficiency.