IC engine

Lower Heat Value of Fuel:

The lower heating value (also known as net calorific value) of a fuel is defined as the amount of heat released by combusting a specified quantity (initially at 25°C) and returning the temperature of the combustion products to 150°C, which assumes the latent heat of vaporization of water in the reaction products is not recovered.

High Heat Value of Fuel:

The higher heating value (also known gross calorific value or gross energy) of a fuel is defined as the amount of heat released by a specified quantity (initially at 25°C) once it is combusted and the products have returned to a temperature of 25°C, which takes into account the latent heat of vaporization of water in the combustion products.

Unit of Heat Value:

Btu = British thermal units; scf = standard cubic feet

Cut-off Ratio:

·         The cutoff ratio is the ratio of the volume after combustion to the volume before combustion.

·         The Air standard efficiency of a diesel cycle is given as:

η=1−1/(R^(γ−1))*[γc^(γ−1)/γ(γc−1)]

·         where,

RoRo is the compression ratio

rcrc is the cut off ratio

·         From the above equation, it is observed that, the thermal efficiency of the diesel engine can be increased by increasing the compression ratio, RoRo, by decreasing the cut-off ratio, rcrc, or by using a gas with large value of γ.

·         The efficiency of a Diesel cycle is always lower than that of an Otto cycle having the same compression ratio.

·         However, practical Diesel engines uses higher compression ratios compared to petrol engines and are often quite efficient.

Supercharging:

In I.C. engines supercharging is the process of improving the volumetric efficiency of the engine by using power of engine. In other words it is process of improving the breathing ability of the engine.

Supercharing provides more air in the cylinder (at higher pressure) so that more oxygen in the cylinder results in better combustion and higher efficiency. A compressor using the engine power compresses the atmospheric air that is intaken by the cylinder.

A suction process that uses the compressed air (large quantitiy of air so that large quantity of oxygen) is called as supercharging.

Types of Supercharing:
1) Centrifugal Supercharging
2) Root’s type Supercharging
3) Vane type Supercharing

Methods of Supercharging:

1)  Independently driven compressor or blower

2)  Ram effect

3)  Underposton supercharging

4)  Kadenacy System

5)  Engine driven compressor or blower

Advantages of Superchargine:

1)  High power output

2)  Better atomization of fuel

3)  Better mixing of air and fuel

4)  Better scavanging of products

5)  Quicker acceleration of vehicle

6)  More complete and smoother combustion

7)  Reduce exhaust smoke

8)  Reduced specific fuel comsumption

Limitations of Superchargine:

1)  Increased thermal stresses

2)  Increased heat losses dut to turbulance

3)  Increased gas loading

4)  Increased cooling requirements of piston and valves.

Centrifugal Superchargine:

The centrifugal supercharging draws its power from the movement of the drive where it is attached. The supercharger powers an impeller or small rotating wheel. The impreller draws the air into a small compressor housing (volute) and centrigual force sends the air into the duffuser. The result is air that is highly pressurized but that travels at low speed. The high pressure and low speed air is then fed into the engine where the additional pressure the engine the abilityu ot burn more fuel and have a higher level of cobustion. This results in a faster, more responsive vehicle due to greater engine volumetric efficiency.

Turbocharging:

 The objective of a turbocharger is to improve an engine’s volumetric efficiency by incresing density of the intake air allowing more power per engine cycle. The turbocharger’s compressor draws in ambient air and compresses it before it enters into the intake manifold at increased pressure. The result in a greater mass of air entering the cylinders on each intake stroke. The power needed to spin the compressor is driven from the kinetic energy of the engine exhaust gases.

In automotive applications boost refers to the amount by which intake manifold pressure exceeds atmospheric pressure. This is representative of the extra air presure that is achieved over that would be achieved without the forced induction. The level of boost may be shown on pressure gauge. The control of turbocharger boost has changed dramatically over the 100-plus years of their use. Modern turbochargers can be use westages, blow-off valves and variable geometry.

In petrol engine turbocharger applicatons boost pressure is limited to keep the entire engine system, including the turbocharger , inside its thermal and mechanical design operating range. To avoid engine knocking (detonation) and the related physical demage to engine the intake manifold pressure must not get too high.

Types of turbocharger

1)  Single turbo

2)  Twin turbo

3)  Twin scroll turbo

4)  Variable geometry turbo

5)  Varibale twin scroll turbo

6)  Electric Turbo

Flash point of fuel:

The flash point of a volatile material is the lowest temperature at which vapours of the material will ignite, when given an ignition source. The fire point is always higher than flash point. Sometimes there is confusition between flash point and autoignition point but both are different because the auto ignition point is that point where temperature of fuel is such that that do not need any ignition source of start ignition, but flash point temperature needs ignition source.

Mean Effective Pressure (mep):

Mean effective pressure (mep) is a quantity relating to the operation of a reciprocating engine and is a valuable measure of an engine’s capacity to do work that is independent of engine displacement.

IMEP: When quoted as an indicated mean effective pressure (IMEP) it may be thought of as the average pressure acting on a piston during the different portion of its cycle.

BMEP: Brake mean effective pressure calculated from measure brake torque.

IMEPg: Gross indicated mean effective pressure calculed from calclated from in-cylinder pressure over compression and expansion portion of the engine cycle.

IMEPn: Net indicated mean effective pressure calculated from in-cylinder pressure over the complete cycle.

PMEP: Pumping mean effective pressue from work moving air in and out of the cylinder, across the intake and exhaust valves. Calculated from in-cylinder pressure over intake and exhaust portion of the engine cycle.

FMEP: Friction mean effective pressure required to overcome engine friction can be thought of as mean effective pressure lost due to friction. Friction mean effective pressure calculation requires accureate measurement of cylinder pressure and dynamomerer brake torque.

FMEP = IMEPn – BMEP

Back Pressure:

Back pressure caused by the exhaust system of an automobile engine has negative effect on engine efficiency resulting in a decrease of power output that must be compensated by descrasing fuel consumption.

Specific fuel consumption:

Power developed per unit fuel is called specific fuel consumption.

Brake specific fuel consumption: It is a measure of the fuel efficiency of any prime mover that burns fuel and produces reotational or shaft power. It is tipically used for comparing the efficiency of internal combustion engine with a shaft output.

BSFC = (fuel consumption rate in grams per second)/(power produced in watt)

Thrust specific fuel consumption: It is the fuel efficiency of an engine design with respect to thrust output. TSFC mau also be thought of as fuel consumption per unit thrust.

Specifif Gravity:

Specific gravity is the ratio of density of a substance to the density of referance substance (generally water).

Specific gravity of Petrol = 0.739 at 15.55 degree celcius (60 degree F)
Specific gravity of Diesel = 0.82 to 0.95 at 15.55 degree celcius (60 degree F)

Scavenging:

Scavenging is the process of pushing exhaust gas charge oiut of the cylinder and drawing in a fresh draught of air or air/fuel mixture for next cycle.

There are 3 types of scavenging

1)  Direct or cross flow type scavanging

2)  Reverse type scavenging

3)  Uniflow type scavenging

Cross Flow/ Loop Scavenging:

Cross flow scavengine rake place with the help of piston movement. Cross flow scavengine, transfer prot (inlet) and exhaust port are situated on the opposite side of the cylinder. The exhaust gas is pushed out by cross flow. The piston head is designed to have a hump shape called deflector. The fresh air enters in the engine cylinder is deflected to the upward by a deflector and pushing exhaust gas down the other side. Before loop scavanging invented, almost all two-stroke engines use this method. In practice the gas flow failed to follow the idealised pattern. The rib of the deflector piston also a poor shape for the combustion chamber with long flame paths and excessive surface are. This method of scavenging has now beed almost entirely replaced by loop scavanging.

Although obsolute for piston ported two stroke engines cross-flow scavanging is now commmon in four stroke engines where their inlet and exhaust valves are mounted on opposite sides of cylinder head. As the inlet and exhaust strokes of four stroke cycle happen on different strokes of piston the upward exhaust stroke pushing gases out is followed by the downward inlet stroke allowing the fresh charge in the flow is as tow isolated isnle flow rather than as an unstable loop.

Advantage-

-    Low Manufacturing cost

-    Good scavenging at low speed and part throttle.

-    Low engine volume for the multicylinder arrangemet.

Disadvantages-

-    Heavy piston with very high heat absorption

-    High tendency to knock

-    Poor scavanging at high speed and full throttle

-    Compulsory water cooling, diffuculty in cooling piston crown.

Reverse tye/ Backflow/Schnuerle porting Scavenging:

Rather than the flow loop being vertical, the gases asre encouraged to move in two horizontal loops. In this method, the inlet and outlet ports are suited on the same side of the cylinde. The fresh charge while entering into cylinder forms a loop and pushed out the burnt gases.

Advantage-

-    Low maintenance

-    The low surface area to the volume of the cylinder hence the heat lodd reduced

-    Good scavenging at full throttle

-    Water cooling system not necessary.

Disadvantages-

-    Poor scavenging at part throttle operation

-    Scavengine time is short.

Uniflow Scavenging:

In this method the scavenging the fresh charge while entering from one side (or sometimes two sides) of the cylinder pushes out the gases through the exit valce situated on the top of the cylinder. In this both the fresh charge and burnt gases move in same direction. Usually in deisel type eingine the directrion of flow is upward but with spark ignition einginies such as the Ricardo dolphin the direction of flow is generally downward with fresh charge entering at top of engine. It is widely used for two stroke engines and locomotive engines.

Advantages-

-    Extended time for valve operation

-    The possibility of mixing is reduced due to uniflow

-    Increse power output

-    Most efficient of all three methods of scavenging

-    Good scavenging at all speed ranges and throttle position

-    Low fuel comsumption compared to other scavenging types.

Disadvantages-

-    Elaborate and costly construction

-    Difficulty in cooling the piston

Air Filters/Cleaners:

A particulate air filter is a device composed of fibrous or porous materials which removes solid particulates such as dust, pollen, mold and bacteria from the air. Filters containing an absorbent or catalyst such as charcoal my alos remove odors and gaseous pollutant such as volatile organic compounds. Air filters are used in application where air quality is important, notably in building ventilation system and in engines.

Photochemical smog:

It is a type of smog or air pollutant derived from vehicle emission from internal combustion engines and industrial fumes that react in the atmosphere with sunlight to form secondary pollutants that also combine with the primary emission to form photochemical smog.

§  For maximum power genration the air fuel raito for a pertol engine for vehicle is of order of 12:1.

§  Air fuel ratio for idling speed of a petrol engine is approximately 10:1.

§  The theoritically correct air-fuel ratio for petrol engine is of order of 15:1.

§  Volatility of diesel fuel oil is indicated by 90% distillation temperature i.e. when 90% of sample oil has distilled off.

§  Stiochiometric air-fuel ratio is chemically correct mixture.

§  Ignition quality of petrol is expressed by octane number.

§  Petrol is distilled at temperature in range of 65-220 degree celcius.

§  Kerosene is distilled at 220-350 degree celcius.

§  Self ignition temperature of petrol is more than 500 degree celcius.

§  Iso-octance has octance number of 100.

§  Octance number is determined by comparing the performance of the pertol wth hydrocarbons of mixture of normal heptance and iso-octance.

§  Centane is a straight chain peraffin.

§  Ethul fluid is used to increase in octance rating of the fuel.

§  Self ignition temperature of diesel oil compared to pertol is higher.

§  Normal heptane accelerates auto ignition.

§  Cetance humber is determined by comparing the performance of diesel woil with the nydrocarbon of mixture of cetance and alphamethyl nepthalene.

§  Violent sound pulsation within the cylinder of an I.C. engines are caused due to detonation.

§  Auto ignition temperatue is that at which it catches fire without external aid.

§  Ignition lag is the time taken by fuel after injection to reach upto auto ignition temperature.

§  The spark plug gap is normally maintained at 0.45 to 0.6 mm.

§  The delay period in petrol engine is of order of 0.002 second.

§  Detonation is caused by peroxides, aldehydes and ketones.

§  If overhead clearance is less then horzontal engine should be selected.

§  Piston rings are plated with chromium, cadmium or phosphate in order to reduce wear and eliminate scuffing.

§  In order to prevent knock in S.I. engines the charge away from the spark plug should have low temperature low density, long ignition delay and rich mixture.

§  To reduce the possibility of knock in the C.I. engines the first element of fuel and air should have high temperatue, high density, short delay and reactive mixture.

§  Accouding to Recordo’s theroy detonation occures due to instataneous aouto ignition of last part of charge to be burnt.

§  The ignition of charge by some hot surgace in the engine cylinder before operationof spark flug is known as pre ignition.

§  Ignition lag is time before actual fuel injection and the pump pluger starts to pump fuel.

§  For best results of efficent comustion, high speed diesel engines need an approximate centane no of 50.

§  Calorific value of diesel oil is 15000 kcal/Kg

§  Carbon resideual in diesel oil should not be more than 0.1%.

§  The sepcific gravity of diesel oil is 0.85.

§  Freezing temperature of petrol is less than -30 degree celcius.

§  The specific gravity of petrol is 0.75.

§  Detonation can be controlled by retarding the spark timing.

§  The efficiency of I.C. engines normally is of the order of 30-35%.

§  Sulphur content in diesel oil should not be more than 1%.

§  The m.e.p. of a diesel cycle having fixed compression ratio with increase in cut off ratio will increase.

§  Ignition timeing of a multi cylinder petrol engine can be adjusted by rotating the distrubutor.

§  Fuel consumption with increase in back pressure will incerease.

§  Leakage past te piston rings and valuve seats in I.C. engines with increase in speed decrease.

§  The function of a distrubutor in an automobile is to time the spark.

§  The system of lubrication used for motor cycle and scooters is by mixing 5% lubricating oil with petrol.

§  The thermal efficiency of a two stroke engine as compared to four stroke engine is less.

§  Diesel engine as compared to petrol engine require bigger flywheel.

§  The minimum value of auto ignition occurs in the region of chemicaly correct fuel-air ratio.

§  The tendency of a diesel engine to knock increase if compression ratio is increased.

§  The tendency of a petrol engine to knock increase by supercharging.

§  The brake mean effective pressure of an I.C. engine with increase in speed wil remain unaffected.

§  In petrol engine, Nitrogen as gets exhausted out without burning and without transformation.

§  The level of fuel in the float chamber of a carburator as compared to the level of the jet in the venturi is lower.

§  In carburator the top of the fuel jet with referance to the level in the float chamber is kept at slightly higher level.

§  Power impulse from I.C. engine are smoothed out by flywheel.

§  Deposition of carbon in petrol engine cylinder would result in increase in compression ratio.

§   If petrol is used in a diesel engine then higher knocking will occur.

§  In turbulence chamber in diesel engine fuel is injected into an auxiliary chamber that is separated from the cylinder by an orifice or throat.

§  For low load operation C.I. engine is more suitable.

§  For the same size and weight a two stroke engine as compared to four stroke engine will generate power about 1.7 times.

§  The thermal efficiency of a semi-diesel cycle having fixed compression ratio and fixed quantity of heat with increase in pressure ratio will increase.

§  In a diesel engine injection pressure developed by injector is of the order of 1400 Kg/M^3.

§  The bi-fuel engine uses liquid fuel during start up and gas as the basic fuel.

§  Morse test us used to determine mechanical efficiency of multicylinder engines.

§  During idling stage gasoline does not flow through the carburator tube because of ventruri vaccum.

§  Thermal efficiency of I.C. engine on weak mixture is higher.

§  In petrol engine the actual pressure developed compared to the preducted maximu pressure is 50%.

§  Octance number of petrol normally used in petrol engine is of order of 80-90.

§  Octance number of petrol available from Indian refineries of the order of 13.

§  In spark ignition engines the knocking tendency can be decreased by adding dopes like tetraethyl lead and ethylene dibromide.

§  Performance number are indicative of th fuels having antiknock quantities superior to iso-octance.

§  Four stroke petrol engines as compared to two stroke petrol engine having same output rating and same compression ratio have higher thermal efficiency.

§  The cetance number of diesel oil generally available is of the order o 55-70.

§  High speed diesel engine need a cetane number o 50.

§  Speed droop is the decrease in engine speed from no load to full load.

§  Cetance number is the measure o ignition quality.

§  Indicated power = Brake power + Friction Power

§  The antifreeze solution commonly used in automibiles is glycol.

§  The power to weight ratio in a two stroke engine as compared to four stroke engine is more.

§  Freezing temperature of petrol is -50 to -30 degree celcius.

§  Injection lag in diesel engine is caused by expansion of fuel oil discharge lines under high pressure, compressibility of fuel and leakage past the fuel-oil plenger.

§  The piston of diesel engine are usually cooled by lubricating oil.

§  The backpressure of petrol engine is usually of the order of 1.2ata.

§  Muffler is used to reduce exhaust noise.

§  The specific fuel consumption is espressed as the fuel consumed per hour per unit brake horse power.

§  Higher calorific value of a fuel is based on the assumption that water is present in vapoir from.

§  In case of compound engine, equal power is developed by each lylinder with a view to obtain uniform turning moment.

§  The compression ratio is kept low in pertol engine compared tro a diesel because higher compression ratio in petrol engine would lead to preignition of fuel.

§  A temperature indicator is usualy procided for automobiles. It indicates temperature of jacket cooling water.

§  The lead-acid type of battery is commly used in automobile application.

§  The gear ration in a differential unit of a passenger car is of order of 3:1.

§  The acid used in automobile battery is H2SO4.

§  Ignition accelerators are substances which increases the rate of preflame reaction and reduce the ignition lag.

§  n-heptane accelerates auto-ignition and iso-octane helps to resist auto ignition.

§  Petrol engines are not suitable for part load operaiton because mechanical efficiency is poor due to increasing internal losses at incresed throttling.

§  The power to weight ratio of diesel engine compred to petrol engine is low.

§  Flash point for diesel fuel should be minimum 49 degree celcius.

§  Vapour lock is complete or partial stoppage of fuel supply due to vaporization of fuel in supply system.

§  By higher octance number of S.I. fuel, it mean that the fuel has lower volatility.

§  Keeping other parameter constant brake power of diesel engine can be increased by increasing the pressur eo intake air.