Combustion engines Chemical energy in fuel converted to thermal energy by combustion or oxidation Heat engine converts chemical energy into mechanical energy Thermal energy raises temperature and pressure of gases within engine, and gas expands against mechanical mechanisms of engine Combustion Internal: fuel is burned within the engine proper (including e.g. rocket engines, jet Engines, firearms) External: combustion is external to the engine (e.g. steam, stirling engine, gas turbine)
Internal Combustion Engine (IC Engine) The internal combustion engine (IC Engine) is a heat engine that converts heat energy (chemical energy of fuel) into mechanical energy (usually made available on a rotating output shaft). The Internal Combustion Engine (also known as IC Engine) is an engine in which the combustion of fuel and an oxidizer (typically air) occurs inside a confined space called a combustion chamber. This exothermic reaction creates gases at high temperature and pressure, which are permitted to expand inside that confined chamber. Thrust produced by this expanding gas drives the engine creating useful work. Application Mainly used as prime mover, e.g. for be the propulsion of a vehicle i.e. car, bus, truck, locomotive, marine vessel, or air plane. Other applications includes stationary saws, lawnmowers, bull- dozers, cranes, electric generators, etc.
Internal Combustion Engines - Construction The three main portions of an IC engine are Head block: Top Part Cylinder Block: Middle Part Chamber or Sump: Bottom Part
Engine Terminology
Piston Cylinder Assembly: It is the assembly for manipulating the working fluid. The assembly is characterized by a piston moving inside the confined cylinder. Inlet Valve: The valve through which air fuel mixture (in case of SI engine) or air (in case of CI engine) is introduced inside the cylinder. Exhaust Valve: The valve through which the products of combustion leave the cylinder. Crank Mechanism: Mechanism to convert reciprocating piston motion to rotary motion. Bore: Diameter of Cylinder. Top Dead Center (TDC): Position of Piston where Cylinder Volume is minimum. Bottom Dead Center (BDC): Position of Piston where Cylinder Volume is maximum. Stroke: It is the maximum distance that the piston moves in one direction. It is the distance between TDC to BDC. Clearance Volume (V c ): Minimum Cylinder volume when Piston is at TDC. Swept or Displacement Volume (Vs or Vd ): Volume swept out by the
Piston as it moves from TDC to BDC. Where d is the cylinder bore and l the stroke Compression Ratio (r v ): Ratio of maximum volume at BDC and minimum volume at TDC. Mean piston speed: the distance traveled by the piston per unit of time: Classification of IC engines Where l is the stroke in (m) and N the number of crankshaft revolution per minute (rpm). Method of Ignition (a) Spark ignition (SI): High-voltage electrical discharge between two electrodes ignites air-fuel mixture in combustion chamber surrounding spark plug (b) Compression ignition (CI): Air-fuel mixture self-ignites due to high temperature in combustion chamber caused by high compression, Diesel engine
The type of fuel Gasoline, Diesel or fuel oil, Gas (natural gas or methane), Liquefied petroleum gas (LPG): mainly propane, propylene, butane, and butylene, Alcohol (ethyl, methyl), Dual fuel (e.g. methane/diesel), Gasohol (e.g. 90% gasoline, 10% alcohol), Biodiesel: cleaner-burning diesel fuel made from natural, renewable sources such as vegetable oils. Number of strokes per cycle (a) Four-stroke: Four piston movements over two engine revolutions for each engine cycle. (b) Two-stroke: Two piston movements over one revolution for each engine cycle. Not in use for its sound pollution and air pollution. The cycle of operation (a) Otto cycle (also known as constant volume cycle) engines, (b) Diesel cycle (also known as constant pressure cycle) engines, (c) Dual combustion cycle (also known as semi-diesel cycle) engines
Valve location (a) Valves in head, (b) Valves in block (c) One valve in head and one in block (less common) Piston movement (a) Reciprocating and (b) Rotary. Number of cylinders (a) Single cylinder engines (e.g. lawnmowers), (b) Multi-cylinder engines. The cooling system (a) Air cooled engine, (b) Water cooled engine, (c) Evaporative cooling engines
Arrangement of cylinders (a) In-line or straight: cylinders in straight line, one behind the other in length of crankshaft. (b) V: two banks of cylinders at an angle with each other along a single crankshaft, angle typically 60-90 0 (c) Flat or opposed cylinder (V with 180 0 ): two banks of cylinders opposite each other on a single crankshaft (small aircrafts) (d) W: three banks of cylinders on same crankshaft (not common) (e) Opposed piston engine: two pistons in each cylinder, combustion chamber between pistons (f) Radial engine: cylinders positioned radially around crankshaft. V In-line radial Flat
The method of fuel injection (a) Carburetor engine, (b) Air injection engine, (c) Airless or solid injection 4-stroke SI (Petrol) engine operation In an internal combustion engine, the piston executes four distinct strokes within the cylinder for every two revolutions of the crankshaft. The four strokes are termed as (i) Intake Stroke (ii) Compression Stroke (iii) Power Stroke (iv) Exhaust Stroke Four stroke engines: the cycle of operation is completed in four strokes of the piston or two revolution of the crank shaft. Each stroke consists of 180 0 of crank shaft rotation.
(i) First stroke: intake or induction Intake valve opens, and exhaust valve closes Piston travels from TDC (top dead center) to BDC (bottom dead center) Volume increases in combustion chamber and creates vacuum Fresh charge is drawn into the cylinder due to suction. For SI engine the charge is a mixture of fuel and air. For CI engines charge is only air (air passes through intake system, fuel is added). (ii) Second stroke: compression Piston reaches BDC, both intake and exhaust valves close. Now piston rises from BDC back to TDC with all valves closed, compresses the charge (air-fuel mixture) raising the temperature and pressure. This stroke requires work input from Piston to the charge. Near end of compression stroke, spark plug fired and combustion is initiated. In SI engines it is induced by spark plug. In CI engines combustion is initiated by injecting fuel into the hot-compressed air using fuel injectors.
(iii) Third Stroke: Expansion or Power Stroke Piston near TDC: nearly constant-volume combustion occurs. Changes composition of gas mixture to exhaust products occurs due to exothermic blast and temperature and pressure increases. High pressure pushes piston away from TDC. Work is done on the Piston as thermal energy is converted to mechanical energy. Piston moves from TDC to BDC, volume increases and pressure and temperature drop (iv) Fourth Stroke: Exhaust blowdown Late in power cycle exhaust valve is opened. Piston moves from BDC to TDC due to momentum gained Pressure differential pushes hot exhaust gas out of cylinder and. through exhaust system when piston is at BDC. Exhaust gas carries away high amount of enthalpy, which lowers cycle thermal efficiency
Near end of exhaust stroke before TDC, intake valve starts to open and is fully open by TDC when intake stroke starts next cycle Near TDC the exhaust valve starts to close and is fully closed sometime after TDC. Period where both intake valve and exhaust valve are open is called valve overlap Indicator and valve-timing diagram for a 4-stroke engine
Figure: Actual Indicator diagram for a four stroke cycle petrol engine. Figure: Valve timing for a four stroke cycle petrol engine.
Figure: Actual Indicator diagram for a four stroke cycle diesel engine.
Comparision of Petrol and Diesel Engine
2-stroke SI (Petrol) engine operation
Figure: Actual Indicator diagram for a two stroke Cycle petrol engine. Figure: Valve timing for a two Stroke cycle petrol engine.
2-stroke CI (Diesel) engine operation
Figure: Actual Indicator diagram for a two stroke Cycle diesel engine. Figure: Valve timing for a two Stroke cycle diesel engine.
Scavenging of IC Engines The process of removing burnt gases, from the combustion chamber of the engine cylinder is known as scavenging. The last stroke of an IC engine is the exhaust, which means the removal of burnt gases from the engine cylinder. It has been experienced that thc burnt gases in the engine cylinder are not completely exhausted before The suction stroke. But a part of the gases still remain inside the cylinder and mix with the fresh charge. As a result of this mixing, the fresh charge gets diluted and its strength is reduced. In a four stroke engine, the scavenging is very effective, as the piston during the exhaust stroke, pushes out the burnt from the engine cylinder. It may be noted that, a small quantity of burnt gases remain in the engine cylinder in the clearance space. In a two stroke engine,, the scavenging is less effective, as the exhaust port is open for a small fraction of the crank revolution. Moreover as the transfer and exhaust ports are open simultaneously during a part of the crank revolution. Therefore fresh charges also escapes out along with burnt gases. This difficulty is overcome by designing the piston crown of a particular shape.
Cross flow scavenging Backflow or loop scavenging Uniflow scavenging
Detonation/Knocking in I.C. Engine It is sharp metallic sound produced by the the impact of pressure wave on the walls of combustion chamber. The waves by their impact against the walls of the chamber set them in vibration which leads to high frequecy noises. It is caused due to the propagation of a high speed pressure waves colliding with one another due to auto-ignition of fuel-air mixture (inside the cylinder). Chief effects of detonation 1. A loud pulsating noise may be accompanied by a vibration of the engine. 2. An increase in the heat lost to the surface of combustion chamber. 3. An increase in carbon deposits. Prevention of Knocking Fuels formulated with tetraethyl lead (TEL) are resistant to autoignition. Detonation of petrol engine is reduced by this process. This is called doping. But unleaded gasoline is used these days due to environmental concerns over air pollution that limits the compression ratios of sparkignition engines to approximately 9.
Progress of Flame Front End gas theory (SIE): Charges near the flame front is heated, attain self-ignition temperature, burns and forms a new flame front. Some unburnt charge (end gas) is raised to self-ignition temp. ahead of the flame front and burns at constant volume with abrupt rise of pressure. Chemical theory (CIE): The entire charge is heated up to ignition temperature, burns instantaneously and releases with rapid rise in pressure or temperature.
AIR - STANDARD CYCLES The accurate study and analysis of I.C.E. processes is very complicated. To simplify the theoretical study "Standard Air Cycles" are introduced Assumptions 1. Cylinder contains working fluid, a constant amount of air treated as ideal gas. No change in composition. 2. The specific heats and other physical and chemical properties remain unchanged at their ambient temperature values during the cycle. (Cold Air-Standard). 3. Instead of heat generation by combustion, heat is transformed from external heat source. 4. The process of heat removal in the exhaust gases is represented by heat transfer from the cycle to external heat sink. 5. All the processes are internally reversible i.e. there is no friction or heat loss. 6. Cycles can be presented on any diagram of properties. 7. There is no Intake and Exhaust Strokes like an actual engine.
Although an air-standard analysis simplifies the study of internal combustion engines considerably, the values of different parameters (pressure, temperature) calculated on this basis may depart significantly from those of actual engines. Air-Standard analysis allows internal combustion engines to be examined only qualitatively. Thermodynamic cycles that adhere to air-standard assumptions are Otto Cycle of Petrol Cycle (for SI Engines) Diesel Cycle (for CI Engines)
Air Standard Otto (constant volume ) or Petrol Cycle The air-standard Otto cycle assumes the heat addition to be occurring instantaneously while the piston is at TDC. Therefore in petrol cycle, heat addition will take place at constant volume. This is due to the fact that the pressure inside the cylinder will rise rapidly during the combustion period as soon as ignition of air-fuel mixture is started by a spark plug. Q in Q out These cycles is applied in petrol (or gasoline) engine, gas engine, and high speed diesel (oil) engine.
Process 1-2: Isentropic Compression. Piston: BDC TDC Process 2-3: Constant Volume Heat Addition. Piston: at TDC Process 3-4: Isentropic Expansion (Power Stroke). Piston: TDC BDC Process 4-1: Constant Volume Heat Rejection. Piston: at BDC Process 2 3: Reversible heat addition at constant volume Process 3-4: Isentropic expression Same expression as Process 1-2 but here the points will be 3 and 4.
Process 4 5: reversible constant volume cooling This cycle is applied in 4- stroke and 2- stroke engines. The thermal efficiency of the Otto cycle: η increased by increasing r, η increased by increasing γ, η independent on the heat added or load. In modern petrol engines (r) reaches a value of 12.
Air Standard Diesel (Constant Pressure) Cycle Air standard Diesel Cycle assumes the heat addition occurs during a constant pressure process that starts with the piston at TDC.
Process 1-2: Isentropic Compression. Piston: BDC TDC Process 2-3: Constant Pressure Heat Addition (1st part of Power Stroke). Piston: TDC BDC Process 3-4: Isentropic Expansion (Remainder of Power Stroke). Piston: TDC BDC Process 4-1: Constant Volume Heat Rejection. Piston: at BDC This cycle is the theoretical cycle for compression-ignition or diesel engine. For this cycle:
Problem-1 Control mass: Air inside cylinder State information: P 1 = 0.1 Mpa, T 1 = 288.2 K Process information: r c = 10 and q H = 1800 kj/kg Model: Ideal gas, constant specific heat, value at 300 K For compression process 1 2 the second law is
State-1 P 1 = 0.1 Mpa, T 1 = 15+273=288 K Compression ratio, r v = V 1 /V 2 = V 4 /V 3 = 10 And Q 23 /m = 1800 kj/kg We know from ideal gas law: P 1 v 1 = RT 1 [R = 0.287 kj/kg-k for air.] or, (0.1*1000)v 1 = 0.287 *288 or, v 1 = (0.287*288)/100 So, v 1 = 0.827 m 3 /kg For isentropic compresion 1-2: = Or, T 2 = 288 * (10) 1.4-1 so, T 2 = 723.4 K For constant volume heat addition 2-3:
For isentropic expansion 3-4: The thermal efficiency, η th The effective pressure, mep W cycle /m = q 23 /m q 41 /m = 1800 C v (T 4 T 1 ) = 1800 0.717*(1287.5 288) = 1083.36 kj/kg
Problem-2 Do it yourself
Process Constant Known ratio P 2 V 2 T 2 Isobaric process Isochoric process (Isovolumetric process) (Isometric process) Pressure Volume V 2 /V 1 P 2 = P 1 V 2 = V 1 (T 2 /T 1 ) T 2 = T 1 (V 2 /V 1 ) T 2 /T 1 P 2 = P 1 V 2 = V 1 (T 2 /T 1 ) T 2 = T 1 (T 2 /T 1 ) P 2 /P 1 P 2 = P 1 (P 2 /P 1 ) V 2 = V 1 T 2 = T 1 (P 2 /P 1 ) T 2 /T 1 P 2 = P 1 (T 2 /T 1 ) V 2 = V 1 T 2 = T 1 (T 2 /T 1 ) Isothermal process Isentropic process (Reversible adiabatic process) Temperature Entropy [a] P 2 /P 1 P 2 = P 1 (P 2 /P 1 ) V 2 = V 1 /(P 2 /P 1 ) T 2 = T 1 V 2 /V 1 P 2 = P 1 /(V 2 /V 1 ) V 2 = V 1 (V 2 /V 1 ) T 2 = T 1 P 2 /P 1 P 2 = P 1 (P 2 /P 1 ) V 2 = V 1 (P 2 /P 1 ) ( 1/γ) T 2 = T 1 (P 2 /P 1 )(1 1/γ) V 2 /V 1 P 2 = P 1 (V 2 /V 1 ) γ V 2 = V 1 (V 2 /V 1 ) T 2 = T 1 (V 2 /V 1 )(1 γ) T 2 /T 1 P 2 = P 1 (T 2 /T 1 ) γ/(γ 1) V 2 = V 1 (T 2 /T 1 ) 1/(1 γ) T 2 = T 1 (T 2 /T 1 ) Polytropic process P V n P 2 /P 1 P 2 = P 1 (P 2 /P 1 ) V 2 = V 1 (P 2 /P 1 ) (-1/n) T 2 = T 1 (P 2 /P 1 )(1-1/n) V 2 /V 1 P 2 = P 1 (V 2 /V 1 ) n V 2 = V 1 (V 2 /V 1 ) T 2 = T 1 (V 2 /V 1 ) (1 n) T 2 /T 1 P 2 = P 1 (T 2 /T 1 ) n/(n 1) V 2 = V 1 (T 2 /T 1 ) 1/(1 n) T 2 = T 1 (T 2 /T 1 )