engine

Engine

Any machine that converts one form of energy into useful mechanical energy is

termed as engine.

Efficiency

• Volumetric Efficiency• Combustion Efficiency• Thermal Efficiency

• Volumetric Efficiency: The ‘volumetric efficiency’ represents the degree of completeness with which the cylinder is re-charged with fresh combustible mixture and varies with different engines and also with the speed.

• Combustion Efficiency: The ‘combustion efficiency’ represents the degree of completeness with which the potential heat units in the charge are produced as actual heat in the cylinder.

• Thermal Efficiency: The ‘thermal efficiency’ governs the percentage of the actual heat units present in the cylinder which are converted into mechanical work.

Horsepower

• Indicated horse power• Brake horse power

• Indicated Horse PowerLet, p = mean effective pressure, N/m2. D = diameter of cylinders, m. L = length of stroke, m. N = revolutions per minute. f = no. of effective strokes, per revolution per

cylinder. n = number of cylinders.

• Force acting on one piston = p/4 Newtons

• Work done per effective stroke = p L /4 newton-meters = joules

• Work done per revolution = p Lf /4 joules

• Work done per minute = p L f N /4 joules

• For the whole engine= p L f n N /4 joules

• Indicated horse power is the power that is calculated theoretically or in other words that is actually generated inside the engine.

• Brake horse power is the power that is available at the engine shaft which is less than the Indicated brake horse power.

• Mechanical efficiency,

Definitions

• Top dead Centre : Position of the piston when it stops at the furthest point away from the crankshaft

• Bottom Dead Centre : Position of the piston when it stops at the point closest to the crankshaft

• Bore : Diameter of the cylinder or diameter of the piston face, which is the same minus a very small clearance

• Stroke : Movement distance of the piston from one extreme position to the other: TDC to BDC or BDC to TDC

Definitions

• Indirect Injection : Fuel injection into the secondary chamber of an engine with a divided combustion chamber.

• Direct Injection : Fuel injection into the main combustion chamber of an engine. Engines have either one main combustion chamber (open chamber) or a divided combustion chamber made up of a main chamber and a smaller connected secondary chamber.

Definitions

• Displacement volume : Volume displaced by the piston as it travels through one stroke. Also known as swept volume.

• Clearance volume : Minimum volume in the combustion chamber with piston at Top Dead Centre.

Definitions

• Ignition Delay : Time interval between ignition initiation and the actual start of combustion process.

• Wide-Open Throttle : Engine operated with throttle valve fully open when maximum power and/or speed is desired.

Definitions

• Compression Ratio: It is the ratio of the volume of the combustion chamber or the cylinder at the bottom dead center to the volume of the combustion chamber at the top dead center.

i.e. Cr=VBDC/VTDC

Classification of Engines

• Internal combustion • External combustion

Internal combustion Engine

• It is a heat engine that converts chemical energy in a fuel into mechanical energy, usually made available on a rotating output shaft.

• Chemical energy Heat energy Mechanical Energy

Classification of Internal combustion Engines

• Type of stroke• Type of ignition• Type of design• Relative position of cylinders• Valve Location• Type of Fuel used• Air intake process• Method of Fuel Input

Type of stroke

• Four-Stroke Cycle. A four-stroke cycle experiences four piston movements over two engine revolutions for each cycle.

• Two-Stroke Cycle. A two-stroke cycle has two piston movements over one revolution for each cycle.

• Six-Stroke Cycle.

Type of ignition

• Spark Ignition (SI). An SI engine starts the combustion process in each cycle by use of a spark plug. The spark plug gives a high-voltage electrical discharge between two electrodes which ignites the air-fuel mixture in the combustion chamber surrounding the plug.

• Compression Ignition (CI). The combustion process in a CI engine starts when the air-fuel mixture self-ignites due to high temperature in the combustion chamber caused by high compression.

Type of design

• Reciprocating. Engine has one or more cylinders in which pistons reciprocate back and forth. Power is delivered to a rotating output crankshaft by mechanical linkage with the pistons.

• Rotary. Engine is made of a block (stator) built around a large non-concentric rotor and crankshaft. The combustion chambers are built into the non-rotating block.

Relative position of Cylinders

Single CylinderIn-LineV EngineW engineOpposed Cylinder EngineOpposed piston EngineRadial engine

Single Cylinder. Engine has one cylinder and piston connected to the crankshaft.

• In-Line. Cylinders are positioned in a straight line, one behind the other along the length of the crankshaft. They can consist of 2 to 11 cylinders or possibly more. In-line four-cylinder engines are very common for automobile and other applications.

• V Engine. Two banks of cylinders at an angle with each other along a single crankshaft. The angle between the banks of cylinders can be anywhere from 15° to 120°, with 60°-90° being common. V engines have even numbers of cylinders.

Opposed Cylinder Engine. Two banks of cylinders opposite each other on a single crankshaft (a V engine with a 180°V). These are common on small aircraft and some automobiles. These engines are often called flat engines.

Sample Problem

• What is the difference between opposed cylinder and V-cylinder configuration?

• W Engine. Same as a V engine except with three banks of cylinders on the same crankshaft. Not common, but some have been developed for racing automobiles, both modern and historic. Usually 12 cylinders with about a 60° angle between each bank.

Opposed Piston Engine.

• Two pistons in each cylinder with the combustion chamber in the center between the pistons.

• A single-combustion process causes two power strokes at the same time, with each piston being pushed away from the center and delivering power to a separate crankshaft at each end of the cylinder.

• Engine output is either on two rotating crankshafts or on one crankshaft incorporating complex mechanical linkage.

Valve Location

• Valves in head• Valves in block• One valve in head and one in block

Valve in block, L head.Older automobiles and some small engines.

Valve in head, I head. Standard on modern automobiles.

One valve in head and one valve in block, F head. Older, less common automobiles.

Valves in block on opposite sides of cylinder, T head.

Type of Fuel used

• Gasoline.• Diesel Oil or Fuel Oil.• Gas, Natural Gas, Methane.• LPG.• Alcohol-Ethyl, Methyl.

Air Intake Process

• Naturally Aspirated. No intake air pressure boost system.

• Supercharged. Intake air pressure increased with the compressor driven off of the engine crankshaft.

• Turbocharged. Intake air pressure increased with the turbine-compressor driven by the engine exhaust gases

Supercharger

Turbocharger

Method of Fuel Input

• Carbureted.• Multipoint Port Fuel Injection. One or more

injectors at each cylinder intake.• Throttle Body Fuel Injection. Injectors

upstream in intake manifold.

Engine Operation Cycle

• Two stroke cycle

• Four stroke cycle

Two Stroke Cycle

• Compression Stroke• Expansion Stroke

First Stroke

With all valves (or ports) closed, the piston travels towards TDC and compresses the air-fuel mixture to a higher pressure and temperature. Near the end of the compression stroke, the spark plug is fired; by the time the piston gets to IDC, combustion occurs and the next engine cycle begins.

Combustion

occurs very quickly, raising the temperature and pressure to peak values, almost at constant volume.

Second Stroke

Very high pressure created by the combustion process forces the piston down in the power stroke. The expanding volume of the combustion chamber causes pressure and temperature to decrease as the piston travels towards BDC.

Exhaust Blow down

At some point bBDC, the exhaust valve opens and blow down occurs. The exhaust valve may be a poppet valve in the cylinder head, or it may be a slot in the side of the cylinder which is uncovered as the piston approaches BDC. After blow down the cylinder remains filled with exhaust gas at lower pressure.

Intake and Scavenging

At some point bBDC, the intake slot on the side of the cylinder is uncovered and intake air-fuel enters under pressure.This incoming mixture pushes much of the remaining exhaust gases out the open exhaust valve and fills the cylinder with a combustible air-fuel mixture, a process called scavenging. The piston passes BDC and very quickly covers the intake port and then the exhaust port. The higher pressure at which the air enters the cylinder is established in one of two ways: supercharger and crankcase.

Four stroke cycle

• Intake stroke or Induction• Compression• Expansion stroke or Power Stroke• Exhaust Stroke

First StrokeThe piston travels from TDC toBDC with the intake valve open and exhaust valve closed. This creates an increasing volume in the combustion chamber, which in turn creates a vacuum. The resulting pressure differential through the intake system from atmospheric pressure on the outside to the vacuum on the inside causes air to be pushed into the cylinder. As the air passes through the intake system, fuel is added to it in the desired amount by means of fuel injectors or a carburetor.

Second StrokeWhen the piston reaches BDC, the intake valve closes and the piston travels back to TDC with all valves closed. This compresses the air-fuel mixture, raising both the pressure and temperature in the cylinder. The finite time required to close the intake valve means that actual compressiondoesn't start until sometime aBDC. Near the end of the compression stroke, the spark plug is fired and combustion is initiated.

Combustion

Combustion of the air-fuel mixture occurs in a very short but finite length of time with the piston near TDC (i.e., nearly constant-volume combustion). It starts near the end of the compression stroke slightly bTDC and lasts into the power stroke slightly aTDC. Combustion changes the composition of the gas mixture to that of exhaust products and increases the temperature in the cylinder to a very high peak value. This, in turn, raises the pressure in the cylinder to a very high peak value.

Third Stroke

With all valves closed, the high pressure created by the combustion process pushes the piston away from TDC. This is the stroke which produces the work output of the engine cycle. As the piston travels from TDC to BDC, cylinder volume is increased, causing pressure and temperature to drop.

Exhaust Blow DownThe exhaust valve is opened and exhaust blow down occurs. A pressure differential is created through the exhaust system which is open to atmospheric pressure causing much of the hot exhaust gas to be pushed out of the cylinder and through the exhaust system when the piston is near BDC. Opening the exhaust valve before BDC reduces the work obtained during the power stroke but is required because of the finite time needed for exhaust blow down.

Fourth StrokeBy the time the piston reaches BDC, exhaust blow down is complete, but the cylinder is still full of exhaust gases. With the exhaust valve remaining open, the piston now travels from BDC to TDC in the exhaust stroke. This pushes most of the remaining exhaust gases out of the cylinder, leaving only that trapped in the clearance volume when the piston reaches TDC. Near the end of the exhaust stroke bTDC, the intake valve starts to open, so that it is fully open by TDC when the new intake stroke starts. Near TDC the exhaust valve starts to close and finally is fully closed sometime aTDC. This period when both the intake valve and exhaust valve are open is called valve overlap.

CI engine

Difference between SI and CI Engine

• Type of fuel used• Type of cycle used• Introduction of the fuel - In SI, through

carburetor/fuel injectors in intake stroke and in CI through fuel pump in combustion stroke.

• Introduction of the air - In CI turbochargers or superchargers are used where as there is no such thing in SI engines.

• Ignition of fuel - Petrol is a highly volatile liquid, but its self-ignition temperature is high. Hence for the combustion of this fuel spark is necessary to initiate its burning process.

• With diesel, the self-ignition temperature is comparatively lower. When diesel fuel is compressed to high pressures, its temperature also increases beyond the self-ignition temperature of the fuel. Hence in the case of CI engines, the ignition of fuel occurs due to compression of the air-fuel mixture and there is no need for sparkplugs.

• Compression Ratio for the case of SI engines, the compression ratio of the fuel is in the range of 6 to 10 depending on the size of the engine and the power to be produced. In CI engines, the compression ratio for air is 16 to 20. The high compression ratio of air creates high temperatures, which ensures the diesel fuel can self-ignite.

• Weight of the engines• Speed achieved by the engine• Thermal efficiency of the engine

• Air Fuel Ratio: The air-fuel mixture is homogeneous throughout in SI while in CI engines only air enters and mixture is heterogeneous in the cylinder.

• Engine Strength: Compression ignition engine has to be built much stronger than an spark ignition engine due to the fact that the Compression Ignition engine will "Detonate" the fuel and will cause it to explode . This is why diesel engines are so much louder than gas engines. Spark Ignition system ignites the fuel and it burns in a rapid but controlled manner that “Pushes" the piston down during the power stroke and puts less stress on engine parts.

Sample Problem

• Write 2 advantages of two stroke engine over four stroke engines and 2 advantages of four stroke engines over two stroke engines

Detonation in SI Engine• During the combustion of the fuel through spark plug, the end charge

auto-ignites itself before the flame front reaches it, hence this abnormal combustion is called detonation or knocking.

• Auto-ignition occurs at a very critical temperature and the time required by the fuel to reach this temperature is called ignition-delay.

• In auto-ignition, the burning is almost instantaneous which results in extremely rapid release energy and hence causing the pressure of the end gases to rise to almost 3 to 4 times.

• This large pressure differential gives rise to severe pressure wave which strikes the cylinder wall and sets it vibrating, giving high pitched metallic pinking or ringing sound.

• To avoid this, a fuel should have high auto-ignition temperature and long ignition-delay in SI engine.

Engine Problems

• Three fundamental things can happen: – A bad fuel mix– Lack of compression– lack of spark

• Beyond that, thousands of minor things can create problems, but these are the "big three." Based on the simple engine we have been discussing, here is a quick rundown on how these problems affect your engine:

Bad Fuel Mix

• You are out of gas, so the engine is getting air but no fuel.

• The air intake might be clogged, so there is fuel but not enough air.

• The fuel system might be supplying too much or too little fuel to the mix, meaning that combustion does not occur properly.

• There might be an impurity in the fuel (like water in your gas tank) that makes the fuel not burn.

Lack of Compression

• If the charge of air and fuel cannot be compressed properly, the combustion process will not work like it should. Lack of compression might occur for these reasons:

• Your piston rings are worn (allowing air/fuel to leak past the piston during compression).

• The intake or exhaust valves are not sealing properly, again allowing a leak during compression.

Lack of spark

• The spark might be nonexistent or weak for a number of reasons:

• If your spark plug or the wire leading to it is worn out, the spark will be weak.

• If the wire is cut or missing, or if the system that sends a spark down the wire is not working properly, there will be no spark.

• If the spark occurs either too early or too late in the cycle, the fuel will not ignite at the right time, and this can cause all sorts of problems.

• Many other things can go wrong. For example:– If the battery is dead, you cannot turn over the

engine to start it.– If the bearings that allow the crankshaft to turn

freely are worn out, the crankshaft cannot turn so the engine cannot run.

– If the valves do not open and close at the right time or at all, air cannot get in and exhaust cannot get out, so the engine cannot run.

– If someone sticks a potato up your tailpipe, exhaust cannot exit the cylinder so the engine will not run.

– If you run out of oil, the piston cannot move up and down freely in the cylinder, and the engine will seize.

Constructional Details

• Block • Cylinder Head• Camshaft• Crankshaft• Cylinder• Piston• Connecting rods

Constructional Details

• Piston Rings• Spark Plug• Exhaust system• Fuel pump• Fuel Injector• Water Jacket• Radiator

Constructional Details

• Glow Plug• Valves• Smart Engine• Catalytic Converter

• Engine blockBody of engine containing the cylinders, made of cast iron or aluminum.

Cylinder Head

• The piece which closes the end of the cylinders, usually containing part of the clearance volume of the combustion chamber.

• It is usually cast iron or aluminum, and bolts to the engine block.

• The head contains the spark plugs in SI engines and the fuel injectors in CI engines and some SI engines.

• Most modern engines have the valves in the head, and many have the camshaft(s) positioned there also (overhead valves and overhead cam).

• Rotating shaft used to push open valves at the

proper time in the engine cycle, either directly or through mechanical or hydraulic linkage.

• Camshafts are generally made of forged steel or cast iron and are driven off the crankshaft by means of a belt or chain.

• In four-stroke cycle engines, the camshaft rotates at half engine speed.

Camshaft

Camshaft

Crankshaft

• Rotating shaft through which engine work output is supplied to external systems.

• The crankshaft is connected to the engine block. • It is rotated by the reciprocating pistons through

connecting rods connected to the crankshaft.• Most crankshafts are made of forged steel,

while some are made of cast iron.

Crankshaft

Cylinder

• Portion of engine block in which the pistons reciprocate back and forth.

• The walls of the cylinder have highly polished hard surfaces.

• Cylinders may be machined directly in the engine block, or a hard metal sleeve may be pressed into the softer metal block.

• Sleeves may be dry sleeves, which do not contact the liquid in the water jacket, or wet sleeves, which form part of the water jacket.

Cylinder

Piston

• The cylindrical-shaped mass that reciprocates back and forth in the cylinder, transmitting the pressure forces in the combustion chamber to the rotating crankshaft.

• The top of the piston is called the crown and the sides are called the skirt.

• Pistons are made of cast iron, steel, or aluminum.

Connecting Rods

• Rod connecting the piston with the rotating crankshaft, usually made of steel or alloy forging in most engines but may be aluminum in some small engines.

Piston & Connecting Rods

Piston Rings

• Metal rings that fit into circumferential grooves around the piston and form a sliding surface against the cylinder walls.

• Piston rings are made of highly polished hard chrome steel.

• The purpose of these is to form a seal between the piston and cylinder walls and to restrict the high-pressure gases in the combustion chamber from leaking past the piston into the crankcase.

Piston rings

TYPESCompression rings : Prevent pressure leakage into

the crankcase.

Ride in grooves in piston head to seal the cylinder

Piston Rings

• Oil Rings : Prevent engine oil leakage to combustion chamber

Piston Rings

Piston Assembly

Spark Plug

• Electrical device used to initiate combustion in an SI engine by creating a high-voltage discharge across an electrode gap.

• Spark plugs are usually made of metal surrounded with ceramic insulation.

Spark Plug

Exhaust System

• Flow system for removing exhaust gases from the cylinders, treating them, and exhausting them to the surroundings.

• It consists of an exhaust manifold which carries the exhaust gases away from the engine, a thermal or catalytic converter to reduce emissions, a muffler to reduce engine noise, and a tailpipe to carry the exhaust gases away from the passenger compartment.

Exhaust System

Fuel Injector

• A pressurized nozzle that sprays fuel into the incoming air on SI engines or into the cylinder on CI engines.

• On SI engines, fuel injectors are located at the intake valve ports on multipoint port injector systems and upstream at the intake manifold inlet on throttle body injector systems.

• In a few SI engines, injectors spray directly into the combustion chamber

Fuel Injector

Fuel Pump

• Pump to supply fuel from the fuel tank (reservoir) to the engine.

• Many modern automobiles have an electric fuel pump mounted submerged in the fuel tank.

• Some small engines and early automobiles had no fuel pump, relying on gravity feed.

Fuel Pump

Water Jacket

• System of liquid flow passages surrounding the cylinders, usually constructed as part of the engine block and head.

• Engine coolant flows through the water jacket and keeps the cylinder walls from overheating.

• The coolant is usually a water-ethylene glycol mixture.

Water jacket

Radiator

• Liquid-to-air heat exchanger of honeycomb construction used to remove heat from the engine coolant after the engine has been cooled.

• The radiator is usually mounted in front of the engine in the flow of air as the automobile moves forward.

• An engine-driven fan is often used to increase air flow through the radiator.

Radiator

Glow Plug

• Small electrical resistance heater mounted inside the combustion chamber of many CI engines.

• It is used to preheat the chamber enough so that combustion will occur when first starting a cold engine.

• The glow plug is turned off after the engine is started.

Valves

• Used to allow flow into and out of the cylinder at the proper time in the cycle.

• Most engines use poppet valves, which are spring loaded closed and pushed open by camshaft action.

• Valves are mostly made of forged steel.• Valve seats and are made of hardened steel or

ceramic.

Valve

Smart engine• Engine with computer controls that regulate operating

characteristics such as air-fuel ratio, ignition timing, valve timing, exhaust control, intake tuning, etc.

• Computer inputs come from electronic, mechanical, thermal, and chemical sensors located throughout the engine.

• In automobiles the same computers are used to make smart cars by controlling the steering, brakes, seats, anti-theft systems, sound systems, navigation, noise suppression, environment etc.

• On some systems engine speed is adjusted at the instant when the transmission shifts gears, resulting in a smoother shifting process.

Catalytic Converter

• It is the chamber mounted in exhaust flow containing catalytic material that promotes reduction of emissions by chemical reaction.

• Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion carbon monoxide, unburned hydrocarbons, and oxides of nitrogen are converted to less-toxic or inert substances such as carbon dioxide, hydrogen, nitrogen and oxygen.

Engine Emissions

• One of the biggest problem with the automobiles.

• Can’t be completely normalized.• Compromises have to be done with the power

to decrease the emissions.

Emissions

• Hydrocarbons• Carbon Monoxide• Oxides of Nitrogen (NOx)

• Hydrocarbons are fuel molecules which did not get burned and smaller non-equilibrium particles of partially burned fuel.

• Carbon monoxide occurs when not enough oxygen is present to fully react all carbon to CO2 or when incomplete air-fuel mixing occurs due to the very short engine cycle time.

• Oxides of nitrogen are created in an engine when high combustion temperatures cause some normally stable N2 to dissociate into monatomic nitrogen N, which then combines with reacting oxygen.

Prevention

• To improve the technology of engines and fuels so that better combustion occurs and fewer emissions are generated.

• The second method is after treatment of the exhaust gases. This is done by using thermal converters or catalytic converters that promote chemical reactions in the exhaust flow.

• These chemical reactions convert the harmful emissions to acceptable CO2, H20, and N2.