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The internal combustion engine is a type of engine in which a fossil fuel is burnt in presence of an oxidizer, usually air, in a combustion chamber. Basic principle is that the high temperature high pressure gases produced during the combustion apply direct force to the pistons present in the combustion chamber. In other words, this force produced is used to move some mechanical component like the piston a certain distance thereby transforming chemical energy to mechanical energy. Some of the fossil fuels used are diesel, petroleum and compressed natural gas.
This mixture of air and fuel is ignited with the help of either a spark plug (petrol or gasoline engine) or a compression ignition stroke (like in a diesel engine).
Gasoline Ignition Process
Gasoline ignition engines generally count on a lead acid battery and a starter to generate a high voltage spark that is used to ignite the air fuel mixture present in the engine cylinders. After starting the engine, the lead acid battery is charged by means of an alternator which is driven by the engine belt while the engine is running. The mixture is compressed to approximately 12.8 bar.
Diesel Ignition Process
In diesel engines, the ignition process involves a compression of mostly air to a very high pressure, more than twice than that of gasoline engines, and just as the peak compression is about to be achieved a small quantity of diesel is injected which facilitates instant ignition.
Figure . Lead Acid Battery
Figure . Alternator
Almost all cars of today have engines that are based on the four stroke cycle or in other words, the 'Otto' Cycle. They have one power stroke for every four strokes of the piston or two rotations of the cam.
Four Strokes Cycle
The four stroke cycle involves intake, compression, combustion (which is the power stroke) and exhaust.
INTAKE- during the intake stroke, the piston descends to the bottom of the piston cylinder therefore increasing volume. A mixture of air and fuel enters the cylinder chamber through the intake port due to the atmospheric pressure or rather the difference in pressure. The intake port or valve then shuts.
COMPRESSION - at this stage, both the intake and the exhaust valves shut, and the piston ascends to the top of the cylinder therefore compressing the mixture of air and fuel. During the upward movement or compression, the temperature of the air-fuel mixture rises by several hundred degrees.
COMBUSTION or POWER stroke - The second part or revolution of the cycle starts from here. When the piston is close or at the TOP DEAD CENTRE, the compressed air-fuel mixture is ignited by the spark plug or by heat generated during the compression stroke (in case of a diesel engine). The resulting pressure forces the piston back down to the BOTTOM DEAD CENTRE.
EXHAUST - The exhaust valve is open at this point and the piston returns to the top to dispel the burnt air-fuel mixture.
Figure . 4 Stroke Cycle
The Otto cycle is an idealized thermodynamic cycle that elucidates the internal combustion spark ignition engine.
The construction involves two components:
Top and bottom of the loop - adiabatic process (no heat gained or loss)
Left and right side of the loop - isochoric process (constant volume process)
The 4 strokes in the Otto cycle are defined by a graph.
1. Process 1-2 is isentropic compression of air
2. Process 2-3 is a constant volume process involving heat transfer to the air
3. Process 3-4 is isentropic expansion, in other words the power stroke.
4. Process 4-1 is the last process which is again a constant volume process in which heat is rejected. .
Figure . Otto Cycle
The Otto cycle represents the Thermodynamic cycle most commonly found in automobile engines.
There are many types of engine layouts, the most common ones being V-Shaped and in lines.
The V-shaped Engine - It is one of the most widely found engine configurations today. Basically, the pistons and cylinders are aligned in two separate banks or planes in such a way that when viewed from along the axis of the crank shaft, they from a V shape.
Some examples are the V-6 and the V-8.
V6 - The 6 cylinders are mounted on a crankcase in two banks of three cylinders each which are at right angles or at acute angles to each other and all the pistons driving a common crankshaft.
V8- the 8 cylinders of the V8 are mounted on a crankcase in two banks of four cylinders each which are at right angles or acute angles to each other and all the pistons driving a common crankshaft.
Figure . V8 Engine
The Inline Engine- All the cylinders in an inline arranged in a row without any offset. The inline engine is also known as a straight engine. There are many types as shown.
Figure . Inline 4 engine
Figure . Inline 6 engine
Unlike gasoline engines, diesel engines do not have a spark plug to initiate combustion of the air fuel mixture. After the air is sucked in, it is compressed to a very high pressure by the upward movement of the piston which results in an increase in temperature to about 700 °C. Diesel is then injected which results in spontaneous combustion. This entire process of self- ignition is known as detonation.
However, detonation is deleterious to gasoline engines. In gasoline engines, detonation is also known as knocking. It occurs in internal combustion engines when the combustion of the air fuel mixture starts off as it is supposed to but the pockets of air fuel mixture explode outside the so called envelope of the normal combustion front. In other words, it is also occurs due to premature combustion.
Knocking occurs at high compression ratios when the charge ignites ahead of the flame front that is way before the piston has reached top dead centre. This results in the formation of high pressure frequency waves which causes the engine to vibrate and can be heard in the form of audible knocks.
Knocking can cause overheating of spark plugs (especially spark plug points) and erosion of the combustion chamber surface. The most common method to avoid knocking is to use fuel of a higher octane number.
Figure . Knocking in the piston chamber
Fuel Injection System
The air/fuel ratio is the amount of air compared to the amount of fuel. The air/fuel ratio affects the power of the vehicle, fuel economy and also the emissions of the vehicle. The ideal air/fuel ratio (although varies from car to car) is about 14.7to 1, in other words, there are 14.7 parts of air in 1 part of fuel. An air fuel ratio of 14.7 to 1 is commonly referred to as a stoichiometric ratio.
Basic Fuel Delivery System - The main purpose of the fuel deliver system is to supply proper volume of pressurized clean fuel to the fuel rail. Fuel delivery starts with the fuel stored in the tank. When the fuel pump is energized it sends pressurized fuel from the tank through the fuel pipes laid out under the car and through a filter, to the injectors.
Injector - The fuel injector is an electromagnetically or electromechanically operated needle valve that when energized, the electromagnet moves the plunger thereby allowing the fuel to be injected in to the piston chamber. The duration for which the injector is open is known as pulse width and its unit is seconds.
Figure . Fuel Injector
Electronic Fuel Delivery System
The Electronic Fuel Delivery System comprises of basic components such as fuel tank, fuel pump, fuel rail, fuel lines, and fuel injector etc. to facilitate movement of fuel. There are also a few other components present such as fuel pressure sensor, fuel temperature sensor, fuel pump driver module and also the parallel pressure relief valve.
Figure . Electronic return less fuel delivery system
Working of the pressure and temperature sensors
The fuel pressure sensor is a type of diaphragm strain gauge device used to measure the pressure of the fuel rail. A vacuum line is connected to the sensor. The sensor uses the intake manifold vacuum as a reference to determine the pressure difference between the fuel rail and the intake manifold. The higher the pressure in the fuel rail, the higher the voltage signal that is sent to the PCM. This input signal is used to adjust the fuel injector pulse width and meter fuel to each cylinder. Some fuel rail pressure sensors also measure fuel rail temperature using a thermistor. The relationship between fuel pressure and fuel temperature is used to determine the possible presence of fuel vapor air in the fuel rail. This signal is used to vary the fuel pressure and avoid fuel system vaporization by changing the speed of the fuel pump. The speed of the fuel pump sustains fuel rail pressure which preserves fuel in the liquid state. Depending on the pressure and temperature, the fuel injector pulse width is either increased or decreased to help provide a stoichiometric fuel mixture.
Fuel Pump and Fuel Pump Driver module
The fuel pump fitted in today's cars is electromechanical in nature unlike the fuel tanks of before which were completely mechanical that were fitted in cars having carburetors, a contraption that is now obsolete. This electromechanical fuel pump is fitted in fuel tanks of cars and its main purpose is to force pressurized fuel or gasoline through the fuel lines to the fuel injectors. It does this by creating a positive pressure in the fuel lines located beneath the car.
Considering the fuel tank is electronically operated, the question arises whether a spark would cause the fuel in the tank to ignite and explode. However, liquid fuel does not explode and hence placing the fuel pump inside the fuel tank and submerging it in liquid fuel is one of the safest places to put it. Also, placing it in the fuel tank makes it least likely to come in contact with or handle gasoline fuel vapor. Modern engines use the concept of pulse width in fuel injectors to control the amount of fuel being sprayed in to the engine.
Usually, cars with an electromechanical fuel pump have an ECU (Engine Control Unite module) which can be programed to shut off the pump, thereby stopping fuel pumping even when the engine is running. In other words, the ECU is programed with a so called Safety Logic that shuts off the pump in the event of a collision or an accident, hence preventing fuel leak from the ruptured fuel lines. ECUs in cars are also programed to shut of the fuel pump if zero or low oil pressure is detected. This usually occurs when the engine has suffered a terminal failure hence risking fire or smoke in the engine compartment.
Figure . Fuel Pump
Fuel Pump Driver Module
In order to regulate the fuel pressure, a device known as the fuel pump driver module is used to alter the speed of the fuel pump. In most cars, this module is located in the rear of the vehicle. It operates by receiving a signal from the Powertrain Control Module (PCM), which in turn is used to vary the voltage sent to the fuel pump.
Figure . Fuel Pump Driver Module
Apart from this, there are a number of sensors that come into play when it comes to deciding the amount of fuel being pumped into the engine. Some of the sensors are:
CKP sensor known as the crankshaft position sensor is used by the Powertrain Control Module to determine when to energize the electromagnetically operated fuel injectors.
Figure . CKP sensor
CMP sensor also known as the camshaft position sensor which identifies when the first piston is on the compression stroke and therefore sequentially times the fuel injectors.
Figure . CMP sensor
MAF sensor or mass air flow sensor, which is used to measure the amount of air in terms of mass entering the engine. It is located in the air path to the intake manifold between the air filter and the throttle body/ butterfly valve.
Figure . MAF sensor
IAT or intake air temperature sensor which is used to calculate changes in load in order to account for changes in mass of air caused due to changes in temperature.
ECT or the engine coolant temperature sensor, also known as cylinder head temperature sensor that is located near the thermostat housing in order to modify air-fuel ratio.
Figure . ECT sensor
HO2S or the heated oxygen sensors located before and after the catalytic converter in the exhaust system of cars. There are used by the electronic engine control system to determine or measure the amount of oxygen in the exhaust system to indicate if the air-fuel mixture is rich or lean. The concept is, the oxygen sensor measures the amount of air outside and compares it with the amount of air in the exhaust gases. This difference is used to generate a voltage signal. The greater the difference, the more the voltage.
Figure . HO2 sensor
AIR INTAKE SYSTEM
Present cars have air intake systems that mainly comprise of three parts such as mass air flow sensor, air filter and throttle body. Also many cars are now fitted with an intake manifold which is responsible for optimally distributing air among the cylinders and therefore maintaining the appropriate air-fuel mixture.
Throttle body- It is that part of the air intake system that is responsible for controlling the amount of air entering the engine by a mechanism of controlling fluid flow through obstruction and constriction. The throttle body has a butterfly valve which obstructs and constructs the flow of air and is moved in accordance with the degree to which the driver presses the accelerator pedal. When the intake manifold is at ambient atmospheric pressure, the throttle body is completely or wide open. Cars of today have an electronic throttle body which means that when the driver presses the accelerator, the manual throttle sends a message to the powertrain control module which in turn controls the position of the throttle valve. In other words, the driver does not have complete control on the butterfly valve. As a result the PCM can control the throttle body to reduce emissions or maximize performance.
Figure . Throttle Body
Air Filter- An filter in a car is located in a specific holder present at the beginning of the air intake system. The air filters present in cars are particulate air filters which are usually made of fibrous paper material and are folded in an accordion style. These air filters are responsible for ensuring that clean air enters the engine through the intake manifold by filtering out dust, pollen, mold and other particulate matter.
Figure . Air Filter
EXHAUST GAS RECIRCULATION VALVE
Exhaust gas recirculation is a 'tactic' that is used by internal combustion engine cars to reduce the content nitrogen oxide in the exhaust gases. This so called tactic or technique is carried out by a device known as the Exhaust Gas Recirculation Valve. What the EGR valve does is, it recirculates a considerable portion of the exhaust gases to the cylinders. When it is added to the cylinders, it increases the specific heat capacity of the piston cylinders which in turn lowers the adiabatic flame temperature. However, in diesel engines, when the exhaust gases are recirculated, they lower the combustion temperature thus reducing nitrogen oxide emissions. Overall, it is helps in increasing the life span of engines through reduced cylinder temperatures. The EGR valve increases the fuel efficiency in cars through a set of three mechanisms like,
Reduced throttling losses
Reduced heat rejection
Reduced chemical dissociation
But, it also reduces the fuel efficiency of cars through a mechanism reduced heat specific ratio.
Figure . Exhaust gas recirculation valve
TURBOCHARGERS and SUPERCHARGERS
Turbochargers and Supercharges fall into the category of forced induction devices which are used to increase air intake in cars. Taking turbochargers into consideration, turbochargers allow more power to be produced by the engines by forcing more air in the intake and therefore proportionally more fuel into the combustion chamber. Turbo chargers are driven by the engine's exhaust gas turbine unlike superchargers that are mechanically driven by the engine belt connected to the crankshaft.
Turbochargers are more expensive as they are more efficient than superchargers but are preferred less due to turbo lag. What the turbo charger does is, it sucks in the ambient air and compresses before it enters the intake manifold. In other words, it is responsible for greater mass of air entering the piston cylinder. A turbocharger also increases fuel efficiency without increasing the power of the engine, by recirculating exhaust gases (rather wasted energy from the exhaust) and shoving it back into the engine intake.
A supercharger on the other hand, is a type of air compressor used for forced induction in the internal combustion engine. The intensity with which a supercharger forces air depends on the engine speed (RPM), in other words, the degree to which the driver presses the accelerator. Superchargers are known to add about 30% more torque and about 45% - 46% more horsepower. Considering that superchargers are responsible for compressing air, which would mean that the temperature of air would increase. High temperature air is less dense; hence the intensity of explosion in the piston chamber wouldn't be as much. So, superchargers are fitted with inter coolers to cool the air before it enters the piston chamber. Intercoolers basically work like radiators facilitating movement of air or water through pipes in order to radiate heat.
Figure . Turbocharger
Figure . Supercharger
There are 3 types of superchargers
Roots superchargers- placed on top of the engines and work by giving discreet bursts and add more weight to the vehicle. These types are usually fitted in vehicles known as hot rods.
Twin-Screw superchargers - Have twin worm gears rotating in an inward direction. Most commonly found on the Ford GT.
Centrifugal superchargers- Most common form of superchargers which uses impeller mechanism to compress air.
AIR CONDITIONERS (in automobiles)
Air condition is a process by which the properties like temperature and humidity are altered to make more favorable conditions. In other words, it makes these conditions favorable by technologically cooling, heating, ventilation and disinfection.
This change in properties is done using a refrigeration cycle.
The refrigeration cycle is based on the second law of thermodynamics. The second law of thermodynamics states that "in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state." Or "the entropy of an isolated system never decreases, because isolated systems spontaneously evolve towards thermodynamic equilibrium -- the state of maximum entropy".
Figure . Single Stage Refrigeration Cycle
In cars, the air conditioning the system is split into two sides, a high pressure side and a low pressure side, also known as the discharge and suction respectively. Air inside the cabin is altered, using certain components of its air conditioning system which are
Compressor - The compressor in cars compresses refrigerant gas to a high pressure high temperature gas. The compressor has a pulley in the front which is run by an engine belt. The compressor is fitted with an electromagnetic clutch that turns the compressor on and off depending on the amount of cool air needed. The compressor clutch when energized, it engages the compressor and starts to drive it.
Figure . Compressor
Condenser- The condenser condenses the refrigerant to a high pressure low temperature (makes it hot to warm) liquid. In other words, the compressor is like a radiator (heat exchanger) which is located at the front of the vehicle. In some cars, the compressors have their own cooling fan. Condensers are very thin as then have to depend on air flowing through them to cool the refrigerant. The condenser consists of a series of tubes and fins to increase its efficiency as a heat exchanger.
Figure . Condenser
Evaporator- The evaporator is located near the cabin and its primary function is heat absorption. Another function is dehumidification. The ideal temperature of the evaporator is around 0 °C because of the low pressure low temperature refrigerant which enters it. As the warm air of the cabin passes over and through the evaporator fins, it causes the refrigerant to boil. This is because refrigerants have a very low boiling point of around -26.3 °C.
Figure . Car Evaporator
Accumulator- The accumulator is basically a vessel that collects the excess refrigerant leaving the evaporator in order to convert the liquid refrigerant to its gaseous state before it enters the compressor as the compressor cannot compress liquid. It also contains a desiccant like silica gel to absorb moisture. This is because moisture and refrigerant combine to form acids which can be corrosive to the system.
Figure . Accumulator
Receiver Dryer- The receiver dryer, like the accumulator is a liquid storage tank that stores excess refrigerant coming in from the condenser. It is used in a thermal expansion valve refrigerant system and is placed immediately after the condenser. Like the accumulator, it also contains a desiccant to absorb moisture. Refrigerant enters the receiver dryer. The vapor rises to the top while the heavier liquid collects at the bottom.
Figure . Receiver Dryer
Apart from these three main components, there are two different types of pressure regulating devices.
Orifice tube- It is found somewhere between the outlet tube of the condenser and the inlet tube of the evaporator. It measure approximately 3 inches in length and is about 0.063 inches in diameter. It is responsible for ensuring that refrigerant flows at a constant rate, filtering the refrigerant and also meters the liquid refrigerant entering the evaporator coil therefore causing a pressure drop.
Figure . Orifice tube
Thermal Expansion Valve- This type of valve, like the orifice also is responsible for pressure drop. However, unlike the orifice, it also has pressure and temperature sensors and is responsible for regulation refrigerant flow. It is located between suction lines and the evaporator inlet and outlet tubes.
Figure . Thermal Expansion Valve
There are two types of refrigeration systems -
Orifice tube type refrigeration system- During stabilized conditions, which is when the Air Conditioner system is not operating, the refrigerant is in a vaporized state and pressures are equal throughout the system. When the Air Conditioner compressor is in operation, it increases pressure on the refrigerant vapor, raising its temperature (according to the ideal gas law). The high pressure and high temperature vapor is then released into the top of the condenser core.
The condenser core, being close to the ambient temperature, causes the refrigerant vapor to condense into a liquid when heat is removed from the refrigerant by ambient air passing over the fins and tubes. The liquid refrigerant still at high pressure, exits from the bottom of the condenser core and enters the inlet side of the evaporator core orifice.
The evaporator core orifice is the restriction in the refrigeration system, that creates the low pressure drop in the evaporator core and creates the low pressure drop in the evaporator core, and separates the high and low pressure sides of the Air Conditioner system. As the liquid refrigerant (R-134a) leaves this restriction, its pressure, temperature and boiling point are reduced.
The liquid refrigerant is at its lowest temperature and pressure now. As it passes through the evaporator core located at the bottom of the contraption, it absorbs heat from the passenger cabin or compartment in the car, airflow passing over the fin sections of the evaporator core. The addition of the heat causes the refrigeration to boil and become gas. The now cooler passenger cabin air can no longer support the same humidity level of the warmer air and the excess moisture condenses on the exterior of the evaporator coil and fins, and drain outside the vehicle.
The Air Conditioning has a cycling switch that interrupts the compressor operation before external temperature of the evaporator core gets low enough to cause the condensed water vapor that is the excess humidity, to freeze. It performs this function by monitoring the low side pressure line. A refrigerant pressure of 210 kPa would yield an operating temperature of 0 °C.
Figure . Orifice Tube Type Refrigeration System
Thermal Expansion Valve Type Refrigeration System- During stabilized conditions, which is when the Air Conditioner system is not operating, the refrigerant is in a vaporized state and pressures are equal throughout the system. When the Air Conditioner compressor is in operation, it increases pressure on the refrigerant vapor, raising its temperature (according to the ideal gas law). The high pressure and high temperature vapor is then released into the top of the condenser core.
The liquid refrigerant now exits from the outlet of the condenser core and enters the inlet side of the Air Conditioner receiver/drier. The receiver-drier is created and designed to remove moisture from the refrigerant. The outlet of the receiver-drier is connected to the thermostatic expansion valve or TXV. The TXV, like the orifice, provides the restriction in this type of refrigerant system, and separates the high and low pressure sides. As the liquid refrigerant passes through this, its boiling point and pressure are reduced.
The liquid refrigerant is at its lowest temperature and pressure now. As it passes through the evaporator core, it absorbs heat from the passenger cabin, airflow passing over the fin sections of the evaporator core. The addition of the heat causes the refrigeration to boil and become gas. The now cooler passenger cabin air can no longer support the same humidity level of the warmer air and the excess moisture condenses on the exterior of the evaporator coil and fins, and drain outside the vehicle. In the thermal expansion valve type refrigeration system, a thermistor measures the temperature of air that has passed through the evaporator core, controls the Air Conditioner clutch cycling. If the temperature of the evaporator core discharged air is low enough to cause condensed water vapor to freeze, the Air Conditioner clutch is disengaged by the PCM.
The Air Conditioner compressor thermal protection switch is designed to interrupt compressor operation if the compressor housing exceeds temperature limits. In case the pressure is too high, the compressor relief valve will open. This would vent the refrigerant to relieve unusually high system pressure.
Figure . Thermal Expansion Valve type Refrigeration System
Refrigerant (R-134a) - R-134a is a refrigerant used in the air conditioning system of cars. Before R-134a, R-12 was used but due to its ozone depletion potential, it was replaced. R-134a has a boiling point of -26.3°C which makes it an ideal refrigerant for air conditioner in cars.