Vehicle Exhaust Gases

Vehicles emit exhaust gases as a result of the combustion of fossil fuels. There are various exhaust gases that are emitted from the engine bases on various conditions that develop within the combustion chamber. These gases can either be treated and then sent to the atmosphere via tail pipe, some amount being used in the purpose of exhaust gas re-circulation (EGR) or sent through the tail pipe without any treatment. Gases such as nitrogen oxides (NO_x), carbon monoxide (CO), hydrocarbons (HC), particulate matter (PM), volatile organic compounds (VOC), and in some cases smaller amount of sulphur dioxide (SO_x) and ammonia (NH₃).

Nitrogen oxides (NO_x):

NO_x refers to NO and NO₂ and it is produced due to the combustion at very high temperatures. At higher temperatures, nitrogen molecule splits up and reacts with oxygen forming NO and later on NO oxidizes to NO_2. Diesel engines produce more NO_x than a gasoline engine because diesel engines have higher combustion temperatures.

Carbon monoxide (CO):

CO is formed as a result of incomplete combustion. Ideally, only carbon dioxide (CO₂) and water (H₂O) should be formed as the products of combustion, but lack of oxygen can result in incomplete reaction of carbon with oxygen and thus CO is formed. Gasoline engines emit more CO compared to diesel engines.

Particulate matter (PM):

Particulate matter is a mixture of solid and liquid particles suspended in the air such as dust, soot (fine carbon particles as dust as a result of incomplete combustion), metal, acids, poly-cyclic aromatic hydrocarbons  and most of them being hazardous. It has both coarse and fine particles. The size of the coarse particles vary from 2.5 µm to 10 µm, whereas the finer particles' size varies from 0.1 µm to 2.5 µm. Finer particles are more in composition compared to coarse particles. Diesel engines emit more PM.

Volatile Organic Compounds (VOCs):

VOCs are formed due to partially burned or unburned fuel. It is a combination of carbon compounds. It is also released due to evaporation of fuel. Gasoline engines emit more VOCs since gasoline is more volatile compared to diesel.

Hydrocarbons (HC):

Hydrocarbons are the unburned fuel which as a result of fuel not burning due to insufficient combustion temperature. It is attributed to the presence of lean air-fuel mixture inside the cylinder. In a lean air-fuel mixture, flame speeds may be too slow for complete combustion to take place.

Inline Fuel Injection Pump (Diesel)

Bosch In-line fuel injection pumps were first introduced in 1927. Since its introduction, it has kept countless number of diesel engines in function. In-line pumps are still widely used in large number of diesel engines, mainly because of its durability and ease of maintenance.

Requirements:

A fuel injection pump is used to supply fuel to the engine at a certain pressure. The pump generates the pressure and supplies the fuel with the right quantity at the desired timing. The pressurized fuel is delivered to the nozzle via a high pressure line. The nozzle injects the fuel inside the combustion chamber. There are various requirements to be met by a in-line pump, such as:

• Timing and duration of fuel injection


• The total volume of fuel to be injected


• The amount of pressure to be created

DESIGN:

The in-line fuel injection system consists of the following components:

• Fuel tank


• Feed pump to supply the fuel from fuel tank to the high pressure pump via a filter


• High pressure in-line pump to pressurize the fuel


• Nozzles to inject the fuel inside the combustion chamber


• Governor to vary the fuel quantity at varying speeds (usually a RSV governor)

In-line pumps can have a set of 2 to 12 cylinders. It is used in various commercial vehicles, agricultural and construction machineries. The maximum injection pressures can vary between 400 bar to 1350 bar based on the pump design.

Design of In-line pump:

It is an aluminium housing which has an internal camshaft. The camshaft is driven via a timing device or directly by the engine. The in-line pump camshaft rotates at the same speed as that of the engine camshaft (i.e) speed of camshaft is half the speed of the crankshaft.

Roller tappets sit over the cam lobes. The number of roller tappets equals the number of cylinders. Above each roller tappet, plunger return springs are placed to assist the plungers in returning to the bottom dead centre (BDC) after each stroke. The plunger is guided inside a barrel where the fuel is pressurized. Plunger has a vertical groove and a helical groove that assist in varying the fuel quantity. The plunger and barrel together are called plunger and barrel assembly.

Delivery valves are seated between the barrel-and-plunger assembly and the delivery valve holder. In the event of delivery stroke, delivery valve cone is lifted from the valve seat due to the high pressure created in the barrel. The delivery valve cone is pressed against the spring provided in the delivery valve holder. The fuel escapes through the holder to the nozzle via a fuel delivery line.

WORKING:

The fuel system layout consists of a feed pump which sucks fuel from the fuel tank and then delivers it to the high pressure inline pump at a low pressure. The camshaft is provided with a separate lobe that drives the feed pump. Diesel is then sent to a filter to remove the unwanted impurities such as dust, corroded particles, water, etc.

Fuel enters the fuel gallery provided in the in-line pump. The fuel gallery is directly connected to the fuel inlet ports in the barrels of all the cylinders.

Plunger stroke phases:

The position of the plunger results in various functions:

Intake Phase

Preliminary Phase

Delivery Phase

1. Intake phase: When the plunger is at bottom dead centre (BDC), the fuel inlet port in any one of the barrels is open and the fuel enters the barrel. This phase is called the intake phase.


2. Preliminary phase: When the plunger starts moving towards top dead centre (TDC), it closes the fuel inlet port and this is called preliminary phase. The fuel is now trapped inside the barrel.


3. Delivery phase: When the plunger continues to move further towards TDC, the trapped fuel is compressed. This increases the pressure inside the barrel-and-plunger assembly and the delivery valve cone is lifted from its seat to allow the pressurized fuel to escape through the delivery valve holder.

Fuel Delivery Variation:

The fuel quantity can be varied based on the position of the vertical and helical grooves. The position of these grooves can be varied with the help of a control rack and control sleeve assembly.

The control sleeve is meshed with the control rack. Translatory motion of the control rack is converted into rotary motion by the control sleeve. The plunger is seated in the groove of the sleeve, therefore the plunger rotates along with the sleeve.

Zero Delivery

Low Speed Delivery

High Speed Delivery

• Zero delivery: To achieve zero delivery, the vertical groove of the plunger should be in line with the inlet port of the barrel. In this position, the pressure chamber in the barrel is directly connected to the fuel gallery throughout the entire stroke from BDC to TDC. Therefore, the fuel in the barrel escapes back to the fuel gallery without getting delivered.


• Partial delivery: Partial delivery of fuel can be achieved by varying the helical groove position in line with the fuel inlet port. Different quantities can be achieved at different positions of the helical groove.


• Maximum delivery: Maximum delivery of fuel can be achieved if neither the vertical groove nor the helical groove is in line with the fuel inlet port.

GOVERNOR:

The top priorities of a fuel injection pump is to supply fuel to the engine at the right time under all operating conditions and at all operating loads. The governor has to constantly vary the control rack positions as the conditions keep changing. Some of the functions of a governor are:

• To precisely meter fuel quantities at various engine loads


• To supply the fuel to the nozzles at the right moment


• To supply fuel for a defined duration of time

Governor Requirements:

The basic function of a governor is to prevent the engine from exceeding its maximum revving speed. Diesel engines may over-rev due to excess amount of air and a governor can be used to cut the fuel supply till the engine speed goes below the maximum revving speed.

Variable Speed Governor (RSV):

RSV governor is used to control the fuel quantity at varying speeds between the idling speed and the maximum speed. It has a flyweight attached to one end of the camshaft. It also has a governor spring pivoted to a tensioning lever which acts against the force of the flyweights. When the speed of the engine is altered, the tension in the governor spring also alters accordingly such that the turning of the tensioning lever is kept in equilibrium with the opposing forces of the flyweights.

The variation in the control lever angle is transmitted to the control rack via fulcrum and guide lever linkages. This helps in varying the fuel quantity. Various speeds can be achieved by varying the control rack movement:

• Starting: The control rack is kept in the starting position by means of a starting spring whose one end is hooked to the control rack and the other end is hooked to the top end of the fulcrum lever. This sets the fuel injection pump to the starting quantity.


• Idle Speed: To achieve idle speed, the control lever is released and made to rest against the low idle stop screw. In this case, there is no tension on the governor spring and it sits in a vertical position. There is literally no force acting against the flyweights, therefore the flyweights start opening at low speed. The sliding bolt is forced to move outside in the right direction, as a result the guide lever is also pushed towards the right. The lever pivots the fulcrum lever to move towards the right and this pulls the control rack towards the idle speed stop. The tensioning lever comes in contact with the auxiliary idle speed spring and this regulates the idle speed of the engine.


• Low Speed: The control lever is pressed to a certain angle. This results in the increase in tension in the governor spring and thus acts against the flyweights, forcing the sliding bolt to slide to the left. This results in the control rack to move towards the left, increasing the fuel quantity, thereby increasing the engine speed. This happens for a brief moment, as the increase in engine speed results in the flyweight to rotate faster and generate a greater centrifugal to act against the governor spring. Equilibrium is achieved between the governor spring and the flyweights' force. The control rack moves towards the right again and the engine speed is kept in control.


• Maximum Speed: The control lever is compressed fully against the maximum speed stop screw.The working is similar to the one explained at low speed. At this point, there is maximum tension in the governor spring.

Hybrid Electric Vehicles

With the dwindling fossil fuel resources, the world might very soon run out of fossil fuels in the next 3 decades. It is imperative that these resources be conserved, hence scientists and engineers come up with solutions to save it. Hybrid vehicles can be one of the alternatives to save fuels. Hybrid electric vehicle uses two forms of energy to provide propulsion, one being the internal combustion energy and the other is a an electric motor powered by battery. Hybrid vehicles significantly reduce emissions and also increase fuel economy. It was Ferdinand Porsche to first introduce a gasoline-electric hybrid vehicle in 1901 by the name Lohner-Porsche Mixte Hybrid.

POWERTRAIN:

There are 3 types of powertrain in hybrid electric vehicles:

• Series hybrids: In a series hybrid, only the electric motor was used to propel the wheels of the car, whereas the Internal Combustion Engine (ICE) was used as a generator to power electric motors to recharge the battery. But these are not efficient for higher speeds, as the motor doesn't generate enough power.


• Parallel Hybrids: In a parallel configuration, both the electric motor and ICE can simultaneously power the drive train as they are connected to the mechanical coupling. A bigger ICE can be installed in this case to generate more power for high speed operation. The ICE also acts as a generator for recharging the battery.


• Power split Hybrids: It is a combination of both the series and parallel hybrids, thus making it more efficient. They are also expensive.

REGENERATIVE BRAKING:

Whenever we apply brakes, a lot of energy is removed from the car. This energy is the kinetic energy dissipated in the form of heat. Regenerative braking is a way of capturing this energy and converting it into useful electrical energy which can recharge the battery.

Plug-In Hybrids:

This is similar to the conventional hybrid vehicles, except that we use rechargeable batteries which can be recharged by an external power supply. It has a plug which can be connected to the electrical grid for recharging.

TOYOTA PRIUS:

It was the first mass produced hybrid car, launched in the year 1997 in Japan. It has a parallel hybrid power train. The biggest advantage of Prius is that it can run solely on electric power up to a speed of 24 kph. The engine does not come into play until the vehicle crosses 24 kph speed. This can significantly reduce the emissions in cities having huge traffic.

It has a unique power splitting device which can efficiently turn the engine for certain speed and load ranges. It is a planetary gear setup which can act as a parallel and as well as series hybrid. It can combine the powers of ICE and electric motor when required. The power split device allows series operation, where the ICE can either be used to power the car on its own for high speed operation or recharge the battery at low speeds.

Prius uses a 1.5 liter gasoline engine which can generate 76 hp and can run at a maximum speed of 5000 rpm. The electric motor can generate 67 hp for 1200 to 1540 rpm range.

Engine Layouts

Engine layout refers to the arrangement of the cylinders in an order. There are various types of engine layouts that can be selected based on various factors such as the available space, stability, etc.

Inline Engines (I6, I4):

The name inline depicts that the cylinders will be arranged in a single line. In this case all cylinders will be arranged in a row connected to a single crankshaft and will be powering the crankshaft. Inline engines can be used in smaller cars where if it is placed transversely can lower the size of the engine hood. But inline engines raise the center of gravity of cars, thus making it lesser stable on cornering.

V Engines:

V6 and V8 engines are more in common in this configuration. In the case of V6, the cylinders are arranged in two banks of three cylinders each set at an angle of 60° or 90°. V6 has become more common in modern cars because of its compactness. It is shorter in length to I4 and can generate more power. All cylinders are connected to a single crankshaft. But we require two crankshafts, one for each bank to operate the valves.

Flat or Boxer engines:

Boxer engines are commonly found in Porsche and Subaru models. The cylinders lay flat at 180° to each other. In the case of a flat 4 engine, two cylinder banks with 2 cylinders each will be firing in opposite direction to each other. The center of gravity of the vehicle is significantly reduced and there are lesser vibrations. It also improves fuel economy. All cylinders are connected to a common crankshaft but require two crankshafts.

W12 Engines:

These are simply a combination of two V6 engines connected to a common crankshaft. It has 3 banks of 4 cylinders each with an angle of 60° between the banks. Volkswagen is the manufacturer of these engines. It is sturdier compared to a V12 engine because it is shorter in length. The recently displayed Bentley Bentayga at the 2015 Frankfurt auto show features a 6.0 L W12 engine which can generate 600 bhp. Bugatti veyron uses a 8.0 L W16 engine which can generate 1000 bhp.

Two Stroke Engine

You would be familiar with the two common internal combustion engines, S.I and C.I. There is one more type of engine known as the two stroke engine that is commonly used for low power applications such as dirt bikes, mopeds, lawn movers, jet skis, etc.

Let's look at some of the advantages of two stroke over four stroke engines. The name suggests that we get power for every two strokes of an engine i.e. for every one revolution of crankshaft, two strokes fire once. Whereas in four stroke engines, the engine fires only once for two revolutions of the crankshaft. Two stroke engines do not have valves, so it doesn't require camshaft thereby reducing the overall weight of the engine. The construction is simpler in two strokes. Two strokes can produce twice the amount of power for a single revolution when compared to four strokes, therefore it has a greater power to weight ratio.



WORKING:

There is an inlet port and an exhaust port. When the piston moves up from b.d.c to t.d.c, the inlet port is uncovered and it allows the air-fuel mixture to enter the crankcase. Simultaneously, the air-fuel charge present in the combustion chamber is compressed and ignited by means of a spark plug.

In the next stroke where piston moves from t.d.c to b.d.c, there is an expansion due to combustion and a lot of heat energy is released. The inlet port is covered and the air-fuel mixture trapped at the bottom in the crankcase is transferred to the upper part of the cylinder through a transfer port. While the piston moves to the bottom, it uncovers the exhaust port through which the exhaust gases escape to the atmosphere. The compressed air-fuel charge pushes the exhaust gases out. A cross-flow design will ensure that a minimal amount of fresh charge could only flow out with the exhaust gases.

The crankshaft starts gaining momentum and the power cycle repeats itself. You can see that there are various simultaneous processes occurring in every stroke.

• When the piston moves up, it compresses the air-fuel charge in the combustion chamber and also captures the heat energy released when the charge is ignited. On the other hand, it also creates a vacuum at the bottom so that a fresh air-fuel charge is sucked inside the crankcase via the inlet port.


• When the piston moves down, it first covers the inlet port and pushes the fresh air-fuel charge from crankcase to the combustion chamber. Then it uncovers the exhaust port and the exhaust gases escape out to the atmosphere.

WHY ENGINE OIL IS MIXED WITH THE FUEL:

You would have noticed in four stroke engines that the engine oil is poured in the crankcase of the engine. But it is not so in the case of two stroke engines. The engine oil is rather mixed with the fuel and stored in the fuel tank. Since the crankcase is not separate in two strokes, it is used to pressurize and send the charge to the combustion chamber. So the crankcase cannot hold the lubrication oils. The engine oil is mixed with the fuel to lubricate the engine components as and when the charge is sucked in.

DISADVANTAGES COMPARED TO FOUR STROKE ENGINES:

• Two strokes are not fuel efficient. During expansion stroke, few amount of fresh charge escapes with the exhaust without getting burnt.


• The rate of pollution is higher. Unburnt hydrocarbons pose a serious threat to the environment.


• The engine parts wear out faster due to lack of an individual lubrication system.

Spark Ignition (S.I) engine

Let's study the movement of piston and how it defines the four strokes of an S.I engine. As you know that there are two valves provided at the top of the cylinder head, viz. the inlet valve and the exhaust valve. Both the valves are operated by the camshaft. The camshaft either opens the valves directly or via a rocker arm. There are four strokes in a S.I engine. They are:-

1. Inlet stroke


2. compression stroke


3. power stroke


4. Exhaust stroke

INLET STROKE:

                                           

During the inlet stroke, the piston moves from T.D.C to B.D.C and the inlet valves are opened. Air-fuel mixture with the stoichiometric air-fuel ratio of 14.7:1 is passed into the combustion chamber because of the low pressure created inside it . The air and fuel was mixed traditionally with the help of carburetors, but fuel injection pumps are replacing the carburetors. The inlet valve remains open for a few angles after B.D.C depending on the design of an engine. The intake valve then closes and the air-fuel mixture is trapped inside the combustion chamber. During this entire process crank rotates 180 degree.

COMPRESSION STROKE:

                                                 

During the compression stroke, both inlet and the exhaust valves remain closed. Now the piston starts moving up from B.D.C to T.D.C making the crankshaft to rotate another 180 degree. The air-fuel mixture inside the cylinder is compressed until the piston reaches T.D.C. The entire volume to which the mixture is compressed is the clearance volume of the cylinder.

Clearance volume = Total volume of the cylinder - Volume swept by piston in the cylinder (swept volume)

A high amount of pressure and heat is generated while compressing the mixture. This atomizes the fuel droplets. The higher the compression ratio, the higher the fuel atomization. Finer droplets results in increased surface area of the fuel droplets and thus helps the fuel burn completely when the spark ignites. Compression ratio of an S.I engine varies from 6:1 to 10:1. The air-fuel mixture is compressed in the range of 8 bar to 14 bar depending on the throttle position and the load.

Compression ratio = volume of the combustion chamber when piston is at B.D.C/ volume of the combustion chamber when piston is at T.D.C

IGNITION:

                         
Just before the piston reaches the T.D.C, the spark plug fires and ignites the compressed air-fuel mixture inside the combustion chamber. The spark plug fires at 15 degree before T.D.C and it varies for different engines.

POWER STROKE:

                                           
Both the inlet and the exhaust valves remain closed. The spark given by the spark plug ignites the mixture and generates a lot of heat and pressure up to 60 bar. It also generates a lot of force that pushes the piston from T.D.C to B.D.C and this rotates the crankshaft another 180 degree. The power generated during the power stroke is stored in the flywheel and given to the transmission. Combustion is the result of a chemical reaction that produces several exhaust gases such as carbon monoxide (CO), Hydrocarbons (HC), particulate matters and several others. All these gases expand within the combustion chamber and thus reduce the pressure inside it.

EXHAUST STROKE:

                                             

The exhaust valve opens and now the piston moves from B.D.D to T.D.C, completing the last stroke of the combustion process. The exhaust gases are pushed outside through the exhaust valve due to piston movement. The pressure drops to 2 to 4 bar inside the cylinder.