Combustion Chambers for Diesel Engines

 The shape of a combustion chamber helps in determining the quality of combustion and therefore the performance and exhaust characteristics of a diesel engine. Appropriate design of combustion chamber combined with piston action produces whirl, squish and turbulence effects that are used to improve the distribution of fuel and air inside the combustion chamber.

The following technologies are used in diesel engines:

• Undivided combustion chamber for Direct Injection (DI) engines
• Divided combustion chamber for Indirect Injection (IDI) engines

Between these two, undivided combustion chamber is predominantly used in vehicles due to their more fuel savings and lesser noise and vibration compared to the divided ones.

Undivided Combustion Chamber (DI):

The direct injection process involves injecting the fuel directly into the combustion chamber. The combustion chamber also relies on the shape of piston crown. Fuel atomization, heating, vaporization and mixing with the air must take place in rapid order.

During the intake and compression strokes, special shape of the intake port in the cylinder head creates an air vortex inside the chamber. Of the combustion chamber designs, the most widely used at present is the w piston crown recess.

The design of a combustion chamber must also ensure even distribution of the fuel inside the chamber so that rapid mixing of air and fuel can take place. A multi-hole nozzle is used in the direct injection process to achieve better atomization of the fuel. The pressure required for direct injection is pretty high at 2200 bar.

In practice, there are two types of direct injection:

• Systems in which mixture formation happens by specifically created air-flow effects
• Systems which control mixture formation virtually by means of fuel injection and largely avoid any kind of air-flow effects

In the latter case, no effort is wasted in creating air turbulence and therefore it helps in more effective cylinder charging and smaller gas replacement losses. However, it demands better nozzle positioning, higher number of nozzle jets and higher intensity of injection pressure in order to provide effective air-fuel mixture.

Direct Combustion Chamber:

As already mentioned, indirect injection engines are far less economical and noisy, along with higher exhaust gas emissions compared to the engines with direct injection technology. As a result, direct combustion chambers are rarely used.

There are two types of processes with direct combustion technology:

• Pre-combustion chamber system
• Whirl chamber system

Pre-Combustion Chamber system:

In the pre-chamber system, fuel is injected into a hot pre chamber recessed into the cylinder head. Pre chambers are much smaller in size compared to the main combustion chamber. The fuel is injected via a pintle nozzle(1) at a relatively lower pressure up to 450 bar. To make sure fuel is impartially burnt, only a small amount of air is supplied to the pre combustion chamber. A specially shaped baffle(3) is positioned at the centre of the pre combustion chamber. The injected fuel strikes the baffle and mixes thoroughly with air.

The partially combusted fuel/air mixture is sent to the main combustion chamber via a connecting channel(4), where it mixes with the available air and burns rapidly. The ratio of pre combustion chamber volume to the main combustion chamber volume is approx. 1:2.

A glow plug(5) is positioned on the lee side of the air flow. A controlled post glow period of up to 1 minute after cold start can help in improving exhaust gas characteristics and reduce engine noise during warm up period.

Swirl Chamber System:

In this process, combustion is initiated in a separate chamber (swirl chamber) that has approx. 60% of the compression volume. The spherical and disc shaped swirl chamber is linked by a connecting channel to the main combustion chamber at a specific angle.

During the compression stroke, air entering via connecting channel is set in a swirling motion. The fuel is then injected so that the air swirl penetrates perpendicular to its axis and meets a hot section of the chamber wall on the opposite side of the chamber.

As soon as combustion starts, the air fuel mixture is delivered under pressure to the main combustion chamber where it mixes with the remaining air. Since the cross section of the connecting channel between swirl chamber and main combustion chamber is larger than the one in the pre-combustion chamber, the gas flow losses are relatively lower in swirl chamber design. This helps in higher internal efficiency and lesser fuel consumption. However, the combustion noise is higher in swirl chamber design.

Homogeneous Charge Compression Ignition (HCCI) engine

We have always heard about two types of engines which dominate the automotive market when it comes to internal combustion engines, namely: Spark Ignition (S.I) and Compression Ignition (C.I). Homogeneous Charge Compression Ignition (HCCI) engine is a blend of both S.I and C.I engines.

We all know that air-fuel mixture in a S.I engine is a homogeneous charge (i.e.) air and gasoline easily mix together at room temperature and require a spark plug to be ignited. Diesel on the other hand is less volatile and C.I engine can combust fuel as soon as it reaches the auto ignition temperature without the help of spark plugs. HCCI engines burn gasoline under the influence of high temperature and pressure during compression. Therefore, HCCI provides us with the best of both worlds.

Working of HCCI:

The working of HCCI is based on the four stroke Otto cycle. During the suction stroke, gasoline and air mixture is introduced in the combustion chamber. It is important to keep the air-fuel mixture much leaner in order to help in effective combustion process to take place. A conventional S.I engine has a stoichiometric air-fuel ratio of 14.7:1. But in a HCCI engine, the air-fuel mixture ranges from 24:1 to 31:1 which makes it easier for the engine to self ignite the charge.

Next follows the compression stroke. Unlike a conventional S.I engine, HCCI engines usually have a higher compression ratio up to 16:1. This helps in achieving the required temperature and pressure to auto ignite gasoline.

Just like a conventional C.I engine, the combustion takes place right at the end of compression stroke in a HCCI engine. The piston then moves from T.D.C to B.D.C to complete power stroke. Exhaust stroke follows the power stroke. The emissions are much lesser compared to both S.I and C.I engines.

Advantages of HCCI:

·         Fuel consumption can be reduced by 15% compared to S.I engine.

·         Emissions are lower due to lower peak temperature.

·         It can run on gasoline, diesel and other alternative fuels.

Disadvantages of HCCI:

·         Higher cylinder pressure can damage the cylinder.

·         Auto ignition can be difficult to control. It can lead to excessive knocking.

·         Power generated is less compared to S.I engines.

·         Cold starting is difficult without a spark plug.

How do gasoline engines differ from diesel engines?

Both gasoline and diesel engines are 4 stroke Internal Combustion Engines, but both work in a different way. These 2 engines can be distinguished by the way they ignite the fuel. While gasoline relies on spark plugs to ignite the fuel, diesel can be self ignited due to the high temperature and pressure inside the combustion chamber.

Another difference between the 2 engines in the compression ratio. The compression ratio of gasoline engines is in the range of 6:1 to 10:1, and it can go up to 12:1 for higher octane gasoline. Diesel engine's compression ratio is between 16:1 to 20:1.

Petrol engines are quick burn engines, which result in faster combustion of the fuel and hence more power is generated compared to diesel engines. Diesel engines are slow burn engines and hence more time is taken to burn the fuel, resulting in higher torque than gasoline engines.

Air-fuel mixture can be sent in the suction stroke of gasoline engines, which makes it a homogeneous mixture engine. Air and diesel don't mix well at room temperature and hence cannot be sent together in the suction stroke of diesel engine, hence it is called heterogeneous mixture engine.

To know more about the working of both petrol and diesel engines, click the link below:

Spark Ignition (S.I) engine

How Car Parts Work: Diesel Engine

Two Stroke Engine

How Car Parts Work: 6 stroke engine

Two Stroke Diesel Engines

Types of 6 Stroke Engines

Single Piston models:

·         The Bajulaz 6 Stroke Engine:

It was invented in 1989 by Roger Bajulaz of the Bajulaz S.A Company in Geneva, Switzerland. It is similar to a conventional engine with 2 additional fixed capacity chambers in the cylinder head. One of the two fixed capacity chambers is a combustion chamber and the other is an air pre-heating chamber. The combustion chamber receives a charge of heated air from the cylinder and simultaneously fuel is injected into it. Both air and fuel mix in the combustion chamber and is compressed and burnt to a high pressure. The high pressure gained is then sent to the cylinder to achieve the power stroke.

Meanwhile, the air pre-heating chamber heats the air to a high degree. The chamber is in the surrounding of the cylinder wall so that the air is heated due to the combustion process inside the cylinder. The heated air is sent to the cylinder during the 5^th stroke.

The fuel consumption can be reduced dramatically by up to 40% and also the pollution is significantly reduced.

·         Velozeta 6 Stroke Engine:

In this type, air is injected at the end of exhaust stroke (4^th stroke) so that the air expands due to the heat in the chamber and provided a supplementary expansion stroke. It is more of a gas scavenging process to clean up the combustion chamber completely before next air-fuel intake stroke. This engine showed 40% reduction in fuel consumption and was developed in the year 2005 by a team of Mechanical engineering students from India; U Krishnaraj, Boby Sebastian, Arun Nair and Aaron Joseph from College of Enginnering located in Trivandrum.

·         Dyer 6 Stroke Engine:

It was invented in 1915 and it uses water as the working fluid after exhaust stroke. Water is injected into the combustion chamber after exhaust stroke which is instantaneously converted into steam. The steam expands and forces the piston down from TDC to BDC to provide an additional power stroke. Later in 2006, Bruce Crower applied a patent for an engine working on similar principle.

·         NIYKADO 6 Stroke Engine:

This engine was designed and patented by Chanayil Cleetus Anil of Kochi, India in 2012. The model works on air injection at the end of exhaust stroke and it claims to be 23% more fuel efficient than a conventional 4 stroke engine.

  

Opposed Piston Models:

·         Beare Head:

It was designed by Malcolm Beare of Australia. This design allows 2 opposing pistons to move in a single combustion chamber. One piston is used for 4 stroke operation at the bottom end and the other opposing piston moves at half the rate of the 4 stroke piston. The opposing piston replaces the valve mechanism by opening and closing the inlet and exhaust ports. It claims to increase the power by 9%.

6 stroke engine

A 6 stroke engine is an improvement over the current 4 stroke engine used in automobiles. Consider a 4 stroke engine with 4 strokes (intake, compression, power and exhaust); a 6 stroke engine provides an extra power and exhaust strokes. The heat of combustion which is leftover after the exhaust stroke (4^th stroke) is used to create an additional expansion stroke. Air or water can be used as the fuel for the 5^th stroke.

Working Principle of 6 Stroke Engines:

The principle of 6 stroke engines is to capture the waste heat from the 4 stroke Otto cycle or Diesel cycle and utilizing the waste heat to generate an additional power stroke and exhaust stroke. Air or water is used as the working fluid for the additional power stroke.

Design of 6 stroke engines:

The design is pretty much similar to a conventional 4 stroke internal combustion engine. The piston moves up and down the combustion chamber. Apart from the existing intake and exhaust valves, there are additional air suction valve and air exhaust valve for the 5^th and 6^th strokes respectively.

WORKING OF 6 STROKE ENGINES:

1^st Stroke (Intake stroke):

Consider a 6 stroke engine with Otto cycle; the intake valve opens and air-fuel mixture is sucked into the combustion chamber due to the piston movement from top dead centre (TDC) to bottom dead centre (BDC). The movement of piston from TDC to BDC creates negative pressure in the cylinder, which sucks in more air-fuel mixture.

2^nd Stroke (Compression stroke):

In the compression stroke, all 4 valves will remain closed. The air-fuel mixture is trapped inside the combustion chamber. Now the piston starts moving from BDC to TDC and compressed the air-fuel mixture inside the combustion chamber.

3^rd Stroke (Power stroke):

Just before the piston reaches the TDC in the combustion chamber, a spark plug ignites the compressed air-fuel mixture. This generates a lot of heat and the piston is forced to move down from TDC to BDC. The expansion of gases releases energy (horsepower) which is used to run the vehicle. All 4 valves remain closed during this operation.

4^th Stroke (Exhaust stroke):

The exhaust valve opens and the movement of piston from BDC to TDC pushes the exhaust gases out through the exhaust valve. Now we are quite familiar with these 4 strokes in a conventional 4 stroke engine. We also are aware of the fact that there is some leftover heat in the combustion chamber after the exhaust stroke. The leftover heat is used in the 5^th stroke.

5^th Stroke (Air Suction):

Fresh air from the atmosphere is sucked in through the air suction valve. The high temperature inside the combustion chamber heats the fresh air and this leads to expansion of air which forces the piston to move down from TDC to BDC to provide an additional power stroke. The air can also be pre-heated before being sent into the combustion chamber. Pre-heated air will result in better expansion.       

6^th Stroke (Air Exhaust):

The air exhaust valve opens and as the piston moves up from BDC to TDC, the expanded air is sent out through the exhaust. This provides better gas scavenging.

In some cases, even water can be injected inside the combustion chamber after 4^th stroke. The water is converted into steam due to heat and creates an auxiliary power stroke.

Advantages of 6 Stroke Engines:

·         Higher fuel economy up to 40%

·         No cooling system required as heat is carried away during 5^th and 6^th stroke

·         Significant reduction in emissions

·         Two power strokes for 6 strokes

·         Can run on multiple fuels, including LPG

Types of 6 Stroke Engines:

There are several designs of 6 stroke engines that work on different concepts to generate an additional power stroke and exhaust stroke.

Types of 6 stroke engines

  

      Related Topics:

• Spark Ignition Engines (Gasoline)

• Two Stroke Engine

• Diesel Engine

• Rotary Engines

• Engine Layouts

• Two stroke Diesel Engine

• Peugeot MCE-5 VCR Engine

• Variable Compression Ratio Engine

Diesel Engine

Just like an S.I engine, C.I engine also comprises of four strokes for the entire combustion process to occur. Unlike a typical S.I engine where air and fuel are mixed and supplied to the combustion chamber, only air is sent during the suction stroke in C.I engine since air and diesel mixture is a heterogeneous mixture. Diesel is injected at the end of compression stroke. C.I engines don't require spark plugs to ignite the fuel.

SUCTION STROKE:

The inlet valve is open during the suction stroke allowing the air from the atmosphere to enter the combustion chamber. The piston moves from t.d.c to b.d.c, thereby creating suction and sucking more air. The inlet valve closes after the piston reaches b.d.c and the air gets trapped inside the combustion chamber. Due to this piston movement, a very low pressure of about 0.1 bar or less is created inside the cylinder.

COMPRESSION STROKE:

Now both the inlet and exhaust valves remain closed and the piston moves upward from b.d.c to t.d.c. This compresses the air inside the cylinder and increases the pressure and temperature. The pressure achieved is around 30 to 50 bar and the temperature reached is about 600 °C. This pressure and temperature is enough to ignite the fuel that will be injected at the end of compression. The compression ratio of a C.I engine varies from 16:1 to 20:1.

POWER STROKE:

Both the inlet and the exhaust valves remain closed. Diesel is injected into the combustion chamber at the end of compression stroke. The rate of combustion will depend on how fine the diesel is atomized by the fuel injection system. The higher the injection pressure, the finer will be the atomization of diesel. The highly pressurized diesel is injected into the heated air and the heat inside the combustion chamber due to compression is enough to ignite the fuel. The intense heat will vaporize the fuel droplets and burn it releasing heat energy. This energy pushes the piston from t.d.c to b.d.c and the energy is used to drive the crankshaft.

EXHAUST STROKE:

The exhaust valve opens and now the piston moves from b.d.c to t.d.c pushing the exhaust gases out. This completes the four strokes of C.I engine. The exhaust gases are sent to the atmosphere through the exhaust pipe.

Peugeot MCE-5 Variable Compression Ratio Engine

The engineers at Peugeot used a conventional rod-crank mechanism. The use of a 2-stage turbochargers helps in improving the engine performance. With the combination of VCR and the turbocharger, the engine’s power is doubled and the torque is tripled compared to a conventional gasoline engine.

The MCE-5 engine’s compression ratio changes according to the driving conditions. This allows the fuel consumption to reduce significantly up to 35%. This makes the MCE-5 concept the most efficient VCR till date. The compression ratio can be varied from 7:1 to 18:1.

The MCE-5 VCR engine is a flexible engine allowing each cylinder to have an independent compression ratio.

The MCE-5 varies compression ratio with the help of a rod-crank mechanism, gears and actuators. As you can see in the figure, a common cylinder head is used for the combustion chamber and the control jack chamber. Control jack is provided for each combustion chamber. Control jack is also known as the secondary piston. The Vertical movement of the control jack is controlled hydraulically. Both the piston and control jack move parallel to each other in the cylinder head.



The control jack is provided with a rack at its lower end which engages with a gear wheel which is pivoted to the smaller end of the connecting rod. The opposite side of this gear wheel also has gear teeth which mesh with a rack on the lower end of the piston. The other side of the piston rack is provided with another rack with finer teeth that drives a small gear wheel. As the piston moves up and down, the motion from the piston is transferred to the crankshaft via the piston rack, larger gear wheel and the connecting rod.

The compression ratio can be varied by moving the control jack. As the control jack moves up, the larger gear wheel pivots about the small end of the connecting rod, thereby pulling the piston down in the combustion chamber. As a result, the compression ratio is lowered. On the contrary, as the control jack moves down, the piston moves up and raises the compression ratio.

MCE-5 is a 1.5 liter engine producing 220 hp and 420 Nm torque. The engine was first used in Peugeot 407 model, launched in 2009 at the Geneva Auto show.

To know more about VCR Engines, click on the following link:

SAAB SVC Engine

Variable Compression Ratio Engines

Variable Compression Ratio (VCR) is a technique to vary the compression ratio of an engine under varying loads. Engine operating at higher loads require lower compression ratio, whereas an engine operating under low loads require higher compression ratio. VCR engines provide the best of both worlds. This adjustment can significantly improve an engine’s fuel efficiency.

Compression ratio is the ratio of total cylinder volume when the piston is at Bottom Dead Centre (BDC) to the clearance volume in the cylinder when piston is at Top Dead Centre (TDC). In order to vary the compression ratio, the clearance volume in the cylinder has to be varied.

Variable compression ratio can be achieved by the following methods:

• By manufacturing an engine with a cylinder head that can pivot about a point.


• By varying the piston head height.


• By varying the connecting rod length.

SAAB Variable Compression (SVC) Engine:

The SAAB Variable compression engine has a cylinder head (also known as monohead) hinged on one end to the crankcase. The cylinder head can be tilted to different angles to achieve variable compression ratios.

Higher compression ratio is achieved at lower loads for higher fuel efficiency and lower compression ratio is required at higher loads to prevent knocking.

The compression ratio varies from 8:1 to 14:1. The engine is employed with a special device known as the eccentric shaft. The eccentric shaft when rotated tilts the monohead (cylinder head) to a certain angle to change the compression ratio. The eccentric shaft is turned with the help of a hydraulic actuator which provided a hydraulic pressure in the range of 60 to 100 bar. The monohead can be tilted to an angle of 4.1 degree.

The SVC is a 1.6 liter 5 cylinder inline engine whose fuel efficiency is 30% better than a conventional gasoline engine. The engine generates 220 hp at 5800 rpm and a maximum torque of 305 Nm at 4000 rpm.





To know more about a different type of VCR engine, Check the following link:

Peugeot MCE-5  VCR Engine

Turbochargers

What is a Turbocharger?

When someone asks you a way of increasing the power of an internal combustion engine, then immediately we start thinking about turbochargers. It can considerably increase engine power without adding any significant weight.

Turbochargers forcefully introduce more amount of compressed air into the combustion chamber. This allows the fuel injection system to supply more fuel into the cylinder and therefore increase the engine horsepower. The advantage of a turbocharger is that it increases the power-to-weight ratio of the engine, making it more efficient.

Who Invented Turbochargers?

The first turbocharger ever designed was patented by Alfred J. Büchi on November 16, 1905. He was a Swiss automotive engineer and is known as the inventor of turbochargers.

How does a Turbocharger work?

A turbocharger is run by the exhaust gases coming out of the engine. The exhaust gases rotate the turbine, which in turn runs the compressor. The compressor draws more amount of air and compresses it. The compressed air is cooled and sent to the inlet manifold. Turbochargers can spin up to speeds in the range of 1,00,000 to 1,50,000 RPM.

The normal atmospheric pressure of air is 1.013 bar. A typical turbocharger can provide a boost of 0.4 to 0.6 bar. Therefore it can provide almost 40% to 50% more air than an engine without a turbocharger.

Turbocharger Design and Operation:

One can find a turbocharger installed in the exhaust manifold of the engine. Turbocharger is made of a turbine and a compressor, both connected with a single shaft. Most of the modern day turbochargers are provided with a wastegate in the turbine side.

The exhaust gases flow through the turbine blades and spins the turbine. The compressor on the other side also starts spinning at the same speed as that of the turbine. The compressor draws the air in and then compresses it due to its centrifugal action.

The cool air entering the compressor is squeezed and heated and then it is blown out. The compressed air passes through a heat exchanger to cool it down, as cooler air has more density.

Now the cooled, compressed air is sent to the engine intake. More amount of air helps in faster combustion and also allows more fuel to be injected inside the combustion chamber. As more fuel is burned, more power is produced.

Purpose of a wastegate in a Turbocharger:

At higher engine speeds, the turbochargers spin faster than in a normal speed. This can result in a higher boost pressure from the turbocharger. Wastegate prevents the boost pressure to cross the maximum limit by bypassing the exhaust gases to the exhaust pipe. This also prevents the turbine from spinning at speeds higher than its maximum limit.

Turbo Lag:

One of the main disadvantages of using a turbocharger is that they don’t provide immediate boost pressure as soon as the engine is started. It takes some time to produce enough exhaust gas to spool the turbines fast enough to generate the required boost pressure. This is known as turbo lag.

Turbo lab can be reduced by using a smaller turbocharger in which boost pressure can be created at lower speeds. But smaller turbocharger won’t be able to generate the required boost pressure at higher speeds and there is more risk of damage to the turbine at higher speeds.

To overcome the problem mentioned in the previous paragraph, some cars are installed with 2 turbochargers. One small sized turbo to provide boost pressure at lower speeds. Whereas the other turbo which is larger, provides boost at higher speeds.

Related Topic:

• Variable Geometry Turbocharger 

Rotary Engines

Rotary engines, also known as Wankel engines is a type of internal combustion engine in which the method of operation is different compared to a conventional four stroke engine. Rotary engine adopts eccentric rotary design to complete the combustion process.

In a four stroke engine, all four jobs- intake, compression, combustion and exhaust takes place in the same volume space of the cylinder. Whereas, in a rotary engine all jobs take place at different parts of the engine housing.

History:

Wankel Rotary engine was patented by a German  engineer, Felix Wankel in 1929. NSU Motorenwerke completed a working prototype of the design in 1957.

Design:

Wankel rotary engine employs a eccentric triangular rotor instead of a cylindrical piston in order to compress and burn the air-fuel mixture. The housing is oval or epitrochoid ( as shown in fig 1, a roulette traced by a point 'p' attached to a circle of radius 'a' rolling outside the circle of a radius 'r' ) in shape. The use of a triangular rotor and the epitrochoid design of the housing creates three different volumes of gas inside the housing.

Fig 1: Epitrochoid explanation

There is an intake port on the upper left portion of the housing, and an exhaust port at the lower left portion of the housing. One or two slot(s) are provided at the mid right portion of the housing to house the spark plugs.

The entire housing is enclosed in coolant jackets. An eccentric shaft (output shaft) runs through the centre of the housing. It has round lobes mounted eccentrically around the output shaft, therefore making the lobes slightly offset from the axis of the output shaft.

A triangular shaped rotor is fixed over the rotor journal provided in the eccentric shaft. Internal gears are provided in the rotor that mesh with the central output shaft or the eccentric shaft.

Each face of the rotor is provided with a cavity or pocket to help in the combustion process. The cavity increases the displacement of the engine and also allows some space to the air-fuel charge.

The apex of each face acts as a seal for the three volumes of gas to the outside of the chamber.

Assembly:

The rotary engine is made of several layers:

• The outermost layer is a layer of coolant jacket. Coolant liquid flows through the passages provided in the jacket.


• The next layer from outside is the oval or epitrochoid housing layer which also has a exhaust port. This layer is very smooth so that it can help the rotor to rotate with minimum friction.


• The next layer is the centre piece which contains the inlet port.


• The centre piece has a circular port in the centre which houses the output shaft with circular lobes. The rotor rotates around the circular lobe and it also has internal gear that is meshed with a smaller gear fixed to the housing.

Working:

As the rotor rotates around the chamber, all three volumes of gas expands and compresses alternately. This alternate expansion and compression of the volumes creates a suction force for the air-fuel mixture, then compresses the fresh charge, then ignites the compressed charge with the help of spark plugs and eventually drives out the exhaust gases.


The offset lobe on the output shaft will rotate three times for every one revolution of the rotor. When the rotor rotates, the three chambers created keep varying in sizes.

• Intake :  Intake starts when one of the apex of the rotor crosses the inlet port. The volume of the chamber expands, thereby drawing in  more air-fuel mixture. The intake stroke is over when the consecutive apex crosses the inlet port and no more air-fuel charge enters the chamber.

• Compression: As the rotor continues its rotation in the housing, the volume of the chamber keeps getting reduced and the air-fuel mixture is compressed.

• Combustion: As soon as the face of the rotor makes it around the spark plugs, combustion process starts. Normally two spark plugs are employed for complete combustion. This produces enormous power that pushes the rotor in the direction that increases the volume of the chamber filled with exhaust gases unless the peak of the rotor crosses the exhaust port.

• Exhaust: Once the apex of the rotor crosses the exhaust port, the combusted gases are free to move out through the exhaust port to the tail pipe. The continuous rotation of the rotor shrinks the chamber volume and this forces the exhaust gases to move out of the chamber. Later the apex reaches the inlet port and the entire cycle starts again.

One salient feature about the rotary engine is that all three faces can act as a combustion chamber and thus three combustion or power stroke per revolution of the rotor. But the output shaft rotates thrice for every one revolution of the rotor. Therefore, one combustion stroke per revolution of the output shaft.


Advantages:

• A four stroke piston engine has a minimum of 40 moving parts including camshaft, crankshaft, valves, rockers, timing gears, etc. Whereas, rotary engines have only three main moving parts (i.e) two rotors and a output shaft.


• The entire operation is smoother. Rotary engines are balanced internally with counterweights to cut down the vibration.


• The power delivery is smooth. For every one revolution of rotor, three combustion strokes are produced.


• Rotary engines are reliable because of the slower moving parts such as the rotor which has one-third of the speed of the output shaft.

Disadvantages:

• Doesn't meet the emission norms set by the U.S government.


• Expensive in production compared to piston engines.


• Higher fuel consumption.


• Lower compression ratio.

Two Stroke Diesel Engines

The main difference between two stroke and four stroke engines is the power produced by both the engines. Two stroke engines fires once in one revolution of the crankshaft, whereas the four stroke engine fires once in 2 revolutions of the crankshaft. Therefore, the two stroke engines are capable of producing more power per revolution.

In a gasoline powered two stroke engine, we know that there is a disadvantage of some amount of gasoline escaping along with the exhaust gases without getting burnt during the process of scavenging.

In the case of diesel powered two stroke engines, only air is used as a fresh charge, so there is no fuel wastage. Diesel is injected with the help of a fuel injector only after the air is compressed up to 1/18th of the cylinder volume. The compression of air increases the temperature and pressure inside the combustion chamber high enough to burn the diesel without the application of spark plugs.

Working:

The two stroke diesel engine has an air inlet port through which air is constantly tried to be pushed inside the combustion chamber with the help of an air pump. Two or four exhaust valves are provided at the top which open at the same time to let the exhaust gases to escape. A diesel injector is provided at the top to inject the diesel at the precise timing. There is no spark plug.

Power Stroke:

1. When the piston starts moving towards the Bottom Dead Centre (BDC) from the Top Dead Centre (TDC), the exhaust valve opens by means of camshaft and the exhaust gases escape out through it.


2. Further moving down, the piston uncovers the air inlet port and allows the air to fill inside the combustion chamber. The fresh air charge also drives out the remaining exhaust gases out of the exhaust valve.

Compression Stroke:

1. After reaching the BDC, the piston starts moving towards the TDC. The exhaust valve is closed and no more fresh air escapes out through the exhaust valve.


2. Further moving up, the piston covers the air inlet port and starts compressing the air trapped inside the combustion chamber to almost 1/18th of its volume.


3. The temperature and pressure rises inside the combustion chamber and just before the piston reaches TDC, diesel is injected inside the combustion chamber.


4. The fuel is burnt and tremendous amount of energy is released and the steps in the power stroke repeats itself.

A two stroke diesel engine must have a turbocharger or a supercharger to push more air inside the combustion chamber. Thus it is more expensive than a two stroke gasoline engine.

Variable Valve Timing

Variable valve timing is a method to improve performance, fuel economy and to reduce emissions by varying the valve lift timing. Inlet valve is used to allow the combustible mixture to enter the combustion chamber. Varying the inlet valve lift timing can lead to a significant change in the performance of the engine.

Engine requires large amount of air at high speeds, therefore a shorter inlet valve lift duration doesn't let the required amount of air to enter the combustion chamber and this affects the performance. On the other hand, at lower speeds if the valves are open for a prolonged time then the fuel would escape unburnt leading to higher emission levels.

Various adjustments that can be implemented:

1. Earlier closing of inlet valve: This is an effective way of increasing the fuel economy. At low load conditions, the amount of combustible mixture required is less, therefore closing the inlet valve earlier allows lesser amount of combustible mixture to enter the combustion chamber. The temperature inside the combustion chamber is significantly reduced, thereby reducing nitric oxide emissions by 25%. But it can increase hydrocarbon emissions. It can reduce pumping losses up to 40%.


2. Late closing of inlet valve: Closing the inlet valve late allows some of the combustible mixture to flow back to the inlet manifold, thus increasing the pressure inside the inlet manifold. The next stroke will allow the combustible mixture to flow in at a higher pressure, thereby reducing pumping losses by 40% at low load. It also reduces nitric oxide emissions by 25%.


3. Earlier opening of intake valve: This is related to valve overlapping where few amount of exhaust gases enter the inlet manifold via the inlet valve and it is cooled. In the next inlet stroke, the exhaust gases enter the combustion chamber, thus helping in reducing the temperature and results in lesser nitric oxide emissions. This is favourable for low load conditions.


4. Earlier closing of exhaust valve: Closing the exhaust valve early can trap few amount of exhaust gases inside. This allows lesser amount of air-fuel mixture to enter inside the cylinder in the next stroke, hence increasing fuel efficiency.


5. Late closing of exhaust valve: Closing the exhaust valve late allows the entire exhaust gases to escape to the atmosphere, hence allowing more air-fuel charge in the intake stroke.

HONDA VTEC:

Honda engines are known for its VTEC (Variable Valve Timing and Lift Electronic Control). It is a combination of mechanical and electrical system allowing the engine to have multiple camshafts. The camshafts are provided with lobes of varying sizes that can alter the timing of valve operation at different speeds.

The basic function of a VTEC system is to allow the inlet valves to open for a longer duration at higher speeds. This will allow more air to flow into the engine, thereby the engine can burn more fuel and generate more power. Rocker arms are used to open and close the valves. The rotation of cam moves the rocker arm and the rocker arm operates the valve. Rocker arm shafts are provided with an oil supply line which comes from the solenoid. The solenoid varies the oil pressure based on the signals from the ECU (Electronic Control Unit). There are 3 cam lobes provided for one cylinder. One big lobe at the centre and 2 smaller lobes set apart by the bigger lobe. Similarly, there are 3 rocker arms, two for the smaller cam lobes and one for the bigger lobe. the oil supply line is connected to the centre rocker arm.

At lower speeds, the rocker arms are operated by the 2 smaller lobes to allow less air-fuel mixture to enter the cylinder. At higher rpm, the solenoid increases the oil pressure and there are pins in the centre rocker arm that are pushed outside and these pins sit inside a slot provided in the other 2 rocker arms. The pins lock the other 2 rocker arms and now all three rocker arms work as a single unit. The rocker arm unit is operated by the bigger lobe and hence allowing more air-fuel mixture to enter the cylinder.

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.

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.