SPECIFIC FUEL OIL CONSUMPTION (SFOC)

Brake Specific Fuel Oil Consumption of a Marine Diesel Engine (SFOC)

What’s Brake in the heading??

Brake simply refers to the measure of fuel efficiency in an engine which burns fuel and as an output delivers or rotates a shaft. It’s unit will be g/Kw.Hr

What if we are asked to calculate Thrust Specific Fuel Oil Consumption??

Thrust SFOC is related to calculation of fuel efficiency in an engine which burns fuel to produce a thrust as an output i.e Rocket, Aeroplanes etc.  It’s Unit will be g/s.kN

BSFOC or SFOC Calculation

As mentioned above SFOC unit of measurement is g/Kw.Hr.

Where

g = Grams (Weight)

Kw = Kilowatt (Power)

Hr. = Hour (Time)

So what exactly are we talking about??

Weight here is referring to the weight of the Fuel. i.e. HFO or it can be MDO

How to calculate this weight??

We all know the formula for density

Density (ρ) = Mass (m)/Volume (V)

Unit of Mass = Kg (Kilogram)

Unit of Volume = m³  (Meter Cube)

From where will we get this data??

Density (ρ)- This is Obtained from BDN (Bunker delivery note). 

Very Important – Density mentioned in BDN is at 15⁰ C but the fuel which is entering the engine is at 125-135⁰C.  Therefore it is important that correction factors are taken into consideration at this time.  Make use of ISO 91-1 Tables to the maximum to get the most accurate results.

Mass is what we are looking to calculate.

Volume = This is obtained by flowmeter. (Consider the number of hours you need to calculate SFOC for and take values accordingly).  Unit here obtained from flowmeter will be in Litres.  Now in order to convert this to m³  (Meter Cube) we have to divide this value by 1000. i.e

m³  (Meter Cube) = L (Flowmeter Reading)/1000

We now have Density and the Volume, so now we can easily calculate Mass as

Mass = Density (ρ)X m³  (Meter Cube). The unit of this calculation may be in Kilogram (Kg) therefore to convert this to Grams (g) we need to multiply this value with 1000.

Next step is to calculate the Engine power.

Before we get into the math it is important that we understand the different kind of power terminology.

Indicated Power – This is the power which is developed within a cylinder of an engine

Brake Power – This is the useful power which is available at the shaft output.

Fuel Power = Mass of Fuel/s x Fuel Calorific Value

Mechanical Efficiency is the ratio of Brake power to Indicated Power.  

Always remember that Brake power is less than Indicated power as brake power accounts the friction losses within an engine.

How do we calculate Brake power and Indicated Power???

Brake power is calculate by using the formula

BP=2πNT

N= Shaft Speed in rev/sec

T= Torque in Nm (Newton meter)

Torque is measured/calculated by use of a dynamo meters.  Usually Torque is measured by using the formula

T = net brake force x radius

Indicated Power:

Primary means to calculate I.P. is by use of an indicator.  Indicator gives you an idea about MEP (Mean effective pressure in cylinder and at the same time provides relevant graph (2 stroke engine) of the fuel injector condition and how good is the compression and combustion.

Indicated Power is calculated by the using the formula

IP = pLAN

Where

p = MEP (Mean effective pressure)                                               

L = is the stroke length

A = Area of the piston

N = Number of cycles per second

How this formula is derived???

We know that Power is calculated by

P= Work done/Time or Work done x Number of cycles

Now to calculate Work Done we use the formula

Work Done = Force x Distance Moved

We all know that Force = Pressure x Area

Therefore we have

Workdone = Pressure x Area x Distance Moved

Therefore Power = Pressure x Area x Distance Moved x Number of Cycles

From where will we get these values???

Pressure or the Mean effective pressure is obtained by means of Indicator. 

Use this formula when calculating the MEP

MEP = S_p x H

Where

S_p = Spring Constant (Pls refer your indicator manual)     

H = is the average height of the graph.  This in turn is calculated by dividing the Total area of the graph by base length of the diagram.

Area is the area of piston

Distance moved is the stroke length (this is obtained from Engine Manual)

Number of Cycles is from your daily counter (You can make use of value from Engine pickup as well)

Always remember N for a four stroke engine is half the value i.e. N/2

So now we have Indicated power per Unit.  Calculate the average Indicated power for all the cylinders i.e

IP_Avg  = IP1 + IP2 + IP3…………+IPN

                N (Number of Cylinders)

So now we have the average Indicated Power of all the cylinders.

On board a ship we normally use the indicated power for calculating the SFOC

Let’s now recall

SFOC = g/KWhr

We have Calculated Grams

We have Calculated KW (Indicated Power)

Hr = Time (Measuring period)

At the end Let’s summarise:

SFOC = g/KwHr

SFOC =        ρ x m³x 1000 (Time in which this mass of fuel was consumed can be 1hr 

                                    MEP x L x A x N x Hr (Time observed same as above)

Important Efficiency Calculation that we need to know

Brake Thermal Efficiency = Brake Power / Fuel Power

Indicated Thermal efficiency = Indicated Power / Fuel Power

Mechanical Efficiency = Brake Power / Indicated Power

Sometimes you may be asked to calculate SFOC making use of the Lower Calorific value correction factor.  This is done as on test bed the fuel used is of different calorific value. (The value of test bed fuel calorific value is available in Engine manual).

In order to determine the caloric value of fuel on board ship, same is sent to the laborartory for testing and the value obtained.  Once this value is available following formula is used to determine corrected SFOC

SFOC x LCV obtained from Lab

              LCV at Test bed

LCV = Lower Calorific Value

LCV can be obtained from the Lab as well as from ISO graphs if available in the manual.

Tie Rods of a Diesel Engine

What is a Tie Rod???

A tie Rod is a Hydraulically Tightened Long Stud which joins different components of an Engine.  The hydraulically tightened tie rods are specifically positioned to maintain a static preloading of the engine block to absorb the dynamic loads generated by the impact from the combustion process and moving masses. 

So what exactly is a Tie Rod Holding???

This rod holds the three major engine components i.e. Cylinder block or entablature, “A” frame, and crankcase in compression and transmits the firing load to the bedplate. The tie rods are fitted through the above mentioned components and are hydraulically tightened so that the whole engine can be held in compression. As per the design these tie rods are placed as close as possible to the centerline of the crankshaft in order to minimize the bending moment of the transverse girder.

We all know that above definition is for a Two Stroke Engine, Can we have a tie rod on a Four Stroke Engine???

The answer is Yes.  Surprised?  Let’s go Back to the school.

Remember Underslung Crankshaft?  Underslung Crankshaft’s are supported by means of main bearing caps i.e they are suspended in the engine frame and supported by Main bearing caps.

Always remember a two stroke engine has a Bed Plate which supports the crankshaft but on a four stroke engine with a monoblock design the crankshaft is suspended in air and supported by main bearing caps.

So if a stud is used to support the underslung crankshaft will it be called a Tie rod???

No.  The main function of a tie rod is to reduce the bending stresses and at the same time act as means to transmit all the load on the bearing caps back to the frame. Such engines may also makes use of the side tie bolts which locate the bearing cap, and prevent sideways movement.

A stud does not transmit any firing forces nor prevent bending moment, but simply holds components together.  For Example a cylinder head is bolted to Engine block by means of Studs.

For such four stroke engines where the Crankshaft is underslung, the load on the bearing caps is transferred back to the frame by the use of tie bolts. The engine also makes use of the side tie bolts which locate the bearing cap, and prevent sideways movement.

ENGINE TIMING AND TERMINOLOGY

Now that we have learnt about engine classification let's now try and understand Engine Timing and associated Terminology.

What is meant by Engine Timing?

Engine timing is the determination of what happens when inside an engine cylinder i.e at what position and at what instance we have Induction, Compression, Ignition and Exhaust.  If all these strokes don't happen at the prescribed/designed time and instance than the engine will not work as it should.

Lets take a simple example.

If we are not experiencing ignition of fuel at a position of piston where it normally occurs than we are likely to experience unfavorable combustion and poor engine performance.  As a result it becomes very important for us to understand as to what is happening when inside an engine cylinder so that we can troubleshoot accordingly.

So what all timing cycles are we looking at?

Universally all the engines have 5 cycles i.e. INDUCTION, COMPRESSION, IGNITION, EXPANSION and EXHAUST .  Let it be 4 stroke engine or 2 stroke engine, both of hem have to undergo these 5 cycles.

So what's the meaning of these 4 cycles?

INDUCTION - The induction system provides the engine with an adequate supply of clean air for good combustion (and for scavenging cylinders on two and four stroke-cycle engines) for all operating speeds, loads, and operating conditions.

COMPONENTS WHICH ARE LIKELY TO BE FITTED IN AN INDUCTION SYSTEM

• On a naturally aspirated four-stroke-cycle engine, the system includes the air cleaner, a precleaner (if used), the intake manifold, and the connecting tubing and pipes 
•  On the two-stroke cycle engine, the system also includes a blower for scavenging air and for combustion.  A four stroke engine may also have a blower. 
• On a turbocharged engine, additional air is supplied by means of a turbocharger, which is exhaust gas–driven. 
• On a supercharged engine a mechanically driven blower is used to supply additional air. An air shutdown valve may be included to allow engine intake air to be shut off completely for emergency engine shutdown.
• An intercooler or after cooler may also be included in the induction system. Since cooler air is more dense, a greater amount of air is in fact supplied if the air is cooled. 
• The intercooler is mounted to cool the intake air after it leaves the discharge side of the turbocharger and before it enters the engine (before it enters the blower on two-stroke-cycle diesels). 
• The after cooler is mounted in the two stroke- cycle diesel engine block so that it will cool intake air before it enters the cylinder ports. Air-to air aftercoolers are mounted in front of the radiator.

At this point it becomes important for us to understand the meaning of these 5 terms:

1. INTER COOLER - Is an air cooling device which is placed between the Engine turbocharger or Supercharger and the engines air intake manifold.
2. AFTER COOLER - Is an air cooling device which is placed between the LAST Engine turbocharger or Supercharger in series and engines air intake manifold.  After Cooler is the last cooling stage before the engine.  We can only say that intercooler is connected between the turbochargers/superchargers in series and the last intercooler which is connected to the engines manifold is called the After Cooler.  Therefore Intercooler is After cooler.
3. RADIATOR - Radiators are heat exchangers which can be used for heating as well as cooling.  Radiators are mainly used in cars and are used for cooling the water which circulates around the engine.  
4. SUPERCHARGERS - Supercharger is  typically driven by a belt or a gear powered by the engine. superchargers are good because the horsepower boost is available immediately however supercharger takes the power away from the engine.
5. TURBOCHARGERS - The turbocharger creates pressure using the already-burnt exhaust gas that is coming out of the engine to turn the blades of a fan that forces the air into the engine. Turbocharger takes a moment to "spool up" from the exhaust gas.  Turbocharger does not take power away from the engine. 

COMPRESSION - Means the reduction in volume and increase in pressure of the air or combustible mixture in the cylinder prior to combustion.  This is achieved by piston which is moving upwards in cylinder.  Also compression of gas naturally increases the temperature.

IGNITION - Means the act of igniting fuel in cylinder.  The fuel is injected into the cylinder through fuel injectors and this is ignited by either a spark plug or by the heat produced in the cylinder due to compression.

EXPANSION - After the ignition the combustion occurs in the cylinder which is nothing but a "blast" which causes a rapid expansion of gases which causes the piston to move downward.

  

Now in many texts you will find one more stroke called the power stroke which defines that blast which occurs in cylinder.  This blast is what defines the intensity of combustion occurring in the cylinder meaning if we were to measure the value of this blast, the greater the value higher the pressure in the cylinder.

EXHAUST - Also there exits another stroke which is called the Exhaust stroke which happens, where all the burnt gases are removed from the cylinder due to the upward movement of the piston.

We can measure certain values within a cylinder to determine the condition of combustion which are PEAK PRESSURE and COMPRESSION PRESSURES. (we will look more into these two terms in my upcoming posts, but first we must understand the difference between two 2 stroke engine and 4 stroke engine)

2 STROKE ENGINE AND 4 STROKE ENGINE

In my previous post on ENGINE CLASSIFICATION we have seen that there exists 2 stroke and 4 stroke engines.  Lets now first of all take a look at the differences between the two. 

Lets try and keep this as simple as possible.  

1. A 2 stroke engine fires once every revolution where as 4 stroke engines fire once every two revolutions.
2. In a 2 stroke engine compression stroke is followed by ignition of compressed air and fuel and on the return stroke the same happens.  In 4 stroke engines there is 1 compression stroke and 1 exhaust stroke.  In compression stroke the fuel/air mixture is compressed prior to blast where as in exhaust stroke the burnt gases are simply pushed out of the cylinder.
3. The firing frequency of 2 stroke engine is every revolution where as for a 4 stroke engine is once every 2 revolution.
4. 2 stroke engines make use of cross heads while 4 stroke engines do not make use of cross heads.  They have the connecting rod connected to the piston by means of gudgeon pin and hence do not have a piston rod.
5. In a 2 stroke engine scavenging takes place during the latter part of the downward stroke i.e expansion stroke and the early part of upward stroke i.e compression stroke whereas in a 4 stroke engine scavenging takes place when the piston is nearing and passing TDC during the latter part of upward stroke (Exhaust stroke) and the early part of the downward stroke (intake stroke).
6. Also majority of 2 stroke engines make use of inlet ports and not inlet valves unlike 4 stroke engines which make use of inlet valves.
7. A major difference can be that of lubrication.  2 stroke engines make use of forced lubrication when it comes to piston to liner motion where as 4 stroke engines make use of splash lubrication. Some of the 2 stroke engines make use of oil and fuel premix like the chain saw engines. 
8. Two stroke engines have higher power to weight ratio than 4 stroke engines
9. Two stroke engines are easier and cheaper to build than 4 stroke engines

Please note that there exists certain number of engines which do not comply to point 6 i.e 2 stroke engines can have inlet valves, it entirely depend upon the design and make of the engine. There are 2 stroke engines being made with Poppet valves in the cylinder head.

Ref. point 4 where it has been said that 2 strokes engines make use of cross head engines, please note that there are certain number of 2 stroke engines which have a construction similar to that of a 4 stroke engine without cross head. This point is also obsolete.  

Ref to points 8 and 9 please note that there were small 2 stroke engines which were chainsaw engines used in bikes in the late 19th and 20th centuries.  These engines were small compact engine with higher power to weight ratio.  Now that these engines are obsolete points 6 and 7 no longer should be used when it becomes to distinguishing between 2 stroke and 4 stroke engines.  (I have mentioned these points only because you will find them in many texts).

The only major difference between the two is that "2 STROKE ENGINE CAN PRODUCE TWICE THE POWER OF A 4 STROKE ENGINE OF SAME SIZED ENGINE AND SAME REVS".

THE 2 STROKE CYCLE WITHOUT CROSS HEAD

4 STROKE ENGINE WITH SPARK PLUG

4 STROKE ENGINE WITHOUT SPARK PLUG

             

Diagram representing a typical spark plug engine
Above image source - how stuff works inc

Figure shows a typical 2 stroke engine with cross head

Now that we have learnt the differences between a 2 stroke engine and 4 stroke engine it is now time for us to understand the TIMING CYCLE.

Always remember that each stroke is one half of a revolution or 180 degree of cam movement.  All four strokes i.e 4x180=720 degrees represent two complete revolutions of the crank for a 4 stroke engine.

TIMING CYCLE OF A 4 STROKE ENGINE

Timing diagram of a typical 4 stroke engine

Lets now take a look what is happening here

INDUCTION STROKE - Inlet valve opens and the piston is moving from Top Dead Center (TDC) to Bottom Dead Center (BDC) at the end of exhaust stroke.  In this stroke air is either sucked into the cylinder due to the build up of negative pressure as the piston is moving down or forced into the cylinder by means of a turbocharger.

Inlet valve is made to open 8 degrees to 17 degrees before the piston reaches TDC as show in the figure above, however this value may vary from engine to engine and it can range from 5 degrees to 30 degrees as per engine design.  This is called Inlet valve opening lead.

Here the question arises "WHY THE INLET VALVE IS OPENING BEFORE THE PISTON REACHES THE TDC"?

The valve is made to open before the piston reaches the TDC because of the following reasons:

• The opening area of the valve is very small once the valve opens i.e leaves the seat
• As the opening area of the valve is small therefore the intake efficiency is low as the intake resistance is high due to smaller opening area
• By the time valve would have opened (if it were to open at TDC), the piston would have moved considerably down the bore, and by the time valve will close, the opening of the valve would have been minimal

This means that if the valve was made to open at TDC or after, than we are introducing less air into the cylinder and if the valve is made to open too early before the piston reaches TDC there is a possibility of exhaust flowing into the intake manifold. 

At the end of induction stroke Inlet valve is made to close after BDC as shown in the figure above (44 degrees after BDC and 59 Degrees after BDC).  This value can however range from 40 degrees to 60 degrees as per engine design.  This is called inlet valve closing lag.

Here the question arises "WHY THE INLET VALVE IS CLOSING AFTER THE PISTON LEAVES THE BDC"?

The valve is made to close after the piston leaves the BDC because of the following reasons:

• Once the piston has reached the BDC the air pressure in the cylinder is still lower than the atmospheric pressure
• Air will continue to flow in the cylinder till the pressure inside the cylinder is lower than the pressure in the intake manifold
• Air intake can be increased by delaying the closure of the inlet valve at the end of intake stroke
• Delaying the closure of the inlet valve forces more air or air/fuel mixture to be introduced into the cylinder in spite of the fact piston is moving upwards

 This means that if the valve is made to close once the piston has reached BDC than we are introducing less air in the cylinder than the actual capacity of the cylinder and if we are to close the inlet valve a lot later than we are flowing the air back into the intake manifold.

COMPRESSION STROKE - Both the inlet and exhaust valves are closed and the piston is moving from BDC to TDC and the air or air/fuel mixture is compressed.

INJECTION STROKE - Fuel injection into the cylinder begins at 10 degrees to 18 degrees before TDC and closes at 10 to 20 degrees after TDC.  This is done in order to ensure proper burning of the fuel.

This means that if the fuel valve is opened too early than we will have early ignition which will reduce power of the engine and if we have late injection than the chances of incomplete combustion are likely.

EXPANSION/POWER STROKE - In this stroke also inlet and outlet valves remain closed and the piston is moving from TDC to BDC.  This happens because of the Blast that occurs due to the ignition of air/fuel mixture in the cylinder.

EXHAUST STROKE - Exhaust valve opens and the piston is moving from TDC to BDC and the exhaust gases are expelled out of the cylinder.

Exhaust valve is made to open 50 degrees to 59 degrees before BDC as can be seen in the figure above however this value can range from 40 degrees to 60 degrees as per engine design.  This is called exhaust valve opening lead.

Question here now arises "WHY THE EXHAUST VALVE IS OPENING OPENING BEFORE THE PISTON REACHES BDC"? 

The valve is made to open before the piston reaches BDC because of the following reasons:

• This prevents the exhaust gases from preventing a high pressure cushion on the piston
• This high pressure cushion would restrict the movement of the piston
• This restriction would take off a certain amount of power from the engine
• Also because of the resistance of the exhaust gas pressure (back pressure), after the piston has passed the BDC the losses on the piston increases

This means that if the valve is made to open as the piston reaches BDC or after, than the pressure inside the cylinder will be lower than the exhaust pressure in the manifold, which would cause a back pressure and introduce exhaust gases from the exhaust manifold back into the cylinder or if we were to open the valve too early before the piston reached the BDC than we are inviting losses in the work done by the engine and as a result the output will be much lower.

At the end of exhaust stroke exhaust valve is made to close 10 degrees and 17 degrees after the piston has crossed TDC as shown in the figure however this value can vary from 10 degrees to 30 degrees as per engine design.  This is called exhaust valve closing lag.

Question here arises "WHY THE EXHAUST VALVE IS CLOSING AFTER THE PISTON LEAVES THE TDC"?   
The valve is made to close after the piston leaves TDC because of the following reasons:

• We do not want any exhaust gases left in the cylinder before the start of next cycle
• If exhaust gases are not fully removed from the cylinder than we are inviting problems in the combustion cycle

This means that if the valve is made to close as the piston reaches TDC or before than we are not allowing all  the exhaust gases produced in the cylinder as a result of the combustion process to be expelled out which may affect combustion in the next cycle and if we close the valve more later as the piston has crossed TDC than the exhaust will be introduced back into the cylinder from the exhaust manifold.

Now what have we missed out here?

If we look into the diagram we can see that both the inlet valve and the exhaust valves are open at the same time i.e the inlet valve opens before TDC and exhaust valve closes after TDC.  Now the duration for which these two valves remain open is called VALVE OVERLAP.

Now what's the use of VALVE OVERLAP?

Valve overlap ensures that the exhaust gases rushing out of the cylinder creates suction, in order to draw in fresh air or air/fuel mixture and the fresh mixture entering the cylinder pushes out the exhaust and give a scavenging effect.

This means that an early overlap may cause exhaust gases being expelled into the intake manifold and a late overlap may cause exhaust gases being drawn back into the cylinder. 

 Having learnt about the timing cycle of a 4 stroke engine let's now take a look at 2 stroke engine timing diagram.

TIMING CYCLE OF A 2 STROKE ENGINE ( A 2 STROKE ENGINE WITH SCAVENGE PORTS AND EXHAUST VALVE)

Figure shows a Typical 2 Stroke Cycle

Valve timing diagram of an engine with Loop Scavenging

KEY - 1-2 - FUEL INJECTION TAKES PLACE (INJECTION STROKE)

2-3 - EXPANSION (POWER STROKE)

3-5 - EXHAUST TAKES PLACE (EXHAUST STROKE)

4-5 - SCAVENGING TAKES PLACE 

4-6 - FRESH AIR IS ADMITTED (INTAKE STROKE)

6-1 - COMPRESSION (COMPRESSION STROKE)

Lets now take a look what is happening here.  

A typical example of an engine with loop scavenging is Sulzer RLB

                                                          Engine with Uni Flow Scavenging with exhaust valve                            Source - disel duck

Lets see what is happening here

• As the piston is moving towards BDC after the power stroke, 110-120 degree after TDC, Exhaust valve opens and the exhaust starts to expel out of the cylinder
•  Before BDC again at 130-150 degrees after TDC, the piston uncovers the scavenge ports in the cylinder, and the fresh air is introduced into the cylinder
• Here at this time both the exhaust valve and inlet ports are open and at this point scavenging occurs where the remaining exhaust gases are expelled out of the cylinder.  This is called VALVE OVERLAP (we have seen this in the four stroke cycle as well)
• The air entering the cylinder is at a high pressure i.e greater than atmospheric pressure, this is supercharged air
• As the piston starts to move up at 130-150 degrees before TDC the scavenge ports close and the entry of fresh air is now stopped
• As the piston moves further upwards at 110-150 degree before TDC exhaust valve closes and the compression stroke starts
• At the end of compression, at 10-20 degrees before TDC ignition starts and fuel is now injected into the hot air in the cylinder which was compressed in the compression stroke
• Once the ignition has occurred, expansion occurs where the piston is pushed downwards and the entire cycle repeats itself

WHAT IS MEANT BY SCAVENGING and SUPERCHARGING?

Scavenging is the process in which air is forced into the cylinder which is called the scavenge air, through ports in the cylinder liner which are called the scavenge ports, which helps to clear the cylinder from gases of combustion.

Supercharging means an increase in air flow into the cylinders of an engine which serves to increase in power output, in addition to being used for scavenging.  So basically what is happening is scavenging is done when the air is admitted into the cylinder under low pressure with the exhaust valves or ports open.  Supercharging is done with the exhaust valve or ports closed where the air is forced to enter the cylinder and thereby increase the amount of air that is available for combustion.  Therefore an engine is said to be supercharged when the scavenge manifold pressure exceed the atmospheric pressure.  

Always remember an engine which is supercharged, of the same give size, can develop more power than an engine of the same size that is not supercharged.

Always remember, as explained above,

1. SUPERCHARGERS - Supercharger is  typically driven by a belt or a gear powered by the engine. superchargers are good because the horsepower boost is available immediately however supercharger takes the power away from the engine.
2. TURBOCHARGERS - The turbocharger creates pressure using the already-burnt exhaust gas that is coming out of the engine to turn the blades of a fan that forces the air into the engine. Turbocharger takes a moment to "spool up" from the exhaust gas.  Turbocharger does not take power away from the engine. 

Lets take a look at what happens when a 4 stroke engine is supercharged?

For a 4-stroke diesel engine to be super-charged, a blower must be added to the intake system since exhaust and intake in an unsupercharged engine are performed by the action of the piston. The timing of the valves in a super-charged 4-stroke cycle engine is also different from that in a similar engine that is not super-charged. In the supercharged engine, the closing of the intake valve is slowed down so that the in-take valves or ports are open for a longer time after the exhaust valves close. The increased time that the intake valves are open (after the exhaust valves close) allows more air to be forced into the cylinder before the start of the compression event. The amount of additional air that is forced into the cylinder and the resulting increase in horse-power depends on the pressure in the air box or intake manifold. The increased overlap of the valve openings also permits the air pressure created by the blower to remove gases from the cylinder during the exhaust event.

Having learnt about the meaning of Scavenging lets take a look at different types of scavenging.

     

1. REVERSE FLOW - In this type of scavenging, there are two pairs of inlet ports, at the front and rear respectively, with two exhaust ports located on either side of the two pairs of inlet ports.  In this type of scavenging the incoming flow of air spreads out like a fan which is deflected downwards and expel the exhaust gases.
2. CROSS FLOW - In this type of scavenging, Intake and Exhaust ports are located on the opposite sides.  The gases in a way flow across the head.
3. LOOP FLOW - This type of scavenging occurs in engines which have exhaust ports just above scavenge ports. As the piston uncovers the exhaust ports at the end of power stroke, the exhaust gas starts to leave the cylinder.  As the scavenge ports are uncovered by the piston, the scavenge air loops around the cylinder and pushes the remaining exhaust gases out of the cylinder.
4. UNI FLOW - This type of scavenging occurs in engines which have exhaust valve located on the cylinder head.  As the exhaust valve opens due to cam action, the exhaust gases are expelled out of the cylinder, and as the scavenge ports are uncovered by the piston, the scavenge air enters the cylinder and expels the remaining exhaust gases.  This type of scavenging is quite common and is quite simple as the flow of air is in one direction that is from bottom to top.

A POINT WHICH MUST BE REMEMBERED BY EVERY ENGINEER:

There exists four stroke engines as well as two stroke engines which can have spark ignition.  

There exists four stroke engines as well as two stroke engines which can have trunk type piston design.

There exists four stroke engines as well as two stroke engines which can have inlet and exhaust valves.

There exists four stroke engines as well as two stroke engines which can be turbocharged. 

 

Engines where mixture of fuel and air is introduced into the cylinder are usually spark plug ignition type engines and engines where only air is introduced into the cylinder are usually diesel engines.

For information on Spark Ignition 2 Stroke engines please click here

BEARING DESIGNATIONS AND MEANING

On board our ships BEARINGS are used in almost every machinery.

But what does a Bearing Mean?

"Bearings are a hardened spherical ball that spins between two surfaces and reduces friction".

We are very well aware of the fact that at the time of ordering or at the time of replacing these bearings there is a Specific number designated to them.  This means that every bearing has been designated with a certain set of numbers and alphabets.  Before getting into the Nomenclature lets take a look at different types of bearings.

Different types of Bearings

• NEEDLE BEARINGS

NEEDLE BEARING

• CYLINDRICAL BEARINGS

     

CYLINDRICAL BEARINGS

• SLEEVE BEARINGS

SLEEVE BEARINGS

• SPHERICAL BEARINGS

SPHERICAL BEARINGS

• LINEAR BEARINGS

LINEAR BEARINGS

• ROLLER BEARINGS

ROLLER BEARING

• RADIAL BEARING

RADIAL BEARING

• PILLOW BLOCK BEARINGS

PILLOW BLOCK BEARING

• TRACK RUNNER BEARING

TRACK RUNNER BEARING

• DEEP GROOVE BALL BEARING

DEEP GROOVE BALL BEARING

• ANGULAR CONTACT BALL BEARING

ANGULAR CONTACT BALL BEARING

• THRUST BEARING

SELF ALIGNING ROLLER THRUST BEARING

TILTING PAD THRUST BEARING

• MAGNETIC BEARING

MAGNETIC BEARING

AND THERE ARE MANY OTHER TYPES OF BEARINGS

Now lets find out as to how we can distinguish between so many different types of bearings without even looking at them.

BEARING NOMENCLATURE

Nomenclature depends from manufacturer to manufacturer.  Nomenclature described below is the most common one.

A bearing number has 4 parts to it which includes four numbers and a set of letters.  The first two numbers stand for type and series, the last two numbers designate size and the letters dictate the variation of the bearing.  Lets now take a look at what actually these numbers are and what do they mean:

The first number OR LETTER designates the bearing type.  The first number can be

1 = SELF ALIGNING BALL BEARING

2= SPHERICAL ROLLER BEARING

3= DOUBLE ROW ANGULAR CONTACT BALL BEARING

4= DOUBLE ROW BALL BEARING

5= THRUST BALL BEARING

6= SINGLE ROW DEEP GROOVE BALL BEARING

7= SINGLE ROW ANGULAR CONTACT

8= FELT SEAL ALSO KNOWN AS WIDE CUP BEARING

32= TAPERED ROLLER BEARING

R= INCH NON-METRIC BEARING (i.e bearing dimensions do not comply with metric system)

N= CYLINDRICAL ROLLER BEARING

NN= DOUBLE ROW ROLLER BEARINGS

NA= NEEDLE ROLLER BEARINGS

The second number designates the series or cross section.  It's the ratio of the bore to the width of the bearing, which also controls the OD of the bearing.  The larger the cross section,  the larger the OD of the bearing.  Second number can be

8= EXTRA THIN SECTION

9= VERY THIN SECTION

0= EXTRA LIGHT

1= EXTRA LIGHT THRUST

2= LIGHT

3= MEDIUM

4= HEAVY

The third number is a set of two numbers indicates the bore in mm of the bearings.  All the bearings are designated with metric standards and not inches.  The third number can be

00= 10 mm

01= 12 mm

02= 15 mm

03= 17 mm

For 20-240 mm BEARINGS = LAST 2 DIGITS X 5 = BORE mm

This means if we have the third number as 04 than it would mean that the bore of the bearing is 4x5=20mm or 08 than 8x5=40mm 

The letters indicate the variation.  These vary from manufacturer to manufacturer but the most common ones are as follows

PLAIN= NO SHIELD

TYPE Z= ONE SHIELD (NORMALLY HAVE METAL SHIELDS)

TYPE 2Z= TWO SHIELDS (ONE ON EACH SIDE)

TYPE RS1= ONE SEAL (NORMALLY R INDICATES RUBBER)

TYPE 2RS1= TWO SEAL (RUBBER SEAL ON BOTH SIDES). 

CD= 15 DEGREE CONTACT ANGLE (FOR ANGULAR CONTACT)

ACD= 25 DEGREE CONTACT ANGLE (FOR ANGULAR CONTACT)

V= SINGLE NON CONTACT SEAL

VV= DOUBLE NON CONTACT SEAL

DDU= DOUBLE CONTACT SEALS

N= SNAP RING GROOVE ON OUTER RING

NR= SNAP RING GROOVE ON OUTER RING INCLUDED

M=BRASS CAGE

J= STEEL CAGE

ZBNR= SHIELD AND SNAP RING ON SAME SIDE

ZNJ= SHIELD ON SIDE OPPOSITE SNAP RING GROOVE

ZNRJ= SHIELD ON SIDE OPPOSITE SNAP RING

BLANK= STEEL CAGE

The bearings also have C2, C3, C4 or C5 designation after the bearing number.  This indicates that the internal fit (the inner race to the ball to outer race) is not standard.  Always remember that bearings with these designations need to be replaced with the same designators i.e a C2 bearing is to be replaced with a C2 bearing only.  A C2 bearing is less than standard clearance, while a C3, C4 and C5 are larger than standard.

For further information on bearings and

designations by SKF Click Here 

designations by NSK Click Here or Here

designations by NTN Click Here

ENGINE CLASSIFICATION

Engine is something which help us Humans give power which can be used to perform any particular task.  Imagine we cranking an alternator continously to get electricity!!! Instead we the humans made what is a diesel engine which will do this work for us.  I would say that an engine is something which transforms one form of energy to another and as a result make our life simpler and easier.

An Engine can be:

HEAT ENGINE - Engines which converts heat energy into mechanical or electrical energy.  Heat engines are usually Prime movers.

EXTERNAL COMBUSTION ENGINE (EC ENGINE) - Engines in which the combustion of fuel which is directly or in-directly responsible for driving the engine, takes place outside the engine.  Perfect example of this type engine would be steam turbines, where fuel is burnt in a boiler and the steam produced from the boiler is used for driving the Turbine.

INTERNAL COMBUSTION ENGINE (IC ENGINE)  -Engines in which the combustion of fuel takes place within the engine, are called Internal Combustion Engines.  Simplicity in engine design, operational costs and fuel economy is what makes these engines more popular and efficient than EC Engines.

We are more interested in Internal combustion engines as on board motor ships we have this type of engine installed.

Internal Combustion Engines are Classified on the basis of:

1. IGNITION SYSTEM - Ignition system can be of two types i.e. COMPRESSION IGNITION ENGINES and SPRAK IGNITION ENGINES. 

• COMPRESSION IGNITION ENGINES (CI ENGINES)- In these types of engines, the heat which is produced due to the compression within a cylinder is so high that it is sufficient enough to cause combustion and as a result there is no other means as such provided to cause the ignition of fuel within the cylinder.  A perfect example of CI Engines is a Diesel Engine.  Figure below illustrates the working cycle of a Diesel engine.  You can also click on this link to get the animated view of the diesel engine.  DESEL ENGINE ANIMATION.  These types of engines are based on DIESEL CYLCLE.

• SPARK IGNITION ENGINES (SI ENGINES) - In these type of engines, it is the Spark Plug, which produces a spark, causes the ignition of fuel within the cylinder.  This type of engine is found in our Cars and Bikes which we ride in our day to day routine.  Figure below illustrates the working of a Spark Ignition Engine.

Both SI engines and CI engines are internal combustion engines and both make use of liquid fuels.  There are a number of differences between the two.
Table below shows major differences between SI and CI engines.

p.s. Compression Ratio is defined as the ratio of maximum volume of cylinder i.e with piston at BDC (bottom dead centre) to volume of the cylinder with piston at maximum compression i.e with piston at TDC (top dead centre).  Watch this video to understand Compression Ratio.

2. Internal combustion engines can further be classified on the basis of OPERATING CYCLES

 

• OTTO CYCLE which is also known as CONSTANT VOLUME COMBUSTION CYCLE- It is the standard cycle which is employed in Petrol engines i.e. our car engines.
• DIESEL CYCLE which is also known as CONSTANT PRESSURE COMBUSTION CYCLE - it is a standard cycle which is employed in slow speed Diesel engines.
• DUAL COMBUSTION CYCLE which is also known as CONSTANT PRESSURE AND CONSTANT VOLUME COMBUSTION CYCLE - It is a combination of both otto cycle and diesel cycle where heat is partly added at constant volume and partly at constant pressure.  This type of cycle is employed in Medium and High speed diesel engines.

3.  Internal combustion engines can also be classified on the basis of STROKES or CYCLES
Cycles in a engine means the following events:

• Filling the Engine Cylinder with Fresh Air
• Compressing the air so much that the fuel vapors which are coming in contact with this compressed air which is now at an elevated temperature due to compression ignites
• Combustion of fuel
• Expansion of hot gases
• Exhaust of these gases after moving the piston

In single word, a cycle comprises  of - INTAKE - COMPRESSION - POWER - EXPANSION - EXHAUST.

Depending on many strokes of a piston are required in completing this cycle, the engines are further divided into 2 classes

• FOUR STROKE ENGINES -An engine which requires 4 strokes of piston i.e. 2 times up and 2 times down to complete one cycle, is a Four Stroke Engine.
• TWO STROKE ENGINE - An engine which requires 2 strokes of piston i.e. 1 time up and 1 time down to complete one cycle, is a Two Stroke Engine.

4. Internal combustion engines are also classified on the basis of PISTON ACTION

• SINGLE ACTING ENGINE - Engines of Single Acting type have one One Piston per cylinder, with the pressure of the combustion gases acting only on the surface of the piston.  Single acting Engines are widely used in Internal combustion engines as well as in many external combustion engines.  This is what we have on Board our ships.

Figure shows a typical Single acting Engine

•   DOUBLE ACTING ENGINE - In this type of engine both the ends of the cylinder and both faces of the piston are used to develop power i.e. cylinder develops power in both upward and as well downward stroke.

Figure shows a Typical Double Acting Engine

• OPPOSED PISTON ENGINES - This type of engine comprises of 2 pistons which are traveling in opposite directions.  The combustion space is in the middle of the cylinder and lies between the two pistons.  These engines have 2 crankshafts as well, where the one piston drives one crankshaft and other piston drives the other.  P.S. each piston is single acting.

Figure shows a Typical Opposed Piston Engine

5.  Internal Combustion engines can be classified according to PISTON CONNECTION

• TRUNK PISTON TYPE - In this type of engine piston is connected directly to the upper end of the connecting rod.  A gudgeon pin or a horizontal pin or wrist pin is what connects the two.  This is the type of engine that we have on board our ships for Medium Speed Engines.

Figure shows a typical Trunk Type Piston

• CROSS HEAD TYPE ENGINE - In this type of engine piston is attached to a piston rod whose lower end is connected to a Cross head, which slides up and down in guides.  Cross head is connected to the connected rod.  This type of engines are mainly used in large 2 stroke engines and in double acting engines.

Animation shows a typical Cross head Engine

At this point of our learning it would be nice to Compare the two engines i.e.

TRUNK PISTON V/S CROSS HEAD ENGINES

On board our ships most of the Diesel generator make make use of trunk pistons where as the main propulsion engine is crosshead engine.  

CROSSHEAD ENGINE V/S TRUNK ENGINE

KEY - 1-EXHAUST, 2-SCAVENGE AIR RECEIVER, 3-EXHAUST VALVE , 4-CYLINDER HEAD, 5 -A FRAME, 6-CYLINDER LINER, 7-PISTON, 8-SCAVENGING AIR PORTS, 9-PISTON ROD, 10-CROSSHEAD, 11-COLUMN, 12-CONNECTING ROD, 13-CRANKCASE, 14-BEDPLATE, 15-CRANKSHAFT, 16-INLET VALVE

Most of the medium and small size engines use trunk pistons.  As the piston is being pushed upwards by the crankshaft and the connecting rod during compression, resulting side thrust which is produced which causes the piston to press against the cylinder wall, first on one side, then on the other as it moves down.  Thus side thrust alternates from side to side as piston moves up and down.  At the top stroke, when the gas pressure is greatest, side thrust is negligible (this happens in trunk type engines as there is a small connecting rod angle).  So, most of the wear take place at the middle of the stroke: making piston skirt increases thrust bearing area, and hence reduces wear.  In medium and small size engines, due to lower gas pressure, unit's side pressure is so small that neither piston nor liner wears much.

In crosshead engines, crosshead takes the side thrust, which will be high in large engines.  Cross head engines have the following advantages:

• Easier Lubrication
• Reduced liner wear
• Uniformly distributed clearance around piston
• Simpler piston construction as gudgeon pin and its bearing are note there

On the other hand Cross head engines can have the following disadvantages

• Greater complication in engine construction
• Added weight of crosshead
• Added height due to the addition of another component i.e. Crosshead
• Need careful adjustments

  6.  Internal combustion engines can also be classified according to CYLINDER ARRANGEMENT

• CYLINDER - IN - LINE ARRANGEMENT - This is the simplest and the most common arrangement.  In this type of arrangement all cylinders are vertically in line.

Figure shows 4 cylinders in line, however the number of cylinders can go up to 12 but the most common is 6.

• V-ARRANGEMENT - If an engine has more than 8 cylinders, it becomes difficult to make a sufficiently rigid frame and crankshaft with an inline arrangement.  Also engine becomes quite long and takes up considerable space.  As a result, V-Arrangement is used for engines with more cylinders, (generally 8,12,16) giving about half-length of engine, more rigid and stiff crankshaft, less manufacturing and installing cost.  Angle between 2 cylinders or banks is kept from 30 degree to 120 degrees (most commonly 40 deg, 75 deg)

Figure shows a typical V-Arrangement Engine

• FLAT ARRANGEMENT -  It is a V-Engine, but this type of V engine has an angle between banks increase to 180 degrees.  This type of engine is mainly used in trucks, buses, rail cars etc.

Animation illustrates a typical Flat Engine

• RADIAL ARRANGEMENT - In a radial arrangement engine or Radial Engine all the cylinders are set in a circle and all point towards the centre of the circle.  The connecting rods of all pistons work on a single crankpin, which rotates around the centre of the circle. This type of engine was used in aircraft engines but now turbines are more widely used. 

Figure illustrates a typical Radial Engine

7.  Internal Combustion engines can also be classified on the basis of FUEL INJECTION

• AIR INJECTION ENGINE - The fuel is injected into the cylinder by a blast of compressed air.  This type of engine was heavy and complicated and now is obsolete.
• AIRLESS (or SOLID or MECHANICAL) INJECTION ENGINE - Fuel is injected into the cylinder, through the fuel valve, by high pressure fuel pump.  At present this is the type of engines which are used.

8.  Internal Combustion engines can be classified on the basis of CHARGING

• NATURAL ASPIRATED ENGINE - In these types of engines, a vacuum is created as the piston moves away from the combustion and as a result draws in fresh charge.  Our car engine is a perfect example of such type of engine. (petrol engines)
• SUPERCHARGED ENGINE - In this type of engine charge is admitted into the cylinder at a pressure greater than the atmospheric pressure.  This high pressure can be produced by a pump or blower or Exhaust Gas Turbocharger.  Our ship's make use of Supercharged engines

9.  Internal combustion engines are classified according to FUEL USED

• HEAVY FUEL OIL ENGINE - These are the engines which can burn high viscosity fuel
• DIESEL OIL ENGINE - These are the engines which can burn Diesel oil
• GASOLINE ENGINE - These are the engines which can burn gasoline as fuel.  These engines can also use kerosene.
• GAS BURNING ENGINE - These are the engines which use gaseous fuels at higher compression.  There are three ways which are adopted to burn these gaseous fuels and as a result these engines are accordingly named.  These engines are:

GAS DIESEL ENGINES - Only air is compressed in these engines.  At the end of compression, gas at high pressure in injected into the cylinder.  With gas, a small amount of fuel termed "pilot fuel" is also admitted into the cylinder to assist in ignition and to cause a smooth and prompt ignition.

DUAL FUEL ENGINE - In these type of engines, gas and air are admitted in the cylinder at the same time and it is the gas/air mixture which is compressed.  At the end of compression, fuel is injected to assist in ignition and cause a smooth and prompt ignition.

Figure shows a typical Wartsila Dual Fuel Engine

HIGH COMPRESSION, SPARK IGNITED GAS ENGINES - In these types of engines, gas and air are admitted in the cylinder at the same time and it is the gas/air mixture which is compressed.  At the end of compression, a spark plug produces a spark which ignites the mixture and causes combustion.

10.  Internal Combustion engines are also classified according Speed

• SLOW SPEED ENGINES - Engines which have rpm less than 300 r.p.m
• MEDIUM SPEED ENGINES - Engines which have rpm ranging from 300-1000 r.p.m.
• HIGH SPEED ENGINES - Engines which have rpm more than 1000 r.p.m.

11.  Internal combustion engines can also be classified according BORE/STROKE RATIO

Figure shows bore and length of stroke

• SQUARE ENGINE - If the bore to stroke ratio becomes 1 i.e if the bore is same as stroke the engine is said to be Square Engine.  In this type of engine Crankshaft web dimensions become less compared to journal and crankpin.

• OVER SQUARE ENGINE (SHORT STROKE ENGINES) - If the bore/stroke ratio is greater than 1, i.e bore diameter is larger than length of stroke.  This allows more valves to be placed in the cylinder head.  These type of engines allow for higher r.p.m and thus more power without excessive piston speed.  These engines have lower friction losses (due to the reduced distance travelled during each engine rotation) and lower crank stress (due to the lower peak piston speed relative to engine speed). Due to the increased piston- and head surface area, the heat loss increases as the bore/stroke-ratio is increased excessively. Because these characteristics favor higher engine speeds, over square engines are often tuned to develop peak torque at a relatively high speed. The reduced stroke length allows for a shorter cylinder and sometimes a shorter connectingrod,generally making over square engines less tall but wider than undersquare engines of similar power.  Source wikipedia

• UNDER SQUARE ENGINE (LONG STROKE ENGINE) -  If the bore/stroke ratio is less than 1 or if the stroke/bore ratio is greater than 1 then the engine is said to be Under square engine.  This means that the length of stroke is greater than the bore.  At a given engine speed, a longer stroke increases engine friction (since the piston travels a greater distance per stroke) and increases stress on the crankshaft (due to the higher peak piston speed). The smaller bore also reduces the area available for valves in the cylinder head, requiring them to be smaller or fewer in number. Because these factors favor lower engine speeds, under square engines are most often tuned to develop peak torque at relatively low speeds.An under square engine will typically be more compact in the directions perpendicular to piston travel but larger in the direction parallel to piston travel.                                                     Source wikipedia

• SUPER LONG STROKE ENGINES - To have better propeller efficiency and better combustion even with lower grade of fuels, lower r.p.m. engines with even longer strokes are gaining popularity.  These engines have stroke/bore ration in the range of 3.

At the end of this blog we can now say that there are 11 Different categories of an Internal Combustion Engine.