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.
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
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
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
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 = Sp x H
Sp = 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
IPAvg = 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.