MOTOR ORAL QUESTIONS & ANSWERS PART-4

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Q – What do you understand by the term sfoc and how to calculate it.

Ans – Specific fuel oil consumption is the measure of mass of fuel consumed per unit time to produce per kW. The marine engine efficiency is usually calculated using the SFOC.

In order to achieve accuracy, the fuel consumption & power developed is always measured over a suitable time period on a good weather.

Calculation of the specific fuel oil consumption (g/kWh, g/bhph) requires that the engine power, & the consumed fuel oil amount, are known for a certain period of time. The engine power(in bhp or KW) can be calculated from the Indicator diagram or from Fuel Pump Index method or from the turbocharger speed. The oil amount is calculated for a few hours to avoid calculation mistakes. The engine parameters should not be changed during the period. Since quantity measurements will be in volume units (m3), it will be essential to know the oil density, in order to convert to weight units (gram). The density is to correspond to the temperature at the measuring point. Density can be calculated on the basis of the bunker specifications. Density at 15 deg is given in the BDN (Bunker Delivery Note). The density at required temperature can be determined with the density correction factor equation. But generally graphs are provided in the manual to find the density at required temperature, where the change in density is shown as a function of the temperature.

Temperature Corrected Density can be calculated with the formula given below:

Temperature corrected Density = Density of Fuel Oil @ 15 degree Celsius * (1- ((t1-15) * 0.00064)

Where,

t1 stands for temperature of oil in bunker tanks in degree celsius,

0.00064 is the correction factor,

volume of oil in m³ (actual sounded volume), is obtained from the sounding table.

Q – How to calculate mean effective pressure with planimeter and indicator card

1) Calculation of engine Power with Indicator diagram

For engines with the indicator drive or MIP-equipment, we can take the indicator diagram which can be used to find the Mean Indicated Pressure (MIP). Don’t get confused between MIP and MEP. Former is the Indicated mean pressure while MEP is the effective pressure available after friction losses in the shaft.

Calculation of the indicated & effective engine power consists of the following steps:

Calculate:

  • The mean indicated pressure, pi
  • The mean effective pressure, pe
  • The cylinder constant, K
  • The indicated engine power. Pi
  • The effective engine power, Pe
  • The mean indicated pressure, Pi

Mean Indicated Pressure, pi

Pi (bar) – Ax Cs/L

A = A is the area of the Indicator diagram measured with a Planimeter in mm².

Cs – Spring constant of the drive in mm/bar (vertical move seat of the indicator stylus from for a 1 bar pressure rise in the cylinder)

L = length of the indicator diagram (atmospheric line)

Q – How to calculate the engine power without indicator cards?

Ans – This question is very frequently asked now a days in MMD orals because all the people onboard very much familiar with the indicator cards but if in case your indicator card machine won’t work in that case how will you calculate the power, so there will be the some method which cannot give you the exact value but at least give you a near by value

Power calculation without Indicator Diagram.

The estimation is based on the nomograms considering engine parameter measurements taken on the testbed. The graphs are provided in the manual

1) Fuel Pump Index method

The fuel pump index is used to find out the MEP from the nomogram graph. Again from the graph, the mep at a specific speed gives the engine bhp.

This method should only be used as a quick estimation, because the fuel oil, as well as the condition of the fuel pump, may have great effect on the index. In particular, worn fuel pumps or suction valves tend to increase the index, & will thus result in a too high power estimation.

Chart I: draw a horizontal line from the observed fuel pump index to the nomogram curve, & then a vertical line down to the observed engine speed on the Chart II. From this intersection a horizontal line is drawn to the effective engine power scale. (This method is specific for the some engines. many other parameters are included to calculate the BHP in bigger engines)

This effective power is used to calculate SFOC of the engine.

Q – Working of different types of governor and explain different terminology related to it

Ans – Governor function is to control the fuel supply with respect to the load or any other parameter which it is suppose to maintain under all the conditions

Different types of governor

1. Speed control governor maintain constant speed irrespective of the load)

2. Load sensitive governor (according to the load it control the fuel supply)

3. Speed & load sensitive governor (mostly used in main engine)

For auxiliary engine we use constant speed governor because we have to maintain the frequency constant all the time irrespective of the load.

M/E is the variable speed governor it sense both the speed and say whichever is higher that we have to control.

Note – Just like in bad weather RPM is dropping but load on M/E is very high so if in that Case governor only sense the RPM so they try to increase the fuel and increase too much load on the M/E which may break any part of your engine (extreme critical condition).

Mechanical governor

Mechanical governor

Mechanical hydraulic governor

Mechanical hydraulic governor

Most of the governor are of centrifugal type governor-

1. If we increase the compression of the speeder spring then your engine rpm has been increased

2. Due to the rotation the weight try to move outwards and on the opposite side spring force will act so these two forces decide whether to the increase the fuel or not.

3. This is the pure mechanical governor which is not used in big engines because force required to move the rack is very high so that much force we cannot obtain from these type of governors that’s why we use hydraulic governor now a days.

Hydraulic type governor

1. When RPM is more and we need to reduce the fuel then oil from line 1 drains out (small quantity) and because of this power piston comes down and fuel quantity automatically gets control.

2. Now if rpm is less then oil from line 2 goes towards the underside of the power piston by a oil pump and increase the quantity of fuel to the engine

3. Basic principle of working is same only we use hydraulic power to move the rack efficiently

4. Any control system must have feedback else it won’t work properly same given in this type of governor.

Note – Most of the time surveyor ask you a simple question that what is droop and why it is required.

In simple terms I can say drop of speed due to change in load that is called droop because when load is increased the spring compression reduces due to which it run at lesser rpm than the previous rpm that was the characteristic of the governor.

Let’s take an example to understand this- your generator is running at 900 rpm 0 % load and now load increase to 100 % so now the rpm becomes 891, so now how to calculate the droop –

%Droop = (No load RPM — full load RPM / No load RPM) x 100

So in this case,

(900 – 991)/900 x 100 = 1 % Droop

Note – Your droop should not be too high else your generator could not be able to maintain the frequency and it will trip

But the main point is that why droop is required – if droop was not there then it is impossible to parallel the two generators.

If droop is zero it means your governor is isochronous governor and two generator which contain two isochronous governor we unable to parallel that governor because they don’t have any line of intersection.

Note- Surveyor favourite question is that you are on watch and you parallel two generators but you notice that one generator is taking more load as compare to the other what could be the reason for that – if you fully understand what I told you above then you can easily able to answer this question.

Note – Most of the people have the confusion that if load suddenly changes then how governor maintains the frequency or rpm, for to understand this lets take an small example

– if you start the ballast pump so suddenly rpm of the generator drops and it may trip your generator on low frequency so that’s why governor has two actions, first action has been taken by the load sensor so that it will supply the excess fuel so that drop in speed should not occur much and fine setting is been done by the speed sensor so that we can maintain the desired rpm.

Q. Why fly weights are driven at the higher rotational speed than the engine?

Ans-

  • Fly weight equipped with the governor  work on the centrifugal force. The centrifugal force acts upwards & the spring force acts downwards.
  • This difference between the spring & the fly weights is known as the governor effect, which is responsible for moving the pilot valve & hence the fuel rack.
  • Higher the governor effect, higher will be the sensitivity of the governor. Hence, to increase the sensitivity of the governor the centrifugal force has to be increased.

Centrifugal Force = mr ω^2

Where, m = Mass of the fly weight

r = radius of the rotation of the fly weight

w = Speed of the rotation of the fly weight.

  • If we increase m ’mass’ then the inertia effect will increase hence the sensitivity will drop as it take more time to act.
  • If we increase r ‘radius’ the governor becomes too big in volume & takes higher power to rotate the governor, therefore drop in the sensitivity is probable.
  • Increasing of ‘w’ is done by increasing the speed of the rotation of the fly weight. Change in the speed gives very high effect as the centrifugal force is directly proportional to the square of the speed, additionally speed increase proves to be more effective, increased sensitivity, less power requirement & compact size of the governor.

Q. What is a dead band?

Ans- Dead band is the change in the speed required before the governor will make a corrective movement of the throttle. In simple words, Dead band is speed of the fluctuation around the steady state speed, in which governor will not initiate the corrective action.

In simple terms it is the silent period of the governor at which governor does not take any action. The dead band effect is decreased by increasing the sensitivity of the governor.

This can be achieved by;

  • Driving the flyweight at the higher speed
  • Use of conical spring
  • Hydraulic amplifiers can be used to amplify the governor effect.
  • Linkages shall not be worn
  • Governor oil should be low viscous oil so as to reduce the drag effect.
  • Less mass of the fly weight by using a hollow weight.

Dead band is not provided to governor, it is because of slow response of mechanical components in a mechanical or hydraulic governor. If the dead band is too less, governor may start hunting.

Q – Explain electronic governor and why it is used so much now a days.

Ans – Electronic governor we are using just for the sake of alternator life (Quick response by the electronic governor with respect to the load variation)

Basic understanding of the electronic governor you will get from the diagram given below –

Main engine governor – one of the important term that mostly surveyors asked that what is deadband in the governor – To understand the concept of deadband lets take an example that your engine is running at 800 rpm and if rpm goes to 790 so governor try to increase the fuel and when rpm goes to 805, governor will try to reduce the fuel, so like this it will keep on increasing or decreasing the fuel so that term is called hunting and we cannot able to achieve the stability so we set the deadband just like if rpm goes to 795 from 800 so governor will not take any action same on the other side if rpm goes to 805 from 800 governor should not take any action so from 795 to 805 governor wont take any action so this band is called dead band.

Electronic governor

1. In rough sea mode we have to reduce the rpm because propeller coming out of water so in that case we have to reduce the deadband and increase the sensitivity of the governor may it cause hunting of the governor but our main aim is to avoid the overspeed of the main engine which is one critical case during the rough weather

Note – Deadband is less our sensitivity of governor is more and vice-versa

2. Function of limiter – Depend upon the condition it will limit the fuel supply to the engine. So that in case of rough weather he will make sure that in any condition

engine should not be overloaded. So limiter will check each and every parameter

before supplying fuel to the engine

3. Scavenge air limiter – Just in case if you don’t have sufficient air pressure in the charger air receiver and you increase the fuel then at the outlet you will get the black some so limiter sense the same and give signal to governor that to reduce the fuel

4. Load limiter – Normally we put the knob of load limiter to 10 means 100 percent. But in the condition of bad weather our rpm is not so high but load already reached to 100 percent so in that case governor won’t release the fuel else our engine becomes overloaded

5. Torque overloading – This thing is not in our hand, it depends upon the nature. We didn’t load the engine as according to the load wise condition but still your torque is more because of the weather condition

Note – Torque overloading means that same power you developed @90 rpm

Power = 2 x pie x rpm(N) x Torque (if your rpm(N) is low but still the power is same it means the value of Torque is high) that means torque overloading this was the worst condition for the engine Torque overloading condition is more dangerous because this was the time your liner cracking, piston ring breaking take place. Torsion meter is also fitted on the ship sometime or it is given in the governor itself. In case of load limiter or torque overloading case governor cut off the fuel but in case of scavenge air limiter it will restrict the flow of fuel not completely cut off the fuel just like in other two cases

Note – So final conclusion is, limiter check all the parameters but it control that parameter or governor work according to that parameter which was on the higher side.

Q – How to test overspeed trip in generator and main engine

Ans – This question belongs to electrical but still surveyor ask this question in motor because this was one of the critical trip, which avoid the engine to run on overspeed.

Overspeed trip don’t have automatic reset, you have to manually reset it once it will get activated

Overspeed set point-

1. In generator it is 10 to 15 % of MCR

2. In main engine set point is not to be more than 10 percent

How to test overspeed trip in generator – In generator governor we have the speed knob setting, so just keep on increasing the speed and finally at one point of time when engine trip on overspeed. But actual overspeed trip is not good for engine so before performing this trip just reduce the setting and perform it because our main aim is to check that the trip is working or not.

How to test overspeed trip in main engine – As we know in electronic governor we have the ramp generator, which generate the false signal and then we can check that on overspeed the main engine will trip or not but before doing this make sure your selector switch should be on test mode. Normally in main engine we never perform the actual overspeed because it may damage the engine so we always generate the false signal.

Note – 1. Governor is the controlling device and overspeed is the safety device. Governor has the modulating control and overspeed has two mode either give fuel or completely cut off the fuel.

2. Overspeed is the separate system not with in the governor, because overspeed occurs when your governor is faulty.

Surveyor mostly ask this question that due to faulty governor overspeed take place and fuel still keep going inside the engine so what will you do-

1. First put the canvas to the turbocharger blower side ( to stop the air supply to the engine)

2. Take the fuel pump rack to zero and also operate the puncture valve on the fuel pump

3. Operate emergency stop (because at that time our main concern is to save the engine)

Once engine has stopped after overspeed then you have to do the complete check of the engine –

1. After taking all the safety precaution go inside the crankcase and remove bottom end bearing

2.Check connecting rod for bend and cracks

3.Crankshaft deflection to be taken

Note — Most of the time overspeed occurs at the time of starting the engine because at that time governor release the fuel in excess quantity so that engine should able to come over inertia, after that governor role is to take back the fuel rack.

In fuel line we have the mechanical stoppers also which restrict the fuel rack to go inside the fuel pump so in that case your governor is okay, but your fuel rack may only go upto 60%

One important point to note that before transferring control from ECR to W/H at that time make sure your fuel rack setting is same both in ECR & on W/H, else if fuel rack setting in E/R is zero so the time you transfer control to W/H from ECR your engine suddenly stops without any warning.