Mean piston speed, its significance on consumption of fuel oil .

Q:-Illustrate mean piston speed, its significance on consumption of fuel oil. Explain how mean piston speed is related to r.p.m. and ideal combustion. Joining an old ship as Chief Engineer, formulate a methodology by which you can ascertain   the optimum use of fuel oil against desired mean piston speed.

 Mean Piston speed: The mean piston speed is a parameter in the power equation which suggests that power can simply be increased by increasing the MPS. But this can only be done up to the limitations imposed by the following consideration.

                  Dynamic forces and moments are generated within the engine system when the reciprocating and rotating masses are in motion. The magnitude of these forces are greatly increased with speed on bearings, bearing bolts and other moving parts and    reaction forces on stationery members are increased due to inertia forces. Similarly the peak turning moment of the fluctuating torque due to the moments of forces and the    reaction torques is also magnified in magnitude failure as a consequence of high   maximum fluctuating stress is a factor which should receive more consideration in the design.

          Speed decreases the service life at those components which form rubbing and rotating. The relative surface speed must be such that the rate of wear of piston rings or liners must be within the acceptable limits speed however is not the only criteria on for wear as it is known that wear on the liner is maximum at top center where the speed is least. Nevertheless, where the working condition is identified as one of extreme with high     temperature and pressure existing and lubrication being in the thin film region speed must be limited for greater safety and reliability. The output at the cylinder is lowered   beyond a certain speed as the efficiency of the gas exchange process sharply drops.    

           The output of a diesel engine depends on its capacity to breath fresh air at the beginning of compression. At a high piston speed the resistance of gases to how in and out of the cylinder increases high level of torsional vibration in the elastic shaft system.

The expression (2Ln) for mean piston speed can be re arranged as follows:  

         Mps  = 2 x Stroke   x n      where n=(N/60)   , N=rpm

           The expression further suggests that besides a power function the mps permits manipulation of other parameters for certain gain for a certain mps and bore diameter, shorter the stroke higher is the speed of revolution. A short stroke reduces the height and weight of the engine considerably.

             Significance on consumption of fuel oil:-

    M.P.S. = 2Ln ; if stroke is constant, then MPS is proportional to N

    Now Power P = P_mLAN

        where, P_m = mean effective pressure

                    L   = stroke

                    A   = area of cylinder

                    N   = revolutions per minute 

      If L & A are constant;

      Then     Power α P_m.N

       Equation of power is also related to displacement and speed of ship.

              Power α Δ^⅔ V³

   where    Δ = Displacement

                V = ships speed

At constant displacement of ship

         power α V³

          but V α p.N

where      p  = propeller pitch

                N = rpm

  but p is constant

Therefore, V α N

Now putting this value in the power equation

   power α V³α N³

Now we can conclude that,

   power α (MPS)³

Therefore MPS is the parameter of the power equation.

But the fuel consumption of a ship depends upon the power developed, indeed the     overall efficiency of power plant is often measured in terms of the specific fuel  consumption which is the consumption per unit of power expressed in kg/kwh

Fuel consumption per unit time α power

                                                       power α Δ^⅔ V³

       Therefore, fuel consumption per unit time α Δ^⅔ V³

       or      fuel consumption/day    =    Δ^⅔ V³               _ tonnes

                                                                                      fuel coefficient

                                                                                                        

        in above equation, at constant Δ and constant value of fuel coefficient,

                               fuel cons./day α V³

                                   but           V α p x N

                               Therefore     V α N   (because propeller pitch p is constant)

             Therefore,  fuel/cons. /day α N³

             We know that,         MPS α N

              Therefore, fuel cans. /day α  (MPS)³

      From the above equation, we can say that the higher the mean piston speed, higher the fuel cans. and lower the MPS, lower the fuel cons.

       Increase and decrease of MPS has limitations due to following reasons:-

1) If MPS is high, time available for fuel combustion is very less, so some unburnt fuel will carry forward to exh. gas due to incomplete combustion; thus fuel consumption increases and thermal efficiency becomes low

2) At low MPS, compression is isothermal, but we need adiabatic compression, i.e. no heat loss or gain. at slow MPS, compression temperature is not sufficient to complete combustion of fuel injected, and unburnt fuel will carry over resulting in to black smoke and higher fuel         consumption.

        How MPS is related to rpm and ideal combustion:-

        MPS plays a great role as far as scavenging, i.e. exchange of gases is concerned. the output of the cylinder is lowered beyond a certain piston speed as the efficiency of the gas exchange process sharply drops. With effective MPS, better scavenging and         compression temp. Can be achieved, resulting in complete combustion of fuel oil          injected. Complete / ideal combustion gives you better power and RPM.

         The methodology by which you can ascertain the optimum use of fuel oil against desired mean piston speed in an old ship:-

            Upon joining an old ship as a chief engineer, the following checks are to be carried out:-

1. Check all the parameters of the engine
2. Take power cards for all the units
3. Check fuel and cylinder lub. oil cons. of engine
4. Check that PMS for m/e is being followed as per maker's instructions
5. Check the maintenance history of main engine

  All above parameters you can compare with previous record, including sea trial records. From above, you will come to know about the status of M/E & its output and fuel cons. For optimum fuel consumption, firstly, better scavenging and better  compression should be there, and mean piston speed is affecting scavenging.

1) Factors affecting scavenging to be maintained.

• Scavenge ports to be kept clean.
• Turbo charger air filter to be kept clean.
• Air cooler both air and water side clean.
• Exhaust valve maintenance as per PMS.

2) Engine parameters must be closely monitored.

• Exhaust temperature and color of smoke.
• Engine peak pressure and performance.
• Inspection through scavenge ports.
• Scavenge temperature.

1.   Fuel System:-

• Fuel purification to be proper
• Fuel pressure to be maintained
• Fuel injectors are in good overhauled and pressure tested condition
• Fuel timing should be proper
• Condition of fuel pump should be good
• Correct fuel temp. to be maintained

1.  Optimise cylinder lubrication by manual setting.
2. Maintain the PMS of M/E.