Power Calculation in Marine 2-Stroke Engines

We have all wondered if our vessels are being utilized to their fullest and if we are getting the maximum out of our Bunkers.

Extensive man hours are spent analyzing the vessel performance data to be able to operate Main propulsion engine at maximum efficiency.

Every analysis relies on few basic measurements taken by the ship’s crew on daily basis-?

1.?????Daily Fuel Oil consumption (FACT- T/Day) – This at times is one of the most sort after reading and most carefully taken due to over bunker claims. The Chief engineer normally would calculate it to the utmost accuracy.

·???????Limiting factors on this calculation is the Temperature correction which changes with every bunker delivery. Especially with the wide range of Low sulfur fuel in the market today this can be significant.

·???????Other factor which can induce error is the absence of malfunction of the Flow meter. If the reading is tank sounding based we can safely assume an error factor of up to 1% to 5%. We will be bit careless and neglect this for our discussion.

·???????There is one more correction to this to be included for engines with unstructured or unknown maintenance schedule; it’s the daily fuel leak off being collected in ER. Normally this is collected in overflow tank and transferred back to the Bunker tank or settling tank. This figure at times can be significant if the vessel history during takeover is unknown and thus needs attention. The daily fuel oil consumption from flow meter or tank sounding must be adjusted for this leak off.

2.?????Engine Torque developed (TACT- N/m) - This is calculated by installing torsion meters on the shafting and along with the RPM readings from Pickup sensor the reading can be displayed in real time to monitor engine thermal loading and making corrections to RPM accordingly.

3.?????Engine Power (PACT- Kw or BHP) – The Engine Performance packages installed on modern vessels can display real time data on the power developed by each unit and the complete engine overall. The data can be utilized for thermal balancing of engines and optimum engine performance with best Specific Fuel oil consumption and Spec Lube oil consumption.

4.?????Engine Revolution Counter (RevsACT- Revolutions) – Most of the vessels are equipped with the revolution counter and this is recorded on a daily basis separately for standby and sailing periods.

Apart from these readings as an operator one should also be interested in the Shop Trail data for the engine and Sea trail records of the vessel. The parameters we would consider in our discussion are-

1.?????MCR Power- PMCR

2.?????MCR RPM- RMCR

3.?????SFOC MCR- SFOCMCR

4.?????Calorific value of fuel used during shop Trial –CV0

There are two approaches to the Power calculation in absence of Power or Torque sensors-

1.?????Power Calculation based on Actual Fuel Consumption-

Output (BHP) = (Fm * ? at Fm * CV1 * 238.889 * 1000)/ (SFOCMCR?* CV0 * 10200)

Fm : Reading of Fuel Oil Flow Meter in Liters/Hr

? at Fm : Density of Fuel oil at Flow meter

SFOCMCR ?: SFOC AT MCR Load (g/BHP Hr)

CV1 : Calorific Value of Fuel In use, (1 MJ/Kg = 238.889 Kcal/Kg)

CV0 : Calorific Value of Fuel used during Shop Trial.

Alternatively the Output can be calculated as follows-

Output (KW) = (FACT * CV1 * 10^6)/ (SFOCMCR?* 24 * CV0 *XLeak *XLoad )

FACT : Fuel Consumption (Tons/ Day)

CV1 : Calorific value of Fuel in Use (MJ/Kg)

SFOCMCR : SFOC AT MCR Load from Shop trial data ( g/Kw Hr)

CV0 : Calorific Value of Fuel used during Shop Trial.

XLeak : Leak Factor (Can be taken as 1 for simplification)

XLoad : Load Factor (Can be taken as 1 for Simplification)

Once we have the Power with us we can calculate SFOC and SLOC for comparing with the engine curves and decision on the adjustment of CLO feed rate, hull cleaning or propeller cleaning can be taken easily.

Another Important Factor to be derived from the engine performance data is the thermal load.

Thermal Load on the engine (%) = (Actual Output/ Actual RPM)/(MCR Power/MCR Rpm)*100

?The thermal load is proportional to the Torque of the engine. To maintain same output with increased external resistance due to hull or the weather the engine has to operate at higher torque thus increasing the thermal load on engine components inside the combustion space. Quite a few piston and liner failures have occurred due to increased thermal load resulting in higher temperatures inside the combustion space. This reduces the viscosity of the CLO and also at times can oxidize it completely reducing the effectiveness. At times the temperatures due to thermal load can carbonize the cooling space of the piston and cause buildup and subsequent damage.

Thus knowing and maintaining the thermal load becomes critical.

Please comment with your views on this along with any suggestions on the above approach. This method is not accurate but will be acceptable with slight error margin.