06-19-2012 08:36 AM
We are running a ship with a MAK 8M20 engine on MDO.
With rising fuel prices it appears to become more and more attractive to trade in vessel speed for lower fuel consumption, however common wishdom seems to be that it is not advisable to run the engine at less then 80% for longer periods of time. Can someone explain what aspects should be considered in this regard, and if there are possibilities to make modifications to the engine to optimise it to run at less power for longer periods of time?
06-29-2012 09:57 PM
Good Evening shoresup
Considering you are running on MDO running at reduced load should not be an issue. Problems arise when the engine is run at low loads on intermediate fuels or IFO because of deposits such as sulfur left behind as the result of incomplete combustion. MAK recommends switching from intermediate fuel to MDO when running an engine below 40% to 50% load for extended periods of time
I would suggest you monitor your fuel consumption, engine load, propeller pitch (if fitted to CP Propeller) and vessel speed to find a setting that values come together to give optimum performance, to find what is most commonly referred to as"Economical Speed".
For example if it is determined that; at 80% load setting the vessel speed is 14 knots and burns .8 tonnes of fuel per hour, at 85% load setting the vessel speed increases to 14.5 knots and fuel consumption increases to 1 tonne per hour, and at 75% load setting the vessel speed drops to 12.5 knots and fuel consumption decreases to .7 tonnes per hour. It can be seen from this example that running at 80% Load will give the best possible speed for the least amount of fuel consumed.
I know this is an over simplified example but should put you on the right track to find the optimum engine speed and load setting for your engine. It has been my experience that the ratio of speed to fuel consumed is a parabolic curve and finding the point which everything comes together at the point of optimal performance has been a challenge for engineers for a long time.
07-03-2012 01:33 PM
Good evening GLEngineer,
Thank you for your feedback. I am not an engineer, but I am looking at it from an operational/business perspective. Besides looking at the most economical speed that also means taking into account added operational costs due to longer trips etc.
The ship is fitted with a FP propellor which probably makes the case a little different. Would you still expect the speed vs consumption curve to be of parabolic shape? Obviously the propellor will be designed for a specific speed and perform less optimal if the operational speed is lower than the design speed, but on the other hand the resistance curve of the ship suggests a reduction of required power of 50% when slowing from 11 knots till 9.5 knots.
07-03-2012 04:24 PM
Good Evening Shoresup
Yes the fuel consumption curve would still be parabolic even with a fixed pitch propeller. The greatest factor in fuel consumption is the design of the hull itself and how much resistance is created by the amount of water the vessel pushes as it moves through the water. It stands to reason the faster a vessel moves the greater the resistance will be and the amount of power required to maintain that speed will be greater.
Most fixed pitch propellers carry the same relative efficiency through out the lower end to the top 2/3rds of speed range. After that the relative slip of the propeller increases and the propeller efficiency drops off.
For your information there are two types of Propeller Slip Real Slip and Relative Slip. Real Slip is the ratio of a vessel's real time speed over the ground divided by the theoretical speed of a propeller and Relative Slip is a vessel's speed through the water taking into account currents and other factors divided again by the theoretical speed of the propeller
The theoretical speed of a propeller in knots can be calculated by taking the (Propeller Pitch in feet times the shaft RPM times 60) and divided by 6080
I should also mention reducing the power output 50% load does not necessarily mean a 50% reduction of the full load consumption it has been my experience that it works out to be somewhere between 65% to 70% of full load consumptions.
I would also suggest the engine consumption be closely monitored over a period of time at various loads would be the best means to find out what is best for you and your buisness plan
I hope my input has helped
07-15-2012 04:44 AM
Dont forget that the most economic specific fuel consumpion per kwH is around 80 to 90% load.
Running extended periods in partial load without regular full load running.
will wear down your engine much faster.
Glazing will apear on your liners causing increased lub oil consumption.
Due to incomplete combustion soot and corbon deposit will increase on the pistong and will polish the cylinder wall.
The turbo charger will require more flushing and expect accelerated wear on the cylinder head due to bigger carbon deposit on the valves.
i dont know what will be more economic in the long run.
the fuel expenses saved or the increased maintenance and overhauls.