Answer EKM Question 22

Q22. Enumerate the causes of Piston Crown burning and subsequent deterioration in engines using Heavy Fuel oil. Also mention the possible reasons and steps to be taken to correct the situation of complete burn out of the Piston crown.

Answer: Piston crown forms the lower part of the combustion chamber which seals the cylinder and transmits the gas pressure to the connecting rod. The crown is subjected to the high temperatures in the combustion space and the surface is liable to be eroded/burnt away. Due to which, the material from which the crown is made must be able to maintain its strength and resist corrosion at high temperatures.

Steel, alloyed with chromium and molybdenum is used, and some pistons have a special alloy welded onto the hottest part of the crown to try and reduce the erosion caused by the burning fuel.
The MAN B&W piston has an 8 mm thick heat resisting layer of a hard nickel-chrome alloy called Inconel welded to the hottest part of the crown to resist the 'burning" of the piston crown.
Causes of Piston Crown burning and subsequent deterioration:
(a) Prolonged low-load Heavy fuel operation:
(i) Direct burning of metal surface is not possible, however, High temperature corrosion from vanadium and sodium, and erosion due to high flame speed and gas speeds is accelerated leading to loss of material.
(ii) Combined effect of above and poor bunting in low loads causes accumulation of unburnt carbon which starts burning in the crown accelerates burning of piston crown.
(b) Inefficient piston cooling: Piston is cooled by lube oil or water. Poor cooling due to scale build up in the cooling surfces in water cooled pistons and carbon deposits in oil cooled pistons leads to higher temperature differential and increased thermal stressing.
(c) High fuel viscosity (inadequate fuel temperature): Too high viscosity - Large droplets of atomised fuel which takes longer to heat up and start burning.
(d) Dribbling of fuel: Leaky fuel injector causes fuel to dribble which results in large burning droplets of fuel depositing on the surfaces of liner and piston.
(e) Too high scavenge temperature: Reduced mass due to high temperature which causes increased compression temperatures and poor combustion of fuel.
(f) Overloading of engine: Injecting too much fuel will lead to increased Pmax and higher temperatures, Higher flame and gas speeds will occur. Poor burning due less time.
(g) Fuel quality issues
(i) Too high calculated carbon Aromaticity Index: Ignition delay followed by rapid rise in  temperature and pressure.
(ii) Water In fuel - Causes steaming, localized erosion.
(iii) Incorrect fuel Injection timing- leading to carbon deposits and after burning.
(iv) Sludge In fuel - Poor burning accumulation of carbon.
Possible reasons and steps to be taken to correct the situation of complete burn out of the Piston crown.
(a) Measurement of piston top land: The piston top land wear down is measured with the help a template which is ratted on top of the piston crown and the clearance between the template and top land is measured with the help of feeler gauge.
(b) Max. Permissible burn-away
For (MAN MC-C)
(i) Standard piston top: 20mm
(ii) Inconel piston top: 8mm
SULZER RT-flex 96C - Standard piston top:10mm
(c) Pressure Test of Piston:  It is important to pressure test the piston prior to cleaning and installation. Invert the piston to reveal the base of the piston palm facing up, Fill the piston with water, Fit the pressure testing tool at the palm of the piston, Connect the air hose to testing tool and supply air pressure up to 7 bar. Shut the air and isolate the piston in this condition for 2-3 hours while checking for any leakages from all contact surfaces and sealing rings, Check crown for any crack or water.
(d) Prevention measures:
(i) To avoid excessive burning of piston crowns and cylinder liners and other associated problems which may result, the following standing orders are to be adhered to.
(ii) The set of power and draw cards taken regularly and study to be made based on the results.
(iii) Fuel injectors are regularly tested and nozzles, springs, push rod etc are replaced at correct intervals.

(iv) Fuel quality is to be maintained well by proper purification, manual draining, Correct supply temperature and viscosity etc.

(v) Air cooler is cleaned at correct intervals.

(vi) Proper cooling water testing and dosing shall be maintained.