Diesel Piston Crown Erosion

Service Engineering Bulletin - SB005

A diesel injection system must deliver (meter) the right amount of fuel, in the right form (atomised), in the
right place (spray pattern), at the right time (system timing). With everything correct, combustion takes
place without liquid fuel being deposited on the combustion chamber surfaces. A thin boundary layer of gas
should be maintained between the surfaces and the burning mixture. In turn, this provides thermal protection
of the combustion chamber by limiting the heat transfer to the components. The boundary layer is very thin
(measured in microns) but the insulation it provides is of the utmost importance (Fig 1).
A malfunction, or the incorrect adjustment of the fuel injection system will often cause fuel to wet the
combustion chamber surfaces (Figures 2 to 4). When this happens, the fuel burns in direct contact with
the surfaces without the protective boundary layer being present. The rate of heat transfer from the
combustion gases increases dramatically and temperatures rapidly exceed the melting point of
piston alloys. Piston crown erosion inevitably follows.
The illustration shows a typical example of crown erosion and melting. An unatomised fuel spray acts
like a ‘flame thrower’, melting and blasting through the crown of the piston. A mixture of moltern crown
alloy and burning diesel has eroded and melted behind the Alfin insert and exited out through the oil
ring groove and down the skirt.
When crown erosion is discovered in a diesel engine, make an immediate and thorough check of the whole
fuel injection system. A piston alloy has not yet been invented that will resist fuel burning on its surface,
and blaming the component’s metallurgy will simply delay identification of the true cause - a fuel
injection system malfunction.