Fundamental Analysis of Thermal Overload in Diesel Engines: Hypothesis and Validation

Abstract

'Thermal Overload' can be defined as a condition under which design threshold values such as the surface temperature of combustion chamber components is exceeded. In this paper, a low λ value is identified as the most probable cause of voluminous flame production, resulting in high surface temperatures of engine components, i.e., engine thermal overload. Test results indicated that the flame became voluminous when the excess air ratio, λ was low, and the exhaust temperature increased from 775 to 1000 • C with λ changing from 1.12 to 0.71. Temperature indicating paints were applied on two piston crowns to investigate the effect of the voluminous flame on component surface temperature. The piston crown with high rates of hot corrosion was very close to matt glaze (much in excess of the design temperature), which proved that high surface temperature and salt deposition on the crown in the heavily burned away regions could have been caused by flame and fuel impingement, respectively. A numerical calculation was presented to estimate the flame temperature for various air excess ratio, which provides a guidance for the operation conditions of diesel engines to avoid engine thermal fatigue due to thermal overload.

Investigation on the Effect of the Gas Exchange Process on the Diesel Engine Thermal Overload with Experimental Results

“…This voluminous flame comes into contact with the combustion chamber components, breaking down the boundary layer and increasing the rate of heat transfer. This does not occur under lean burning conditions, where the flame is more compact and a cushion of air exists between the flame and the component surface [11].…”

“…However, very little evidence has been published which considers the probable causes of thermal overload of the engine. As the component temperatures are dependent on the flame size within the combustion chamber, its relationship to the air-fuel ratio was previously investigated using a flame visualisation test rig [11]. The hypothesis indicated that the thermal overload was influenced by high surface temperatures due to contact with a voluminous flame, which itself was a likely consequence of a low air-to-fuel ratio [11].…”

“…As the component temperatures are dependent on the flame size within the combustion chamber, its relationship to the air-fuel ratio was previously investigated using a flame visualisation test rig [11]. The hypothesis indicated that the thermal overload was influenced by high surface temperatures due to contact with a voluminous flame, which itself was a likely consequence of a low air-to-fuel ratio [11]. One possible reason for this phenomenon is that there is a deficiency of active radicals present in the combustion zone, which will slow down the flame propagation rate, thus contributing to make it more voluminous.…”

Development of a Diesel Engine Thermal Overload Monitoring System with Applications and Test Results

“…The test results prove that high surface temperature and salt deposition on the crown in the heavily burned away regions could have been caused by flame and fuel impingement respectively. As in an ideal scenario, the flame should be compact and should not come in contact with the piston crown or exhaust valve [18]. Piston crown temperature estimation was carried out on a 7RTA847 engine (Sulzer, Winterthur, Switzerland) where high rates of hot corrosion were evident on a unit and also on a unit where piston crown wear rates were normal.…”

“…This high temperature flame when close to or in contact with any combustion chanter component will result in an increased rate of heat transfer through the component and its consequent failure. Detailed information on the test rig development and test results analysis can be found in [18]. Piston crown temperature estimation was carried out on a 7RTA847 engine (Sulzer, Winterthur, Switzerland) where high rates of hot corrosion were evident on a unit and also on a unit where piston crown wear rates were normal.…”