Heat Losses

The importance of the degree of insulation at full engine power: As a consequence of the reduced temperature gradients in the substrate, heat transfer to the cooling water will be reduced. Fig. 7 shows how the heat flux to the cooling water in the cylinder head is reduced with increasing insulation of the heat exposed components. Initially the engine had no coating. At 29 hrs the piston was coated with a 1 mm thick ceramic coating and at 51 hrs the cylinder head, too, had been coated with ceramic, but in this case with 2 mm thickness. After a further run of 11 hrs the heat losses to the cooling water had been reduced to almost 70% of the level when the engine was run without insulation. The reduction obtained in the last period is attributed to a closing of the outer surface of the coating.

Comparison between heat losses of the individual components: The effect on the heat losses due to ceramic coating of the cylinder head and the piston crown is shown in Fig. 8. The percentage change shown is relative to the heat losses in the uncoated engine. Fig. 8 shows that temperature changes in the insulation layer cause a variation in the heat flux from 1.6% reduction to 3.2% increase.

For the cylinder head there was a clear reduction in the heat flux at all running conditions. The relative reduction was 12% at full brake effect and rotating speed. At low rotating speed there was about 2% reduction of the heat flux at all brake effects. At intermediate rotating speeds the heat flux decreases with lower brake power and becomes higher than in an engine without ceramic coating. With full brake effect at intermediate and high rotating speeds the changes for the three components are equal to about 6% reduction. At the lowest rotation speed the reduction is about half of the before mentioned value. It can be concluded that the total heat losses at full engine power are reduced by. 6%. The total heat losses can be divided into a reduction to the cooling water of 9%, and an increase to the cooling oil of 3%.

Considering the results from tests with coating on the cylinder head, it is probable that with a thicker crating on the piston crown it is possible to obtain a reduction instead of an increase in heat losses to the cooling oil. Furthermore, a surface treatment closing porosities in the outer layers of the coating should provide an additional reduction of heat losses to the cooling media.

Discussion: Of the candidate thermal barrier coatings, zirconia seems to be the best choice for the combustion chamber of a diesel engine. Zirconium dioxide has a special advantage because of its thermal coefficient of expansion, which is near that of cast iron, and it has a very low thermal conductivity. Impregnation or some surface treatment to reduce surface penetration is necessary in order to improve thermal impedance.

The variation of temperatures over the surface is much larger on the ceramic coating than on the cast iron substrate, but does not reach the same depth in ZrC>2 as in the cast iron.

Changes of heat flux in the liners are rather small when all components are coated with ceramic. With a 1 mm coat on the piston crown and a 2 mm thick coating on the cylinder head, the heat flux increases on the piston crown and decreases on the cylinder head. This is seemingly a paradox, but it looks like a coating with an untreated surface needs a certain thickness in order to reduce the heat flux.

Heat losses at full engine power with coatings on both piston crown and cylinder head have been reduced by 9% to the cooling water but increased by 3% to the cooling oil. The size of the water cooling system can thus be reduced, which results in lower parasitic losses of the water pump and fan.

It is a big advantage that a thick ceramic coating on a component reduces both temperature level and temperature gradients. This means that the material can withstand higher mechanical stresses while being subjected to lower thermal stresses. The result is that thermal fatigue of the component will be reduced or eliminated.

A new coating with a porous surface reduces the compression level and air intake rate of the cylinder. While the fuel consumption is reduced at low rotating speeds, it is increased at high rotating speeds. Insulation of only the piston crown has a beneficial effect on the fuel consumption. Apparently fuel injection parameters for the higher temperature insulated case are very important.

When the cooling effect of the cylinder liners and the piston crown is reduced such that the material temperature in these components is increased, the specific fuel consumption will increase. But here, also, the fuel consumption decreases when the cooling of the piston is reduced and its surface temperature increased. The pressure of incoming air and combustion gas energy are increased when using ceramic coatings, but the smoke number will decrease. With the increased combustion gas energy available, the turbocharger can be better adjusted to the changed conditions, steam production from the bottoming cycle steam boiler increased or turbocompounding introduced. These waste heat utilization schemes can also be combined.

0 0

Post a comment