Applications

The methodology has been applied in a number of studies. These included: 1) analyses supporting the experimental program discussed above, 2) effects of progressive insulation of various in-cylinder components to determine the best methods of insulation, 3) study of efficiency potential of turbocharged intercooled engines with and without exhaust heat recovery, 4) study of effects of liner insulation, 5) cyclic surface temperature swings and their dependence on material properties and engine operating conditions, and 6) studies of thermal shock in ceramic components. Of these, items 3) and 6) are discussed below, while the others have been described in prior publications given in the list of references.

The developed methodology was applied to the analysis of a number of key issues in LHR engine design (Morel et al, 1986). Among the key findings of that study were:

most thermal efficiency benefits are obtained by piston insulation, less by head insulation, and only very small benefit is obtained by liner insulation;

the small liner insulation gains are offset by lower volumetric efficiency and by higher piston/ring/liner temperatures and the attendant tribological problems;

turbocharged engines benefit meaningfully from insulation (five percent efficiency gain) even without additional heat recovery devices.

Another example application is a study of thermal shock in a ceramic-coated valve. A sudden change in engine speed and load produces a change in the rate of heat flux from in-cylinder gases to the adjacent structural components. As a result, a thermal transient is produced in the components.which is sometimes referred to as thermal shock. This thermal transient generates a moving front of. sharp temperature gradients which propagates through the structure and produces high local stresses. An associated effect is the creation of a moving distortion pattern. As a consequence of these effects, severe load/speed transients are considered to be highly adverse engine operating conditions, which can lead to early material failures.

The valve considered in this investigation, shown in Figure 12, was made of steel coated with 0.060" thick layer of plasma sprayed zirconia. The finite element model representing the valve is shown in Figure 13. A transient event was set up in an IRIS simulation to represent a realistic rapid increase in engine fueling rate from a light load to a full load (subject to governor smoke limit). As a result of the load increase, the valve temperatures rose monotonically over a period of about 1000 engine cycles. The thermal stresses in the coating also increased during that period, but this increase was not monotonic. As seen in Figure 14, a large compressive stress overshoot occurred, peaking after around 40 engine cycles. Similarly, on a reverse transient from high load to idle there was a substantial tensile stress overshoot.

CONCLUSIONS OF METHODOLOGY SECTION 1. A comprehensive methodology has been developed for the design analysis of LHR engine concepts. It incorporates several new advanced heat transfer models and integrates these into a single coupled simulation.

2. An experimental program on diesel engine heat transfer produced a detailed set of experimental data on cooled and ceramic-coated engine configurations. The predictions of the methodology were compared with the data and they were found to be in excellent agreement.

3. Insulating the engine with ceramics was found to reduce substantially both the peak heat flux and the time average heat flux.

4. Applications of the methodology to thermal shock in ceramic-coated valves showed that substantial stresses occur during engine transients, which exceed stress levels found during steady state conditions.

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