Psz Abovetherings

Figure 9. Cast iron cylinder block with solid ceramic cylinder bore insert

thermal expansion coefficients allowed for maintainence of an interference fit in the assembled components, where desired, over the full range of operating conditions.

Ceramic Insert Components

The various insert components evaluated in the Phase II investigations were: (1) a cast iron cylinder liner with an interference fitted PSZ liner covering the bore ID above the top ring reversal point (Fig. 9); (2) a cast iron cylinder head with an inserted 5 millimeter thick headplate which contains the full valve seats (Fig. 10); and (3) the

Figure 10. Cast iron cylinder head with solid ceramic headplate insert

Figure 11. Steel articulated piston top and solid ceramic piston bow! insert steel articulated piston top with a variety of piston bowl inserts (Fig. 11).

Based on the previous coating test results, the full length cylinder bore solid ceramic insert concept was not evaluated. The selected above-the-rings cylinder bore insert insulates the combustion chamber when the piston is around top dead center and the major portion of the heat release is occurring. This approach does not require the rings to ride over a discontinuity in the bore. This component has been subjected to hundreds of hours of hot engine testing without failure.

The inserted headplate cylinder head has also been a very successful component but with a qualification. Finite element analysis of this component indicated maximum temperature levels of 1500 - 1700 F. In addition, high thermal stresses were predicted at the center portion of the plate on the combustion face, Under engine testing, the inserted headplate has completed just over 500 hours. Early in the testing, a fine crack was observed across a thin section at the center of the plate. The original crack does not appear to have grown and may very well be the required stress relief for this design. From a long term durability point of view, the predicted operating temperature levels may be a problem for partially stabilized zirconia.

Piston bowl inserts of two different PSZ materials and a PSZ/alumina composite material have been evaluated. All of these one piece bowl inserts developed cracks in a relatively short period of time. The areas where cracks developed were generally consistent with the high stress regions identified by finite element analysis. Three design iterations significantly increased component reliability but still an acceptable value was not obtained. For a component such as this to be successful requires a marked improvement in the current state of the material - it's Weibull modulus and its strength. This may be true for many of the ceramic materials being considered for these engine applications.

Ceramic Insert Conclusions

Conclusions regarding solid ceramic inserts in the adiabatic diesel engine are:

(1)The ceramic insert engine concept provide a further reduction in heat loss and a slight improvement in fuel consumption over the coated engine concept.

(2)Like the coatings, the long term durability of solid PSZ inserts fabricated from currently available material may not be adequate at operating engine stress and temperature levels.

(3)Again, the long term durability of the cast iron engine structure operating at the elevated temperatures of the uncooled ceramic insert engine is a significant concern.

(4)Finally, the ceramic insert engine is seen as being more complex to fabricate and assemble than the baseline, water-cooled cast iron engine.

Structural Ceramics

The longer term, higher risk structural ceramic concept (Phase III) utilizes stand alone structural ceramic components. This concept has the potential to provide further reductions in heat loss and improvements in fuel consumption. The ringless piston is a key feature of this concept. If successfully developed, major friction reductions are projected and liquid lubrication in the high temperature piston/cylinder area is avoided.

Structural Ceramic Components

The structural ceramic components evaluated to date have been silicon nitride cylinders, ringless pistons, piston pins, and valves (Fig. 12). These components have operated successfully in a single cylinder engine for over 250 hours. No signs of cracking or wear have been observed. At present, engine blowby is marginally acceptable at high speed (4500 rpm) but unacceptably high, by an order of magnitude, at low speed (1500 rpm). However, motoring friction mean effective pressure is up to 10 psi lower relative to the water-cooled baseline engine with rings. If

Figure 12. Silicon nitride structural ceramic components blowby objectives are achieved, further friction reductions are proj ected and the impact on brake specific fuel consumption would be substantial.

Structural Ceramic Conclusions

The evaluation of this concept has only just begun. Some initial results have been encouraging—in particular, the structural integrity and the motoring friction reduction—but several design and material challenges remain. Blowly control is critical. High temperature lubrication needs much attention. Elimination of the liquid lubricant in the high temperature piston/cylinder area does not make the problem go away. Experience to date indicates that some non-liquid lubrication will be required at ceramic/ceramic or ceramic/metal interfaces. Current materials need to be improved if high reliability components are to be designed. And lastly, but most importantly, the ceramic components must be cost effective. A reasonable estimate is that these materials today would cost $50— 200/lb. on a volume, finished component basis. These low heat rejection concepts are competing with engines that contain critical components that have finished $ per pound costs of $0.50—1.00/lb. for cast iron components such as the cylinder head and block, $1.50— 2.50/lb. for aluminum components such as the piston and intake manifold, and $5,00—6.50/lb. for steel components such as the valve ,• crankshaft and camshaft.

References

1. W. R. Wade, P. H. Havstad, E. J. Ounsted, F. H. Trinker and I. J. Garwin, "Fuel Economy Opportunities with an Uncooled DI Diesel Engine", IMechE Paper No. C432/84, SAE Paper No. 841286, 1984.

2. P. H. Havstad, I. J. Garwin and W. R. Wade, "A Ceramic Insert Uncooled Diesel Engine", SAE Paper No. 860447, 1986.

Figure 12. Silicon nitride structural ceramic components

3. W. R. Wade, P. H. Havstad, V. D. Rao, M. G. Aimone and C. M. Jones, "A Structutal Ceramic Diesel Engine - The Critical Elements", SAE Paper No. 870651, 1987.

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