Ceramic Components in Passenger-Car Diesel Engines

Peter Walzer, Hartmut Heinrich & Manfred Langer, Volkswagen AG;Pos tBox3180 , Wolfsburg 1, West German),.

Abstract Components for diesel engines are being developed from various ceramic materials. The properties of the most interesting materials, the process technology and the designs of the components are described. Experimental results with thermal insulation of combustion chamber components and with wear resistance shielding of valve train components are being reported. The response of turbochargers using low density ceramic rotors is tested and the temperature loads on ceramic particulate filters during regeneration are being measured.

The Ceramic Materials In Table I the ceramic materials under consideration for automotive engines are listed. Properties of special impor- tance, when compared with the metallic materials presently in use, are underlined

The oxide ceramic materials alu- minium titanate. AI2TiO 5. and zircon- ium dioxide, Zr02, are already familiar from other technical applications. The principal advantage of these materials over metal is their low thermal con- ductivity; this means it is possible to achieve highly effective thermal insu- lation. However. the low strength of aluminium titanate creates the need for designs in which the ceramic can be supported as a heat shield in a metallic structure. Zirconium dioxide is of considerably higher strength and may thus be used for self-supporting com- ponents. In addition, this ceramic has a high hardness which might yield better wear resistance in sliding contacts with metal surfaces.

The material aluminium magnesium silicate. AMS, has particularly low coefficients of thermal expansion and

high resistance to thermal shock. These properties make use of this material advantageously for applications in which the component is subject to high, rapidly changing thermal loads, but where mechanical stresses are low.

Sintered silicon nitride, S-Si3N4, and sintered silicon carbide, S-SiC, are new non-oxide ceramics. Compared to the materials mentioned earlier, these cer- amics exhibit higher thermal stability, which even exceeds that of metallic supera l loys at t empera tu res above 1270K, combined with favourable thermal shock resistance. This means that these ceramics can be used as structural materials for components which are subject to high mechanical and thermal stresses. As their density is only 1/3 of that of metallic superalloys, components made of these ceramics have additional advantages where rotary and oscillating motion is involved.

Not shown in the table are ceramic fibres made of aluminium oxide which may be used to reinforce non-ferrous metals. Cast aluminium parts which are

~ ty Ultimate Flexural Strength

(at 800K) (at 1400K)

Material " ~

Density

MN/m 2 MN/m 7

530 300

450 450

70 20

600 300

40 20

10 3 kgJm 3

Coefficient Thermal Y°ung's of Thermal Conductivity Modulus Expansiona Coefficient X

(at 1260K) (300-1260KI (at 1260K)

109 N/m 2 10 -6 mlrnK W/InK

300 3.2 12

400 4.5 40

12 0.6 1

2OO 9.8 2.5

23 3.0 2

S-Si 3 N,~ 3.1

S-Si C 3.15

AMS 2.2

Zr O 7 5.7

AI7 O3-Ti O 7 3.2

Table I Properties of ceramic structural and insulating materials

reinforced with fibres to 35 - 40 percent by volume achieve three to four times the rigidity of cast parts without fibres and would therefore be suitable for conrods, for example. However, these materials are still too expensive for economical application in the fore- seeable future.

Thermal Efficiency of an Insulated Combustion Chamber Figure 1 gives the results of a cyclic process calculation for a combustion chamber with metallic and with ceramic walls. The upper graph indicates the influence of the choice of material on the level and the profile of the surface temperature of the combustion chamber wall.

The 3 mm ceramic lining means that mean surface temperatures are approx. 200K higher, but the temperature fluctuations remain considerably smal- ler than those of the working medium when considered over the cycle. Here it must be noted that the process is far from being adiabatic, but even in a well- insulated machine an intensive heat exchange occurs between the working medium and the wall.

The effects of this" higher mean surface temperature on the principal operating parameters can be read off from the lower graph. The increase in thermal efficiency is obviously only small, as the increased wall temperature primarily causes redistribution of wall and exhaust-gas heat. Corresponding to the decrease in heat transmission to the wall is an increase in the heat dissipated with the exhaust gas. The hotter com- bustion chamber walls dissipate more heat to the fresh air drawn in such that considerable charge losses occur in the

MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986 75

case of a naturally aspirated engine. In this case a supercharged engine was investigated. Here the supercharger is able to take more energy from the greater exhaust-gas energy and can compensate for the charge loss via increased supercharging. In passenger car diesels this usually is only valid at low engine speeds where the opening pressure of the waste gate is not being reached. In the case discussed here the opening of the waste gate, therefore, was set to a higher pressure and it was

Working Medium and M e a n C o m b u s l i o n - c h a m b e r Wall l"emperalure

K f I i ,3,, . . . . . . . . . ~,o.~0- = . . . . 2ooo I~ - ~ - ~ o,,,,,,,.. ...............

. - - . 3ram Celomic Lln,n 9

1500 t - ~,

/ f~7 I "%G_~j I ooo

~ - - - , / ' - - ~ - " - ' - , . . . . . ~ . . . . 4

BDC TDC BDC TDC BDC Crankshaft A n g l e

Influence on Operating Parameters

Ceramic "-~ L i n i ng ~ e°~'

1.= . , , f ie+°. , >.J.~;;~'k::::.~_:Z~'~ To r q ue .Ou I p u t 1 mO ~1 ~J! I | °pec/tic Fuel Co

o.8 i l .... . . 0.6

400 600 800 1OO0 K Mean Combustlon-chamber Waft Temperature

Fig. 1 Calculation of the influence of ceramic insulation on the operating parameters in the diesel engine with exhaust gas-operated turbocharger. N --- 4500 rpm, Equivalence ratio 0 ----- 0.8 (full load).

4 + - 9 0

Fig. 2 Ceramic components for in- sulation of the combustion chamber. Swirl chamber, piston insert, valve coating made of AI2TiO s, cylinder liner and cylinder head cover made o f ZrO z.

assumed that the engine design is able to withstand the final combustion pres- sure which has risen by approximately 10 bar.

In order to verify these calculated results in practice, the combustion chamber of a supercharged swirl-cham- ber diesel engine was insulated with ceramic layers. Figure 2 illustrates the various components used. On average it was possible to install a 3 mm thick insulating layer on approx. 80% of the combustion chamber surface.

Table II shows a comparison of measurement results on the insulated engine with those of a standard engine. Identical quantities of fuel were in- jected and identical timings were used. However, the engines differed in terms of compression ratio, charge-air pres- sure and waste gate setting. In the case of the insulated engine this setting was selected to be higher to the extent that the exhaust-operated turbocharger can supply a higher charge-air pressure from the increased exhaust-gas energy. A lower compression ratio was selected as a more favourable cold starting ability was expected.

Both engines achieve virtually the same final performance which thus corresponds to the calculation. The lower smoke number of the insulated engine may also permit an increase in the quantity of fuel injected and thus a further slight improvement to perform- a_nce. Approximately 13% less heat was dissipated to the coolant. Whether or not this may contribute to a usable reduction in the power consumption of the fan or in the radiator area must be the subject of further investigations. The answers given finally have to take into consideration, too, the fact that in most cases the ability of a diesel engine to heat the car properly is already limited.

Raled Power : Output I kW Soot BOSCH-Units T e Turbine K

PL bar Qwott kW

US City -Test : Fuel Economy miIGal

CO g/mi HC g/mi NO x glmi

Engine Basic Insulated

51 50 3.0 2.0

1000 1070 1.65 1.75 46 40

41 39 1.0 1.0 0.2 0.15 1.1 1.4

Table II Measurement result of the supercharged diesel engine with and without ceramic insulated combustion chamber. (In both cases experimental vehicles were used).

The results obtained from the US test regarding fuel consumption, CO and HC emissions do not exhibit any noteworthy differences. It might be possible to reduce the higher NOx emissions to some extent by retarding the injection timing in the case of the insulated engine. This result, however, shows that for a diesel engine which has to meet low NOx emission levels as prescribed for cars, every insulation has to be considered with caution.

The results for full insulation as measured and discussed above certain- ly cannot justify the use of ceramics in the combustion chamber of a passenger car diesel engine for thermodynamic reasons alone. In further tests it has to be shown if additional advantages such as reduced particulate formation or the ability to burn diesel fuel with lower cetan grades might result. In the fol- lowing two sections, areas are discus- sed where the insulation of a specific combustion chamber component may lead to cost and functional advantages.

System or Component Cost Reduction It is easy to estimate the benefit of using ceramics in areas where more expen- sive metallic materials can be replaced or where it is possible to dispense with cooling equipment.

Figure 3 shows on the left a piston from a swirl-chamber diesel engine with ceramic insert and on the right the piston of a DI diesel engine with ceramic swirl collar.

With the piston of the supercharged swirl-chamber diesel engine, cooling of the aluminium piston by way of oil injection in the area of the piston crown might be necessary due to the higher thermal loading in comparison with the naturally aspirated engine. In turn, this oil cooling demands injection nozzles, a higher-capacity oil pump and, possibly, an additional oil cooler. Although the

a

Fig. 3 Pistons with ceramic inserts. Material AI2TiO s.

76 MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986

insulated pistons are more expensive than the standard versions, lower system costs may be attained if the low thermal conductivity of the ceramic insert reduces the heat development in the pistons to such an extent that there is no need for elaborate cooling.

Figure 7 shows the ring groove temperatures, measured at full load in each case, as a function of engine speed. Pistons were measured with and without ceramic heat shields and both with and without cooling by way of oil injection. (The temperatures were determined using N T C resistors in the piston and non-contacting data transmisson by way of intermittently coupled alternating magnetic fields). Of

critical importance in this are the ring groove temperatures. It has been shown that these are still achieved even at nominal load by the piston with ceramic heat shield. In the exhaust gas the elimination of cooling meant that temperature increases of up to 60°K occurred (It.

In the case of the swirl collar in the DI diesel engine, use is made of the high thermal stability of the ceramic. Com- bustion cavities with recessed rim or with a swirl collar bring the promise of more favourable compromises with regard to efficiency, emissions and noise. However, with these designs, the temperature loading is approx. 60K higher than with open combustion

Tempera ture in K 5 6 0

5 4 0

5 2 0

5 0 0

4 8 0

4 6 0

4 4 0

4 2 0 - -

40O 1 0 0 0

Without Heat Shield o with Cooling • without C o o l i n g / ~ "

I

/ ~ / _ . . , , , , -o - - / / f . / . / / . /

oj -o • without Cooling

2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0

Engine Speed in rpm

Measuring Point

t Coollng

Fig. 4 Component temperatures With ceramic insulation of the piston. Example: Swirl-chamber diesel engine with exhaust gas-operated turbocharger.

"-1

I

I

Pao/ /

/) ,Ooo \\ ..-- ' ."

Stress :~e:mS'lperes,iv e in MN/m 2

T e m p e r o t u r e i n K

Fig. 5 Optimization of the shape of a ceramic insert. Example: Ceramic lining of the combustion cavity of a direct-injection piston.

cavities. In the case of aluminium pistons, the resulting additional stresses under operating conditions lead to problems with the service life. Here again, an inserted ceramic ring would appear to permit better solutions.

In order to keep the oscillating masses as small as possible, it is advisable to select the less dense AI2TiO 5 for the ceramic inserts in the piston. However, as this material has a tensile stresses of 91 M N / m 2 and the compressive stresses o f - 2 5 3 M N / m 2 which the ceramic is prefabricated as a dish or ring and is cast in with the metallic material. In this, the dimension and the shape of the insert must be selected to ensure that the ceramic is predominantly kept under pressure given the thermal expansions to be expected in operation and that these are at the same time kept within the elastic range of the aluminium. Figure 5 shows two examples of designs for a ceramic dish with combustion cavity. Extrap- olations show that in the right dish the tensile stresses of 91 MN/2/M 2 and the compressive stresses o f - 2 5 3 M N / m 2 still exceed the permissible values for A12TiO 5, whereas the left-hand shape designed after further optimization should only achieve permissible stres- ses of between 34 M N / m 2 a n d - 194 M N / m 2. The effect of the thermal insulation is to reduce temperatures of 1000K at the ceramic surface to 550K in the aluminium.

Investigations in driving operations are still required to indicate whether or not reliable engine operation can actually be achieved under all standard condi- tions. So far, experimental results over several hundred hours have been con- ducted on the reinforced pistons of the swirl-chamber turbocharged diesel, with tests having been performed partially on the engine test bench and partly in the vehicle under road driving conditions. After this testing the surface of the ceramic inserts of all 4 cylinders was covered with a network of fine cracks. These cracks could well be predomi- nantly attributable to therr0al shock. They indicate that the quality of the ceramic insert must be fuaJaer improved in order to guarantee the running performance which is normal in series production.

Improvement of Mixture Preparation In diesel engines, the ignition of the injected fuel is delayed as long as the temperatures of the combustion chamber walls are still low following a cold start. The fuel quantity which is injected during this ignition delay burns im- mediately. The steep pressure rise this

MATERIALS & DESIGN VoL 7 No. 2 MARCH/APRIL 1986 77

causes leads to severe knocking noise, which is a well-known phenomenon. With a ceramic swirl chamber the thermal capacity is reduced to approx- imately 1/2 of that of the metallic chamber and at the same time the dissipation of heat into the surrounding metallic structure is reduced. The consequence of this is that the ceramic swirl chamber attains the operating temperatures faster and thus the mixture preparation of the warm engine is accomplished earlier.

Figure 6 shows a ceramic swirl chamber. The material AIETiO.~ was again chosen for this chamber. For use in the engine the low strength means, as it does with the pistons, that a design must be selected in which the ceramic is held in a cast metallic surrounding. However, it is possible to use grey cast iron for this instead of the expensive nickel alloy in standard applications.

The results of noise measurements

Fig. 6 Ceramic swirl-chamber in- sert. Material AI~TiO 5.

taken during the warm-up phase are given in Figure 7. Measurements were taken of the loudness of an engine with ceramic insulation as compared to the standard engine. Reduction of the loudness by roughly 15% is achieved in the first 3 minutes, whereas the ceramic engine is no longer of any advantage in terms of noise level once full operating temperature has been attained after 20 mins. (Loudness is determined from objective data such as the sound level, spectrum and time function and ex- presses human sensitivity to the intensity of a source of sound. The numerical value is doubled if there is a sensation of double the loudness).

Reduction of Wear Rate For the engine drive train ceramics might be of interest because of the following properties. The hardness of all engineering ceramics is considerably higher than that of metals. In addition ceramics have low adhesion tendency against metals. Final ly there are indi- cations that ceramic sliding dry against metals results in lower friction than metal against metal.

F rom this it would be reasonable to expect advantages in respect to wear rate and mechanical losses in areas where sliding contacts with reduced lubrication possibilities exist. The valve train is such an area. Here the compo- nents are normally subject to high sliding velocities, with only minimum lubrication during the first minutes following a cold start.

Figure 8 shows a valve tappet with a ZrO2 adjusting plate. The surface is polished to a roughness of 0.03 pro. After

- - Standard

- - - Ceramic

Loudness in sane G

Measurement Conditions I d l i n g : 9 0 0 rpm

Start Temperature: - 10°C D i s t a n c e : 0 . 5 0 m in Front of Radiator

t : r equence R a n g e as of 180 Hz LS-4 o

Loudness S = 2 ,0 LS- Loudness Level (According to Poulus~E.Zwicker)

r Immediately Following

0

~>. 20ra in after Cold Starting i t Cold Storting

10 20 30 40 5 0 0 10 20 30 40 50 Time in s Time in s

Fig. 7 Influence of a ceramic swirl-chamber on noise emission in cold starting.

tests running against the standard metal camshaft no wear on the tappet could be measured and the wear at the camshaft had been reduced. The effects on the mechanical losses were too small to be identified in' the measurements.

The technology to fabricate ZrOz plates as shown has been well estab- lished. The at tachment to the metallic component, however, still needs better solutions. So far clipping, if possible, and vulcanization have given the best results.~ 2~

Response Time of the Exhaust- Operated Turbocharger The diesel engine can however also benefit indirectly from the use of ceramics in the exhaust-operated tur- bocharger. The response behaviour of a turbocharger today still is unsatisfac- tory, particularly with regard to the frequent load changes which are typical of passenger-car operation. This res- ponse time can be shortened by using ceramic with only 1/3 of the density of the metallic version for the turbine rotor. As less energy is released in the event of a burst, the housing can also be designed with thinner walls.

Figure 9 shows the results of an extrapolation of the stresses which would have to be withstood by a ceramic rotor. The critical operating condition is the cold start with imme- diate acceleration to full load. The figure illustrates the maximum tensile stresses which occur following cold starting as a function of the operating time. The maximum stresses occur 27 seconds after the start, the locations of the maximum stresses are in the area of the corner radius, blade to blade, at the hub centre and at the transition to the main journal. The high stresses indicated can only be withstood by the non-oxide ceramics such as S-Si3N 4 or S-SiC.

Figure 10 shows the prototype of a

Fig. 8 Valve tappet with adjusting plate from ZrOz.

78 MATERIALS & DESIGN Vol, 7 No. 2 MARCH/APRIL 1986

Max. Princ. Stress

~N/m2 ~ _

200

100

0

(/, /

...,"

10

1

31

,..°..." ..°.-'" °....-'

Material S-Si C i

20 30 40 5 0 " Sl Time in s

Fig. 9 Stresses in the ceramic turbine rotor of an exhaust gas-operated turboeharger. Example: Cold start, material S-SiC.

Temperature 1600 ~ i~ 1

K 1400

1200

1000

800

600

400 -"

0

E2oh~uSk~a?:~loTe£perature 950K /'~ f ~ A f~/A~ /

Fig. 13 Temperature rise during regeneration in individual matrix channels of an AMS particulate filter.

radial turbine made of ceramic. The material used was S-SiC. A design is being investigated with integral rotor and the connection to the metallic shaft is to take place at the "cold" end.

So far, ceramic rotors of this design have survived circumferential speeds of up to 385 m/s and gas temperatures of up to 1250 K without rupturing. At present the tests are being continued using a passenger-car diesel engine under real driving conditions. Accel- eration measurements in Figure II indicate that the response behaviour of the ceramic turbocharger has actually improved.

Porosity and Temperature Require- ments of Particulate Filters Finally, ceramic can also play an essential part as a material for partic-

ulate filters for diesel engines. Future governement regulations demand that certain limits in soot emissions are complied with in the operation of diesel engines. One of the most widely dis- cussed proposals for achieving these low values is the particulate filter. The purpose of this is that it should retain, accumulate and burn at increased temperatures as many soot particles as possible from the exhaust gas. What is required is a material which is suitably porous for filtering, which has a high resistance to thermal shock and from which it is possible to manufacture a filter at low cost and with a large surface to volume ratio.

Such a soot filter is shown in Figure 12. The material used is a special form of the glass ceramic AMS, similar to that which is used for the catalytic-

Fig. 10 Ceramic turbine rotor, mat- erial S-SiC.

.8 I Charge-o r Pressure / S in bar 1.6 6s//

1.4 / / ~ Ceralmic I Tutbocharget

1.2 / [ I Standard Turbochorger

1.o~1. ~ ' ' J 00 1 2 3

Time in s

mbo

4 5

Fig. 11 Response time of a turbo- charger with ceramic turbine rotor compared to standard version.

Fig. 12 Diesel particulate filter, ma- terial cordierite AMS (2Mg0.2AI2Os5 S fO2), right side loaded with carbon deposits.

converter support in spark-ignition engines. The required porosity is at- tained with 40 to 50% material density. The unit illustrated with a honeycomb matrix can be produced using the extruding method. The ducts at each end are sealed alternately such that the gas must pass through the porous duct wall and in doing so it deposits some of the soot.

MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986 79

Experiments show that such filters accumulate over a driving distance of 100 km between 20 to 30 grams of particulates. The filter can be regen- erated by burning these carbon deposits off. The peaks and the distribution of the temperatures of this burning pro- cess put severe requirements on the filter.

Figure 13 shows results of the temperature measurement of a regen- eration process, in which thermocouples have been installed in different locations over the filter. Obviously, the regener- ation of the whole filter lasts much longer than that of an individual chan- nel, which means that there is a flame front propagation through the filter. The temperatures yield 1300K if there is, as in the measured case, enough exhaust gas to cool the burning process down. If in real driving, however, the engine is being put to idling whilst the filter is still regenerating, with the smaller gas masses being even higher regeneration temperatures may occur. This behaviour might require ceramics

with even higher temperature capacity and thermal shock resistance than presently used for catalysts, c3~

Mass Production of Ceramic Components The stage of development discussed so far referred to individual items. These components were carefully selected and pre-tested before being subjected to the load cycles indicated. Ceramic materials are brittle and their strength is affected to a great extent by irregular- ities such as inclusions or micro-cracks in the material or on the surface of the component. The answer to the question as to whether or not, and if so, ceramic components can be used as standard in the app l ica t ions invest igated here, therefore depends not only on their functional advantages but also on questions as to which extent the scatter in the material can be restricted from a production point of view, and which economical methods can be established for performing, if possible, a 100% check of the components. The fact that

the prospects for usage are promising is proven by the examples where ceramics have already slipped into the engine almost unnoticed. These are AI203 as the spark-plug insulator. ZrO2 as the electrolyte of the X - s e n s o r and A M S as a catalytic-convertor support.

References

1 Interner Entwicklungsbericht 5/82 Karl Schmidt GmbH. Stuttgart

2 Fingerle, D. Anwendungkeramischer Bau- teile bei Nutzfahrzeug-Dieselmotoren. Techn. Akademie Esslingen 7173/64.011

3 Schweimer, G.W. Filter fur Dieselabgas und deren Regeneration. Techn. Akademie Esslingen 7173/64.011

Acknowledgements This article - 1985, SEA Paper No. 850567 (SP-610). - is reprinted with kind permission of the Society of Automotive Engineers Inc.

80 MATERIALS & DESIGN Vol. 7 No. 2 MARCH/APRIL 1986