STATE OF THE ART

The aluminium piston is currently the state of the art for passenger car (PC) diesel engines. Steel pistons are under development [1] and are close to start of production. In order to directly compare the behaviour of the two piston concepts in a passenger car application, Mahle performed a system comparison of steel and aluminium pistons in a turbocharged diesel engine [2]. The investigation tar-geted the frictional and thermodynamic differences between the two while main-taining emissions values. For this pur-pose, an aluminium piston with cooled ring carrier was compared with a so-called TopWeld steel piston. The friction was measured using the indicating method [3], the piston temperatures were captured online during operation [4],

and a thermodynamic assessment and exhaust gas analysis were performed.

COMPARISON OF THE STEEL AND ALUMINIUM PISTON DESIGNS

As a material, steel is characterised by the following properties as compared with aluminium: : reduced thermal expansion : increased strength : greater density : reduced thermal conductivity. These properties must be taken into con-sideration and exploited when implement-ing a design for a steel piston concept, which is the only way that they can con-tribute to reducing CO2 or to improving engine performance. For the steel piston, the wall thickness can be reduced greatly due to its higher strength, ❶. Conse-

quently, the weight of the piston group can be the same or even lower with a steel piston concept. The reduced oscil-lating masses may make it possible to eliminate the balance shafts.

The steel piston also allows the cooling gallery to be positioned higher [5], thus reducing the top land height. In both cases, a reduction in compression height becomes possible. The reduced compres-sion height can be used to extend the length of the conrod in an existing engine concept, for example, while keep-ing the swept volume the same. This reduces the maximum lateral forces and therefore the friction forces on the piston skirt. It is also possible, however, to take advantage of the reduction in compres-sion height by adjusting the displacement of the engine (rightsizing) and the com-bustion chamber geometry. For a new

AUTHORS

DR. SIMON SCHNEIDER is Project Manager Corporate

Advanced Engineering for PC Diesel Technology at the Mahle International

GmbH in Stuttgart (Germany).

DIPL.-ING KAI SCHREER is Project Manager Pre-Development HSD Steel

Pistons at the Mahle GmbH in Stuttgart (Germany).

DIPL.-ING. HOLGER EHNISis Development Engineer in

the Engine Test Laboratory at the Mahle International

GmbH in Stuttgart (Germany).

DR. STEFAN SPANGENBERG is Director Product Development

Engine Systems and Components Europe at the Mahle GmbH

in Stuttgart (Germany).

DEVELOPMENT PISTONS

32

Pistons

SYSTEM COMPARISON OF ALUMINIUM AND STEEL PISTONS FOR PC DIESEL ENGINESIn recent years, the steel piston has proven to be significantly superior to its aluminium counterpart under the

special operating conditions of commercial vehicle engines. The greater strength of steel, in particular, is a

primary factor due to the prevalence of high mechanical loads. Recently, it has been increasingly considered

whether the use of steel pistons would also be advantageous in passenger car diesel engines. Mahle investigated

this topic through a system comparison in a turbocharged diesel engine.

development of an engine series, the reduced compression height can directly reduce the overall height of the engine, thus decreasing the installation space required. This can have a positive effect on the cw value and pedestrian protection for the vehicle as a whole.

The lower thermal expansion of steel furthermore allows the installation clear-ance to be tight while maintaining suffi-cient operating clearance when the pis-ton is hot. Since frictional losses can be avoided with low piston overlap, this provides an advantage especially under high loads.

OPERATING BEHAVIOUR

The pistons compared in this study are developed to the point of series production

and their clearance is optimised for each concept. Both feature a DLC-coated piston pin of the same diameter and the same ring pack optimised for frictional loss. The operating values of the pistons are compared in the operating map for identi-cal nitrogen oxide emissions (achieved by adjusting the EGR rate) and identical 50 % heat release points. For full-load opera-tion, the comparison is made only for identical 50 % heat release points.

❷ shows the difference in friction for the two variants in the operating map of the test engine. The steel piston has a friction advantage under high loads of up to 0.1 bar friction mean effective pres-sure (FMEP), which corresponds to as much as 3 g/kWh break specific fuel consumption (BSFC). Under low loads, the frictional loss behaviour of the steel

and aluminium variants can be consid-ered essentially comparable (measure-ment accuracy ∆FMEP = ± 0.03 bar). The equivalent level of friction in this com-parison is achieved with an aluminium piston with relatively high installation clearance. As the clearance is reduced, the frictional loss advantage of the steel piston becomes more pronounced.

Differences in frictional losses occur particularly when the piston cooling is shut off. The frictional loss is then neutral only for a limited set of conditions, as the piston temperature increases signifi-cantly for both variants (e.g. at 1500 rpm and 50 Nm at the bowl rim by 35 °C for the aluminium piston and by 60 °C for the steel piston). For ranges with some-what higher loads, the rise in tempera-ture increases, leading to overlap due to thermal expansion of the aluminium pis-ton even though the high installation clearance. The frictional loss then rises sharply and causes an overall increase in fuel consumption – despite the advantages of the thermodynamics and the oil pump drive (e.g., at 2000 rpm and 100 Nm by 9 g/kWh BSFC). This behaviour is less severe for the steel piston, but Mahle re -commends that the piston cooling is not shut off, particularly for steel pistons, because otherwise the engine oil can deteriorate strongly on the inside of the cooling gallery and beneath the centre of the combustion bowl.

Com

pres

sion

hei

ght

Top

land

Wall thicknessat piston bowl

Coolingchannel

Com

pres

sion

hei

ght

Top

land

Wall thicknessat piston bowl

A B

Coolingchannel

❶ Comparison of the geometry of the steel piston (A: TopWeld) and the aluminium piston (B: piston with cooled ring carrier)

10I2013 Volume 74 33

Pistons

Due to the tighter installation clear-ance of steel pistons, no acoustic issues are generally expected for a cold engine, but even in the warm state the engine remained acoustically unobtrusive.

COMBUSTION

The indicated specific fuel consumption (ISFC) in all cases is significantly lower with steel pistons than with aluminium pistons. The operating map improve-ments due to thermodynamic advantages range between 4 and 8 g/kWh. The ther-modynamics are affected by the follow-ing parameters: : Blow-by quantity: For measurements

with identical ring packs, the steel pis-ton results in 15 to 45 % less blow-by. About 30 % (at partial load) or 10 % (at high load) of the advantages in fuel consumption can be ascribed to the difference in blow-by.

: Higher wall temperatures in the com-bustion chamber: The tested configura-tion exhibits a difference of about 50 °C in the maximum component tempera-ture at the bowl rim for steel and alu-minium pistons with identical cooling (steel piston: maximum 430 °C, meas-ured near the surface). The difference is even greater at the centre of the

bowl, at 90 °C, as this area is more dif-ficult to cool with steel pistons. The first ring groove shows the same tem-perature level for both piston types (maximum 190 °C). The tests indicate only a minor effect of piston tempera-ture on fuel consumption. The loss dis-tribution from the pressure curve anal-ysis indicates similar or slightly lower wall heat losses for the steel piston.

: Reduced top land volume: The first piston ring on the steel piston can be placed at a higher position than on the aluminium piston. The smaller top land volume is advantageous for CO emis-sions of the steel piston and has a posi-tive influence on the effective com-pression ratio for the same combustion chamber geometry. It is beneficial to reduce this volume, which makes this a system advantage of the steel piston.

The combustion process of the steel pis-ton is characterised by a shorter duration in the second half of the combustion cycle as a result of the effects described. The 90 % heat release point is up to 5 °CA ear-lier, with the centre of combustion at the same location. ❸ shows an example of loss distribution for an operating point with identical nitrogen oxide emissions and the same position of centre of com-bustion, as well as the fuel consumption

results at a few selected operating map points (1 to 7), which represent a “normal operation”.

PISTON TEMPERATURES

The temperature distribution in the alu-minium and steel piston are fundamen-tally different. In the aluminium piston, the heat is distributed more uniformly due to the high thermal conductivity and larger material cross sections, and is then dissipated by the cooling oil. The heat transport in the steel piston, in con-trast, is rather limited and takes place primarily by means of the cooling oil.

Starting from this behaviour, the effects of various parameters on the piston tem-perature were investigated. This is shown as an example for the bowl rim of the pis-ton at 2000 rpm and 250 Nm, ❹. The base temperature (series settings) is 260 °C for aluminium and 277 °C for steel. It is immediately evident that the influence of the parameters on the temperature is similar for both variants (similar gradient of the temperature curves). There is, how-ever, no setting for which the aluminium piston would attain the component tem-peratures of the steel piston. A variation in temperature of up to 15 °C can be achieved by means of the EGR rate, and

❷ Friction difference in the operating map, shown as the difference in friction mean effective pressure (FMEP) over indicated mean effective pressure (IMEP) and engine speed (positive values: steel piston has lower friction) (full load points (1 to 9) examples with increasing speed of the full load curve, mapping points 1 to 7 as a representative selection for a “normal operation”)

DEVELOPMENT PISTONS

34

around 20 °C if the centre of combustion is shifted to an extreme extent.

Piston cooling can have a significant direct effect on the piston temperature without negatively affecting the engine thermodynamics to any substantial degree. A change of 70 °C in the cooling oil temperature changes the bowl rim temperature by about 35 °C. The effect is nearly linear. Fuel consumption and engine friction are barely altered, ❺. For a difference in cooling oil temperature of 30 °C, the steel and aluminium piston would have the same bowl rim tempera-ture. The change in cooling oil flow rate has the potential to vary the bowl rim temperature by up to 50 °C. An opti-mised oil flow rate provides the opportu-nity to adjust the piston temperature in a targeted manner with a reasonable level of effort. This is better achieved for the steel piston than for the aluminium pis-ton. For small oil flow volumes, the steel piston exhibits a friction advantage of 0.04 bar (corresponding to 1 g/kWh BSFC). This is due to the fact that the tempera-ture at the skirt rises by 15 °C for a smaller oil volume flow, and the reduced oil viscosity has a positive effect on fric-tion. In contrast, small oil volume flows are critical for steel pistons with respect to cooling channel coking and surface scaling.

CHALLENGES FOR SERIES-PRODUCTION READINESS

Since the steel piston has a substantially higher temperature level than the alumin-

ium piston, as demonstrated, scaling can occur at severely thermally loaded loca-tions such as the bowl rim. This scaling layer and the scaling scars that form can become initiation points for bowl rim cracks during subsequent operation. Another challenge is the tendency of the piston cooling oil to coke in the cooling channel and at the inner form of the pis-ton. The coke oil deposits reduce the cool-ing efficiency and thus additionally aggra-vate the temperature problems at the bowl rim. The soot input from combustion also has a rather significant effect on oil aging and the tendency for carbon build-up.

The development of steel pistons for engines with aluminium crankcases pre-sents another challenge. Due to the dif-

ference in thermal expansion between steel piston and aluminium cylinder block, the operating clearance increases as the temperature rises, and the piston may strike the cylinder wall with greater impact. This can be counteracted by optimising the piston installation clear-ance, the shape of the piston, and the piston pin offset.

MAHLE PASSENGER CAR DIESEL STEEL PISTONS

The so-called Monotherm piston has been proven and tested millions of times in commercial vehicle engines for over ten years and will be employed in the first passenger car diesel engines in

❸ Loss distribution for the operating point 1500 rpm, 200 Nm (left), fuel consumption advantage of the steel piston at selected operating points (right)

300

280

260

0 10 20EGR [%]

T bow

l rim

[°C

] 300

280

260

Temperature cylinder 2bowl rim, thrust side

2000 rpm,15.98 bar IMEP

Aluminium pistonSteel piston

Baseline setting(ISO HR50, ISO NOx,poil = 2 bar, Toil = 90 °C)

8 12 16 20HR50 [°CA a TDC]

T bow

l rim

[°C

]

300

280

260

10 20Oil flowfor four pistons [l/min]

T bow

l rim

[°C

] 300

280

260

50 70 90 110Toil piston [°C]

T bow

l rim

[°C

]

❹ Changes in piston temperature at the bowl rim by varying engine parameters and by varying piston cooling

10I2013 Volume 74 35

2014. The thermally decoupled and flexi-ble shaft of the one-piece forged piston has the greatest potential for CO2 savings at the lowest weight, ❻.

Another concept in the Mahle passen-ger car steel piston portfolio is the so-called TopWeld piston, made from two parts joined together. Characterised by a closed cooling gallery and an attached skirt, this piston type is suitable for the highest peak cylinder pressures. Its greater rigidity allows for smaller wall thick-nesses, especially between the combus-tion chamber and the cooling channel, which in turn makes it possible to opti-mally cool the bowl rim.

The consistent ongoing development and optimisation of the advantages of both piston concepts results in the new so-called MonoGuide piston, which is

also a two part joined piston. Analogous to the Monotherm piston, it is character-ised by a flexible, decoupled skirt. This gives the piston excellent seizure resist-ance and, together with the skirt that extends to the ring area, optimal guid-ance in the liner with good noise behav-iour. This type of piston is therefore also perfectly suited for use in engines with aluminium crankcases.

OUTLOOK

It is expected that the steel piston will find wider application in passenger car series production in addition to the alu-minium piston. The attainable fuel con-sumption advantages and the possible use at maximum combustion pressures will surely be a motivating factor.

REFERENCES[1] Baberg, A.; Freidhager, M.; Mergler, H.; Schmidt, K.: Aspects of Piston Material Choice for Diesel Engines. In: MTZ worldwide 73 (2012), No. 12, pp. 26-30[2] Schneider, S.; Ehnis, H.; Schreer, K.: Analyse von Aluminium- und Stahlkolben – Vergleich von Reibung, Kolbentemperatur und Verbrennung. International Stuttgart Symposium, 2013[3] Deuß, T.; Ehnis, H.; Freier, R.; Künzel, R.: Friction Power Measurements of a Fired Diesel Engine Piston Group Potentials. In: MTZ worldwide 71 (2010), No. 5, pp. 20-24 [4] Schäfer, B.-H.; Schneider, V.; Geisselbrecht, M.: Real-time Kolbentemperaturmessungen mit einem auf Telemetrie basierenden Datenübertragungssys-tem – Messtechnikapplikation und erste Ergebnisse. 10th Stuttgart International Symposium, 2010[5] Stitterich, E.; Geisselbrecht, M.; Künzel, R.: Influence of cooling channel design on piston tem-perature of HSD engines. 13th Stuttgart International Symposium, 2013

❻ Mahle passenger car steel piston range (A: Monotherm, B: TopWeld, C: MonoGuide)

198

196

194

192

190

188

Indicated specific fuelconsumption

ISFC

[g/

kWh]

290

280

270

260

250

50 70 90 110Toil piston [°C]

Temperature of bowl rim thrust side, cyl. 2

T bow

l rim

[°C

]

18

16

14

12

50 70 90 110Toil piston [°C]

Ratio of engine oil enthalpy(for energy balance)

*Heating of base engine oil necessary

2000 rpm15.98 bar IMEP

Aluminium piston

Steel piston

Baseline setting(ISO HR50, ISO NOx,poil = 2 bar, Toil = 90 °C)

X oil t

otal [

%]

1.00

0.98

0.96

0.94

Friction mean effective pressure

*See note

*See note

FME

P [

bar]

❺ Variation of temperature of the piston cooling oil (base engine at a constant 90 °C): effects on piston temperature, fuel consumption, engine friction, and overall proportion of the oil heat in the split of losses

DEVELOPMENT PISTONS

36

10I2013 Volume 74 37

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/// SCIENTIFIC DIRECTORProf. Dr. Helmut Tschöke

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Simultaneous InterpretingGerman and English

Heavy-Duty, On- andOff-Highway EnginesEvolution or Revolution – Quo vadis?

8th International MTZ Conference

5 and 6 November 2013

Ludwigsburg | Germany

NEW DIESEL AND GAS ENGINESReducing emissions andfuel consumption

MIXTURE FORMATION AND SUPERCHARGINGNew solutions and systems

ENGINE AND SYSTEM OPTIMIZATIONHybrid systems and new solutions

/// SCIENTIFIC DIRECTOR

Prof. Dr. Helmut TschökeOtto-von-Guericke UniversityMagdeburg