SYSTEM COMPARISON OF

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1 SYSTEM COMPARISON OF steel & ALUMINIUM PISTONS FOR PC DIESEL ENGINES In 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. 24

2 Authors DR. SIMON SCHNEIDER is Project Manager Corporate Advanced Engineering for PC Diesel Technology at Mahle International GmbH in Stuttgart (Germany). DIPL.-ING KAI SCHREER is Project Manager Pre-Development HSD steel Pistons at Mahle GmbH in Stuttgart (Germany). DIPL.-ING. HOLGER EHNIS is Development Engineer in the Engine Test Laboratory at Mahle International GmbH in Stuttgart (Germany). DR. STEFAN SPANGENBERG is Director Product Development Engine systems and Components Europe at Mahle GmbH in Stuttgart (Germany). 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 targeted the frictional and thermodynamic differences between the two while maintaining emissions values. For this purpose, 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 steel piston concept. The reduced oscillating 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 compression height can be used to extend the length of the conrod in an existing engine concept, for example, while keeping 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 compression height by adjusting the displacement of the engine (rightsizing) and the combustion chamber geometry. For a new 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 c w value and pedestrian protection for the vehicle as a whole. The lower thermal expansion of steel furthermore allows the installation clearance to be tight, while maintaining sufficient operating clearance, when the piston 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 identical nitrogen oxide emissions (achieved by adjusting the EGR rate) and identical 50 % heat release points. For full-load operation, the comparison is made only for identical 50 % heat release points. 2 shows the difference in friction for the two variants in the operating map of As a material, steel is characterised by the following properties as compared with aluminium: :: Reduced thermal expansion, :: Increased strength, :: Greater density, and :: Reduced thermal conductivity. These properties must be taken into consideration and exploited when implementing a design for a steel piston concept, which is the only way that they can contribute to reducing CO 2 or to improving engine performance. For the steel piston, the wall thickness can be reduced greatly due to its higher strength, 1. Consequently, the weight of the piston group can be the same or even lower with a Top land Cooling channel A Wall thickness at piston bowl Compression height Top land Cooling channel B Wall thickness at piston bowl ❶ Comparison of the geometry of the steel piston (A: TopWeld) and the aluminium piston (B: piston with cooled ring carrier) Compression height autotechreview October 2014 Volume 3 Issue 10 25

3 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 ) the test engine. The steel piston has a friction advantage under high loads of up to 0.1 bar friction mean effective pressure (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 considered essentially comparable (measurement accuracy ΔFMEP = ± 0.03 bar). The equivalent level of friction in this comparison 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 significantly for both variants (e.g., at 1,500 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 somewhat higher loads, the rise in temperature increases, leading to overlap due to thermal expansion of the aluminium piston 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 26 thermodynamics and the oil pump drive (e.g., at 2,000 rpm and 100 Nm by 9 g/ kwh BSFC). This behaviour is less severe for the steel piston, but Mahle recommends that the piston cooling is not shutoff, 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. Due to the tighter installation clearance 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 improvements due to thermodynamic advantages range between 4 and 8 g/kwh. The thermodynamics are affected by the following parameters: :: Blow-by quantity: For measurements ❸ Loss distribution for the operating point 1500 rpm, 200 Nm (left), fuel consumption advantage of the steel piston at selected operating points (right)

4 with identical ring packs, the steel piston 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 combustion chamber: The tested configuration exhibits a difference of about 50 C in the maximum component temperature at the bowl rim for steel and aluminium pistons with identical cooling (steel piston: maximum 430 C, measured near the surface). The difference is even greater at the centre of the bowl, at 90 C, as this area is more difficult to cool with steel pistons. The first ring groove shows the same temperature level for both piston types (maximum 190 C). The tests indicate only a minor effect of piston temperature on fuel consumption. The loss distribution from the pressure curve analysis 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 emissions of the steel piston and has a positive influence on the effective compression 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 piston 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 earlier, with the centre of combustion at the same location. 3 shows an example of loss distribution for an operating point with identical nitrogen oxide emissions and the same position of centre of combustion, 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 aluminium and steel piston are fundamentally different. In the aluminium piston, the heat is distributed more uniformly due EGR [%] Oil flow for four pistons [l/min] HR50 [ CA a TDC] Temperature cylinder 2 bowl rim, thrust side 2000 rpm, bar IMEP Aluminium piston Steel piston Baseline setting (ISO HR50, ISO NO x, p oil = 2 bar, T oil = 90 C) ❹ Changes in piston temperature at the bowl rim by varying engine parameters and by varying piston cooling ISFC [g/kwh] Indicated specific fuel consumption Temperature of bowl rim thrust side, cyl FMEP [bar] X oil total [%] 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 contrast, 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 temperature were investigated. This is shown as an example for the bowl rim of the piston at 2,000 rpm and 250 Nm, 4. The base temperature (series settings) is 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, however, no setting for which the aluminium piston would attain the component temperatures of the steel piston. A variation in temperature of up to 15 C can be achieved by means of the EGR rate and 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, 5. For a difference in cooling oil temperature of 30 C, the steel and aluminium piston would have the same bowl rim temperature. The change in cooling oil flow rate has the Friction mean effective pressure *See note Ratio of engine oil enthalpy (for energy balance) *See note *Heating of base engine oil necessary 2000 rpm bar IMEP Aluminium piston Steel piston Baseline setting (ISO HR50, ISO NO x, p oil = 2 bar, T oil = 90 C) 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 autotechreview October 2014 Volume 3 Issue 10 27

5 ❻ Mahle passenger car steel piston range (A: Monotherm, B: TopWeld, C: MonoGuide) potential to vary the bowl rim temperature by up to 50 C. An optimised oil flow rate provides the opportunity 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 piston. 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 temperature at the skirt rises by 15 C for a smaller oil volume flow, and the reduced oil viscosity has a positive effect on friction. 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 28 Since the steel piston has a substantially higher temperature level than the aluminium piston, as demonstrated, scaling can occur at severely thermally loaded locations 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 piston. The coke oil deposits reduce the cooling efficiency and thus additionally aggravate 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 presents another challenge. Due to the difference 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 clearance, 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 10 years and will be employed in the first passenger car diesel engines in The thermally decoupled and flexible shaft of the one-piece forged piston has the greatest potential for CO 2 savings at the lowest weight, 6. Another concept in the Mahle passenger car steel piston portfolio is the socalled 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 thicknesses, especially between the combustion chamber and the cooling channel, which in turn makes it possible to optimally cool the bowl rim. The consistent on-going 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 characterised by a flexible, decoupled skirt. This gives the piston excellent seizure resistance and, together with the skirt that extends to the ring area, optimal guidance in the liner with good noise behaviour. 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 aluminium piston. The attainable fuel consumption 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 [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 [4] Schäfer, B.-H.; Schneider, V.; Geisselbrecht, M.: Real-time Kolbentemperaturmessungen mit einem auf Telemetrie basierenden Datenübertragungssystem Messtechnikapplikation und erste Ergebnisse. 10th Stuttgart International Symposium, 2010 [5] Stitterich, E.; Geisselbrecht, M.; Künzel, R.: Influence of cooling channel design on piston temperature of HSD engines. 13th Stuttgart International Symposium, 2013 Read this article on