Additive Manufacturing Enhances GTDI Pistons

Selective Laser Melting may help manufacture future gasoline-engine pistons with enhanced heat-transfer properties and reduced weight.

April 1, 2016

Miguel Angel Reyes Belmonte, Sam Akehurst, Colin D. Copeland, Drummond Hislop, George Hopkins, Adrian Schmieder, Scott Bredda

Piston regions for conjugated HT-FEA 3D-CFD modeling.

The trend toward downsized gasoline turbo-charged direct-injected (GTDI) engines has raised peak combustion chamber pressures to 160 bar (2321 psi) and temperatures above 2000°C—nearly the limit of traditional materials and design. More than ever, effective thermal management is vital for durability and combustion stability.

Techniques such as adding oil-squirt jets to spray lubrication oil under the piston crown, while simple and effective, are often not sufficient for increasingly high-power-density engines.

In order to better understand the heat transfer and mechanical stress effects inside a downsized GTDI engine, the authors conducted a conjugate heat transfer and finite-element analysis. The intent was to study the piston design possibilities afforded by the use of the Selective Laser Melting (SLM) additive manufacturing process and its impact on further increasing engine efficiency, combustion stability and phasing. Potential benefits using SLM also exist in reducing piston mass and friction.

Model set-up

For the study, an aluminum piston in 2618A T6 alloy was used. An internal cooling gallery was added near the top land and first ring groove to improve piston thermal management and analyze the benefits on engine overall performance. A flat-top piston crown was selected over more complex geometries that may be possible with the SLM process, in order to target high temperature regions near the piston bowl.

Figure 2: Piston mesh for HT modeling (cooling gallery in blue).

The piston model was divided into the components shown in the main illustration. Due to the complexity of defining a full 3D-CFD conjugate heat transfer simulation including the combustion chamber flow field, it was decided to impose heat transfer coefficients obtained from previous research at the different piston elements. A temperature level of 1,000°C and 120 bar (1740 psi) were chosen as combustion gas conditions at the piston crown. A normal force of 70 kN (7.8 t) was considered at the piston pin bore for later FEA analysis. Finally, a mesh with a base size of 3 mm (.19 in) was selected for the HT modeling; see Figure 2.

In order to analyze the influence of introducing a cooling gallery near the top land, a heat transfer analysis was repeated for the same piston model with and without the cooling gallery. Heat-transfer study results were analyzed in terms of temperature and heat flux ratios at each of the piston regions. To simplify the analysis, the same color scale was used for both cases.

As can be observed, temperature differences between both models are evident, showing the higher temperature at piston top land and rings for the non-cooled case, as was expected. The lower temperature observed at those regions for the cooled piston is beneficial to prevent abnormal combustion events such as knocking or pre-ignition.

Table 1: Piston temperature distribution. Comparison between CFD modeling and literature survey data (temperature in °C)

Average temperature results are summarized in Table 1, the main differences observed on temperature distribution across the piston can be explained due to the fact that literature survey data for the cooled piston have been obtained from diesel piston suppliers. This is because cooled pistons for gasoline engines have not been widely used and there was not enough detailed information on that topic.

Table 2: Heat transfer ratio comparison (in %) between CFD modeling and literature survey data.

As shown in the table, modeled temperature agrees with the results found in previous works, biggest differences have been observed predicting oil temperature at the outlet of the cooling gallery. Modeled results show lower temperature at the oil outlet — a consequence of simplifying the complex phenomena of heat transfer to the oil due to the shaking effect. The temperature reduction found near the top land will help to optimize position of piston rings; this possibility and its benefits are studied in the next section.

Shown in Table 2, the heat transfer ratio found for the original (non-cooled) piston matches with the data observed on the literature survey. Regarding the heat transfer ratio calculated for the cooled piston, it was observed that the cooling gallery was not removing enough heat compared with previous works. The same fact was already detected from the lower oil outlet temperature obtained from the temperature analysis presented on Table 1. This difference can be explained due to the simplification assuming a constant value for the oil heat transfer coefficient, instead of doing a more complex analysis.

FEA stress analysis

Figure 3: Stress distribution for piston design that includes a cooling circuit.

Following conjugate heat-transfer CFD modelling, further piston modelling work was completed by undertaking FEA (Finite Element Analysis) by means of ANSYS R14.5 software. In order to do so, two load conditions were considered. Figure 3 shows the stress distribution map for the piston including a cooling gallery. Three regions have been chosen to study in detail von-Misses equivalent stress; these regions correspond to the piston crown, piston undercrown and piston bore.

In addition, safety factors (SF) have been calculated at each region using fatigue material data. This data is available at 200, 250, 300 and 325°C, and in the case where the temperature falls between two data sets, the fatigue limit at the higher temperature is used in order to give conservative values of SF. It was found that the weakest piston region of the three is the pin bore. Meanwhile the lower temperature due to increased cooling in the region of the new oil gallery increases the safety factor above 2.

As it was observed from piston heat-transfer analysis, temperature near the top land was reduced by around 50°C for the piston that included a cooling gallery. That temperature reduction would allow moving the top ring groove up closer to the crown, which would reduce piston crevice volume.

Figure 4: Top ring groove-position modification.

It has been widely studied that the piston crevice region, where the flame cannot penetrate, is the largest contributor (up to 80%) to engine-out hydrocarbon (HC) emissions. Meanwhile, an 86% decrease in piston top-land volume can decrease HC emissions by 20 to 40%. The authors analyzed different scenarios of modifying top ring groove position and focused on two iterations: moving the groove up by 1 mm and by 2 mm, as is depicted in Figure 4.

Figure 5: FEA for piston after moving up first ring.

FEA was performed to determine mechanical stresses distribution on piston design due to the ring movement; see figure 5. The ring groove fatigue strength is improved, as this region is approximately 50°C cooler than at the standard piston. Also, as the groove is moved up, the wall section increases where it is not directly adjacent to the cooling gallery, thus improving the stress condition further.

Taking into account the standard piston analyzed in this paper had a top-land height of 8.25 mm (.32 in), moving first ring groove up 2 mm (.07 in, a 25% reduction in top land height) will lead to an HC emissions reduction of approximately 10%.

SLM and piston lightweighting

Utilizing the SLM manufacturing technique could potentially enable lighter-weight pistons. The process is capable of creating complex internal geometries that include tailored internal voids. Less aluminum is used, and structural integrity of the piston maintained by including lattice structures above the pin bore.

Figure 6 shows the kinds of lattice structures that could be introduced internally using SLM. Heat transfer and FEA analysis were repeated for the piston model that included an internal region meant to be representative of the lattice structure. In order to perform this analysis, two approaches were adopted: First the lattice structure is considered as simply a void and secondly the same void is treated as possessing half of the material density.

On first approximation, it was considered an overly aggressive assumption to introduce two voids on the studied region. Under that assumption, piston weight was reduced 12%. For these studies the same two cases as explained before were analyzed. In order to perform a conjugated heat transfer analysis of revised piston design, the same heat transfer coefficients in the previous section were used.

Equivalent von-Misses stress was analyzed in detail at the same three numbered piston regions that were presented before. The assumption was that two voids inserted on the piston model have little effect on the crown peak stress or fatigue safety factor. However, it was detrimental at the undercrown and pin bore. Including a cooling effect from the oil gallery is unlikely to improve this, as this would only increase the fatigue life adjacent to the crown and ring grooves.

For this simulation, the void was modeled as a separate bonded piece assuming a 50% material density (aluminum) to the rest of the piston. However, the cooling gallery would mean the crown, undercrown and ring grooves would run significantly cooler. Thus they would have a higher fatigue limit than was taken into account for this analysis. Under this assumption, piston mass was reduced by 9% and the same loaded case setup and boundary conditions presented previously were repeated.

Stress is reduced at region 2 and 3 under the assumption of considering the lattice structured region as same material than the rest of the piston, but assuming half of its density. Peak stress at the top of the piston remains more or less unaltered whatever assumption was employed. Meanwhile, the peak stresses at the pin bore are within the safety material limit.

Next steps and conclusions

In order to complete the lightweighting analysis on the piston work it is planned to repeat the heat transfer and FEA stress analysis with the same piston model but introducing a real lattice structure.

That structure will be built by means of SLM manufacturing process and will be placed at the region where a void had been considered for this paper.

The influence of introducing a cooling gallery placed near the top land was studied and temperature reductions between 30°C and 50°C were observed. The temperature reduction observed after introducing a cooling gallery makes the piston less sensitive to knocking. The top land temperature reduction also could help to increase engine compression ratio and/or reduce piston height, which would result in friction reduction. In addition, the observed temperature reduction will allow a more optimal first ring grove position thereby reducing the crevice volume.

The study suggests the potential for reducing piston mass by introducing lattice structures that are able to be built by utilizing SLM. As a first approach, it was assumed the lattice structure as a void and it was found that stress near the pin bore and void region rose significantly. As a second approach, it was assumed that lattice structure region could be represented by means of same material but using half of its density.

Under that assumption, peak mechanical stress and material fatigue limit are found near the piston bore region. However, the authors expect piston bore stress will be far from the material limit once realistic lattice structure properties replace the void assumption.

This feature is based on SAE Technical Paper 2015-01-0505 authored by: Miguel Angel Reyes Belmonte, Sam Akehurst and Colin D. Copeland (Univ. of Bath), Drummond Hislop, George Hopkins and Adrian Schmieder (HiETA Technologies Ltd.) and Scott Bredda (GE Precision) titled, “Improving heat transfer and reducing mass in a gasoline-engine piston using additive manufacturing.”