MMLV: Aluminum Cylinder Block with Bulkhead Inserts and Aluminum Alloy Connecting Rod 2015-01-1238
The Multi Material Lightweight Vehicle (MMLV), developed by Magna International and Ford Motor Company, is a result of US Department of Energy project DE-EE0005574.
The project demonstrated the lightweighting potential of a five-passenger sedan while maintaining vehicle performance and occupant safety. Prototype vehicles were manufactured and limited full-vehicle testing was conducted. The Mach-I vehicle design, comprised of commercially-available materials and production processes, achieved a 364 kg (23.5%) full-vehicle mass reduction. This resulted in environmental benefits and fuel economy improvements.
A significant factor in the overall MMLV mass reduction was the decrease in the powertrain system weight from 340 kg (conventional) to 267 kg (MMLV). This enabled the application of a 1.0-liter three-cylinder engine as the main powerplant. By downsizing the engine, and by implementing material changes within the engine, the weight of the dressed engine was lowered by 29 kg.
This paper examines the results from studies performed on two key MMLV powertrain components: an aluminum engine block with steel bulkhead inserts; and a connecting rod forged using a high-strength aluminum alloy. Both can yield significant weight savings compared to their conventional iron and steel equivalent components.
For the MMLV, a small-displacement, high-output engine was desired for the powertrain. Based upon a prototype1.0L I3, the MMLV variant was cast in aluminum instead of gray iron. As the aluminum block structure was not sufficient to carry the loads associated with operation, reinforcement was needed. A bulkhead insert (patent-pending) was designed to carry the loads from the head deck through to the crankshaft area while maintaining stable thermal growth characteristics in the main crank journal. The optimum geometry of the insert was found to have an I-beam cross-section; macro-serrations were added to enhance the bond between the block and bulkhead insert.
This study found that the cast aluminum block with the bulkhead inserts reduced the weight of the I3 block by 9.5 kg; this represents a 42% savings compared to a block cast in iron. The study also determined that bulkhead inserts can be designed to carry 95% of the main bearing loads generated from the combustion process. Additionally, the casting process allowed for a high interfacial bond to develop between the block and insert. Finally, a bulkhead fatigue bench test was used to validate the design and accurately predict failure location in the bulkhead insert.
The results of the study have been used to guide a decision to continue the evaluation of the aluminum block with bulkhead inserts.
The aluminum connecting rod study found that, with a careful selection of material and processes, it is feasible to produce aluminum connecting rods for high volume OEM vehicles. Further, the study found that a recently developed high-temperature, high strain-rate forging process does improve the ductility and fatigue strength of a 2618-T6 aluminum alloy.
CAE analysis was used to confirm that a connecting rod can be designed using the 2618-T6 alloy to meet fatigue safety factor as well as clamp load and fastener requirements. The analysis determined that the Al alloy reduced the weight of a connecting rod by over 200 g (for a total saving of 700 g for the I3 engine); this represents a weight savings of 40% compared to conventional steel alloy rods.
As with the block, the initial study on the aluminum connecting rod indicates that further material, design, and component verification is warranted.
