Searching for the Low-Cost, Power-Dense, Aluminum Engine

Jeff Blackburn and Jeff Thrasher
Click here to see this story as it appears in the September/October 2017 issue of Metal Casting Design & Purchasing

Aluminum alloys play a significant role in the vehicle light-weighting movement, with applications ranging from aluminum sheet body panels to cast aluminum powertrain and structural components. While most industry experts agree aluminum content will continue to increase in the cars and trucks of tomorrow, the competition for market share among materials and manufacturing methods is intense in an industry driven by innovation but constrained by very high cost sensitivity. A new approach to forming closed deck aluminum engine blocks could substantially lower cost compared to what has been previously achieved.

Vehicle light weighting is a key contributor to OEM efforts to comply with future CAFE (Corporate Average Fuel Economy) requirements, but it is not the only one. Producing smaller displacement engines could offer improved fuel economy without sacrificing performance, often through the introduction of turbo charging. This increase in power per unit of engine displacement (specific output) means greater demands on the system as a whole, and a particular challenge lies in achieving sufficient strength and rigidity in the engine block head deck area. Historically, engine designers have had to choose between the robust closed head deck engine design formed via relatively expensive low pressure casting processes and an open head deck design formed via the more economical high pressure diecasting (HPDC) process. While attractive from a cost perspective, HPDC historically has been capable of producing only the open head deck design, which lacks the lateral cylinder wall support and rigidity of the closed deck option. As a result, when highly specific output is mandated, the engine designer often has had little choice but to develop a closed head deck design formed via the low pressure method and face a premium over HPDC that can exceed $80 per part produced.

Aluminum Engine Block Forming Methods
For years, aluminum has been a popular choice in engine block material in both mass market and niche applications. The material’s potential to deliver substantial weight savings over conventional iron blocks has seen it evolve into the material of choice for small and mid-sized engines. The fundamental difference between HPDC and low-pressure casting is that in HPDC, the steel forming tool can be used repeatedly (generally producing 80,000 parts or more before replacement) whereas the low pressure method usually sacrifices all or part of the tool (made of compressed sand mixed with binding agents) with each part produced. Inherent in the reuse of the HPDC mold is the requirement that all casting geometry be open in one direction so the forming tool can be pulled out of the casting after the aluminum solidifies. Casting geometry that is not open in one direction is considered undercut or closed geometry.

The key advantage of low-pressure casting is this ability to cast “closed” geometry, offering greater design flexibility than HPDC. However, the required sacrifice of the tool with every casting made, coupled with a substantially longer cycle time and greater floor space requirements drives a tremendous cost penalty: with an $80/casting premium over HPDC, a low-pressure manufacturing strategy can mean $100 million or more in incremental annual cost on a high-volume engine program.

Additionally, the low-pressure process generally cannot produce the very thin walls achievable in HPDC and thus typically produces a heavier product, to the chagrin of vehicle light weighting engineers.

Open and Closed Deck Engines
The terms open deck and closed deck refer to the absence or presence of material bridging the top of the cylinder walls to the peripheral body of the engine block. If this bridging material is not present the engine is considered an open deck design. If this bridging is present, it is considered a closed or semi-closed deck design. One can imagine how the closed deck design presents a fundamental problem for a casting process requiring removal of the steel forming tool.
A new method of creating the desired undercut closed deck geometry while using conventional HPDC tooling (no loose or lost cores) relies on casting dams the full depth of the water jacket channel.

These dams form the necessary strengthening bridges between the cylinder bores and the peripheral body.

If left as formed, these dams would block the flow path of the coolant along the length of the engine in the water jacket channel.

It is a subsequent machining step that transforms the dams into bridges, providing closed deck-style cylinder support while allowing the coolant to flow unobstructed below the bridges and along the cylinder walls.

The machining to remove the coolant-blocking portion of the aluminum dams is done through openings in the side faces of the engine block. To minimize the diameter of the holes, a woodruff style “T” cutter geometry is used. This preserves the valuable surface area on the sides of the block for mounting bosses and cast-in features while stiffening the area of connection to the cylinder wall, all while maintaining the desired water jacket cross sectional size.

This additional woodruff machining introduces incremental cost, but would consume just a small fraction of the original $80 per piece savings, while producing similar geometry.

The machined holes in the sides of the block are then plugged using core plugs. These plugs (sometimes referred to as frost plugs) have been used as a low cost, reliable method of sealing such low pressure passages in engine blocks for more than 50 years.

An added benefit of this new semi-closed deck HPDC forming method is that the dams introduced provide a marked improvement of the flow path for the molten aluminum during the casting phase. CAE simulation indicates this considerably improved flow pattern will improve cast material properties in the entire head deck face, especially the problematic fire ring area; simulation also points toward significant quality improvements in cylinder wall porosity, potentially easing implementation of spray bore technology where it is sought.

The Challenges and Potential Payoff
Other challenges remain in the diecasting of engine blocks with increased specific power, among them a need for greater strength in the bulkhead/bearing cap area. However, innovation and technology are attacking this problem, as well, with additive manufacturing methods now greatly increasing the cooling capability in today’s HPDC tooling, in turn delivering improved mechanical properties in these critical cast areas.

Today’s engine designer needs to consider many factors. However, in a world where savings of even a few dollars in piece price represent a notable victory, the possibility of removing $80 from the price of an engine block could provide a strong incentive to explore this new solution.