Small, In-line Passenger Vehicle Engines: Cast Iron with the Same Weight as Aluminum

Multiple Authors

Although compacted (vermicular) graphite iron (CGI) was first observed and patented in 1948, the first series production CGI engine wasn’t launched until 1999. The nearly 50-year development cycle, from innovation to implementation, was primarily due to the narrow stable range for the reliable series production of high-quality CGI.

Tupy, with foundries in Brazil, Mexico, and Portugal, has been a pioneer in the development and growth of CGI; however, there has not yet been a high-volume application of CGI for small in-line petrol/gasoline engines. Therefore, in order to demonstrate the potential benefits of CGI in this highest-volume sector, the foundry group collaborated with the engine design firm Ricardo and SinterCast to convert the cylinder block of a modern, state-of-the-art three-cylinder gasoline engine from aluminum to CGI. The revised engine was simultaneously upgraded to a 48-volt hybrid configuration to further demonstrate the potential of CGI in hybrid and range-extender applications. 

Cylinder Block Design

Over the past 30 years, the minimum wall thickness in cast iron cylinder blocks has progressively decreased from 4.5 mm (+1.5 mm / -1.0 mm) produced in green sand molds to the current series level of 3.0 mm (+/- 1.0 mm) produced in fully enclosed core packages. The present study took a further step forward, establishing 2.7 mm as the nominal wall thickness (+/- 0.8 mm). While it is possible to make further reductions, the present study acknowledged that the benefit gained by further reducing all of the minimum walls from 2.7 mm to 2.5 mm is one of diminishing returns—the task is challenging, and the weight reduction is marginal. Instead, the present study introduces a creative new approach to achieve the next quantum step in weight reduction. First, the overall architecture was re-imagined, incorporating low-density plastic covers for the lower crankcase housing and oil sump; and second, CGI grade GJV 550 was introduced to enable substantial reductions in the thickness of the heaviest load-bearing sections of the block.

The reference engine for the present study was a state-of-the-art 1.2 liter, in-line three-cylinder gasoline engine, launched in 2016, based on an aluminum cylinder block. The objective was to convert the cylinder block from aluminum to CGI while maintaining the performance, total weight, and power density of the engine. In order to assemble a running engine for durability testing, the outer dimensions and bore centers of the CGI cylinder block were maintained to allow components to be used from the donor engine. The weight of the original aluminum cylinder block, shown in Figure 1(a), was 14.05 kg/31 lb (16.59 kg/36.6 lb including bearing caps and fasteners). The fully assembled aluminum engine weight is 92.0 kg.

The high pressure diecast aluminum cylinder block, produced with no internal sand coring, comprised a metal volume of 5,230 cubic cm. Minimum wall thickness was 4.0 mm, and there were several heavy sections throughout the block. For reference, direct material substitution with CGI in the existing aluminum design would result in a weight of approximately 38 kg (83.7 lb). The first design analysis, based on conventional lightweighting techniques, showed that the maximum possible reduction of wall thicknesses in the existing architecture could lead to a CGI block weight of approximately 30 kg (66.1 lb); still an increase of 40-45% relative to the aluminum engine. It was clear that a completely new design approach was needed. The task therefore began to identify and eliminate all unnecessary mass and to modify the architecture to apply plastic casings for the lower crankcase housing and the oil sump.

The first CGI design variant was developed with the use of OptiStruct software with the cylinder block being subjected to eight different load conditions, including firing, bending and torsional loading. The block design was iteratively modified to produce an architecture that provided the same stiffness as the original aluminum block, with a minimum volume of material. The resultant design is shown in Figure 2 (a). The initial design was analyzed for bore distortion and ring conformability, head mounting stresses and local stress concentrations, together with computational fluid dynamics (CFD) analysis to predict peak temperatures in the cylinder bore walls. Regions with low safety factors were modified by altering contours, section thicknesses, and fillet radii to approach the safety factors and stiffness profiles of the original aluminum block. The final design of the CGI cylinder block, together with fracture split main bearings and CGI ladder frame is shown in Figure 2 (b). Nominal wall thickness for both the cylinder block and the ladder frame was 2.7 mm +/- 0.8. The fully assembled cylinder block, with the plastic outer casings affixed, is shown in Figure 2 (c).

Over the eight load cases investigated, the final CGI design provided 92% of the stiffness of the original aluminum block, and 96% for the six most dominant load cases. Likewise, cylinder bore distortion was similar to the original aluminum engine and satisfied ring conformability with 1 μm oil film. 

The minimum bore wall thickness in the inter-bore area adjacent to the cored water-cooling passage was set at 2.0 mm, which is consistent with the wall thickness applied in a high-volume CGI gasoline engine manufactured by the foundry. 

Bore wall temperatures displayed a normal profile with typical temperatures of approximately 200C (392F) and peak temperatures remaining below 230C (446F).

The weight reduction offered by the CGI cylinder block concept was augmented by the novel use of durable plastic casings that enclosed the lower crankcase and also served as the oil sump. The casings were fabricated from PA66GF30 plastic with a density of 1,690 kg/m3 and were designed to incorporate the oil galleries, the oil cooler mount, the timing case and the lower oil pan. The use of low-density plastic casings provides a significant weight reduction contribution to the CGI design where the parent metal density is approximately 7,150 kg/m3 but does not confer any significant benefit in aluminum designs where the substitution of metal by plastic would require thicker wall sections for bottom-end durability and where the parent metal density is approximately 2,700 kg/m3 The ladder frame was initially considered in both CGI and aluminum versions, however, the aluminum version resulted in increased bearing distortion due to thermal expansion differences. For this reason, combined with durability considerations, the engine was developed with a CGI ladder frame.

The final consolidation of weights for the different cylinder block configurations is summarized in Table 1. The use of CGI, together with the novel hybrid iron-plus-plastic design concept, provided weight parity for the cylinder block assembly, and therefore for the fully assembled engine.

Proof Of Concept Durability Testing

The project concluded with a 100-hour dynamometer test to validate the durability of the design concept. The test was divided into six segments including a four-hour break-in period with engine speeds ranging from 1,250 to 5,000 rpm; four segments of 20~25 hours at progressively increasing load levels; and a four-hour segment at full load of 5,000 rpm and 180 Nm of torque. Regular borescope inspections and performance checks were conducted during the initial break-in period to ensure proper mating and bedding-in of pistons, rings and bore surfaces. Thereafter, borescope inspections were conducted every ten hours, and the engine oil was changed at the 50-hour interval, according to standard test procedures. The test cycle is summarized in Table 2 while the test bed is shown in Figure 3.
The dynamometer test successfully validated the mechanical viability of the concept. The fuel consumption (25.8 kg/h) and blow-by (66-74 L/min) under 5,000 rpm full load conditions, and the Friction Mean Effective Pressure (0.25 bar) at 2,000 rpm, were all within the normal range of the aluminum series production engine. All of the major components demonstrated solid durability with no evidence of deformation or cracking in the cylinder block or in the fracture split main bearings. Likewise, there was no evidence of blow-by leakage around the cylinder head gasket, and the piston crowns and under-crown positions were in good condition upon visual inspection at the conclusion of the test. The piston rings were also in good condition at the conclusion of the test with piston ring gaps ranging from 0.23 to 0.28 mm, well-within the engine designer’s top ring specification of 0.15-0.30 mm. The final bore roundness was similar to that measured before the start of the test and as shown in Table 3, was generally within the engine designer’s guideline of 10 μm at the conclusion of the test.

Beyond the major engine components, the outer plastic casing also proved to be durable during the proof-of-concept test. The inner and outer surfaces of the left-side plastic casings, at the conclusion of the engine test, are shown in Figure 4.

Foundry and Machining

Upon receipt of the structural design, MAGMASOFT simulations were conducted to establish and optimize the mold filling. The primary objectives for the casting process were to avoid carbides in the thin outer walls; to avoid turbulence that could lead to casting defects or cold shuts; and, to minimize residual stresses, particularly at the junction of thick and thin walls at the bottom end of the block. The simulations led to a gating system that fed the iron directly into the main bearings to maximize the heat flow into the cylinder bore walls. Additional mass was also added above the machined top deck level to increase the metal flow-through, ensuring low-nodularity CGI microstructures in the thermally loaded cylinder bores. Higher liquid metal temperatures were also directed to the thin outer regions and the rear gearbox flange to minimize carbide formation. Figure 5 shows the simulated metal flow velocity and the temperature profile.

Following the first prototype casting trials, additional mass was added to some sections of the tooling to optimize castability. As shown in Figure 6 (a), the main addition was to the top deck to ensure controlled cooling of the cylinder bore walls. This additional mass was removed during machining, resulting in no increase in the weight of the finished cylinder block assembly. Some additional material was also added to machined bosses, the mounting flanges for the plastic casings and to the rear gearbox flange. Again, the majority of this material was removed during machining. The simulations also showed the need to increase several fillet radii and the thickness of some ribs to alleviate residual stresses. Together, these changes resulted in a weight increase of approximately 500 grams compared to the original FE design, maintaining weight parity with the original aluminum block. The dimensional analysis of the as-cast block and ladder frame, shown in Figure 6, show that the main structural areas of the block are within the casting tolerance of +/- 0.8 mm for this prototype. The tolerance during series production would be +/- 0.5 mm.

The cylinder block and ladder frame castings were produced by additive manufacturing of individual cores that were assembled to form fully enclosed core packages. The fully assembled core packages, with metal filters and running systems incorporated directly in the core packages, were placed into greensand flasks on the standard production line. The sand cores used conventional silica sand and resin compositions while Cerabeads® ceramic sand with increased resin content was used to ensure the robustness of the 2.0 mm thick (25 mm tall) inter-bore water jacket core. Following the coating process, the inter-bore water passage was 2.5 mm thick. As shown in Figure 7, the gating system included stabilizing bars at either end of the casting to minimize residual stress and to protect the rear flange during shakeout.
The molds were poured with series production iron, prepared in 14-ton medium frequency induction furnaces, and poured from 1,500 kg (3,307 lb) ladles on the standard series production molding line at the foundry group’s production facility in Saltillo, Mexico. The liquid metal was controlled by the CGI process control technology, using thermal analysis parameter settings developed by the foundry to achieve the desired low nodularity in the thermally loaded cylinder bores, together with higher nodularity in the main bearings and outer walls to maximize strength, NVH and durability. The core packages and as-cast castings are shown in Figure 8.

The as-cast cylinder blocks and ladder frames were subjected to magnetic particle testing before and after shot blasting to ensure that all castings were sound. In total, 12 sets of castings were produced to provide set-up blocks for machining, machined components for NVH modal testing and engine assembly for the 100-hour durability test and, display castings. 

Material Properties

In comparison to aluminum alloys used for high-pressure die casting of cylinder blocks, CGI provides more than double the fatigue strength, double the elastic modulus (stiffness) and approximately 75% higher tensile strength. In comparison to conventional gray cast iron, CGI provides approximately 75% higher strength, 50% higher stiffness and double the fatigue strength. These superior mechanical properties provide the basis for weight and size reduction, improved engine durability and, in new applications, increased performance.

The international ISO 16112 standard for CGI specifies six grades of CGI ranging from 300 MPa to 500 MPa, increasing in increments of 50 MPa. In order to maximize the weight reduction potential, the present study leveraged the high cooling rate in the thin walls of the cylinder block and ladder frame to achieve ultimate tensile strengths of more than 550 MPa in the main bearings and more than 600 MPa in the 2.7 mm nominal outer walls of the block. By strategically modifying the mold filling and gating, the overall objective was to ensure slow cooling of the cylinder bore walls to ensure low nodularity (for efficient heat transfer and good machinability), while exploiting the rapid cooling in the outer sections of the block to maximize strength and stiffness.
As shown in Figure 9, the microstructure in the thermally loaded cylinder bore walls contained approximately 10% nodularity with more than 90% pearlite, as evaluated by Image Pro-Plus image analysis software using the nodularity evaluation guidelines in the ISO 16112 standard. This structure provides the optimum combination of thermal conductivity for efficient heat transfer, together with strength, stiffness and Brinell hardness for bore dimensional integrity and tribology. The nodularity values in Figure 9 (a) were obtained from the as-cast block, such that the uppermost value of 18% nodularity represents the top deck of the block after machining. The bore microstructure satisfies all OEM Specifications for CGI cylinder blocks currently in series production.

In conventional gray cast iron, thin (<4 mm) walls frequently develop degenerated (D-type) graphite that leads to a reduction in strength. However, with CGI, fast-cooling sections solidify with higher nodularity resulting in increased strength, stiffness and ductility. The ultimate result is that the tensile strength in the thin walls of conventional gray cast iron can decrease below 250 MPa while the tensile strength of CGI can increase beyond 600 MPa. The design of the cylinder block and the gating system were developed to exploit this natural cooling rate phenomenon. Figure 10 shows that the nodularity in the fracture split region of the main bearing was approximately 30-35%. Test bars prepared from the main bearings, according to the ASTM E8/E8M standard, provided tensile strengths between 550 and 600 MPa.

Conclusion Summary

The re-imagined CGI version of the engine demonstrates the potential for small gasoline engines with cast iron cylinder blocks to be weight neutral with aluminum while providing all of the traditional benefits of cast iron, including design flexibility, superior mechanical properties and durability, parent bore running surfaces, fracture split main bearings and reduced cost. Environmentally, CGI cylinder blocks are more recyclable than aluminum and consume less energy than aluminum during the manufacturing phase. From the life cycle perspective, the requirement for aluminum cylinder blocks is that the weight reduction relative to cast iron must provide sufficient reduction in fuel consumption and CO2 emissions during the life of the vehicle to pay back the higher manufacturing energy. However, when the cast iron engine is weight-neutral, the life cycle energy payback for aluminum is meaningless. 

The present study demonstrates the widespread operational, economic and environmental benefits of CGI in small, low-cost gasoline engine applications. With weight parity and positive NVH contributions, the design philosophy introduced in this paper offers new opportunities for primary drivetrains, hybrid engines and range extenders.