Archive | 2019
Substitution of cast iron engine components with aluminium alloys: a life cycle perspective
Abstract
Environmental sustainability is nowadays one of the most important global challenges. It is common that the amount of CO2 emissions is being used as a measure of the environmental impact of vehicles. As a result, manufacturers focus on producing lightweight car components in order to minimize the weight of the vehicles and maximize the fuel economy. As a consequence, car manufacturer designers have started to favour low density materials. However, it is usually the case that the energy footprint of the materials as well as the processes involved in the manufacturing of automotive components is often not assessed. This study focuses on the validity of the claim that lightweight materials are associated with enhanced environmental sustainability by making a full assessment of the energy consumption and CO2 emissions during the manufacturing and usage stages of diesel and petrol engine blocks made of cast iron and aluminium. For this purpose, inputs from over 100 world experts from across the automotive supply chain have been taken into consideration. Our results show that the usage of lightweight materials is often associated with higher energy consumption and CO2 emissions. More specifically, the 1.6L aluminium alloy engine block examined only seems to compensate for the additional energy consumed during their manufacturing process after 200,000 km of on-the-road driving compared to the one made of cast iron. Similar trends are observed for the CO2 emissions. Introduction According to recent reports [1], road transport is responsible for about 20% of the total CO2 emissions in the EU and has increased by more than 20% since 1990. This has led to legislation encouraging the production of lightweight cars in order to reduce the on-the-road emissions. As a result, there is a general perception that lower density materials will contribute towards the reduction of the CO2 footprint of automobiles. Moreover, when it comes to recycled materials, e.g. aluminium (Al), it is more than common that the energy input required from ancillary processes used in the recycling stages is often being neglected or underestimated [2]. Recently, researchers have focused on the big picture and introduced the term “embodied energy”, which is indicative of the energy required for the production of materials using ores and feedstock. Each product has a number of life phases, namely; material production, manufacturing, transportation and use. According to Ashby et al. [3] the “use” phase of an automobile is the most dominant in terms of energy consumption. However, in the second part of this investigation a comparison is being made between the energy used for the production of 14 kg steel bumper and a 10 kg aluminium one. Their results show that the energy required for manufacturing the bumper made of aluminium is 5 times higher than the corresponding value for the one made of steel. Moreover, the extra amount energy required for the aluminium bumper can be offset after 250,000 km of on-the-road driving. The high embodied energy of aluminium compared to steel is attributed to the energy intensive electrolysis and bauxite conversion stages. In a similar study, Sorger et al. [4] demonstrated the potential of using cast iron (CI) for manufacturing cylinder blocks. They suggested that CI can significantly contribute towards ecological sustainability and energy balance. The authors clearly highlighted the importance of evaluating the entire product lifecycle (“cradle-to-grave”) instead of solely focusing on the “use” phase. As shown in Figure 1 the energy requirements and CO2 emission for a crankcase made of cast iron are much lower than the corresponding values for the Al casting processes. Finally, the energy savings during the use phase of the lighter Al crankcase were found unable to offset the additional energy demand of the manufacturing phase during the lifecycle of the product. Figure 1: Manufacturing phase – energy requirements and CO2 emissions for the production of a cylinder crankcase (including consideration of the global recycling rate according to Gesamtverband der Aluminiumindustrie e.V. (GDA) [5] In this investigation we perform a full assessment of the energy requirements and CO2 emissions of the “manufacturing” and “use” phases of a 1.6 in in-line 4-cylinder engine block. For this purpose, have compared the cases of (a) a cast iron engine block and (b) an aluminium engine block. Our results show that there substituting cast iron with aluminium would not contribute to neither energy efficiency nor environmental sustainability as far as the product lifecycle is considered. Methodology In order to obtain the required data for this study we performed a wide literature review and contacted more than 100 experts in the automotive industry (engine design consultancy firms, foundries, mining/machining/heat treatment/recycling/impregnation companies, and primary alloy producers). As expected, it was not been feasible all times to collect the required energy data from the aforementioned companies; thus when those data were not available we obtained the required from the multiple sources in the literature. The selection of the engine type under examination was based on the investigation of Trechow [6] who forecasted that by 2016 4 cylinder engines would increase from about 58% of the world-wide market to about 71%. Moreover, both OEMs and automotive suppliers we contacted suggested that both petrol and diesel 1.6 L in-line 4 cylinder blocks can be characterised as the representative engines of modern vehicles. In order to select appropriate weight for the four aforementioned engine types we took into account the fact although CI is about 3 times denser than Al, it also characterised by superior mechanical properties (i.e. strength/density and Young’s modulus/density ratios). Consequently, CI allows for more compact designs with thinner cross sections. Based on an industry survey we conducted, we selected a 9 kg weight differential and 11 kg differential between the petrol and diesel engine blocks respectively. Taking into consideration the above and the fact that CI is about 3 times denser than Al, it can be concluded that the volume occupied by the CI block is about 55% less than the corresponding volume of the Al block. This results in a reduction of the weight of the ancillary components. Initial reports based on accepted industry standards have shown that a 5-10% weight reduction can yield 6% fuel savings [7]. However, more recent reports ([8], [9]) indicate that, instead of 6%, a 4.6 % might be achievable while occasionally fuel savings can be as low as 3%. According to a NRC report [10], for 1% and 5% reduction, fuel savings of 0.3% and 3.3% can be achieved respectively. In this study, the value of 4.6% has been adopted. Embodied Energies There are discrepancies in the literature regarding the energy required for the formation of primary materials. Allwood and Cullen [2] have suggested values of 170 GJ/tonne and 35 GJ/tonne for primary aluminium and iron respectively. On the other hand, online sources and investigations suggest values ranging between 50 and 100 GJ/tonne for primary aluminium and 35 GJ/tonne for primary iron. In order to select an appropriate value we draw the full lifecycle of each material and calculated the energy/mass in each step of the process as illustrated in Figure 2. The similar process was followed for iron. According to our calculations 98 GJ and 17 GJ are required for the production of 1 tonne of aluminium and iron respectively. Figure 2: Process flow steps for primary aluminium production and corresponding energy content required to produce 1 tonne of aluminium Besides raw material, most of the foundries we interviewed used recycled material to makeup the metal charge. The CI foundries interviewed used a high proportion of steel scrap as charge material. Steel scrap was also mixed with scrap from End of Life (EOL) components and fettled methoding systems. In this investigation we considered that in CI foundries the metal charge consisted of 91% recycled material which, depending on its provenance, had an energy content of 10 GJ/t or 4 GJ/t respectively. The Al alloy foundries interviewed used various percentages of recycled material. Low Pressure Die Casting (LPDC) foundries were found to use 100% primary material and at the same time performed no in-house recycling. On the other hand, Low Pressure Sand (LPS) foundries used both secondary ingot and inhouse recycled A319 alloy (~35%). Moreover, recycled foundry ingot was used to offset losses; thus we can claim that 100% of the charge material was recycled. In High Pressure Die Casting (HPDC) foundries a high proportion (~27%) of internal scrap was added to A380/383 secondary foundry ingot. Based on the aforementioned recycling rates and assuming the best case scenario for Al foundries, we considered values of embodied energy equal to 32, 24 and 25 GJ/tonne for the LPS, LPDC and HPDC processes respectively. In addition to primary and recycled materials additional materials have to be used in each one of the casting processes considered in this study (CI, LPS, LPDC and HPDC). In Al alloy foundries CI liners are being used which are either cast in or pressed. According to the feedback received from OEMs participating in our survey pre-machined liners were used. We considered that for the cast liners 95% recycled scrap iron was used which result in an embodied process energy equal to 188 MJ or 12 GJ/tonne for the set of four liners. Moreover, additional alloying elements were used in ach process type. In Al alloy foundries copper (13.5 GJ/tonne) and silicon (122 GJ/tonne) [11] were used while in CI foundries ferrosilicon (1.6 GJ/tonne) was added to enhance the grain structure and thus the quality of the finished component. Standard sand casting and Low Pressure Sand casting are burdened with additional energy associated with the mining, preparation, recy