- Introduction
The vehicle manufacturing sector, as a dynamic arena, has undergone various changes, influences and effects, since the advent of the European Industrial revolution, and majorly so, from the input of Henry Ford’s – Assembly line model. Through its involvement in the process of vehicle manufacture, it actively contributes to the global economy, by way of revenue allotment, industry development and employment opportunities, in addition to contributing to varied environmental impacts and effects. It is involved in the development, design, manufacture, selling and marketing of vehicles, motorcycles and mopeds amongst other transport equipment. Pertinent, is the presence of the auxiliary arm of manufacturing, which majorly pertains to the maintenance and repair of automobiles i.e. fueling (gas) stations and repair shops. The vehicle engine is a crucial part of the overall system structure of a vehicle or automobile, crucial in the conversion of fossil fuel, into kinetic energy for forward movement (Zamagni, 2012). The effectiveness of the engine has a broad impact on the environment, as it ensures that proper combustion occurs and so air pollution is minimized. The engine also ensures the longevity of other parts of the car by ensuring that the power supply to each component is according to specifications.
- Vehicle Engine: The Crankshaft
The ‘Crankshaft’, at times abbreviated as ‘crank’, pertains to the engine part, which is crucial in translating the engine’s reciprocating linear piston motion, into one of rotation. So as to perform this core task, the crankshaft utilizes ‘crank throws’ (crankpins). These are additional bearing parts whose axis is usually offset from the crank, which is where the connecting rods of each cylinder, are attached through their big ends. The whole ‘crankshaft’ system is connected to a flywheel, which aimed at reducing the ‘four-stroke cycle’s pulsation characteristic. Additionally, it is at various instances attached to a torsional damper at the opposite end, for reducing the torsion vibrations, which often occur at the crankshaft’s general length (Ed Ashby & Jones 1992). This area of tension, is usually from the farthest cylinders, which is at the output end, and thereby instrumental in acting on the metal’s torsional elasticity.
The crankshaft is composed of a linear axis, on which it rotates, in addition to a number of bearing journals, which ride on the system’s main bearings. These bearings are replaceable and are usually held in the vehicle’s engine block. While possessing two end-supporting bearings, it is fundamental that the system also have other supporting bearings. This is due to the crankshaft’s sideway’s motion, which translates to a constant shift of pertinent cylinders, in a vehicle’s typical multi-cylinder engine. This is fundamentally what brought about the development of V8 engines, whose shorter crankshafts are more efficient and structurally effective than their straight-8 engine counterparts. A major disadvantage, with respect to the latter, was the intolerable amount of flex endured with the utility of higher compression ratios.
These increased volumes of compression, though being crucial in enhancing rotational speeds, would inadvertently wear out the system. This is the main reason why high performance vehicle engines have more numbers of main bearings. Crankshafts, just as the other composite parts of the current vehicle engine, have undergone through an evolutionary journey. From medieval Roman and Middle East contributions, by way of various systems, to current scientifically and technologically enhanced models, vehicle engine modification has been an inevitable aspect. Crankshafts are either made in monolithic (a wholesome entity) form, or by way of assembly, from various composite pieces. While monolithic crankshafts are more common, assembled systems are preferred either for smaller or larger vehicle engines (Bugayev et al. 2001).
Forging, as well as casting, is implemented through utility of steel metal, which in contemporary systems, is more inclined towards casting in ductile steel. With their lighter weight and enhanced inherent dampening, as well as their enhanced compactness (with respect to their overall dimensions), forged crankshafts are the favorite choices of most manufacturers. Steel, as an alloy or iron metal, is different, and hence less of a preferred contemporary choice for most vehicle parts manufacturers.
- ‘Cradle-to-Grave’ Life cycle
Life-cycle assessment, the ‘cradle to grave’ analysis, refers to the technique of assessing the resulting environmental impacts of a given product’s impacts. This is with regard to all phases of such a product’s life i.e. the raw material extraction process, through the processing, manufacture/ assembly and distribution. The aspects of material utility, maintenance and repair, as well as either recycling or disposal are also factored in. Thus, LCAs do aid in the avoidance of a narrow outlook, with regard to environmental impacts and concerns. LCA’s goal is to fully compare the wide ranging environmental effects, as assigned to various products. This is with the core intention of improving such processes as aforementioned, as well as the provision of sound and informed decision-making processes, in addition to pertinent supporting policy.
A holistic assessment is required, towards establishing a given product’s life cycle. Two main types of LCA are utilized i.e. Consequential and Attributional LCAs. The former, involves the process of identifying environmental consequences which may result, due to a system’s proposed change, or a given decision. This is inclusive of both economic and market implications, with the latter pertaining to the establishment of burdens which are associated with a products manufacture and production process, as well as overall utility (Zamagni 2012). Another form of LCA is the Social form, which being under development, seeks to assess the resulting social implications as well as social impacts of a given decision or pertinent change of a process.
- Steel as an Alloy: Extraction and Manufacturing process
The utility of alloy steel, which contains additional alloying elements in its core composition, is favored with respect to the manufacture of crankshaft and/ or engine parts. These elements are added intentionally, with the aim of modifying the metal’s inherent characteristics. Common alloys include amongst others: niobium, manganese, titanium, chromium, vanadium, nickel, molybdenum and boron. Carbon, in addition to other elements, act as the hardening agent, vital in preventing the crystal lattice, in the iron atom, from sliding past upon the other. Through a variation, with respect to the amount of alloying elements, as well as their form i.e. in the precipitated phase as solute elements, control is achieved of various crucial fundamentals. The overall ductility, hardness and tensile strength, of the resulting alloy is therefore tampered, to the desired quantities and characteristics.
The increase of carbon content, steel as a metal is made stronger and harder than raw iron, with the only drawback being the former’s ductile nature.
Those alloys which posses ratios higher than 2.1% of carbon, with this being dependent on either the processing phase, or overall content of other elements, are referred to as cast iron. To be noted is that cast iron even under hot temperatures, is not malleable and thus, the only way of working on it, is through casting. This process is performed where the cast iron has both properties of good casting characteristics and a lower melting point. Wrought iron, contains lesser volumes of carbon, and thus is distinguishable from steel. Iron, mined in the form of an ore i.e. hematite and magnetite (as iron oxides), is extracted from the ore through removing the oxygen present (Degarmo, Black & Kohser 2003).
This is achieved through combining the ore with carbon, which is a favored, chemically compatible partner by way of smelting. Cast iron’s melting point is at approximately 1,375 °C, where liquid state of structure is reached. It is at either this liquid phase, or its solid form that carbon is readily dissolved. During the smelting process, pig iron is produced, with this however containing much more carbon than the preferred content of steel. A subsequent step reduces this excess carbon, as well as other existing impurities. Further still, is the addition of other elements which aid in producing steel containing the desired properties. Manganese and nickel are crucial elements in strengthening its tensile quality, with chromium increasing both its melting point and hardness. Vanadium is another element added, to increase the steel metal’s hardness, while at the same time reducing the overall effects of metal fatigue.
Vanadium micro-alloyed steels are utilized majorly, due to their air-cooling capacity, especially after achieving great strengths, without the addition of heat treatment. The exception here is the surface hardening of bearing surfaces, with the alloy’s low content making this a cheaper material, as opposed to high alloy steels. The utility of carbon steels necessitates the additional process of heat treatment, for desired results to be achieved. Iron crankshafts are utilized mostly in cheaper production engines, where load-weight is lighter (Crawford 2011). Through machining out of a billet, usually of high quality vacuum re-melted steel, crankshafts can also be modeled, as is shown in the appendix.
- Vehicle engine component: Crankshaft Eco-Audit
As showcased in the above, LCAs are critical in the assessment of a given product’s overall contributions, with regard to environmental impacts and effects. In conducting the Eco-Audit, one of the factors, is a holistic approach to the component’s full ‘cradle-to-cradle (recycling option). This thus pertains to auditing the entire process of component manufacture, from the raw material extraction and conversion, to the manufacturing/ assembling phase, to the distribution and recycling or disposal phases. In eco-auditing, one evaluates the pertinent component, with the aim of identifying both implementation gaps present, with regard to the management system, as well as overall environmental compliance. Additionally, is the aspect of utilizing various corrective measures requisite, towards mitigating or even reversing resulting cases of negative environmental impacts.
As aforementioned, crankshafts are machined, out of billets, with this providing for components which utilize higher quality steels. These metal alloys, are quite expensive due to the large quantities of materials/ elements removed by way of milling machines and lathes. Additionally, is the high cost of raw materials, as well as the heating treatment that is additionally required, with the heating process, necessitating the utility of more fossil fuels. This does inadvertently add to the overall impact of the environment, by way of carbon emissions. High performance crankshafts, especially billet crankshafts, usually require the process of nitridization, which being slow, is also costly, during the hardening process (Funatani, 2004). However, an advantage exists where crankshafts, even when severely damaged, can be repaired. This is through a welding operation, before commencement of grinding.
- Strength enhancement: The Process of Nitro-carburizing
Carburization is also utilized, due to its resulting effect on the high stresses emanating from Hertzian contact. With regard to counterweights, some expensive crankshafts, built for high performance, utilize tungsten alloy, though cases of depleted uranium have been used also. A cheaper option is the utility of lead, whose density is much lower than tungsten (Funatani, 2004). Steel making processes do result in huge volumes of gaseous, liquid and solid wastes being generated. Additionally, vast amounts of pure sand need be mined, with extensive water recycling being necessary, so as to minimize waste water production. With coal being a core component of the processing method, by way of enhancing the achievement of a consistent carbon-iron ration, the mining processes used, are of adverse effect to the environment.
The crankshaft’s mechanical properties are enhanced through various methods such as nitro-carburizing, where toughness/ straighten-ability and strength are achieved. This is achieved by way of utilizing a designed material composition, which is developed without use of thermal treatment. The resulting advantage is a 16% increase of higher fatigue strength, in addition to the inherent straighten-ability and machinability. This is achieved through the adjustment of the alloy’s core material composition, so as to optimize the inherent micro-structural control. Ferrite strengthening is suitably achieved through the addition of some limited quantity of MO. As aforementioned, there is the normalizing treatment is omitted, thus the CO₂ emitted is reduced by up to 22%. Produced during the processes of post-hot-forging, the reduced levels showcase some level of environmental consideration (Asai et al. 2009).
- Vehicle Crankshaft Nitro-carburizing: An Eco-Audit
The process is performed at sub-critical temperatures, involving the diffusion of carbon and nitrogen, into the carbon steel’s surface. This results in a harder case with a soft inner core, whose compound surface area is thin. This surface area/ layer is both corrosion and wear resistant, with the additional advantage of not being brittle. Below this layer, the crankshaft’s fatigue resistance is enhanced by way of the thin case. Fatigue processes, anti-seize properties and adhesive wear resistance are improved, through the diffusion of both carbon and nitrogen. The process, known as Lindure, has the optional choice of adding oxygen, in the low temperature-utilizing surface treatment. The low temperature means that the overall production of carbon emissions is reduced, hence fewer CO2 and MJ footprints (Asai et al. 2009).
Ion nitriding may also be used, as it achieves the same result with the use of lower temperatures. The difference between the two processes is that ion nitriding relies on charged ions from gases to diffuse the carbon and nitrogen. This ionized gas is referred to as plasma, hence why the process may be referred to as plasma nitro-carburizing. This process is recommended because it results in less distortion of the crankshaft; this is mainly because of the extremely low temperatures of around 420o C that do not allow formation of chromium nitride, which distorts the metal (Asai et al. 2009). This attribute of nitro-carburizing, of not changing crystal structure of components, is why it was selected over case hardening. The process is also efficient in terms of energy consumption and pollutants discharged. The eco-feasibility of the crankshaft should factor the amount of energy input during manufacture, and whether the output from the component over its lifetime repays this input (Zamagni 2012). The process time is significantly low and can be reduced further by the pre-preparation of a nitride layer to be used. This reduces the time cost of manufacturing the nitro-carburized component.
Environmentally, through a reduced need for temperature utility, less of CO2 is emitted, with the nitro-carburizing element bringing about an increase of the overall component’s end-of-life aspect. The nitro-carburized crankshaft will be 85% more resistant to corrosion than it would have been before the process. The crankshaft will also be resistant to corrosion from salt water, which increases the lifespan of the component. A finish may be applied on top of the crankshaft in the form of a layer but this will only be for aesthetic purposes. The extended lifespan of the component makes it eco-friendly since it reduces the volume of waste material (Zamagni 2012). It can thus be concluded that using steel alloy, then nitro-carburizing it, would help improve the eco-feasibility of vehicle crankshafts.
- References
Asai, T, Takitani, Y, Sano, N, & Matsumoto, H (2009), "Strength Enhancement of Nitro-carburized Crankshaft Material", SAE Int. J. Mater. Manuf., vol. 1(1), pp. 204-210. [doi:10.4271/2008-01-0431].
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Bugayev, K, Konovalov, Y Bychkov, Y, Tretyakov, E Savin & Ivan V (2001), Iron and Steel Production, The Minerva Group, Inc.
Crawford, RH (2011), Life Cycle Assessment in the Built Environment, Taylor and Francis, London, UK.
Degarmo, EP, Black, JT, Kohser, RA (9th Ed.) (2003), Materials and Processes in Manufacturing. Wiley Publishers.
Funatani, K (2004), “Heat Treatment of Automotive Components: Current Status and Future Trends”, Trans. Indian Inst. Met., vol. 57(4), pp. 381-396.
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