PTWA/RSW is providing the perfect alternative to cylinder liners or oversized pistons in remanufacturing, and is rapidly becoming the production solution for OEM engine block manufacture.
In the search for higher fuel efficiency and lower CO2 emissions, engine design is moving towards more compact and lighter engine blocks. Thinner walled cylinder bores and the use of aluminium in engine blocks limits the ability to remanufacture blocks using traditional techniques such as boring oversize and using a larger piston or fitting a cylinder liner to return to using a standard sized piston.
The PTWA (Plasma Transferred Wire Arc)/RSW (Rotating Single Wire) process involves machining a small amount of metal off the engine block cylinder bore, then replacing it by spraying molten metal in a rotary motion down the bore.
However this technology is not just limited to the remanufacturing of engine blocks. It is also being used by many global automotive OEMs in Euro 6 compliant engines instead of cast-iron cylinder liners because it has many advantages, such as engine block weight reduction, friction reduction, material cost savings, reduced emissions and oil and fuel economy improvements. Using the correct process and specification, PTWA/RSW can also be sprayed directly onto aluminium engine blocks, eliminating the requirement for cylinder liners altogether. Subsequently during the remanufacturing process, the PTWA/RSW coating is then machined off and reapplied, returning the engine block to an almost new condition.
Autocraft Drivetrain Solutions is Europe’s only independent engine remanufacturer to possess PTWA/RSW technology for cylinder bore coating. The recent £3.85m investment at its factory in Grantham, England, enables the company to provide PTWA/RSW spraying, development and research to its global automotive OEM customers, as well as its use within Autocraft’s engine remanufacturing business. Autocraft has pioneered the Thermal Spray process over 12 years of development in conjunction with Ford, GTV and Flame Spray, and it now has the facility and licence to use the patented PTWA/RSW process without restriction for all its customers, on both new and remanufactured engine blocks.
The PTWA/RSW Process
In remanufacturing, engine blocks are stripped down from the core engine, inspected, cleaned and prepared by machining a small amount of metal off the cylinder bore to remove any imperfections. The Rotating Single Wire (RSW) torch sprays up and down the cylinder bore, making several passes to ensure correct thickness of PTWA coating. Autocraft’s unique Twin-Torch system applies a two layer PTWA coating; an AL Ni chemical bond coat followed by a carbon steel PTWA top coat. The bond coat is required on cast iron in place of the mechanical roughening process already proven for aluminium substrate. There is currently no proven method to prebore a surface profile on a brittle substrate such as cast iron, for satisfactory mechanical adhesion. To overcome this Autocraft uses the bond coat to form a chemical bond between the substrate and the top coat.
The molten steel wire tip is atomised through the nozzle of the torch and droplets are accelerated towards the substrate (cylinder bore wall) by compressed air. This leads to solidification of the particles on the prepared surface and formation of lamellar coatings with porosity defined by strict specifications. The blocks are then CNC honed to return the cylinder bore back to its original size, ready to take a standard-sized piston.
The PTWA/RSW process results in improved efficiency of the engine due to the defined porosity coating providing additional retention in oil volume, especially in the highly loaded areas of top and bottom dead centres. This leads to reduced friction and improved heat transfer from the cylinder wall into the cooling water circuit. The cylinder surface temperature is reduced by up to 30°C, improving combustion and reducing unwanted emissions.
Autocraft’s highly developed process enables PTWA to be sprayed directly onto cast iron with a base coat. This is a unique feature for engine block reclamation as the coating can easily peel without the specialist knowledge of block preparation, spray depth and honing.
Due to the nature of the PTWA/RSW process, overspray will be a significant problem for all manufacturers. Autocraft’s patented methods of limiting and removing the overspray have been developed over a number of years. Whilst an inconvenience when spraying new blocks, overspray is one of the greatest challenges for remanufactured blocks, as it is critical that the existing machined faces and oil ways are protected as they cannot be re-machined, resulting in high levels of scrap. With the PTWA/RSW of new blocks, the critical faces and oil ways are machined after spraying.
Autocraft’s PTWA/RSW machine is versatile and flexible, and can be fixtured up for many varieties and sizes of in-line and Vee engine blocks. Autocraft has the capacity to fully coat over 80,000 cylinder bores per annum, and spray and hone cylinder bores or liners from 65mm to 250mm.
The Future of PTWA/RSW
Many of the global automotive car manufacturers are using PTWA/RSW technology in the engine blocks of new vehicles because there are many benefits. Blocks are being switched from using steel or cast iron liners to an all-aluminium construction, representing a significant weight reduction for the manufacturer. This results in not only a material cost saving, but also leads to better oil and fuel efficiency and contributes to Euro 6 compliance. Autocraft is in a unique position, being the only independent engine remanufacturer to have this capability to recover most types of engine blocks, and also produce new, all aluminium linerless blocks. Having pioneered the technology over 10 years, Autocraft is already partnering with major OEMs to assist them with their development of PTWA/RSW.
In addition to remanufactured engines, Autocraft also provides medium to low sized batches of new engines for service application using both PTWA/RSW and new blocks. This enables the engine plants’ production engines to focus on supply to new vehicles, ensuring sales are not limited due to the availability of engines.
The Financial Benefit of PTWA/RSW
Recovering and reusing engine blocks through the PTWA/RSW process also relieves OEMs from purchasing large amounts of All Time Requirement (ATR) stocks. Currently, many OEMs interrupt production runs of engine blocks to produce a batch for remanufacturing for use after the end of the engine’s production life. The calculation of the amount of blocks needed is complex, order too many and the OEM is left with obsolete stock and high storage costs; order too few and a new batch of engine blocks is required to be cast and machined, often when tooling is no longer available a number of years later. The ability to recover engine blocks using PTWA/RSW results in a much lower number of blocks being required in the longer term, and a reduced cost exposure to the OEM.
Autocraft’s unique method of limiting the overspray on both new and remanufactured blocks leads to less residual overspray, meaning reduced maintenance of the PTWA/RSW cell (less frequent cleaning of cell, extraction system and torch maintenance etc.) resulting in increased uptime and higher throughput. This in turn makes the minimal overspray easier to remove, as it does not require manual removal or ultra-high pressure water jetting for removal. This in itself reduces material cost (using less steel wire), saves on additional capital and process cleaning costs, and results in a lower, more competitive piece cost for a PTWA/RSW engine block versus new.
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