The two-stroke large bore diesel engine is recognised as the most economical and reliable prime mover for the marine fleet with a long running time. Many parts of the diesel engine are being currently critically reviewed with the purpose of increasing their durability and reliability with a minimum of maintenance requirement.
The exhaust system is a cost-intensive part of the engine and its operation under extreme load conditions forces the need for engineered materials, maintenance and reconditioning services for these components.
Expensive materials are required to endure erosion at high temperatures, pressures and corrosion deposits, Nimonic alloy being the most renowned material for this application but the standard valve spindles are manufactured out of heat-resistant steel with a hard-faced seat area. The cost of these materials, and the need to reduce service intervals, are a significant proportion of the price of an exhaust valve, justifying the need for effective maintenance and reconditioning procedures. The usual procedures involve the welded satellite recharge of valve seats and High Velocity Oxygen Fuel (HVOF) application of cermets onto a valve spindle.
The purpose of the OFIENGINE project was to increase the durability of the exhaust system and the global engine, by the development of a new Oxy Fuel Ionization (OFI) thermal spray technology able to generate the technically required coatings to fight the identified wear mechanisms, and also to compete successfully with the current processes in cost, reliability and industrial affordability.
The objectives of the project were:
- to increase the durability of critical exhaust components (valve spindles, seats, etc.) and other pieces of two-stroke diesel marine engines by the use of advanced coatings;
- to reduce the cost of manufacturing coated components;
- to increase the number of suppliers offering these services and thus increasing the effective quality of the components used in marine diesel engines;
- to develop the technology of manufacturing marine engine components (valve spindles, valve seats, piston rods, cylinder cover and connecting rods) with improved technical, economic and service characteristics using novel thermal spraying techniques;
- to develop the new thermal spraying equipment for producing the components for marine transport application.
The project was divided into seven groups of activities:
- specifications: a complete data collection and compilation of the specifications is performed on the existing practices and desirable product properties;
- development of the new thermal spraying equipment: the objective is to develop, design and manufacture the prototype oxy-fuel ionisation (OFI) unit;
- development of OFI coatings and procedures: the coatings are developed, evaluated and compared to coatings applied by conventional HVOF (high velocity oxygen fuel) and HFPD (high frequency pulse detonation) spraying;
- manufacturing of the coated marine engine components: the objective concerns manufacturing and testing the marine engine components;
- testing of the developed coated marine engine: the objective is integration of the developed marine engine components and testing in the industrial partners' system;
- mathematical modelling of thermal spraying process and optimisation: the objective deals with the modelling of a pressurised diffusion flame, which includes the interaction of the high-velocity and high temperature plasma-flame exiting from the Laval nozzle with the surrounding gas at atmospheric pressure and with the substrate, as well as the kinematic and thermal behaviour of powder particles injected within the jet;
- dissemination and exploitation of results: the objective is awareness raising and providing information to the main stakeholders, and these are research and expert institutes, industry managers, policy-makers and main environmental and technology associations.
The consortium agreed to focus the work on the development of an OFI prototype for the production of novel WC-Co and Cr3C2-NiCr based coatings to assess the wear and corrosion protection needs of three main diesel engines components: valve spindles, piston crowns and piston pins.
The design of the OFI spray system has been accomplished following a modular concept. The peripheral installations (i.e. cooling system, powder feeder and gas lines) were carefully designed / selected and manufactured to fulfil the technical requirements of the new gun prototype and ensure the operation of the system in a safe and reproducible way.
The control software has been developed following a user friendly concept, which will allow the operation of the system under different levels of access, i.e. full access as process designer or 'engineer modus', limited access as basic operator, visitor, etc. In general, the system will allow the specification of set points, the monitoring of present values, selection of gas lines, plasma parameters and powder feeder parameters, execution of calibration procedures, development and management of safety protocols, printing, recording and file saving.
The new gun prototype was manufactured and validated for the development of high quality cermet type and amorphous / nanostructured steel coatings. Nevertheless, the same also features a flexible modular design which should offer the possibility to tailor the gun configuration for materials with the most different chemical and physical nature. Three of the demonstrators scheduled in WP1 for marine engines have been manufactured, i.e. piston crown, piston pin and valve spindle.
According to the predicted results from the third simulation step dealing with the mixing of the plasma-flame jet flow with the ambient gas, the gas jet velocity (250 m/s) and temperature (180 K) are slightly increased by the adding of a plasma source. This increase can be explained by a better mixing of the fuel and oxidant thanks to a higher turbulence level reached with the plasma source and the higher temperature of the ignition flow that favours the appearance of the methyl radical. The predictions showed a rather good agreement with the measured external gas flow temperature and the photographs done of the shock waves.
The last simulation step dealt with the prediction of the injection and the cinematic and thermal behaviour of particles in the flame. A particle melting model was implemented in the CFD code fluent. The model was