Diversions

RT-FLEX96C: The World’s Largest and Smartest Diesel

October 21, 2013

The Wärtsilä RT-FLEX96C – the 14-cylinder, common rail version of the RTA96C – is an absolute beast; weighing in at 2,300 tons, it’s the largest and most powerful reciprocating engine in the world. What’s rarely appreciated outside of maritime engineering circles, however, is that it’s also one of the smartest.

The 14-cylinder RT-FLEX96C was developed to meet a need for increased power in the maritime shipping industry. Previously, the practical maximum size for shipboard engines was capped at 12 cylinders; by re-engineering the crankshafts and thrust-bearing mid-gear structures of the existing RTA96 engines, while stiffening the individual cylinders, Wärtsilä was able to greatly increase the torque and output power of their engines without compromising durability from the increased stress. At this time, the RT-FLEX96C produces a sustained torque of 7,603,850 newton meters (or around 5,608,300 lbs ft), more than enough twisting force to shatter older crankshaft designs.powerchart

The larger, stronger 14-cylinder variants output 80,080 kW at 127 rpm (see chart), making them the most powerful engines in service.

While meeting the engineering challenge of surviving its own output, the engineers behind the RT-FLEX96C introduced a number of efficiency and operational improvements. Size isn’t everything; proportionally, these behemoths are among the best of their class in fuel usage and energy valorization.

The  RT-FLEX96C Common Rail

common rail flow schematicWith engines of this size, efficiency and longevity are key design considerations. Moving the RTA series away from the classic camshaft design, Wärtsilä builds the 13- and 14-cylinder models entirely around a proprietary common rail system known as the RT-flex.

Common rail technology offers a number of performance advantages over camshaft-centered designs, in terms of engine control. For one, mechanically controlled injection pressure is determined by engine speed, making low-speed camshaft engines inherently less efficient. Second, engineers were practically limited in the number of injections achievable per cycle. While it is certainly possible to expand the array of pumps and injectors per cylinder – and many highly efficient diesel designs do just that – it is more mechanically difficult to achieve.

wartsila-sulzerThere are maintenance considerations, as well; primarily, lower-pressure injection results in less thoroughly atomized fuel. This increased knocking, vibration, and overall engine wear. Combating this with multiple injection pumps per cylinder yield diminishing returns, as the added complexity both increases cost and introduces more failure points to the design.

Replacing camshafts with a common rail injection and valve actuation system sidesteps both of these constraints. First, all fuel injectors are supplied by a single, high-pressure accumulator – this is the “rail”. (In the 14-cylinder Wärtsilä RT-flex96C, it’s one rail per array of 7 cylinders). The fuel pumps, therefore, maintain a single target pressure rather than varying by speed or timing. This dramatically increases efficiency of combustion at low speeds, as compared to feeding mechanical energy to separate injection reservoirs via a camshaft.

Having a common reservoir also means consistent pressure across the lifetime of each individual injection event. This results in more thorough atomization of fuel, increasing combustion efficiency while reducing vibration, explosive events, and emissions. Both in terms of fuel efficiency and maintenance costs, consistent injection pressure yields major cost savings.

While Wärtsilä was able to achieve respectable efficiency gains through refinement of their camshaft-based models, redesigning the RTA96C around their RT-flex common rail technology yielded longer service lifetimes, reduced operating costs, and notable emissions reductions. Field performance of the RT-FLEX96C has reinforced Wärtsilä’s decision to build future 13- and 14-cylinder engines of this type exclusively around their RT-flex common rails and injection controls.

As a bonus, electronic control means that existing engines can be more easily tuned or retrofitted to future efficiency goals. It’s worlds easier to patch software than redesign an entire camshaft-based engine.

Waste Heat and Energy Valorization in the RT-FLEX96C

waste heat flowchartOne powerful tool engineers use in refining overall efficiency across systems is valorizing wasted heat and energy. By capturing wasted outputs and turning them to productive purposes, systems function more holistically. This reduces net costs be deprioritizing or eliminating redundancies, lowering net costs, and boosting global efficiency.

Exhaust gas is a convenient source of heat for shipboard systems. In shipboard installations of the RT-FLEX96C, exhaust gas is used for electrical generation, both through an exhaust-gas power turbine and by capturing waste heat in a dedicated recovery plant. This plant uses the formerly wasted heat to power steam turbines for electrical generation – up to 12% of main engine power – and turn a shaft motor to augment propulsion.

waste heat plantThis isn’t a novel engineering approach, but what distinguishes the RT-FLEX96C in terms of waste heat valorization is the way it’s designed to accept lower air intake temperatures. Tuning the engine’s turbochargers to ambient air temperatures (i.e., outside the ship) increases available exhaust energy, while altering air flow through the engine to match that which would result from warmer intakes, such as when intake air is source from inside the engine room.

Increasing exhaust energy, without impacting wear or performance, gives engineers more heat to work with. The available energy can then be turned to shipboard heating, power generation, and propulsion, while simplifying maintenance by reducing the load on redundant systems.

While it’s tempting to focus on the sheer size and power of the RT-FLEX96C series engines, they are equally impressive for achieving smokeless operation, incredible fuel efficiency for their size, and new levels of waste heat and energy valorization. Wärtsilä – and their peers in the maritime engineering world – are achieving both increased power and decreased impact.

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