Injection technology can minimise methane slip on dual-fuel engines, while common rail retrofits can prolong the life of diesel engines and cut emissions
As shipping increasingly operates gas-powered ships with dual-fuel engines on board, the issue of methane slip contributing to greenhouse gas (GHG) emissions is likely to become more prominent. Methane slip comes from lean-burn engines and originates from incomplete combustion on the cold cylinder walls, combustion chamber surfaces (known as quenching), and losses from gas exchange and from the combustion chamber crevices. All this can be minimised through better fuel injection into the engine.
Woodward L’Orange has developed a new injector concept that is robust, economical and highly adaptable for future fuel use and which minimises methane slip. These are high-pressure gas direct injectors for pure-gas and dual-fuel engines. They augment the company’s existing injectors and enable a precise pilot injection quantity in gas or diesel mode. Both technologies were successfully introduced to the market on Wärtsilä engines.
Woodward L’Orange’s new dual-fuel injector family can inject high-pressure gaseous fuels and several low-energy liquid fuels, such as methanol and LPG.
Combustion with direct gas injection has advantages that justify the use of the more advanced technology. Injected at high-pressures, the gas is ignited by a diesel pilot and burned heterogeneously, similar to typical diesel combustion. The gas is completely converted and methane slip reduced to an absolute minimum, the company said.
This combustion process also eliminates knocking and thus specific cylinder outputs, typical of diesel engines, can be attained. High efficiencies and transient response are also achievable via direct injection without throttling.
This new injector family is designed for high-speed engines, but can easily be adapted to medium-speed units.
The injector concept has proved reliable during hundreds of hours on a range of single cylinder engines. Woodward L’Orange said compliance with future emissions regulations is achieved and its technology allows the power densities and dynamic performance that ship operators expect from diesel engines.
Challenges in design
In designing the new injectors, Woodward L’Orange had to consider the challenges of methane slip. Typically, diesel pilots are used to ignite the air-gas mixture; this meant its new injector family needed to be used for diesel injection as a back-up system in dual-fuel applications.
To fully optimise the combustion process, flexible timing of gas and diesel injection is required. To comply with emissions regulations in the pure diesel mode, the diesel injection side must supply the same injection pressure (2,200 bar) and hydraulic features equivalent to a pure diesel injector. The diesel side of the new injector is almost the same as in its common rail diesel counterpart. The only compromise when designing the diesel injection side was the reduction of the accumulator volume, to obtain room for the gas side components.
The gas side is designed to provide 500 bar gas injection pressures. The high-pressure is necessary to allow supercritical gas flow (with pressure ratios more than two), even against peak cylinder pressures (around 250 bar). This enables a defined mass-flow throughout the injection event. In this way, the mass-flow is not sensitive to engine back-pressure. The nozzle layout is designed to provide concentric gas injection jets around the diesel pilot.
To provide as much gas as possible in close proximity to the needle seats, accumulation volumes inside the injector are optimised.
Woodward L’Orange reduced pressure losses to a minimum by analysing the injector using computational fluid dynamics. The goal of losing less than 10% of the rail pressure, from the injector inlet down to the sac-hole, was achieved by providing large flow cross-sections inside the injector.
Another challenge in designing the gas side of the injector was the need for unrestricted diesel-only operation. Without having pressurised media on the gas side, the needles need to reliably seal against peak cylinder pressures in the diesel mode.
Springs which are connected to the gas needles by means of tightly guided spring spindles provide the force necessary for sealing during the diesel mode.
The main fuel needles are actuated by an independent hydraulic valve, which can be located with high flexibility to allow packaging that fits the customer’s engine concept. To save space in the inner region of the cylinder head, the valve is placed on top of the injector.
For liquid low-caloric fuels, like methanol and ammonia, the required injection quantities are significantly higher than in a diesel injector; therefore, a separate accumulator can be integrated into the injector design. This means hardware changes are minor when switching from one fuel to another.
Retrofitting diesel engines
For shipowners wishing to remain with diesel engines while reducing their environmental footprint, a retrofit solution is available from Heinzmann, which includes an electronic fuel injection (EFI) system.
Ship operators can undertake a common rail retrofit for engine sized 500-10,000 kW and for applications with variable load and speed. They can then benefit from advanced electronic fuel injection, including fuel savings, lower lubricant oil consumption and decreased emissions. Heinzmann says the return of investment can be less than two years, assuming more than 5,000 operating hours per year.
“This combustion process eliminates knocking and thus specific cylinder outputs, typical of diesel engines, can be attained”
These retrofits improve engine efficiency and prolong their lifetime, according to the company, which also provides the monitoring, control and safety features required for efficient marine applications.
It has now completed the retrofit of Swedish Maritime Administration (SMA)’s 43-year old icebreaker Ymer. Heinzmann implemented a common rail retrofit to the ship’s five main Pielstick 2.2 engines, each generating 3.5 MW. System installations and commissioning were completed without any major modification to the engines.
Following the retrofit of the first engine in 2013 there was a two-year-test period that proved the targeted 5% fuel saving could be met. The retrofit involved the complete replacement of the fuel system, mounting of a local operation panel and the integration of the Heinzmann control, monitoring and safety unit into the ship’s automation system.
A further 2.5% fuel saving was achieved via an additional turbocharger upgrade. The vibration level fell by 30% and maintenance costs also decreased. This encouraged SMA to authorise the retrofit of the other four engines by end Q4 2016.
Further fuel saving potential was achieved by reducing the engine speed in part load from 485 rpm fixed speed down to 360 rpm variable speed. This resulted in fuel saving of more than 4%, while lube oil consumption decreased by 50%. After completion of the commissioning of all five engines, successful integration into the ship automation system was verified during sea trials. Following these successes, SMA is now considering retrofits for the remaining sister vessels.
High pressure dual-fuel injector specifics
Woodward L’Orange’s new injector concept has the following specifications: