A radical new injector system

22 Jun 2017

A fuel injection system currently under development promises to challenge the dominance of common rail technology by providing a simple and fuel efficient alternative, writes Dag Pike.

The RKI injection system is a self-pressurizing fuel injector which can inject fuel into the combustion chamber at a greater pressure and in smaller droplet sizes than conventional systems. This is claimed to enable more complete combustion and leads to a higher thermal efficiency, as well as dramatically reduced emissions of both NOx and particulate matter.

Another claimed benefit of the RKI system is the wide spectrum of fuel sources that it can use, from heavy fuel oil to bio-diesel, enabling ships to maximise their price to thermal efficiency ratios constantly in response to global fuel pricing. RKI claims that when the injectors are optimised in a modern large-bore, two- or four-stroke engine, the engine will be capable of exceeding the current emission and fuel regulations, giving ships the ability to use only one fuel source for both propulsion and electricity generation.

The RK injector receives its initial activation from the piston’s compression. Incorporated into the design of the injector is a pressure multiplier that develops the very high injection pressures, forcing the fuel into the combustion chamber at a significantly higher pressure than current systems through up to 180 small holes, each one with a diameter of about 0.01mm. Developer RKLAB claims that with this creates optimal combustion conditions within the chamber.

The dispersal nozzle of the RK injector injects the diesel in 180 tiny droplets under a pressure of 5,000 bar or more, in a unique spray pattern which further improves the combustion process. This compares with the current, commonly used injection systems which use between 4-8 holes in the injector nozzle. This atomisation of fuel particles improves the mixing of the air and fuel in the combustion chamber, which improves the combustion process but also lowers temperature and compression at which combustion occurs. The company claims that it is the more complete combustion of the fuel at lower temperatures and lower combustion compressions that delivers the three main benefits of the RK injector system, giving better fuel economy, a reduction in the particulate matter and a decrease in the production of NOx.

Natural feedback

The system also boasts inherent fuel optimisation via a natural feedback loop: as the injector takes its initial signal (compression pressure) from the combustion chamber it will only provide the amount of fuel that the chamber will effectively burn. Thus it is the natural combustion curve of the fuel that will dictate the rate of fuel being added, optimising the rate of fuel injection and never over-fuelling. This does not preclude the ability to electronically control the RK injector, which can be controlled through the solenoid valves in the same way as a conventional injector and will be able to rely on similar diagnostic tools to monitor and control the combustion process.  

There are two distinct phases to the RKI injection process: an initial phase prior to ignition and a second phase after combustion commences. In the initial stage a small amount of fuel is injected into the compressed air, starting the combustion process. This fuel acts like a spark plug in a petrol engine, igniting the rest of the fuel. The second phase is a response to the significant increase in pressure in the combustion chamber following the ignition, after which the remaining fuel is injected as quickly as possible. In combination, these features allow feedback from the combustion process to govern the behavior of the fuel injector.

Lower particulate and NOx emissions can thus be achieved from the engine without the need for ancillary pollution control measures, such as particulate traps and exhaust gas recirculation. These pollution control measures usually reduce engine power and are parasitic on fuel consumption.

The injector does not have a mechanical method for controlling the timing of the injection. Instead timing is driven by combustion pressure changes in the cylinder, although electronic control is provided to reset the injector ready for the next cycle. The RK injector has been designed to achieve more effective mixing of the fuel and air than can be achieved with the current conventional fuel systems on the market and RKI’s commercial potential is based on addressing this “major problem” of traditional diesel engines in a new way.

Low-pressure pump

The RKI requires only a low-pressure fuel supply pump. As the high pressure is generated locally within the injector, the injection rate can be coordinated, with the combustion processes occurring in the chamber. This responsiveness to the combustion process is not possible with conventional fuel injectors that are either mechanically or electronically synchronised with the rotation of the engine.

The injection pattern is radial, compared with the inverted conical patterns commonly achieved with conventional injectors. This allows changes to the shape and flow patterns in the combustion chamber. The system operates with a flat crown on the lightweight piston and requires little swirl in the air flow to ensure satisfactory mixing. The injector has been specifically designed to be fast acting to enable timely response to combustion conditions.

RKLAB is preparing to open a new facility in Finland, expanding its central research and development function and establishing a specialist marine and rail division. The new centre will work closely RKLAB’s main powertrain consultancy in the UK, Powertrain Technology, and their development partners Mazda and VTT.

The marine power sector is an ideal segment for RKLAB to develop first, the company suggests, given high levels of customisation and the significant challenge the industry faces in achieving compliance with emissions and fuel efficiency regulations.