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Joining module is key to fuel injector assembly

Joining module is key to fuel injector assembly A small company in Norfolk is manufacturing a novel, economical, fuel injection system for small petrol engines. The precision engineered injector is assembled using an electromechanical joining module to accurately position and hold the components during laser welding.

If you have ever struggled with starting a petrol lawnmower or small outboard, you might have wondered why it doesn't start as easily as your car!  The carburettor is most likely the answer.  Modern auto engines are almost always equipped with fuel injection but most small petrol engines are still using a Venturi carburettor based on the design by Wilhelm Maybach in 1885. Being relatively simple devices, carburettors are much cheaper to manufacture than conventional fuel injection systems, which are too complex and expensive to be used on low cost engines.

That was the case until Scion Sprays, a small Norfolk company, developed a new system that is both low cost and simple enough to be viable on small engines from 50 to 250cc, a range that covers everything from lawnmowers and small marine outboard motors to motorcycles and scooters.

The system, based on research into the electrostatic atomisation of fuels by Jeff Allen, a founder and director of Scion Sprays and previously Chief Engineer at Lotus Engineering, has been developed into a fuel injection system that eliminates the need for expensive high-pressure fuel pumps and complex electronic controls.  

Conventional fuel injection systems use pulse width modulation, which determines the amount of fuel injected per engine cycle by varying the time during which fuel is injected. This demands precision sensors and a high-pressure fuel pump combined with a sophisticated, electronic control system. The Scion Sprays system uses pulse count injection (PCI), which injects a small, geometrically fixed volume of fuel repeatedly into the engine intake manifold. The number of pulses determines the amount of fuel injected in each engine cycle. Typically, the pulse volume is 1µl and the pulse frequency greater than 1kHz. In this way, the injector acts as both pump and flow meter.

The PCI injector is constructed as a simple, positive displacement pump that needs to be assembled with repeatable precision to ensure consistent performance. Richard Hoolahan, Manufacturing Manager at Scion Sprays, commissioned Rivercircle of Peterborough to design a rig that would accurately assemble the components of the injectors and hold them in place during the laser-welding phase of the process. Rivercircle specified the relatively new electromechanical NCFT joining modules from Kistler Instruments. These modules are simple to install requiring only a 3-phase electrical supply and have integral piezoelectric force and absolute encoder displacement monitoring capabilities that simplify the precise programming of home position, return stroke, joining stroke and an intermediate position. Of special importance to Scion Sprays was the 0.005mm repeatability of the NCFT units under real world, production conditions ensuring that the injector pulse volume of 1µl is maintained during volume production.

The assembly rig, designed by Rivercircle, has been further developed by Scion Sprays and is now being used for a limited production run of the injectors. With the assembly process now proved, it is planned to install six rigs to produce 30,000 injectors a year. Even before the production unit is fully installed, a European manufacturer of 50cc, two-stroke motor scooters has placed a substantial order. In addition to reliability and simplicity, the Scion Sprays PCI injectors make a valuable contribution to environmental protection by reducing emissions of HC+NOx by 35% and CO by over 88% compared to the standard carburettor fitted engine and minimises the amount of unburned oil in the exhaust.

Although reliability and easy starting are important benefits of the PCI injectors, reducing environmental damage is equally important with some sensitive, recreational areas already restricting the use of 2-stroke powered vehicles because exhaust emissions are damaging flora and fauna. In particular, some inland waterways and wetland areas are considering an outright ban on small two-stroke outboard engines unless the manufacturers take urgent action to minimise the environmental impact of their products.
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