Accurate fuel flow check

In-process control of critical hole drilling operations helps cut car emissions

Six tiny holes and the ability to drill them with great accuracy have won a 1997 Queen’s Award for Technological Achievement for special machinery maker Amchem of Birmingham.

Tighter European Commission emission regulations are forcing vehicle makers to find ways of cutting pollution. The generally accepted method is tight control of the fuel mixture entering the combustion chamber and ways of achieving that are the subject of continuing research worldwide.

Amchem believes it has found a ready solution with the development of an in-process flow monitoring and measuring system which it will sell as an option on its range of electro-discharge machines.

It claims that the closed-loop system provides adaptive in-process control of critical hole drilling operations to give an accuracy of +/-1% on final fuel flow. This compares with +/-5% on mechanical hole-drilling methods and +/-3% on existing open loop electrode machining.

Spark erosion, electro-discharge machining (EDM), where a computer-controlled spark erodes the metal, is the slow but reliable method used by aeroengine companies such as Rolls-Royce and Pratt & Whitney for machining hard materials. But it is also used by car companies where the value of machining accuracy outweighs higher machining costs.

Up to six holes in an injector nozzle, each no more than 75microns in diameter, control the flow of fuel into the combustion chamber and are a key factor in determining a vehicle’s emissions performance. The actual number of holes depends on engine type.

To develop the in-process flow measurement system, Amchem had to model the flow of fuel through each hole in turn. As substitute fuel it chose to use the existing supply of deionised water which forms the non-conductive coolant and lubricant in EDM systems.

The resulting computer model is used as a reference during production to keep the nozzle drilling process within tolerance.

Developing the model was not a simple matter of adding the flow results for one hole to successive holes. The sum of the flow through all six holes is what counts. And various machining parameters such as the timing of the pulsed DC voltage used to maintain a spark can affect surface finish.

Electrode wear is another problem which can cause inaccurate holes. The electrode in an EDM system provides the reference shape from which the spark jumps through the deionised water on to the workpiece.

Amchem has developed a novel method of preventing wear by reversing the electrical polarity of the system to ensure even wear of the electrode. Injector nozzle drilling apart, the company reckons the development will be invaluable in all EDM machining applications.

A further challenge was the need to develop flow measurement instruments which can cope with the very low flow levels through such small holes.

Standard flowmeters take around 30 seconds to stabilise but a hole takes just 15 seconds to drill, says Dave Suttie, development and applications manager.

The development was completed with the change from CNC control using analogue circuits to a PC-based servo system. `We are getting much more feedback and far better control of the drilling process,’ says Suttie.