June 2021

JUNE 2021 AFTERMARKET 31 Frank Massey/ ADS Low Pressure Gauge with flow control valve, sight and pressure evaluation Gauges or scopes Exactly why would a scope fanatic like me advocate the use of gauges? Where low pressure is concerned, they offer a definitive conclusion. Look at the gauge I designed many years ago, it embodies all the aspects I have just discussed; visual examination of flow characteristics, with isolation taps to check rise and decay, and proof pressure. Remember, a pressure below atmosphere is not a vacuum. It is pressure differential with a value below 1 bar. How can you have a negative pressure? Not possible guys. We live with an atmosphere at a pressure of 1 bar, pressure therefor flow, will always be high to low. Let us move on to the high pressures experienced with both common rail and direct gasoline injection which can be assessed in similar ways. Once again, we need to apply the same criteria as with low pressure priming and remember I did include oil and water measurement for consideration. This is where the scope comes into its own, evaluation of high-pressure response when delivered from a mechanical pump. The true essence of CR and GDI is independent control and delivery of fuel across engine load and demand. In simple words rate of change. Measurement and interpretation The good news is that pressure sensors are usually accessed easily. Because they convert pressure into a linear voltage response, they are also easily interpreted with a scope. A little housekeeping first guys. You must have a scope with a minimum electronics industry standard bandwidth of 25mhz, and use a high sample rate. I suggest 10m/s or more. Do not drop the sample rate to clean up the image. You can filter once the data is in the buffer. Also, a floating measurement across the sensor will help, i.e., signal and ground. The sweep time can be up to 50 seconds or so providing you sample rate is high. As you zoom in for closer inspection you will be dividing your sample rate yet retaining enough data for accurate evaluation. Assuming all the priming tests have been carried out, you need to establish several critical functions. The rate of pressure rises from key on engine crank (KOEC), and key on engine run (KOER). There are subtle differences across systems. However, the later systems will reach 260bar plus in well under 500m/s. A bit more housekeeping; It is essential that the cranking system has been evaluated fully. Also clarify correct battery fitment, health and charge status, current consumption and rotation speed. Without battery health confirmation, KOEC HP pump tests are invalid. Then we assess the behaviour of pressure at idle speed, as this will be affected by injector faults, such as, delivery balance, atomisation, and leakage. Unstable pressure can also result from control actuator faults and adaption deviation. Also take note of combustion noise. Another topic for the future here is NVH vibration monitoring of combustion. Evaluation To evaluate full system pressure, or proof as I call it, you must manually take control of the HP pump. This is however getting more difficult and should not be undertaken without full knowledge of how the pump is controlled. That said, we monitor rise time to peak pressure which is always 4.5v. Why? Because that is the limit of the system and sensor output. The actual pressure achieved could be much higher, we simply cannot confirm it. The final aspect of the HP system evaluation is decay time, or system leakage. Some systems with Piezo injectors like Bosch should not leak while others do over a very predictable time. Therefore, you can confirm system leakage, with no intrusion, with clean hands, in minutes. Next month: Frank walks us through some waveform images. This means less words more pictures!