Common rail diesel and pressure evaluation

Is the pressure getting to Frank? It doesn’t seem that way, but he is certainly getting to grips with common rail diesel issues

By Frank Massey |

Published:  18 June, 2021

For this month’s topic I am going to revisit a subject which still attracts a lot of interest, but with perhaps a more forensic approach; Common rail diesel and pressure evaluation using Pico scope with WPS pressure sensor. Before we get into a detailed overview of pressure waveform analysis, let’s explore some of the other important tools at our disposal.
Firstly, Communication. This is something that is becoming a lost art, especially with social media and online booking. Never overlook the opportunity to question the driver of the problematic vehicle. Note the expression ‘driver’ as often it may be booked in by someone else. Here lies the first problem, opinion, description, expectation, and cost. This is where, in my opinion, the most skilful member of your team is needed. Apart from the obvious technical information, you need how, where and when. You need to establish if they value their car, and if they do not, they will not value you.
Next comes with what I call common-sense; observation. Cars cannot speak but they do tell a story. This could be the most valuable part of your forensic skills, looking for evidence of cause. Non-intrusive examination has many forms, including a thorough serial interrogation. Do not be drawn towards DTCs without knowing why. Look for adaption or correction data, deviation of reliable known values, once again note ‘reliable.’

All hydraulic systems, fuel, oil and water, require a reservoir of the appropriate substance. Just think about that one for a while, many do not, at their cost. It must travel from one location to another. Obvious, yes, so how are you going to check it?
Pressure may be the obvious answer however, flow, and rate of change, (rise and decay time) are just as important. Let me throw a curve ball in here; Do you have one electric supply pump or several? The laws of physics dictate they consume current, so let us explore how observation of current will predict the physical environment of fluid flow.
Initial inrush; How much current is required on start-up can suggest a faulty pump, a restriction in flow or incorrect viscosity. The continual current rate proves a similar conclusion; Too much might suggest a restricted filtration system or blockage. Too little may indicate a worn pump, low pressure or lack of fluid. All of this can be checked from a control fuse easily accessible and quickly. We love quick, and accurate.
The hidden problem overlooked my many is experienced when relying on serial data alone or even pump current and flow rate when measured blind. Are there cavitation or voids in the flow stream? The void represents a pressure differential, my favourite subject, between supply and demand. This brings us to pressure gauges and the scope.

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.

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.

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