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.’
Reservoir
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.
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.
- Expert training, Italian style
I have just returned from Modena, delivering a two-day advanced technical training seminar to 21 of Italy’s top independent technicians. I was travelling from across Italy and beyond, attending PCB Automotive’s international training programme, focusing on gasoline direct injection and common rail diesel.
The essence of the training was the advanced application of Pico oscilloscope to diagnose and gain predictive evidence for a repair solution. The technicians were particularly interested in the techniques of high pressure pump testing pioneered by ADS several years ago.
Attending the event were diesel and gasoline specialist and Italy’s most respected Bosch trainer. No pressure then! The session began with an explanation of the importance of scope performance when gathering data at high resolution requirements. Acquisition, storage, display, test lead bandwidth and advanced triggering with math channel analysis.
As expected Bosch had a different political agender towards our pump proof testing. It was however received with enthusiasm and great interest once its accuracy and simplicity was experienced.
The session on ignition evaluation was of great interest especially when focusing on primary current ramping and slew rate. Further discussion on burn time and slope completed the morning session.
The four-course lunch was a typical treat of local Italian hospitality and cuisine. The afternoon session included the evaluation of load request with air mass and Lambda response. By comparing response and rise time evaluation it is possible to predict and confirm complex fuel delivery and combustion anomalies.
A full eight-hour day concluded with hybrid cart racing north of Bologna, followed by the inevitable pizza and Mr Moretti’s finest.
Data and interest
Day two and common rail diesel with a similar agenda, with a strong focus on Math analysis of pressure and volume control against fuel delivery pressure. We have accumulated a vast database of rail pressure profiles, rise and decay times across all diesel manufacturers. This was of great interest to a Bosch pump and injector repair specialist. Approving nods and smiles throughout supported our confidence in this form of diagnostic analysis.
There was great interest in the examination and influence of rail pressure and air mass response time. Converting digital air mass into a current profile enabled delegates to understand the speed and simplicity with which a technician can determine the source of an error, either hydraulic or sensor input.
The day concluded with certificates and a photo session. Teachers usually get apples from students so imagine my joy with a bottle of wine and finest olive oil.
With a 6.30am flight in the morning and 50 kilometres back to Bologna, it was a very nice gesture from Luca my host to take me for a special evening meal at a villa outside Modena.
I found it intriguing to be asked by several of the delegates, with my international training experience, where the best technicians are to be found. I responded including them with the very best, not just because of their technical fluency, but especially their mutual respect and co-operation with each other, sadly lacking with some in the UK.
Using Math
To conclude let’s look at an example of using Math to predict hydro mechanical function against rail pressure.
The system example; Bosch cp1h, volume control single point, solenoid injectors. Sample taken from stable idle, part load snap open throttle, return to idle.
Channel a blue;
Rail pressure profile idle, snap open, rtn to idle.
Channel b red;
Volume control valve ground duty control
Channel c green;
Volume control valve, current vis rail pressure
Channel d black;
Air mass meter profile vis rail pressure
Math Channel purple;
Math duty volume control
The focus of this analysis was to establish the rail pressure hydraulic response time against air mass load signal, expressing the primary control device, volume, as current and math duty conversion.
To understand the values in this test enables an unequalled understanding of the systems capability in delivering fuel vis time.
The next challenge is to sync a second 4 channel scope monitoring compression using WPS, with the 4-stroke cycle overlay, noting the secondary pressure increase when fuel is injected. Then we really are ahead of the curve.
- DENSO diesel engine efficiency with i-ART
DENSO is using its i-ART pressure sensors to deliver optimal fuel injection control. The design of the miniature pressure sensors, which monitor injection activity from within the structure of the injector itself, allows a high level of fuel injection accuracy, monitoring the amount and timing of the process with 1/100,000 second precision. The technology ensures modern diesel engines offer a cleaner, quieter and more fuel-efficient drive.i-ART has already become diesel technology of choice to some of the largest manufacturers in the world, including becoming a key selling-point for Volvo’s eco-friendly Drive-E engine.
https://www.denso.com/global/
- Creating a buzz
This all-makes tool connects to the common rail injector whilst on or off the vehicle to run an electronic diagnosis and is designed to complement the sealed rail kit and false actuator kit. Once connected, the Buzz Tool measures the resistance/inductance of the injector coil, checks the insulation of the coil to the injector body and drives the valve to make it 'buzz'. The tool is supplied with a set of eight adaptors for the whole range of known CR solenoid injectors.
- Volkswagen Polo: Engine management light illuminated and glow plug light flashing
A 2010 Volkswagen Polo with a 1.6 litre common rail diesel injection engine (engine code – CAYC) was reported to have an engine management light and glow plug warning light both illuminated.
A diagnostic scantool was connected to the vehicle’s data link connector and the following error code was stored in the electronic control module:
- New starters and alternators from Autoelectro
Autoelectro has released a number of new-to-range references. Starter motor part numbers include examples for the Mazda 2 1.5 petrol (2015-), Audi A4 1.8 petrol (2011-), Mitsubishi ASX 1.6 diesel (2019-) and Dacia Duster 1.6 petrol (2015-). Alternators in the mix include products for the Suzuki SX4 1.0 petrol (2016-), Porsche 718 2.0/2.5 (2016-), Kia Picanto 1.25 petrol (2017-) and Renault Trafic 2.0 diesel (2019-).
www.autoelectro.co.uk