PART TWO: Combustion past, present and future
Part two
By Frank Massey |
Published: 27 August, 2020
Frank continues his look at combustion complications and throws the net wider to include the impact of peripheral systems
With the hindsight of the previous issue, much of what I stated in part one seems to be playing out! A great number of fellow professionals using the Automotive Support Group have been remarkably busy preparing their workshops, re-equipping, and marketing to their customers.
David re-opened ADS on 18 May, with a couple of non-direct workshop staff still on furlough, me included. Initial indications support a healthy restart.
Reflecting on my thoughts in part one, it became obvious that to discuss combustion control with current drivetrain development is not possible, without including all peripheral systems. This includes camshafts, valvetrain and all.
I shall begin part two with a discussion on each of the possible options when diagnosing combustion issues. Not necessarily in logic order as this is often dictated by many factors, such as fault condition, fault frequency, driver input style, vehicle condition variation, environment influences, accessibility, tool requirement, access to test data, and software access. It should not be due to time or cost either, but in the real world it always is.
Visual inspection is a valuable and much ignored asset. I accept the difficulties with current vehicle-built restrictions, but it does however provide some obvious answers as-to vehicle care and servicing. Collision damage, water ingress and corrosion are very often the root cause of many problems. Pay attention to evidence of previous repairs too. There will always be tooling witness marks on fasteners where this is a factor.
Serial data has become a powerful diagnostic asset, especially when used in conjunction with data logging. Relying simply on fault codes is not recommended as combustion errors are just that, an error not yet defined as a fault. It would be nice if all software platforms could provide access and actual, specified and correction data. Unfortunately, they do not!
A suggested log list would include:
- A time or event stamp
- Load request
- Engine speed
- AFR correction
- Bank 2 sensor correction
- Exhaust gas temperature
- Ignition set back or retard
- Fuel pressure
- Injected fuel quantity
- Turbo boost correction
I have previously recommended the use of a professional fully programmable injector test bench. This is essential, although I did qualify my remarks due to some obvious restrictions like temperature, vibration, different mounting stress and delivery pressure.
What I’m saying is that to match the operating environment perfectly is improbable, as with intermittent problems there is also the issue of opportunity. We can, however, simulate some exact and accurate data, delivery volume, atomisation, and inductance. Invariably this will identify a large percentage of faults.
Availability
Exhaust gas analysis can also be a poisoned chalice. Its reaction delay is unacceptable for fast transient events, and it averages the exhaust gas sample across all cylinders. Using an infrared thermal gun is perfect for cylinder temperature imbalance, if you can see the engine that is.
Now we arrive at the oscilloscope, and like all other options under consideration must be available at the point of error. By that, I mean monitoring the most applicable components. I discussed ignition profile in part one and could discuss this and other subjects through several articles. The reaction to combustion effects are instant, so what about our friends the oxygen sensors?
They should react quickly, especially the wideband variety. They are, however, subject to correction and do average exhaust oxygen content across several cylinders. The bank two sensors are more informative of oxygen anomalies, assuming the problem is consistent and present long enough.
A technique we introduced probably 30 years ago, which was actually dismissed by some at the time, was to look at the air mass meter profile against lambda response. This is still a perfectly accurate means of assessing fuel delivery at the critical point of demand. Conducted under a snap throttle test, it can determine fuel delivery issues. A point to bear in mind here is one I made several paragraphs ago; The PCM will only identify errors based on sensor range error, so if the PCM cannot see it, you get no DTC! Savvy?
Still focusing on scope application, the real genius of the next option is with the application software, vibration monitoring. I’m attempting to simplify this subject as an overall option package. What it offers is an incredible non-intrusive real time response to cylinder contribution, that sounds like a lot of words describing combustion to me.
Let’s consider the simple logic here. A rotating engine has a given mass energy, the more cylinders there are, the smoother the engine. That’s part of today’s challenges. With smooth running adaption, cylinder select and active engine damping, not forgetting noise cancelling technology, identifying a problem is not so easy anymore.
The mass energy is a combination of the rotation and reciprocating components. These are, by nature, always in conflict. They do, however, form a repetitive vibration pattern. This will vary for each engine design number of cylinders and operating condition. This pattern is determined by disturbance or orders. A single crankshaft rotation is the first order. Each cylinder combustion event is contributing to what is considered combustion orders or how many events occur per crank rotation. So, a half order will occur every 180 degrees, and a second order every 720 degrees.
Therefore, if each combustion event releases the same energy or mass to the piston crown, we can monitor it in real time with an 3D accelerometer mounted on the driver’s seat frame. Yes, it is really that easy!
The engine configuration and number of cylinders is input together with serial engine RPM into the software wizard. The output is measured by frequency and mass in milli gravity.Any deviation in combustion contribution will be displayed as a change in mass value.
So, there are our diagnostic options, success today is often achieved by several diagnostic methods. The real skill is choice and interpretation of the evidence that is the difficult bit. I have chosen simple images to demonstrate the principles. Keep well, and see you in the next issue.
- Part two The good and THE GREAT
In part one, we looked at the start of the ‘diagnostic process.’ The first steps were customer questioning, confirming the fault and knowing the system and its function. These help the technician to build the ‘big picture’ necessary to repair the vehicle correctly.
In this article we will look at the next four steps.
Step 4: Gather evidence
It is easy to overlook this step as many technicians think of it as the overall ‘diagnosis.’ However, once the technician understands the system, gathering evidence will provide key information. This step is normally best carried out with the use of test equipment that does not mean the dismantling of systems and components.
Many technicians have their own favourite tools and equipment but this list can include (but not limited to)
the following:
Scan tool – It is always best practice to record the fault codes present, erase the codes, and then recheck. This means codes which reappear are still current. Remember that a fault code will only indicate a fault with a circuit or its function. It is not always the component listed in the fault code that is at fault
Oscilloscope – An oscilloscope can be used for a multitude of testing/initial measuring without being intrusive. Some oscilloscope equipment suppliers are looking at systems within high voltages hybrid/electric vehicle technology. The waveforms produced by the test equipment can be used when analysing the evidence and may indicate that a fault exists within a system. An understanding of the system being tested will be necessary to understand the information. This may even include performing sums so all those missed maths lessons at school may come back to haunt you. It may take time to become confident analysing the waveforms, so be patient
Temperature measuring equipment – This can include the use of thermal imaging cameras. Most systems that produce energy/work will also produce some heat. The temperatures produced vary from system to system. Examples include everything from engine misfires to electrical components, as well as air conditioning system components and mechanical components such as brake and hub assemblies. The possibilities are endless and results can be thought provoking.
Emission equipment – By measuring the end result, an exhaust gas analyser can show you if the engine is functioning correctly. The incorrect emissions emitted from the exhaust help indicate a system fault or a mechanical fault with the engine
Technical service bulletins – Many vehicle manufacturers produce technical service bulletins (TSBs) that are generated by a central point (usually a technical department) from the information that is gathered from their network of dealers. Some of these may be available to the independent sector either through the VM or through a third party – It’s always worth checking if these exist. They may indicate a common fault that has been reported similar to that the technician is facing. Some test equipment suppliers may provide TSBs as part of a diagnostic tool package
Software updates – Many vehicle systems are controlled by a ECU. Most vehicle manufacturers are constantly updating system software to overcome various faults/ customer concerns. Simply by updating the software can fix the vehicles problem without any other intervention of repairing a possible fault. This is where having a link to a vehicle manufacturer is vital in repairing the vehicle
Hints & tips – Most technicians will have a link or access to a vehicle repair forum where they can ask various questions on vehicle faults and may get some indication of which system components are likely to cause a vehicle fault
Functional checks – Vehicle systems are interlinked and typically share information using a vehicle network. The fault may cause another system to function incorrectly, so it is vitally important that the technician carries out a functional check to see if the reported fault has an effect on another system. By carrying out this check the technician again is building the big picture
Actuator checks – Most systems today are capable of performing actuator tests. The technician can perform various checks to components to check its operation and if the system ECU can control the component, often reducing the time to the diagnosis, by performing this task the technician can identify whether it is the control signal, wiring or component or it is sensor wiring. This function can be used in conjunction with serial data to see how the system reacts as the component functions
Serial (live) data – The technician can typically review a vehicle system serial data through a scan tool. Having live data readings to refer to can help you review the data captured. Using actuator checks and viewing the serial data can also help the technician to identify a system fault
Remember to record all the evidence gathered so it can be analysed during the next step in the diagnosis. We can’t remember everything. If the technician needs to contact a technical helpline they will ask for the actual readings obtained recoding the data gathered will help.
Step 5: Analyse the evidence
Analysing evidence gathered during the previous steps can take time. The technician needs to build the big picture from all the evidence gathered during the first few steps. You need to analyse the information gathered, and decide on what information is right and wrong.
This step may rely on experience as well as knowledge on the product. You should take your time – don’t be hurried. Time spent in the thinking stages of the diagnosis can save time later. Putting pressure on the technician can lead to errors being made. It may be necessary to ask the opinion of other technicians. If the evidence is documented it may be easier to analyse or share between others.
Step 6: Plan the test routine
After analysing the evidence gathered it’s now time to start to ‘plan’ the best way to approach to the task or tasks in hand.
The technician should plan their test routine, decide on what test equipment should they use, what results are they expecting, if the result is good or bad and which component should they test next.
Document the plan – this enables you to review decisions made at this stage in the next step. The technician may not always get it right as there may be various routes to test systems/components. The test routine may have to be revisited depending on the results gathered during testing. Documenting the test routine will provide a map. Also, don’t forget to list the stages, as this is something that could be incorporated into an invoicing structure later.
The technician should indicate on the routine what readings they expect when they carry out the system testing. This can be generated by their own knowledge/skill or the expected readings may come from vehicle information which they have already sourced. If the information is not known at the time the test routine is planned, then the test routine may highlight what information is required and what test equipment is needed. You shouldn’t be afraid to revisit the plan at any time and ask further questions on which direction the tests should take. If the plan is well documented and the technician becomes stuck at any point, they can pause the process and revisit later. Also the information can then be shared with various helplines that support workshop networks.
Step 7: System testing
The technician then follows their pre-determined plan, if it is documented they can record the results of the test(s) as they follow the routine.
Many technicians tend to go a little off-piste when they get frustrated. Having the routine documented can keep the technician on track and focused on the result. If the routine is followed and the fault cannot be found the technician may have to go back to the analysing the evidence or planning the test routine. The technician shouldn’t be scared of going back a few steps, as I said previously analysing the evidence takes practice and can be time consuming, not to be rushed.
Summing up
Remember to follow the process. It is easy to be led off track by various distractions but don’t try to short circuit the process. Some steps may take longer than first thought to accomplish than others. Some distractions may be outside of your control, and it may be necessary to educate others. Practice, practice, practice. Refine the process to fit in with your business and its practices, the business could align its estimating/cost modelling to the process, being able to charge effectively and keeping the customer informed at each stage of the process.
Coming up...
In the next article I will be looking at the next four steps which are; Step 8: Conclusion (the root cause), Step 9: Rectify the fault and Step 10: Recheck the system(s). The last article in this series will indicate the final three steps and how to fit them all together in order to become a great technician and perhaps succeed in Top Technician or Top Garage in 2018.
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