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.

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.


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