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Frank Massey looks at how to get to grips with ignition combustion diagnostics which is an increasingly complex subject

Ignition primary good earth path

Published:  22 October, 2018

Let me begin with the simplest of all overviews: There are four possible causes of combustion failure or malfunction; and please note my refusal to use the word misfire. The word is meaningless without a definitive confirmed diagnosis. The four possible causes are:

  •  Insufficient delivery of ignition energy
  •  Incorrect fuel air composition
  •  A mechanical defect
  •  A control error

I also have a very firm opinion as to the selection and reliability of diagnostic tools when looking at ignition. Serial diagnostics has improved so much that I am prepared to use it, with the qualification that it is for guidance only. For example, identifying a cylinder, the number of misfire events, ignition timing angle, coil saturation time, and in some cases, spark burn duration, ignition set back etc.

I am excluding for the moment other vital supporting data such as exhaust oxygen sensors feedback, exit temperatures, load request data, etc. what I am focusing on is it possible to accurately assess the spark energy delivery characteristics other than with an oscilloscope? The answer is no.

Critical characteristics
The reality is that given the complexity and integrated reliance on other system components and function, it is often a mandatory requirement to apply several tests to accurately confirm not just failure symptoms , but cause too. The four  critical characteristics of good ignition energy delivery are:
Burn time. Critical observations; Exposure time, slope, and spark turbulence are all vital in understanding the transition of energy across the spark delivery path. Expect smooth transition at idle, turbulence on load.

 Ignition primary current profile. Critical observations; Peak value fall time of inductance, vital when confirming if the problem of poor energy delivery is a primary or secondary issue. Expect near vertical transition with undershoot.

Coil ringing. Critical observations;  This is the electrical resonance that is characteristic when switching current within an inductor. It determines the winding insulation integrity, and predicts a breach or shunt that will reduce the spark burn time and lead to eventual ignition failure. Expect three events.

 Firing line voltage. Critical observations;  Strictly speaking this is normally viewed in primary, however can be assessed in secondary where access is not available to pin 1. The shape or ignition profile would more normally resemble a wardrobe on it side rather than stood up! It represents the circuit load from the coil tower and includes the spark plug. Expect direct ignition 30v, waisted spark 40v, rotating ignition 50v.

I am not aware of any actual measurement data existence within manufacturers TBs, or be available serially. This is something I looked at over 30 years ago and was ridiculed by many.

Incorrect air fuel composition is an equally complex subject,  however  this can be in part successfully complemented with the use of serial data. Monitoring live data during a dynamic drive cycle can reveal possible symptoms, but not the cause of a problem.  Air fuel ratio, load request, fuel trim, fuel delivery status, ie homogenous or stratified, all of which should be evaluated against request, actual, and corrected data.

It doesn’t end there though. The hydraulic and mechanical dynamics of fuel delivery through the injector must be fully understood and tested. Intake air turbulence, swirl flaps, tumble and drumble characteristics, are all dramatically affected by valve and intake carbon deposits,  especially on direct injection systems that do not have the advantages of dual port injection. Mechanical defects are probably as complex to confirm as disassembly or removal is often needed to fully examine the serial or oscilloscope evidence. Many engines now require spark plugs to be precisely torqued to ensure the ignition energy is delivered to an exact position within the cylinder. How would you diagnose that?  Ignition energy across the plug would be normal, load request data would match, air fuel ratios normal.

Incomplete combustion or an anomaly as I call them could be the result of mechanical or cooling defects. These can  include piston crown mapped oil cooling jets, cylinder cooling, valve seat irregularities, valve to piston timing, injector flow and atomisation issues, or ignition energy incompatibility with the internal cylinder condition.

The only useful evidence may include, cylinder misfire count, oxygen sensor irregularities, excess oxygen and increased HCs in the exhaust stream. Any of you guys out there remember the good old days when 4-gas exhaust analysis was the craft of every professional engine tuner?

So here we are around 800 words to just touch on an incredibly complex subject. In the past the cure would have been relatively straightforward. Today it will destroy the engine, with prestige manufacturers like Porsche currently suffering multiple major engine failures. Techs need to think smarter.

Ignition primary poor earth path

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