Combustion past, present and future

Part one

By Frank Massey |

Published:  14 August, 2020

Combustion problems have been with us since the dawn of the internal combustion engine, and they continue to occur as technology changes

I’m sure none of us could have foreseen just where we are today, no more than it is possible to predict where the future will take us. We can, I believe, have confidence in several probabilities.

The future requirement for vehicle repair and servicing is high quality work, and I predict an even more discerning demand from the public for skilful independents to provide that service. It is inevitable that some businesses will cease trading, with the loss of employment for many skilful technicians.  The market will find its own natural level, and I do not think artificial stimulation will change it. This is, however, the pivotal moment when professionally structured, financed and client-focused businesses will take pole position, and find the opportunities in our future.

As for now, it hurts and hurts a lot. So, lets re-focus for a moment on those skills which will be essential. We hear daily the vital importance of testing and the evidence led by science which will result in the quickest possible success. It is this message I and others have applied to vehicle diagnostics for the last few decades. I hope now it may convince other sceptical techs how vital it is.
So, I’m going to reflect on the past, present and future with this topic on combustion problems.
For the sake of accuracy, it is better not to use the term misfire. Rather, go with ‘combustion anomaly’. This description is broad enough to include a combination of mechanical, ignition, and fuel delivery options. As to the cause? This can be, and often is, intermittent in nature.

The past provided options which sadly have been dismissed by many of today’s techs. These ranged from the obvious visual inspection of the sparking plug to just listening to the combustion note in the exhaust, where combustion transients could clearly be detected. These were usually associated with fuel quantity, atomisation and flame spread. More on that later.

One of the greatest assets was four gas analysis. This was an acquired art, the bible for all discerning tuners. Has it been stolen? Have we lost it? Where has it gone? It is still there, you just need to know where to look. It is hidden, often disguised in the belief that onboard diagnostics will find problems for us. It would be a mistake to rely entirely on that process, so Back To The Future McFly!

Why not drill the pre-cat manifold, install a rivet nut and install a hose ferrel? Now you can blow the dust off your four or five gas analytical skill. Interesting fact: The expression lambda is taken from a mathematical calculation of the four key gases; CO, HC, CO2, O2, (splint calculation).

Optimum air fuel ratio for a given engine design is achieved when CO and oxygen balance at around 0.5%, (off load). Producing the highest CO2 value at 16-17%. HCs should be exceptionally low at 50-100ppm. These values may subtly change depending on the sophistication of fuel delivery and engine design.

Let’s now focus on a phenomenon experienced visually when looking at the spark profile with an oscilloscope. The spark line should display a relatively smooth transition with a slightly negative slope or bow across its entire duration. The spark duration is of paramount importance. It is not dependant on the total energy value (joules) and may vary from 2-4m/s, dependant on system design. The firing line voltage (pressure) is determined by the value of resistance in the firing line circuit. Coil ringing or resonance at the end of the burn time reflects on the integrity of the secondary coil windings. Tom Denton reflected on this recently on a Facebook post. Go and find it, then come back. I’ll wait!

The function I am interested in is the small rise or ‘hump’ after the burn line and just before the coil ringing. From this you could predict the cylinder fuelling. A large hump indicated a lean combustion event. This could of course change with each individual burn. The ultimate way to see this is with a digital phosphor oscilloscope.

My thoughts now pass to the injectors, where previous approaches would have dictated removal and bench testing. I do accept this is still an option, however current thought process is often dictated by time and cost rather than diagnostic expedience. Injector design is in my opinion the area where the most advancement has taken place with fuel delivery. The mid 1980s saw around 2-4bar delivery with port injection, with single point manifold injection at 1 bar, but I’m trying desperately to forget about them. Current pressures can reach 700bar with homogenous and stratified control.

Now here is a statement you must not ignore.  Bench testing alone cannot be relied upon to determine an injector performance error, full stop. Tolerances of 2-microns dictates that any inappropriate removal technique avoiding correct tooling will stress the injector virtually guaranteeing fuel delivery anomalies. The use of bio-ethanol fuels and FOD will damage the internals, so here we are at the crux of my topic, what are the tools and process we could use to prove combustion issues.

Keep well guys from all at ADS, and remember to look up the Automotive Support Group on Facebook; It’s there to help you.

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