June 2020

JUNE 2020 AFTERMARKET 27 EYEBROW www.aftermarketonline.net 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! This shows a normal combustion profile in both primary and secondary circuits. Slope is slightly negative with a smooth transition. The end of burn hump is minimal followed by a normal coil oscillation This shows a sudden drop in firing line voltage, which represents a drop in electrical load within the combustion cycle. The two main reasons are a shunt across the spark plug due possibly to carbon fouling, or a unstable combustion environment, oily or rich AFR. it also possible the ignition energy has found an alternative route to ground. It then recovers with a slightly elevated hump 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. Injectors 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. Next issue: Scoping AMM output against lambda response, serial data logging and NVH analysis