Forum » Diesel Tuning Fundamentals » Injector timing and afr

Injector timing and afr

Diesel Tuning Fundamentals

Forum Posts



Tech Articles

Discussion and questions related to the course Diesel Tuning Fundamentals

= Resolved threads

Page 1

On this module it notes that injector timing effects smoke output. With an afr meter connected will this then affect the afr based on the higher level of unburnt fuel?

Or does it remain the same due to initially having equal fuel mass per airmass?

You will see a leaner AFR with the same amount of fuel as you increase timing and get closer to MBT. This happens because the fuel spends more overall time in the cylinder with more timing, this allows the fuel to absorb more heat and auto ignite. Some fuel would have not been ignited with less timing producing smoke and sneaking out of the cylinder to be read by your AFR sensor. So to sum it up you'll see a leaner overall AFR and make more power with higher timing values as you approach MBT. You'll also See lower EGT's with timing increases as you get closer to MBT.

As for start for injection and injector duration in relation to AFR. To many things that could give a wrong reading and this is why.

Not all diesel engines mix oxygen and exhaust gas very well. We call them trapped air pockets and if able to read the AFR very fast it will show this. Also every small hole in the exhaust system before the o2 probe will draw in outside air due to the nature of a diesel engine exhaust stroke it pulsated a load more compared to a petrol engine.

As for to advanced injection. It will smoke as well as diesel will hit the wall and will stay on the wall for some time before it will burn or end up in the oil sump sticking to the cylinder liner.

We build race engines with massive injector making a AFR of less than 13.1 on low end burning off all diesel soot as soon as it hits oxygen in the outside air as things get very hot it will burn easy. On hi end we can hit maybe 17.1 AFR ratio as we run out of fuel delivery in a given injection angle in relation to crank angle. So we still have oxygen left but unable to get the diesel fuel in the limited time hi RPM present. So a o2 reading is nice to know where you stand upon mid end RPM on what fuel gets burned but dos not tell the whole story. Also diesel will burn any where as long as we got heat and oxygen often creating a afterburner in the exhaust manifold spiking EGT.