[Blue Angel]

Having just bought my first diesel (2016 BMW 535d) and being a huge gearhead, I'm wondering about how diesels work with regards to fueling, A/F ratios, etc.

In a gas engine the "gas pedal" is basically telling the throttle how far to open and how much air to allow into the engine. Based on this, the computer calculates how much fuel to add based on how much air mass the engine is breathing and a number of other fine-tuning factors. The engine runs rich during cold starts to warm up the catalyst faster, and to keep combustion temperatures and knock under control under high load.

As far as I know a diesel runs lean all the time. The strategy is different in that the "gas pedal" actually controls the amount of fuel that goes into the engine, and in modern diesels there's a programmed delay in the rate of fuel added to allow the turbocharger to keep up with the air required to burn it efficiently (again, as far as I know).

My questions are:

1. Do diesels ever intentionally run rich, and if so when? I know there are EGR strategies to keep emissions in check, but does the Air/Fuel ratio play a part as well? Too rich results in excessive soot from unburnt fuel, but is there a strategy as to how rich they need to run or do they always run as lean as possible? There is also the regen cycle that puts additional fuel into the DPF to burn off the collected soot, but I don't consider that as it's all done after the engine.

2. Efficiency vs. load. A gas engine has a maximum efficiency (BSFC) load at any given speed, and is less efficient with more or less load applied. Do diesels work this way as well, or are they more efficient the more load they're under?

If there's a website that clearly explains all of this please feel free to direct me to it instead of regurgitating it all here. Thanks!