Modern Diesel Fueling Strategies
 

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!

A diesel engine usualy has no throttle.
It always takes in as much air as possible.
Power is adjusted by injecting more or less fuel.
Less fuel = less power
More fuel = more power (up to a point)

Modern diesels use EGR to make the engine run richer without adding more fuel.
This results in lower NOx raw emissions, wich need to get converted to nitrogen and water by the SCR catalythic converter.
The SCR cat needs an urea solution to do that, so by needing to convert less NOx into N2 and H2O, it saves you DEF.

Downside of running close to stochiometric is that the engine produces more soot particles.
These particles get trapped and burned in the particle filter.
To not clog up the filter over time, it needs to burn the particles, wich happens when driving fast on the highway for a while or with a regeneration cycle.
In said cycle the engine injects additional fuel after the main combustion to get the exaust gas temperature up and burn off particles.
To avoid unnessesary regeneration cycles, the ECU avoids running stochiometric or even rich.

Diesels have a point where their BSFC is best, but as they have no throttle, they are more efficient under low load than gasoiline engines.
This results in great fuel economy.

Turbo-lag has not been much of an issue anymore, with the variable-nozzle turbochargers becoming mainstream on Diesels.

I disagree, it has become less of an issue, but it's still there.
A modern turbocharged engine doesn't have the throttle response of a naturaly aspirated engine with ITBs.

I said not much of an issue, but I didn't say it was not an issue at all.


Even a naturally-aspirated engine with just one throttle-body often has a better throttle response, no wonder Volkswagen makes naturally-aspirated versions of the T-Cross in Brazil for regional exports, as some countries such as Mexico and Bolivia are quite hilly, and the turbo-lag is felt harder in La Paz and Mexico City than in Brasília for instance.

Oh, I'm quite aware of that.
The response of my Toyota MR2 and a recent model Audi TT is a difference like night and day.
With the throttle response of an engine like the TT's, the MR2 would be undriveable at the limit.

The higher-revving nature of a gasser tended to dictate the need for a larger turbo. Didn't you notice how turbochargers for Diesel engines used to be proportionately smaller than those for a gasser? No wonder often the turbocharger of a low-revving tractor-truck would be used for adaptations to cars.

I believe modern turbodiesels have artificially sluggish response purely for emissions reasons. They only add fuel at a rate slow enough for the turbocharger to completely keep up, since running even slightly rich results in huge increases in particulates and soot.

This was evident when I drove truck for a lumber yard in my younger years. The old '90 (I think) International 466 flatbed 5-ton was a joy to drive, and the throttle response was very good - it was like the tach was simply attached to the throttle pedal with a cable. You could put that engine anywhere you wanted with speed and precision, making it an absolute pleasure to rev-match. (For those not aware, you only use the clutch to get it moving in the lowest gear, and it's all clutchless shifting from there on out).

A newer International 466 they leased, a '96 I think, had an electronically controlled fuel delivery system and felt completely disconnected compared to the older truck. It took me a long time to adjust to driving it, and even though it had air conditioning (and an advertisedpower increase) I still preferred driving the old '90.

Having said all that, the engine in my 335d is quite responsive. It's sitting in front of an automatic transmission, so it's pretty much blasphemy to speak about throttle response... if it was a manual, OK. :D

EDIT: Forgot to mention that the older trucks would spew black smoke on every shift as the turbo caught up to the fuel being dumped into the engine by the mechanically actuated fuel pump. The newer electronic engines had no such black smoke between shifts, but they seemed to take FOREVER to spool back up and make power.

Turbochargers are specified based on flow rates. 300 hp is 300 hp, regardless if its attached to a Honda or a Peterbuilt. It's not quite that simple when you get into pressure maps and what not, but people shopping the scrapyard for Powerstroke turbos to boost their budget projects aren't usually too fussy. :)

The old '90 International 466 I drove way back was only rated at 250hp if I remember right. That figure is nothing special for a gas four-banger 1/4 the displacement these days.

I'm very curious how these modern diesels control for adding power... if the air is available is it just a matter of adding more fuel? I know how to tune gas engines, but diesel is a completely unknown concept for me.

This car must have a very effective EGR system then, because I have put about 15,000km on it since buying it a year ago and have yet to top up the DEF tank. That, or the DEF tank is just huge?

So far I have been completely unaware of any regen cycles taking place. From what I've read it should be obvious when a regen cycle is happening as your mileage will suffer terribly. I've noticed nothing out of the ordinary so far, and I don't think I use the car on the highway more than most people would. My commute is actually very slow, travelling about 50MPH with the cruise on for about 15 min. which doesn't seem like much time/load to facilitate the burning of soot in the DPF? When I do have it on the highway I tend to drive it harder just for that reason, but it's not a regular thing.

...and is the reason I was so interested in this car. For a big heavy AWD sedan it's phenomenally frugal. I average about 8-8.5 L/100km (27.5-29 MPG), and do much better on long trips. I got 5.7 L/100km (41 MPG) calculated at the pump on a 3.5 hour highway trip in January with four brand new snow tires, averaging around 60 MPH.

One thing I'm curious about is cold weather operation. A gas car runs rich to warm up the catalyst, which can take quite a while with light-footed driving, or while idling, and can result in fuel contaminating the oil. I don't believe diesels have this issue and would be much more efficient in the winter because of that, no? I didn't notice much change in fuel economy in the cold last winter and I'm wondering if that's why?