View Single Post
Old 11-18-2021, 10:59 PM   #10 (permalink)
Blue Angel
EcoModding Apprentice
Blue Angel's Avatar
Join Date: Feb 2013
Location: Ottawa, ON, Canada
Posts: 190

Previous Car - '12 Chevrolet Cruze Eco MT
Team Chevy
90 day: 44.29 mpg (US)

535d XDrive - '16 BMW 535d M-Sport
Thanks: 17
Thanked 59 Times in 38 Posts
Originally Posted by Autobahnschleicher View Post
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)
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.

Originally Posted by Autobahnschleicher View Post
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.
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?

Originally Posted by Autobahnschleicher View Post
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.
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.

Originally Posted by Autobahnschleicher View Post
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.
...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?
2016 BMW 535d
4100lb XDrive Eco-Yacht
  Reply With Quote