Modern Diesel Fueling Strategies

[Piotrsko]

The only way I know my golf is in regen mode is the idle climbs to 1300 rpm or there's slight bog on heavy acceleration and the cooling fans go to high speed. Cruising above 50, you'll never notice, but the flow and temperature might be high enough to break down the particulate size. On a schoolbus I drove there was a regen light that would go on every so often.

If you don't notice your oil turning black from unburned fuel, you're not looking at the engine oil

[Autobahnschleicher]

Originally Posted by Blue Angel

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?

1. Modern diesels work about the same as oldschool ones in that regard, but they do add some EGR especialy under low load to keep NOx down

2.That's what the EGR is supposed to do, the filling intervalls can depend on driving conditions though.

3.You might not even notice the car initiating a regen cycle when driving on the highway as it just injects a little more fuel after combustion to get the exaust gas temps up enough.
Plus highway driving means you might not even need regen cycles at all as there your EGT goes up sufficiently on its own.

4. Yep, diesels can be very efficient, yours is actualy not that efficient by diesel standards.
Take a look at the Passat 1.9 TDi or a Lupo 3L

5. Diesels hate the cold.
As they are so efficient under low load and move that much air, they take a long time to warm up.
In idle they might not even heat up at all, so if it gets very cold where you live, consider a block heater.

[Blue Angel]

Originally Posted by Piotrsko

If you don't notice your oil turning black from unburned fuel, you're not looking at the engine oil

No worries there... the oil is B L A C K when I change it! I've decided to change the oil in this car at 10k km intervals instead of the recommended 15k. The timing chain on this engine is a very very big job... one I don't plan to do!

[cRiPpLe_rOoStEr]

Originally Posted by Blue Angel

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?

It's usual for the tanks to hold enough DEF for such long mileages, but it's also worth to consider cars often resort to both EGR and DEF while heavy-duty trucks and buses usually have either one or another, so a car would use fewer DEF proportionately to the fuel consumption, even though the DEF tank of a truck might hold a volume proportionately closer to that of the fuel tank.

[Blue Angel]

Originally Posted by Autobahnschleicher

1. Modern diesels work about the same as oldschool ones in that regard, but they do add some EGR especialy under low load to keep NOx down

Good to know!

Originally Posted by Autobahnschleicher

2.That's what the EGR is supposed to do, the filling intervalls can depend on driving conditions though.

Makes perfect sense.

Originally Posted by Autobahnschleicher

3.You might not even notice the car initiating a regen cycle when driving on the highway as it just injects a little more fuel after combustion to get the exaust gas temps up enough.
Plus highway driving means you might not even need regen cycles at all as there your EGT goes up sufficiently on its own.

Right. I've read that a good highway blast is good for modern diesels from time to time, and that's probably why. This car enjoys the left lane anyway.

Originally Posted by Autobahnschleicher

4. Yep, diesels can be very efficient, yours is actualy not that efficient by diesel standards.
Take a look at the Passat 1.9 TDi or a Lupo 3L

I could also ride a scooter.

I'm just comparing this car to its gasoline powered siblings, or other comparable cars with gas engines. It was only available in this AWD 3L six configuration in North America, but in Europe is was available with several four cylinder diesels that are all quite a bit more efficient than this.

The newest Passat TDi sold in North America was the 2015 model, and the most fuel efficient version was EPA rated at 30 MPG City, 42 MPG hwy, 34 MPG combined:
https://www.fueleconomy.gov/feg/Powe...=1&rowLimit=50

Compare that to the 535d at 26/38/30 combined:
https://www.fueleconomy.gov/feg/Powe...=1&rowLimit=50

Compared to the 535d the Passat TDi is smaller, 600 lbs lighter, only FWD (no AWD option), 118 hp less powerful, 3 seconds slower to 60 MPH, and doesn't even have an automatic transmission (the auto drops it to 33 MPG combined). For an additional $12.50/month in fuel compared to the automatic TDi, I'll take the 535d. Especially in Canada where AWD is important three months of the year.

A more even comparison to that Passat TDi would be the 2016 BMW 520d, the same car but with a 184 hp 2L diesel. Compared to the 520d XDrive the Passat TDi still has all the disadvantages listed above, except it's only slower to 60 by about 1/2 a second. Extrapolating from published European data the 520d would test about the same as the automatic equipped Passat TDi - 33 MPG combined.

All that said, if the Passat TDi is considered an efficient car, the diesel BMW 5-Series is a very efficient car since it seems to do more with the same amount of fuel. Shame more European diesel models were not made available here in North America.

Originally Posted by Autobahnschleicher

5. Diesels hate the cold.
As they are so efficient under low load and move that much air, they take a long time to warm up.
In idle they might not even heat up at all, so if it gets very cold where you live, consider a block heater.

I did quite a bit of research on this car before buying, and a good chunk of that was looking into winter performance. It didn't seem to be an issue, and based on my experience last winter it doesn't look like it will be.

Ottawa can get pretty cold, though last year was warmer than usual. We had a few nights down to -25C (-13F) and I'm pleased to report it started absolutely perfectly each and every time. It will ask that you wait 5-6 seconds for the glow plugs to warm up, then it will crank and start like a champ. It did sputter/vibrate once as if one cylinder wasn't firing, but a blip to 1500 RPM solved that quickly. If we have a more typical winter this year I hope the same remains true at -30C (-22F). If so, it gets a huge gold star for cold weather insensitivity.

With heated steering wheel and seats, engine warm-up time isn't as much of a concern. Having said that, I'm absolutely shocked at how quickly this thing warms up. I'm not 100% sure, but I think there must be a resistance heater in the HVAC because it's blowing warm air within a couple minutes of setting off - but that's not the impressive part.

What really gets me is how quickly the engine gets up to temperature. The dash doesn't have a water temperature gauge, it has an oil temperature gauge, and after my easy ~22 min 22km (14 mi) commute to work the oil temp is right in the middle of its operating range (roughly 110C or 230F). On really cold mornings the oil temp doesn't quite make it to the middle of the gauge, but since oil temp generally trails coolant temp I'm reasonably confident the engine is fully warm. That's with the HVAC set to auto to prioritize heat in the cabin, and fuel efficiency seems to be not far off what I see in warmer weather to boot.

Winter heat is just an absolute non-issue with this car, which is in total contrast to my last car - a gas 2012 Chevy Cruze Eco 6MT. That car would never warm up if driven gently on really cold days, even at highway speed (there are lots of complaints from people even with the less efficient automatic equipped versions). On my short and easy commute I never got any appreciable heat out of it unless I left it in 5th (or sometimes 4th!) gear... burn more fuel, make more heat. I designed an elaborate plug-in heater system for that car consisting of the 200W OEM oil pan heater, a few stick-on pad heaters, a 1000W thermosyphon coolant heater and a 12V Prius water pump. With all that going on the engine would be just OK for commutes to work, but the physics were obvious; there just wasn't enough waste heat coming out of that little turbo 1.4 unless you deliberately drove it inefficiently.

So to wrap this up, for what it is I'm seriously impressed with the economy of this car given its lack of apparent shortcomings. It's spacious and supremely comfortable year-round in a difficult climate, an AWD tank in the snow, powerful and quiet, handles well, and is impressively efficient (to me at least). Purchased used for less than 1/3 its original price with 88k kms it seems like a good value so far. Considering I put premium fuel in my Cruze it barely costs more to fuel this car with diesel.

[Blue Angel]

Originally Posted by cRiPpLe_rOoStEr

It's usual for the tanks to hold enough DEF for such long mileages, but it's also worth to consider cars often resort to both EGR and DEF while heavy-duty trucks and buses usually have either one or another, so a car would use fewer DEF proportionately to the fuel consumption, even though the DEF tank of a truck might hold a volume proportionately closer to that of the fuel tank.

Makes sense. This car was asking for an oil change when I bought it, so I wonder if it got an oil change and a DEF top-up when it was returned at the end of its lease and they forgot to reset the oil life monitor? Possible I guess.

I've had a jug of DEF sitting in my garage since I bought the car assuming it will run out. Now I'm curious to see how far it will go before needing a refill. Also curious if that 9.5L (2.5 US Gal) jug will even top up the tank?

OK, so a little Googling says BMW claims 10k miles on a fill, so it must have been topped-up right before I bought it. 15k kms is getting close to 10k miles.

[cRiPpLe_rOoStEr]

I never drove any vehicle fitted with SCR, as in my country it was initially fitted to heavy-duty commercial vehicles and the Agrale Marruá in 2012, and only in 2017 or 2018 it became available on some Peugeot/Citroën commercial vans. The first vehicles mostly focused on private customers fitted with SCR here are the '22 Jeep Compass and the recently-released Jeep Commander, yet now I see more of the flexfuel versions of the Compass because most of its buyers don't seem so willing to deal with SCR. I must confess I'm also not so fond of SCR, yet I look at it from a perspective similar to what 2-stroke motorcycle owners look at those automatic oiling systems with a separate oil tank instead of relying on premix.

[Isaac Zachary]

Diesels can be "tuned" by adjusting the injection timing as it happens before TDC like the spark ignition in a car. The effects are similar.

Regen burning isn't always provided by a fuel injector in the exhaust. The fuel can be added to the exhaust through the engine's injectors if they are activated during the exhaust stroke.

Diesels tend to be more efficient than gassers at low loads due to lack of throttle. However, as far as I've researched, diesels are most efficient at full throttle. The more you spool up the turbo and the more fuel gets injected the hotter the combustion. The hotter the combustion the more energy can be extracted from it.

There is no throttle so adding EGR doesn't help efficiency like in a gasser, because in a gasser the EGR allows the throttle to be open more.

The only reason to run rich is to "roll coal". Even at full throttle diesels don't reach stoichiometric because there's no good way of completely mixing all the fuel into the air and you wouldn't gain any power as a result.

Even without EGR, a diesel will normally run cooler than a gasser even at full throttle because they run lean enough that the extra air is effectively an inert gas just like exhaust gas is.

[Piotrsko]

What about lean mixtures running hotter? Ever fry a piston top in a VW bug? In the diesel F250, you watch EGT on hard pulls because the aluminium stuff tends to melt.

[Isaac Zachary]

Originally Posted by Piotrsko

What about lean mixtures running hotter? Ever fry a piston top in a VW bug? In the diesel F250, you watch EGT on hard pulls because the aluminium stuff tends to melt.

That's somewhat of an old wive's tail.

In my 1972 1600cc VW Super Beetle with mostly stock air cooled engine (stockish cam grind, stock VW heads, stock 34-4 Solex carb, stock oil bath air cleaner, stock VW heat exchangers, stock VW muffler, etc. etc. etc. just electronic distributor and heads machined for higher CR, did adjust pistons for .030 quench),

I ran the compression ratio up to 10:1, ran lean, about 16:1 or leaner at cruising (with the aid of an O2 sensor and AFR gauge) and adusjusted the ignition as far advanced as possible (most power without pinging) with a digital timing map and drove it mostly over steep mountain passes at full throttle and never had any problems at all. No pinging, no knocking, no fried pistons. The engine ran like a charm.

Why? Because leaner doesn't always mean hotter. Hot is around stoichimetric. A little leaner, like 15:1, is still hot. Go richer, 13:1, 12:1, etc. and it gets cooler. But going leaner, 16:1, 17:1, 18:1, (at 18:1 my Bug started misfiring) also runs cooler. In fact I actually had a hard time keeping my oil temps up as 180°F was the hottest I could get, and head temps never wanted to go over 325°F either.

I did make sure I got at least 12:1 at full throttle, but I drove all over, both going up steep mountain passes and along long stretches of road at highway speeds and never had any problems. I also averaged 30mpg.

I'm not the first VW aircooled owner to do this. Lots of others have found that they can do the same thing. Many airplane owners have found their planes run better leaner and cooler at loads below 75%.

The lean = hot and rich = cool depends on the context. Are we building a race engine and are getting AFR's of 13:1? Then yes, leaner is hotter and richer is cooler. And maybe you could do 16:1 in a race engine, but you wouldn't have the power.