Hypermiling in a Diesel

[8$PG]

Hello!

This is my first diesel-fueled car I've owned (BMW 320dA). With the gasoline-engines it was fairly easy to get better fuel economy than the spec. but with this modern diesel I'm having a real challenge keeping it above the advertised mileage. Mostly my driving consists of long runs on the freeway with cruise control at 115-120km/h and some city/urban driving at 50km/h and through roundabouts and some traffic lights. My girlfriend takes the car to the gym now and then, short runs. Only city driving the average consumption is at around 6-7L/100km. At freeway speed it's under 5L/100km. With this kind of driving the average is pretty much right on the advertised combined figure.

With the gasoline-engines using lots of throttle when usable or none at all have helped but with the diesel I'm not so sure. Going downhill and when idling and even at constant speed on level road the consumption is really low. When accellerating alot the consumption gets much higher.

I realize keeping the speed down would do alot but it's difficult on the freeway without causing "a scene". I'm a bit curious how the couple in the Passat tdi got 84mpg (2.7L/100km). Whan kind of driving technique is required to reach this number in a Diesel? Driving at a constant 60km/h? My personal best in this car is 4.4L/100km (which is the advertised highway mileage).

Feel free to input any thoughts.

[Lug_Nut]

Originally Posted by 8$PG

I'm a bit curious how the couple in the Passat tdi got 84mpg (2.7L/100km). What kind of driving technique is required to reach this number in a Diesel? Driving at a constant 60km/h?

For my own personal best (2.9l/100km) it was not to far from the 60 kph number you posted.
The highway course we were assigned (and on which we were monitored to prevent driving too closely) had a maximum speed of 65 mph and a minimum of 45 mph. There was also a minimum and maximum time to perform each of the repeated loops. I found that an average of 47 mph (75 kph) was just fast enough to complete each lap within the maximum time allowed. A slower speed would have used less fuel, but the time per lap would have taken too long, and I wouldn't have received credit for the distance.

[8$PG]

2.9 is very good for a golf. I need to keep the speed above 70km/h to be in 6:th gear.

What do you think would be the most fuel efficient? Driving 60km/h in 5:th gear at 1400rpm or 72km/h in 6:th gear 1250rpm.

[Lug_Nut]

2.9 l/100km is better considering it was a Passat, not a Golf.

Is your turbocharger equipped with a variable inlet? These are susceptible to a condition we call "surge" in which the pressures produced during full accelerator pedal application, and the relatively slow reaction of the variable vane control at that low an engine speed, can cause turbo damage. Don't try to floor the accelerator at such slow engine speeds for long duration high load use. Shift down a gear or two to accelerate from 1200 or 1400 rpm in 5th or 6th. In 1st, or even 3rd, gear the car will accelerate quickly enough that you won't be at the low rpm and high load condition for long, but leaving the car in 6th at 1400 rpm and driving up a hill with your foot on the floor isn't good.

72 kph will require 44% more power due to air resistance than 60 kph. I don't think that engine operation at 1250 rpm will be 44% more efficient (liters per 100 km) than 1400 rpm.
It will be thermally more efficient as the engine will be at nearer to 100% of it's capability at 1250 rpm and pushing 44% harder, than at a lesser percent of what is likely a higher power capacity at 1400 rpm. From a grams of fuel burned per KwH produced reference the faster road speed and slower engine speed is more efficient.

Is it possible to drive at 1100 rpm in 6th at 65 kph? That would be more efficient in both specific heat energy produced per unit of fuel, and also in distance traveled per unit of fuel.

[8$PG]

I will try that in manual mode, but I think it will downshift automatically at 68km/h just like in auto-mode. The manual mode isn't that unconditioned as in my previos honda 5-speed but this box is using lock-up more firmly even when pressing the accellerator hard.

Actually the engine is very torque-capable at low rpm:s. On-ramps are quick and easy at half accellerator with gearbox still in 6:th gear and lock-up engaged. Flooring it (or close to it) will certainly mean a quick downshift so I don't think the turbo is in danger there.

In 5:th gear at 60km/h the rpm was about 1300-something actually not that different from 72 in 6:th.

I was unaware of variable inlets, I thought the inlet would want to breathe as easy as possible. There is however a short bypass tube that leads through a valve directly from the engine bay into the air filter box, but that part of the filter is spotless clean so I think this is just a failsafe bypass that normally never engages.

The best I was able to get recently is 4.2l/100km on a single trip through stockholm driving the speed limit (or slightly above) and some 10-minutes of traffic jam. I'm expecting to do less sometime this summer.

I was also able to determine that the AC (when clutch engaged) adds ~0.5 momentarily on the freeway. To think that some people run it all year constantly...

[8$PG]

Originally Posted by 8$PG

I will try that in manual mode, but I think it will downshift automatically at 68km/h just like in auto-mode.

It downshifts at 65km/h so driving 67 is possible in manual mode. To get to manual mode you have to go through sport mode and that makes the transmission downshift first which is unconvinient. To avoid this you have to do this at stand still and then shift manually through the gears.

[Piro]

Originally Posted by 8$PG

I was unaware of variable inlets, I thought the inlet would want to breathe as easy as possible.

A variable intake geometry turbocharger is fairly common now.

Essentially, at low RPMs, where the exhaust flow is low, the vanes expand in the turbo to make the passage more restricted, thereby increasing pressure, which helps spin up the turbine faster. As RPMs climb, the vanes contract in, leaving a larger path for airflow, to keep the engine free flowing, and of course, the turbo is still spinning as the overall pressure to the turbo is similar.

The idea of course is to reduce turbo lag, but not run out of puff at the high end. Kind of like having a small and large turbo at different points in the rev range.

Anyone is free to correct me if I explained it poorly.

[Cogeneration]

To the OP. Does your BMW 320dA have a VNT turbo ? If yes then Piro's post is correct. With the L/100km that you've posted I say you're doing pretty good mileage wise. If you bought it new then the mileage will get even better as the engine loosens up with miles driven. Finding the kph sweet spot is the key. I have a 2011 Golf TDI. One thing I do to lower the fuel consumption is when at a traffic light stopped I put it in neutral. This shows the consumption going from 0.9 L/hour to 0.7 L/hour. Not a drastic drop but it helps. As for coasting down hill in neutral foot off brake it burns more fuel as opposed to coasting down hill in gear.

[trollbait]

As for coasting down hill in neutral foot off brake it burns more fuel as opposed to coasting down hill in gear.

What is going to happen after getting downhill, or any coast, needs to be factored in. Coming to a stop or needing to slow down anyway, then the fuel cutoff in gear will save more fuel. If not, the engine braking could lead to more acceleration after the coast. Burning more fuel than was saved keeping it in gear.

[Draigflag]

I use the neutral technique, like trollbait says, the need to accelerate again arrives sooner and more often if left in gear. In neutral, the car will roll freely faster and therefore further than in gear.