Low Revs and Preventing DMF Failure

topdost

Why to worry so much , get it removed , when it get faulty . Get the car remapped and that it your are sorted .

Will pass MOT

Johnmcl7

Ultrasonic wrote: »

I'm well aware of the relationship of revs, engine load and fuel consumption (i.e. BSFC). However, the fuel efficiency in mpg (as distinct from engine efficiency, a measure of the power produced for a given amount of fuel) is higher at 30 mph in my car in 5th gear than 4th (or 3rd or 2nd or 1st). This is easy to confirm with the trip computer.

I seriously doubt that the fuel economy part of that sentence is true. Have you tested it with your trip computer (assuming you have one)? I would be very surprised if you don't get better fuel efficiency in 4th gear at the very least. The debate re. flexibility is one I'm aware of but if possible I'd rather not take this thread off topic with that.

Yes, I have had tried it with both the 1.9 TDI cars I had (a Mk I Skoda Octavia with the 90bhp 1.9 TDI and a Mk II Seat Toledo with the 150bhp 1.9 TDI) and the trip computer definitely did favour driving in high gears and low revs but I didn't find it very accurate. Measuring fuel used I found driving that way used more fuel compared to using a lower gear and sitting at slightly higher revs.

Trying to get back on topic, I know that I get best fuel economy with low revs in high gears (and FWIW VW recommend exactly that here), what I am trying to work out is whether, with the engine showing no sign of distress, if it may still be causing long term harm.

I don't think there is any solid evidence on DMF failures as carrying out a scientific study on them would be tricky given the miles involved. The belief with low revs is that even if it feels ok (no juddering), the DMF is still having to work harder to smooth out additional vibrations so it's doing more damage than higher revs. Gunning the car hard with racing starts is believed to also do more damage.

Personally I can't say based on my own experience as I've yet to have a DMF fail on me. My current car has a DPF (I know the 1.9TDI's do not) so that's more of a concern to me when driving although it has behaved itself so far.

John

forgotmyname

I posted about this a while back, Lower revs does not mean better fuel economy.

In my 6 speed diesel mondeo below 60mph in 6th gear gives worse consumption that changing down to 5th.

I repeated the test at 40mph in 4th and 5th gear. 4th gear gave better fuel consumption compared to 5th. I read the throttle position at the same time and 5th gear used 18% throttle travel, Where 4th only required 11% due to the extra torque available. This gave better fuel consumption.

Below 1500rpm is not good for the DMF. Depending on the car and the power/torque curve. 2000rpm is probably close to the best consumptio.

But every car will vary slightly.

Strider590

forgotmyname wrote: »

I posted about this a while back, Lower revs does not mean better fuel economy.

I'd go with this too, I've been running an OBDII scanner on my car..... At 50 in 5th, it's mpg is between 11 and 28, drop into 4th gear and suddenly it's 34 to 70+ mpg.

It's about driving within the rpm range where the cars runs most efficiently (ie the point between max torque and max power).

Notmyrealname

Ultrasonic wrote: »

I never do that. My question is specifically about travelling at a steady speed.

Which is pretty much nigh on impossible in an urban areas.

Not if you depress the clutch pedal at the same time.

So you're not in full control of the vehicle.

I seriously doubt that the fuel economy part of that sentence is true. Have you tested it with your trip computer (assuming you have one)?

Yes in 44 tonne lorries to cars.

5th is only beneficial at 30MPH if you do a constant speed on a flat gradient. As soon as you get a rise in gradient or have to vary your speed AS YOU DO IN THE REAL WORLD, it is worse.

here), what I am trying to work out is whether, with the engine showing no sign of distress, if it may still be causing long term harm.

Please look at the URL. It is THE AMERICAN VW website. US roads, especially urban ones, are a lot different to our own. They are a lot flatter, straighter and emptier on the whole.

forgotmyname

Ah the US way of thinking. We need a road through that mountain.

Do we make the road 20 miles longer and go around? + $2 billion

Do we go through the mountain? +$4 billion

Or do we just flatten the whole mountain? +$100 billion but we get to use lots of explosives. And we can borrow some if we want to play in the back yard.

It sounds like my kind of place.

Ultrasonic

atrixblue.-MFR-. wrote: »

By feeding the lower oxygen exhaust gas into the intake, diesel EGR systems lower combustion temperature, reducing emissions of NOx. This makes combustion less efficient, compromising economy and power. The normally "dry" intake system of a diesel engine is now subject to fouling from soot, unburned fuel and oil in the EGR bleed, which has little effect on airflow but can cause problems with components such as swirl flaps, butterfly valves , where fitted. Diesel EGR also increases soot production, though this was masked in the US by the simultaneous introduction of DPF's.EGR systems can also add abrasive contaminants and increase engine oil acidity, which in turn can reduce engine longevity

Thanks for that detailed description. Does running at lower revs increase the chances of the intake being fouled with soot? I guess it comes down to engine conditions that are less efficient resulting in more fouling?

Ultrasonic

topdost wrote: »

Why to worry so much , get it removed , when it get faulty . Get the car remapped and that it your are sorted .

Will pass MOT

Well the short answer would be that it is obviously cheaper if it never fails at all!

When you say 'get it removed' do you mean having it replaced by a SMF? That is something I'd at least consider but I'd rather avoid the need for the decision. How would a remap help?

Ultrasonic

Strider590 wrote: »

I'd go with this too, I've been running an OBDII scanner on my car..... At 50 in 5th, it's mpg is between 11 and 28, drop into 4th gear and suddenly it's 34 to 70+ mpg.

What applies in one car won't necessarily apply in another, it will depend on the specific BSFC for the vehicle.

Strider590 wrote:

It's about driving within the rpm range where the cars runs most efficiently (ie the point between max torque and max power).

That is not my understanding. The highest engine efficiency is usually at close to full load at the torque peak, but this does not correspond to maximum fuel efficiency (mpg) - two very diifferent things. The point of maximum engine efficiency is of most significance when considering fuel efficient acceleration, but not so much for steady speeds (when much lower power is required).

I did for what it's worth try driving around with revs close to the rpm where the the torque peaks (I think ~2000 rpm in my car) as I'd read advice similar to above. This did not result in better fuel efficiency based on my trip computer. Having read a bit more I now think I understand why. The text under the BSFC plot I linked to earlier gives the best explanation that I've seen.

Johnmcl7

Ultrasonic wrote: »

I did for what it's worth try driving around with revs close to the rpm where the the torque peaks (I think ~2000 rpm in my car) as I'd read advice similar to above. This did not result in better fuel efficiency based on my trip computer. Having read a bit more I now think I understand why. The text under the BSFC plot I linked to earlier gives the best explanation that I've seen.

As I said above, I wouldn't use the trip computer to measure fuel efficiency as I never found it accurate (it was the same on both 1.9TDI cars I had, both Mk IV Golf platforms). It was generally too optimistic and it favoured certain driving styles which didn't actually yield better fuel economy. The only you way you can accurately measure your fuel economy is by how much fuel the engine is using which you can check when you fill up.

John