Diesel 'Sweetspot' ?Pedal to the Metal Vs Low Revs Coking Up?

legomanx51v

christy02 wrote: »

Nice dash. What car is it?

I really want to know what car this is!

Moneymaker

2011abc wrote: »

Ok so we all have diesels to save a few bob on fuel .But if you drive TOO 'steadily' you get that pile of black smoke the next time on your journey you floor it.Does that build up eventually cause problems over weeks and months ?Should you give her at least some welly every journey?Is it better to just average a slightly higher speed/revs to avoid this?Any advice appreciated.

(I drive an earlyish 1.3 Multijet with no dpf and can get up to an indicated 90mpg if REALLY slow (55mph on Mways)which brim to brim works out at about 76/78mpg.Normally get closer to 70mpg (actual mpg) though.Its a Panda which has started every morning for coming round to three years now (40-45k miles done to see 141k on clock now)Only oil changes (done every 6k miles) but Pre NCT work was a bit pricey at around 600 euro (front shocks and wishbones mostly and that was self sourcing parts and shopping around for labour)...These cars are fairly fond of front shocks (and tyres and tracking).But considering it probably saves me 50 a month compared with a one litre petrol Im keen on it.

No we don't.

The Dagda

legomanx51v wrote: »

I really want to know what car this is!

Honda Civic.

CiniO

Moneymaker wrote: »

No we don't.

Most of us do anyway.
At least I do have diesel to save on fuel and tax.
Otherwise I would have chosen petrol.

CiniO

The Dagda wrote: »

Honda Civic.

That's right sir

corkgsxr

Moneymaker wrote: »

No we don't.

If it wasnt tax and fuel cost id probably settle for a 3-3.5 inline 6 petrol.

jca

Another thread decended to ****e..... Byeee.

dr.fuzzenstein

jimgoose wrote: »

Automatics were always somewhat cognisant of throttle position, altering the shift-pattern (sometimes too subtly) depending on whether it was getting a bootful or a barman's fart. This was done via a vaccum sensor in the intake and in later incarnations often further modulated by "sport" and "comfort" modes, or similar. Also in the old, old days you had a switch on the floor under the accelerator pedal which whomped the thing down a cog con mucho gusto when it was floored. Nowadays of course you have sensors everywhere, everything connected to everything-else and fancy-dan dual-clutch automated manual setups, affording quite a degree of control even in an auto.

Not always, my 1984 Merc had the switch, Mercs always had that switch there, but my 1973 MKIII Cortina (3 speed auto, lol) did not.
The Merc you floored it, hit the switch, waited, waited, waited and it would drop two gears at the same time. Terrible!
The Cortina would drop a gear if you had the throttle 80-90% pressed, it knew it beforehand.
I miss that 2 liter Pinto engine.

shedweller

I recall seeing a thing called a bsfc chart but each one is unique to an engine. They are a graph of rpm vs fuel use and speed etc.
Anyone got more on this?

Bpmull

shedweller wrote: »

I recall seeing a thing called a bsfc chart but each one is unique to an engine. They are a graph of rpm vs fuel use and speed etc.
Anyone got more on this?

I used these on a project I done in college this year. As you said they really just compare rpm to fuel usage. There is obviously then a peak where the car is most efficient fuel wise and it drops off gradually if revs are too high or low. I got all my info from scientific journals which only students can access. But I'm sure google will bring up plenty for them.

shedweller

Bpmull wrote: »

I used these on a project I done in college this year. As you said they really just compare rpm to fuel usage. There is obviously then a peak where the car is most efficient fuel wise and it drops off gradually if revs are too high or low. I got all my info from scientific journals which only students can access. But I'm sure google will bring up plenty for them.

Yeah i do remember searching for specific ones but to no avail. As you say, scientific journals would have more info but not for us uneducated oafs!!
Could always shoot off an email to my car manufacturer...

jimgoose

dr.fuzzenstein wrote: »

Not always, my 1984 Merc had the switch, Mercs always had that switch there, but my 1973 MKIII Cortina (3 speed auto, lol) did not.
The Merc you floored it, hit the switch, waited, waited, waited and it would drop two gears at the same time. Terrible!
The Cortina would drop a gear if you had the throttle 80-90% pressed, it knew it beforehand.
I miss that 2 liter Pinto engine.

Yeah, most Fords always had the vacuum sensor. The floor-switch seems to have been a German thing - IIRC BMWs used to have them as well?

jca

jimgoose wrote: »

Yeah, most Fords always had the vacuum sensor. The floor-switch seems to have been a German thing - IIRC BMWs used to have them as well?

The fords had a separate kick-down cable that ran from the carb to the auto box. Simple but very effective system.

GTE

2011abc wrote: »

Ok so we all have diesels to save a few bob on fuel .But if you drive TOO 'steadily' you get that pile of black smoke the next time on your journey you floor it.Does that build up eventually cause problems over weeks and months ?Should you give her at least some welly every journey?Is it better to just average a slightly higher speed/revs to avoid this?Any advice appreciated.

Taking a look at the exhaust recirculation value (EGR) is usually a reasonable thing to do. It is a valve which regulates exhaust gas which on a diesel is sooty and it can clog. That said, the warnings about EGRs that I have read generally come from the USA which apparently can have dirty diesel compared to us.

It is good practice to burn the soot off the EGR by ensuring you get some full engine temp, high rev running every once and a while but my particular gearbox has the engine sitting high enough in the revs on the motorway that I do not need to bother (3000+). My engine is also modest in the amount of horses it plants onto the roads so I am full loud pedalling it most of the time anyway.

I do have an older generation of diesel so I have less (or none) of the issues that the relatively modern units suffer from. It strikes me from here and the engine forums I read that any low rev gear changing which causes issues is causing issues with weak components that have been fitted to an otherwise reliable way of making car move such as causing issues flywheels or particulate filters.

firemansam4

I noticed on the mpg counter on my diesel that i was getting significantly less mpg in 6th gear driving at 100 kph than i was in 5th, dont think I would use 6th unless on a motoway now.

Also some modern diesels are prone to DPF failure and require a long sustained drive every now and then, and im pretty sure this needs to be done over 2000 revs

Cleveland Hot Pocket

firemansam4 wrote: »

I noticed on the mpg counter on my diesel that i was getting significantly less mpg in 6th gear driving at 100 kph than i was in 5th, dont think I would use 6th unless on a motoway now.

Also some modern diesels are prone to DPF failure and require a long sustained drive every now and then, and im pretty sure this needs to be done over 2000 revs

A lot of people who bought diesels for de cheep tax and to save a few cents at the pump are blissfully unaware of both of the above points!

Phil_Lives

The sweetspot for a diesel engine is usually anywhere above stall speed and not on the turbo.

pred racer

Don't worry about it......just keep er lit:D

Gazzmonkey

Phil_Lives wrote: »

The sweetspot for a diesel engine is usually anywhere above stall speed and not on the turbo.

Are you sure about that? I would think that driving at such a low rev would cause DMF issues among others things.

Popoutman

In general the sweet spot is when the turbo is pressurising the intake sufficiently for the volumetric efficiencies of using a turbo, to kick in. Too low rpm and too much throttle (labouring) is not a good idea as you can put the turbo outside of the envelope it was designed for. A few hundred rpm higher and it'll be golden.

Most VAG diesels that spot is between 1800 and 2200 rpm. The turbo spins fast enough to give enough pressure to be more efficient even though there are more injection events per second.

Jesus.

Popoutman wrote: »

In general the sweet spot is when the turbo is pressurising the intake sufficiently for the volumetric efficiencies of using a turbo, to kick in.

In the quiet words of the Virgin Mary............. come again?

Phil_Lives

Gazzmonkey wrote: »

Are you sure about that? I would think that driving at such a low rev would cause DMF issues among others things.

I don't see how driving at a constant speed/revs would do any damage to the dual mass flywheel.

corkgsxr

Phil_Lives wrote: »

The sweetspot for a diesel engine is usually anywhere above stall speed and not on the turbo.

Mm perfect for blocking dpfs. Whole point of the tread. Better to drive higher revs or just blast every once and a while

Phil_Lives wrote: »

I don't see how driving at a constant speed/revs would do any damage to the dual mass flywheel.

The question is whether the engine is labouring or not. And if its labouring the dmf could be shuddering to smooth everything out

Slattsy

How often to these diesels need a good blast (high revs / low gear) to clear the dpf?

We chatting once a week, once a month...

Phil_Lives

corkgsxr wrote: »

Mm perfect for blocking dpfs. Whole point of the tread. Better to drive higher revs or just blast every once and a while



The question is whether the engine is labouring or not. And if its labouring the dmf could be shuddering to smooth everything out

At a constant 80 kmph on a motorway in 6th gear over 20 or 30km there is no danger to either dmf or dpf and the engine will be barely ticking over with turbo providing boost but the engine hot enough to clear away the particulates.

corkgsxr

Phil_Lives wrote: »

At a constant 80 kmph on a motorway in 6th gear over 20 or 30km there is no danger to either dmf or dpf and the engine will be barely ticking over with turbo providing boost but the engine hot enough to clear away the particulates.

Likes of a 1.5/1.6 diesel produce very little heat when not producing alot of boost.

id say thats too low revs to clear the dpf. Just not enough heat. Maybe in summer but iv had 1.6 diesels and there a cold place to be In winter. Unless you horse it they take miles to produce heat from the cabin heater

Phil_Lives

Agreed that it takes time for an engine to heat up. My small capacity diesel takes a while to heat up but I'm doing 30km each way each day of which 15km is motorway and another 10km clear moving open road so the dpf is getting cleared. I don't speed much as it makes little difference to my arrival time even though the motorway is derestricted and could go as fast as I like.
I'm confident that my DPF is being cleared and cold temps won't be a problem for me this week with over 30 degrees forecast for most of the week.

jca

corkgsxr wrote: »

Likes of a 1.5/1.6 diesel produce very little heat when not producing alot of boost.

id say thats too low revs to clear the dpf. Just not enough heat. Maybe in summer but iv had 1.6 diesels and there a cold place to be In winter. Unless you horse it they take miles to produce heat from the cabin heater

One thing I don't miss having gone back to petrol is the lousy heater performance from the diesel and the sticking vanes in the turbo and the knocking flywheel and the juddering clutch...... Yep sure am glad to be rid of it. Petrol DSG I love it...:D

GTE

jca wrote: »

One thing I don't miss having gone back to petrol is the lousy heater performance from the diesel and the sticking vanes in the turbo and the knocking flywheel and the juddering clutch...... Yep sure am glad to be rid of it. Petrol DSG I love it...:D

Reads like your diesels issues are fierce model specific.

jimgoose

bbk wrote: »

Reads like your diesels issues are fierce model specific.

You need big, pig-ignorant Yank petrol engine. You do. You really do.

dr.fuzzenstein

jimgoose wrote: »

You need big, pig-ignorant Yank petrol engine. You do. You really do.

And here is the post that explains that running a 535i really is cheaper than a 520d using extremely dodgy logic and while you're at it, you might as well go for the M5.
So, if you want a 520d, buy an M5, by boards logic, its cheaper!;)

shedweller

Not much coking up going on here. Must be because of the pedal to the metal posting!
Keep er lit lads! Feck all science here eh? (Loling at the thought that actual science might happen here)
Feckin thread fail lads. Zero science.

jca

shedweller wrote: »

Not much coking up going on here. Must be because of the pedal to the metal posting!
Keep er lit lads! Feck all science here eh? (Loling at the thought that actual science might happen here)
Feckin thread fail lads. Zero science.

No one turned up at the Aldi car park,so you came home and got on boards instead.

jimgoose

dr.fuzzenstein wrote: »

And here is the post that explains that running a 535i really is cheaper than a 520d using extremely dodgy logic and while you're at it, you might as well go for the M5.
So, if you want a 520d, buy an M5, by boards logic, its cheaper!;)

You know it makes sense, Rodders! This time next year...

Jesus.

jca wrote: »

One thing I don't miss having gone back to petrol is the lousy heater performance from the diesel and the sticking vanes in the turbo and the knocking flywheel and the juddering clutch...... Yep sure am glad to be rid of it. Petrol DSG I love it...:D

I've always said I prefer diesels. However, it is true that when you're knocking about town and you're rarely on a Motorway, they can be a bit rough. Their natural habitat is eaytin' up the miles out on the open road.

Caliden

Jesus. wrote: »

I've always said I prefer diesels. However, it is true that when you're knocking about town and you're rarely on a Motorway, they can be a bit rough. Their natural habitat is eaytin' up the miles out on the open road.

Totally agree with this. Ended up trading cars with the missus as I'm normally going between 60 and 80km/h these days to work on b-roads whereas she's on the m50 for 40 minutes.

I found I was getting mid to low 30's (mpg) in the diesel (120d) and switched to a ford fiesta where I was getting high 30's to 40mpg.

Overall a nicer ride too as it's nippier off the line without ragging it.

corkgsxr

Went testing mine with torque ob2 and discovered van uses much less diesel in all conditions if its above 1500 revs.

And on open road between 100 and 110 kph is best

D'Agger

I've been wondering about this myself lately - recently purchased a pessat cc and it's prompting me to go into 6th gear when I'm at 90km - now, to me, the revs aren't high enough in 5th for me to be moving into 6th unless I'm at 110km - I can only assume it's better for fuel efficiency to change when the car suggests but I think doing it will force the engine to labor slightly and I don't want that

MuppetCheck

D'Agger wrote: »

I've been wondering about this myself lately - recently purchased a pessat cc and it's prompting me to go into 6th gear when I'm at 90km - now, to me, the revs aren't high enough in 5th for me to be moving into 6th unless I'm at 110km - I can only assume it's better for fuel efficiency to change when the car suggests but I think doing it will force the engine to labor slightly and I don't want that

I've a Leon FR and its asking me to do it under 80! You can feel the car under stress. I've also noticed that doing what it says makes the car feel a little a little chocked.

corkgsxr

Eg

if im in 4th at 1300 revs its nearly 10L per 100klms
In 4th at 1500 revs its down at 3.5-4 10L per 100klms

90-110 kph. The difference is only 0.5 L per 100 klms

but if you go to 120 its a good jump.


I think the difference is below 1500 its showing vacuum. Even at bang on 1500 its round 1psi. So having abit of boost seems a huge benefit

CJhaughey

Can you not feel where the engine is happiest?

corkgsxr

CJhaughey wrote: »

Can you not feel where the engine is happiest?

What I thought was best was lower in the revs.

it seems like your reving higher than is needed but is much better on fuel that way

CJhaughey

corkgsxr wrote: »

What I thought was best was lower in the revs.

it seems like your reving higher than is needed but is much better on fuel that way

Revs won't kill many engines though, you should know that from riding bikes!
Lugging is much, much worse.

corkgsxr

CJhaughey wrote: »

Revs won't kill many engines though, you should know that from riding bikes!
Lugging is much, much worse.

Ya but the bike engine feels different. Its not smooth untill 7-8k I dont make proper power till 10k. It keeps making power till 16k

even and petrol cars iv had over 4.5/5k it feels like your dogging it

Over 2 in the van feels like it should be wasteful. Just a change of driving habit needed.

peasant

One word of wisdom ...it's called peak torque

Look up the specs of your engine and find out at or between which revs it has its highest torque ...that's where it's at its most efficient.

On some engines it's at a rev spot, on others in a rev band ...meaning on some engines you can only ever be in one gear at any given speed to have peak torque ...with others you have the choice of two or even three gears and still be in the peak torque range.

Max power will be nearer the end of the rev band ...so if you want real oomph you'll have to rev it ...but there goes your fuel efficiency.


Engines that have to work for a living (trucks) usually have the peak torque sweet spot marked on the tachometer

jca

peasant wrote: »

One word of wisdom ...it's called peak torque

Look up the specs of your engine and find out at or between which revs it has its highest torque ...that's where it's at its most efficient.

On some engines it's at a rev spot, on others in a rev band ...meaning on some engines you can only ever be in one gear at any given speed to have peak torque ...with others you have the choice of two or even three gears and still be in the peak torque range.

Max power will be nearer the end of the rev band ...so if you want real oomph you'll have to rev it ...but there goes your fuel efficiency.


Engines that have to work for a living (trucks) usually have the peak torque sweet spot marked on the tachometer

The most sensible post in this thread.

CiniO

D'Agger wrote: »

I've been wondering about this myself lately - recently purchased a pessat cc and it's prompting me to go into 6th gear when I'm at 90km - now, to me, the revs aren't high enough in 5th for me to be moving into 6th unless I'm at 110km - I can only assume it's better for fuel efficiency to change when the car suggests but I think doing it will force the engine to labor slightly and I don't want that

For manufacturer it's better if you change to 6th at 90km/h.
Slightly better fuel economy gives them lower CO2 rating, so their car sells better.
DMF, clutch and other cruicial engine parts wrecked after few years, also give better sales in parts department of this manufacturer.
It's a win-win for them.

gctest50

Very vaguely related - the ball of pain and expense carbon in the oil causes on some engines




http://www.assuredperformance.ie/assets/images/Presentation-1.6-hdi-carbo-issues.pdf

jca

gctest50 wrote: »

Very vaguely related - the ball of pain and expense carbon in the oil causes on some engines




http://www.assuredperformance.ie/assets/images/Presentation-1.6-hdi-carbo-issues.pdf

There's no way that engine was serviced properly. The engine is effectively scrap. Every 30,000 miles for the first two services is ridiculous especially a small turbo diesel.

jimgoose

jca wrote: »

There's no way that engine was serviced properly. The engine is effectively scrap. Every 30,000 miles for the first two services is ridiculous especially a small turbo diesel.

What oil is recommended by the manufacturer for those things?