View Full Version : Driving efficiently... how do you do it?
Following a conversation in the office, I came to the .Org to post the below, but given what I read when I logged in, I thought it more appropriate to wait a while. Anyway...
Conversation in the office about fuel economy, and a couple of the lads in the office now hate my guts. As I recently posted, the bike has been sold, and as some know, I've replaced it with a turbodiesel car. The reason the lads don't like me is that one of them is getting 18mpg from his car, whereas I get 51mpg without even thinking about it.
As I'm pretty new to the turbodiesel world, I thought I'd come & see if anyone had any tips for driving more economically (so I can wind them up more).
I'm aware of coasting, keeping revs as low as practically possible whilst accelerating, leaving the brakes alone, using 5th where possible etc. I'm also aware that on over-run, the engine uses practically zero fuel (useful when you live in an area with a large number of hills).
But is the rev-counter the same as most, where generally peak torque happens at the top/centre of the rev-counter? Also, driving "on boost" from the turbo is likely to be more efficient but that's probably going to be a fine balancing act (as fuel usage could go up with more forced induction from the turbo).
Anyone got any tips for driving a TD engine economically?
TIA. :)
Jackie_Black
04-10-09, 12:57 AM
Basically the turbo uses loads of fuel (but diesels are still very frugal). Built up areas kill MPG but you'll always get over 40 and if you are good you'll see 70 mpg or thereabouts on a motorway, my old cavalier TDI used to do about 67 on a run so new ones must be ace. Oh and rev relatively low and change up early to save the most gas 2k-3k change etc. Diesels make all their torque at about 2500rpm. I loved the money saving but couldnt stand the noise and engine characterisitics thats why i got a 528 auto petrol beamer. Smooth as silk and 24mpg round the doors with 30 on a run.
Oh and rev relatively low and change up early to save the most gas 2k-3k change etc. Diesels make all their torque at about 2500rpm.
I've been changing under 2k and running on average around about 2200RPM today (since the conversation). It's a 1.9l engine, so plenty of pull at low revs.
My commute consists of mainly NSL area's, so built up areas aren't going to be a problem. It's not a new engine, but there's sod all wrong with it. 10 year old engine, and it'll happily idle iself all the way up a 1in7 hill (no feet on any pedal)!!
MiniMatt
04-10-09, 01:04 AM
Bit numb with the other news, never met him, can't say as I agreed with much he wrote but damn if we ain't lost a character with a good heart.
Anyway, I guess this thread is serving the keeping mind off it principle.
So 50 is about what I've gotten from just about every diesel engine I've owned from 1.9td pug/citroen units to 2.5 VAG plants. And yeah, it's not something you really have to think about, I guess it's something about the power delivery that just makes you drive in a more relaxed way and shift up early rather than race to peak power in a petrol. That said, a lot of the newer engines feel suprisingly petrol like.
Personally when in "just get me home it's raining and I'm miserable" mode I find myself shifting up very very early. Peak torque is going to be in the handbook or a website someplace but I've never driven a diesel car that isn't plenty happy shifting up at sub 2k rpm. Hell, they'll change all the way up to 5th/6th on tickover even if you're quite brutal with dumping the clutch. I'd think that driving off boost where possible is likely to be more economic - boost gets more air in in-order to allow more fuel to be pumped in, at off boost levels I'd imagine the mixture ratio is still the same it's just that the airbox is able to supply all the air that's needed.
Jackie_Black
04-10-09, 01:05 AM
On the motor I had I always found if I kept it between 2-3k it was happiest and most frugal. Of course they all feel different so if you can rev less and get away with it then thats good. The cavalier needed to be on boost to actually move and was terribly flat without it so for safety i was always between 2 and 3. If the revs dropped too low I always felt I was labouring the engine too much and the vibration was awful.
Steve_God
04-10-09, 02:08 AM
Most of those above you've got are pretty much it, one other essential one:
If you're coming to an uphill, and you're currently on a downhill, speed up slowly as you are coming down the hill (using minimal additional fuel as gravity will be doing most of the work) and allow that extra speed to carry you up the hill that little bit more, therefore using less fuel on the uphill climb! :)
yorkie_chris
04-10-09, 01:06 PM
Er, A*, it's a diesel, they don't have a throttle!
And valves open for longer? Woot? On a petrol engine with carbs, then loads of boot at low revs means the gas speed in the intake tract drops (mass flow still low, pressure rises up to 0 vacuum i.e 1 atm, gas flow is slow), meaning you don't get proper atomisation and you get a slow, innefficient burn.
Also with a carb'd petrol engine, if you have an accelerator pump, then that boot on the throttle will squirt a bit shot of petrol straight in.
I've been changing under 2k and running on average around about 2200RPM today (since the conversation). It's a 1.9l engine, so plenty of pull at low revs.
Don't be fooled into thinking that you have to change 'as early as possible'. A labouring engine will use more fuel than a slightly higher revving one.
The best thing you can do to save fuel is to plan ahead. Early observation at junctions, adjusting your speed early so you can time your arrival at junctions/lights etc and avoid stopping if possible. Make sure your tyre pressures are correct, empty your boot, switch off the aircon etc etc etc.
There are so many things you can do but planning is the most effective (shown to save between 10 and 25%) when I've conducted assessment drives.
metalangel
04-10-09, 02:29 PM
Don't know what car you've got, I've got a 1.9TDI Octavia and get good mileage out of it, if you accelerate gently and coast to slow down instead of braking later then you should do well.
Something I've been told, though, is to make sure you do give the engine something of a workout. My engine reportedly gets gunge in the turbo (VVT) unless you give it some welly on a regular basis. This amounts to shifting at 3,000rpm where possible (redline is 5k) which generally means I get up to 30mph in 2nd then just shift into third and carry on cruising...
Light throttle control is key to economy.
I.e when pulling away, using mild throttle, but allowing revs to rise it far better than giving it a little beans, but changing at low rpl.
Too low an rpm labours the engine, uses more fuel, cabbies do this requently. Damages the engine and uses more juice so never understand it.
Also, when say your on the motorway, you slow down to 60mph, leave it in 5th/6th, just slightly apply pressure on the throttle and it'll sllloooooowly increase speed, but watch that mpg reader go to over 70mpg ;)
Using the work 1.9 DTi, even though its an estate, i've had 725 miles from a full tank, and regulary see 45-50mpg at 90mph odd... I love it. I hate Dervs, but for road driving you really cannot beat them. Especially as the VXR is lucky to get 300 miles per tank and averages 27mpg on easy driving. The works beast does on average 52mpg. Epic economy.
Mind you, i've had the VXR down to 6.9mpg as well, but no matter how hard i try in the derv, it won't go below 16mpg, even sat on the redline in 2nd :lol:
Alpinestarhero
04-10-09, 02:43 PM
6.9 mpg!!!!!!!
:cool:
my brother was looking at RX-8's. he was appalled to disocver the mpg of those on a good day!
Thats at theoretical top speeds of course ;)
Add another 100hp to the engine and watch that drop into the 4's ;) And thats just from a 4 pot 1998cc turbocharged 237bhp engine :lol:
Imagine what tuned 4 pot Japanese motors are running (I.E FQ400) as standard... Or even a big lump 6.1 V8 LS7 engine as in the VXR8 and Corvette :lol: Then bung a supercharger on that.
1mpg at full chat :D
Alpinestarhero
04-10-09, 02:54 PM
Thats at theoretical top speeds of course ;)
Add another 100hp to the engine and watch that drop into the 4's ;) And thats just from a 4 pot 1998cc turbocharged 237bhp engine :lol:
Imagine what tuned 4 pot Japanese motors are running (I.E FQ400) as standard... Or even a big lump 6.1 V8 LS7 engine as in the VXR8 and Corvette :lol: Then bung a supercharger on that.
1mpg at full chat :D
and thats the American Way :salut:
Mechanically - keep the car well serviced in terms of engine oil and air filter.
As others have said, avoid labouring the engine.
Find out what peak torque is in terms of rpm (it won't be at the top of the dial) and I'd suggest aiming for that when cruising and climbing hills.
Obviously don't go any faster than you have to.
Anticipation.
Avoid needless gearchanges. No need to skip gears but minimising changes does improve economy.
Take any unnecessary weight out of the car.
Don't use aircon, nor lights more than you have to.
Keep records of your fuel consumption and try different things to see what it likes or doesn't like.
There's a limit as to how efficient an engine can be so keep in mind that it's a case of diminishing returns, you can keep putting more effort into your driving but you can only get so much mpg. On a 10 y/o TD 50 mpg sounds reasonable (missed what the car is?).
Alpinestarhero
04-10-09, 03:54 PM
With regards to aircon;
a while back, i cant remember where i read it, but apparently in general, below roughly 50 mph its more economical to have windows open to cool the car. Above roughly 50 mph the aircon is more effiecient
(missed what the car is?).
some kinda chavvy pug :lol:
na, its a peugeot something or another 1.9
With regards to aircon;
a while back, i cant remember where i read it, but apparently in general, below roughly 50 mph its more economical to have windows open to cool the car. Above roughly 50 mph the aircon is more effiecient
I believe it's closer to 30 mph. However, switched off and windows shut is the most efficient (although damned uncomfortable at the wrong time of year).
Alpinestarhero
04-10-09, 04:09 PM
I believe it's closer to 30 mph. However, switched off and windows shut is the most efficient (although damned uncomfortable at the wrong time of year).
yea, something like that. Obviously, its situation dependant :lol: i suppose though, if you keep the windows shut and the air-con off when its a hot sumemrs ay, you'll decide its too hot to drive anyway and therefore not drive
no fuel used at all!
-Ralph-
04-10-09, 09:20 PM
Drive a car with a fuel computer that displays instant consumption. You'll notice that there is a direct correlation between the position of your right foot and the reading on the gauge and getting great fuel consumption can be that easy. When your foot is off the pedal and the car is rolling it reads 999 mpg, the more time you can have the gauge reading this the better.
When you start to climb a hill, let the car slow down by 5 mph, rather than pushing the accelerator pedal further to maintain speed.
Use momentum when you have it and let the car run on with your foot off the pedal or just touching it as much as possible.
Use the brakes as little as possible, all you're doing is taking energy from the fuel you have just burnt to get up to that speed, and converting it into heat, so come off the pedal early when approaching junctions and let the car slow down gradually
The last technique also gives you loads of time to plan ahead, or for those red lights to change to green, if you can plan ahead so don't have to stop at the junction, you don't have to waste loads of energy putting heat into the brakes, at then don't have to burn loads of fuel to get back up to speed again.
Anyone got any tips for driving a TD engine economically?
TIA. :)
It's simple, get yourself one of those fully expensed company cars like mine and never worry about fuel consumption again :D
Imagine what tuned 4 pot Japanese motors are running (I.E FQ400) as standard... :D
Cruising at a steady 85 - 90mph on German autobahns was enough to drop both my Evo V's fuel consumption down to 20ish mpg. Not exactly economical cars to drive to Austria in, so I'm glad the buyer was footing the fuel bills.
barwel1992
04-10-09, 11:32 PM
hmm my dad drives his 1.9 twin tubo diesel sarb 9.3 aero and gets about 60mpg and thats driving normal so thats bloody good
i did how ever manage to empty the MT03 tank in 60 miles but got 120miles when on the M way doing 10mph more than i was when i was giving it some so i think its all down to how smooth you are
The first rule, as others have said, is minimise the energy demand, then worry about how to get the energy second. Slow down, avoid braking etc etc.
This graph is typical of that size of TD engine for car use. The left axis is BMEP, which is simply torque per litre (or "load") expressed in technical units. Engine speed is across the bottom. The 3 red lines correspond (more or less) to constant power lines, 20/25/30kW.
The values shown on the plot are brake specific fuel consumption in gms/kW.hr , or in other words how much fuel is burnt to give the same amount of energy output.
As you can see, if you need a certain power (say 25kW), you get lower fuel usage if you use higher load and lower engine speed. This guideline applies more or less at all power demands in a typical TD engine, it's the way they are designed. It doesn't apply in gasoline engines in quite the same way, the BSFC maps look a little different.
Note that once you go above a certain power demand it becomes impossible to run at the lowest BSFC levels, in this engine it is around 27kW, others will differ, so keep the power demands down.
Also note that this map shows where the engine is most efficient, but that still doesn't over-ride rule #1 to minimise the energy demanded in the first place. Efficiency isn't the same as economy.
Any modern TD engine will run happily at low speeds/high loads without harm. It is true however that occasional regular use at higher speeds is desirable for issues such as valve rotation etc.
http://pics.tdiclub.com/members/tdimeister/engine_map.jpg
PS - some diesels do have intake throttles in order to make EGR systems work and to increase exhaust temps for particulate trap regeneration etc.
-Ralph-
05-10-09, 08:38 AM
Lots of informed and very scientific stuff...
I know who I'm coming to see when I've got a car problem!
It's simple, get yourself one of those fully expensed company cars like mine and never worry about fuel consumption again :D
+1, I bought a petrol car 'cos at the time diesel was 12p a litre more expensive, and the petrol car was £3k cheaper. I still would have saved £300 a year in fuel, but it would have taken 10 years and 200k miles to get the extra £3k back. Then I changed job a got a fuel card, so now I've saved still 3 grand buying the car, can put my car allowance in my pocket, and don't give a monkeys how much petrol it uses. Result!
Any modern TD engine will run happily at low speeds/high loads without harm.
Does that always hold true?
I'm happy to accept that down to say the idle speed, but it feels 'wrong' if you're revs are below that and you're loading the engine?
Great post by the way (still trying to understand it all)
-Ralph-
05-10-09, 09:43 AM
Does that always hold true?
I'm happy to accept that down to say the idle speed, but it feels 'wrong' if you're revs are below that and you're loading the engine?
Can't do that in any TD that I've owned without using the brakes to slow the engine down to below idle speed, there is always enough torque to accelerate the car until the engine is at or above idle speed. My last was a Ford TDCi and if you turned into a corner in second gear and your speed dropped low enough to need first gear, the car would actually start pulling you round the corner. In sixth gear it would run along on it's own at idle speed, without your foot on the accelerator pedal, makes motorway driving in traffic queues dead easy, just select the right gear for the speed of the traffic and let the car run.
So at 20 mph in 6th gear up a hill at ~500 rpm and you think it will just pull?
I have a Mazda 6 2.0TD (143bhp engine) and always get 46-48mpg average from a full tank of fuel on long runs, even when 4-up and loaded.
The car is really high geared (well I think so!) - about 30mph/1000rpm in 5th and 35mph/1000rpm in 6th. So 70mph on motorway is 2000rpm - exactly where peak torque is produced. Nice!
One tip for economy...... use the cruise control if your car has it. The key to economy is maintaining momentum and the cruise control is better than I am at doing that :thumbsup:
So at 20 mph in 6th gear up a hill at ~500 rpm and you think it will just pull?
Nope!!
Apart from noise/vibrations and shoite fuel consuption, engine bearing damage is likely at that sort of load too :lol:
See i disagree with cruise control. It applies throttle more when coming up a hill.
When really, your speed should increase prior to the hill, then lift off and the car will slow going up the hill until your at the top.
I hardly ever use CC as i find it to use a lot more juice in the company cars.
Nope!!
Apart from noise/vibrations and shoite fuel consuption, engine bearing damage is likely at that sort of load too :lol:
What you say is my understanding too - but it's been said above that driving at too-low revs doesn't cause a problem. So I'm asking for clarification.
Alpinestarhero
05-10-09, 01:16 PM
i saw a graph scrollig up the page
i couldnt go any further cos i went a bit giddy with joy
:lol:
-Ralph-
05-10-09, 02:46 PM
So at 20 mph in 6th gear up a hill at ~500 rpm and you think it will just pull?
No, but at idle speed up an incline it will pull, so what kind of numpty do you have to be to get your car into that situation anyway?
Unless you select the wrong gear, use the brakes to slow the motor down below idle speed, or try to get up a 1 in 4 without giving it any more gas, that just ain't gonna happen.
A small petrol engine will drop to 500 rpm in traffic, simply because it doesn't have enough torque at idle speed to pull the car along and the driver hasn't bothered to changed down, a diesel engine won't unless the driver has done something particularly stupid to make that happen, in which case they deserve to knacker the motor.
Apart from noise/vibrations
Which should be giving you a pretty good clue that you've done something stupid, your engine ain't happy, and your gonna cause some damage.
Alpinestarhero
05-10-09, 03:03 PM
I wondered what was really bad about letting the engine labour. I knew it was wrong, and I figured the vibrations were the bad point. Didnt make the connection with bearing surfaces though
another thing to get me paranoid, wooooo...
Just keep your engine spinning easily, don't let it labour or rev to high. 500rpm below and 1000rpm above peak torque seems about right for my particular car.
Control over the loud-pedal is key. Drive like there's a raw egg between the sole of your boot and the pedal and try not to crack it.
Didnt make the connection with bearing surfaces though
I forget the correct term for it, but my understanding is that modern bearings need a reasonable oil pressure to be properly lubed. Needs to be above idle speed (and preferable a few hundred rpm above that) I'd suggest.
It's okay saving fuel, but the amount you'll save will be peanuts compared to the cost of an engine rebuild :thumbsup:
Just drive so it feels and sounds right and quit worrying :thumbsup:
Alpinestarhero
05-10-09, 04:33 PM
I forget the correct term for it, but my understanding is that modern bearings need a reasonable oil pressure to be properly lubed. Needs to be above idle speed (and preferable a few hundred rpm above that) I'd suggest.
It's okay saving fuel, but the amount you'll save will be peanuts compared to the cost of an engine rebuild :thumbsup:
Just drive so it feels and sounds right and quit worrying :thumbsup:
Ah, see I learnt about the pressure from a youtube video posted in my Epic Win thread all about engine oils ages ago (it won the approval of SuperMegaMod Spiderman!). I tend to keep my SV engine above 4000 rpm if I can...bit hard sometimes in heavy traffic though
also, i agree on the money saving bit...I'd rather waste a bit of fuel and not have the hassle of an engine rebuild!
Sorry for any misunderstanding, I tend to use shortcuts in details. When I say low speeds I didn't mean ridiculously low speed, just "lowish" in the overall speed range, so if the BSFC map minimum is around 1750rpm then once you're going below maybe 1500rpm you might want to start thinking about dropping a gear.
Diesels will pull the car along at low idle somewhat differently to a petrol due to the way the idle speed control or governors work.
Bearing oil film thicknesses aren't much of an issue in any self-respecting modern engine. This is a bit of an historic thing carried over from old engines which had pretty poor oil pumps, often not very good bearing geometry and not very sophisticated materials to call on ("white metal" etc.), not to mention indifferent oil and primitive filters (if any!).
Again I wouldn't recommend habitually running at full load and 500rpm, but anything 1500rpm up will be no problem.
-Ralph-
05-10-09, 07:00 PM
Diesels will pull the car along at low idle somewhat differently to a petrol due to the way the idle speed control or governors work
I noticed the fly by wire system on the TDCi actually added gas for you from idle, you could set off from traffic lights and as the clutch started to bite the car would give a little blip up to about 1250 revs without you even touching the accelerator pedal. You could if you were really bored rev the engine in stationary traffic just by lifting the clutch up and down and fooling the car into thinking you were about to move off. :p Once you were on the move it stopped interfering. Anyone who learns to drive in one of these things, must have absolutely no clutch control skills, the computers are almost driving the car for you.
Many petrol cars also have a "fast idle" function linked to the speedo signal, as soon as it exceeds a set value (very low speed) the idle speed is upped. It makes the driveability much smoother when going on/off the throttle, and helps manual gearchanges etc. Some cars have quite complex idle speed control when electrical loads are applied to prevent stalling, improve battery charging and aircon performance when in stop/start traffic.
On my Yaris it can be a bit annoying because it will go in and out of fast idle when trickling along at walking pace in traffic.
-Ralph-
05-10-09, 09:45 PM
On my Yaris it can be a bit annoying because it will go in and out of fast idle when trickling along at walking pace in traffic.
The majority of people are not 'tuned in' enough to their cars to notice this kind of stuff, they just drive on oblivious (not including 'the majority' of people on this site because bikers in general are fairly well tuned in).
-Ralph-
05-10-09, 09:50 PM
See i disagree with cruise control. It applies throttle more when coming up a hill.
When really, your speed should increase prior to the hill, then lift off and the car will slow going up the hill until your at the top.
I hardly ever use CC as i find it to use a lot more juice in the company cars.
I was thinking that too, but before posting I decided that it depends upon the terrain. If it's fairly flat cruise control will do better than you will, but on the likes of the M6 through the Lake District, or M74 through Dumfries and Galloway, manual control allowing the car to slow down when climbing and speed up when downhill, whilst keeping your foot still, you get better MPG than the CC.
Well, thanks to some in this thread, I've barely changed my driving, and reckon it's gone up by around another 10mpg - putting me on a rough guess 60mph (without working it out - I'm waiting to put more diesel in for that).
Embee, in particular, thanks for the graph. :) I was actually running slightly lower RPM than 1500 most of the time. Now I'm generally 1500 minimum (it'll do 50 happily at 1500 on flat ground in 5th). Also if acceleration is needed I'll drop a gear to suse slightly higher revs briefly.
More than triple the fuel efficiency of a colleagues Seat Leon, with a slightly bigger engine. He's not happy. :)
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