blakep82
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| posted on 5/11/12 at 01:03 PM |
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Turbos and fuel consumption
here's a thought, similar to that Cadillac thing a few years ago that had a V8, but when cruising only ran on 4 cylinders
I WILL one day get a skyline (lol) one day...
was thinking though, Turbos compress air, feeding more into the engine, needing more fuel to be put in.
could it be possible to get one of these boost controllers, or a switch to open the dump valve to reduce the boost to zero (or nearly) when cruising
on a motorway, to basically run the engine as a 6 cylinder N/A engine, and therefore reduce fuel consumption a little? fuelling must depend on the
amount of air taken in, so less compression, less fuel?
just wondering, can't get my head round it.
[Edited on 5/11/12 by blakep82]
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
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MikeRJ
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| posted on 5/11/12 at 01:26 PM |
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Under light cruise conditions the boost is already pretty much zero on something with a big engine.
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balidey
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| posted on 5/11/12 at 01:28 PM |
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I won't comment on the 'how', but more importantly..... 'why'?
So you want to buy a skyline but make it run under powered? Why? To save fuel? IMHO you don't buy that car and then worry about fuel
comsumption.
And realistically, I would think the difference in fuel useage would not be as great as you think.
If you want to save fuel then buy the Skyline, leave it as it is.
Buy a 2nd hand diesel mondeo and use that when you don't want to use the skyline.
Dutch bears have terrible skin due to their clogged paws
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flibble
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| posted on 5/11/12 at 01:48 PM |
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I'll take an (uneducated) guess and say that it would make no difference, cruising at 100mph in car X must take a certain defineable amount of
BHP, wether the throttle is barely open as your turbo is helping you, or as the turbo has been 'turned off' - a slightly wider open
throttle to compensate.
As above also, staying at 70mph with a decent sized engine the turbo will barely be spooling.
That said, I've wired in a button for my kits saab engine that will pretty much do as you ask, mainly installed incase friends wanted to drive
it and I don't want themn to have full bhp available 
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roadrunner
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| posted on 5/11/12 at 03:15 PM |
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quote:
Originally posted by balidey
I won't comment on the 'how', but more importantly..... 'why'?
So you want to buy a skyline but make it run under powered? Why? To save fuel? IMHO you don't buy that car and then worry about fuel
comsumption.
And realistically, I would think the difference in fuel useage would not be as great as you think.
If you want to save fuel then buy the Skyline, leave it as it is.
Buy a 2nd hand diesel mondeo and use that when you don't want to use the skyline.
But why have two cars when one, if possible can do both types of driving.
It makes sense to me .
I have a decent sized turbo petrol, but I dont hoon it around all the time because it costs to much in fuel. But it is fun when you can give it the
beans when the occasion arrives.
Brad.
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britishtrident
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| posted on 5/11/12 at 03:42 PM |
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A turbo on a petrol engine has virtually zero effect on fuel consumption at cruising speed. Because it recovers waste energy a smaller engined
turbo car will use less fuel than a larger capacity version with an engine of equivalent power.
On a diesel engine a turbo will improve fuel consumption under most driving conditions.
Unless a particular engine is really type is really inefficient or very efficient fuel consumption depends more on weight and aero
drag than anything else.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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hughpinder
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| posted on 5/11/12 at 04:05 PM |
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It depends on the engine tuning. A turbocharged engine has better thermodynamic efficiency, so it should use less fuel for a given power output
compared to a non turbo. To actually get this good fuel consumptoion when cruising, the gear ratios/engine power curve on light throttle should be
tuned to just overcome air resistance. Most performance cars are not tuned this way, actually producing somewhat more power so you can have a quick
take off.
Regards
Hugh
ETA - I'm not certain on this bit - I believe the EGR valve allows you to circulate some gasses back to the engine, and since these have lower
oxygen content thn the fresh air, less fuel is used to get the stoichiometric ratio and so consuption is improved.
[Edited on 5/11/12 by hughpinder]
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Bare
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| posted on 5/11/12 at 04:35 PM |
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quote:
Originally posted by britishtrident
A turbo on a petrol engine has virtually zero effect on fuel consumption at cruising speed. Because it recovers waste energy a smaller engined
turbo car will use less fuel than a larger capacity version with an engine of equivalent power.
On a diesel engine a turbo will improve fuel consumption under most driving conditions.
Unless a particular engine is really type is really inefficient or very efficient fuel consumption depends more on weight and aero
drag than anything else.
Spoken like someone who has not lived with a turbo worth owning.
After 30 unbroken years of Turbo Saabs, I have a fair experience with turbo fuel consumptions :-)
Those Turbo cars INHALE gasoline.. period . Can't afford it?? Don't buy one.. It's That simple Kids.
Diesel efforts are smelly, polluting pedestrian 'shopping' cars in my part of the world, not worth a thought.
Yes there is a semi equivalent fuel consumption to a similar hp AND torque producing larger NA engine.
But that's usually a significantly larger unit, due to the large torque products of a turbo'd engine..
Turbo ECU's also use fuel enrichment as charge cooling to some degree.
My Accord V6 (3.5l) uses only slightly less fuel than My Saab 9000 both have similar hp ~275.
And the Honda has much less Torque . Saab boost is currently at 20 psi .
But the Saab 's power is erm thrilling in a manner the V6 can't match...consequently 'boost' gets used.. often.
Which is THE crux of the matter:
Fuel consumption will be comensurate with the pleasure it gives and we are all greedy pigs in that :-)
[Edited on 5/11/12 by Bare]
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sdh2903
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| posted on 5/11/12 at 04:37 PM |
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as above, my focus st is barely boosting at sensible cruising speeds. Good job really as it already has a drink problem.
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britishtrident
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| posted on 5/11/12 at 07:14 PM |
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It is pretty well established that Honda horses are smaller than those quoted by European manufacturers so take any Honda power claims with a
pinch of salt. It was also been demonstrated by the turbo era in Formula 1 that if you dribble enough fuel through a turbo charged petrol
engine you can get almost unlimited power. A turbo is essentially a waste heat recovery unit, in a rabid balls to the wall turbo engine the
mapping deliberately creates extra waste heat to be recovered by the turbo which in efficiency terms the equivalent of fitting an afterburner to
a turbo jet.
If an manufacturer wants to build a 160 hp car it can be powered by a moderately mapped 2.5 litre V6 or highly tuned 1.8 4 cylinder
or a low pressure turbo 1.8 4 cylinder -- if driven in identical fashion the low pressure turbo will return about 15% better
consumption than the v6.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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MikeRJ
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| posted on 5/11/12 at 08:22 PM |
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quote:
Originally posted by hughpinder
It depends on the engine tuning. A turbocharged engine has better thermodynamic efficiency,
Not when the turbo is being used, in the case of performance engines anyway. If you do a bit of research you'll find the Brake Specific Fuel
Consumption (BSFC) of a petrol turbo is most always higher than that of a normally aspirated engine producing the same power, because there are extra
pumping losses, lower CR and fuel enrichment is required to cool pistons etc.
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britishtrident
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| posted on 5/11/12 at 10:52 PM |
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Yes the twist in the tale is the improved fuel efficiency of a 160ps turbo 4 compared to 160ps V6 occurs when it is at low loads and not
boosting.
Let's take a medium sized car assume it takes 40ps to maintain a constant speed of 70 mph
The 160ps 1.8 turbo engine will either be off boost or working at very low pressure the fuel comsumption will be the same as a 1.8 120ps
Att the same speed the 160ps 2.5 v6 will allso be working with a low throttle opening but will have the mechanical friction, windage and pumping
losses of an additional two cylinders.
At full throttle the v6 will be slightly more efficient than the turbo 4 but not by much.
These are manufacturers extra figures not real world mpg for the Rover 75
Extra Urban MPG
120ps 1.8 ltre naturally aspirated engine 46.3 mpg
160ps 1.8 ltre turbo engine 46.3 mpg
140ps 2 ltre V6 engine 40.1 mpg
160ps 2.5 ltre V6 engine 40.4 mpg
131ps 2 ltre Turbo Diesel 60.7 mpg
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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blakep82
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| posted on 6/11/12 at 12:31 AM |
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thanks for the replies everyone
so it seems turbos do slightly improve economy then? under gentle conditions though.
the reason i asked, i was watching something last night, it said turbo's improve enconomy, which i thought was unlikely, but then, as said, i
hadn't thought about needing less throttle to accelerate the same as a non turbo.
all quite confusing, but if the turbo won't really be spinning much at motorway speeds for example, then its all good. but its still to do with
engine speed rather than road speed though.
what Brad (roadrunner) said was exactly what i was thinking though.
as for those saying stuff like you don't buy a car like that to worry about fuel, can't afford one, don't buy it etc, well, yes,
correct, and thats why i haven't bought one yet.
its a car i've always wanted, they're cheap enough now (£4-5 will get you a reasonable fairly standard R33 GTS-T) and i've got a
fair way to go to sort it, but thats basically my goal for the next 12 months, to be able to afford one, and to get it.
i've had a crap few years, turned 30 last week, and have now suddenly come over all positive and optimistic, which is something i've
actually never felt in my life. er, newly single (i suspect we may possibly have both been holding each other back a bit) so now's the time to
concentrate on myself, get the jobs i've always wanted, try and get somewhere in life and yes, even a diesel car/van, and that's sort of
my own personal marker, however....
... you don't need to know all that. point is, i was watching something about turbos, and they said about the improvement in economy, which i
didn't expect. and lets be honest, there's not one of us on here that would pass up saving a few quid if we could! hence the question
it won't be any time soon. 12 months at the very soonest, more likely a lot longer though
[Edited on 6/11/12 by blakep82]
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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britishtrident
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| posted on 6/11/12 at 08:03 AM |
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The ultimate example of the Jekyll & Hyde fuel consumption of a high pressure turbocharged petrol engine is the FIAT 500 Twin Air,
driven very gently it returns truly remarkable mpg but floor the loud pedal and the fuel consumption of the tiny engine would embarrass a Jag.
Understanding the effect of throttle on petrol engine efficiency is the difficult bit to get ones head around, and also the fuel efficiency of
the vehicle as a whole is not quite the same as fuel efficiency of the engine.
Strange as it may seem light throttle screws up the efficiency of a petrol engine, the throttle literally strangles the engine's volumetric
efficiency to control the power output. In contrast a diesel engine which has no throttle but regulates the power by controlling the fuel is very
efficient at low power and idle.
A diesel engine will sit idling all day on the proverbial thimbleful of fuel (ok slight exaggeration :-) ) while the equivalent petrol
engine will need more like a bucketful, Even after factoring in the higher specific energy content of DERV compared to pump petrol the difference
in fuel efficiency at very light loads and low RPM is huge.
Going back to petrol engines this brings us back to the low pressure turbo v larger engine and the Cadillac (and F1 Mercedes engine)
trick of cutting out cylinders to save fuel. To maintain a steady 70mph cruise the engine needs to supply 40ps, To supply enough air to develop
this power level the 1.8 litre engine will be working at a wider throttle opening than the 2.5.
But because the cylinders are smaller on the 1.8 the wider throttle opening will fill the cylinders more so the volumetric efficiency (and
hence cycle efficiency) are higher and the smaller engine will require less fuel.
[Edited on 6/11/12 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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beaver34
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| posted on 6/11/12 at 05:31 PM |
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quote:
Originally posted by blakep82
thanks for the replies everyone
so it seems turbos do slightly improve economy then? under gentle conditions though.
the reason i asked, i was watching something last night, it said turbo's improve enconomy, which i thought was unlikely, but then, as said, i
hadn't thought about needing less throttle to accelerate the same as a non turbo.
all quite confusing, but if the turbo won't really be spinning much at motorway speeds for example, then its all good. but its still to do with
engine speed rather than road speed though.
what Brad (roadrunner) said was exactly what i was thinking though.
as for those saying stuff like you don't buy a car like that to worry about fuel, can't afford one, don't buy it etc, well, yes,
correct, and thats why i haven't bought one yet.
its a car i've always wanted, they're cheap enough now (£4-5 will get you a reasonable fairly standard R33 GTS-T) and i've got a
fair way to go to sort it, but thats basically my goal for the next 12 months, to be able to afford one, and to get it.
i've had a crap few years, turned 30 last week, and have now suddenly come over all positive and optimistic, which is something i've
actually never felt in my life. er, newly single (i suspect we may possibly have both been holding each other back a bit) so now's the time to
concentrate on myself, get the jobs i've always wanted, try and get somewhere in life and yes, even a diesel car/van, and that's sort of
my own personal marker, however....
... you don't need to know all that. point is, i was watching something about turbos, and they said about the improvement in economy, which i
didn't expect. and lets be honest, there's not one of us on here that would pass up saving a few quid if we could! hence the question
it won't be any time soon. 12 months at the very soonest, more likely a lot longer though
[Edited on 6/11/12 by blakep82]
you may get one for 4-5k but i would be worried about the running costs of them in terms of parts and matinance you need to remember you running
something that had supercar slaying performace but now costs 4k like a ford focus, but will still cost supercar prices to run
thats what i found when reading up on them
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gtr_garner
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| posted on 8/11/12 at 09:26 AM |
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My r33 gtr did 15-20mpg on a decent run.
It equated to 170 miles of mixed driver per fill up and this never changed unless i was on a track.
So if you want one of these cars and are bothered about the fuel dont get one!
I ran one as a daily for 18 months and it cost me a small fortune - about £1k a month in fuel, oil changes and either necessary modifications (read
fixes) or me finding spare money to improve the car.
Also factor in costs of buying a ready modified car - 6 days after buying mine the engine blew, down to shoddy rebuild before, luckily the previous
owner coughed up 50% of the bill which he didnt have to but still cost me £4k. Then the clutch went, another £1200. Basically the car spent more
time off the road than i could believe but when it was on it i never had a smile off my face.
I would have one again though without a shadow of a doubt!
Stop trying to make it use less fuel and enjoy the car
The gearing of the car does help with the fuel consumption however, on the gtrs anyway - 5th at 100 is about 4.5k rpm on 18in wheels with 275/35
tyres.
It is one of those cars that if you want to do properly dont have it as your main car, it will kill you ha.
[Edited on 8/11/12 by gtr_garner]
[Edited on 8/11/12 by gtr_garner]
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coyoteboy
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| posted on 8/11/12 at 11:08 PM |
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As with most things, its just not that simple. Fuel consumption, power, efficiency etc all vary with so many variables that you can't generalize
as most people here are. Turbos require lower compression on homogeneous charge engines to counter the boost, stratified/direct injection engines are
a bit different bit not that common yet. Lower compression means lower efficiency off boost as efficiency is directly related to compression ratio.
Under certain conditions turbo engines may make better efficiency than Na cars for the same performance but only with careful tuning. There are a few
reason for the move toward small engines and turbos instead of larger engines, such as lower weight.
But to modify a normal turbo engine to run without boost will do nothing for efficiency. Trust me, I know from experience.
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blakep82
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| posted on 8/11/12 at 11:37 PM |
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let me just say again, at no point did i say i was concerned about the fuel! i just wondered, thats all! lol
but thank you for the info
[Edited on 8/11/12 by blakep82]
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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