What is more fuel efficient? On boost, or off?
Discussion
I don't know if it is different for petrol and diesel but if someone could answer for both please?
What is more fuel efficient? Constant speed while on turbo boost, or constant speed whilst off boost?
Obviously off boost = lower revs, which is more fuel efficient, but is the fact that on boost more air is in the cylinder and therefore causing a "bigger bang" going to make it more efficient on boost? (because you can use less fuel (mixed with more air) to make the same sized bang as if you were on boost?)
Or am I totally off the mark and should get my coat?
Sorry if that above is a bit mixed up, I was struggling to explain what was in my head.
What is more fuel efficient? Constant speed while on turbo boost, or constant speed whilst off boost?
Obviously off boost = lower revs, which is more fuel efficient, but is the fact that on boost more air is in the cylinder and therefore causing a "bigger bang" going to make it more efficient on boost? (because you can use less fuel (mixed with more air) to make the same sized bang as if you were on boost?)
Or am I totally off the mark and should get my coat?
Sorry if that above is a bit mixed up, I was struggling to explain what was in my head.
More boost = high volumetric and thermal efficiency?
(Correct me if i'm wrong, i did mostly brayton cycles rather than otto cycles!)
Edit: So basically a 1L engine with boost vs a 1.6L engine without...The 1L should be more fuel efficient at equal horsepower at equal revs (with a whole other boatload of assumptions...)
(Correct me if i'm wrong, i did mostly brayton cycles rather than otto cycles!)
Edit: So basically a 1L engine with boost vs a 1.6L engine without...The 1L should be more fuel efficient at equal horsepower at equal revs (with a whole other boatload of assumptions...)
stabbed rat said:
I don't know if it is different for petrol and diesel but if someone could answer for both please?
What is more fuel efficient? Constant speed while on turbo boost, or constant speed whilst off boost?
Obviously off boost = lower revs, which is more fuel efficient, but is the fact that on boost more air is in the cylinder and therefore causing a "bigger bang" going to make it more efficient on boost? (because you can use less fuel (mixed with more air) to make the same sized bang as if you were on boost?)
Or am I totally off the mark and should get my coat?
Sorry if that above is a bit mixed up, I was struggling to explain what was in my head.
Are you the stupid version of the other rat?What is more fuel efficient? Constant speed while on turbo boost, or constant speed whilst off boost?
Obviously off boost = lower revs, which is more fuel efficient, but is the fact that on boost more air is in the cylinder and therefore causing a "bigger bang" going to make it more efficient on boost? (because you can use less fuel (mixed with more air) to make the same sized bang as if you were on boost?)
Or am I totally off the mark and should get my coat?
Sorry if that above is a bit mixed up, I was struggling to explain what was in my head.
geeteeaye said:
stabbed rat said:
I don't know if it is different for petrol and diesel but if someone could answer for both please?
What is more fuel efficient? Constant speed while on turbo boost, or constant speed whilst off boost?
Obviously off boost = lower revs, which is more fuel efficient, but is the fact that on boost more air is in the cylinder and therefore causing a "bigger bang" going to make it more efficient on boost? (because you can use less fuel (mixed with more air) to make the same sized bang as if you were on boost?)
Or am I totally off the mark and should get my coat?
Sorry if that above is a bit mixed up, I was struggling to explain what was in my head.
Are you the stupid version of the other rat?What is more fuel efficient? Constant speed while on turbo boost, or constant speed whilst off boost?
Obviously off boost = lower revs, which is more fuel efficient, but is the fact that on boost more air is in the cylinder and therefore causing a "bigger bang" going to make it more efficient on boost? (because you can use less fuel (mixed with more air) to make the same sized bang as if you were on boost?)
Or am I totally off the mark and should get my coat?
Sorry if that above is a bit mixed up, I was struggling to explain what was in my head.
Slinky said:
More boost = more fuel ..
Yes but it is also the point at which the engine is at its most efficient. Highest volumetric efficiency is achieved roughly when max torque is achieved, with lowest frictional losses. Put simply, you need more fuel to accelerate and the most efficient way to do this is "on boost" or around the max torque band.
It is also more efficient to accelerate quickly up to speed as you arrive at a steady cruise more quickly as opposed to "driving economically" like a granny where you end up using not quite as much fuel but take a lot longer to get up to speed, so in affect more fuel overall.
Cruising speed is most efficient at low revs, by a mile.
briers said:
Yes but it is also the point at which the engine is at its most efficient. Highest volumetric efficiency is achieved roughly when max torque is achieved, with lowest frictional losses.
Put simply, you need more fuel to accelerate and the most efficient way to do this is "on boost" or around the max torque band.
It is also more efficient to accelerate quickly up to speed as you arrive at a steady cruise more quickly as opposed to "driving economically" like a granny where you end up using not quite as much fuel but take a lot longer to get up to speed, so in affect more fuel overall.
Cruising speed is most efficient at low revs, by a mile.
So are you saying, lower revs and off boost for cruising? For example, my car (MINI cooper D cruises at 60 at 2k rpm dead on, if it was geared differently, lets say 250 rpm lower at 60 (just off boost) would it be more efficient than how it is now?Put simply, you need more fuel to accelerate and the most efficient way to do this is "on boost" or around the max torque band.
It is also more efficient to accelerate quickly up to speed as you arrive at a steady cruise more quickly as opposed to "driving economically" like a granny where you end up using not quite as much fuel but take a lot longer to get up to speed, so in affect more fuel overall.
Cruising speed is most efficient at low revs, by a mile.
stabbed rat said:
So are you saying, lower revs and off boost for cruising? For example, my car (MINI cooper D cruises at 60 at 2k rpm dead on, if it was geared differently, lets say 250 rpm lower at 60 (just off boost) would it be more efficient than how it is now?
Most fuel efficient cruising should be at minimum power to equal air resistance, rolling resistance and drivetrain drag - lets call this X bhp. If you retuned a turbo car to be producing X bhp on boost (whereas previously it hit X bhp in top gear off boost), it would be more efficient ie higher MPG. And possibly a pig to drive. This could be achieved with a smaller turbo that spooled up at lower revs maybe?
(Please correct if wrong)
Edit: its all rather simplified. I'm assuming you are tuning it properly, not being a numpty.
Edited by alephnull on Friday 8th April 00:33
stabbed rat said:
So are you saying, lower revs and off boost for cruising? For example, my car (MINI cooper D cruises at 60 at 2k rpm dead on, if it was geared differently, lets say 250 rpm lower at 60 (just off boost) would it be more efficient than how it is now?
Not in all cases. Wind resistance is a big factor at 60mph and above. You will see a big tail off above these speeds for mpg. The wind resistance met needs power to cut through it. It isn't a steady cruise, the engine needs to generate power against higher resistance and therefore is often more efficient running on boost at these speeds, especially in a low powered heavy car, it can achieve much better economy on boost. The line is fine, but it is geared like this because of wind, weight and power and possibly to conceived need to overtake without changing gear for the majority of women.
The same can be said for other car examples. My wifes rav4 diesel is more efficient in 5th than 6th at 60mph because of the gearing. Big heavy car ticking along in 6th at 1.4krpm. My car is geared to do about 160mph so the is very little difference in mpg between 60 and 75. But in my wifes rav if can be as much as 8mpg.
alephnull said:
stabbed rat said:
So are you saying, lower revs and off boost for cruising? For example, my car (MINI cooper D cruises at 60 at 2k rpm dead on, if it was geared differently, lets say 250 rpm lower at 60 (just off boost) would it be more efficient than how it is now?
Most fuel efficient cruising should be at minimum power to equal air resistance, rolling resistance and drivetrain drag - lets call this X bhp. If you retuned a turbo car to be producing X bhp on boost (whereas previously it hit X bhp in top gear off boost), it would be more efficient ie higher MPG. And possibly a pig to drive. This could be achieved with a smaller turbo that spooled up at lower revs maybe?
(Please correct if wrong)
Edit: its all rather simplified. I'm assuming you are tuning it properly, not being a numpty.
Edited by alephnull on Friday 8th April 00:33
It was more a curiosity thing, as well as the fact that my brother has a MINI cooper D too and his is geared differently (1750-1800rpm) at 60mph and just off boost.We drive totally different roads and driving styles so it would be hard to just compare.
Check the stats too. A lot of modern common rail / piezo injectors are now producing boost at lower RPM's. It was common site to see a lot of diesels produce at 2.2 - 2.8k rpm. Now it is more like 1.6/1.8 - 2.5k. For economy reasons and advancement in valve design and fuel delivery and metering from piezo injectors etc etc...
Edited by briers on Friday 8th April 00:50
briers said:
Check the stats too. A lot of modern common rail / piezo injectors are now producing boost at lower RPM's. It was common site to see a lot of diesels produce at 2.2 - 2.8k rpm. Now it is more like 1.6/1.8 - 2.5k. For economy reasons and advancement in valve design and fuel delivery and metering from piezo injectors etc etc...
It is 1750-1800 on our carsEdited by briers on Friday 8th April 00:50
ETA - where boost start (not the range)
Theoretically a boosted engine is more thermodynamically efficient an therefore can be more fuel efficient. The issue comes down to mapping, and how the engine has been designed.
As previously posted, that R32 will be horrific on fuel as it comes on boost, whereas something like a VW TSi is mapped to be fuel efficient up to a certain rev point or throttle position.
Many OEMs map cars to be rich when on boost (at higher throttle points) to protect the cat from overheating, so in the real world it very much comes down to the specific car and situation.
As for diesels, they work very differently to petrol engines: they are always on wide open throttle (they don't have a throttle!) so when they are off boost they can't benefit from the positive effects of +100% VE, whereas when they come on boost they fill the cylinders above 100% VE and still only use a tiny amount of fuel, so as far as I understand diesels have no real drawbacks to being on boost, and this is part of the reason why diesels have small turbos (that and they make sod all power off boost).
As previously posted, that R32 will be horrific on fuel as it comes on boost, whereas something like a VW TSi is mapped to be fuel efficient up to a certain rev point or throttle position.
Many OEMs map cars to be rich when on boost (at higher throttle points) to protect the cat from overheating, so in the real world it very much comes down to the specific car and situation.
As for diesels, they work very differently to petrol engines: they are always on wide open throttle (they don't have a throttle!) so when they are off boost they can't benefit from the positive effects of +100% VE, whereas when they come on boost they fill the cylinders above 100% VE and still only use a tiny amount of fuel, so as far as I understand diesels have no real drawbacks to being on boost, and this is part of the reason why diesels have small turbos (that and they make sod all power off boost).
deveng said:
and this is part of the reason why diesels have small turbos (that and they make sod all power off boost).
A range rover was being put through its paces on a test track when just off the track there was a hump/bridge on the way back to the garage area. With the turbo's boost disabled the car couldn't make it over the crest. 
Turbicharged petrol and turbo diesel are two entirely different things as far as fuelling goes, esp at part-load.
For best economy in a turbo petrol engine you want to keep out of boost as much as you can - because under boost conditions the fueling will start to go 'rich' to prevent detonation etc, even if the load does not justify the amount of fuel burnt. In an un-boosted petrol engine, best economy usually occurs with W.O.T. at lowest revs possible to maintain speed - ie high volumetric (breathing) efficiency, only just burning enough petrol to maintain the load. Worst posible case is a petrol engine, starting to boost, but part-throttled to limit power. This is bad because it is subject to lots and lots of pumping losses: restrictions on the intake (throttle plate) and the exhaust (some turbo back pressure), and maybe even excess fuelling, just to maintain a marginal load.
However:
Turbodiesels run the intake essentially 'unthrottled' - i.e. wide open, no throttle butterfly so power demand = fueldemand, more or less, and manufacturers make the turbo spool as low-down as possible becasue it increases the effective compression ratio = better efficiency. Totally different...
For best economy in a turbo petrol engine you want to keep out of boost as much as you can - because under boost conditions the fueling will start to go 'rich' to prevent detonation etc, even if the load does not justify the amount of fuel burnt. In an un-boosted petrol engine, best economy usually occurs with W.O.T. at lowest revs possible to maintain speed - ie high volumetric (breathing) efficiency, only just burning enough petrol to maintain the load. Worst posible case is a petrol engine, starting to boost, but part-throttled to limit power. This is bad because it is subject to lots and lots of pumping losses: restrictions on the intake (throttle plate) and the exhaust (some turbo back pressure), and maybe even excess fuelling, just to maintain a marginal load.
However:
Turbodiesels run the intake essentially 'unthrottled' - i.e. wide open, no throttle butterfly so power demand = fueldemand, more or less, and manufacturers make the turbo spool as low-down as possible becasue it increases the effective compression ratio = better efficiency. Totally different...
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