I've rewritten this a few times, as I've confused myself a bit trying to explain what was an initial single question.

Fortunately, these rewrites have cut a lot of explanatory waffle down to two basic questions:

Firstly, on the same platform, do different engine sizes have max MPG at different speeds, or is 55MPH (or thereabouts) is the optimum speed for max MPG for all engine sizes for a particular car?

(Presumably, very aerodynamic cars will have a higher optimal efficiency speed to cars with poor aerodynamics.)

 

Secondly, using the Octavia as an example, is the 1.0 TSI Octavia is the most economical TSI Octavia when compared to other petrol variants throughout the performance envelope of the 1.0 TSI?

(For example, I suspect that at 110MPH, the 1.5 TSI will be more economical than the 1.0 TSI, which is probably starting to get a bit red in the face at this speed. Is that assumption correct?)

 

Does anyone have any info/opinions on these two questions?

If they could all do the same economy and emit the same emissions with the same models and different engine capacities from a 1.0 - 2.0 petrol / 113 ps - 300 ps then the VW Group would not have to have cheated the old tests and would have found it easy to get the models like a Golf certificated for the WLTP with equally low emission figures.

They had to take the likes of the 1.5 TSI ACT that they thought would be fine and change the engine management and get the results but have them behaving terribly.

 

They are making 1.0 TSI's with Mild Hybrid if they have a DSG and 1.4 TSI's as plug in Hybrids, and 1.5 TSI's / TFSI's have ACT / COD.

Edited 2 July, 20223 yr by roottoot

41 minutes ago, roottoot said:

If they could all do the same economy and emit the same emissions with the same models and different engine capacities from a 1.0 - 2.0 petrol / 113 ps - 300 ps then the VW Group would not have to have cheated the old tests and would have found it easy to get the models like a Golf certificated for the WLTP with equally low emission figures.

They had to take the likes of the 1.5 TSI ACT that they thought would be fine and change the engine management and get the results but have them behaving terribly.

 

They are making 1.0 TSI's with Mild Hybrid if they have a DSG and 1.4 TSI's as plug in Hybrids, and 1.5 TSI's / TFSI's have ACT / COD.

Yes I realise there are differences, and it gets complicated with hybrids, but the questions remain. (Rephrased)

 

1) Is the 1.0 TSI at 110MPH still more economical than the 2.0TSI at 110MPH? (My assumption is that it is not, though to be honest I have no science to back this up. It's basically a gut feeling)

2) Are all similar cars most economical at a particular speed? (Which I accept may vary with different shapes of car.)

 

Now it might be that the answer to 1) is "Yes of course!", I'm just assuming a 2.0 TSI is working a lot less hard at 110MPH than the 1.0 TSI in the same body shell is.

Same with 2).

 

But I don't know, hence the question.

45 minutes ago, EnterName said:

Yes I realise there are differences, and it gets complicated with hybrids, but the questions remain. (Rephrased)

 

1) Is the 1.0 TSI at 110MPH still more economical than the 2.0TSI at 110MPH? (My assumption is that it is not, though to be honest I have no science to back this up. It's basically a gut feeling)

2) Are all similar cars most economical at a particular speed? (Which I accept may vary with different shapes of car.)

 

Now it might be that the answer to 1) is "Yes of course!", I'm just assuming a 2.0 TSI is working a lot less hard at 110MPH than the 1.0 TSI in the same body shell is.

Same with 2).

 

But I don't know, hence the question.

 

Short answer, 'no'. 

 

Long answer is one or several books long. 

 

Couple of facts, max mpg occurs at an incredibly low speed ie 20, 30 or at most 40 mph. 

Small engines, being pushed to 100 mph or more may well richen up to protect itself rather than overheat key parts. 

 

Aero is the most important factor over about 50 mph. 

 

I miss my old Audi A4 which seem to be doing over 60 mpg at legal and more and with the biggish tank could do almost a thousand miles with its 130 hp 1.9 pd engine. 

 

As to efficiency and cheap to run my Zoe is on another level costing about 1.4p a mile where as my 1. 4 TDI must be around 14p a mile with diesel at £2 a litre. 

 

Hypermilng on road and track is a real skill and different drivers in the same car can make several percentage points difference. 

 

2 hours ago, lol-lol said:

Couple of facts, max mpg occurs at an incredibly low speed ie 20, 30 or at most 40 mph. 

Small engines, being pushed to 100 mph or more may well richen up to protect itself rather than overheat key parts. 

 

Non fact!, all petrol engines for the last 25 years + have run closed loop lambda fuelling which maintains the mixture at the stochiometric ratio.

 

Petrol engines are at their most efficient on wide open throttle, most efficient does not mean lowest fuel consumption, vehicles built for economy record breaking will accelerate on full throttle to somewhere near to the point of max torque and then coast down to a lower speed and accelerate again, probably between 1500 and 4500 RPM, it will depend on the engine, the cam and valve timing, turbocharged or normally aspirated but the principal remains the same.

5 minutes ago, J.R. said:

 

Non fact!, all petrol engines for the last 25 years + have run closed loop lambda fuelling which maintains the mixture at the stochiometric ratio.

 

 

That may be true for car engines.  It's certainly not the case for motorcycles.  In fact, even the recently introduced wide band lambda sensors on BMW bikes aren't used to run the system in closed loop throughout the rev range. 

9 hours ago, Schtum said:

 In fact, even the recently introduced wide band lambda sensors on BMW bikes aren't used to run the system in closed loop throughout the rev range. 

Wideband lambda sensors are also fitted to some Skoda engines such as the EA211 1.4TSI and 1.5TSI.

Edited 3 July, 20223 yr by PetrolDave

12 hours ago, lol-lol said:

 

Short answer, 'no'. 

 

Long answer is one or several books long. 

 

Couple of facts, max mpg occurs at an incredibly low speed ie 20, 30 or at most 40 mph. 

Small engines, being pushed to 100 mph or more may well richen up to protect itself rather than overheat key parts. 

 

Aero is the most important factor over about 50 mph. 

 

I miss my old Audi A4 which seem to be doing over 60 mpg at legal and more and with the biggish tank could do almost a thousand miles with its 130 hp 1.9 pd engine. 

 

As to efficiency and cheap to run my Zoe is on another level costing about 1.4p a mile where as my 1. 4 TDI must be around 14p a mile with diesel at £2 a litre. 

 

Hypermilng on road and track is a real skill and different drivers in the same car can make several percentage points difference. 

 

This is what I suspect.

In similar bodies, different capacity engines will have an optimum speed/gear combination for peak MPG.

If I simplify it by removing driver gear selection and use a DSG, then presumably it would be possible to plot a curve showing speed and MPG, that would rise, and then fall, perhaps like a crude bell curve.

What I'm curious about is seeing these curves, because it's not something I've seen any data on.

My own personal perspective is to favour larger capacity engines for high vehicle speed efficiency, and small capacity engines for low speed vehicle efficiency.

But as I said, I have no data to back this up.

The crude MPG figures given for vehiclesis no help and that's been replaced by the even cruder WLTP economy figure, which gives an overall fuel efficiency (and CO2 emission) rating.)

Bentley , Audi and even VW had nice big engines with many cylinders and they can shut down half of them and still nip along.  Ford and others as well.   Petrol or diesels.   Then there was Overboost.  Good for kidology of emissions.    VW had a V6 engine and a V5 one then came up with a 1 4 TSI (1,390 cc) that performed as well for bhp / Nm torque but using much less fuel.   Load more oil maybe, but ok not if not thrashed. 

 

 

 

 

Volvo / Geeely developed things properly when Stefan Jacoby head of VW USA huffed off to be their CEO.

(probably whistle blowing on VW Group and Defeat Devices with TDI's that he introduced to the USA.)

 

 

 

 

Edited 3 July, 20223 yr by roottoot

11 hours ago, Schtum said:

 

That may be true for car engines.  It's certainly not the case for motorcycles.  In fact, even the recently introduced wide band lambda sensors on BMW bikes aren't used to run the system in closed loop throughout the rev range. 

Not Lambda 1.0 on wide open throttle but still kept within a richer value for emissions.