A little PH experiment I would like your help with

[scubadude]

Megane 1.9 130hp diesel

Towing RIB and 3up full of dive kit max 65mph it gets 35mpg
Plodding around town 0-40mph it gets 40mpg
Super cautious 60-65mph it gets 67mpg
Normal cruise 70-75mph it gets 54mpg
Fast cruise 85-90mph it gets 50mpg
Fast A-road driving 50-90mph it gets 45mpg
Roofbox seems to be worth 4mpg at normal cruise

[zeppelin101]

scarble said:

I think higher load cycles have shown worse for certain pollutants, if not worse for economy and if you do the comparison NEDC yourself rather than taking the published then you nullify the effect of an incorrect load.

Well yes, put more load on the engine (whether's through vehicle mass / speed / accel requirement) then it will be operating at a condition where the gaseous emissions will be higher.

Similarly, you could take a classic Mini and stick it on the dyno and set the dyno load to be the equivalent of a Range Rover and watch the thing still at the first accel! Quite useful for simulating accels up defined gradients or just seeing how a powertrain would behave in a different vehicle etc.

The load is determined before the car sees a dyno and then input into the dyno to determine the aero contribution, rolling resistance etc and that is where the issue lies. These aren't like the "cheap" chassis dyno's used by aftermarket places to measure torque, these are calibrated pieces of equipment which are designed to simulate road conditions. But as usual, st in = st out. Put the right numbers in and it does what it's supposed to!

[scarble]

Oh I see, they're precise dynos are they, obviously nothing like the emissions dynos we use
What I mean is a high load cycle as in, requires lots of heavy acceleration or lots of high speed cruising.
The NEDC is very light on acceleration and very low speed for the majority of the cycle, new cycles will be higher load.

[RobM77]

By the way, OP: have you seen the PH MPG wiki?

[Fastdruid]

scarble said:

The NEDC is very light on acceleration and very low speed for the majority of the cycle, new cycles will be higher load.

And the urban has a large portion of being stationary. Hence why almost everything has stop-start as it can make a huge difference.

[scarble]

Indeed
and everything is DI downsized and turbo because it gives a CO2 and economy benefit at moderate loads. But the Particlegeddon is looming!

[Fastdruid]

scarble said:

But the Particlegeddon is looming!

On a related note, a friend posted on a mailing list:
I was in a presentation yesterday looking at the longer term schedule for emissions regulations and diesel engines.I found myself wondering where in the timeline the "just give up" box was. The aftertreatment cost and complexity is going to get crazy at the Euro 6.2 emissions level. (current cars on sale are Euro 5)

[pherlopolus]

I wonder what the emissions would be on a small gas turbine running a genset for an EV? I guess that's a bit far off the track of whether your MPG matches the government required test criteria one though.

[scarble]

Silly question.. are there actual MPG limits or just emissions? I should probably know but.. :shrug: (I know there isn't a :shrug: but there should be)
CO2 has been the whipping boy in recent years but as always you don't get anything for nothing, the push to reduce CO2 has driven us all to diesels and DI petrols so particulates, especially the <10um (that's a mu for micro ok?) are probably on the up maybe? The funny thing is the (probably) more dangerous <10um (still a mu) particles aren't even legislated and afaik they won't be any time soon either
So if we really want to save the planet we need to go to larger displacement n/a PFI petrol engines with high cr, happy cams and proper intakes.
Or we could all switch to ethanol.
Or nitromethane.

[zeppelin101]

scarble said:

Silly question.. are there actual MPG limits or just emissions? I should probably know but.. :shrug: (I know there isn't a :shrug: but there should be)
CO2 has been the whipping boy in recent years but as always you don't get anything for nothing, the push to reduce CO2 has driven us all to diesels and DI petrols so particulates, especially the <10um (that's a mu for micro ok?) are probably on the up maybe? The funny thing is the (probably) more dangerous <10um (still a mu) particles aren't even legislated and afaik they won't be any time soon either
So if we really want to save the planet we need to go to larger displacement n/a PFI petrol engines with high cr, happy cams and proper intakes.
Or we could all switch to ethanol.
Or nitromethane.

10 micrometres is large. There are two phases to particulates, the second of which is what is measured as they are the harmful ones that stick around. Anything below 23nm generally speaking is gaseous and reactive. Anything over 23nm is considered "sooty".

Legislative counters burn the sub-23nm stuff off and just measure anything over the threshold.

[CraigyMc]

pherlopolus said:

CraigyMc said:

It must be being misreported.

It honestly makes little or no difference what speed I go on the motorway (within reason)

Looking ahead and choosing the time of day makes more of a difference than 50 or 80 mph

Do you agree that drag goes up as the cube of the speed?

[scarble]

It doesn't, it goes up as the square of speed but the power to overcome goes up as the cube of speed

@Zep, I probably mean <10nm then

[PoleDriver]

I did a 250 mile run yesterday. A3, M25, M1 and around Northampton. Stuck in very slow moving traffic a few times, up to 85-95 for a fair time, average speed 61 MPH.

Car is BMW E39 2.8 touring

Average consumption 34 MPG.

Not sure how that relates to the manufacturers figures?

[pherlopolus]

CraigyMc said:

Do you agree that drag goes up as the cube of the speed?

absolutely, and I also agree that engines have a sweet spot in the rev range where they are most efficient.

I'm not making an arbitrary point based on physics, I'm making an inexpert observation based on my experience.

[MartinQ]

I have performed a very accurate scientific experiment. Over the last three days I measured the mpg at a constant 70mph one day, then 0.8 and 0.9 leptons. I used the OBC, but this shouldn't matter since I just wanted a comparison. I used the M1 between J25 and 27 on my daily commute and used the cruise control to maintain speed. I took the average from each direction to remove any elevation change effect (quite significant as it happens with approx 100m vertical distance between the two junctions).

70mph: 56.7 and 48.7mpg. Average: 52.7mpg.
0.8 leptons: 48.7 and 45.5mpg. Average: 47.1mpg.
0.9 leptons: 40.3 and 37.6mpg. Average: 39.0mpg.

The car is a BMW 123d with quoted extra urban mpg of 64.2 and combined of 54.3.

I'll try 60mph tomorrow if my patience allows.

ETA:
60mph: 70.6 and 54.3mpg. Average: 62.5mpg.

Edited by MartinQ on Saturday 7th December 06:57

[scarble]

Good stuff!
Can you do the same in an n/a petrol car please?

I'd do it myself but on my commute I spend more time accelerating and decelerating than cruising..

Do you reset the trip at a set location once you've reached cruising speed to eliminate the influence of any variation in acceleration prior to the .. experimentation?
As I suspect transients have a significant influence on economy.

Edited by scarble on Friday 6th December 09:24

[CraigyMc]

It's always tricky to do comparisons. The following factors have an impact:

State of the car:

• Engine Coolant temperature (cold = more friction)
• Gearbox temperature (cold = more friction, too hot = more friction)
• Tyre temperature (cold = more rolling resistance)
• Tyre pressures (low pressures = more rolling resistance, pumped up to max sidewall = least resistance, but best grip in between that pressure range)
• Specification of the car (options = heavier = harder to carry up hills and to accelerate from rest)
• Wheel options (bigger wheels = more drag)
• Tyres fitted (wider tyres usually = more drag, construction has an impact on rolling resistance)
• Weight of any cargo (usually pushes rolling resistance up, but may also lower aerodynamic drag by pushing the ride height lower)
• Options turned on/off. Anything using electricity causes the alternator to increase resistance on the engine.
• Air conditioning: when on obviously causes more fuel to be used to power the compressor.
• Windows: All need to be totally closed not to cause aerodynamic drag.
• State of the heating/ventilation/aircon vents/fans. Some cars close them entirely when not in use, cutting aerodynamic drag.
• DPF regen cycles use fuel but don't occur all the time - are they in the test or not?

Atmospheric conditions:

• Temperature (cold = less economy)
• Precipitation (wet = tyres need to move the water = more rolling resistance)
• Precipitation (wet = some brakes auto skim = more rolling resistance)
• Air Pressure (more = more aerodynamic drag)
• Wind (tailwind = lower indicated airspeed = less drag, headwind = higher indicated airspeed = more drag)
• Wind (crosswind = more rolling resistance as the tyres cope with it)

Road conditions

• Going uphill = more energy needed from the engine, going downhill less energy needed from engine. Even if you start and end at the same elevation with a hill in the middle, it doesn't balance out.
• Bumps = cause rolling resistance (turned into heat in the suspension).
• Heavily cambered roads cause more rolling resistance, as you need to stop the car rolling off the side.

Driver inputs

• Driving the car economically versus uneconomically.
• Using the brakes at all tends to hump economy.

I'm sure there are dozens of other criteria, but the above is part of the reason why real-world results and the NEDC test cycle aren't all that similar.
If the market demand is for them to be accurate, one way to do it would be to use feedback from the real world to publicise actual results (eg. all cars have devices that report fuel economy) -- but the manufacturers don't want that, and it'd be expensive for all involved.

Cycle cheating is a bit of an art form too.

[MartinQ]

I've updated my post above with the 60mph run. Not too bad for a 200bhp car! Although I may have got it into the low 30s on the A road back home.

Anyway, I reset the computer as I hit the target speed and engaged CC. I'll be able to try my 330Ci for an NA petrol comparison, but as I'm off next week it may have to be the week after.

[blindswelledrat]

A little interesting update for this.
Ive been doing an identical journey lately from High Wycombe to Sheffield in identical traffic and at more or less the same speed (85-90 where possible but up and down from 65-90 depending on traffic)

Ive done it in two different cars:

2011 Peugeot 5008 1.6D. Theoretical mpg 45 urban 65 EU and 55 combined: Pretty consistent return for that journey: 41 mpg

2004 Saab 9-3 aero 2.0t. Theoretical MPG: 25 urban 45 Eu 35 combined: Return 34mpg

Given that the diesel is more expensive the difference is marginal. It's infuriating because the Peugeot was bought for my wife with the sole purpose of it being economical. Definitely not worth the sacrifice with the st underpowered engine.

Edited by blindswelledrat on Monday 27th January 11:22

[Bill]

It's a big car with a small engine being made to accelerate at speeds the aerodynamics affect. I'm not surprised tbh, but it'd be interesting* to see how they compare at a steady 70mph.


* Not for you, obviously.