fuel economy of larger diesel engines

[whitenemesis]

If it's a lab test on a rolling road then it's really easy to have an accurate volumetric cylinder in place to hold the fuel.

I'd love to hear their technical argument for basing a fuel consumption figure on anything other than the actual volume of fuel consumed.

I have no doubt they'd get laughed out of any court.

To expand on how they do the calculation..

"1.4.3. The fuel consumption, expressed in litres per 100 km (in the case of petrol, LPG or
diesel) or in m3 per 100 km (in the case of NG) is calculated by means of the following formulae:
(a) for vehicles with a positive ignition engine fuelled with petrol:
FC = (0.1154 / D) · [(0.866 · HC) + (0.429 · CO) + (0.273 · CO2)]
(b) for vehicles with a positive ignition engine fuelled with LPG:
FCnorm = (0.1212 / 0.538) · [(0.825 · HC) + (0.429 · CO) + (0.273 · CO2)]
If the composition of the fuel used for the test differs from the composition that is assumed for the calculation of the normalised consumption, on the manufacturer's request a correction factor cf may be applied, as follows:
FCnorm = (0.1212 / 0.538) · (cf) · [(0.825 · HC) + (0.429 · CO) + (0.273 · CO2)]
The correction factor cf, which may be applied, is determined as follows:
cf = 0.825 + 0.0693 · nactual
where:
nactual
= the actual H/C ratio of the fuel used
(c) for vehicles with a positive ignition engine fuelled with NG:
Fcnorm = (0.1336 / 0.654) · [(0.749 · HC) + (0.429 · CO) + (0.273 · CO2)]

(d) for vehicles with a compression ignition engine:
FC = (0.1155 / D) · [(0.866 · HC) + (0.429 · CO) + (0.273 · CO2)]
In these formulae:
FC = the fuel consumption in litre per 100 km (in the case of petrol,
LPG or diesel) or in m3 per 100 km (in the case of natural gas)
HC = the measured emission of hydrocarbons in g/km
CO = the measured emission of carbon monoxide in g/km
CO2 = the measured emission of carbon dioxide in g/km
D = the density of the test fuel.
In the case of gaseous fuels this is the density at 15 °C."

 

[LTD]

To expand on how they do the calculation..

Urban cycle test explained

The urban test cycle aims to simulate driving conditions in and around town. It is carried out on a rolling road in a laboratory with an ambient temperature of 20 degrees Celsius (°C) to 30°C.

D = the density of the test fuel.
In the case of gaseous fuels this is the density at 15 °C."

Flawed test.

The density of the fuel should be calculated at the same ambient temperature as the room (assuming normalisation of all items) unless the temperature of the fuel is accurately maintained at 15C

 

[whitenemesis]

When all is said and done these are (as has already been stated) 'standardised' tests and as such provide a tool for comparing vehicles and their fuel consumption.

Where things go amiss is when we (the consumer) then uses that data to assess the efficiency of our particular vehicle 'in the real world'.

The variables introduced by the 'real world' are too numerous and uncontrolled

 

[LTD]

Ergo, the advertising of any vehicle based on fuel economy should be banned as the figures achieved are not in any way, shape or form, realistic in a real-world scenario.

 

[elisetype25]

While past experience means I have always suspicious of manufacturers mpg figures, I have found the official Mercedes Figures to be very accurate.

I admit that I have adjusted my driving style since buying the car and have taken a little more care with my right foot.

As mentioned earlier in the thread we have a tendency to use the power available rather than what is needed and this will greatly affect consumption.

Here are a few observations and figures derived from my totally unscientific tests.

The car is a E220 BE with the 5 speed auto. I have now done 33k miles in a little over 1 year. The official figures were 34.5mpg urban, 57.6mpg extra-urban and 46.3 combined. incidentally, my car always underestimates economy usually by between 0.5 and 1 mpg (computer reads 46.3mpg, calculated will be just over 47mpg).

New, the car returned around 43mpg for the first 5k miles or so, rising to 46mpg as the weather warmed up and was hitting nearly 50mpg at the height of summer. Driving around locally most journeys would be 10-20 mins and the car averages above 40mpg, significantly better than the official urban figure. My lowest return for a tank was very early in the cars life at 39mpg.

My commute to work is 90 miles each way on a mixture of A, B roads but is maily dual carraigeway and motorway, travelling at the speed limits 46mpg is easily achievable. I have on a couple of occassions driven with a heavier right foot this drops economy to around 41mpg.

The last couple of weeks having hit the 10k business miles that means the expenses rate per mile drops, I have experimented driving at 60mph on 70mph roads, the difference is shocking. The journey takes 8 minutes longer (1 hr 53m v 1hr 45m on a relatively clear run) but the consumption falls from an average 46mpg to an impressive 54mpg. I'm sure I could get closer to 60mpg with little effort.

Other than speed and driving style, the other real influence on mpg is weather conditions, my observations are that rain, wind can each knock a couple of mpg off the expected return, so travelling at the speed limits on a wet and windy day can easily see economy drop from 46 to 42mpg.

Other more obvious factors are extra drag from roof racks (these account for 10-15% more consumption on their own, and are even worse if you add bikes or a roof box). Tyre pressures are the other critical factor, with a couple of PSI easily knocking another few mpg off.

While not defending maufacturers claimed figures, it is very easy to pass the blame for the root cause back to them rather than looking at how we drive. Cars regularly overtake me on motorways travelling well in excess of the speed limit and this will mean they are probably using 30%+ more fuel for the same distance, and you can often find yourself just behind them when traffic becomes congested. Some variation in figures will result from the inherent variances associated with manufacturing tolerances involved in producing highly complex products in large numbers and as such there will always be the odd engine which under or over performs against expectation.

My experience with the car shows that if I drove it like I used to drive my old Subaru, I would probably be lucky to get 35mpg from the car, but it's a big family estate and doesn't warrant that sort of style, and I am now looking at averages closer to and over 50mpg.

We can each choose how fast we drive, the degree to which we anticipate traffic and road junctions, how often we check the tyre pressures and whether we can be bothered to remove the roof rack. If you want to achieve manufacturers figures it can be done, you just need to choose to do so.

If you want performance and reasonable economy buy a Lotus, I get 30-40mpg out of my Elise (less than 800kg) and its much better in the corners than the Merc.

Regards, Neil

 

[Number_Cruncher]

It's UNECE Regulation 101

http://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/r101r2e.pdf

The "road load" verification is described in here;

http://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/r083r4e.pdf

Appendix 4a - Appendix 1 describes dynamometer requirements

In short, as stated above, aerodynamic drag IS included.

There's no technical problem in collecting exhaust gases and measuring them - it's much more sensible, much less open to abuse, and much safer than attempting to measure the amount of fuel used.

I have great confidence that the engineers of each manufacturer submitting vehicles for testing against these regulations have been through the regulations checking to ensure a) that the regulations don't give any unfair advantage to their competitors, and b) to optimise their own products to get he best results they can.

Even if the numbers reported by the tests cannot be replicated in real life, the standard test does allow some BROAD conclusions to be made. Yes, there may be some anomalies, but, I'm absolutely sure these standard tests are better than allowing manufacturers to specify the tests!

 

[whitenemesis]

The "road load" verification is described in here;

http://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/r083r4e.pdf

Appendix 4a - Appendix 1 describes dynamometer requirements

In short, as stated above, aerodynamic drag IS included.

There's no technical problem in collecting exhaust gases and measuring them - it's much more sensible, much less open to abuse, and much safer than attempting to measure the amount of fuel used.

I have great confidence that the engineers of each manufacturer submitting vehicles for testing against these regulations have been through the regulations checking to ensure a) that the regulations don't give any unfair advantage to their competitors, and b) to optimise their own products to get he best results they can.

Even if the numbers reported by the tests cannot be replicated in real life, the standard test does allow some BROAD conclusions to be made. Yes, there may be some anomalies, but, I'm absolutely sure these standard tests are better than allowing manufacturers to specify the tests!

Agree 100%

 

[Beno]

Agree 100%

I work at a research facilty and I can state that there is no mechanical/remapping etc changes made to production vehicles to get low fuel results. We are regulaly inspected by the ministry of transport as are all motor manufacture's. The figures are derived by driving a fixed cycle (Not representable in the real world) so to achive the fuel economy figures supplied woul be very difficult.

 

[DITTRICH]

MPG Figs

Many years ago, mercedes used to quote fuel consumption for 90kph and 120kph as well as the other measures. I found these to be a useful comparison for the performance of my E300D Estate. Nowadays they are not allowed to provide these figures but I think they were really useful. 42.2mpg @ 90kph and 32.1mpg @ 120kph. Simulated urban driving was calculated at 30.1mpg.
And this is with a 4 speed automatic. I once achieved 45mpg on a trip from London to Wales.

 

[Sp!ke]

No it's not. The test is only for a short duration so measuring the gaseous output is far easier than measuring the fuel consumed, which might vary a lot in percentage terms due to the short test.

ok, now this confuses me.

Why is it that some cars are fantastic on fuel but dont have very good emmission figures?

Mrs Sp!kes Volvo S60 for instance can achieve 60mpg on my commute if I am careful, a respectable figure even for a Smart car. The volvos VED is £200 per year and the Smart zero based upon emmissions???

 

[wemorgan]

Could it be that gear ratios in larger cars are set for more efficient motorway cruising than smaller cars which may be geared for low speed acceleration?

So many large cars have a claimed top speed >120mph, more like >150mph for quite a few Mercedes on this forum......60-70mph might be less than 2000rpm.

 

[Anders]

I have only had my non Blue Efficiency C220 for 5000 km so the average may not be too reliable but somewhat disappointed that it will do no better than 35-36 mpg over a 450 km non-stop journey at 150 kph cruising speed. For comparison my M5 did 23-24 mpg and that was driven faster and more aggressively.

 

[Charles Morgan]

I drove a non turbo diesel today that averaged 42mpg on a 210 mile journey that I did in 3.5 hours, with one decent delay and a 10 minute pitstop. While slow to accelerate, as most other cars on the road were driven by people who made precisely zero use of their more modern cars performance, I wonder if in reality most progress has been absolutely pointless over the last 20 years. The only time I felt I needed more power was when I had a van on my rear and I couldn't show him a clean pair of heels.

 

[whitenemesis]

Drove to Olly's and back this morning, 180mls, approx 3hrs (driving time) and averaged 52mpg. The A34 and M4 were nice a clear, so tootled along at 70-80mph.