Interesting Item On Audi Diesel Fuel Economy

[06G220D]

I just saw this in the Honest John motoring advice section of todays Telegraph.

Q. "I took delivery of a new Audi A4 Avant 2.0 TDI 143 manual in August and having now covered more than 5,000 miles I am disappointed with the fuel economy. My previous A4 always returned at least 50mpg. The new model seems reluctant to deliver 40mpg. Audi says that I should be getting 52.3mpg Combined, but I have also read that the particulates burner uses more fuel. Logic would surely suggest that lower emissions figures should not be achieved at the cost of poorer fuel efficiency."

M.H., Bingham

A. The diesel particulate filter (DPF) will use more fuel, particularly if you fill with cheap diesel rather than super diesel such as BP Ultimate, and particularly if you drive at low revs for economy. Most of all, you'll use a lot more fuel if you do a lot of short runs from cold and town running. You might find that increasing your motorway speed, perhaps from 65mph to 70mph, has a beneficial effect because the engine will not then have to over-fuel to burn off the particulates in the DPF.

[dgman]

that somewhat follows the advise in the lex owners manual which says not to use 6th gear under 75mph.

[ihpj]

With the inclusion of DPFs we will have to adapt our driving styles to take into account its 'peculiar' behaviours. I don't think it is a bad technology, but it works in a particular way so we will have to understand that cars with such fitted will need to be driven slightly differently than we are used to/expected.

[kenfoxy]

There NOT now seems to be a direct relationship between CO2 and fuel economy.

Yet another example of incorrect fuel figures issued by manufacturers.

[fairygodmother]

The cars are actually tested by an independent company and are not issued by the manufactures. Don't forget that all the consumption figures you see are taken in a laboratory and are done the SAME each time. Below is how they measure each car:

Cycle:

The urban test cycle is carried out in a laboratory at an ambient temperature of 20oC to 30oC on a rolling road from a cold start, i.e. the engine has not run for several hours. The cycle consists of a series of accelerations, steady speeds, decelerations and idling. Maximum speed is 31mph (50km/h), average speed 12mph (19km/h) and the distance covered is 2.5 miles (4km).

Extra-Urban Cycle:

This cycle is conducted immediately following the urban cycle and consists of roughly half steady-speed driving and the remainder accelerations, decelerations, and some idling. Maximum speed is 75mph (120km/h), average speed is 39mph (63 km/h) and the distance covered is

4.3miles (7km). Combined Fuel Consumption Figure:

The combined figure presented is for the urban and extra-urban cycle together. It is therefore an average of the two parts of the test, weighted by the distances covered in each part.

[aido]

Never knew that - that's interesting to know how they actually get those numbers :)

[Parthiban]

"Logic would surely suggest that lower emissions figures should not be achieved at the cost of poorer fuel efficiency"

What a moron! So he'd prefer to get cancer than pay a bit more on fuel? That guy needs beating with a stick :shutit:

Yep that is very interesting info - I wonder if car manufacturers set up engines and gearing to get the maximum out of that test, and then not achieve it in the real world. With the disparity in price between diesel and petrol now, if you do a lot of short town journeys, a diesel must work out more expensive?

[Matus]

There NOT now seems to be a direct relationship between CO2 and fuel economy.

you think that the smell coming out of diesel exhaust is pure CO2?

[johnboy]

So Lexus arent the only ones having trouble with there diesel engines

[Kazi]

Give the car chance, its only done just over 5000 miles. I remember reading from the majority of IS220d owners on here that the car gets better mpg the more you drive it...10k and above....

[johnboy]

Give the car chance, its only done just over 5000 miles. I remember reading from the majority of IS220d owners on here that the car gets better mpg the more you drive it...10k and above....

I agree totally, modern engines are built to much closer tolerences, so take a lot longer to loosen up, approx 10k, unfortunately customers arent always told this, so they expect to get the quoted mpg after 1000 miles.

But it does appear that the modern fuel systems are getting cleaner, but has an effect on fuel economy.

[Scarlet Pimpernell]

My new car will beat the official figures if I try, so I'm not one who thinks the figures are unachievable.

My old A4 took 10000 miles to loosen, and then could beat the figures every time. The Lexus did too, from 5-9000 miles then went down hill....

When I tested the new A4, it was just about achieving the claimed figures with only a few miles on the clock. it comes down to 2 things:

Your driving style

The "mechanics" of the car and how well it's running (injectors etc).

Modern diesels are very complex - they have to be to beat the emissions rules.

The quote about slower driving means more fuel is a larf...

The comments about the Lexus manual saying use 6th over 75 is to do with the tall gearing, not economy. The engine labours at anything less than 70 on the non-Sport badly - which is not good for the engine or box. It must feel like it's pulling a truck...

[dgman]

My new car will beat the official figures if I try, so I'm not one who thinks the figures are unachievable.

My old A4 took 10000 miles to loosen, and then could beat the figures every time. The Lexus did too, from 5-9000 miles then went down hill....

When I tested the new A4, it was just about achieving the claimed figures with only a few miles on the clock. it comes down to 2 things:

Your driving style

The "mechanics" of the car and how well it's running (injectors etc).

Modern diesels are very complex - they have to be to beat the emissions rules.

The quote about slower driving means more fuel is a larf...

The comments about the Lexus manual saying use 6th over 75 is to do with the tall gearing, not economy. The engine labours at anything less than 70 on the non-Sport badly - which is not good for the engine or box. It must feel like it's pulling a truck...

on page 109 of the owners manual it clearly states that changing up to 6th after 75mph gives optimum fuel economy ,that suggests too me that the engine runs cleaner above a certain amount of rev's and that in turn may reduce the amount of cleaning cycles that the dpf would require.as you say the modern day diesel engine has become a complex machine trying on one hand to give very good fuel economy,on the other low emissions and particle filtration.the specific driving style required to meet the requirements of the dpf has not been fully explained by the manufacturers and that could be a major factor in the varied fuel economy posted by drivers of dpf equiped cars.i think that we are getting towards the technical limits of a diesel engine in todays climate.

[kenfoxy]

The cars are actually tested by an independent company and are not issued by the manufactures. Don't forget that all the consumption figures you see are taken in a laboratory and are done the SAME each time. Below is how they measure each car:

Cycle:

The urban test cycle is carried out in a laboratory at an ambient temperature of 20oC to 30oC on a rolling road from a cold start, i.e. the engine has not run for several hours. The cycle consists of a series of accelerations, steady speeds, decelerations and idling. Maximum speed is 31mph (50km/h), average speed 12mph (19km/h) and the distance covered is 2.5 miles (4km).

Extra-Urban Cycle:

This cycle is conducted immediately following the urban cycle and consists of roughly half steady-speed driving and the remainder accelerations, decelerations, and some idling. Maximum speed is 75mph (120km/h), average speed is 39mph (63 km/h) and the distance covered is

4.3miles (7km). Combined Fuel Consumption Figure:

The combined figure presented is for the urban and extra-urban cycle together. It is therefore an average of the two parts of the test, weighted by the distances covered in each part.

Thank you for this information, I stand corrected. Do you happen to know if the VCA select vehicles at random or are they presented by the manufacturers for testing? Using 20- 30 degrees is OK as a relative measure across vehicles, I doubt it is an average cold start temperature for the UK or even Europe.

[dave1]

There was an article about this in the Sunday Times last year.

Link