I agree with your comments about short journeys. For the first 10 miles or so while my car is warming up, the consumption is high. For my 40 mile commute, it is the next 30 miles that gives a reasonable average.

My car is the detuned 3 litre which because of the efficient dynamics package is, in the auto version at least, more economical than the pre-LCI 520d. I know because I get a lower business fuel rate than a colleague with a 520d.

Regards,

 

My car is a manual, though I have to admit that at the moment average journey length is about 10 miles so this may account for the problem. Also the car has only done 4000 miles so hopefully it will get better as time goes on.

 

I've always believed that bigger engines are often more efficient in a lot of real-world circumstances... Not that the 520d is 'underpowered' but it must work harder than a 525d/530d and this can increase consumption. Individual driving style has the greatest impact on economy though - Reading the road and using the brakes and accelerator appropriately makes a large difference!

 

working harder might well be true at the top end of performance, but perhaps its not so much of an issue at lower and average speeds. I read years ago on a book on the Austin Mini, that it used approx 8 hp (out of the 34!) to do a constant 40mph on the flat. If it takes say, 50hp for an E60 to do 60mph on the flat, it would be the same for both engines. Would working a 163hp motor to 50hp be that much harder than a 200hp motor to produce same power?

In theory, each pair of crank rotations on a LCI 2.5 (3 litres) will draw 3 litres of fuel and air, where the 2.0 will draw (a more obvious!) 2 litres. So, the 3.0 ought to be less efficient.... unless the 3.0 revs lower. On a 520d 2k revs give you 70mph.

There must be some trickery here that only the designers fully understand!

 

Completely agree with your comments. Having said that the new LCI 520d should be capable of 'very good' economy figures - certainly consistent high 40's on a combined cycle, and easily into the 50's on a motorway cruise.

The new LCI 525d (detuned 3.0d) is considerably better on both the economy front & in terms of performance over the old pre-LCI 525d (2.5d). This is down to a number of factors Inc. Efficiency dynamics.

So, at a slight tangent, how long do we recon it will be before the 525d's detuned 3.0 engine is replaced with the new 2.0 twin turbo (204 hp, 400 NM, emmissions 138) as now been fitted in the new 123d? The updated 2.0d (177 hp) was first seen in the 1 series I believe. Surely this engine is stepping on the toes of the detuned 3.0d engine - more hp, same torque, considerably reduced emmissions?

 

Indeed, just one problem with the new TT engine... It's missing 2 cylinders!

Seriously though, I expect the new 5 (F10) to utilise this engine and the 3.0 engine will receive some mild extra tuning to differentiate... I don't think there'd be room for 5 diesels? (2.0t (520), 2.0tt (523?), low-tune 3.0t (525), high-tune 3.0t (530) & 3.0tt (535))

 

It's a combination of things and the capacity comparison is a little too simplified... Indeed, the 525d/530d are geared higher and remember they have 2 additional cylinders to 'assist'.

From memory my 530d is over 80mph at 2k rpm. Not sure if the auto has a longer final gearing...

 

The auto is indicating about 83-84mph @ 2k rpm.

Regards,

 

You're forgetting the throttle here (or valvetronic for some engines). Closed throttle means low volumetric efficiency - 3l engine may only be able to suck in 0.3l of air. Also with turbo (diesel or petrol), with WOT and full boost, you may be getting 4-6l of air into a 3 litre engine depending on boost pressure. Even a non turbo engine will never achieve 100% volumetric efficiency even at very low revs - there simply is not enough time for a cylinder to fill to atmospheric pressure when it's on the intake stroke. Volumetric efficiency also goes down with rpm - high rpm means less time for engine to suck in air even at same throttle. Engine computer will then work out how much fuel to inject - usually to achieve a stoichiometric burn (ie as efficient as possible), but sometimes rich mixture as extra fuel will cool the engine (turbo petrol engines do this a lot). Or cut off the fuel completely on overrun if you close the throttle (ie the engine is just pumping air). Valve timing also plays its part - a high power engine may well open the intake valve well before the exhaust valve is closed to try and improve volumetric efficiency at high revs, but this can mean pushing unburnt fuel/air mix straight out the exhaust valve.

So, as you yourself conclude, engine capacity is not a direct indicator of economy, although it certainly influences it. All things being equal, a 2l engine will need more throttle to maintain a steady 40mph cruise than a 3l engine, and therefore the fuel burnt is not simply 2/3. Probably be very similar in this case - main difference being more effieciency losses in the bigger engine.

Andrew

 

so the lcis have better engines i knew that, the 520d was a efficiency dynamics, but didnt know this was the case over the diesil range, mines a 2006 oct model, with reasonable mpg after ecu remap