diesel mpg

Post by **irish.david** » 08 Mar 2007, 22:26

I don't really agree with the calculation used above comparing petrols and diesels. There is much more to be taken into account than just the diesels better volumetric efficiency. To name a few, the diesel has a turbo, different gearing, different cc and of course petrol and diesel have different calorific values. The 1/3 difference between the engines mpg's shown above is just a coincidence.

My opinion is that a turbo diesel engine is the best engine for a van, unfortunately VW didn't fit the right one with the 1.6 TD. With the weight of the van the poor engine will be running on a fairly high boost most of the time which will undoubtably shorten it's life. I'd recommend getting a 2.1 and gassing it or get a knackered diesel and fitting a 1.9TDi from another car.

Dave

Post by **airhead** » 08 Mar 2007, 23:58

bigbluebus wrote:anyone any bog standard 1.6 TD figures?

Yeah, I get 35mpg average.

Post by **HarryMann** » 09 Mar 2007, 01:19

I don't really agree with the calculation used above comparing petrols and diesels. There is much more to be taken into account than just the diesels better volumetric efficiency.

What you say maybe be right, but no-one mentioned volumetric efficiency. Simon mentioned CR and I mentioned throttling losses, both known to be the chief thermodynamic reasons for the diesel's better effciiency.

Offset against that the pumping losses of the injection pump, not insignificant.

Its the turbo that changes volumetric effciiency most markedly, but thats not directly related to efficiency.

Look at the newly released diesel Polo, at 70+ mpg, that's thermal effciiency doing a lot of that, as weight and aerodynamics are offering diminishing returns now motor makers know how to use wind-tunnels and design stiff light structures.

Post by **airhead** » 09 Mar 2007, 02:13

So on that basis you reckon with electronic injection and put into a VW polo, the JX engine could be getting around the 70mpg mark? Sounds unlikely to me. More modern engines as another point to mention, are also built to higher tolerances, with much more precise control of all aspects of the engines running. s a result, the modern diesel engine is smoother, quieter and much more efficient both from a power and a fuel perspective.

Post by **Russel** » 09 Mar 2007, 07:09

The most significant improvement in modern SMALL diesles(not trucks)is the fact of using direct injection.The compresion is much greater thus giving a much better combustion and far more talk.
The electronics will give a sharper response to things and cut emissions but not always better efficiency.
On the last trip i have just done the van with the elec. TDI was by far the heaviest on feul even though it was the lightest and on the least aggresive tyres.

Post by **HarryMann** » 09 Mar 2007, 10:27

So on that basis you reckon with electronic injection and put into a VW polo, the JX engine could be getting around the 70mpg mark?

We're back to that long thread a while ago about the pros and cons of thermodynamic cycles, Diesel, Otto and modified Otto (Atkinson and Miller
http://en.wikipedia.org/wiki/Atkinson_cycle)

The detail implementation of these basic heat cycles, to make a real engine, is the other aspect. The aim - to get as close to the 'ideal' thermodynamic efficiency as possible.

Neither Diesel nor Ottto/petrol follow the ideal cycles exactly, which are Diesel ~ heat addition at constant pressure and Otto ~heat addition at constant volume.

Development is being done in two areas, modifying the heat cycle itself and also perfecting the mechanical implementation of the engine. There is cross-over between them which isn't always obvious...

One reason that VW have returned to direct injection is that the heat loss in the pre-chamber of indirect injection diesels reduces cycle efficiency and there are also inherent pumping losses (compressing the air through the pre-chamber's nozzle).

Another and the more often quoted, is that with electronic injection, heat addition and combustion can be controlled much more accurately, so the advantages of the Ricardo pre-chamber (indirect injection) - pre-mixing and flame control - can be dispensed with.

But they're both the same in reality, minimising the inherent cycle inefficiencies yet retaining the practical advantages (better combustion).

Whilst we've always marvelled at the intricacy and accuracy of mechanical injection pumps and nozzles, ultra high-pressure electronic injection has highlighted how far short of the mark they really were ...

..almost like pissing into the wind with your fingers crossed that it doesn't gust at the wrong moment

The real improvements will come when a major manufacturer makes the move away from piston engines, to one that can compress the gas much more quickly, and expand it more slowly. That is, one with very asymmetric compression and power strokes. A new type of rotary engine is needed, but not based on the heavily compromised Wankel design, which has done more to put industry off rotary engines than to attract them to it.

Post by **amulet** » 09 Mar 2007, 10:36

yep, that was certainly the case, mine downs gallons like Gazza downs pints.

Still, I suspect foul play - it's amazing what some people will do to win a £5 bet on whose van was gonna give the best mpg stats...

Post by **syncroand101** » 09 Mar 2007, 10:49

amulet wrote:it's amazing what some people will do to win a £5 bet on whose van was gonna give the best mpg stats...

Which reminds me, I paid for that last fill up in Spain, €45 I believe. Mind you, that probably only got you 50miles.

Post by **amulet** » 09 Mar 2007, 11:40

I prefer being able to drive those fifty miles in less than 5 hours, unlike some people

actually, make that 10 hours if it was up hill!

can I pay you in dirhams please, still got a few left over...

Post by **syncroand101** » 09 Mar 2007, 12:27

amulet wrote:I prefer being able to drive those fifty miles in less than 5 hours

Yeah, I can't imagine it was much fun on the back of the tow truck.

Post by **amulet** » 09 Mar 2007, 12:48

oooh you bitch.

Post by **Russel** » 09 Mar 2007, 17:34

Well thats the only time he got better mpg than us.He never seems to mention his friendly beer swiging tow truck drivers.I wonder why,maybe a bit to friendly

Post by **HarryMann** » 09 Mar 2007, 19:02

I'm wanting to see the point to fills and corresp. odometer mileages, then I can arbitrate as to who was or wasn't cheating.. and get the real mpg plotted out to see the trends as you all trucked across continents..

.. but then, i suppose a full set of unexpurgated (

) figures is asking a bit too much

Post by **amulet** » 09 Mar 2007, 23:06

yup, afraid so, Jake and Russel were too busy whispering sweet nothings to each other on their CB's and I too preoccupied sampling Morocco's finest export...

Post by **HarryMann** » 09 Mar 2007, 23:13

Very respectable figures anyway, for all of you with those roofs, racks and that much gear on-board.

I think a mean mpg figure should go in the Wiki, against Expedition 1.9 Tdi Campers - fully loaded.

What should it be?