Diesel power vs Petrol and maybe auto vs manual

[MBManInKen]

Having a bad day Yesterday. Been raining all month and things just not going quite right at the moment...

Sorry to hear that, hope things will improve soon!

I agree 15% isn't marginal. I'm now a bit confused as the figures I have seen today are by mass (pound) also and work out to 6%.

But we need by volume, not by mass.

I've been trying to find a proper source for calorific values, but don't seem to be able to find the physics paper I was looking for .

Anyway, I should have realised that things are a bit more complicated as the exact composition and blends of fuels as sold at the pump are not exactly the same as using some pure derivative in a lab. For example, the following:

http://www.branz.co.nz/branzltd/publications/pdfs/SR118(2003).pdf

reports the net calorific value of diesel as 35.8 MJ/l and regular petrol as 32.2 MJ/l, a difference of only about 11%. Still a quite meaningful difference, although lower than I initially reported.

 

[crockers]

Sorry to hear that, hope things will improve soon!



But we need by volume, not by mass.

I've been trying to find a proper source for calorific values, but don't seem to be able to find the physics paper I was looking for .

Anyway, I should have realised that things are a bit more complicated as the exact composition and blends of fuels as sold at the pump are not exactly the same as using some pure derivative in a lab. For example, the following:

http://www.branz.co.nz/branzltd/publications/pdfs/SR118(2003).pdf

reports the net calorific value of diesel as 35.8 MJ/l and regular petrol as 32.2 MJ/l, a difference of only about 11%. Still a quite meaningful difference, although lower than I initially reported.

Yes but do you prefer automatics or manuals....lol:bannana:

 

[hawk20]

No. I think they do not want to produce a diesel engine.
what they are trying to propose is a different class for diesel and petrol runners with exactly the same rules in each class.

Yes, because they cannot keep up with the diesel.

 

[hawk20]

Interesting to compare the ML420 diesel and ML500 petrol. 0-62 and top speed are almost identical, the diesel is only around 4 mpg better on the combined rating and £1000 or so more expensive.

ML500 does 0-62 in 6.9secs. The diesel does it in 6.8secs. So very similar. But although 4mpg more miles per gallon doesn't sound much, we are talking an ML500 that does only 21.1 mpg combined against 25.5 mpg for the diesel. And that is 20% better. Or about £450 p.a for the average driver doing 12k miles per annum.

Given that the vast majority of cars/suvs with engines this big will not be bought by private buyers but usually paid for by a mixture of shareholders and taxpayers money (as the costs are tax deducted), I suppose a few hundred either way probably won't make much difference.

 

[glojo]

What about the automatic gearbox. We sometimes hear from the testosterone kings that it must be a manual be we are talking cars that are used on our highways and bye-ways, not the race track; although we are seeing more and more sophisticated automatic gearboxes on the race circuits.

John

 

[recycled]

What about the automatic gearbox. We sometimes hear from the testosterone kings that it must be a manual be we are talking cars that are used on our highways and bye-ways, not the race track; although we are seeing more and more sophisticated automatic gearboxes on the race circuits.

John

They are not automatics, they are sequentials or clutchless manuals.
Might have an eletronic clutch but it is still a manual. i.e no torque converters present.

 

[recycled]

Yes, because they cannot keep up with the diesel.

well obviously if you are competing against twin turbos and a bigger MPG and greater torque ,how can you keep up? imagine one pit stop while you have to do two or three

 

[glojo]

They are not automatics, they are sequentials or clutchless manuals.
Might have an eletronic clutch but it is still a manual. i.e no torque converters present.

Here we go again,
My Sprinter is exactly that, a clutchless manual, but to a lay person it is an automatic.

Customers hoping to specify Sprintshift, the new automated manual transmission available on the facelifted, common-rail diesel, Mercedes-Benz Sprinter....... What's more, it's only available on models with anti-lock brakes.

John

 

[recycled]

Well we are not lay people now john are we? .
The gearbox is not only about the stick shift. More components underneath as well.

 

[glojo]

Well we are not lay people now john are we? .
The gearbox is not only about the stick shift. More components underneath as well.

To me if the thing changes gear without the need to touch the gear lever then it does it automatically.

I an still in awe listening to the Sprinter double de clutching when changing gear, you can just hear the revs changing as first it goes into nuetral and then matching exactly the engines revs to the next gear, amazing technology.

John

 

[recycled]

I am not too sure with what merc are saying .Much like BMW smg which is not a sequential. as it does not go up gears in consecutive order. .
Anyway with them racing cars you still have to shift the gears manually. forward for upshifts and back for downshifts

 

[fredfloggle]

What about the automatic gearbox. We sometimes hear from the testosterone kings that it must be a manual be we are talking cars that are used on our highways and bye-ways, not the race track; although we are seeing more and more sophisticated automatic gearboxes on the race circuits.

John

I feel that many newer engines are much more suited to an autobox - particularly Merc diesels - actually I only bought an auto because of my wife , but I'm very glad I did.

 

[crockers]

Also - with a Merc...if one buys a manual it does limit its resale attraction as the majority of Mercedes are automatic.....

On a differing topic - when I visited Stuttgart (1998) they told us that the largest number of cars they sell are silver 22% followed by Obsidian Black (Is that the one with the green fleck?)....

Wonder if silver is still no 1..

 

[Benzowner]

What an interesting read and some useful technical bits(yawn) There have been many comments regarding economy of petrol vs diesel, performance of both and the best bit calorific values of diesel vs petrol, and the costs of a petrol car vs the equivalent diesel, if there is such a thing. But no one has mentioned the longevity of a diesel engine against a petrol. You often hear of diesels with mega mileage on the original engine, but a petrol engine normally requires head work at around 100 to 120k miles. My own personal favourite, if it was available, would be a 4 to 4.5 litre diesel auto.

 

[BTB 500]

My own personal favourite, if it was available, would be a 4 to 4.5 litre diesel auto.

That's exactly what the GL420 and ML420 are, wonder if they will put that engine in proper cars too?

 

[MBManInKen]

Yes but do you prefer automatics or manuals....lol:bannana:

See post number 2 .

 

[crockers]

See post number 2 .

well you actually said

"I was one of those ignorants who used to think autos are for boring elderly types

. How wrong I was!

"

Doesn't say which you prefer ....just that you now like autos....

 

[MBManInKen]

well you actually said

"I was one of those ignorants who used to think autos are for boring elderly types

. How wrong I was!

"

Doesn't say which you prefer ....just that you now like autos....

I prefer autos. Definitely!

 

[Dryce]

Well we are not lay people now john are we? .
The gearbox is not only about the stick shift. More components underneath as well.

So how do you classify a CVT (which may also have an automatic clutch that is used only when stationary)?

The Vauxhall and Renault robotised gearboxes (Easytronic and Quickshift) both simulate a traditional auto with torque converter when idling with the brake released. Many drivers wouldn't be able to tell the difference between a regular auto and Easytronic. (Quickshift uses different control method).

I don't think you can say that an auto gearbox has to have a torque converter and epicyclic gears.

An auto gearbox IMHO is one where there is optionally no user intervention required to change ratios in forward driving. Ie. stick in drive and use the brake pedal to stop and the accelerator to go.

The only non-traditional auto I have seen that is actively sold as opposed to just listed as an option is the VW/Audi DSG. And it is being sold as a feature. The manufacturers are much more coy about the robotised manuals.

 

[hawk20]

IMO to call the DSG gearbox a manual is a bit of a stretch of logic. In many ways it is more complex than the traditional auto and a lot is going on that is being operated by computer. I suppose we need a new name such as semi-automatic. It is true that the driver tells the DSG box when to change gear, but then so can he with Tiptronic and the latest flappy paddles. And even with kickdown. The old boundary lines all get a bit blurred as technology advances. A quick look at how it works helps to see how complex it is. The Wikipedia description is nice and clear: -
The Direct-Shift Gearbox (Direktschaltgetriebe) is a dual-clutch gearbox designed by BorgWarner and initially licensed to Volkswagen Group (which owns the Volkswagen, Audi, SEAT, Bentley, Bugatti, Lamborghini, and Škoda brands). By using two clutches, fast shifts can be achieved, and the torque converter of a regular automatic transmission is eliminated.
The engine drives two clutch packs. The outer clutch pack drives gears 1, 3, and 5 (and reverse). The inner clutch pack drives gears 2, 4, and 6. Instead of a standard large dry clutch, each clutch pack is a collection of four small wet interleaved clutch plates. Due to space constraints, the two clutch assemblies are concentric. Because the alternate clutch pack's gearing can be pre-selected (predictive shifts taking place while the other section is in use), un-powered time while shifting is avoided because power is simply switched from one gearbox to the other. The DSG takes about 8 milliseconds to upshift. The quoted time for upshifts is the time the wheels are completely non-powered.
Once the driver has selected D for drive, the transmission's first clutch is engaged and the first gear is selected on the first shaft. The driver instructs the car to accelerate. As the car accelerates, the transmission's computer lines up second gear on the second shaft (which is connected to the second clutch). Depending on the amount of power being requested by the driver (full throttle or normal driving) the car then upshifts. During this sequence, the DSG disengages the first clutch while engaging the second clutch (all power from the engine is now going through the second shaft), thus completing the shift sequence. This sequence happens in 8 ms, and there is practically no power loss.
Once the vehicle has shifted up to second gear, third gear is lined up and is pending. Once the time comes to shift, the second clutch disengages and the first clutch re-engages. Downshifting is similar to upshifting but in reverse order. The cars computer senses the car slowing down or more power required, and thus lines up a lower gear on one of the shafts not in use, and then completes the downshift.

Clearly DSG is very complex and a lot is being done ‘automatically’ for us in the background. If the makers let the computers already involved, decide when to change gear -- and let drivers choose a newly provided ‘D’-- then in my view it will be fully automatic. Whether or not it has a torque converter seems to me to be totally irrelevant.
The VW website and Wikipedia give lots of information
http://en.wikipedia.org/wiki/Direct-Shift_Gearbox