Diesel power vs Petrol and maybe auto vs manual

[recycled]

Good so why do diesels need turbos then?i mean every modern diesel car has got one or two bolted to it?

 

[R2e]

As a longtime petrolhead recently converted to diesel, I have to say the thing I like about the C270CDI is driving it! It has more performance than my previous E240 (W210), but the delivery is totally different and somehow more relaxing. Don't know why, maybe it's the massive amount of torque. The one strange thing is when resuming cruise on the cruise control, when it hits the set speed the fuel is obviously cut in a much more abrupt way than happens with the petrol engine. Catches me out every time.

I have a little game with myself on the M25 (early morning trips) to average over 60mph from home to office, a distance of 61 miles, for the best fuel consumption showing on the trip. So far the best I've done the 61 miles is at an average of 64mph (includes non motorway driving each end of course) and 57.1 mpg showing at the end of the trip.

 

[Dieselman]

Good so why do diesels need turbos then?i mean every modern diesel car has got one or two bolted to it?

As well as for the obvious reason of cramming more air a turbo also reduces cylinder temps due to greater airflow and reduces emissions.

Don't forget the engine runs super lean-burn under all but full load conditions.

 

[Diesel Benz]

Good so why do diesels need turbos then?i mean every modern diesel car has got one or two bolted to it?

To me both the diesel and the gassed need a turbo (one or more), you just cannot find it from a gasser that often. A gasser with a turbo has the disadvantage that the gasser runs on constant fuel-to-air-ratio which forces the turbo run very slow at low loads. The diesel fills just as much air at all loads which allows the turbo to reach good boost quicker (although there always is some turbo lag).

I start with supporting the view that everybody is allowed to have their own preferences but some statements so far were not quite objective.

Diesel has the advantage of higher thermal efficiency and it improves with a well designed turbo (all big trucks run a turbo diesel, why not a naturally aspirated gasser). A gasser has the advantage that it can be built to run at higher RPM, specifically when compared to direct injection diesels. This allows higher max power (power is 2*pi*rpm*torque) while diesel fuel does not burn fast enough.

But why should we compare a turbo diesel to a turbo gasser or same sized gassers etc? I would start with the same cost (where it is better to remain comparing same body types). I'd take an example of E320CDI against E350 gasser. The German price is 49.206 euros against 49.504 for the gasser. These cars are equipped exactly similarly. So what do you get for the money (forget the 300 euro higher price for the gasser)?

- Both reach 250 km/h top speed,
- 0-100 km/h acceleration 6.8 versus 6.9 seconds for the gasser,
- fuel consumption average 7.3-7.6 against 9.7-10.2 l/100 km (meaning the CDI has better mileage, if your used to mpg figures).

The peak power is higher on the gasser by 200 versus 165 kW, so why the acceleration gain for the CDI? It is from the average power over the applied engine rpm range at the acceleration case. This of course comes from the huge torque at low RPM. True that torque alone does not mean too much anything, the same for peak engine power. The average engine power over the RPM range is important, the diesel characteristics also make every day driving a pleasure.

Considering automatic transmission, it may be true that MB manual boxes have better longevity than auto boxes, the problems with early 7G-tronic and some cooler leaks on early W211s may give a wrong view though. But why do you want to have a radio and a CD player in the car when those can break?

Auto box cannot reach the economy of a manual box but for an average driver it gives not only comfort but you will be able to accelerate to the figures given for the car while doing that with the manual box asks for rally driver capabilities and hard use. The auto box is a bit more expensive (specifically as long as it is as rare as in Europe, in US cars they are often the same price, if not the opposite) Quite clear benefits for each one and then pretty much a personal view on the preferred emphasis.

Clearly local factors need to be taken into account, the tax on diesel fuel versus gas varies etc.

 

[MBManInKen]

Can I just say that I have no complaints about the amount of torque available in my S500?

I'll say it again: why can't we let everyone have their own preferences without religious wars over which is "best"?

I like the immense smoothness and silence of my V8, with a broad range of rpms available for acceleration and heaps of torque across a broad range.

I don't like diesel smells (and stains) and despite the fact that modern diesels have come a very long way, I have not yet seen one that is as silent, vibration-free and smooth as the same car with a large petrol engine.

I tried my own car in diesel version: very nice, but for me no match to the V8.

Now, this is what I like, clearly other people might have quite different preferences and requirements and good on them I say!

 

[MBManInKen]

i might consider an auto but i have two merc autos and they were both unreliable

I was surprised by this - I thought Merc auto boxes had a good reputation? Am I missing something?

While not statistically relevant, I am on my 4th Merc auto and never had the slightest problem with any of them, I'm happy to report.

 

[Diesel Benz]

I was surprised by this - I thought Merc auto boxes had a good reputation? Am I missing something?

While not statistically relevant, I am on my 4th Merc auto and never had the slightest problem with any of them, I'm happy to report.

I wouldn't be too sure about the longevity of manual boxes either. One also should take into account that all high power engines only have the auto box option. How would a manual survive in a 65AMG? Just about all taxi cars have an auto box, one should take into account the average mileage too when comparing. Of course each one can be made to last. Still I would not say one being simply better than the other.

 

[MBManInKen]

You also say petrol engines are faster. Generally true for top speed due to revving higher whilst running lower gear ratios to act as a TORQUE multiplier, but for a given power output they always accellerate more slowly due to lack of TORQUE.

I think the main problem with the argument about torque is that it ignores the simple fact that while diesel is more energy-rich in terms of energy per volume, diesel engines have smaller workable rpm ranges available to them.

So, while the petrol torque peak is lower, for a similar engine it is usually spread over a greater rev range. Therefore, less gear changes and production of high levels of torque for longer for the petrol car. The focus of diesel owner on peak torque is as much of a red herring as the focus of petrol owners on horse power in my humble opinion.

And as Recycled says, there is a bit too much apples with oranges comparison going on in these types of debate in terms of engine sizes and aspiration type.

 

[recycled]

I think the autos do not just like me. The C180 classic auto was by far the most troublesome of the lot. It kept not moving, especially in the morning, took an age to remember whether it was in reverse or drive and just generally made noises of all kinds and violenly vibrated at standstill. i just gave up.
My brothers W201 gearbox gave him even more problems.
But his E36 an E39 BMW tiptronics were spot on.
Maybe i had a bad box, then again. BMW and even ferrari still offer manuals for their powerful V8 engines.
Why can merc not give us that option in the AMG.
The corvette is a 7.0L V8 with a manual so big engines is no excuse
A manual can survive in an 65AMG

 

[Dieselman]

while diesel is more energy-rich in terms of energy per volume,

Marginal.

diesel engines have smaller workable rpm ranges available to them.

But much longer gearing to compensate.

So, while the petrol torque peak is lower, for a similar engine it is usually spread over a greater rev range. Therefore, less gear changes and production of high levels of torque for longer for the petrol car.

Less gear changes in a diesel, not more. Strangely due to the longer gear ratios due to the greater available torque.

And as Recycled says, there is a bit too much apples with oranges comparison going on in these types of debate in terms of engine sizes and aspiration type.

Well, in that case keep it simple.
Use the calorific value of the fuel consumed, ignore all other aspects such as the volume of injested air, although for a given engine size a petrol injests more air due to running at higher rpm due to the shorter gear ratios and higher available rev limit.

Of course your V8 is smoother than a straight six diesel, it's got two more cylinders. Who said compare like with like.? Try a V8 diesel fitted with piezo injectors, you might be surprised.

I personally only have a preferrence, I don't expect your preferrence to be the same, but we need to stick to thermodynamic facts.

 

[Diesel Benz]

So, while the petrol torque peak is lower, for a similar engine it is usually spread over a greater rev range. Therefore, less gear changes and production of high levels of torque for longer for the petrol car. The focus of diesel owner on peak torque is as much of a red herring as the focus of petrol owners on horse power in my humble opinion.

MB should actually use the gasser RPM gauge on a diesel car too, with the same scale and then put below x1000/1.5. Then one would see a similar available range of rpm and torque.

The gasser may run roughly up to 7500 rmp if a diesel runs to 4500, that is already more than a factor of 1.5. The point is that it makes no difference if the available engine rpm range is from 1000 to 3000 or 2000 to 6000 (not typical figures but simple to calculate).

Like mentioned, the power at a given RPM makes a difference. Even if MBManInKen claims the gasser having larger available torque range, it is actually the opposite in my example of E320CDI versus E350, same priced cars, if you take the range in the form that matters, the ratio, not the absolute RPM range (in the example, one has a range of 2000 while the other has a range of 5000 but the first one would have twice longer rear differential ratio). Assume both engines had constant torque and the same peak power, they would have the exact same acceleration performance!

Now on the V8 gassers, to me it looks like those truly have a wide torque range. Don't know if it is because of lower tuning than the smaller gassers typically have or is it something else.

You may guess I'm favouring diesels a bit but we have a gasser in the family too. And to promote gassers further, one can say that for the highest powered engines there is no other option. Also the diesel noise is still there even with the current piezo nozzles, specifically noticeable from outside. But in almost all aspects the modern diesels are totally different from the old ones and the experience from the old ones may give quite wrong impression. Not necessarily always for the new engines, through added complexity I doubt the same longevity is possible as from the old W123 diesels or similar.

 

[Bobby Dazzler]

BMW and even ferrari still offer manuals for their powerful V8 engines.
Why can merc not give us that option in the AMG.

This almost certainly isn't the only reason, but will no doubt account for the lack of a manual AMG in part...

The high-power BMW and Ferrari engines are high-revving but fairly low on torque compared to AMG engines, and both have traditionally sold more manuals than autos.

MB on the other hand traditionally sell more autos than manuals, and AMG engines tend to be relatively low revving* but produce more torque.

Therefore the cost of producing a manual gearbox that can reliabley deal with the torque produced by an AMG's is probably difficult to justify.

The latest AMG 6.2 V8 engines rev higher than the previous generation V8s, but IIRC do not rev quite as high as the MPower 4.0 V8 and 5.0 V10 engines.

The corvette is a 7.0L V8 with a manual so big engines is no excuse

It's probably out of a truck!!

 

[timskemp]

Simple rule I've built up is that two similar size and weight cars driven in the same manner, a diesel will use 75% of the fuel of the petrol one. I'm averaging about 34mpg overall in the E class, my S60 (2.0T petrol, 180bhp ish) averaged 24mpg overall. The A170CDi I had averaged about 38mpg.

The biggest influence on economy is the driver.

 

[MBManInKen]

Ooh, what a grumpy answer!

Marginal.

I said by volume. Diesel weighs more than petrol. By mass, petrol actually does better (marginally), but by volume diesel has about 15% more energy than petrol. As it is the volume of fuel in the engine that is of relevance, this difference is both (a) relevant and (b) significant.

But much longer gearing to compensate.

Less gear changes in a diesel, not more. Strangely due to the longer gear ratios due to the greater available torque.

Sorry, that is not what I was talking about at all. I was referring to the fact that you can sustain a given gear longer in a petrol car, which is precisely the reason why they end up with higher horse power output (one of those red herrings I mentioned) and higher top speeds.

Well, in that case keep it simple.

That was what I attempted to do, but clearly still it confused people .

Of course your V8 is smoother than a straight six diesel, it's got two more cylinders. Who said compare like with like.? Try a V8 diesel fitted with piezo injectors, you might be surprised.

Well, I did not advocate anywhere comparing my V8 to a straight six diesel, did I?

The point is that the s-class is available in two 6 cyls, one petrol, one diesel. Of those two I still prefer the petrol. Furthermore, there is no V8 diesel available for the car model of my choice - a fairly typical situation for the type of car I drive.

In addition, having the choice, I prefer the V8 5L over the 6 cyl petrol car. Can I have my own preference, please? No where did I say that what works for me should work for others.

I personally only have a preferrence, I don't expect your preferrence to be the same, but we need to stick to thermodynamic facts.

Right, so where did I get my facts wrong, please? I found this a rather un-called for comment TBH.

 

[recycled]

.



It's probably out of a truck!!

May be out of a truck but it is very fast and it works.
The carrera GT one is not from a truck though

 

[crockers]

I am very happy with my C320 cdi auto - its smooth - effortless and I get around 40 mpg... If I floor it it leaves most others for dead --

I would love a go in a 535 tdi - but if I had to choose between a 535 tdi and a M5 - with someone else paying all running costs I would go for the Petrol - M5...

:bannana:

 

[crockers]

auto - v manual

I didn't actually mention this part of the debate..

I used to always love manuals over autos - but after having had autos now for 10+ years - I wouldnt want to go back to a manual...

I have had autos with buttons etc to change gears on the steering wheel and have found after a few weeks I would just leave it in D and not bother...

The only auto I have driven that cries out to have the paddles flapped is a golf gti with DSG - now that is the best automatic I have driven - I only wish that VWs patend runs out soon or other companies can liscense it...A 320 CDi with DSG --- mmm that may be the best of both worlds

BUT....this is all my personal opinion and everyone is entitled to their own opinions - lets face it wouldnt it be an awful world if we all liked the same thing...

 

[janner]

To me both the diesel and the gassed need a turbo (one or more), you just cannot find it from a gasser that often.

Watch this space...

 

[recycled]

I didn't actually mention this part of the debate..

I used to always love manuals over autos - but after having had autos now for 10+ years - I wouldnt want to go back to a manual...

I have had autos with buttons etc to change gears on the steering wheel and have found after a few weeks I would just leave it in D and not bother...

The only auto I have driven that cries out to have the paddles flapped is a golf gti with DSG - now that is the best automatic I have driven - I only wish that VWs patend runs out soon or other companies can liscense it...A 320 CDi with DSG --- mmm that may be the best of both worlds

BUT....this is all my personal opinion and everyone is entitled to their own opinions - lets face it wouldnt it be an awful world if we all liked the same thing...

IIRC the DSG is actually a manual in that it has an electronic clutch.
Tiptronics are autos trying to be manuals.
Sequentials/semiautomatics are manuals trying to be autos.

The main difference between one and the other is one has a clutch and the other uses a torque converter so it is irrelevant whether it is the driver or the electronics that depress a clutch a clutch present makes it a manual box.

 

[glojo]

Watch this space...

I trust all those bits will be assembled in the right places, on the right vehicle and be at the right place in mid August?

John