As diesel engines produce 50% more torque compared to similar sized petrols' and diesel torque curve is fairly constant over a long range (due to turbos), my guess is that diesel engines should work better with automatic transmission.
Now since I personally nevered owned either auto or diesel cars, I welcome comments from experts.
|
You asked a similar question on HJ comrade movilogo over a month ago,
for which you received 20 very good (IMO) replies.
I would add (FWIW) that you need to decide what particular make & model you are thinking about,
then hire that car for a few days to see how you get on with a diesel auto.
|
>> As diesel engines produce 50% more torque compared to similar sized petrols' ..........
As I've said before, the torque which matters is the torque at the driving wheels. Because a diesel has a higher gearing than a petrol the torque at the driving wheels is a lower proportion of the engine output torque than it is with a petrol. I just wish car manufacturers would quote torque at the driving wheels instead of (or as well as) the engine output torque.
|
"I just wish car manufacturers would quote torque at the driving wheels instead of (or as well as) the engine output torque. "
In which gear ?
And it still would be virtually meaningless without a test drive.
|
>> In which gear ?
In all gears if possible.
>> And it still would be virtually meaningless without a test drive.
It would have a lot more meaning than quoting engine output torque. If you knew the torque at the driving wheels you could do a direct comparison between petrol and diesel variants of the same model.
|
>>As I've said before, the torque which matters is the torque at the driving wheels.
>>Because a diesel has a higher gearing than a petrol the torque at the driving wheels is a
>>lower proportion of the engine output torque than it is with a petrol. I just wish car
>>manufacturers would quote torque at the driving wheels instead of (or as well as) the
>>engine output torque.
If this is true (I'm not arguing one way or the other here) why is it a diesel powered car wears out a pair of tyres on the driven wheels faster than petrol if the petrol has a greater turning force on the road ?
Last edited by: gmac on Thu 25 Mar 10 at 14:47
|
>> ......... why is
>> it a diesel powered car wears out a pair of tyres on the driven wheels
>> faster than petrol ............
If it's a front wheel drive car the extra weight of the diesel engine might have an effect on tyre wear.
|
Front engine, RWD diesels rip through rear tyres quicker than petrols.
I'm not sure diesel engines are that much heavier than equivalent petrol either. The cast iron block is rare if at all used these days.
Last edited by: gmac on Thu 25 Mar 10 at 15:13
|
>>As I've said before, the torque which matters is the torque at the driving wheels
...which is why most diesel powered cars in real world driving conditions feel more powerful
(unless you include V8 exotica etc.)
You have mentioned this several times L'Es, and each time you seem hamstrung by the 'theory' rather than practice.
What you're implying is that the better in-gear & general driveability of (most) diesels is somehow an illusion & that you're revealing this as some sort of myth by insisting that gearing dilutes any advantage.
If that's so then many thousands of drivers (and road testers & motoring journalists etc.) have been deceived over the years & diesels aren't really more torquey at all - what deluded fools we've all been!
|
>>I give up!
Assuming that's not meant as 'give in' - can you explain why diesels feel more & are more torquey to drive then - despite this stuff about gearing you bring up?
The simple fact is that despite (generally) higher overall gearing, diesels produce much more torque at the rear wheel than a petrol at similar revs - it's one of the characteristics
& no amount of gainsaying will make your petrol Focus the equivalent of a similarly endowed diesel of the similar bhp.
The gearing argument is misleading you I'm afraid to say!
|
>> >>I give up!
>>
>> Assuming that's not meant as 'give in' - can you explain why diesels feel more
>> & are more torquey to drive then - despite this stuff about gearing you bring
>> up?
In case L'esc really has given up, my interpretation is that he didn't actually say the torque at the wheels for a diesel is lower than that of a petrol, just "a lower proportion of the engine output torque". So if the diesel engine is twice as torquey, but the gearing as only a bit higher, then the diesel's torque at the wheels is still higher. But it would be nice to know how much higher, hence L'esc's request.
But I'm only guessing.
|
|
Now come on Focus, that's unnecessarily reasonable. How can we have a decent disagreement with you coming in and being so balanced and thoughtful?? ;-)
|
>> Now come on Focus that's unnecessarily reasonable. How can we have a decent disagreement with
>> you coming in and being so balanced and thoughtful?? ;-)
Apologies.
Anyone got Mr X's email? :-)
|
I think the power-to-weight ratio figure gives something along similar lines to what L'escargot is driving at.
To calculate torque at the wheel you would need rpm. When you apply rpm to torque you then get back to power.
To use some real world numbers.
1.6 HDi (1560 16v turbo)
Power 111PS
Torque 285Nm (Specific Torque 182.69Nm/litre)
Power-to-weight 72.13bhp/ton
1.6 (1598 16v)
Power 120PS
Torque 160Nm (Specific Torque 100.13Nm/litre)
Power-to-weight 79.73bhp/ton
Last edited by: gmac on Thu 25 Mar 10 at 20:06
|
>> In case L'esc really has given up my interpretation is that he didn't actually say
>> the torque at the wheels for a diesel is lower than that of a petrol
>> just "a lower proportion of the engine output torque".
Thanks, Focus. You've hit the nail on the head.
I've given up trying to explain, but I certainly haven't given in that I'm wrong on this subject. I put a lot of thought into composing my posts, usually altering them and fine-tuning them before I click on the Post Message button, so that they convey exactly what I mean and aren't in any way ambiguous or open to interpretation. Unfortunately, in so many cases, it's obvious from some of the replies that the same amount of thought hasn't gone into the reading of them.
Last edited by: L'escargot on Fri 26 Mar 10 at 09:17
|
L'esc, although you're right as far as it goes, I don't think you draw the correct conclusion from the facts.
I can understand your reasons for not liking diesels, even if I think most of them are misguided, but diesels (and forced induction petrol) DO drive with far more grunt at low revs than an 'equivalent' NA petrol. Every larger diesel I've driven (1.6 litre +) has pulled strongly from fairly low revs, whereas smaller petrol engines tend to really need revving to perform. You're right that the crank torque will be divided more in a particular gear, but it's normally so much higher to begin with that you're left with an advantage anyway.
A petrol turbo, or a larger petrol (2.5 litres +) will have exactly the same pleasant driving characteristics with lots of low rev 'shove', but for many of us the diesel is a better overall compromise just because of fuel economy. It's not better for your preferences, so you don't need to own one - the joy of living in a country where we're free to chose.
|
>> I can understand your reasons for not liking diesels
Has L'esc actually stated this? He might well have done, but I don't think it's in this thread, and my memory's not that great :)
|
|
You're right, not in this thread. Sorry Snail, didn't mean to put words in your mouth.
|
Well, L'Es, this is what you said & I read it very carefully:
As I've said before, the torque which matters is the torque at the driving wheels. Because a diesel has a higher gearing than a petrol the torque at the driving wheels is a lower proportion of the engine output torque than it is with a petrol.
Which matters not a jot because it's still (even with higher gearing) greater than comparable
petrol engines. It's entirely a moot point - TD engines major in torque & it's that characteristic
you feel when driving them - i.e. excellent in gear acceleration from lowish rpm.
So what is the point you're trying to make? I'd genuinely like to know.
|
>> You asked a similar question on HJ comrade movilogo over a month ago,
Well, I didn't ask this question! My question was bit different there.
www.honestjohn.co.uk/forum/post/index.htm?t=83248
:)
>> I just wish car manufacturers would quote torque at the driving wheels instead of (or as well as) the engine output torque
Wheel torque = Engine torque * final drive ratio * gear ratio
Last edited by: movilogo on Thu 25 Mar 10 at 15:23
|
The whole purpose of gearbox is to multiply torque on wheels. As diesels produce more torque, it should theoretically require less gears compared to petrol counterparts (which produces less torque and thus require more torque multiplication).
Electric motors produce constant torque at any RPM, so electric cars don't really require a gearbox (except final drive).
So, in theory, a flat torque producing diesel does not need so many gears as of petrol engines.
Some cars have same 4-speed gears (ie with same gear ratio) for both petrol and diesel variants - which is surprising.
That means petrol auto model need to frequently hunt for gears where as diesel auto can cruise comfortably without much change.
|
Petrol or diesel - which works better with autos? - Dog
