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Old 24th May 2011, 20:46   #61
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Default Re: Torque

Quote:
Originally Posted by Born2Slow View Post
One of the main reasons being diesels run quite lean even at full power. From whatever information I could get on the internet it is never richer than 1:20 as the diesels starts to smoke. So that is 30% less fuel compared to a petrol.
Great! Spot on, though I think its better to think in terms of air utilization, rather than fuel (partially) burned. The figure I remember is 20% air is unutilised.

The diesel (under steady state conditions) is effectively 20% smaller than the petrol.

Quote:
Originally Posted by vina View Post
Wouldn't a lean mix cause higher NOx emissions?
.....
What am I missing?
Not missing anything. That is one of the problems of the diesel.

Quote:
Originally Posted by pjbiju View Post
As far as the energy values are concerned, we need to remember that we buy diesel/petrol by volume. And the density of diesel is higher than petrol. So in effect we get more energy/kg for diesel.
I think you mean energy/ ltr.
But chemical reactions don't care whether you bought your fuel by the Kg or ltr.

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Sutripta
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Old 24th May 2011, 21:02   #62
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Default Re: Torque

Quote:
Originally Posted by Sutripta View Post
Great! Spot on, though I think its better to think in terms of air utilization, rather than fuel (partially) burned. The figure I remember is 20% air is unutilised.




The diesel (under steady state conditions) is effectively 20% smaller than the petrol.
any particular reason why this is so? Does this happen even with very low rpm engines where there is plenty of time available for combustion?

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Originally Posted by Sutripta View Post
Not missing anything. That is one of the problems of the diesel.
Is this a problem for diesel fuel or is this a problem for the diesel engine?

Also, with NOx emissions remaining high, how does the Euro-4/5 etc. work? Is there a mechanism to add some amount of fuel to the exhaust to take care of NOx in the catcon?
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Old 24th May 2011, 21:15   #63
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Default Re: Torque

Quote:
Originally Posted by Sutripta View Post
...
I think you mean energy/ ltr.
But chemical reactions don't care whether you bought your fuel by the Kg or ltr.

Regards
Sutripta
Yes, Energy/litre. And it matters since for a given amount (volume) of fuel injected/mixed, diesel has more energy that can be released by burning it. This is one of the reasons why diesel engines give us better mileage. Is it not?
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Old 24th May 2011, 21:21   #64
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Default Re: Torque

^^^
Hi,
Thats the problem:- you have to burn it. Properly.

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Sutripta
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Old 27th October 2011, 14:06   #65
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Default Petrol vs diesel torque characteristics & driving pattern

The debate between petrol-heads & diesel heads are unending. Both these engines have their own advantages & disadvantages. In this thread I have put the power & torque graphs of similar sized naturally aspirated petrol & CRDi with almost similar power ratings. Due to confidentiality issues I have removed the power/torque scale. Please don't ask anything regarding which engines, where etc.

Name:  Torque.bmp
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Size:  841.8 KB
Name:  Power.bmp
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Point 1:
Starting from 1000 RPM to all the upto 4500 Diesel rules. Diesels can produce almost twice the torque at certain RPMs. However in the lag zone 1000-1500 the petrol performs better (except a few lag free diesels) inspite of having the same torque. Why? Firstly the gearing of diesels are taller and then they also suffer from higher kerb weight. In real world Diesels pull 1.5X times harder after the lag zone.

Point 2:
Now where does the petrol score? Inspite of having shorter gearing, the petrol pulls longer. Thanks to its 6500 RPM which can even be enhanced to touch 7000 at your own risk. For example in the 2nd gear the diesel is fine till 65 but runs out of breath at 75. The petrol on the other hand will pull all the way till 85. Similar story extends in all gears.
So it is only post 4500 that petrol has the advantage. Not only because it delivers more torque in that region but also coupled with shorter gearing gives a tremendous advantage.

Point3:
Now in case of normal driving, where the RPM is limited in the range of 1000-4000 it is diesels by a huge a margin. Lets say for example on a diesel car you are traveling at a speed of 100kmph in 5th gear. It will have better acceleration than a petrol traveling in 4th gear. So to emulate the oil burner the petrol needs to fall back in the 3rd gear which IMHO is impractical.

Point 4:
The increase in acceleration on downshifting in a petrol is much more than a diesel. That is because in a diesel when you downshift, you get the advantage in gearing but you loose out on torque as diesels produce less torque at higher RPMs. In other words if you see the power graph of the diesel, it is almost same in the 3000-5000 Range. So if you are in 3000 RPM and you downshift to get in the 4000 RPM zone there is hardly an increase in the available power. In a petrol it is not so. It is only 5500 RPM there is no point downshifting.

At the end of the day it is a personal preference regarding petrol and diesels. I like the characteristics of the diesels and I stand by it.

Last edited by oxyzen : 27th October 2011 at 14:09.
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Old 27th October 2011, 14:42   #66
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Default Re: Petol vs diesel torque characteristics & driving pattern

Absolutely agree with you.

I'm in favor with the diesel engine characteristics. I have driven both the engines extensively and I fee the drivability is better in the diesels when compared to the petrol. Yes, I know the turbo lag with kick in and irritate which doesnt happen with the petrol but guys we can use the gearing wisely to stay in the power band of the diesels.

Accelearyion once in turo range is fantastic and i guess the petrol engine cant give that feel of the kick while accelerating.
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Old 27th October 2011, 15:06   #67
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Default Re: Petol vs diesel torque characteristics & driving pattern

Please check realistic performance curves given here in:
http://www.team-bhp.com/forum/techni...ft-petrol.html (Torque & Power Graph of Indian Cars - Starting with Maruti Suzuki Swift Petrol)
http://www.team-bhp.com/forum/4x4-te...ml#post2243806 (DDIS in 1990 Gypsy sj410, will it work?)

Things have been discussed in detail. The poower band of diesels is much narrower I believe, going by these images. The flat torque curve and the near linear power delivery makes petrol engines highly predictable. This is not the case with diesels. There is nothing called linear power delivery in diesels. Spirited driving requires petrol engines.
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Old 27th October 2011, 15:28   #68
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Default Re: Petol vs diesel torque characteristics & driving pattern

Diesel is good for people who dont like to press accelerator hard and worry about fuel efficiency (meaning for people who dont rev their engine much). The pulling power at the lower rpms make them happy.

Whereas you can enjoy a petrol engine only if you forget about the fuel efficiency and push the engine rpm to the higher side.
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Old 27th October 2011, 15:58   #69
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Default Re: Petol vs diesel torque characteristics & driving pattern

Thanks for the info but these graphs here are also realistic with points take at every 250RPM interval.

Quote:
Originally Posted by specialist1968 View Post
Things have been discussed in detail. The poower band of diesels is much narrower I believe, going by these images.
Power band in this diesel is 3000-5000. For petrol it is 5500-6500. I think powerband of diesel is wider. Please understand you need to keep the engine in the powerband to extract the maximum performance. For a petrol it is too high & too narrow.

Quote:
Originally Posted by specialist1968 View Post
The flat torque curve and the near linear power delivery makes petrol engines highly predictable. This is not the case with diesels. There is nothing called linear power delivery in diesels.
Agreed.

Quote:
Originally Posted by specialist1968 View Post
Spirited driving requires petrol engines.
Spirited driving needs a good driver regardless of the make (petrol / diesel).
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Old 27th October 2011, 17:28   #70
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Default Re: Petol vs diesel torque characteristics & driving pattern

@Oxyzen: The things you are trying to state have already been covered/discussed on an existing thread http://www.team-bhp.com/forum/techni...l-engines.html (Do Diesel engines generate more Torque than Petrol engines?)

Cheers
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Old 27th October 2011, 21:13   #71
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Default Re: Petol vs diesel torque characteristics & driving pattern

It depends on individual preferences, however, I would like to put down a couple of scenarios :

1) Diesel. Lets say a diesel sedan/SUV like Fortuner/Vento is cruising at 100 kmph on highway. They are right there in powerband, so overtaking = flexing right foot. And if the other vehicle also starts to accelerate, its simple, flex the right foot further.
Very practical and effortless. Takes stress out of driving on highways without dividers. Reasonably safe overtaking is possible as one spends less time on the wrong side.

Now a petrol car like, City/Fiesta ( 1.6 ). Car is doing 100 kmph. A quick overtaking would require a downshift ( or may be two ) but it can pull long so overtaking multiple cars vehicles in right gear can be done. Still not as effortless as diesel, but worth the effort ( especially for an enthusiast ).

2) City scenario.
Diesel. Turbo lag + higher gearing + heavyniess ( compared to petrol ) does not make it that nice.
Also the narrower powerband can play spoilsport. You want to close that gap near a junction ( traffic island/circle ). The vehicles ahead also are accelrating a bit. Here you might have the shove, but the powerband which is narrow will dictate a gearshift which is not convenient.

Here petrol wins hands down. you can easily manage to close the gap due to lack of turbo lag as well as bigger powerband.

To summarize :
City
Petrol :
Larger powerband.
Relatively lighter car with lighter controls ( mainly clutch )
Less gearshifts required when immediate acceleration required.

Highway
Diesel :
Great torque
Easy overtaking
Heavier car ( relative to petrol ), stiffer suspension ( due to heavier engine ), and is less risky than petrol in water logged areas.

For city, I would prefer a petrol, for highway a diesel.
But heart in heart, I love petrols. Its not matter of always being the quickest, but working hard through engine and gearbox gives higher satisfaction.
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Old 28th October 2011, 08:30   #72
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Default Re: Petol vs diesel torque characteristics & driving pattern

I got a diesel, reason being : Fuel Economy.

Had fuel been cheaper, I wouldn't haven even looked at a diesel or a turbo petrol. Direct Injection petrol which revs to the sky would do for me. Practicality be damned.
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Old 28th October 2011, 09:56   #73
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Default Re: Petol vs diesel torque characteristics & driving pattern

First of all a 1000 low end for diesel superiority is a bit too low. Diesel do not get much of a turbo assist below 1800rpm or so.

Secondly, with a lower redline I will expect Diesels to have a taller gearing. So the straight conversion has to be tempered with this.

Finally, it is accepted that petrol engines have a more linear power delivery.
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Old 28th October 2011, 11:19   #74
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Default Re: Petol vs diesel torque characteristics & driving pattern

I love diesels, I have always preferred driving diesels over petrols. Reasons -

1) Low end torque makes its driving in city soooooo easy! I dont like to rev, I like effortless performance. In diesels, the torque at lower rpms means you can shift at 2500 rpm & still accelerate.

2) Push in the back feeling when turbo kicks in!

3) Of course, fuel efficiency!
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Old 28th October 2011, 11:35   #75
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Default Re: Petol vs diesel torque characteristics & driving pattern

I like diesels on highway and love petrols in the city. When you slow down for a speed breaker with a little incline, I just hate my Vento, you have to be in 1st gear. It will take eternity in 2nd gear. A petrol would rev quickly.
If petrol was cheaper and turbos were available, I wouldnt have bought a diesel.
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