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Old 29th October 2011, 12:08   #91
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Default Re: Petrol vs diesel torque characteristics & driving pattern

Oh I am so sorry I totally forgot we have a car called CIVIC. Yes my friend it might be going to 100, and people may go up to 85 on 1.2l motors. But then you will be on revlimiter,isnt it?
And your next gear is likely to miss that green zone .
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Old 29th October 2011, 20:29   #92
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

Actually you can tune any engine to give (at extremes of tuning range)

. Flat torque over 90% of the RPM range, with lower peak torque
. Peaked torque at the upper range of RPM.

In general for a particular engine configuration; be it petrol or diesel; the area under the torque curve is more or less constant. So if you consider a normal tune where the peak torque is say, 100 over 50% of the RPM, you can get a torque of 50 over 90% or a torque of 200 over 10% of the range (figures are arbitrary, but detailed calculation can be made). So choose what you want and tune the engine accordingly.

For street cars it is preferable to have a flat torque curve over 70% RPM band, while for performance cars you peak it in a 20% range. For extreme performance you can even achieve higher peak torque in a 10% band, but that will require 2 to 3 times the number of gears to remain in the torque band.

Coming back to the thread heading. Diesel engines were used mostly for heavy transportation, hence were designed to have extremely high torque at the lower end. Keeping in power band is no problem - just add more gears. Some big trucks have upto 18 gears compared to normal 5 in our cars. The torque can be so high that in low speed ship engines the propeller is driven directly with no gear box! Mind you at one time diesel engines were designed and used for aircraft, but were soon overtaken by high speed petrol engines once high octane fuel became available.

With modern trend of improving speed and FE at high speed, diesels are mimicking petrols. They have good FE, higher RPM range and bad low end torque. This is offset by turbo charging the diesel to give more power, in tune with stated aim of high speed travel. Older diesel designs had extremely high low end torque, but no RPM range worth speaking of. This is why small 40 to 60 HP engines could drag a 2 ton vehicle through mud and sand where modern 150+HP engines end up with a clutch burn. The only saving grace for diesels is that they are inherently more efficient so offer better FE in all states of tuning.
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Old 29th October 2011, 23:53   #93
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Default Re: Petrol vs diesel torque characteristics & driving pattern

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Originally Posted by dustom_99 View Post
AS PER GRAPH MY FRIEND,AS PER GRAPH! Which hapns to be your graph,without any public data.
Weird privacy policy that stops you from disclosing data which is already available all over net (gear ratios,tyre size,engine specs).
So what is the problem then. If it is available all over the net. By bother about this one?

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Originally Posted by dustom_99 View Post
I dont know where did you get data from,and I agree some cars will make it to 80 may be even 85,but beyond 70 it is quite a labour even for a petrol mill(of usual cars),and they take lot longer to get to 80 in 2nd as compared to the situation where you pass the sweet spot just enough that the next gear will fall rite onto the sweet spot or just below it.
Try doing a 70-85 in 2nd gear and then in 3rd gear & then let me know.
What do you think is the sweet point? Max torque point? You are wrong. The sweetest point is the max power point.
There was a thread in Team-BHP regarding Polo cup where the speed, RPM and several other parameters were logged. The polo was constantly in the 4000RPM range although its peak torque is in 1500-2500 Range.

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Originally Posted by dustom_99 View Post
And I could not understand the policy behind plotin acc v1000?whats that for?
Its acceleration vs speed graph. It says which gear you should be in at what speeds to extract the most.
It gives an indication of in gear acceleration in petrol vs diesel.
And lot more.

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Originally Posted by dot View Post
Looking at these plots, if I were the driver of the red car and to take advantage of the peak torque and acceleration, I would be in 3rd gear by the time speed reached 50kmph.
But (at 50kmph), the graph shows maximum acceleration is possible in 2nd gear. So why not stick to 2nd. Yes you produce more torque in 3rd the higher gearing means less is transmitted to the wheels.

There is a balance between producing higher torque & extracting more mechanical advantage (lower gearing). My aim in here is to find the balance. I would shift to 3rd at 70. Beyond which the 3rd gear shows better acceleration than 2nd.

Quote:
Originally Posted by dot View Post
In the blue car I would be in 2nd and will continue in that gear till speed reaches 80 odd.
Spot on!

Last edited by oxyzen : 29th October 2011 at 23:54.
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Old 30th October 2011, 05:56   #94
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Default Re: Petrol vs diesel torque characteristics & driving pattern

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Originally Posted by dustom_99 View Post
And your next gear is likely to miss that green zone .
I agree, you hit the rev limiter by pushing cars in 2nd gear when speeds like 80-100 are achieved. However, when the next gear is engaged, the rpm automatically falls back to the peak torque zone.
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Old 30th October 2011, 09:24   #95
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

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Originally Posted by Aroy View Post
In general for a particular engine configuration; be it petrol or diesel; the area under the torque curve is more or less constant. So if you consider a normal tune where the peak torque is say, 100 over 50% of the RPM, you can get a torque of 50 over 90% or a torque of 200 over 10% of the range (figures are arbitrary, but detailed calculation can be made). So choose what you want and tune the engine accordingly.

For street cars it is preferable to have a flat torque curve over 70% RPM band, while for performance cars you peak it in a 20% range. For extreme performance you can even achieve higher peak torque in a 10% band, but that will require 2 to 3 times the number of gears to remain in the torque band.

The torque can be so high that in low speed ship engines the propeller is driven directly with no gear box! Mind you at one time diesel engines were designed and used for aircraft, but were soon overtaken by high speed petrol engines once high octane fuel became available.

With modern trend of improving speed and FE at high speed, diesels are mimicking petrols. They have good FE, higher RPM range and bad low end torque. This is offset by turbo charging the diesel to give more power, in tune with stated aim of high speed travel. Older diesel designs had extremely high low end torque, but no RPM range worth speaking of. This is why small 40 to 60 HP engines could drag a 2 ton vehicle through mud and sand where modern 150+HP engines end up with a clutch burn. The only saving grace for diesels is that they are inherently more efficient so offer better FE in all states of tuning.

Agreed, but wont the tuning affect gearing ? And if the engine is tuned for higher rpm performance, wont it reduce the overall life of engine ? Lets say a Cruze is tuned to have all its grunt above 2500 rpm till 5000 rpm. But this would obviously reduce engine life as more often than not, people would be driving a in one gear lower than normal to be in the power band.

Forget ships, IIRC, even trains have so much torque that they do not have gearboxes. This is what I have read which could be wrong, so do correct me if I have made a mistake.

I am bringing in the diesels only.
In interests of higher FE at higher speeds, etc. the grunt is more or less skewed towards higher end of rpm. What this has lead to ? Low bottom end torque and can we say spoiled engine life ?
Earlier units which were so strong in low ends were known to cross 3-4 lakh kms. The new modern CRDi units are yet to earn such a solid reputation. Yes, the stress of such high pressure injections and turbo do play a role.

But what exactly have we achieved ? Low end torque gone, higher no. gearshifts at lower speeds, relatively reduced engine life ( and clutch too ) for mainly high speed FE. Not a good trade off IMO.

Last edited by aaggoswami : 30th October 2011 at 09:27.
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Old 30th October 2011, 11:43   #96
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Default Re: Petrol vs diesel torque characteristics & driving pattern

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Originally Posted by oxyzen View Post
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.

Attachment 833050
Attachment 833051
I'll mention a recent example, we went for a Sunday morning drive, A Civic MT with K&N and my Laura 140 TDI DSG. On all counts of performance the Laura out performed the Civic, be it acceleration or roll on (the Diesel surely in not even close to the joy of high revving a Petrol engine). The NVH is not much of an issue with latest Diesels, here the Laura was quieter because the Civic had the K&N filter. My point is Diesels are closing the gap real fast, the latest Diesels have much wider power band with power arriving as low as 1500rpm for most German Diesels and they now feel happier revving close to redline. Add to that the bonus of FE and pleasure of overtaking at 1700rpm in 6th gear with no downshifting and still managing great combination of acceleration and FE.

I am a Dieselhead for life, for me as an all in one package for daily usage nothing beats the combination of a smart Automatic Transmission mated to a Modern CRDi engine. Getting FE close to budget Diesel hatchbacks and still possessing enough grunt outperform the Petrols with the convenience of AT.

Last edited by .anshuman : 30th October 2011 at 11:44.
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Old 30th October 2011, 13:46   #97
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

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Originally Posted by aaggoswami View Post
even trains have so much torque that they do not have gearboxes. This is what I have read which could be wrong, so do correct me if I have made a mistake.
Interesting thread.

BTW, you are right in the sense locomotives do not have gearboxes but that's not because of high torque which they obviously do possess. It is because their engines (called prime movers) are mated to a generator (alternator) that in turn produces electricity to drive the traction motors. The motors are coupled to the axles thru a reduction gear. As to why diesel-electric locos have generators and not clutch and gearboxes like automobiles read the IRFCA site which is fascinating.

Cheers!

Last edited by R2D2 : 30th October 2011 at 13:48.
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Old 30th October 2011, 13:55   #98
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

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Originally Posted by aaggoswami View Post
Agreed, but wont the tuning affect gearing ? And if the engine is tuned for higher rpm performance, wont it reduce the overall life of engine ? Lets say a Cruze is tuned to have all its grunt above 2500 rpm till 5000 rpm. But this would obviously reduce engine life as more often than not, people would be driving a in one gear lower than normal to be in the power band.
The idea was to share the knowledge that an engine may be tuned to suit your requirements, that is it is easy to get good low end torque at the expense of high speed performance, or go the other way and sacrifice the low end torque for better high speed performance. One advantage of broader torque curve is that you need less number of gears - the Petrol Geep had only three. Another case where good low end torque is not limited to diesels. Similar is the case with the original Jonga petrol engine.

Low speed diesels have a phenomenal life. For a transporter whose vehicle will cover 5-10L km in its life time, long life of the engine is important. For a personal vehicle, where even 1L km in an ownership period of 5 years (before changing the vehicle) is rarely achieved, of what use is long life. Hence higher FE and higher speeds are preferred. Yes today's designs have resulted in low torque at the lower end, but most users prefer high FE over drivability anyway.

This reminds me of comparison between the Pajero and Fortuner. Fortuner scored in most areas, but was beaten soundly in the low end torque, a major plus point if you are going to traverse bad muddy roads regularly. The Pajero also scored on hill climbing abilities, again a tribute to its low end torque.
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Old 30th October 2011, 14:24   #99
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

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Originally Posted by Aroy View Post
This reminds me of comparison between the Pajero and Fortuner. Fortuner scored in most areas, but was beaten soundly in the low end torque, a major plus point if you are going to traverse bad muddy roads regularly. The Pajero also scored on hill climbing abilities, again a tribute to its low end torque.
Pajero uses shorter gearing to compensate for the lack of grunt. Fortuner in comparison has way more grunt from as low as 1200rpm but you may not feel it due to tall gearing. I always tend to drive the Fortuner in a gear higher compared to any other car, just because of the super low end.
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Old 30th October 2011, 15:00   #100
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

My two cents:
Since diesel engines (no spark plug) have to compress air more (to ignite the diesel fuel) than petrol engines, they have a longer travel (read stroke) of the piston, which in turns means a bigger crankshaft, since the connecting rod (between piston and crank) determines it.
Now if the piston exerts same force in either cases (petrol & diesel), the diesel one will exert more torque on the crankshaft than the petrol one.
This is just the most fundamental and basic difference. There are however trade offs.
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Old 30th October 2011, 15:18   #101
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

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Originally Posted by aaggoswami View Post
And if the engine is tuned for higher rpm performance, wont it reduce the overall life of engine ?
Engine life dies not exactly depend on how it is tuned. It depends on how it is run. Running the engine at a gigher RPM definately eats up the usable life. Not only because you are clocking more revs but also because the operating condition at higher RPMs are more severe.

But dont sweat. The Vehicles have a lot less life than the engines. Other parts may give up including engine auxillaries. I have not noticed significant wear even after running the modern engines at maximum RPM upto 500hrs.

But yes higher RPM results into quicker detoriation.
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Old 30th October 2011, 15:23   #102
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

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Originally Posted by aaggoswami View Post
Forget ships, IIRC, even trains have so much torque that they do not have gearboxes. This is what I have read which could be wrong, so do correct me if I have made a mistake.
Most trains have a Diesel-Electric system
The Diesel engine is actually a big generator. It generates electricity which in turn runs a motor.

An excellent thread: http://www.team-bhp.com/forum/commer...ve-diesel.html

Last edited by bblost : 30th October 2011 at 15:25.
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Old 31st October 2011, 00:13   #103
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

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Originally Posted by Valkyrie View Post
My two cents:
Since diesel engines (no spark plug) have to compress air more (to ignite the diesel fuel) than petrol engines, they have a longer travel (read stroke) of the piston, which in turns means a bigger crankshaft, since the connecting rod (between piston and crank) determines it.
Now if the piston exerts same force in either cases (petrol & diesel), the diesel one will exert more torque on the crankshaft than the petrol one.
This is just the most fundamental and basic difference. There are however trade offs.
Cant agree.
It is not so simple to increase torque just by changing the bore/stroke ratio.
The basic assumption that is flawed here is that pistons will not exert the same force in petrol & diesels. You can hypothetically assume the mean effective pressure is same. And then the Petrol with its larger cross-sectional area will exert more force. But the diesel will compensate like you mentioned here.
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Old 31st October 2011, 08:31   #104
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

@oxyzen:
That's why I said, its a fundamental difference.
For the same bore, this is applicable, and if you look at the heat per kg values of petrol and diesel, they both are same.
And diesel engines are slower too, because of the inevitable high mass of the engine block to sustain such high pressure. Hence they are not as free revving as their petrol counterparts. So if a petrol gives you high torque at a specific rpm, the diesel will give the same or more at a lower rpm.
While I am no expert at engines, all this is complicated, and further research of the topic might help.
what I pointed out was a basic difference on account of the definition of torque.
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Old 31st October 2011, 10:35   #105
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

If you go by the argument that more piston area gives more torque then the "over square" design will have better torque for the same displacement compared to long stroke engine, but most of the torquey engines have long strokes. So may be torque is a function of the pressure, piston area and the duration of the pressure, all factors maximized in slow speed diesels.

Controlled injection as in computerized engines can be used to prolong both the quantum and duration of power generating combustion stroke.
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