[CPH]

Originally Posted by Shan2nu

See, diesels aren't low revving by choice. Unlike petrol, diesel engines cannot run a low compression ratio. To create this high ratio, the engine needs to run a much longer stroke, which means the piston has to travel a much greater distance to complete 1 revolution. So diesel engines achieve similar piston speeds at lower revvs compared to relatively shorter stroked petrols.

Just to give you an example, an F1 engine with 39.7mm stroke runs a mean piston speed of 25.14m/s @19000rpm where as an 84mm stroked F20 S2000 engine will achieve 25.2m/s@9000rpm.

Add to this the stronger but heavier internal parts and it bcoms even more difficult to produce a high revving diesel engine.

Lastly, as i've already said before, almost all the desired diesels in the world (be it production or race cars) use "Forced Induction". So more than anything, its the turbo that is actually making these engine produce the kind of torque that they do, which is why you see so many turbo diesels in the market.

So dont expect diesel performance from a petrol or vice-versa. Decide what kind of engine you prefer and enjoy it.

Shan2nu

The Fiat group engines are very similar on the bore to stroke ratios on petrol and Diesel. Their 2.4 5 cylinder units are even identical.

A number of PSA and VAG units too.

[neoonwheels]

I love petrol cars for the free revving nature of the engine. But then on the basis of fuel prices, the more we rev, the big hole we get in pocket. If the petrol is fun from 3K-6K rpm (typical range), diesel is fun from 2k-4k (Turbo based). But in heart I still love a petrol car.

[Shan2nu]

The Fiat group engines are very similar on the bore to stroke ratios on petrol and Diesel. Their 2.4 5 cylinder units are even identical.

A number of PSA and VAG units too.

But compression ratio wont be similar and at what rpm do the petrol and diesel produce their max power?

Shan2nu

[CPH]

Originally Posted by Shan2nu

But compression ratio wont be similar and at what rpm do the petrol and diesel produce their max power?

Shan2nu

Of course they aren't having similar compression ratios because of the much higher flash point of Diesel it can deal with higher compression ratios as long as we talk conventional petrol egines.

Petrol engines produce the power higher up than most Diesel engines, but the gap is closing due to variable vane impellors.

[GTO]

Another important point:

Turbo-chargers!

All the diesels that we speak about, from the performance perspective, are turbo-charged! In fact, I can't think of a single common-rail diesel sold in India without a turbo. Unfair to compare a turbo-diesel to an N/A petrol no?

Two examples of a turbo-petrol versus a turbo-diesel:

1. Octavia vRS versus Octavia 1.9 TDi

2. The currently sold Laura 1.8 TSI versus the Laura 1.9 or 2.0 turbo-diesels.

Now we're talking . We will agree that the diesels are immensely more practical, but after a spirited drive in either, 9 out of 10 BHPians will walk toward the turbo-petrols. Guaranteed.

Originally Posted by GTO

Are you kidding? I own a high-revving petrol and a torquey diesel. The sheer joy of high-revving a petrol, and working through the gears on a curvy circuit, long straights and fast corners is UNMATCHED! Sure, diesels have better torque and offer superior driveability for daily use. Yet, for driving pleasure alone, nothing comes close to a crisp petrol engine at 7,000 - 8,000 rpms (dream on, diesels) mated to a slick manual gearbox.

And yes, there's the orchestra of a good exhaust with a petrol too. Absolute music to the ears.

[Shan2nu]

Another important point:

Turbo-chargers!

All the diesels that we speak about, from the performance perspective, are turbo-charged! In fact, I can't think of a single common-rail diesel sold in India without a turbo. Unfair to compare a turbo-diesel to an N/A petrol no?

Exactly. Even in the WTCC championship, the diesels are FI while the petrols are NA.

The BMW and Chevy produce 280bhp@8300/8500rpm and 245-269nm@5800/7250rpm while the Seat Turbo Diesel does 280bhp@4000rpm and 450nm@2500.

And as we've seen before even if you take 2 cars with identical power to weight ratio, the torque figures do make a big diff in acc and when you compare 245/260nm vs 450nm, the petrols are bound to find it diff.

Seat - WTCC - Fia World Touring Car Championship

BMW - WTCC - Fia World Touring Car Championship

Chevy - WTCC - Fia World Touring Car Championship

Shan2nu

[harishF1]

Diesel units have always be known for its power at lower RPMs. Check out vehicles like the Caterpillers. All such vehicles have amazing power at low revs. I am aware that the gearing ratios play a huge role, assuming cetris paribus, a diesel unit is better for high power operations. Petrol units have been peppier and a popular choice for speed. Some of the figures mentioned about Audi may not be the actual as all vehicle are undertuned for safety by the OEM and the extent to which it was undertuned. We may not really know the actual engine performance by comparing the figures on leaflets.

[anilisanil]

A bit off topic, but posting any way because I do not want to start another thread. Somewhere I read on this forum that petrols are faster than Diesels because they can be revved up to higher RPMS, I am sure this is not the case.

Because I know that a spark ignition engine gains lesser momentum per stroke than a CI engine does which is why a diesel engine delivers same speeds as a petrol does at lower rpms.

But my question is why are petrols such fun to drive and what is it that it is lacking in Diesel engine for them to deliver lesser power out of similar displacement. I did read about the requirement for Forced Induction in Diesels, but just wanted to discuss the exact requirement behind that?

What is it behind that petrol engines are achieving and what is IT that is lacking in Diesel engines, I personally believe that a diesel engine would be more powerful than petrol ideally. Correct me if I am wrong, and please enlighten me!!

[alpha1]

Originally Posted by anilisanil

A bit off topic, but posting any way because I do not want to start another thread. Somewhere I read on this forum that petrols are faster than Diesels because they can be revved up to higher RPMS, I am sure this is not the case.

Because I know that a spark ignition engine gains lesser momentum per stroke than a CI engine does which is why a diesel engine delivers same speeds as a petrol does at lower rpms.

But my question is why are petrols such fun to drive and what is it that it is lacking in Diesel engine for them to deliver lesser power out of similar displacement. I did read about the requirement for Forced Induction in Diesels, but just wanted to discuss the exact requirement behind that?

What is it behind that petrol engines are achieving and what is IT that is lacking in Diesel engines, I personally believe that a diesel engine would be more powerful than petrol ideally. Correct me if I am wrong, and please enlighten me!!

Throttle response and higher range of RPM (low to red-line).
So while staying in the same gear you can go to higher RPM, and consequently higher speed.

Diesel driver will have to shift to next gear to achieve the same.

[musicmanaman]

Originally Posted by anilisanil

A bit off topic, but posting any way because I do not want to start another thread. Somewhere I read on this forum that petrols are faster than Diesels because they can be revved up to higher RPMS, I am sure this is not the case.

Because I know that a spark ignition engine gains lesser momentum per stroke than a CI engine does which is why a diesel engine delivers same speeds as a petrol does at lower rpms.

But my question is why are petrols such fun to drive and what is it that it is lacking in Diesel engine for them to deliver lesser power out of similar displacement. I did read about the requirement for Forced Induction in Diesels, but just wanted to discuss the exact requirement behind that?

What is it behind that petrol engines are achieving and what is IT that is lacking in Diesel engines, I personally believe that a diesel engine would be more powerful than petrol ideally. Correct me if I am wrong, and please enlighten me!!

I'm no expert, but my 2 cents is this, Petrol engines are simpler, lighter and can reach a higher RPM compared to Diesel's. Diesel's engines are heavier, cause they need to be stronger as the ignition happens due to extrem compression and without a spark plug. Buttom line is Petrol engine can reach higher RPM's, need less maintanence and are less complicated atleast in the basic sence. A good performence sports bike is according to me the ultimate example of Petrol engineering at it's best.

[anilisanil]

Originally Posted by alpha1

Throttle response and higher range of RPM (low to red-line).
So while staying in the same gear you can go to higher RPM, and consequently higher speed.

Diesel driver will have to shift to next gear to achieve the same.

I am asking the reason behind this.
Also, I do not agree with that gear change thing in fact a petrol engine needs more gear changes for transition from speed A to speed B.

I think it is wrong to link RPM to speed, for the same speed a petrol engine requires greater rpm than diesel. correct me if i am wrong

Buttom line is Petrol engine can reach higher RPM's, need less maintanence and are less complicated atleast in the basic sence. A good performence sports bike is according to me the ultimate example of Petrol engineering at it's best.

I am asking why a petrol engine of same cc can develop more power than a diesel engine of same cc and am also asking what has to be done to diesel engine for it to develop the same power as a petrol engine. I think you got my question wrong!!

[GTO]

Originally Posted by anilisanil

Somewhere I read on this forum that petrols are faster than Diesels because they can be revved up to higher RPMS, I am sure this is not the case.

Rubbish. High rpm alone isn't the make-it-or-break-it of performance. The more important factors are power & torque output, weight, gearing etc.

E.g. The BMW 320d outruns the 320i petrol, whether in a 0 - 100 sprint or in-gear acceleration. The former does need an earlier upshift and doesn't revv as high as the petrol.

But my question is why are petrols such fun to drive

In a petrol:

- You can revv higher. That's more fun in itself

- There's more engine braking on tap

- A higher rpm range offers more flexibility say, in the middle of a fast corner (where a diesel might need an upshift)

[oxyzen]

Originally Posted by GTO

In a petrol:

- You can revv higher. That's more fun in itself

- There's more engine braking on tap

- A higher rpm range offers more flexibility say, in the middle of a fast corner (where a diesel might need an upshift)

Very well summarised by GTO.
But more engine breaking on tap is is not always true. Well theoretically it should be but all my perceptions were changed by Chevy Beat. It has one of the worst engine braking capabilities. I am not sure if it is due to heavier flywheel or taller gearing but braking is way less than others.

With diesel there is only one advantage and that is good enough to sweep off anybody off the feet. MAD TORQUE RUSH.

However we all know the answer to this debate is Turbo Petrol. Best of both the worlds.

[alpha1]

Originally Posted by anilisanil

I am asking the reason behind this.
Also, I do not agree with that gear change thing in fact a petrol engine needs more gear changes for transition from speed A to speed B.

I think it is wrong to link RPM to speed, for the same speed a petrol engine requires greater rpm than diesel. correct me if i am wrong



I am asking why a petrol engine of same cc can develop more power than a diesel engine of same cc and am also asking what has to be done to diesel engine for it to develop the same power as a petrol engine. I think you got my question wrong!!

It does not.
Let say you are doing 40 kmph at 3rd gear. at 2000 RPM.
In diesel car - if you need to race to 120kmph you will need to increase the engine RPM to 2000 x 120/40=6000 RPM.

Sadly there are no diesel engines that can rev that high.
So you will shift to higher gear.

On my petrol car I can go upto 140, without changing the gear, because my petrol engine can rev till 7000 RPM.

And ground speed is directly related to engine speed and gearing ratio.
What else can it depend on?

[jassi]

I can rev the nuts of my petrol santro and get it to 70 in 2nd and 130+ in 3rd gear (agreed its lightly modded) - I am guessing on a similarly powered diesel it will require gear changes all the way to 5th? Guess automatics make life easier when trying to extract performance out of a diesel!