Petrol Engine or Diesel Engine: Your Choice?

[Gurrala74]

Re: Petrol or Diesel???

Vipul, Codeman

I agree with you 100%.

I took a test drive of Skoda RS petrol some time back and no words to explain its kick !! was awesome

This all debate is due to the price difference between petrol (75/-) and diesel (45/-) and like you people said if diesel was par with petrol price, no body would have opted for diesel cars including me !!

But one thing I would like to say about diesel car that may be because of its engine weight I feel like it is much stable on the road at high speeds. I was bit afraid to drive the same speed in petrol same car

So there are advantages and disadvantages with petrol and diesel both, hence it depends on personal liking that which is better for them.

 

[Sam Dinkar]

Re: Petrol or Diesel???

@codename47: Very nicely said. Any Goan on this thread would definitely say that petrol is better

 

[codename47]

Re: Petrol or Diesel???

@Gurrala74 - When you say the car is stable in diesel at higher speed then a petrol engine , it could be the engine mounts and the poor wheel alignment.

diesel engine takes long time to go at higher speeds , in my car 180 kmph comes soon enough. ( 0-180 kmph in abt 10 seconds) and I feel no vibrations or unstable.

@Samdinkar - Thanks brother :)

 

[raj_5004]

I repeat - boils down to personal preferences. Preference can be seen in the poll above!

 

[visran]

I would any day opt for a modern Diesel as it is more efficient, more cleaner, and more fun to drive(read torque)

 

[nozzlering]

Re: Petrol or Diesel???

The clatter sound is more prominent when a diesel engine is started, but it dies out as the engine warms up. And with advanced technology and better insulation, present day diesel is much more silent. In any case, petrol engine is much silent than diesel engine.
Petrol engines can be supercharged, but with limitation. Turbo kicks in and at some point it has to cut off. On the contrary, its much easier to turbocharge a diesel engine.
Diesel engines are sturdier built. The requirement of high compression ratio, high fuel pressure forces the manufatcurers to build them to last longer.
While petrol engines undoubtedly have much greater acceleration from 0-100 when compared to any diesel engine, how many times do we do this? Infact we do 50-70 or 70-90 most of the time in 4th or 5th gear. And believe me a turbo diesel is much faster than a petrol counterpart in the given situation. Please leave aside petrol or diesel supercars, or modified cars. I am talking about the stock cars available on road.
Diesel fuel is cheaper. And its true that most people go for diesel cars, because of this reason. And as mentioned by dharmesh, we're struck by mob mentality. People are buying without considering there requirements. But above is only on the basis of economics. If I like diesel engines, I am going to buy it anyways, irrespective of my monthly running. Same goes for people who like petrol engines.
Diesel engines give low end torque. At low speed, when its really needed. Why petrol engine is not delivering same? Because at low speed, its efficiency is terribly low. It makes driving a whole lot comfortable (foot work is reduced considerably).
Petrol engines are cheap to maintain. But diesel is also not far behind. Present day diesel engine maintenance cost has reduced considerably. Having said this, I must say, diesel engine parts last longer than that of petrol engine. Reason is simple. Operating temperature and speed is higher in case of petrol engine.
And we can go on saying in favor and against. The debate is never ending. I have always driven petrol engine, and like to drive my wagon R. But after driving a diesel engine, I find more at ease while driving an oil burner.

 

[Gurrala74]

Re: Petrol or Diesel???

@Gurrala74 - When you say the car is stable in diesel at higher speed then a petrol engine , it could be the engine mounts and the poor wheel alignment.

I am talking about the petrol car which is less weight and diesel engine with more weight. As I know more weight in the front will be more stable !!

 

[nozzlering]

Both types of cars are stable. Designers and engineers compensate all factors and limitations to take care of stability factor.

 

[VivekRj.Dieselhead]

Advantages of diesel over petrol

* Better mileage

* Cheaper fuel

* The more daily miles clocked - the faster the returns

* Returns for an average of 65 km per day would be approx 18 months

* There is no significant difference in part prices between Petrol vs Diesel.

* Service costs would be similar {based on above fact}

* A diesel never needs a tune up.

* A diesel will normally outlast a petrol engine by a considerable margin.

* Higher torque at lower RPMs - thus the usage in trucks, tow vehicles, Even Train Engines, etc. However, this also has a negative connotation regarding power - less BHP from a diesel engine as compared to a petrol engine of the same engine capacity.

* Petrol engines suffer significant loss in lubrication based on the fact that petrol is a corrosive agent as compared to diesel.

* Compression ignition engines - easier to turbocharge in a diesel engine. In a petrol engine if the fuel-air compression ratio is on the higher end during the inlet stroke, then the mixture undergoes premature ignition on the compression stroke. This is easier in a diesel cylinder, as the inlet stroke only involves air, and not fuel - thereby giving the turbocharger component more lee-way in terms of pulling in air.

* Modern common-rail diesels now have service intervals of between 10,000 - 15,000 kms, are fairly refined and usually quicker. Up to 1 - 1.5 lakh kms, today's diesels are just as reliable as petrols.

* A diesel engine does not require an ignition system at all and can start as long as fuel is fed into the engine. This itself makes a 'traditional' diesel engine way more reliable than a petrol.

*A diesel engine (similar size and application) is also more fuel efficient.

*A diesel produces it's power in a more practical place in it's rpm range. Thats the advantage of a diesel.



In layman's terms,

A diesel produces it's power in a more practical place in it's rpm range. Thats the advantage of a diesel.

The HP was a figure made up by james watt to sell his engines so that his customers will have some sense of the engine's capacity. HP at a given rpm = (Torque at that given rpm x given rpm ) / 9550 . It is a made up number but since we all use the same made up number we use it as an yard stick.


It should be evident from the above formula that the HP is a continuously varying figure as you vary the rpm or in layman's terms, as you are pressing the gas pedal.


We all know Work done = force x distance moved . Let us call this torque.

We all also know that Power = work done per unit time. let us call this horse power.

The most important thing to grasp about HP when it comes to engines is that there is a rpm associated with that HP. With out which the HP figure is invalid. It is an unusable number.

SO we always say, X HP at y rpm. If we say X HP , we might as well be saying my car's power is 45 mangoes. It becomes an unusable measurement ( that is the point I am stressing )


Let us take a petrol and a diesel engine that produce the same Max HP . Let us further assume that the diesel has twice the torque of the petrol.

From the above formula for Horse Power it must logically follow that for the petrol engine to produce the same HP as the diesel, it must move twice the distance in the same time. Since distance covered by the engine is angular and not linear, the petrol must rotate twice as many times in a given unit of time as compared to the diesel.

There fore you see that for comparable Max HP petrol and diesel engines, the red line rpm where the petrol will produce the max HP will be around twice of that where the diesel produces it's max HP.

So why should there be a red line for any engine. The simplistic (albeit inaccurate ) answer would be that beyond a certain rpm the engine will simply self destruct due to inertia associated with the up and down motion of the pistons and the rods etc.

For the lighter petrol engine the unbalanced forces are smaller and they can afford to rotate faster before reaching the point of self destruction ( a higher red line ) . But the thing that makes it lighter also makes the engine weaker and unable to bear the high compression explosion cycles associated with the diesel engines. If you do not have high compression/explosion your torque is lesser. So the thing that helps petrols rev faster also makes them less torquey.




SO IF YOU ARE RACING, YOU SHIFT AT THE REDLINE, IF YOU ARE LOOKING FOR ECONOMY YOU SHIFT AT THE MIDDLE OF THE TORQUE CURVE.


With a petrol car your torque curve is flat and not bell shaped like a diesel so you almost want to shift at the redline, in diesel you can distinctly feel the torque letting off and you tend to shift in the torque band.


Now, why are diesels costlier than the petrols.
(1) There are more moving parts than petrol
(2) High strength alloys are needed as the cumbustion temperature is much higher and stress is much higher
(3) The components need better technology . Like VGT. Imagine having to change the angle of attack of the turbo vanes when spinning at 20000 rpm.
(4) Market demand


PS: Interesting fact . Otto Diesel designed the diesel engine to run on peanut oil. It took the petroleum industry almost 30 years to come up with a fuel that could replace the pea nut oil.

 

[nozzlering]

A nice long writeup. Though I disasgree on various points.

Advantages of diesel over petrol

* Better mileage

* Cheaper fuel

---------------------------

* Higher torque at lower RPMs - thus the usage in trucks, tow vehicles, Even Train Engines, etc. However, this also has a negative connotation regarding power - less BHP from a diesel engine as compared to a petrol engine of the same engine capacity.

* Petrol engines suffer significant loss in lubrication based on the fact that petrol is a corrosive agent as compared to diesel. DISAGREE. Petrol being lighter (in terms of fractional distillation process). It tends to dilute/ break the thin film of lubricant on the liner and piston rings. Petrol is not corrosive.

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* A diesel engine does not require an ignition system at all and can start as long as fuel is fed into the engine. This itself makes a 'traditional' diesel engine way more reliable than a petrol. Reliable in a sense that while other might not work, it will work, say during monsoon flooding in Mumbai. But todays diesel engines use lot of automation like the petrol engines.

*A diesel engine (similar size and application) is also more fuel efficient.

*A diesel produces it's power in a more practical place in it's rpm range. Thats the advantage of a diesel.


You have given a mathematical formula for explanation in layman's terms!! I feel we should either give a proper mathematical explanation, or totally avoid it.
In layman's terms,

A diesel produces it's power in a more practical place in it's rpm range. Thats the advantage of a diesel.
---------------------------
From the above formula for Horse Power it must logically follow that for the petrol engine to produce the same HP as the diesel, it must move twice the distance in the same time. Since distance covered by the engine is angular and not linear, the petrol must rotate twice as many times in a given unit of time as compared to the diesel. What is this?

There fore you see that for comparable Max HP petrol and diesel engines, the red line rpm where the petrol will produce the max HP will be around twice of that where the diesel produces it's max HP.

-------------------------

Now, why are diesels costlier than the petrols.
-------
(3) The components need better technology . Like VGT. Imagine having to change the angle of attack of the turbo vanes when spinning at 20000 rpm. Turbo vanes' angle is not changing. There is a set of stationary nozzles. And in VGT, there angle changes, changing the gas pressure acting on turbo vanes.

 

[TSIVipul]

Re: Petrol or Diesel???

Dharmesh - Please feel free to come and drop by. I'd love to take you for a spin. I've taken few of my mates for a ride , all they say after some nice acceleration is - WHAT THE HELL WAS THAT! I simply smile and say just 14 pounds of air (14psi boost) and 7000 RPMs in a sync.

Is this invitation valid for the ones who drive petrol cars too?

As I said before , If diesel was 80rs a litre people in India would have hated it and loved the petrol , Hard to digest but bitter truth

It looks like you are now on the way to become a SAADHU,speaking a lot of bitter truths!!

That's true and that is the reason of the sudden surge in sales of diesel cars. . Even if monthly running is low, people are blindly buying diesel car. I have heard people saying , my next car would be diesel car only as petrol prices are sky rocketing. And their use is just on weekend.

Nothing new,people here are also buying diesel to just keep them parked most of the time.

Frankly speaking it is only few of us in the millions of Indians who consider all aspects and do detail review of what we have to buy as per our need. But else it is all Herd's mentality.

+100..

I took a test drive of Skoda RS petrol some time back and no words to explain its kick !! was awesome.

This is what I have been saying for so long.No diesels can ever match the turbo petrols.

But one thing I would like to say about diesel car that may be because of its engine weight I feel like it is much stable on the road at high speeds. I was bit afraid to drive the same speed in petrol same car

Dear,stability comes from suspension and weight distribution and a stiff chassis,not form engine.Instead the nose heavy diesel cars are always less agile or say "eager to take sudden steering inputs" as compared to petrols.You need to drive any tightly sprung petrol car and then see its magic.

So there are advantages and disadvantages with petrol and diesel both, hence it depends on personal liking that which is better for them.

+100..

@codename47: Very nicely said. Any Goan on this thread would definitely say that petrol is better.

Oh yes,I remember,in Goa you can get petrol for about 55 Rs/Lt.
They really knows it very well.

I repeat - boils down to personal preferences. Preference can be seen in the poll above!

I am sure buddy,you haven't went out of wits yet!!

I would any day opt for a modern Diesel as it is more efficient, more cleaner, and more fun to drive(read torque)

Fun to drive can come by torque only!!
Eh,as said by many "To each his own".

I am talking about the petrol car which is less weight and diesel engine with more weight. As I know more weight in the front will be more stable !!

Dear,the nose heaviness of any car results in imperfect weight distribution,less agility feeling and of course,the increased weight has positive effect on braking.

 

[VivekRj.Dieselhead]

DISAGREE. Petrol being lighter (in terms of fractional distillation process). It tends to dilute/ break the thin film of lubricant on the liner and piston rings. Petrol is not corrosive.

Actually i meant that, in terms of lubricating properties, its way behind diesel, which may lead to early engine breakdown. Whereas diesel has more lubricating properties which is good for engine parts.

todays diesel engines use lot of automation like the petrol engines.

I agree they do, but still they have an edge over petrol engines.

Turbo vanes' angle is not changing. There is a set of stationary nozzles. And in VGT, there angle changes, changing the gas pressure acting on turbo vanes.

I was considering VGT.

Thanks for raising these points.

 

[nozzlering]

VivekRj.Dieselhead Actually i meant that, in terms of lubricating properties, its way behind diesel, which may lead to early engine breakdown. Whereas diesel has more lubricating properties which is good for engine parts.
Yes. being corrosive gives a totally different meaning. Lubrication is always good for engine parts. But diesel gets to see only the cylinder liner, head and at most, top piston rings after injection. If diesel leaks into lube oil (by any means), engine parts wont be happy at all. Having a viscosity higher than petrol, it has lesser effect of eroding the lube film layer on the liner and piston ring (I mentioned in earlier post).

I agree they do, but still they have an edge over petrol engines. We have been debating over this . And I am with you on this statement.


I was considering VGT. Yes in VGT, or any turbo, the rotating vanes will always have fixed pitch. Only the pitch of nozzles is variable in VGT, and fixed in case of FGT.

 

[VivekRj.Dieselhead]

Re: Petrol or Diesel???

If diesel was 80rs a litre people in India would have hated it and loved the petrol , Hard to digest but bitter truth

Sorry to say this but still world is dependent on diesel for major transportation. I think diesel is costing nearly same as petrol in most of developed countries, but still they use diesel trucks and other diesel powered machines for their respective purpose, that is because diesel engines has more power than petrol engines. If diesel would not have had an edge over petrol ones then it could have vanished like coal did in the past.

 

[VivekRj.Dieselhead]

Re: Petrol or Diesel???

If diesel was 80rs a litre people in India would have hated it and loved the petrol , Hard to digest but bitter truth

Sorry to say this but still world is dependent on diesel for major transportation. I think diesel is costing nearly same as petrol in most of developed countries, but still they use diesel trucks and other diesel powered machines for their respective purpose, that is because diesel engines has more power than petrol engines and more efficient as well. If diesel would not have had an edge over petrol ones then it could have vanished like coal did in the past. Cng is still not that much conventional like diesel is, though it provides cheapness of diesel and peppiness of petrol.

It surely would not make any difference in india too, if it would become rs80/lt. Still, huge torque and power lovers will buy the diesel vehicles. And one more thing above mentioned example shows only the need of diesel in real world. Imagine if petrol gets vanished, there would be no major changes, rather it will make every auto manufacturer to innovate new things to make diesel engines even better. But if somehow diesel would.......... You can imagine what would happen until some other fuel like diesel gets invented.