The Technical Truth? :Petrol vs Diesel

[jat]

Originally Posted by Magma

wow
man, i definetely bit off more than i can chew! (thanks busa,jat,ripper ,reignofchaos and all)
this is a full blown crash course on diesel vs petrol!
well now you can understand my confusion about considering a diesl engine though i dont drive much! ( just 50kms weekly only in the city!!)
what do you guys think ? do you guys see Any problem(expensive maintenence etc etc ) for me buying a diesl engine car though i have lower running in the city?

That the beauty of this site ;-)

The car you want depends whether your heart rules you or head.
Considering your driving distance and being only in city, petrol is will be more economical and practical (head factor) although driving experience may be slightly different (heart factor).

We are comparing apples and oranges here. How do you decide what set up in a petrol versus diesel constitutes relative equality and then, having decided that, how do you decide they are not equal and that one is better?

Well you are correct here, but hey,in the same vein, I feel that While we are talking new gen diesels, we are much much far away from the new gen petrols, obviously because of our fixation on FE. The Toyota/Honda petrol engines (IDSI,VVT), or even for that matter the VTVT verna engine is much smoother than diesels, IMHO.

You consider the same segment of cars. For example take the hatch-backs - Getz, Swift etc then C-segment e.g. Verna, Fiesta etc and their development over a few years. This will give you the trend of performance and money being put in research and what is going to be in future.

If you see the trend, only the Honda is going the diesel way with their NHC engine (good low end torque and FE is fantastic - I am a diesel guy, but I am really impressed by their engine). No other car maker has taken any interest in developing petrol engines with bulk of torque lying in lower rpm band (statistically speaking) and increasing fuel economy. There are petrol engines which have fantastic performance and fuel economy. But they are far expensive than these diesels and again the torque characteristics are for racing type and not day to day driving in city. Also we all know that they can not be fitted to the C-segment and hatch-back which form the bulk of revenue gettes for Indian car makers.

[BUSA]

Here is a comparission between a diesel Fabia and a petrol Mini.

Mini Cooper Vs Skoda Fabia Rs Video

Check it out, says something about the torque

[Jomz]

Originally Posted by kpzen

Fuel Pump of a CRDi engine is what creates the majority of price difference...

Fuel pump..??? Those Injectors are so complicated that each costs more than a fuel pump.

The rails in a CRDi are forged From one piece to stand pressures of 1600 bar + while the max fuel injection pressure in gasoline is 350 bar.

Then diesel engines runs higher compression and higher pressures, each component, Cylinder block, Cylinder head, crankshaft, conrod, everything is beefed up to handle that higher pressure.

Then there is BOSCH.. who is a sort of monopoly in diesel injection systems and charges a premium price. There is no other option but to transfer that price to the customers.

Then the drivetrain is beefed up so that it can handle the higher torque

The suspension is beefed up to handle the extra engine weight.

Turbo is costly, but there are a few suppilers so that OEM's can go for bidding and select the cheapest and the one which lasts longer..

Even the fuel injection pipes.. which carry 1600 bar pressure are costly.

Originally Posted by jat

Also fuel injection equipment (PD or CRD or anything else) have to be fuel tight at such high injection pressure with no seals but only metal to metal contact. They are honed and mated to each other perfectly (none of the parts can be interchanged with another set). To attain such perfection, the manufactering cost is higher than petrol fuel injection equipment.
The plunger and barrels of fuel pumps and needle and guides of fuel injectors surfaces have to be very hard compared to petrol system while the core has to be elastic, a very difficult manufactering process.

Additionaly, to generate such pressures, you have to accelerate those plungers of the fuel pumps (TD / CRD and others) at a very high rate. Which means the cams have to be designed very strong to bear such force of accelaration. Also the surface have to very hard to bear the impact of the plunger rollers hitting back on the cams while the core has to be elastic to avoid breaking and cracking. And these cams are additional to intake and exhaust valves cams.

Not only the cams have to be stronger, but the shaft on which these cams are mounted, the chain or gear drive for driving those cams / camshaft have to be stronger to bear these extra torques (high acceleration means high torque means higher torsional stiffness for shaft and gears (for layman - more strength)).

Also, with diesels, it is difficult to scale down the fuel injection parts to very small sizes. (That was the one of the biggest headache for the designers in manufactering small diesels for a long period of time)

Now you can think the manufactering cost involved into making diesels. There are many more complex system affecting the cost of manufactering but these are bulk cost affecting the price difference.


And these extra torque is again has to be shared by crankshaft which means that they also have to be slightly heavier than petrol brothers, theoritically :-)

There is no doubt that the makers try to take advantage of more economical operation of diesel cars but then in this competitive world, they can not hold it for long. (Just see what TATA INDIGO has done to car prices)

The high pressure is in the rail and the pipes leading to it.. The pump doesn't have to develop that much of pressure.the pump has a non return valve on the top of it.. after which there is high pressure. only when the pressure drops the valve acts and fuel is admitted into the rail to build up prssure,the torque change in the pump is nominal can be achieved by just increasing the width of gears by 10% and that doesn't change the costs of that.

But the fuel circuit has to withstand high pressure and it is really costly.
The injector is really complicated with the soleniod and the soleniods have to do a lot of cycles along the life of the engine.those stuf is really costly.

[rippergeo]

We have new gen diesels in india, that outperform petrols of similar cubic capacity and price, thats all I'm trying to say!

and they do so while giving

1. excellent mileage
2. cost effectiveness due to our govt's fuel pricing policies
3. flexibilty to work on highways and cities alike
4. reliability comparable to petrols
5. the fun to drive factor with turbo boosts and shovel loads of torque


Cons
1. The extra lac that you pay upfront(minor change in EMIs)
2. Increased NVH(slight for example Verna D)
3. lower rev range
4. Less tunability(in india atleast)

all the manufacturers are coming out with good common rails, The DICOR, the TDCi in the endeavour, the CRDe in the scorpio.

No one seems to be doing anything in the petrols, they continue to flog the same tech to us.

[vibzez]

I had a M. Zen first, then an Esteem. For me I could never imagine a diesel getting the better of a petrol, in either in performance or driveability. But then I got a Scorpio and a Swift D.. My concepts have changed. In my opinion, from the day to day driving I do, taking all things in consideration, I would go in for a diesel.

Also would like to point out, that their is a huge difference in the driveability of the vehicles in the diesel segment too. The Swift has a narrower power band than the Scorpio. The Scorpio just impresses me so much with the zest shes got from even near idle rpm's. Infact it is easier to drive in the city. People having a Scorpio would subscribe to this view.

[ramkya1]

Originally Posted by vibzez

I had a M. Zen first, then an Esteem. For me I could never imagine a diesel getting the better of a petrol, in either in performance or driveability. But then I got a Scorpio and a Swift D.. My concepts have changed. In my opinion, from the day to day driving I do, taking all things in consideration, I would go in for a diesel.

Also would like to point out, that their is a huge difference in the driveability of the vehicles in the diesel segment too. The Swift has a narrower power band than the Scorpio. The Scorpio just impresses me so much with the zest shes got from even near idle rpm's. Infact it is easier to drive in the city. People having a Scorpio would subscribe to this view.

I have experienced the transition in diesel technology. Way back in 1987 I had an amby with a BMC engine (God I hated having to stop for changing water in the radiator) then switched to petrols as maintaining and driving a diesel was a pain. Changed to Fiat, M800, Zen, Tata Estate (was big, showy with lots of space, was also a pian to drive in traffic, but the diesel engine was much better than the amby, technology has moved on, but nowhere near the petrols) Again switched to petrols; Esteem (short time) Santro and Ford Fusion. After 2000 the Diesel technology has improved, I have driven for long drives friends Diesel cars and went for the plunge, found them to be grately improved, finally bought a Scorpio CRDe, in August 07.

The CRDE is a genertion ahead, completely agree wtih VIBZES, a pleasure to drive in the city and also highways. In comparision to the BMC amby & the Tata Estate the CRDE is a new era; very smooth, goes off the block similar to petrol. I've done 14K km on the Scorp in 4/5 months, its good on the hills too as I've mentioned in my recent Shirdi travelogue, where petrol Fiesta 1.4 struggled, the scorpy climbed with ease.

From experience of Scorp users, maintenance also is relatively cheap. In fact M&M has home-brewen the technogy, has not transferred all the costs to the buyer and given a powerful diesel with a good cost. It's going to be launched in UK too, already introduced in the US. How much it would sell abroad will depend on lots of things.

Happy Driving,

--Ramky

============

[kpzen]

Originally Posted by Jomz

Fuel pump..??? Those Injectors are so complicated that each costs more than a fuel pump.

The rails in a CRDi are forged From one piece to stand pressures of 1600 bar + while the max fuel injection pressure in gasoline is 350 bar.

Then diesel engines runs higher compression and higher pressures, each component, Cylinder block, Cylinder head, crankshaft, conrod, everything is beefed up to handle that higher pressure.

Then there is BOSCH.. who is a sort of monopoly in diesel injection systems and charges a premium price. There is no other option but to transfer that price to the customers.

Then the drivetrain is beefed up so that it can handle the higher torque

The suspension is beefed up to handle the extra engine weight.

Turbo is costly, but there are a few suppilers so that OEM's can go for bidding and select the cheapest and the one which lasts longer..

Even the fuel injection pipes.. which carry 1600 bar pressure are costly.

My bad..I meant injectors only...Sorry.
Geomy kuttan - Very well written..

[Magma]

Originally Posted by jat

That the beauty of this site ;-)

The car you want depends whether your heart rules you or head.
Considering your driving distance and being only in city, petrol is will be more economical and practical (head factor) although driving experience may be slightly different (heart factor).

.

Please Elaborate : " economical and practical"

[rippergeo]

Originally Posted by Magma

Please Elaborate : " economical and practical"

I suppose Mr Jat means its economical because of the premium you pay initially for a diesel
BUT
if you're going for EMI's it wouldnt make a massive difference to your payment.

i'm not so sure about practical!
the whole point is- the diesels are more practical than petrol in the way they deliver their power. everything that the engine can give, it gives lower down in the rev range, and it gives good horsepower with a higher torque than what a similar sized petrol engine can give.

In a drag race-a spirited petrol driver may be able to get ahead because he can hold each gear longer.
BUT
in a twisty circuit, where the petrol keeps falling out of the powerband during cornering, the diesel will get ahead because its usable power is available earlier.(see the link in BUSA's post about the fabia Vs MINI)

thats the kind of situation you will find on our crooked highways and crowded streets. you will need your car to pull from low down in the rev range say 1500rpm or so and take you to 3000-4000rpm(by which time you run out of space to drive anyway)
now,
note that this is precisely where the petrols start to produce their peak power(3000-4000) most petrols need to rev higher, so by the time you rev up the petrol to its peak performance, the lowly turbodiesel or the Indirect injection NA diesel would have slipped by(at a relaxed 2500rpm)
I agree, the petrol will rev faster on its way to 3000-4000rpm, but not fast enough to catch a common rail turbodiesel which would have already reached its target of 1500-1700rpm(from 1000rpm at idle)

the other way (the fun way)of driving the petrol would be- keep it in the powerband i.e rev it hard and keep the revs above 3000. FE suffers and the engine strains itself. i hope no one argues about engine strain because my understanding is- faster the engine parts move, higher the stresses. doesnt matter if the engine can handle it, if it can run at a lower RPM without knocking, the engine will be happier

The other thing

A diesel is not costlier to maintain than a petrol

I know because- we have owned
1. an M800 with a 1000cc Daihatsu engine(rocket)
2. Zen D(cheap thrills)
3. mahindra MM540(offroad fun)
4. Premier padmini- 138D(sideways Fun)
5. Accent CRDi(the original petrol beater)
6. Swift D(most fun on wheels I've had)
7. Verna D(Diesel dragster)

PS I've had a lot of fun in petrol cars too- but that was when i was parked in some shady lane with my Girlfriend

[rrnsss]

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 continously 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.

[Magma]

@ripper
so as your post suggests : am i right in assuming that if im not worrying about the inital premium i have to pay for a diesel car then there is no problem in goin for it even if a drive in the city and that too only 50kms weekly???
Jat and busa please jump in too!
(busa i remember you posting that petrol would make a better choice for a case like mine: please explain)

[jat]

magma:

As the guys have already explained, the difference in the cost of diesel and petrol cars is comparitively quite high as compared to difference in cost of fuel. Also you can get a better option (more luxuries) for lower cost (eg - compare the fiesta or verna or getz or swift diesel and petrol)

Also AFAIK, the interest rates on loans for diesels are higher than petrol ones.

If you don't care for price and the options thrown in with petrol ones, you can always opt for diesel engines. But as you are going to drive for such short distance, that, you may not be able to enjoy much of difference in driving experience and practically, the diesel won't give you the thrill.

Also mechanics and petrol pump attendants are more familiar with petrol cars and less likely to make mistakes. There are still nuts out there who keep on making mistakes, and that includes the authorised service centres guys.

[BUSA]

You can buy Modern diesel engines for what ever the running be but if your running is too short and just around 50kms a week, that around 7kms a day then a diesel doesnot make sense.

You would be paying premium of 80k for the diesel and it will take lots and lots of time for you to recover that extra paid for the diesel. You should settle for a petrol, your running is very less. However if you are ready to pay the extra premium for the diesel then you can buy the diesel. Diesels tend to have better resale value. Maintainance of modern diesel's are not that expensive comapared to what they were for old school diesel's.

busa i remember you posting that petrol would make a better choice for a case like mine: please explain

[jat]

Originally Posted by rrnsss

In layman's terms,


...............
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.
..............
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.

There are many torque-bhp discussion on this site. Please check following links for further discussion.

http://www.team-bhp.com/forum/techni...-movement.html

http://www.team-bhp.com/forum/techni...ue-vs-bhp.html (Torque vs BHP?)

http://www.team-bhp.com/forum/techni...que-power.html (Engine Output and Torque power)

Also I would add few points here:

1. The low rpm (or rpm ratio to be precise) is due to the fact that diesel is compression ignition and not spark ignition. If you try to get high rpm, the at low rpm, there is insufficient compression to burn fuel and if want enough compression at low rpm, then the peak pressures at high rpm goes beyond limits.

2. Rarely do have any engine with unbalanced force by design. To confirm this, let any engine keep on running with no fuel (car rolling with gear engaged - not a good practice although) and you will notice that there is no vibration at all even if the car is rolling at high speed. But moment the engine is firing, even on idling, there will be vibrations. The vibrations comes from unbalanced moments and firing.

3. Nicolaus Otto is associated with petrol engines and Rudolf Diesel is associated with diesel engines.

regards,

rks

[amangujral06]

what ever you may say..the diesel engine huge size ,weight and noise is the biggest issues yet not solved in lower segment cars..not everybody can afford Octavia(diesel) or merc..one doesn't even come to know when they pass through somebody..but sure knows when a Scorpio or indica is coming..good things come for a price..but it is easy for a middle class guy to afford a petrol car with almost no maintainence rather than getting a heavy elephant who needs to be fed more