The Technical Truth? :Petrol vs Diesel

[tsk1979]

Old octy 90bhp does not have Pump dusse(PD) or unit injectors. That comes in the Laura.

[Technocrat]

Yes, from whatever I have heard the Skoda Diesel doesn't use CRDi technology..

Anyways i think we are going OT. The discussion is what is major difference between Petrol & Diesel for layman & why should he pay more for the Diesel?

The answer IMHO is the lower running costs which compensates for the higher initial price

[Jeep]

The huge diff in Petrol amd Diesel models can be partly attributed to 'Target Pricing' strategy - pricing it considering what the customer is willing to pay (I think the old Maruti Zen also was priced accordingly!)

[abhibh]

Originally Posted by directinjection

Octy diesel may be named TDI but the technology is at par with CRDI and in fact more advanced since it uses high pressure unit injectors that must be incorporated in the engine head design itself

Only Laura TDi is Par with CRDi as it uses PD (Pump Dusse). And those injectors turns out to be more costly than CRDi unit. There have been talks that VW might consider developing Common Rail Engines. (Starts searching the bookmarks for the link stating VW Common rail engines thingy)

Link: VW Media Services

VW (including Audi) and DaimlerChrysler (especially Mercedes-Benz) are cooperating on the diesel engine front in a programme called ''Bluetec" (VW is sticking to the name "Clean TDI" while DaimlerChrysler is using the ''Bluetec'' name it seems) to produce new emission-cleaning technology aimed especially at reducing Nitrogen Oxides. Common rail is integral to this which is largely why VW is moving to it. They are serious about cracking the North American market and it looks to be a great step forward that gives them a chance of doing so as well as having a big impact in other markets including Europe.

[DirtyDan]

Originally Posted by jat

The bulk cost is due to fuel pumps and injectors. Then you got turbocharger, cams for fuel pumps or timing device (ECU), heavier camshaft or drive for fuel pumps (due to very high acceleration of fuel pump plunger for high injection pressure) etc etc. Also combustion products of diesel are more corrosive than petrol, the valves are of slightly different alloys.

Excellent answer, Mr. Jat. Some of us actually READ it! Others flounder.

[ananthkamath]

Other than the supply-demand equation and various other economics which is beyond my understanding, diesels are more expensive because:

1) Higher weight = more raw material cost

2) More precision parts = more engineering & machining costs

3) More involved engine controls & emission controls = vastly higher engineering costs

Its worth it because diesels have more HP/liter than gasoline (atleast the new ones), better driveability excellent fuel economy and more tuning potential.

[hellspawn]

the diesels being priced 70-80k more than the petrols is actually a marketing strategy.if the companies price the petrols and the diesels at the same price,the diesels will cannibalize the petrols sales.so companies price the diesels higher so that people who actually want a diesel for their fuel economy buy them as they would be willing to pay a premium over the petrols.
if a person drives 20kms a day and the petrols and diesels are priced the same,he would still buy the diesel.but since diesels are priced higher,he would opt for the petrol.on the other hand,a person travelling 100kms a day would overlook the petrol and buy the diesel as the savings in his running costs would overshadow the premium he pays for the diesel.

there is no better example to prove the petrol cars failure in front of a diesel car when they were priced similarly.
skoda had launched the octavia 1.9tdi and the octavia 2.0petrol both at a price of 10.69lacs .the diesels ate completely into the petrol sales to an extent that skoda discontinued the petrol variant only to launch a 1.8tpi to cater to actual enthusiast and priced it higher than the diesel.

[dvswift]

Originally Posted by akshay1234

direct injection - you sure your not confusing tdi with pump duese (pd)? i think it is the older type of diesels with a turbo before common rail.

Akshay, The entire VW group cars (except new AUDIs) run on the Pumpe-Duese technology (Unit Injector in German). TDi is a marketing name given for the PD. Don't obviously confuse this with the TDI you see on the Indigos - they are totally different.

[Thad E Ginathom]

If you take two essentially basic engines, just for example, like an industrial engine or ones found in fishing boats (not the highly complex and advanced systems in our modern cars), the first thing to notice about the diesel would be --- no electrics!

Even when we return to think about our complex car engines, whatever we add to the cost in diesel-specific parts, we must remember to subtract the whole high-tension spark plug circuitry and timing mechanism.

[jat]

The diesels injection pressures have come a long way from initial stages (air blast injection with injection pressure of about 50~70 bar) and are approaching 2000 bar now-a-days. Compare these with the petrol injection system and think about the cost of equipment to bear these high pressures for long time. They have to be stronger and heavier.

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)

[Thad E Ginathom]

That's a very interest and clear review of the mechanics --- understandable for a layman like me.

Nice post: thank you

[Technocrat]

^^
I agree.. Excellent post Jat very easy to understand & quite informative

[Jeep]

Hats off, Jat. No wonder why a 1.1L diesel engine is still not around, else, price the car aptly and it will be a hot-seller!

[groom]

as long as you are comapring MPFi to CRDi.. petrol engine cost is cheaper... try comparing Gasoline Direct Injection with Common Rail direct injection ....
the gap is the as narrow as possbile ...

diesel engines are heavier.. due to virtue they operate at high combustion pressures and temperatures ...
valves are now more or less same between these ... but the FIE (Fuel injection system) highly differnt ...

MPFi - FIE runs at around max 3bar and fuel line can even be made of Fibre plastic but CRDi varies from 1200bar to 2400bar ... this really needs materils that can withstand such high pressures .. 2400bar is like, point the injection spray at your fingers and they will be cut ...

now when you comapre the same with Gaoline direct injection - the operating pressure are now up to 200bar (as in case of SAAB and Mitsubishi engines).. the efficency of these are almost same as diesel engines and can push the compression ratio of gas engines from 9.6 to almost 12 ... such Fuel injection systems are again very expensive ... and more over the control of the injector needs to be extremely precise .. hence the solenoid/piezo inside the injector is engineered to high levels ...

so diesel engine have this delta ... even if u dont consider the eletronic diesel control.. the diesel engine needs GOVERNOR ... (speed/torque control device) which adds to cost...

[jat]

I would like to clarify few points here:

1. The query raised was for cars available in India and I don't thing Gasoline direct injection cars are commonly available in India. If the Gasoline DI cars are made available, the cost difference won't be much. But still the fuel injection system manufactering cost for diesels would be higher.

2. 200 bar injection pressures for diesels was long time back in 20s and 30s. Therefore, the gasoline engine makers do not have to carry out heavy research on building new technology for fuel equipments. They carry research on modifying the system. On the contrary, diesel engine maker are still struggling for finding not only the cheaper materials to withstand such pressures, but also the processes to reliably make such equipments. Hence gasoline fuel system cost will be less.

3. The fuel injection control (timing) is more delicate in diesels as there is no spark plugs. But the these system, although a bit complex doese not add so much to the manufactering costs.

4. The thermal efficiency of diesel engines are much higher than gasoline engines. (highest eff achieved has been around 55% for commercial engines)

5. (OT) HP water jet (about 4000 bar) is being used to cut steel and other materials in manufactering processes (special cases) although it is not common due to high cost involved.

6. Regarding governor, please elaborate.
The cars already have ECU for precise fuel management system and I don't think the cost of software can make so much difference in petrol and diesel cars cost (somebody with specialising in this branch, please help). Governors are required where seperate precise speed control is required and where speed is affected by many external factors (e.g. generators, petroleum pipeline pumps etc). In fact, fitting a gorvernor on a car engine can make it sluggish (it doesn't matter if it is a petrol or diesel)