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Old 22nd October 2011, 16:30   #1
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Default Petrol versus diesel cars - reliability

I have heard people express both opinions - that petrol cars are more reliable and others say that diesel is more reliable.

Back in the 1980s when I was a kid and we had a debate as to whether to put a diesel engine into our Ambassador, I remember my grandfather (who was an engineer) saying that Petrol engines are far more reliable than diesel. He said that diesel engines would need to be re-bored more frequently, and could be re-bored fewer times over its life-time.
He used to also say that the were more trouble prone and expensive to service.

Now I live in the UK and there is this same debate going on at motoring forums as to whether petrol or diesel engines are more reliable.

The consensus there is that there was a golden period for diesel engines between 1995 - 2005 where they were very reliable (more so than petrol). However modern diesel engines have too many unreliable components, both electrical and mechanical, to meet stricter emission norms as well as to provide smoother (petrol like) driving characteristics.
These include Dual Mass Flywheels (on the clutch), DPFs (Diesel Particulalte filters), EGR (Exhaust Gas Recirculation), Turbo chargers, Common rail diesel injectors, etc. which can go wrong and are very expensive to repair. Hence the opinion is that unless you do more than 15,000 miles a year, you do not save enough money on fuel to meet the expenses of potential repairs associated with a diesel engine.

What do people think of the diesel engines in Indian cars? Are they of the generation considered to be of the golden age of diesel engines (one generation earlier than current western models) and hence devoid of all these potentially trouble-some systems? Hence could they be considered more reliable than petrol engines?

Or is it a myth that diesel cars can ever be more reliable than petrol? Even if the engine itself is more reliable could it be that a petrol car as a whole will be more reliable than a diesel car, because the higher NVH levels of the diesel car will put more strain on the platform and result in more general reliability issues?
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Old 23rd October 2011, 20:52   #2
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Default Re: Petrol versus diesel cars - reliability

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Originally Posted by rohantn View Post
between 1995 - 2005 where they were very reliable (more so than petrol).
Can you please quote some diesel engines which were more reliable to comparable petrol engines?
Quote:
DPFs (Diesel Particulalte filters)
Apart from the fact that they require maintenance in about 1,00,000 kms, I dont see how thay are not reliable. Other than that, very.. very few CRDi cars have DPF's in India. Its Euro 5 IIRC.
Quote:
EGR (Exhaust Gas Recirculation), Turbo chargers
Umm.. doesn't petrol engines have these as well? Especially EGR's.
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Common rail diesel injectors
IMO, the injectors are one of the most reliable parts in a diesel engine. Such is the precision of engineering and manufacturing these days.

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

I personally think that, considering the fuel prices, and the power/torque/NVH characteristics of diesel engines these days, a modern crdi diesel engine makes more sense to a person whose monthly running is ~1-.15k kms. In everyway.

- IN INDIA

Last edited by dhanushs : 23rd October 2011 at 20:53.
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Old 23rd October 2011, 21:11   #3
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Default Re: Petrol versus diesel cars - reliability

Quote:
Can you please quote some diesel engines which were more reliable to comparable petrol engines?
Peugeot, VW, Merc engines diesels were always more reliable since there was relatively fewer electronic components
Quote:
Apart from the fact that they require maintenance in about 1,00,000 kms, I dont see how thay are not reliable.
We dont see half the issues here, glow plugs work harder, turbo engines run at higher speeds hence more hotter, fuel pumps work harder to finer tolerances. Our speeds are lower and hide some of these things. DMF clutches are known to wear out faster here due to our driving habits and traffic conditions.

Quote:
Other than that, very.. very few CRDi cars have DPF's in India. Its Euro 5 IIRC.
Correct and things will get even messier since cars are used in mainly stop start conditions

Quote:
Umm.. doesn't petrol engines have these as well? Especially EGR's.
IMO, the injectors are one of the most reliable parts in a diesel engine. Such is the precision of engineering and manufacturing these days.
They are but high tolerances and extremely high pressure makes it intolerant of bad fuel and poor maintenance
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Old 23rd October 2011, 21:50   #4
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Default Re: Petrol versus diesel cars - reliability

Diesels are reliable if you keep them running(with periodic maintenance).
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Old 23rd October 2011, 22:42   #5
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Default Re: Petrol versus diesel cars - reliability

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Originally Posted by ajmat View Post
.. makes it intolerant of bad fuel and poor maintenance
Yes, fuel quality is the biggest killer of Diesel engines. The supply chain is scam ridden as there is too much money to be made.
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Old 23rd October 2011, 22:55   #6
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Default Re: Petrol versus diesel cars - reliability

The innova Diesel in our garage has never seen any reliability issues despite being a 6yr/90,000Kms old vehicle. Same goes for the Qualis which was occupying her place before. We have never heard of anything called unscheduled maintenance or breakdowns with those Diesel engines. Touch wood! I think the modern day engines are built to last... be it petrol or diesel. The only turnoff is the service intervels, which are quite frequent than a petrol engine.

This is solely based on my personal experience with Toyota Diesel Engines. I don't know if modern day Diesel engines from other Manufacturers also offer the same kinda reliability.

Last edited by Warwithwheels : 23rd October 2011 at 22:57.
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Old 24th October 2011, 01:25   #7
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Default Re: Petrol versus diesel cars - reliability

If you have seen the Qualis & Logan in Bangalore, which are used by call centers and the Meru Taxis respectively, you may feel a lot more reassured about diesel engine reliability and running.

The Qualises are known to run cleanly in excess of 5,00,000 KMs and are supposedly good for 10,00,000 KMs. The claim is that even today used Qualis are in demand.
I have sat in Logans which have upwards of 2,75,000 KMs and you would be hard put to belive that they have so much running, relative to the way they appear and ride.

And i have asked the drivers, they mention that beyond routine service as per company schedule nothing extra has been done.

The engines are supposed to be good for 4,00,000 - 5,00,000 KMs on the Logan.
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Old 24th October 2011, 10:28   #8
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Default Re: Petrol versus diesel cars - reliability

Hi,

From personal experience large capacity diesel engines with low pressure turbos are more reliable than petrol. These engines produce low horse power ( for there large displacement ) and good low down torque. The tend to be sluggish ( compared to their petrol counterparts ) but have loads of lugging power. These kind of engines are mostly seen in commercial vehicles or in few passenger vehicles like the Mahindra Logan/Verito or Mahindra Bolero etc.

The new gen low capacity high turbo boost engines ( Low displacement high HP high rail pressure ) are relatively more delicate ,very sensitive to fuel quality. These engines do not handle abuse very well and need to be immaculately maintained. ( e.g. Old Hyundai Accent CRDI/ Verna CRDI ).
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Old 24th October 2011, 11:21   #9
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Default Re: Petrol versus diesel cars - reliability

This is quite an interesting thread and I have a couple of points to raise here. Keep in mind, I'm not of the belief that either engine is better or worse - the differences I point out are merely characteristics of the intended usage.

Diesel Engines:

a) Diesel is cheaper than petrol - this had to be the first point

b) Higher torque at lower RPMs - thus the usage in trucks, tow vehicles 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.

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

d) 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.

e) I'm not sure, but I believe diesel engines have higher emissions than petrol engines - not a plus, but something to note.

Diesel engines are sturdier, in my opinion, as they are based on compression ignition - necessitates thicker cylinder walls (this is pure extrapolation, please correct me if I'm wrong), thereby leading to heavier engine as compared to petrol engine in terms of weight. Does this make it more reliable? I'm not sure - but since commercial vehicles are almost exclusively diesel based, I would tend to think they are more reliable. In a business scenario, I would imagine cost of repairs is a significant driver towards my choice of transportation.
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Old 24th October 2011, 12:54   #10
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Default Re: Petrol versus diesel cars - reliability

This is nice purists discussion, not contaminated by cost of fuel (As its different in different parts of the world, and in US Diesel not used so much).

I always believed that Diesel cars needed more repairs as i have seen my fathers official Jeep needed to go to the official company's repair floor once every 2 months, where as our petrol FIAT would hardly need any visit to the garage.

Diesel Jeep also rattled much more then the petrol ones also petrol ones were used in distress, i remember one of the senior colleges of my father had a petrol 4x4 and when anyone's jeep is stuck anywhere he would come with his petrol 4x4 and pull it out, never was another Diesel jeep called (Not sure why).

Diesels also needed to run regularly (not sure what happens if you keep them in the garage for most part of the month). So it was easy for the FIAT to be parked for 5 days a week but not ok for the Jeep to be not running regularly.

When i was buying my first car, thought about Diesel car as well, and as my job involved traveling outside India it meant the car would be un-used for long periods of time (Sometime months) and i was recommended a Petrol as this was not something that was advisable in a Diesel car.

All these are experiences from close to 20 years ago and hence may not apply today as technology has changed.
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Old 24th October 2011, 14:01   #11
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Default Re: Petrol versus diesel cars - reliability

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Originally Posted by Anarchy83 View Post
- but since commercial vehicles are almost exclusively diesel based, I would tend to think they are more reliable. In a business scenario, I would imagine cost of repairs is a significant driver towards my choice of transportation.
I think the main reason why commercial vehicles use diesel is not because they are more reliable. It's because of the price of the fuel and the torque that a diesel engine can produce.

Since commercial vehicles in India run almost exclusively on diesel, it would be good if we excluded them from this discussion. Let's instead focus on vehicle categories where we find both petrol and diesel vehicles.
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Old 24th October 2011, 18:26   #12
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Default Re: Petrol versus diesel cars - reliability

Diesels used to require more frequent oil changes, be smokey, rattley and slow. Of course, that's yesterday.

Modern common-rail diesels now have service intervals of between 10,000 - 15,000 kms, are fairly refined (though still not close to petrols) and usually quicker. Up to 1 - 1.5 lakh kms, today's diesels are just as reliable as petrols.

However, we still have to gauge:

- How the pumps of all these common-rail engines do. Few of us have done over 1.5 lakh kms on modern diesels

- How long the turbos last. That's something the petrols don't have to worry about, as most of them are naturally-aspirated.

That said, considering that most first-time owners sell their cars way before 100,000 kms, I don't think reliability ought to be a decision-making point between diesel & petrol.
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Old 24th October 2011, 19:37   #13
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Default Re: Petrol versus diesel cars - reliability

It was true with the old Amby and the Fiat diesel the oil Change was quite often. Now a days the Vento need the change after every 15K kms. Other Ritz,Dzire and Swift needs once in 10K. The emission of Old cars and new CRDI cars are much different and hardly we see the black smoke in CRDI. Mostly people sell it with in 100,00kms of ODO. But the taxis are running 3L and even above 4L kms. So the age of diesel is different now a days.
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Old 24th October 2011, 22:08   #14
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Default Re: Petrol versus diesel cars - reliability

A different take on the diesel car. A popular saying is that the diesel car is to be regularly driven ( meaning daily). Suppose a person keeps it as a week-end car only, but drives for longer distances on week ends. Say roughly totalling up to 1500-1750 km on those eight days. Does it do any harm to diesel engine that is idle for 5 days and goes for longer distances for only 2 days in a week? Does any of BHP 'ian has experience belonging to this bracket?
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Old 24th October 2011, 23:04   #15
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Default Re: Petrol versus diesel cars - reliability

Quote:
Originally Posted by rohantn View Post
I have heard people express both opinions - that petrol cars are more reliable and others say that diesel is more reliable.
.....
I think we should distinguish between (un)reliability and service life. And we're not getting into scheduled service intervals.

My non data backed up feeling would be
The non CRDI (ie jerk or rotary FIP) diesel was more reliable than the petrol of its day.
Diesels then essentially being for commercial use, had service life (and total cost of ownership) as one of its most important goals. And it was met. So diesels had longer service life than the petrol. (My opinion:- the petrol could also have the same service life if it was given as much importance during design and manufacture. It is just that it was not given that importance)

Nowadays, with CRDIs for cars, I feel these will be slightly less reliable than the petrols simply because there is slightly more to go wrong. And the fuel system of a modern diesel is truly of aerospace standard, work under very adverse conditions, and are thus more likely to go out of specs.

Service life will be the same as contemporary petrols because the design goals and end usage are the same (engines for passenger cars).

Regards
Sutripta
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