[donc]

I don't believe we've answered this question yet. Is anyone here qualified to answer?

Ummm... actually... yes... and the answer remains, it's a moot question (or more correctly, an incorrect statement / implication) - mostly because it's subject to so many inequitable variables. There is no definitive answer, not the least reason being because fuel type is not the determinant of engine life (think lubricant quantities and quality; torque dynamics, vibration and harmonics, the quality and durability of components and construction; driving habits, bearing qualities and construction, kilometres travelled -v- time in service (running hours), RPM, external environmental considerations, forces generated by third-party devices such as turbo chargers etc. etc. etc.).

My credentials for this opinion are - Licensed Motor Mechanic (NSW - Diesel & LPG certified); Member of the Institute of Automotive Mechanical Engineers; Automotive Engineering Specialist (Steering, Suspension & Brakes) - IAME.

Am more than happy to debate this with anyone similarly qualified...

[MRH130]

Ok, if the question is simply "does it last longer and if so why", the answer is that yes, for a number of reasons Diesels tend to last longer. That is far from a hard and fast rule, but it tends to be the case, at least historically.

Firstly, pretty much all of the major components of the engine have to be built more solidly to cope with the greater compression forces involved. i.e. a strong petrol engine will normally run a compression ratio of 10 to 1, whereas a diesel will normally run closer to 17 to 1, and will frequently have a turbocharger pushing more pressure through it. This stronger build is why diesel engines tend to be heavier and this does have some benefits (like theoretical longevity) and detriments (like handling).

Next, as discussed, diesels tend to rev lower than the average petrol, reducing actual piston movements and crankshaft rotations to travel any given distance compared to a petrol engine. That's not the whole story though, and something that can drastically shorten the life of a petrol engine is very high RPM (i.e. 5000-8000) which can, depending a bit on the quality of the lubricant in the engine, exceed the ability of the oil to protect vital components from wearing against each other. This is why, when subjected to the abuse of production car racing, a petrol engine that would normally be capable of 300000-400000 km might only last 4000-5000km. The overall revolutions that engine produces will be much lower than if it had done 400000 of road use, but the greater stress of the high RPM causes extra wear. Obviously, the average diesel engine will never, ever get near the 6500 RPM limit of the petrol engine in the i30. My car, for instance, has made several visits to the limiter that have put the engine, and the lubricant, under a different kind of stress to anything a diesel road car engine would ever experience. I doubt that many members ever take their CRDis past 4000 rpm, as in my observation there is little to be gained from doing so except making a lot of noise. The power and torque of the diesel is achieved lower in the rev range.

Due to their higher efficiency and heavier build, diesels are both more resistant to overheating and more capable of surviving overheating when it happens. There are no alloy heads to crack, for instance.

Falcon taxis often cruise to 600000km on one petrol engine, because they are always warm (most engine wear occurs when cold) and because the power delivery is similar to a diesel and they rarely get over 3000 rpm. Heavy trucks can do a million ks on one engine because they rarely get over 2000 rpm - they're very heavily built and very understressed.

I have a number of petrols that have travelled large distances and are still very strong. One in particular, a Volvo 240, which is only a 4 cylinder and has copped a fair spanking from me in it's life, has just hit 400000km and runs like new, using no oil between changes.

Having said all that, if a diesel is not well maintained or well built, it will fall over at an early age. Another brand we used to have at my dealership had an endemic problem with the diesel engine they used in their commercial vehicle and I lost count of the number we replaced because they prematurely wore out.

Another thing that I think has contributed to the historic longevity of diesels is their simplicity. No ignition system, just the mechanics of the engine and a pump squirting fuel at it. This simplicity means that less diesels get thrown away for non-engine related problems and also means that they run "ok" even when badly out of tune. If you disconnect some sensors on fuel injected petrols, the car will run like a pig, which puts more stress on everything.

This advantage has been reduced by the advent of electronically controlled common rail diesels, where they now have as many sensors and meters as any petrol engine and rely on high level equipment such as very high pressure pumps and piezo injectors. As the fuel consumption, power, refinement and driveability have improved, the simplicity has disappeared and I wonder whether this will actually reduce the longevity of modern diesels to a point closer to their petrol stablemates.

If anyone thinks that diesels are inherently more reliable, talk to anyone with a Holden/Isuzu Jackaroo/Trooper/Bighorn with the Common Rail 4JX1 engine. These have catastrophic issues with control systems and some of those contribute to the massive internal wear from which they suffer.

It's simplistic to try to boil it down to one being better than the other. Like I always say, it's horses for courses and each has their place.

At the end of the day it comes down to how well it's built, how it's maintained and how it's driven. All things being equal a diesel is built tougher and should last longer. However, it's not correct to say that petrols are a short-term option or that they are inherently inferior. I plan to keep my petrol i30 for a long, long time and I have no doubt that it will go the distance. I'm sure if I had bought a diesel it would have gone the distance too.

Nobody has chosen "wrong" and nobody has chosen "right". Whatever works for you is the "right" choice.  

[Dazzler]

Beautifully explained Michael.. I spotted this earlier in the day but only had time now to give it the attention it deserved..

[LuciferDarklord]

Personally I have seen a lot of older diesels retire earlier than they should because repairs to the fuel injection system cost more than the worth of the vehicle.  Also in this case people tend to run them into the ground, and poor injector performance or injection pumps being out of calibration tend to wash the oil off the bores, and low compression results.  Another common problem with older diesels is blocked air cleaners, once again - not enough air, washes the bore down.

Another observation I have made is that engines originally designed to be petrol that have been 'retrofitted' by the manufacturer to run on diesel, tend to have severe structural issues due diesel 'knock' - and example is Chev V8 diesels that were very popular for landcruiser and patrol conversions.  They crack the block around the main bearing webbing.  Nissan LD28 and LD20 seem to fair a little better, but they are'nt as long lived as SD25 or SD33.  Having said that I've also seen numerous SD33 rebuilds that have all sorts of problems getting the rings to bed in.

There is way too many variables to make a strict scientific observation, but generally from what i've seen if looked after a diesel will tend to last longer than a petrol.  I still reckon I was onto something in the previous page where under light loads or overrun - induction and compression stroke, diesels having no vacuum and more often on boost, tends to balance out inertial loads on the conrod and bearing.  Donc, do you see any strength in this hypothesis?

[2i30s]

compression ratios can also play a part in the life of an engine,very high compression ratios and high revs usually mean a shorter engine life. IMO. 

[beerman]

I'm not aware of any evidence supporting a premis that diesel powered engines last longer than petrol powered counterparts... to the contrary, I suspect most taxi-cab owners would argue against this supposition.

Problem with using Taxis is that none of them run on petrol; they all run on LPG which has its own reputation for increasing engine life.

[Lakes]

i might have said something in this post @ some time, but that is now obsoleat, as after talking to my brother in law B4 Xmas he told me after 100,000k most cars loose a lot of value. so as both Diesel or petrol will most cirtainly last 100,000k if we sell b4 to try to minimise our loss's, would not matter what you had. but just my oppionion.
i still have not tried to trade or sell my i30 as yet and 85,000k is coming up fast. it is runing perfect. we can disregard ol donc as seems he was a fly by night man
i will tell you what i have seen, i saw one guy i knew buy a brand new petrol ( brand not important ) he then drove it 105,000k & the poor motor had never had one oil change, i checked his dip stick for him once after 100,000k & it looked like fresh sprayed road tar, it did not apear to even dribble off dip stick. i told him change it but he had no time, then 5,000k latter his motor got a bad sqeel anothyer guy said he thought it was the fan belt slipping but it was his main end bearing or what was left of them. cost him a total engine rebuild.
if you tried this with a diesel could be bad but i think they would not start long b4 100,000 as the soot build up would thicken the oil up  a lot, most diesels won't run with thick weight oil.
this is just my oppionion it does not mean i'm right
cheers

[Ultralights]

All interesting comments, but none on the topic of fuel,  Diesel is essentially a light oil, and hence, at startup, the upper cylinders are lubricated as the fuel is injected/inducted. negating the effect of oil draining off the cylinder walls, whereas Petrol is a solvent, and does a good job of removing residual oil.    also, in the broad strokes, Diesels are built far stronger as the engine essentially is running on detonation, if you pull apart a diesel compared to petrol engine, you will find all bearings, bearing journals, crank,rods, valves, etc are all a lot stronger and heavier. oh, and they run hotter, LPG engines last a similar distance, as seen in taxis, as LPG does not remove residual oil from cylinders, and also produces very little in the way of particulates in combustion to pollute the oil, hence the oil remains cleaner for far longer in dedicated LPG engines.  such as most taxis.

[agentr31]

bah... diesels last longer... its pretty easy to see why

open one up and EVERYTHING IS BIGGER AND STRONGER...

high compression for a petrol is 11~1 for a diesel its 22~1

the thing with the 'diesel is an oil practically' it doesnt really get time to do much as its injected just before TDC

[rustynutz]

bah... diesels last longer... its pretty easy to see why

open one up and EVERYTHING IS BIGGER AND STRONGER...

high compression for a petrol is 11~1 for a diesel its 22~1

the thing with the 'diesel is an oil practically' it doesnt really get time to do much as its injected just before TDC

I know you're just generalising......  
Ours are actually 10.1:1 for the 2.0 litre petrol & 17.3:1 for the 1.6 litre diesel.....

[agentr31]

thats why i said "high compression for"

[noels_hobby]

grandfather was a tractor mechanic
i recall part of a conversation on this some time ago, petrol v diesel engines
the diesel engine tractor was heavier because it was a higher compression engine than the petrol counterpart and needed thicker walls for the compression and extra heat
the diesel lasted longer because both sides of the rings were lubricated upper with diesel fuel which is oily and lower with engine oil. as with valve guides. the old petrol tractor thou started on super and run on kero. petrol consumed twice as much fuel than the diesel and was more expensive to maintain  the diesel engine had no electrics apart from the starter everything was mechanical.
the crdi i30 is a high pressure diesel injection how much diesel would be there too lubricate any thing and how long the engine will last is anybody s guess. i would be happy it lasted 400 000k's 
noel