Engine Longevity

Hello from a new member to this group...here are my thoughts on this one..

I guess traditionally the answer is basically temperature in all it's guises and the effects it has- mainly with the oil. Firstly the combustion chamber (and thus the head and oil temperatures) are lower in a CI (diesel engine). Lower temperatures preserve the integrity of the oil. Diesels used to generally run slower and their maximum BHP was much further down the rev range than a petrol engine, although nowadays that isn't the case. Slower RPM's mean less frictional heat and consequently less wear. Another factor is oil dilution. Petrol engines that require enrichment when cold will einevitably dilute the oil with unburned fuel that escapes past the cold pistons and rings into the sump. Oil dilution reduces it's effectiveness and consequently -more wear. Although this does happen to some extent in a diesel too- the effect is much less pronounced because of the limited amount of excess fuel needed when cold. Also diesel dilution is less harmful to the oil. I think if you want a petrol engine to last- don't scream the balls off it when cold, and keep changing the oil! :-)

That's my 5p's worth...

-MartinT

You sure ? The thermodynamic efficiency of a diesel is better than a petrol, because the difference between absolute inlet and outlet temperatures is HIGHER not lower.

Petrol sump oil dilution will usually evaporate, diesel oil sump oil dilution does not ( we make instruments for measuring it, as an engine failure predictor) If the viscosity of the sump oil drops too low, which it notoriously does in some rough cycle conditions, the oil is too thin to support the journal bearings under load, and the engine collapses in short order.

I reckon you're right about lower rev ranges, but a diesel is usually built more strongly, to resist the higher pressures of the diesel cycle, and that is probably the greatest contribution to longevity.

Steve

In message , steve writes

When I asked some question here earlier, I was advised that people changed the oil in diesels regularly. Is this why?

With someone like me who uses the vehicle intensively in spring and autumn, with it sitting idle in the path for the other 11 months, do I really need to change the oil often?

With the old petrol 110, I did about 1k miles at most per year and never changed the oil over about 10 years. Do I have to change my ways?

Mind you, my priorities and usage may change now that I've measured the mpg of this 200tdi. I'm getting 32.8 - about 10 mpg better than the old

2.25 petrol one.

I reckon that with engine technology these days the life expectancy of a diesel OR petrol is down to how well and how often you maintain your engine.....

I seem to remember that 2.25 diesel engines never lasted as long as

2.25 petrol engines simply because you had to work the knackers off the diesel to get any "performance" out of it.

Peversely though, the 200tdi has a reputation for being extremely long lived when maintained properly. I can vouch for this (hang on while I touch some wood......there we go) the 200tdi in my RRC has just knotched up 140K, and still doesnt use any oil.

Dave

A good friend has a 'from new' M reg Discovery 300TDi getting pretty close to 300,000 on the original engine. It still runs extremely well and a year or so ago his local specialist measured the oil pressure as "pretty much as new". The body is gently rotting away but the engine is solid.

I suspect that when the body rot starts causing serious MoT issues the engine will find its way into an old Defender / 90.

160K on my uprated-intercooler 300TDi, it's done 80K since that went on. I've never once had to add oil to the engine, and it's never let me down. The top intercooler hose was replaced some time ago and the alternator front bearings are on the way out but other than that, it's been a cracker.

The thermodynamic efficiency is better with a diesel because it has a higher compression ratio but as it cannot burn a stoichiometric amount of fuel and that fuel is still burning as the piston is on its down stroke much of this benefit is lost. Because of this its burn temperature is actually lower than the petrol engine as well as rejecting the heat at a lower temperature.

The main advantage is its volumetric efficiency is better, so that when it is at part load it is far more efficient than a throttled petrol engine. Also diesel has about 10% more calorific value than petrol so you get more energy out of a litre.

Yes I'm sure that was true of many diesels, I wonder if the same will be true of these modern offerings which are nearer petrol (spark ignition) in their characteristics because of the effect of the turbo. It's probably moot for most cars with a life of 13 years meaning

Happy new year Steve and all afl

AJH

"AJH" wrote > (SNIP)

I wonder about that too. Until fairly recently all Diesel engines were designed for commercial vehicles so therefore had to have longevity designed in, thicker bearings, stronger rods etc, but the modern car diesel engine revs (a bit) more like a petrol and needs light efficient internals, and the design concentration is on high output, mpg and NVH not longevity. I doubt they will last any longer than a modern petrol engine and probably less if they aren't looked after properly with regular oil changes etc. That said, a modern designed petrol engine usually last a very long time.

Thats reassuring! As soon as we're into the spring i'm going to visit the boys in Allisport. If all goes pear shaped I've got a spare engine in the barn!

Who did you have the intercooler with, worth having??

Dave

The intercooler is a Jeremy Fearns one fitted by Turbo 4x4 in Reading, and yes it's very worth having, I just had a large intercooler and a pump tweak to suit and the difference was very noticeable. It'll hold

85 on the motorway and accelerate much more readily. It ends up being like a TD5 but without losing the low-down torque that the TD5 apparently lacks.

Since the early 1980's is not really 'recently'. Think of the Montego and Cit BX diesels. I reckon modern diesel car engines up to common-rail ones last longer than those old clunkers ever did. The complexity of new CR engines is likely to work against reliability if not longevity all other things being equal. However, all things might not be equal and it is equally possible that they are better built for a longer life than ever before. As always it is likley that a proportion of any engine model will fail within 100,000 miles while a near equal proportion are capable of lasting 250,000 miles or more but few will actually hit that due to factors other than engine failure.

Huw