I have noticed that correspondents on this forum often quote oil consumption rates for modern engines. In the past, only old bangers used oil ? a measure of an engine in good condition was that it did not need the oil topping up between changes.
My first car, a Triumph Mayflower which cost £10 of carefully saved pocket money, left a smokescreen behind when it pulled away, because the piston rings were worn.
But all my subsequent cars have needed no oil at all. My Volvo on 290,000 does 20,000 miles a year and I never see the oil level dropping by so much as half a millimetre.
Why is it normal for some modern close-tolerance engines to burn oil ?
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Cliff - once the engine is properly bedded in, I don't think it IS normal. My 306 HDi uses a scarcely measurable amount between 6000-mile changes, probably <100ml. I suspect the makers are just protecting themselves against the owners of occasional rogue motors which do lose oil somehow.
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Modern engines should not burn much oil once run in. If nothing else it is not good for emissions and emission control system (lambda sensor, cat etc.).
Manufrs. always quote fairly high oil consumption figure in the handbook (1 litre per 1000 miles is typical) to keep down warranty claims.
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Not wishing to divert your thread but you mentioned the Triumph Mayflower:-
I had one which also burned copious amounts of oil and, as I was working in a garage at the time, we decided to strip the engine down. Having removed the spark plugs and head nuts from the (all aluminium) engine we found the head wouldn't shift.
We had some rings that screwed into plug threads, so inserted two after filling the bores with penetrating oil and hooked up the engine lift pulley.
24hrs later - no change. Lifted front of car with engine hoist and left over weekend - no change. Gave up.
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Could have tried starting it with the head nuts undone a full turn ;-)
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We are told constantly that the modern car engine is built using precision machinery to very fine tolerances and therefore should not burn any oil between 6k changes.
In practice however this is not case and a large number seem to actually conform to the high consuption figures quoted by the manufacturers and do actually burn 1 litre per 1000 miles, which quite frankly is just not acceptable.
I have had personal experience of this myself with a 2.8 Audi A4 burning oil at this level from new up until it was sold at 35k without deviation, (which should be well past it`s running in period).
IMHO the reason certain modern engines burn oil excessively :-
1. Modern engines are designed with the sole purpose to run at maximum fuel efficiency and as such tend to operate at higher
temperatures, particularly where turbo charged.
2. To maximise fuel efficiency fast moving components like pistons are designed so that they are not only lighter but shorter (less skirt below the gudgeon pin) to reduce frictional
losses within the bores.
3. Modern synthetic oils are now much thinner with 0W-40 being specified on many high performance engines, which would also contribute to potentially higher oil consumption.
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Don't understand your point 1.
How does running at higher temperatures potentially cause more oil consumption - modern oils maintain their viscosity range under extreme conditions, so is the oil not suitable for the engine?
Point 2.
Understand the lighter bit but frictional losses are caused by the rings which they haven't got rid of.
IIRC It's because of the huge forces the reciprocating parts generate, by lightening them you can reduce the strength of the crank and crankcase which gives you comparatively large weight savings.
This still doesn't explain why the same engines consume oil at differing rates.
The first car I ever bought new I insisted on being trailered to my door - such naievety! When the cars are shipped they are loaded/unloaded with speed being the priority.
Let's face it - as long as the cars aren't marked who knows what goes on.
Oil consumption at the rate you describe is unacceptable, I'm surprised that Audi didn't sort this under warranty.
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>>Manufrs. always quote fairly high oil consumption figure in the handbook (1 litre per 1000 miles is typical) to keep down warranty claims.>>
As has already been mentioned by Aprilla and VAG are among the many manufacturers that quote these sort of figures in their handbook, so to complain is totally futile.
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Surely the worst oil-burning cars (and I would call 1 litre/1000 a bad case) would not pass an emissions test? Quite apart from fouling the lambda sensor?
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I don't believe even a litre/1000 miles will cause any emission test problems. The average car is burning a litre of light hydrocarbons (petrol) every 7 miles or so, so burning a litre of heavier hydrocarbons (oil) over a thousand miles is unlikely to have any measurable effect.
Regards
John S
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But does it burn it cleanly, John? Oil-burning engines always used to make blue smoke. Does the cat cope with that as well?
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In response to Malcolm L.
The crucial thing to realise with piston rings is they have reduced in width by more than 50% (from approx 2mm down to 0.6mm) to reduce frictional losses in search of the holy grail of maximum fuel efficiency.
This highlights the essential need for precision machining tolerances and helps explain why the Japanese (who have proved they work to the finest tolerances in the world) engines burn the least oil.
IMHO it also helps explain (against popular belief) the reason to ensure that these narrower rings are allowed to bed down properly in their bores, by running a new engine in along the sensible guidelines suggested by H.J. and help explain the difference in oil consumption between identical engines. I confess that I thrashed my company Audi A4 2.8 from day one.!!!
There is an interesting thread on piston/piston ring design trends on the following URL from paragraph 6 :-
www.babcox.com/editorial/ar/eb90329.htm
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Andrew
When the oil enters the combustion chamber via the piston rings or inlet valve guides at that sort of rate, then yes, it will be cleanly combusted - it's a miniscule amount per revolution. As I say, compare the fuel burn rate to the oil burn rate. It won't affect the cat. I've been told by an MOT tester that even engines buring much more than a litre/1000 miles still pass the emissions test for HC.
Smoking oil burning engines are, as you say, combusting the fuel badly. But these are burning 100% fuel oil, not just burning a very small amount of oil in a petrol engine. The heavier the fuel, the more difficult clean combustion is. Hence all the recent developments in diesel injection technology.
As an aside, think about lubricating valve guides and rings. The inlet is subject to vacuum, which will tend to pull oil down the guide. The rings clearly get sufficient lubrication as engines last for so long, and this must surely leave a film on the cylinder walls. Thought of like that it's perhaps a surprise that oil consumption is as low as it is!
Regards
John S
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John,
What you say makes good sense, however I slightly disagree with your first sentence. Oil passing into the combustion chamber via the piston rings does not necessarily burn cleanly.
The part of the piston that extends above the top ring has to be deep enough to provide adequate support for the ring. This leaves a shallow pocket around the piston, above the top ring, in which any trapped lubricating oil or diesel fuel tends to escape the combustion process. This unburnt or partly burnt matter can find its way out at the wrong point and pass into the exhaust to emerge as an unwanted emission.
To help overcome this problem, Cosworth came up with a piston design, 10 or 15 years ago, that somehow forced air into the 'pocket' to keep it clean and thus reduce emissions. I can't remember the details and don't know if it went into production.
The unwanted 'pocket' is a main reason for engine designers to keep the height of the piston above the top ring as small as they can, along with the need to minimise piston mass of course.
Incidentally, far more attention than ever before is now given to designing rigidity into engine cylinder blocks because modern emissions regulations are so strict that just a miniscule deformation of cylinder bores can lead to a reduction in ring-to-bore sealing. This applies especially in diesel engines with their very high cylinder loadings but also reads across into spark-ignition engines. It's an important point that I never see mentioned in discussions on emissions.
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Dizzy
Fair comment. Yes, oil trapped in that region tends to cause problems - sticking top rings on 2-strokes being one example. The key point I was making, though, is that oil consumption has to be far worse than 1 litre/1000 miles to have any effect on emissions, even if it's not burned very cleanly.
I perhaps should have qualified the answer a little. As I'm sure you agree, the usual first cause of high oil consumption is worn inlet valve stem seals and/or valves and valve guides. This allows oil into the inlet port and (up to a point) this will tend to burn cleanly in the combustion chamber. Wear here is more obvious on overrun. Manifold vacuum is higher and there's less petrol being burned, hence the classic blue smoke. Given that the MOT emissions test doesn't include overrun, this isn't a problem 'till the valves, guides etc are very worn.
However, once the bores and rings are worn, then exactly the problem you describe comes to the fore. Too much oil is left on the bores, and this won't burn cleanly. By this stage though the engine is on its last legs; oil consumption is very high and the engine is smoking under load.
Regards
John S
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