Been wondering this for a while....
Why do some engines mangle themselves when the cambelt snaps, and others just need a new belt?! I'm aware that the older Vauxhall 8-valve engines and some Fiat units (Seicento?) are "non-interference".
Why can't all engines be built like this, so people only have to pay the £200 for a new cambelt-kit, instead of a thousand or more for a recon engine?
The main reason I bought a Nissan Almera was because it's got a chain instead of a belt. Why don't manufacturers stick with chains? Am I being cynical in thinking it's because the dealers will get more money from continuous belt-changes, and all the engine repairs after snapped belts?!
Also, which cars on the road today have "non-interference" engines?
Any comments welcome!
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Why do some engines mangle themselves when the cambelt snaps, and others just need a new belt?! I'm aware that the older Vauxhall 8-valve engines and some Fiat units (Seicento?) are "non-interference".
The valves occupy the same space (But not at the same time!) as the pistons.
Why can't all engines be built like this, so people only have to pay the £200 for a new cambelt-kit, instead of a thousand or more for a recon engine?
I think the 'advantage' of interference engines is better compression but I will need one of our experts to check that for me. Also I'm sure you could get better compression without resorting to this design.
Why don't manufacturers stick with chains? Am I being cynical in thinking it's because the dealers will get more money from continuous belt-changes, and all the engine repairs after snapped belts?!
This is my personal opinion, but a lot of people will tell you it's due to noise issues. Personally I wish my car was chain and if a kit was available to convert it I'd be first in the queue.
Also, which cars on the road today have "non-interference" engines?
Not sure about this. It's mostly small cars with low capacity engines, I think.
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I agree, there seems to be no pattern, 4 valve per cylinder engines can be NI, and all sorts of capacities.
I can see why diesels can't be as the pistons usually almost touch the head and the cumbustion takes place in a dished piston. Maybe the same is true for Interfrence petrols, a low clearance with a dished piston with everything machined to create charge turbulance.
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Its all to do with:
Compression ratio
Piston design
Head design
I head using inclined valves is less likely to be interference than one using orthogonal valves. Piston crown design is also a factor. Obviously a higher compression makes intereference more likely. Its not really related to capacity.
Whether or not an engine is intereference is not something that is likely to pre-occupy the designers - they have much higher priorities.
Chains don't really make any more noise, but they are more costly to build and deprive dealers of an income stream!
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Modern chain-cam engines are quiet, but early engines were not and the introduction of the rubber belt was primarily for noise and vibration reduction. Manufacturing processes are better and tolerances are tighter now so the trend is a gradual swing back to chains. The angles and positions of valves in the head affects the efficiency of the burn and hence emissions, these are more of a priority to manufacturers than whether the piston meets the valves when the belt snaps, after all the belt snapping is not a normal function of the engine. The fact that belts need changing and chains don't (not for a very long time anyhow) raises revenue for the dealers, is a lucrative by-product of the design, but not part of the original plan IMHO.
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90% of the the choice between chain and or belt is down to cost at factory production level, and dealer income after sale.
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90% of the the choice between chain and or belt is down to cost at factory production level, and dealer income after sale.
If that were the case, why would any manufacturer ever produce a chain-cam engine, a belt-cam is obviously cheaper to produce and raises more dealer income ?
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I think Kith's post is spot on.
When compared with the problems of fluid dynamics, heat transfer, combustion kinetics, pollutant formation, and tribology, making non-interference engines is way down the list of priorities.
The 8 valve Vauxhall engines tend either to miss completely when the belt snaps, or, to snap the rockers, which, thankfully, have a designed in weakness to allow this to happen without causing more expensive damage.
Roll on camless engines, with computer controlled valve actuators.
Number_Cruncher
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I came a accross a very interesting old Norwegian single cylinder, marine 4 stroke diesel. With no camshaft. The valve push rods were opperated off a large diameter cam that was on the crankshaft. In 30 years of working with engines i'd neverv seen that before
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I don't see how it could have been 4-stroke. The camshaft rotates at half crankshaft speed, because a cylinder on a 4-stroke fires on every alternate revolution, so the valves have to open and shut accordingly. Was it a semi-diesel perhaps?
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Could have been a valved two-stroke;having said that,there have a fair number of two-stroke diesels with inlet valves,either mechanically operated or just by airflow-called "uniflow" engines with the exhaust ports uncovered by the piston towards the bottom of the stroke.There is,of course,one type of four-stroke that could never suffer "interference" in any condition-side-valve.
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I read in a article recently that when the chain needs replacing in a 2.8 Sharan that it's an engine and box out job followed by splitting them to get to the chain. I'm under the impression that HDi's were designed so that if the belt goes it breaks the valve rockers rather than the camshaft, I'm sure that if this is true they'll still be times when the damage is more extensive. What sort of prices are charged for chain replacements? I would have thought it's more involved than a belt even if they don't need doing as often.
Steve.
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Xantia HDi.
Buy a Citroen and get to know the local GSF staff better...
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Do the VAG TDI engines have this built in safety measure or do they just trash themselves?
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it was 4 stroke ohv i think there was some trickery involved with the cams i could not fully investigate but i think it was on some sort of sleeved couping on the crank somethink like the Honda vtec engines
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There is a 4 smoke engine for model aircraft (British design too) which utilises a rotating part of the bore liner at the top of the combustion chamber to open and close inlet and exhaust ports.. A very clever design, and impressive animation is on the net somewhere, but I can't find it. Being British, it probably went bust?
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www.rcvengines.com/rcv60sp.htm
Ah here it, is, rotary valves.
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Thanks for all responses - interesting stuff!
Someone asked about cost of chain replacements - it'd be a lot more than a belt replacement, because chains are designed to last the life of the engine - my old G-reg Sunny had 115,000 miles on the clock when I sold it, and still had the original chain.
I heard of a new-model Almera 1.5 where the chain skipped a notch, and it cost a small fortune to replace the chains (16v engines normally have 2 chains) and tensioners. Chains are a better choice for people like me who don't mind changing oil and plugs, but don't really want to fit a piece of rubber that might wreck the engine if fitted wrongly!
Anyway - my other question which nobody's answered yet- which modern cars have non-interference engines? I've heard that some Mazda MX-5s did, likewise the old Cavalier 1.6, and one of the Fiat engines used in the Seicento. Any others out there?
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I don't think there is a definitive list. Here was a link I found with a quick google. It has a PDF at the bottom which states the recommended belt change times for ALL cars! It is american, but my Bora (Jetta) 2.0 was listed at 60K, whereas my dealer is suggesting 40K inline with VAG's new belt replacement policy.
www.familycar.com/CarCare/TimingBelt.htm
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VW Bora (51) 2.0 SE
VW Touran (54) 1.9 TDI
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The very clever model aircraft engine is a variation on sleve valve design popular with some aircraft engine deigners to get round many of the problems associated with poppet valves.Those of us at a certain age will remember "valve bounce" !
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I think you will find nearly all engines with belts fiited now are interference type. can't think of any off hand that are not.
In the pursuit of more economy, multiple valves to assist this, smaller engines and high compression etc it is inevitable that it is the case!
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camshaft design must be an important factor - high valve lifts and opening durations must increase the likelhood of valve to piston contact
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I saw an interesting design on an ancient car [sorry, can't remember what] in the Gaydon Heritage museum which had the cam driven by neither belt nor gears nor chain but a vertical shaft from the front of the crank. It passed through the dynamo!
Modern cambelts should last the life of the engine as long as the things it drives [tension pulley, water pump etc. don't seize or disintegrate. I never change them. My Passat's was still going strong at 240,000.
At Gaydon there was a Hart racing engine on display - its thin cambelt was no wider than my index finger. The belts on our relatively unstressed Ford Zetec 1.6 and Audi 2.8 are much thicker, and wider than my thumb. I expect them to last the life of the engines with the help of an occasional squirt of belt dressing.
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The 1.2 8v engine fitted in the Punto is non-interference, I think the same engine is still available in the Grande Punto and Panda.
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I read somwhere that Nissan are loosing a fortune on camchain warrenty work on Almera's-Smaller,thinner,cheaper etc.Vauhaul had the same problem not so long ago.I think everthing is down to a price nowadays.I recon if they can put a man on the Moon they could make a cambelt that would last the life of the car.But then they couldnt charge you loads of money for replacing them every 50-60K.
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Thank god I got an Almera with the old GA-series engine rather than the new QG series! Having said that my mum's thinking of buying a newer Almera with the QG engine. I've only heard a couple of horror stories, both about the 1.5 litre - anyone know if the 1.8 is equally affected?
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Why not just change the belt when recommended then worry not?
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\"Nothing less than 8 cylinders will do\"
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Why not just change the belt when recommended then worry not?
It's not so much the belt, but the tensioners and associated pulleys that tend to fail first. More often than not, Vauxhall cambelts (for example) before seperate tensioners appeared and the belt had to be tensioned by turning the offset waterpump could manage 80,000 miles before needing changing. Then along came the nylon tensioners which started failing prematurely, so Vauxhall rather than re-engineering the tensioner, lowered the interval to 40,000 miles.
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The engine with a vertical shaft to the cam was a pre-war MG(Wolseley had them as well);they leaked oil like a sieve.NSU had an interesting drive which consisted of three eccentrics-easier to say than explain!
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The original concept was that cambelts were cheaper than chains, lasted at least as long as chains, could withstand misalignment better than chains, needed less maintenance than chains and were quieter than chains. Didn't quite work out like that though.
Pushrods, that's what we need.
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Nah!!!;what we want are sidevalves.
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Never had a timing belt go on my horse and cart...
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"The original concept was that cambelts were cheaper than chains, lasted at least as long as chains, could withstand misalignment better than chains, needed less maintenance than chains and were quieter than chains. Didn't quite work out like that though. "
well 4 out of 5 aint bad.
------------------------------
TourVanMan TM < Ex RF >
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The first engine that I can recall with a cambelt was a 2.0 Ford Pinto. Belt breaks, car coasts to a halt, no engine damage. A service item shouldn't fail and cause catastrophic damage, the design solution should be to increase the factor of safety, perhaps two belts, perhaps more frequent inspections, perhaps more frequent changes. Or perhaps a belt that is designed to last the life of the engine (ha!!).
If it's an engine out job, the cost of changing a belt is ridiculous. Probably cheaper in the long term to run a nice V8 with pushrods. Is that a business opportunity? Replacing destroyed humdrum diesels with nice 3.9s or similar? MOT emissions tests might be a problem though.
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So does anyone have a list of all 'non-interference' engines? I am making the assumption that they are no longer manufactured, except in places like India perhaps?
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The nissan td27 uses timing gears no belt or chain much better wish more were like this, chains are great if the oil is changed regular if not they stretch and snap.
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So does anyone have a list of all 'non-interference' engines? I am making the assumption that they are no longer manufactured, except in places like India perhaps?
Fiat FIRE engine as fitted to the latest model Fiat Panda.
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One of the main reasons we bought a 1.1 Panda.
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