I think all Diesels are 'interference' due to the high compression ratio.
There are Diesel engines that use drive gears rather than belts or chains (VM Mottori).
|
|
There is nothing fundamentally wrong with a cambelt, they are often quieter than chains and chains can be problemmatic at higher mileages. The main problem is some manufacturers not being specific enough when belts should be replaced, designing engine installations where it's difficult to do and designing interferance engines.
|
|
|
I think all Diesels are 'interference' due to the high compression ratio. There are Diesel engines that use drive gears rather than belts or chains (VM Mottori)
Yes thats about thats good with them!
There is a company that makes a aftermarket kit for the Land Rover TDi engine to convert it from belt to gears, expensive
Most larger diesels use gears can be noisy
Loads of diesels are non interference
Cambelts are a good system if the particular car your worried about has a known problem then inspect & change more often
|
|
|
|
I know this may have come up before, but why don't manufacturers change to other then cambelt / chain setup.<<
Probably expense. I know Honda put gear driven cams in the VFR750 motorcycle many years ago and it placed the retail price of that model at the higher end of it's market.
The durability of any cambelt is purely down to how much the manufacturer is prepared to invest in the product. A colleague had an Astra belt fail at 32k miles, 4k short of the specified mileage. Yet Vauxhall shrugged that off as not their problem.
For peace of mind I have just had the belt replaced on my daughters Fiesta 1.25, it is just short of 9 years old and completed 42k miles. And it was used mainly for 5 mile max trips for the first 7 years of it's life. Not only that the mechanic said the belt would have lasted longer as it was in good condition and the tensioners were all still in good working order.
Ford recommend 10 years or 100k miles on this model, so you can see there is a big disparity between manufacturers spec. and product quality.
|
The belt load required to open the valves is one factor. Diesel engine valves require a greater force to open them than do petrol engine valves. I'm not sure exactly why. I think it's partly because of the greater acceleration of the valves as they open.
--
L\'escargot.
|
The belt load required to open the valves is one factor. Diesel engine valves require a greater force to open them than do petrol engine valves. I'm not sure exactly why. I think it's partly because of the greater acceleration of the valves as they open. -- L\'escargot.
That's a new one on me, can't see there being any difference
|
That's a new one on me, can't see there being any difference
It depends on the valve lift and the cam profile.
--
L\'escargot.
|
>> That's a new one on me, can't see there being any >> difference It depends on the valve lift and the cam profile. -- L\'escargot
Same goes for petrol then
|
Some diesels (ie VAG TDI/PD engines) use the timing belt to drive the injector pump.
Thats why some diesels place a higher load on the belt...
------------
MoneyMart
Current car: 55-reg Audi A4 2.5 V6TDi Quattro flappy-paddle
|
|
|
|
I expect this will show my ignorance here but: Cam belts and to a lesser degree chains continually seem to be main a cause of engine failure, which on older cars can mean scrapping. When I was a lad all the cars I owned had push rod engines and as such these sort of problems didn't exist. I know push rod technology is old -- although I think some big US V8's still use it. Someone please tell me why this design dissapeared --- I've no doubt there is a good reason, most likely performance -- twin overhead camshafts, 16/24 valves? also economy perhaps.
Sorry if this sounds stupid but the cost of changing belts on older cars must be getting more and more uneconomical. £300 cost on a car worth £500? Now if that car had a push rod engine that would go on & on...... Living in the past - yes!
|
|
The latest Camaros and I believe the new Corvette still use push rod engines the 5.7 v8 certainly is.
|
|
|
Push rod had there cams driven by chains ...sorry but these can be VERY Problamatic with age
Thin OHC came about with higer revs being reqired and they are a lot smoother & less moving bits but ultimately nothing wrong with push rods
Buy a KA, still has push rods (well the 1.3 does)
|
|
Does anyone know how far we are away from getting electrical or pneumatically operated valves on road cars?
|
This has been mentioned fairly recently but until component quality and reliability is up to Lexus standards I'm not buying.
Might be some gains in efficiency and it might reduce weight by eliminating the need for VVT but at what cost?
As an example, I wonder if in hindsight, VW would have preferred to save a few pfennigs less on their cambelt tensioner mechanism?
|
|
|
|
pneumatic operated valves are necessary for high revving f1 engines but not practical for every day engines.fine while you maintain system pressure,but as soon as you lose it-bang(a lot of f1 engine failures are loss of pneumatic pressure)you regularly see the systems being topped up during pit-stops.
|
|
|
|
Gentlemen, (apologies to any ladies here) if you want the performance of a modern engine, especially a diesel at a price most of us would be prepared to pay, a belt drive is essential. To get the best out of it, spend a little time finding out the weaknesses and then address these points. My old thread "when to change a cambelt" may help.
The Ford petrol pushrod engine is the only one I know of which sounds like a diesel after a few thousand miles. Not many people would be prepared to live with '50s engine technology these days.
659.
|
Not disagreeing with you but the designers/manufacturers of the engines introduce the weak points, VW being a case in point.
Technology doesn't exactly seem to be marching forward on this particular aspect, I can't remember a belt replacement interval being extended - they're all moving the other way which would suggest that something's wrong somewhere.
I don't want 50's technology to ensure reliability, I want to know that cambelt/valve technology is keeping pace with development in the auto industry - which doesn't appear to be the case.
|
Fair point, but engine technology in terms of performance is moving forwards at a rapid pace - a modern Di diesel will give an SFC (specific fuel consumption) which would be unachievable only a few years ago.
All of the gizmos we buy have their wrinkles, and cars are certainly no exception. The wise guy seeks out and addresses their weaknesses - then enjoys the good performance on offer. This forum is a good place to look.
659.
|
Problem with pushrods is all the extra intertia in the valve train. If you want valves to open and close quickly (i.e. high rpm) then they're not good. Plus there are more components so its a more expensive design.
Belts are OK if well engineered, trouble is the accounts get their cold clammy hands on the design and you end up with a cheap solution and the belt driving the waterpump into the bargain.
|
If I may: the mass (weight) of valves plus rockers plus tappets and pushrods - the 'reciprocating mass of the valve train' - has to be overcome perhaps 100 times a second by the valve spring. Cam opens the train positively but the spring has to close it. When the spring can't close it in the available time, what occurs is valve bounce or 'float', nature's own rev limiter. Potential rpm can be increased with stronger valve springs, but wear on all components is increased thereby. One reason why heavily tweaked engines don't last as long.
It seems obvious to me that belts were adopted for ohc valve gear because they are cheaper and quieter. It might also have looked at one time as if they would be easier to change, although that obviously isn't always so. Chains become audible after a while and can grow quite noisy. But as far as I know VVT can only be arranged with chains, so perhaps they are going to come back.
|
"But as far as I know VVT can only be arranged with chains, so perhaps they are going to come back. "
I don't beleive so, the VVC engine in the MGF and other MGR vehicles is belt driven.
|
"But as far as I know VVT can only be arranged with chains, so perhaps they are going to come back. " I don't beleive so, the VVC engine in the MGF and other MGR vehicles is belt driven.
Oh dear. I believe you. Yet another unrepairable Heath Robinson device... awfully clever but what about high-mileage neglected examples?
|
"Oh dear. I believe you. Yet another unrepairable Heath Robinson device... awfully clever but what about high-mileage neglected examples?"
The VVC fitted to the K Series appears to be quite reliable, I have not heard much bad about it. Unfortunately the fact it's fitted to a K Series is enough to put me off one.
|
I agree... belt replacement on K series is a doddle.
Nothing wrong with that design!
------------
MoneyMart
Current car: 55-reg Audi A4 2.5 V6TDi Quattro flappy-paddle
|
|
Performance is taken for granted, let's face it no-one's going to buy a real carp car from new unless it's very cheap.
However, technology moving forward at a rapid pace is increasing TCO which isn't so good.
Not sure where the happy medium is any more, I drive an Accord CDTI which I'm totally happy with apart from the sub 40mpg fuel consumption.
I moved from a Passat TDI so the engine is a generation ahead yet gives worse consumption for a similar sized car with similar drag co-efficient so pardon my scepticism.
|
The best diesels have all been designed on the mainland of Europe - the Americans and the Japanese have had no interest in fuel consumption until very recently. Choose the European mainland country of origin for your diesel car according to your taste and pocket.
659.
|
|
|
|
malcolm,i am suprised your CDTI accord gives you sub 40mpg,does your driving involve a lot of town driving.On my daily dual carriageway/motorway commute i regularly get 39 to 40 mpg from my petrol mazda 6.
|
So am I! I moved from the Passat as although the economy was good (43-44mp), it had a penchant for warranty work which got somewhat tiresome.
I don't drive any differently to the Passat, maybe a tad slower on motorways and I struggle to hit 40mpg. I wonder if the CO2 figures that were published which I took to be a measure of the car's efficiency have been unduly manipulated.
General concensus is that diesel Accords are a little thirstier than expected.
|
|
|
|
Not disagreeing with you but the designers/manufacturers of the engines introduce the weak points, VW being a case in point. Technology doesn't exactly seem to be marching forward on this particular aspect, I can't remember a belt replacement interval being extended - they're all moving the other way which would suggest that something's wrong somewhere.
The XUD replacement interval moved up from 54K to 72K miles towards the end of it's life. For most PSA HDIs the replacement interval is 100K miles.
|
|
|
|
if you want the performance of a modern engine, especially a diesel at a price most of us would be prepared to pay, a belt drive is essential. >>
The 2.0 and 2.2 Ford units in the Mondeo and X-Type are chain.
|
>>factors affecting cambelt lifespan>>
Driving cylinder specific high pressure injection pumps off the camshaft as per VAG PD engines.
|
Wasnt Rover designing a camless engine based on the K series?? Powertrain are aparently still working on it with Nanjing.
Does this get away from the cambelt/chain?
|
|
I believe the quietest engines in the past used gears rather than chains. If properly lubricated they last for ever, but are presumably much more expensive.
|
Some interesting stuff here. Gives some reasons for the proliferation of belt drives on modern engines.
www.diagnosticengineers.org/B5-13.htm
|
Some interesting stuff here. Gives some reasons for the proliferation of belt drives on modern engines. www.diagnosticengineers.org/B5-13.htm
Thanks for that link, very interesting. (I'm not being sarcastic, I mean it!)
|
|
>> if you want the performance of a modern engine, especially a diesel at a price most of us would be prepared to pay, a belt drive is essential. >> The 2.0 and 2.2 Ford units in the Mondeo and X-Type are chain.
Vauxhall, Saab and Nissan also use chains in some or all of their engines. Chains can break, but they generally last a lot longer than belts. A cam chain usually gets noisy when its worn, giving some warning that its on its way out; belts tend to snap with no warning.
Its not uncommon for owners of petrol engined Saabs to still be on their first cam chain at 150 to 175k miles plus.
|
Also the 4cyl and V6 Mondeo/X-Type petrols are chain.
The fundemental difference between a chain and a belt is that the chain is designed to last as long as the engine, they can fail early though so can big end, piston ring, valve spring etc. On the other hand a belt is a consumable that causes catastrophic damage if it fails.
The benefits of belts are really cost alone, packaging is often stated as a benefit however motorcycle engines have chains and space is certainly a premium in motorcycle engine design. In fact I think it is only Ducati that use belts.
|
Although there are exceptions, as has been pointed out, it is the development of the low cost high efficiency diesel which has tipped the economic balance in favour of the belt drive. If an extra timed rotating component such as a fuel pump is to be included in the drive, the costs increase considerably for the chain(s) with associated guide rails and tensioners, wheras the on cost for a belt drive is usually limited to one extra idler and a longer belt.
It's possible that this situation may change in the light of some common rail diesels not requiring a timed drive to the high pressure pump, but I rather doubt whether chains will appear in large numbers in budget diesel engines.
The most evil chain drive of all (unprotected rear motorcycle drive chain) has been replaced with a belt drive by Harley D. and a few others, to good effect.
659.
|
The most evil chain drive of all (unprotected rear motorcycle drive chain) has been replaced with a belt drive by Harley D. and a few others, to good effect.
The belt has not caught on where efficient transmission of power is required, i.e. sports / racing bikes, with the amount of money spent on developing state of the art raod and racing motorcycles these days, if there was a better way someone would have done it by now.
|
If an extra timed rotating component such as a fuel pump is to be included in the drive, the costs increase considerably for the chain(s) with associated guide rails and tensioners, wheras the on cost for a belt drive is usually limited to one extra idler and a longer belt.
Driving such a pump from a cam belt is totaly illogical because it puts further stress on a critical area. If the camshaft/s is/are belt driven then a secondary drive for the pressure pump is clearly a better option.
|
How many PSA type XUD engines were made?
659.
|
|
Millions I should think. And I believe they're still made under licence in India to power generators.
|
|
And despite the fuel injection pump running off the cambelt, have an excellent cambelt failure record. I think this is largely due to the excellent tensioner design.
|
The design of the drive is indeed good on this engine - I know it extremely well as I was working for the company which made the fuel pumps at the time. A 21 tooth crank wheel, solid metal tensioner and idler, both with sealed bearings, good wrap angles, 1" wide belt and a freedom from drive resonances all make this engine durable. If they had not driven the water pump from the same belt, it would have been perfect.
As it is, as long as the pump holds up, the drive will run for far longer than the recommended change interval. Other manufacturers' products often fall down in some respects.
659.
|
Tremendously interesting to hear about it from an engineers perspective. I didn't know the amount of teeth on a crank wheel made much difference, is this because the wheel is therefore broader and stresses the belt less? or something to do with traction/grip?
very good point about the water pump. I now drive a honda after years with an XUD and i notice honda don't like to run the pump from the cam belt. Do othe Jap manuf. do the same?
|
The number of teeth on the crank wheel is a critical factor determining the life of the drive. A low number of teeth (19 is usually the minimum) gives fewer engaged teeth to transmit the drive torque and also poorer engagement of these teeth, as the belt has to bend to a smaller radius. It tends to do so polygonally rather than in a smooth circle due to the presence of the teeth. Add to this the fact that more force is transmitted via the engaged teeth at a smaller radius and that belt degradation is increased due to more flexure with small wheels (throughout the drive as eveverything is in ratio), small wheels are bad news.
A trend to twin-cam engines with both camshafts belt driven has tended to yield a crop of small-wheel designs, as of course the cam wheels become very large with a decently large drive and can prevent a low bonnet line. Our shrewd friends at PSA on their newer diesels drive one camshaft (as on the XUD) and the other from it, via an internal chain.
Finally, the question of drive resonances is a black art, but such resonances can be very destructive. I found that a 21 tooth crank wheel, being a prime number, tended to excite far fewer resonant effects that a 22 tooth wheel, as is used in some other diesels.
659.
|
I found that a 21 tooth crank wheel, being a prime number,
Erm... 21 isn't a prime number. ;-)
|
Dead right, of course; I meant an odd number. The XUD work was done a very long time ago, and my involvement was concerned with the modifications to the rotary fuel pump drive end bearing needed to take the belt load. In the course of this, much testing was done on the XUD camshaft drive, and it was difficult to make it fail. Maybe the prime number was the number of teeth on the belt - I can't remember now.
659.
|
|
But for the fact that more teeth on the wheel ensures less flex and a more positive drive, I would have though that the relevant point would not be the specific number of teeth on the wheel or on the belt rather that the number of teeth on the wheel is not directly divisible into the number of teeth on the belt so it take many more than just one revolution for a given point on the wheel to come into contact with a given point on the belt thus evening out variances in tooth pitch, groove width, belt width, etc.
|
This feature is always incorporated in toothed drives of any kind, belt or chain if competently designed and certainly applies to the XUD. It's dead easy to arrange anyway; just move or enlarge/reduce the size of an idler to allow one extra or one less tooth.
I should also have said that bigger wheels necessitate a longer belt, meaning that there is more material included in the drive. This gives less wear per "unit rubber" and a lower temperature rise.
659.
|
|
659, your post in the related thread in Technical Matters could be taken as advice to anyone buying an aftermarket cambelt. Is it intended as such?
|
It might be better for me not to comment further, although the comments in that thread were made as a result of much testing and represent my opinion.
I'm afraid in these days of litigation, I'll just do a Haynes (sigh!) and suggest that in cases of doubt, buy your belt as an OE part from the vehicle manufacturer.
659.
|
I agree with 659's point about smaller crank pulleys being likely to reduce cam-belt life owing to the smaller bend radius and higher levels of bending stress.
I hope I can add a little info about the belt vibration.
A side effect of increasing the number of teeth on the pulley is to increase the excitation frequency (but to reduce its magnitude) driving the cam belt in bending vibration. The driving mechanism is the chordal action - or polygonal bending between teeth. As the belt tension is also reduced with larger pulleys, for a given belt section, shaft torque, and pulley centre distance, the resonance frequency is reduced (think about guitar strings!). i.e., large pulleys make bending resonance of the belt more likely.
Changing belt width is, interestingly, not helpful in changing the bending natural frequency of the belt, because extra mass is added exactly in line with any added stiffness.
Adding belt thickness does increase bending stiffness faster than adding mass, and hence increases bending natural frequency *but* adding thickkness makes the bending stress under the applied bend radius of the crankshaft pulley much higher. A design parameter ripe for a trade-off study!
Number_Cruncher
|
That's good to hear. I'm due a belt change at the moment but have put it off due to temporary lack of funds. :)
Plus side to running the water pump from the timing belt is that should the aux belt fail (far higher loads on it), it's possible to continue driving for a short distance without the risk of overheating the engine.
Wonder how many HG failures have been caused by people driving after the fan belt has snapped?
|
|
I've got a Vauxhall Corsa approaching its 5th birthday and 25k on the clock - should I be booking it in for its cambelt?
|
Your Corsa should have had its cambelt changed at 4yrs, regardless that it's only done 25,000 miles.
Vauxhall specify every 4yrs / 40,000 miles (whichever comes sooner) for a cambelt and tensioner change.
Unless of course your engine has a camchain? What size engine is it?
|
|
It's the 1.0 three cylinder
|
It's the 1.0 three cylinder
No worries then, as that's the Suzuki engine fitted with a camchain.
|
"How many PSA type XUD engines were made?"
Figure that stick in my mind is 7.5 million - but I don't know where I got it from.
Incredible number of manufacturers used it also.
Only figure I could find was that about 4000 were made per day at the peak - HDi production is apparently 8000 per day.
--
Phil
|
An absolutely brilliant engine in its day. and still pretty good today.
The earlier ones (Pre 1997) were great, all mechanical fuel injection, no ecu and minimal electronics.I think the only thing that was energised when the engine was running was the stop solenoid on the fuel pump (aside from the usual stuff).
|
|
Don't they still make 'em whats the 1.9D engine in the C15 van & in the base berlingo thingys
|
|
Yes, i think you're right.
|
|
|
|
Not many people would be prepared to live with '50s enginetechnology these days.
Absolutely right ! I can't believe we are even discussing pushrod engines here ! The US are not particularly known for cutting edge automotive technology now are they !
I dont see why people panic about cam-belts these days, they are a lot better than in the bad old days of the fabric reinforced belts used on the 70's & 80's cars. In my opinion, if you follow the manufacturers guidelines as to when to change them there are very rarely any problems. The only problem I see is if you buy a car that has been 'clocked' and then you dont change the belt till what you think is the correct mileage !
|
|
Well - a lot of topics have been covered in this thread - but can I just say 'thanks' to those who responded to my original question.
|
|
|
|
|
|
|
|
|
