Many years ago,when cogged belts were introduced,( by I think,Glas),they seemed to me to be a very good idea.
Their simplicity and silence were most appealing.Also their potential for simple and easy replacement.
Naturally,car designers soon found a way of making their replacement almost prohibitively difficult,I have an XUD here which is an all time classic,but even the very visible belts on my Alfasud sprint were not entirely meccano.
But,I have never had one break on me,possibly because ,in spite of the inconvenience,I have them changed either when the manufacturer says,or sooner,together with ancilliary bits and bobs if I dont trust their overlong estimates of service life.
Engine speeds seem to control this,Deisel belts seem to last longer than petrols.
But chains............I have had two break,and been forced to change many others,with sprockets,mainly because of noise.
The problem,it seemed to me, was the use of monoplex chains,instead of duplex ones.Perhaps the cars wth duplex chains never caused a problem,so I dont exactly recall which they were.
It does occur to me,however,that,with modern twin camshaft engines,yards of duplex chain flailing about is hardly an elegant solution.
So why not go back to nice,quiet,cogged belts but specify them a bit harder,and change them at,say,50000 miles?
Useless to ask for them to be made accessible for cheap changing,as the motor trade make most of their money from their workshops these days.
Robin
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Why not go all the way and use gears.
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This has set me thinking, why use mechanically linked solutions at all? Am I right in thinking that some high performance designers have gone down the route of electro(nic)controlled pneumatic/hydraulic operation?
It must be even possible to design the electronics to be fail safe, no bent valves or broken pistons. Completely variable timing, integrated with the ECU, possibly running a multi cylinder engine on subsets of operational profiles, eg town mode, motorway cruising, performance days, caravan towing or even say 4 out of 8 cylinders?
Absolutely no hope of a garage maintaining, just install the Start/ reset button as a customer option, people accepted it on the computer for quite a few years ( altho XP is a marked improvement!).
Insiders tell us when we we can expect to see it on the steets.
pmh (was peter)
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Peter,
I'm a bit out of touch regarding non-camshaft valve operation now, having retired almost three years ago, but this is what I said on the subject earlier ...
Unitary valve operation has been investigated by most engine companies over the past ten years or more but there are still problems to be overcome.
Operation can be via electrical solenoids or hydraulic feed but, whatever is used, space has to be found for it. With a camshaft, a closing valve is actually helping to turn the camshaft and therefore reducing the engine power needed to open another valve further along, but you don't get this benefit with solenoid operation so the solenoid is going to be quite large and you will have four of them per cylinder in a four-valve engine if you want full control of the valves.
There is also the problem of accelerating a solenoid-powered valve in a controlled manner, which is what you get with the ramp on the camshaft lobe. This is especially important as the valve closes because you don't want it hammering into the seat. Some designs have included a cushioning spring but this has the disadvantage that the valve can be held by the spring in the part-open position on failure of the solenoid or its electrical supply and this could cause the valve to be hit by the piston. This and much else has to be resolved before we will see unitary valve operation on car engines.
Regarding Martin's suggestion of using gears -- these are used on lots on heavy-duty engines, for earthmovers, trucks, gensets, etc., but they are expensive to produce, hard to make run quietly and require very exact positioning.
Also, the gear teeth are normally helical, i.e. angled, and this brings its own problems as it puts an axial load on the gears and hubs that needs to be catered for in the design. This is not just a matter of letting the axial force be taken by the cylinder block because any sudden back-running of the engine, even instantaneous, will send the load the other way and perhaps fire the gears through the gear-chain cover! (I am aware of this happening on a fork lift truck engine).
Sorry folks, this has drifted into 'technical'.
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>> So why not go back to nice,quiet,cogged belts but specify >>them a bit harder,and change them at,say,50000 miles?
The later, 1999 onwards? Ford Focus is 100,000 miles until scheduled change of cam belt. Earlier Focus was much lower mileage before change scheduled.
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"Why not go all the way and use gears."
Think of the direction changes.. round horizontally to round vertically to rounddd horizontally = 2 sets of gears = 2 sets of possible noise and definite power losses.. and possible complexities.
Porsche used gear dirven Overhead camshafts on the Carrera 4 in the late 1950- early 1960s - the final developmenet of the Porsche flat 4 engine prior to the 911 engine. It took 15 hours for a skilled Porsche technician to set the timing!
Perhaps an extreme case.. 2 sets of twin ohc per bank of cylinders (hence the 4) but gives an idea of the complexities involved.
Now add to that infrequent poil changes on the gear teeth and cam belts start to look very attractive.
(No I have never had a Carrera 4 - wish I had the money:-)
madf
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Reading through a 1965 Motor magazine the new NSU Prinz is described as having a pair of eccentric con rods driving the camshaft. Claimed to be much quieter than a chain. How's this work then?
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Truck engines have gear driven camshafts..
--
groups.msn.com/honestjohn/problems.msnw?Page=1 - Pictures say a thousand words.....
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