Why aren't more cars fitted with non-interference engines?
Discussion
More of an emissions/performance penalty
One benefit of tightening emission regulations is engine design especially in the area of the cylinder head and timing must be improved which to a petrolhead means you need minimal work to the head to get decent power gains. The ford duratec being a emerging favorite in the kit car circles
One benefit of tightening emission regulations is engine design especially in the area of the cylinder head and timing must be improved which to a petrolhead means you need minimal work to the head to get decent power gains. The ford duratec being a emerging favorite in the kit car circles
A "non-interference engine" means that the piston at top dead centre is lower than a fully open valve. Such a design obviously means you need either a very low compression ratio (not good for power); valves that don't open very far (restricting air flow); or very narrow, long, cylinders (increasing piston movement speed and hence wear of various components).
All in all, it's better to just have a cam-chain (although that's surprisingly rare, for some reason) or replace your cam belts on time.
All in all, it's better to just have a cam-chain (although that's surprisingly rare, for some reason) or replace your cam belts on time.
Edited by kambites on Monday 16th June 08:44
biglepton said:
My Mazda 6 MPS has a timing chain fitted rather than a snappy belt that needs regular changes. Can't understand why more manufacturers don't fit chains.
Because that would take money away from their dealer network, reducing the amount they'll pay for a franchise? [/cynic]biglepton said:
My Mazda 6 MPS has a timing chain fitted rather than a snappy belt that needs regular changes. Can't understand why more manufacturers don't fit chains.
When it's come up before it seems 2 reasons. 1 Noise. Chains make a bit more noise. 2 cost. Apparently it's a PITA to design a proper chain drive and get it all designed to keep it's tension under all conditions etc. Whicle the belt is easier and cheaper to make work.It is certainly true that chains aren't a "no failure" solution.
The chain itself may not fail very often, but the pretensioner is essentially the same device which would be used on a belt (although with a cog rather than a pulley wheel) and in the 10 years I've been driving, I've had two tensioners fail and no belts. Neither tensioner breakage damaged the engine because I was lucky and the belt didn't slip but a tensioner going on a chain drive engine will very quickly destroy the chain and hence the top end of the engine.
The chain itself may not fail very often, but the pretensioner is essentially the same device which would be used on a belt (although with a cog rather than a pulley wheel) and in the 10 years I've been driving, I've had two tensioners fail and no belts. Neither tensioner breakage damaged the engine because I was lucky and the belt didn't slip but a tensioner going on a chain drive engine will very quickly destroy the chain and hence the top end of the engine.
kambites said:
A "non-interference engine" means that the piston at top dead centre is lower than a fully open valve. Such a design obviously means you need either a very low compression ratio (not good for power); valves that don't open very far (restricting air flow); or very narrow, long, cylinders (increasing piston movement speed and hence wear of various components).
[/footnote]
Pistons don't have to be flat on top. I would have thought a designer could cut a bit away for the valve and add a bit elsewhere to maintain the volume.[/footnote]
mekondelta said:
Just wondered why the MX5 seems to be one of the only cars to be fitted with a non-interference engine? If the cambelt snaps, the engine internals supposedly aren't damaged. This should be standard on all engines surely?
Or is there a cost/engineering/performance penalty?
Arent most Toyota engines non-interference?Or is there a cost/engineering/performance penalty?
herewego said:
Pistons don't have to be flat on top. I would have thought a designer could cut a bit away for the valve and add a bit elsewhere to maintain the volume.
I believe that pistons are generally designed to rotate in their bores to make sure that they wear evenly so you'd have to make any cut-out symetrical. You can't have sharp edges because they cause nasty pre-combustion so you really have quite limited design possibilities when it comes to piston shape. Of course a funny shaped combustion chamber probably wont give you such even combustion either. Toyota 3SG(T)E engines both n/a and turbo are non-interferance, like the mx5 they have cut outs in the piston to clear the valves.
The 3sge in its day was one of the most powerful 1998cc engines available and still returned good mpg, the 1989 engine managed 158bhp and a realistic 30mpg in the celicas & MR2's, 1994 version raised the bar to 173bhp and same mpg. Plenty of modern 2 litres (unless turbo'd) can't boast stats as good as that.
The 3sge in its day was one of the most powerful 1998cc engines available and still returned good mpg, the 1989 engine managed 158bhp and a realistic 30mpg in the celicas & MR2's, 1994 version raised the bar to 173bhp and same mpg. Plenty of modern 2 litres (unless turbo'd) can't boast stats as good as that.
kambites said:
herewego said:
Pistons don't have to be flat on top. I would have thought a designer could cut a bit away for the valve and add a bit elsewhere to maintain the volume.
I believe that pistons are generally designed to rotate in their bores to make sure that they wear evenly so you'd have to make any cut-out symetrical. You can't have sharp edges because they cause nasty pre-combustion so you really have quite limited design possibilities when it comes to piston shape. Of course a funny shaped combustion chamber probably wont give you such even combustion either. Some valves do and you actually have valve rotators for this reason, i've only seen these on large engines though.
Dave!
kambites said:
It is certainly true that chains aren't a "no failure" solution.
The chain itself may not fail very often, but the pretensioner is essentially the same device which would be used on a belt (although with a cog rather than a pulley wheel) and in the 10 years I've been driving, I've had two tensioners fail and no belts. Neither tensioner breakage damaged the engine because I was lucky and the belt didn't slip but a tensioner going on a chain drive engine will very quickly destroy the chain and hence the top end of the engine.
Its also true that chains strech more than belts too.The chain itself may not fail very often, but the pretensioner is essentially the same device which would be used on a belt (although with a cog rather than a pulley wheel) and in the 10 years I've been driving, I've had two tensioners fail and no belts. Neither tensioner breakage damaged the engine because I was lucky and the belt didn't slip but a tensioner going on a chain drive engine will very quickly destroy the chain and hence the top end of the engine.
kambites said:
It is certainly true that chains aren't a "no failure" solution.
The chain itself may not fail very often, but the pretensioner is essentially the same device which would be used on a belt (although with a cog rather than a pulley wheel) and in the 10 years I've been driving, I've had two tensioners fail and no belts. Neither tensioner breakage damaged the engine because I was lucky and the belt didn't slip but a tensioner going on a chain drive engine will very quickly destroy the chain and hence the top end of the engine.
Yes, a tensioner failure will let the chain jump a tooth and can cause engine damage - that's why you'd check the tensioner & tensioner rails every 20,000 miles or so to ensure they're still up to the job, or get them replaced. If you don't then I've got no sympathy when it fails. Even if you spend the £100 every 3-5 years to replace it as a precaution it's surely worth it. I've only needed one tensioner in 8 years as the diaphragm was leaking.The chain itself may not fail very often, but the pretensioner is essentially the same device which would be used on a belt (although with a cog rather than a pulley wheel) and in the 10 years I've been driving, I've had two tensioners fail and no belts. Neither tensioner breakage damaged the engine because I was lucky and the belt didn't slip but a tensioner going on a chain drive engine will very quickly destroy the chain and hence the top end of the engine.
The only 'chain' or 'tensioner' faults I've heard about on our old e34 M5s have happened when the car's been in for a service and the car has been rolled forward/backwards with the tensioner off - the chain has jumped forward/backward and when it's all put back together and started you get a very rough running engine for a bit and if you do nothing about it there & then you'll end up with a lovely bag of M-engine bits all over the floor.
kambites said:
herewego said:
Pistons don't have to be flat on top. I would have thought a designer could cut a bit away for the valve and add a bit elsewhere to maintain the volume.
I believe that pistons are generally designed to rotate in their bores to make sure that they wear evenly so you'd have to make any cut-out symetrical. 
rotating pistons! I think not. Munter said:
kambites said:
herewego said:
Pistons don't have to be flat on top. I would have thought a designer could cut a bit away for the valve and add a bit elsewhere to maintain the volume.
I believe that pistons are generally designed to rotate in their bores to make sure that they wear evenly so you'd have to make any cut-out symetrical. rotating pistons! I think not. 
Still can't see you managing to make piston surfaces far off flat though.
Edited by kambites on Monday 16th June 09:32
mmm-five said:
Yes, a tensioner failure will let the chain jump a tooth and can cause engine damage - that's why you'd check the tensioner & tensioner rails every 20,000 miles or so to ensure they're still up to the job, or get them replaced. If you don't then I've got no sympathy when it fails. Even if you spend the £100 every 3-5 years to replace it as a precaution it's surely worth it. I've only needed one tensioner in 8 years as the diaphragm was leaking.
My (belt) tensioner failures were both newish tensioners. I had the Corrado's belts done and about 4k miles later the tensioner went. Had it (and the belt) replaced again and it went again. Dodgy batch of components maybe? Dunno. Neither time did any damage, just made the engine sound like a diesel. 
kambites said:
Munter said:
kambites said:
herewego said:
Pistons don't have to be flat on top. I would have thought a designer could cut a bit away for the valve and add a bit elsewhere to maintain the volume.
I believe that pistons are generally designed to rotate in their bores to make sure that they wear evenly so you'd have to make any cut-out symetrical. rotating pistons! I think not. Still can't see you managing to make piston surfaces far off flat though.
planetdave said:
I don't believe that the purpose of that is to make it a non-interference engine though is it? I think most manufacturers would rather chase clean burn patterns to improve power and/or economy than design their engines to cope with a failure mode which will generally only happen well outside warranty anyway.
Edited by kambites on Monday 16th June 09:51
kambites said:
herewego said:
Pistons don't have to be flat on top. I would have thought a designer could cut a bit away for the valve and add a bit elsewhere to maintain the volume.
I believe that pistons are generally designed to rotate in their bores to make sure that they wear evenly so you'd have to make any cut-out symetrical. You can't have sharp edges because they cause nasty pre-combustion so you really have quite limited design possibilities when it comes to piston shape. Of course a funny shaped combustion chamber probably wont give you such even combustion either. Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff