Freebe

here is are 2 questions I have always not understood. As an architect/military pilot/aircraft maintenance officer

1. why does any engine have an overlap of valve displacement and piston head top movement? why create the problem of valves hitting the pistons under any situation. seems like simple engine design, but what are the trade-offs that make engine design allow this condition?

2. why don't ALL vehicles have chain timing belts? (had an old VW Rabbit airport car where the "belt" broke and you know what happened). when producing 1000's of engines, the cost of a more substantial chain is pennies.

onfireTTS

Both questions are very complex and over my pay grade.

What I have anecdotally seen over the years with our race stuff :

1. Interference engine design is more complex, but typically is more efficient for power (and fuel mileage for those that care), due to the timing events vs squish area. Basically you can make the intake and exhaust valve ride the piston position much closer for velocity and volume control. Helps VE. Probably a Compression design benefit also.

2. I personally like chain drives. Gear drives have their place as well since stretching over time is much more limited vs a full roller chain drive which is better than belt chain drives. All metal.

The synthetic belts are cheaper, lighter, quieter ,etc,etc which is beneficial in their realm. They do require more maintenance and are much more problematic if they get a fluid on them from a leak. Probably many more have broken past their shorter duty life vs steel and have caused more engine damage in interference engines.

Cheers

Jim M.

And when your producing millions of widgets those pennies add up. Years ago VW eliminated a nylon button in their manual transmission, cost was less than $0.05. But the result was a shaft in the transmission could slip and wear a hole in the transmission case and all the oil would leak out. Learned this one the hard way. I had to drop the transmission, weld up the case and move on. Manufacturers don't care after the sale, just means more after sales parts sold.

CodyzWorld

wait, so our cars have belts and not chains?

991.2?

onfireTTS

Chain

Freebe

the area "volume" associated with the cylinder at top of stroke is a space planning problem.

one alternative is small depressions in the cylinder head associated with getting the volume correct and making sure that the valves NEVER protrude into displaced piston path. Having a depression in the piston head may cause other problems. If anyone has comment on this, please jump in.

as always, there has to be more obvious reasons that I am missing the reason to understand this design decision.

worf928

A high static compression ratio (typically 8:1(-ish) or higher) coupled with valves that open into the combustion chamber lead to an interference design. In very simple terms: A low-compression design will not make good power/volume numbers without a lot of forced induction. Valves with low lift (how far they open) don’t let as much air into the cylinder.

Timing belts were used because chains are noisy and because of inertia. Neither of these are considerations today. Chain systems also require more sophisticated tensioning systems.

It is a myth that timing belts are inherently less robust than chains.

And before yaw’ll froth at the mouth look at my user name. The 928 has the longest timing belt ever used in a production car and 99% of everything you’ve ever heard about 928s ‘breaking’ timing belts is 99% wrong.

NiteJav

Its a bit of a misnomer that chains are better then belts. The scary thing with chains is they depend highly upon the durability of the chain tensioners and chain guides. Audi/VW was notorious for having sh!tty chain guides that were made of plastic that get very brittle over time and many heat cycles. One those things break the chain has more 'room' and then you can say goodbye to your valves.

onfireTTS

Depends on the year and platforms. I’ve seen small block Chevrolet chain belts (not as good as chain rollers) go + 100,000 miles with no issues but some timing “slop”.

Earlier synthetic belts on other platforms, required a change out by the OEM wayyyy before 100,000 miles. Some broke and destroyed engines.

synthetics have improved over the years.

Fluid on a synthetic is a potential problem.

All depends on the design goals and specs.

worf928

You mean like this?

NiteJav

Bingo!