Timing Gear

[Berg]

Hi!
I once saw a pic of Ferrari 049 engine with its timing-gear. The question are:

1. Do all other engine use timing gear i/o timing-chain/belt
2. What are the advantages of timing gear compared to timing-chain/belt?
(weight, rigidity, time-lack, accuracy?)

Berg

[manchild]

All engines are now on gears - some are even said to have doubled (Renault, on both sides of the engine)

Belts were last used on turbo's and than in the late '80s early '90s there was transitional period with chain or gear timing but soon they've all switched to gear timing.

I think that huge increase of rpm that used to be not more than 12.000 for decades caused switching to gear timing. When Renault invented pneumatic valve "spring" and when it was applied on normally aspirated engines the rpm increased very much that temperatures and forces/drag (compression ratio) were too high for belt or chain.

For example – Renault had belt timing on the last generation of their turbo engines but one must know that turbo engines had compression ratio of almost half of what normally aspirated engines have 6+ to 1 compared to 12+ to 1.

[RH1300S]

AFIK the answer is "accuracy" of timing. Valve timing is critical in any engine - I must assume that at very high revs with very high specific outputs it becomes hyper critical.

[DaveKillens]

A chain or belt is relatively simple, and if the belt width can be accomodated, relatively easy packaging issues. That is, compared to having to build in idler gears for the cam drive. But I would suspect that a belt isn't as tough as gears when run at maximum load and RPM. I have to suspect the loads and harmonics that a belt experiences at high RPM's is fierce.
Straight cut idler gears for camshaft drive have been around for decades, they are ultra reliable and tough, and if properly packaged, have minimal impact on engine dimensions. They probably have less inertia compared to a belt.
I would believe a belt adds dimension to an engine package, and is more lkely to fail under high load and RPM. Gears are more difficult to build into an engine, and add parts. But compared to belts, probably less likelyhood of failure, and the ability to handle more abuse.

[Apex]

When a belt or chain runs at high rpm around a small pulley the centrifugal acceleration is immense, with out even transmitting any power. This would cause the belt / chain to stretch and the timing would change. With such high RPM a Flat belt would be the only type to use – Flat belts can’t really transmit much power, which the cams require.

[Guest]

Belt is more likely to slip due to high speed revving. Also rapid acceleretion may cause the belt to slip. Like the tires shape recovery of the belt becomes a problem.

[riff_raff]

Timing gears are most accurate and efficient. But gears are also most suceptible to failure due to torsional vibration effects between the crank and cams. Cosworth found this out the hard way with their DFV engine. Their solution was to put a torsionally compliant gear assembly in the camshaft gear train. Honda did something similar with the cam gear drive on their superbike engines a few years ago. They inserted an idler gear that was split in two and spring loaded ( it's called a "scissor gear").

Belts are less accurate and durable than chains or gears, but provide a dampening effect through hysteresis within the rubber belt carcass.

Roller chains are more accurate than belts, but due to their greater mass, they are inertia limited. Chains also provide a small measure of dampening, due to the friction present between the sliding links/rollers.

Belts and chains are also more capable of absorbing dimensional changes between the crank and cams, as the engine heats up.

Each drive system has it's benefits and deficiencies, but spur gears seem to be the choice of current F1 engine designers.

[manchild]

First version of Renault V10 with belts
(notice how simmilar postion and shape of exaust is with modern f1 engines)

[ginsu]

[quote="manchild"]First version of Renault V10 with belts
(notice how simmilar postion and shape of exaust is with modern f1 engines)


The exhaust header looks ancient. Look how much space they wasted with those! It must have been much easier to construct compared to the
nightmarish inconel headers we have today.


Also, the Honda S2000 uses an interesting chain/gear hybrid assembly where the camshafts are gear driven by a 'intermediate gear' that is split with gear teeth on one side and is chain driven on the other.