A Question I couldn't answer...
 

... my dad was driving us home yesterday, and we were in the Echo (he WANTED to drive... *shrug*) and stopped at a light, the engine started to vibrate as i find most autos do when you hold the brake and leave it in drive. i popped into neutral (as i always do) 'cause i hate that, and he asked me "why does it vibrate?"... i never thought about that, and didn't give him much of an answer...

so, i said i'd ask, and here i am asking. "why does it vibrate?"

Re: A Question I couldn't answer...
 

Geez, Keechster, you should know the answer to that one:

Because it's a piston engine! ;)

Re: Re: A Question I couldn't answer...
 

LOL... :)

That could be about anything, especially with an economy car (not that it's poor quality, but you save money by "putting up" with vibrations and such). One thing to consider is that with most autos, when you are at a complete stop with the brake pedal depressed, it will idle at a lower rpm than if you put it into park or neutral. Maybe there is something with that car which causes different vibrations at lower rpms.

---jps

yeah the rpm does decrease as the engine's idle is spinning against more force... but i was wondering if it had more to do with the transmission...?? as when it's in neutral, all the inner workings then are free to spin...:confused:

well the same thing happens in my automatic Protege, as in any engine (rotary included), when the engine is forced to rev below its 'normal' idling rpm, the engine will shake and jerk slightly. When you leave the auto in D when stopping, the tranny fluid, which, in normal condition, will deliver power from the flywheel to the transaxle, does the complete opposite, and the tranny fluid will slow down (but not stop, since it is just liquid) the flywheel, which in turn slow down the engine....

i think i answered your question, or expressed my opinion at least :p

yeap, that's it =) auto trannies are basically the equivalent of a flywheel turning a fan that blows fluid over another fan, however the blades are a lot closer together and the slippage is a lot less than you would expect. they separate a little bit when a dead stop but is still engaged so you can take off when you need to and wait for the tranny to re-engage. (the above comments forego any cvt type trannies and clutch-type automatic trannies)