Confused about RPM and transmission

[jjguitar]

I hope someone can clear this up for me.

I am confused about RPMs and gearing in a car. This is how I understand it to be:

(We'll use and automatic as an example)...When starting from a dead stop, I am in first gear. After I hit a certain point, the transmission will switch to second gear, which will make the RPM gauge suddenly go down a bit. However, wouldn't the engine's rotation speed keep increasing? I always thought that if the gas pedal has a constant pressure, the engine rotation speed will increase evenly, and only the transmission determines the rate at which the tires spin...the RPM of the actual engine does not decrease with the RPM gauge. However, when they say that the overdrive gear (4th) increases gas milliage by lowering the RPMs, then my theory does not make any sense. Could someone explain what I have wrong here?

Thanks,
Justin

[swedish]

Could someone explain what I have wrong here?

Pretty much all of it. A tach (rpm guage) that doesn't show the actual rpm's is like a metric clock. As you go through the gears, the engine will rev from idle to wherever shifting is done (by the tranny or you, auto or manual). Overdrive saves gas by going into a higher gear. This can be done with a manual too, if you rev a certain car out in 4th, you can be doing 90, but you can go 55 in 5th and save alota gas.

[jjguitar]

Thanks for the reply. I have one question...by what means does the transmission tell the engine to start from idle again each time it switches gear?

[ivymike1031]

perhaps this simple explanation will clear things up a bit-

the transmission (when it is not slipping), allows the ratio of engine speed to wheel speed to be one of several values (each gear allows a different ratio). In your lowest gears, the engine (crankshaft) rotates many times for each rotation of the tires. In your highest gears, the engine speed is much closer to the tire speed. Example (using completely made-up values):

Let's say that your transmission is a 4-speed, and that the ratios are
1st gear - 8:1
2nd gear - 5:1
3rd gear - 2.5:1
4th gear - 1:1

That means that if you're in first gear, and the tires are spinning at 100rpm, then the engine will be spinning at 800 rpm (for the purposes of this explanation - there are more details to consider in "real life"). As you drive faster and faster, your tires will turn faster and faster, and so will the engine, and the ratio will remain constant. If your tires sped up to 200rpm, then the engine would turn at 1600rpm.

Only by changing gears can you change the ratio. Let's say that you accelerated to some speed at which the tires were turning at 500rpm, and you were still in first gear - then for this hypothetical case the engine would be spinning at 4000rpm. To go any faster, you'd have to REALLY rev it up, or... change gears. Put it in second, and the ratio drops to 5:1. Now (with your tires still spinning at 500rpm), the engine speed will be 2500rpm.

If you accelerated further, until the tires were spinning at 1000 rpm, the engine would be turning at 5000 rpm. Shift again (to 3rd), and the engine speed would drop to 2500 rpm.

There is plenty more info available online - a good place to start is here:
http://www.howstuffworks.com/transmission.htm
http://www.howstuffworks.com/automatic-transmission.htm

[jjguitar]

Thank you for clearing that up!!

Justin

[Steel]

and the way automatic trannys switch gears is totally different than that of a manual. They DO have clutches in them (a bunch of small ones) and its a planetary gear system, so a certain set of gears is moving at all times. When it shifts, what happens is that the tranny locks up one set of gears while simultaneously allowing others free, so tehre is no point where the engine is disconnected from the tranny (under power) or else the rpm's would skyrocket and an auto tranny would last all of a week. The clutches are primarily for starts from a dead stop, idling, parking, whatnot. iirc, of course

[jjguitar]

But how does the engine know to slow its rotation speed when the gear shifts? How does it get this signal?

[ivymike1031]

it's bolted straight to the transmission - it doesn't have any choice in the matter. Ask yourself this - when you're riding a mountain bike and you change to a higher gear, how do you know to pedal more slowly? Because you can't keep pedaling fast, even if you want to!




"why'd the kid next door cross the road? Because his dick was caught in the goddamn chicken, that's why!"

[jjguitar]

OK I get it now, thank you!

[Steel]

and the tranny knows when to shift from computers. That's all one really needs to know, unless you want to get into complicated computer stuff.