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Unread 10-03-2013, 11:14 PM   #1
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Default Rpm and gear shift

Hello
I'm reading about cars and how cars work, i faced something i couldn't understand which related to engine rpm and gear shift.

In first gear the engin rpm for crankshaft is high (let say 3000rpm) when shift to second gear the engin rpm drops (2000rpm) and so on.. I'm talking in general for old or new cars (manual or auto transmission)

I know about the gear ratio between crankshaft and drive shaft.

My question is
1. how the engine knows the gear is shifted to higher gear so that the engine will slow the rpm and the rotation in the crankshaft..
2. how the engine slow the rpm.

Thank you

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Unread 10-03-2013, 11:34 PM   #2
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Let's say you are driving in 1st gear at 3000 RPM.

If you then shift into 2nd gear, because the gear ratio is higher geared, that is, the car moves faster for the same RPM, the engine speed will drop because at that RPM the car has to be moving faster, but of course the car can't suddenly jump to that speed, the engine must adjust to the change in gear ratio. This means, in this case, since the car can't accelerate instantly the engine speed gets dragged down. The engine doesn't know what speed it needs to run for the gear change, the engine just isn't powerful enough so it is the component in the equation that must give.

It is possible that for a very powerful car, that a quick gear change will cause the wheels to slip on the road surface as the powerful engine DOES have enough power to try to accelerate the car instantly, but no traction means the tires will spin instead as the car can't accelerate that quickly.

Each gear change up means the wheels must turn faster for the same engine RPM.

For each gear change down, the engine must run faster for the same road speed.

Just like pedaling a bicycle with more than one speed. You change up gears so that you don't have to pedal too fast in order to get your road speed up. On hills you shift down so that you can use the same "power" to climb a hill, but at a slower speed. Same deal with a car.
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Last edited by Mr Car Guy; 10-03-2013 at 11:36 PM.

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Unread 10-04-2013, 12:06 AM   #3
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Thank you mr car guy..
I did not mean how the engine knows to what speed to drop to, i meant how the engine knows to drop in general.
As you mentioned the engine must adjust to the change in gear ratio..
Is there sensor or someway tells the engine to slow (adjust) its stroks times when gear is shifted And how it works.. Thank you

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Unread 10-04-2013, 12:33 AM   #4
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If the transmission is a manual, when you shift you should release pressure on the accelerator when you depress the clutch, this closes the throttle and the engine speed drops. If you are up shifting, the RPM needs to drop for the higher gear anyways so when you let the clutch up the RPM has dropped and the clutch engages smoothly.

If the transmission is an automatic, you do not need to easy up on the accelerator when it shifts, the transmission shifts so quickly that the RPM just drops even though you are still depressing the accelerator.

Later model cars retard the ignition timing so the shift is softer on the transmission.
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