Automobiles - why not to remain in 1st gear?

[sk1329]

Automobiles -- why not to remain in 1st gear?

why not to remain in 1st gear ??

 

[russ_watters]

Welcome to PF!

1. Car engines have a limited RPM range, so they wouldn't be able to go fast in only 1st gear.
2. Engine friction is dependent on engine speed, so it is higher the lower the gear.

 

[sk1329]

Since the first gear gives me the most acceleration why do I need to switch gears?

 

[russ_watters]

Did you read either of the answers I gave you?

 

[sk1329]

forget about the 2nd reason you gave because it is inevitable...but can't we increase the range of rpm

 

[sk1329]

if no why can't we inc. rpm of engine

 

[russ_watters]

forget about the 2nd reason you gave because it is inevitable...but can't we increase the range of rpm

First of all, please speak in correct English and complete sentences. It is difficult to understand when you write so poorly.

Second, please be more descriptive. It is difficult to understand you if you won't explain yourself.

Third, if you want to learn, you can't just discard answers you don't like. Both answers I gave you are real.

Now: no, RPM can't be increased indefinitely, partly because of #2 (the engine will burn itself up because of the increased friction), partly because the airflow into and out of the engine have to increase with RPM and they are limited by the size of the intake and exhaust pipes, valve ports, etc.

 

[sk1329]

Thanks for the reply.
I'll keep in mind to write full sentences
In 2nd reason you said something about Engine speed,so what actually engine speed here means ??(Are you talking about rpm only or something else)

 

[russ_watters]

Engine speed is expressed in RPM, yes.

 

[sk1329]

thanks sir

 

[Kozy]

On a basic level:

"Why change out of first gear?"
"To go faster than 30mph"

For any engine speed, the parts inside the engine are moving. As well as moving, they also have a mass. Given a mass and velocity, they have forces. The faster you spin the engine, the higher the forces get until eventually, something breaks. Added to this is the fact that gasoline engines can only produce torque over a narrow range of speeds, so most car engines will be limited to around 8000rpm because of these factors. You then need the gearbox to provide a tractable thrust curve between 0mph and whatever the cars power and aerodynamics dictate as the maximum speed. If you used one gear for this the car would stall when trying to pull away, just try it for yourself using 5th gear to pull away!

In comparison, something like the Tesla Roadster's electric motor will generate useable torque from 0rpm to 14,000rpm, so it only needs one gear and can produce the same sort of thrust curve as an SI engine with a gearbox.