Engine rpm and gear shift

[Mech2325]

Hi everyone,

Can anyone please tell why do the engine rpm decrease as the speed of the car increases

 

[miningman]

because less fuel is being burnt??????

 

[Compositepro]

Because the gear ratio between the engine and wheels has changed.

 

[3DDave]

I want this car.

 

[berkshire]

Contrasting this , I think of the old Daffodil by DAF in Holland. With the variomatic drive that used to run down the road sounding like a boat motor, because the engine rpms never changed.
B.E.

You are judged not by what you know, but by what you can do.

 

[enginesrus]

Sounds like the transmission is a cvt type. As car speed increases the pulleys slowly change the ratio from low to higher.
And as the speed of the car increases the engine speed would start slowing, to a point though I'm sure they have something to determine the load on the engine and will compensate and not allow the rpms to go too low and put too much load on the engine. If it is over loading the engine then there maybe something wrong with the transmission control unit and not compensating for the over load condition.

 

[SomptingGuy]

Transmission control strategy.

Steve

 

[waross]

Can anyone please tell why do the engine rpm decrease as the speed of the car increases

Time for tough love.
I reject this hypothesis!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[3DDave]

Thinking more of it, how fast does the engine need to go to get the car to stop?

 

[dgallup]

I think the OP, who probably does not have English as a first language, is referring to the fact that when we accelerate from a stop the engine RPM's are higher due to the need for power to accelerate the vehicle. However, as we near cruising speed, the transmission (auto, manual or CVT) is shifted into higher gears and engine speed does indeed drop below what it was while we were accelerating. This is because the power requirements to maintain constant speed is quite low and for best fuel efficiency we want low engine speeds. Obviously, for any given gear ration, engine speed and vehicle speed are directly proportional.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[waross]

I wonder if the OP has any engineering training?
That is a strange question for someone signing in as (Mechanical).

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[NormPeterson]

OP needs to define his question better. As in, under what driving conditions; otherwise there's too many ways to guess wrong about what he's looking for.

His post history suggests that his discipline is Boiler & Pressure Vessel. People I knew with that as their day job (back when I had one of those things) generally weren't very familiar with automotive engineering.


Norm

 

[BrianPetersen]

As usual with this sort of question, the original poster vanishes, leaving the rest of us wondering what it was about the simple underlying concept (transmission and ratio-change mechanism/logic in this case) the original poster did not understand.

I would wager that the original poster has never operated a vehicle with a manual transmission. I just finished a little road trip in a vehicle with a manual transmission, and as soon as I'm done this little break, I have to go out and do another errand with that same vehicle with a manual transmission.

 

[NormPeterson]

Not gonna take that wager . . . most of my co-workers during the last 30 years of my career didn't "drive stick" either. Some had never even attempted to learn. Heh . . . since about 1972 we've ONLY owned stick-shift cars.

Since OP was last seen on 08 Oct, maybe one of the first three responses guessed correctly, in which case it'd have been nice to have some feedback. Oh well . . .


Norm

 

[mikeofBelAir]

Noirm,
So mandating manual transmissions and all driving instructions in cursive could cripple the nation????

 

[NormPeterson]

I'd settle for requiring a basic understanding of the concept behind transmissions of any kind having more than one forward ratio in it.


Norm

 

[CWB1]

There's a difference between being able to operate a manual transmission and owning one. Its much the same difference as fake news and truth, full vs semi-truth. Last I knoew roughly half the US population can drive a stick despite only ~15% of new cars being sold with them. Similarly, cursive writing, patriotic symbols, and other news-flavors of the week were only ever removed from a small percentage of schools in the US.

As I'm sure most everybody that has ever worked in the auto industry was told when they started - forget what you think you know about cars and learn.

 

[OhhEnnEmm]

We won't even be able to have this discussion when it all goes electric and loses it's charm

 

[NormPeterson]

If it ever does go 100% electric it won't be just this automotive discussion that will be lost.


Norm

 

[RodRico]

OhhhEnnEmm,

It's going to be quite a long time before going all electric makes sense in the US. If you calculate the joules of energy delivered across the nation via fossil fuels, you'll find we would need a huge expansion of power generation and distribution to satisfy the need (even after accounting for the improved efficiency of electric versus internal combustion). Many point to climate change as the impetus for the infrastructure expansion, but that only makes sense if the electrical power is produced from clean renewable resources yet only 17% of our *current* electrical demand is satisfied by such sources today. Finally, in the time required to expand our clean electric infrastructure, internal combustion efficiency and emissions will be improved substantially, especially if we can make biodiesel or ethanol in sufficient quantities. Near term, we'll see hybrids come to dominate US car sales as they simply make a lot of sense due to their improved efficiency and emissions without all the infrastructure demands. We will then enter a long period of evolution as hybrids continue to improve while electric infrastructure is cleaned up and expanded. In the end, we may never end up all-electric. Even if we do, it will be quite a long time, so there's no need for anyone participating in these forums to worry their knowledge will be obsoleted in their lifetime.

Rod