Random car question for those of you knowledgeable on the subject

Discussion in 'Off-topic' started by Feral, Apr 25, 2017.

  1. Feral

    Feral Active Member

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    I'll preface by saying I know very little about the inner workings of automobiles. I can own and maintain one. That's about it.

    For this scenario the car in question has automatic transmission.

    I have to drive to the mountains this weekend and I've been mentally going over my route. I am aware of engine braking. I know that I should manually switch to gear 1 on my automatic for steep slopes in order to retard acceleration. That way I do not ride my brakes.

    I know that after a point this stops fuel flow to the engine and a vacuum is formed or something. What I can't wrap my head around is why does RPM increase?

    RPM increase under normal circumstances means increased speed. But in a low gear during engine braking you are retarding speed. Why does the RPM rise?

    Is it because the wheels are turning which is transferring energy to the engine via the transmission? I don't know.

    It's not important. I'm just curious and trying to picture this all taking place internally.

    Any answers are welcomed.
     
  2. a_springfield

    a_springfield Well-Known Member

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    Correct. Wheel speed increases engine speed.

    An automatic has variation in the ratio because the torque converter "slips" an manual will have a set ratio for a given gear.

    Do not use 1 on an automatic to go down hills. If you need to either turn off over drive or go to the highest number you have.
     

  3. Feral

    Feral Active Member

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    Interesting. Thanks. It made sense in my head but that doesn't always mean it's right.
     
  4. Craftsman

    Craftsman Well-Known Member

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    You are discussing a topic called 'engine braking'. The engine becomes an energy sink instead of a source. this works with gasoline engines because the valve timing and cylinder arrangement is such that the engine essentially becomes a large air compressor. Desiel engines have to have an external exhaust valve (a "Jake Brake")installed because they are net zero compression when running. That is the source of the deep "braaappppppp" you hear when a truck uses engine brakng.

    Be careful when downshifting to a lower gear. You can easily drop into a ratio that will over-rev your engine based on the wheel speed. Drop one gear at a time and watch your engine RPMs. Some modern transmissions will deny a shift if it will over-rev the engine, but it is best not to count on that.
     
  5. Feral

    Feral Active Member

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    Thank you. I have heard about over revving so I am going to be sure to watch my speed when downshifting.
     
  6. AzB

    AzB Well-Known Member

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    I wouldn't use engine braking on an automatic transmission to control speed on a hill unless the circumstances were extraordinary.

    Basically, replacing brake pads is far cheaper than replacing a transmission.

    Manual transmissions generate a lot less heat and have far fewer moving parts, and therefore can handle engine braking much more capably.

    There are some auto transmissions that are capable, but they are rare and not found on many passenger cars.

    Az
     
  7. Feral

    Feral Active Member

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    I'm more concerned with reaching the bottom of the mountain and not having any brakes left to stop with

    I've read that minimal use on modern automatics causes negligible wear and tear on the transmission. Assuming you shift properly that is.

    It'll be the Smoky Mountains I'm driving around as a reference.
     
    Last edited: Apr 25, 2017
  8. jsaund22

    jsaund22 Ninjaneering Computers

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    This. I lock out my sixth gear when in the mountains, but I drive an F350 diesel - my trans is made for it. I can also put mine into tow/haul mode to enable the exhaust brake.

    Normal passenger cars - I would just use the brake, but don't ride it. Only on very severe downhill runs would I even consider locking out overdrive on a passenger car.
     
  9. Feral

    Feral Active Member

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    I'm unfamiliar with how severe the roads of the Smoky's can get. I'll report back after navigating them the first time around.

    I'll try just lightly using the brakes the first time around. If I must I'll switch to the #2 gear (my car only has 1 and 2 other than D).
     
  10. Clark

    Clark Well-Known Member

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    I have to say that I love the gear lock out feature on my F-150. I can tell it to go no higher than any of the 6 gears and use that to control my speed on hills. I've found though that the effects don't become noticeable until you're down at gear 3, and gear 2 tends to rev the engine higher than I'm typically comfortable with, still though, it's quite useful.
     
  11. jsaund22

    jsaund22 Ninjaneering Computers

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    Word. It works really well on my Super Duty with tow/haul engaged too. That exhaust brake is pretty darn impressive. We took a 13,000 pound camper to the state park on the Cumming side of the Buford Dam last summer, and coming down one of those hills from Cumming it did a darn impressive job of keeping me at 45mph with no braking required.
     
  12. jsaund22

    jsaund22 Ninjaneering Computers

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    This sounds like a good plan. Just experiment with those gears before you get to the mountains. Put it in 2nd and drive it until you think the RPMs are high enough, then note the speed. Don't downshift if you're going over that speed in the mountains.
     
  13. Feral

    Feral Active Member

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    I will do that today.
     
  14. Feral

    Feral Active Member

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    Turns out my car was more advanced than I anticipated. It's actually automatic/manual. I never bothered with the manual side and assumed it was just 1 and 2 but it's actually a "+" and a "-".

    I tinkered with it today driving home. Figured out the range of every gear. It'll be easier than I anticipated with this.
     
  15. Malum Prohibitum

    Malum Prohibitum Moderator Staff Member

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    Do you have a tachometer? What vehicle are we discussing?

    Smokeys? Probably won't need to do this at all.

    Are you towing or something?
     
  16. Malum Prohibitum

    Malum Prohibitum Moderator Staff Member

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    No need to "ride your brakes" in any event. Give them a jab and bring your speed down, then take your foot off the brake. Give it a jab again if needed. Riding your brakes is not good.
     
  17. Feral

    Feral Active Member

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    Correct, it has a tachometer. It is a 2014 Kia Forte. No towing involved.

    I'm just unsure of how steep the inclines may be in the Smoky Mountains so I would rather be cautious than be caught blind.

    The thread initially started due to my curiosity as to why RPM increased when engine braking. But it evolved into a lot of information.

    I do know the cabin I am staying in is atop quite an incline. That may be the steepest part, but it is brief.
     
  18. Casual User

    Casual User Member

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    Good news, that vehicle probably has something called a Hill Descent Control. HDC. Works with the ABS, and can be pretty handy when going down snow ice covered hills, or even steep gravel, muddy roads. This side of the Rockies you aren't too likely encounter anything where an occasional light pressure on the brakes will be a problem.
     
  19. Nemo

    Nemo Man of Myth and Legend

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    Yeah, don't ride the brakes. Might try the cruise control. They will frequently downshift where necessary and not over do such. Let the computer do the work.

    Othewise, just slow down alot and then off the brakes to let the brakes cool. Don't slow from 60 to 50 on brakes and the run back up, brake down type. Slow from 60 to 30 on brakes, let off brakes and no gas, coast up to brake needing speeds again. Modern brakes don't have much overheating problems anyway.

    Just drive reasonable up there and you will be fine. Pay more attention to the curves and drop offs on the other side of the guard rail and it will be no problem for you.

    Nemo
     
  20. Fallschirmjäger

    Fallschirmjäger I watch the watchers

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    IF your car is modern, it may actually prevent you from selecting a gear that will over-rev the engine.
    On my blubaru if I'm going 70 and select 3rd gear it just chirps at me and laughs. 4th will put the tach in the orange zone, but it'll go if it's needed.