?

iceax

hi all was taking the little woman out for a driving lesson in the car today only her second day out, and she's not doing too bad.

any way we were doing a hill start and was telling her the story of gears and the hand break etc .. she asked would the car move down the hill if any of the gears were sellected so I went to show her and ..
Guess what I sellected 2'nd gear and no handbreak on and the car moved down the hill even "in gear" is this correct for a new car with only 9k on the clock it never pulled a trailer by the way and never abused.

omb0wyn5ehpij9

Yeah that is correct. That is how all cars are

unkel

You forgot to take your foot off the clutch

Abelloid

!?! Was the engine running?

iceax

thanks all ,, she is reading the mails now and she is getting a laugh

1 car was off
2 in 2 nd gear with no Clutch 3. i told her it did not make a difference what gear it was in.
So i tell her that 1st gear is the best gear to use if parked on a hill ( if the hand brake was not working ) . ha hah

galwaytt

yep it's normal: think about it: if the engine would NOT turn over, when using gravity (the hill)........then you'd never to bump-start a car either.......

peasant

emmm ...you managed to turn the engine backwards, not a good idea really

qz

I may be missing something here, but ever considered the concept of gravity?

iceax

maybe im not a good teacher on the matter to a new learner " she asked the question of which gear would i leave it in for added safety and i said it did not mather ,, but i JUST learned my self it does Make a difference ... ps i still do not under stand gravity but a lead foot i do know about...:o

dr.fuzzenstein

If the car moved downhill when in gear and the engine didn't turn over, you'd be in trouble.;)

peasant

iceax wrote: »

ps i still do not under stand gravity

Gravity is what makes your car roll down the hill when not in gear. What stops it when it is in gear is the compression of the one cylinder that is currently in its compression cycle.

In a petrol car compression is about 8 or 9 to 1, in a diesel it's about 20 to 1.

If the force of gravity is bigger than the resistance of the air compression in the cylinder then your car rolls even when in gear ..for a petrol a bit of an incline will do that, for a diesel you need a proper hill.

Del2005

The only sure way to stop a car rolling down hill is to turn the wheels towards the kerb.

I can never remember the correct way to point them till I've stopped on a hill though.

CiniO

I was learnt to leave it on 1st gear if the car is facing up the hill, and leave it on reverse gear, if car is facing down the hill.

But to be honest from logical point of view it shouldn't make any difference.

CiniO

peasant wrote: »

Gravity is what makes your car roll down the hill when not in gear. What stops it when it is in gear is the compression of the one cylinder that is currently in its compression cycle.

In a petrol car compression is about 8 or 9 to 1, in a diesel it's about 20 to 1.

If the force of gravity is bigger than the resistance of the air compression in the cylinder then your car rolls even when in gear ..for a petrol a bit of an incline will do that, for a diesel you need a proper hill.

Obviously I might be wrong, as it's after midnight, and I had good few pints, but AFAIK in 4 cylinder engine, each cylinder in certain moment of time is in one of four cycles (intake, compression, power, exhaust).
Both compression and power cycles are cycles with all valves closed.
So in short words, only if the force that makes the car roll down is bigger then a compression on 2 cylinders, then the car will roll down.

PS. It's not exactly gravity force which makes car roll down.
It's only one of the components of the gravity force.
The other component is the force that car is putting onto the ground.
(in other words - the steeper the hill, the lighter the car is).

peasant

CiniO wrote: »

compression on 2 cylinders,

..one would compress, the other would expand, no?

-Chris-

CiniO wrote: »

Obviously I might be wrong, as it's after midnight, and I had good few pints, but AFAIK in 4 cylinder engine, each cylinder in certain moment of time is in one of four cycles (intake, compression, power, exhaust).
Both compression and power cycles are cycles with all valves closed.
So in short words, only if the force that makes the car roll down is bigger then a compression on 2 cylinders, then the car will roll down.

PS. It's not exactly gravity force which makes car roll down.
It's only one of the components of the gravity force.
The other component is the force that car is putting onto the ground.
(in other words - the steeper the hill, the lighter the car is).

That's a very detailed way to be incorrect.

Peasant's right.

CiniO

peasant wrote: »

..one would compress, the other would expand, no?

That's correct. But how you call it doesn't make too much difference.

Hence that we are talking about engine which is turned off.
As I said before there are 4 cycles (intake, compress, power and exhaust).
Intake and exhaust cycles, are the cycles where at least one valve is open.
Compress and power cycle, are cycles where all valves are closed.

We were talking about what resistance engine makes to stop the car from rolling from the hill.

So in both compress cycle and power cycle, engine makes resistance.
As in compress cycle, air in cylinder has to be compressed, and in power cycle air in cylinder had to be expanded. Both above cause resistance.
Considering it's typical 4 cylinder engine, at any moment in time we have 1 cylinder in compression cycle, and one in power cycle.

That causes, that there's 2 cylinders causing resistance against car rolling down. One has to be compressed, other one expanded.

That makes the difference, as you mentioned that resistance against car rolling comes only from one cylinder. But in real it comes from 2 cylinders.

-Chris-

Does it take the same energy to expand a container of air as it does to compress it?

Do we really need to worry about the gradient of the road and detail that the vector of force acting on the car is not a directly down gravitational force?

My physics is rusty at this point, but I'm sure that's a very over-detailed way to discuss the fact that the OP's car rolled on a hill with the engine in gear.





OP, there's nothing wrong with your car that caused this to happen. It will happen with virtually every car if the gradient's steep enough.

Anan1

-Chris- wrote: »

OP, there's nothing wrong with your car that caused this to happen. It will happen with virtually every car if the gradient's steep enough.[/I]

True, but it would have taken a steeper hill to make the car roll in first or reverse. Like peasant, i'd have concerns about turning over an engine backwards.

-Chris-

Absolutely, and if it had 12 cylinders instead of 4, it'd take a steeper hill again.


I'd say the OP would be wise to keep an eye on the car alright, he may have introduced problems by turning the car over backwards, but the fact the car rolled is not an indicator of an issue with the car.

-Chris-

CiniO wrote: »

So in both compress cycle and power cycle, engine makes resistance.
As in compress cycle, air in cylinder has to be compressed, and in power cycle air in cylinder had to be expanded. Both above cause resistance.

OK, bearing in mind that the engine wasn't running, it was the wheels that were turning the crankshaft...

How much effort does it take to expand a cylinder of air, and how much energy does it take to compress one?

Are they even remotely similar figures?

CiniO

-Chris- wrote: »

Does it take the same energy to expand a container of air as it does to compress it?

In my opinion - yes.

Do we really need to worry about the gradient of the road and detail that the vector of force acting on the car is not a directly down gravitational force?

I just mentioned it, to correct what Peasant said that it was gravity force. And in real it's only part of gravity force.
To be exact it's:

My physics is rusty at this point, but I'm sure that's a very over-detailed way to discuss the fact that the OP's car rolled on a hill with the engine in gear.

We can always move the thread to physics forum

OP, there's nothing wrong with your car that caused this to happen. It will happen with virtually every car if the gradient's steep enough.

I agree with that. .It's normal.

CiniO

-Chris- wrote: »

OK, bearing in mind that the engine wasn't running, it was the wheels that were turning the crankshaft...

How much effort does it take to expand a cylinder of air, and how much energy does it take to compress one?

Are they even remotely similar figures?

IMHO they are exactly the same figures... But obviously I might be wrong.

-Chris-

CiniO wrote: »

In my opinion - yes.

OK, and will your opinion also take into account that the force of pulling the piston down (and the vacuum that will create) will probably overcome the force of the springs on the valve and cause one of the valves to open, removing the vacuum?

Are you talking about a perfectly sealed chamber or a working engine?


Have you allowed for static friction in your force diagram?


Seriously, the OP's question is comprehensively answered. I think you should stop now.

CiniO

Sorry Chris - I just realised I was completely wrong about it.
Compression cycle builds up the pressure in the cylinder, and during power cycle, this pressure is released. So not only there's no force needed for power cycle, it actually creates the force.

So for me it looks like, that pistons in two cyllinders (in intake and exhaust cycle) run freely. Piston in cylinder which is in compression cycle, needs force to compress the air. And piston in cylinder which is in power cycle, produces the force. If these forces were the same - the whole thing should pretty much run freely, only resisted by friction.
But actually force needed for compression and force coming from decompression are not the same. They are not linear.


Only what is the same is the amount of energy needed for compression, and that is released from decompression (minus fraction).

So actually only what is causing car not to roll down, is the difference in force during compression and decompression during difference phase of them. Hence in different moment in time, one might be bigger then the other.
That's acutally why you can really feel something like steps when car is rolling down on gear. It's something like if you were trying to push the car on gear. It makes resistance, but if you push it strong enought, engine will just go to next cycle and stop again.

-Chris- wrote: »

OK, and will your opinion also take into account that the force of pulling the piston down (and the vacuum that will create) will probably overcome the force of the springs on the valve and cause one of the valves to open, removing the vacuum?

That's impossible.
Valves are controlled by camshaft, and they can't open when pushed by pressure. If they did there would be no chance engine work.

Are you talking about a perfectly sealed chamber or a working engine?


Have you allowed for static friction in your force diagram?

Working engine needs to be perfectly sealed chamber (within limits) - otherwise it wouldn't work.
Obviously friction adds to it as well, but that's what the whole idea of engine oil is, that this friction is minimal.

Henry Ford III

CiniO wrote: »

...in other words - the steeper the hill, the lighter the car is.

What? I thought weight was a constant?

hi_im_fil

Ye're mixing up mass and weight now

Henry Ford III

hi_im_fil wrote: »

Ye're mixing up mass and weight now

I didn't go to mass, and have lost some weight. Is this significant?

-Chris-

Sorry CiniO, I'm bowing out. The OP's question was answered by post 2, and Peasant's elaboration was a nice attempt to give the OP more information to work with.

The fact that you've brought this thread to this level of pedantry (while still being wrong), is depressing.

I'm done.


@iceax, I hope we were of help.

corktina

OP...get her some driving lessons.....:rolleyes:

CiniO

-Chris- wrote: »

Sorry CiniO, I'm bowing out. The OP's question was answered by post 2, and Peasant's elaboration was a nice attempt to give the OP more information to work with.

The fact that you've brought this thread to this level of pedantry (while still being wrong), is depressing.

I'm done.

I have to admit I was wrong in the beginning. You told me I was, I was trying to defend my theory, but finally I understood I was wrong.
At least I hope what I've written in the last post was right.

For me it was always interesting things, and I find it nice to chat with someone about it.
If you didn't say I was wrong, I'd probably never corrected my way of thinking. That's the way you learn things.
I don't find it depressing.
Maybe Peasant will be back here, and will share his opinion on this

-Chris-

I can still see (what I think are) mistakes in your last post, but I think my head's about to explode...

MalteseBarry

iceax wrote: »

hi all was taking the little woman out for a driving lesson in the car today only her second day out, and she's not doing too bad.

.

I can't be the only person to be surprised, in 2011, at someone referring to their partner as the "little woman". Are you an especially big man?

-Chris-

Please drop that train of thought. I'm sure no offense was meant.

MalteseBarry

-Chris- wrote: »

Please drop that train of thought. I'm sure no offense was meant.

I make it a golden rule to not take offence where it is not meant, and especially not to take offence where it is meant !
As said, I am surprised to find someone referring to their partner as "the little woman" in 2011, but an far from offended by it.

-Chris-

Well then I'm sure you'll drop it as requested and we'll hear no more of it.

Thanks.

Wetbench4

CiniO wrote: »

I have to admit I was wrong in the beginning. You told me I was, I was trying to defend my theory, but finally I understood I was wrong.
At least I hope what I've written in the last post was right.

For me it was always interesting things, and I find it nice to chat with someone about it.
If you didn't say I was wrong, I'd probably never corrected my way of thinking. That's the way you learn things.
I don't find it depressing.
Maybe Peasant will be back here, and will share his opinion on this

I thought this was the funniest thread i've read in a long time. Do you always turn into a mad scientist after a few pints? Its only happened to me once or twice lol. I was kinda worried halfway through that you might get behind the wheel to do some crazy-backwards-down-a-hill experiment after your few pints lol:D