What Stops an Engine when It is Off?

[Jonny Z]

Compression? Internal friction?
What I am getting at is, if you park a car on a slop, with the manual tranny in 1st or R, with no parking brakes, is it possible for the car to slowly over come the compression and friction and starts to inch away? Let's assume the clutch is fine and does not slip.

 

[johnd]

Sure, even a slight hill can overcome compression and turn a motor over and it doesn't care which direction either. That, incidentally, is why it is not a good idea to do this on hills.

 

[porcupine73]

Yep, always use the emergency brake. In extreme cases on some engines it can cause the timing belt to jump a tooth or more.

 

[Jonny Z]

Well here is the thing:
My garage is a mess right now because I was working on a project. I have to leave my car in the drive way. There is sleet / snow sub-freezing temperature coming my way. I have to be at work Monday. I didn't want my parking brakes to freeze in the wet and cold weather, so I left it in first, facing up the drive way slope, then posted this question. After reading the first reply, I went out and slapped a couple of wooden blocks (which I made to block the wheels when the car is on the ramp) under the rear tires.
It is 66 degrees out right now, hard to imagine what would come on Monday morning (20s and sleet).
By the way any of you Northerners had the parking brakes frozen on you? I am probably over preparing. I also got a can of Prestone Deicer in the trunk. You should see the look on the fella's face when I asked for it at AutoZone. But these once-a-year ice storm in Central Texas send a lot of biz to the body shops, which I am trying to avoid.

 

[K-Man]

Parking brakes can freeze at any temperature below freezing once the car gets a few miles on it and even a little bit of moisture gets into the cable system. Been there, done that. My experience is that the parking brake on newer cars will not be sufficient to hold the car on any kind of significant grade by itself anyway. Your solution isn't ideal, but it is the best under the circumstances.

 

[Lazy JW]

Quote:

---

...By the way any of you Northerners had the parking brakes frozen on you?...

---

Yup. And if your (drum) brakes are hot you can warp them as well. I rarely use a parking brake, if I have to park on a hill that concerns me I will use chocks as you have done. Turning the front wheels into a curb can help also.

Internal friction is what holds it in place. Compression bleeds out after a while. I had a worn-out engine in my F-350 that would go chug-chugging downhill when it had a load of firewood on. It actually takes a pretty heavy rig and a mighty tired engine to do this. I have never seen a light automobile that would do it but I suppose it can happen.

Chocks are the ultimate solution. I use them regularly.
Joe

 

[eljefino]

Having replaced an e-brake cable recently, I can report there are rubber grommets to keep water out.

However, they are imperfect, as the inner cable rusted to the outer sheath.

You have to ask yourself, is it frozen with water, or frozen with rust?

I like to use my parking brake at least weekly to keep the rust from sticking together, and to keep whatever (graphite?) lube is in there moving around and coating stuff.

But I avoid using it in slushy icy storms if at all possible and agree with the chock idea. If you let the clutch out so the car rolls up onto the chocks, the tires will grip the chock with their weight and hold them down better. Keeps people from kicking them away.

If you move the car now and set the e-brake you haven't splashed water in everything so you're probably good if you want to use it.

I would focus on other storm-related things, like staying home away from the bozos.

 

[Jim 5]

The ultimate thing that holds it is compression, but it's really about mechanical advantage and gearing.

It's primarily the gearing having a multiplication effect on compression that stops the car from moving. If you leave the car in 1st, then 1st will gives you something like a 4:1 ratio of engine speed to tire rotation. This is why your car can only go maybe 20 MPH in first gear. Of course when the wheels of the car are trying to spin the engine (in your scenario and in a "push starting" scenario) then this mechanical advantage (that helps the car get moving from a stop) will work in reverse to make it harder to overcome the engine compression. This is because the tire rotation in first is trying to rotate the engine 4 times the RPM's of the tires.

Now if you left it in 5th gear which is an overdrive, the car's more likely to move because for every rotation of the tires, you're only trying to rotate the engine 9/10ths of a rotation (instead of 4 times in 1st). That takes a little less than 1/4th as much force as if you had it in a 4:1 first gear. Anybody who's had a dodgy car (like I did) in their youth or otherwise has learned this trying to push start it.

My first vehicle had to be push started by me fairly routinely on icy streets. If I left it in first, then released the clutch, the back wheels would lock up and skid on the ice. I found it would start very nicely in 3rd (summer) and 4th (winter) because the wheels were not trying to rotate the engine at 4x wheel speed, it was more like 1:1 in 4th.

Accordingly, leaving the car in first gear or reverse (which is geared even lower usually than first) would give the most holding power.

 

[Lazy JW]

Quote:

---

The ultimate thing that holds it is compression, but it's really about mechanical advantage and gearing...

---

And how long does the compression hold in a parked engine?
Joe

 

[Jim 5]

Lazy's right, I would agree that internal friction is also a significant factor, and that would account for a good percentage of the holding power. On the other hand, bleed down after sitting ends the very second the engine starts to rotate again - when the vehicle starts to move. With as little as 1/2 of a crank rotation you'll get compression in the next cylinder that's coming on to its compression stroke(assuming 4 stroke engine with at least 4 cyl). So if you're in 1st gear with a 4:1 ratio, it takes 1/8th of a tire rotation (to create 1/2 of a crank rotation)to move from the bled down (from sitting) cylinder to the next one ready for the compression stroke in the firing order.

Anybody that's yanked on a chainsaw with a flooded spark plug and felt the yankback from compression has felt this. Now pull that plug out of that same chainsaw and yank on it with only internal friction holding it. It just spins freely and easily to clear the flooded cylinder.

Having cranked over a v8 (that I was rebuilding) by hand with and without plugs in it, my rough guestimate is that it was at least 2-3 times as hard to crank it with plugs in than without...even cranking slowly with a breaker bar and a socket on the harmonic balancer, 1/8 of a crank rotation at a time.

Internal friction does account for a good chunk of the holding power, but on an engine in good condition, the very second it starts to rotate even a little, the compression builds fast and hard, and that's what holds it.

For those reasons, my own opinion is still that compression and to a lesser extent, internal friction is what holds you when you're stopped. The more mechanical (dis)advantage you create by putting it in a low gear, the harder it's going to be to move those wheels.

 

[Chris142]

---

And how long does the compression hold in a parked engine?

---

A few seconds.

 

[John K]

I always use my parking brake and have never had one freeze, in over 50 years. That way you never have to worry about the rust issue and it keeps it in good working order. I have had manual transmissions pop out of gear and found the car in the middle of a parking lot, and I'd rather not put all the stress on Park pawl with automatic transmission. Have also had cars pop out of park. I like the belt and suspenders solution myself, i.e., not rely on just one method.

 

[Wrehch]

There are two thing's I alway's do. When I get in a vehicle I put my seat belt on and before I get out I apply the parking brake. Like John K. I have never had one freeze on me.

As for how long a cylinder can hold compression. A few seconds if your lucky.

 

[alreadygone]

If it isn't compression that will hold on a hill, and it will for indefinite periods of time. Would anybody like to pull the plugs and try it?

See park brakes freeze quite often, and it's not even that cold that often here. Usually older vehicles where cable end seals are damaged. Best solution? Slide the ash pan with HOT ashes, maybe a few coals from a wood stove under there for a few minutes!

Bob

 

[Jonny Z]

I am sticking with no parking brakes, leaving it in 1st, and a set of Blitz chocks I picked up for $3 at Wallys.

The woodens ones swollen up by water.

 

[Dave H]

I personally use the parking brake in my wife's truck, along with leaving it in first or reverse (dependent on which direction I parked). My Maxima is an auto, but I still use the parking brake on steep hills.
You've got the right idea with wheel chocks, though. Our truck's parking brake is kinda tempermental at times. You have to pull the handle out far enough (to a certain "click"), or it won't be engaged at all. And sometimes it's easy to not pull it out far enough. Basically, it's either on or off. No middle ground. This is why I use both methods, and also carry a set of cheap plastic chocks in the bed (belt and suspenders idea).

One thing with the Prestone De-icer, though. It may not be a good idea to keep it in the trunk. If your door locks are frozen, wouldn't your trunk lock be, too? I keep a can in the house.

Dave

 

[Jonny Z]

Quote:

---

One thing with the Prestone De-icer, though. It may not be a good idea to keep it in the trunk. If your door locks are frozen, wouldn't your trunk lock be, too? I keep a can in the house.

---

Here is the only time when remote keyless entry / trunk release is useful

 

[John K]

I used to have a Ford Ranger with manual tranny that didn't hold all that well on steep hills when in gear. Unfortunately the parking brake wasn't entirely effective either, it held better going forward than in reverse. Sometime when I forgot to release the parking brake I could back out of a parking space and when I tried to go forward it wouldn't move until I release the parking brake.....