Let’s say we are doing something similar to an engine compression test. We have disconnected the ignition system, depress the gas pedal all the way in and crank the engine.
Gas pedal on the floor means that the ECU goes into cylinder flush mode and no fuel is injected. Also it will allow more air to come in and cause higher cylinder pressures. And since we have disconnected the spark plug cables, the engine is not going to start.
Now supposing we are lucky and as we stop cranking the engine, one of the cylinders stops at exactly TDC, during the compression cycle, both valves closed. An engine in good condition would register something like 100-150 psi.
How long will the cylinder keep that compression in?
My uneducated guess is not very long. The seal at the bottom (ring to cylinder wall) isn’t a perfect seal. I think there’s actually a gap, filled by an oil film, and that film will give way to the pressure, and the compressed air will leak out pretty fast.
I don’t think so. If that were true, you couldn’t park a manual transmission car in gear without the parking brake. The compression does eventually bleed out, letting the car roll, but depending on the condition of the engine and the size of the vehicle and hill, etc. it’s usually something that happens over hours to weeks not seconds.
I suspect that the situation in the OP simply doesn’t happen. If the engine came to rest with a piston right at TDC on the compression stroke, the pressure in the cylinder would just push the piston back and rotate the engine ever so slightly until the force exerted by the pressure pushing the cylinder back is in equilibrium with the rotating resistance of the engine. I’m guessing that equilibrium pressure is going to be only a few PSI, not hundreds of PSI. I suppose then the question is how long until that little bit of residual pressure leaks out.
ETA: Of course, you can just try it by rotating the engine by hand… if you let go of the tool midway though a compression stroke, the engine will bump backwards instead of staying where you left it.
If the car starts to roll the cylinders will start to build compression, the weight of the car is not enough to compress the cylinders unless the hill is steep enough.
The weight of the car isn’t enough to compress the cylinders very much, but that tiny bit of compression is the only thing holding your car in place. Try pulling the spark plugs out of your engine and see how well the engine holds your car in place with zero compression. It’s pretty much indistinguishable from leaving it in neutral.
Agreed, if you take compression out of the equation all you are left with is engine friction (which is minimal) and the resistance from the valvetrain. You can turn an engine with no spark plugs with minimal effort.
But then there’s the gearbox. 1st gear usually has a reduction ratio of 3.5 to 4 and final gear ratio is 4 to 4.5.
So whatever resistance is offered by the engine is multiplied by a factor of 16 by the gearbox. This is more than enough to hold the car from rolling, as long as the slope isn’t too great.
The question was how long could a fully charged cyl hold it’s compression, A static car in gear pointing down hill is trying to build compression. I have pulled sparkplugs on hundreds of cars in my life and not one time have I ever heard the tell tale release of pressure you get imediately after pumping up a cyl for a compression test.
I do a leak down test on my plane’s engine every year. this is an air cooled engine so the rings are going to be “looser” than a water cooled engine. If the rings and valves are in good shape the number registered stays steady while it’s monitored. Granted it’s a brief period of maybe 15 seconds.
But consider what a hydraulic jack goes through it’s really a function of what the cylinder rings are designed for.
A few seconds. Crank it over by hand and hold it against the compression that builds up (i.e., don’t let it rotate backwards on you) and you can hear the hiss of the air leaking out of the cylinder. Doesn’t take long for it to stop hissing.
An airplane engine would hold compression well past a few seconds. If you can hear it hissing then it’s generally a valve seating issue. I’ve never been able to hear air leaking into the crank. Unless a ring is busted off it will keep compression. When they wear they hold compression at a lesser value.
Turning an engine over with the spark plugs out is not as easy as you make it sound. The friction is high at start up. There is very little lubrication. The bearings are sleve bearings and will only have a thin coat of oil on them if the engine was ran a short time before, same with the cylilnders.
When I was driving VWs I serviced them my self. I always did a compression test. Removed all the spark plugs first. Before putting in the plugs back in I would set the timming statically. I had to turn the engine over by hand with a wrench.
With the transmission in 1st or reverse gear the friction is multiplied.
It doesn’t take hours or weeks for pressure to leak out of a cylinder; it’s gone in a few seconds. If you don’t use a parking brake, the only thing holding your car in place is static friction between engine parts and the spring action in the valvetrain.
If the engine is exactly at TDC as you state, then cylinder pressure can’t exert any torque on the crankshaft in either direction. Imagine putting a wrench on a bolt such that the free end of the wrench is pointing straight toward you - and you push that free end of the wrench straight toward the bolt. That’s what’s happening here.
One type of diagnostic test known as a leakdown test (there are other kinds of leakdown tests) uses a calibrated orifice supplied with compressed air at a fixed pressure. This is connected to the cylinder, and the pressure on the cylinder side of the orifice is monitored. This gives an indication of how quickly the cylinder is leaking air. (pressure difference across the orifice is proportional to the square of flow rate) A cylinder can leak a fair amount and still be OK. Certainly enough to bleed of the air of compression in a few seconds. By listening at the exhaust manifold, intake manifold, and crankcase breather you can get some impression of whether excessive leakage is due to a valve problem or a ring problem.
Wow, really? I still time my VW staticly, and I turn the engine by hand with the sparklers still in it. I grab the generator pully with my right hand and turn it until the points just open up.
I will add to the many posters who have done this on a leakdown test. On my tester I can shut off the air supply to the gauge set with a ball valve. It takes less then five seconds to leak all of the 80 lbs of “compression” That my compresser put into the cylinder. I seriously doubt that the compression will stay in the cylinder for more then five seconds. It does not stay up for over five secondes even in a freshly rebuilt engine.