Fuel Injected Engine Idle Speed

[E_Dawg]

OK so I made the GIANT LEAP from a '81 152 to the '02 172R

. Ohhhhh moving map GPS; leather seats; double the range; seats that actually slide.....

Anyways there's something in the POH about making sure the engine 'idles' prior to takeoff. What defines engine idling? What if, at idle power, the RPMs vary +-200RPM or so? What defines 'not idling'? And can you still fly it, just without using full idle power?

 

[EatSleepFly]

I think what it means is that if you pull the power back and it idles rough (sorta sputtering, is the best I can describe it), it probably has some vaporiztion happenin'. When you taxi into position, throttle up to full power for a few (or sometimes even several) seconds until the sputters stop and its smooth. Then you're good to go.

 

[ananoman]

I no experience with the new 172, but I remember reading about problems with the new engine. I think it tends to run rich at idle. If you pull it to idle and it sputters, try to lean it just a little bit and see what happens. I thought that they were supposed to be placarded or new suppliments were supposed to be made for the POH.

 

[Eagle]

What it means is to check that it does idle. after all of your run ups etc, the last powerplant check is pulling the throttle all the way out to see that it idles, if it is adjusted wrong you may cut off all the fuel, and shut down.

this will identify a few problems all of which should keep you on the ground.


Also, on the lycoming 320-360-540 engines, you should not be sitting on the ramp with the engine running below 1000rpm. The crank shaft splashes through the pool of oil and this is how the lower half of the engine gets lubed.(low pressure side) if you run at 600rpm, it will not splash it around enough, (of course if it is a rental airplane who cares

unless you are the guy driving when it gives out.)

After start and you have engine oil pressure, you should lean the engines to just under cut off, this will keep your plugs cleaner, just richen it up before you do the run up or it will stall out.

 

[ananoman]

I cannot think of any engine, other than small engines used on lawn mowers that rely only on 'splash oiling'. Even lawn mowers usually use a spinning gear to throw oil up into the engine. Some old low powered agricultural engines also had a projection on the connecting rod bearing cap that acted like a spoon and would scoop up oil and throw it around.

The only engines that can get away with this type of oiling are relatively low powered. All modern 4 cycle engines of even moderate power have a pressurized oiling system. Oil is sucked out of the oilpan by a gear pump and forced through internal passages in the engine block to oil all the moving parts. The crankshaft spins plain main bearings, but does not actually touch them, it floats on a film of oil forced into the bearings by the oil pump. Some of the oil then runs out of the bearing back to the oil pan. Some is used to lube the connecting rod bearings by flowing through passages drilled through the crankshaft. The wrist pins that hold the pistons to the connecting rods can be lubed by a hole drilled the length of the connecting rods. Pressurized oil also lubes the cam bearings, inflates the hydraulic valve lifters that self adjust for wear, and lube the rocker arms by flowing through the hollow pushrods. Only a very few parts of the engine are lubed by oil splashing around. The cylinder walls and cam lobes are two that I can think of. Oil is not thrown out of the crankcase, but thrown out of the bearings on the spinning crankshaft. For extra protection oil spit holes can be put on connecting rods to spit oil into opposing cylinders or oil jets can squirt up to cool the bottom of the pistons.

Several of the POH that I have looked at say that O-320 and O-360 engines can run on 2 quarts of oil. The dipstick only goes down to 4 quarts, so why you would do this, I do not know. I do think that any engine with an 8 qt. oil pan would be unable to run on 2 qt. of oil if it relied on the crank splashing oil around.

Lawnmowers can get away with such a primitive system because they make such little power and really don't run very much. Any type of high powered 4 stroke has to have a pressure system to survive. If metal to metal contact occurs, the engine will rapidly self destruct. Engine designers often try to go to great lengths to keep the crankshafts of high performance engines out of the oil in the crankcase. When the crankshaft spinning at 2700 RPM (much higher in cars and as high as 16,000 in some motorcycles) hits a pool of oil it results in a loss of engine power and a rise in oil temperature due to friction and places a needless stress on the engine. Deeper oil pans, 'knife edged' counterweights, baffles, or even dry sumps can all be used to help minimize losses due to the spinning crank hitting oil sloshing around in the bottom of the engine.

 

[E_Dawg]

For what it's worth (and actually the reason I asked)

I was in Vegas in 105F heat. I did my idle check and sure enough, the engine quit (most likely from vaporization).

Anyways I intentionally flooded it and restarted it, then ran it up to 1800RPM for a while before I took off. I did a quick idle check ('bout 2 sec at idle for fear of having it quit again).

....which leads me to another question. In that situation where you know it's vaporization due to the intense heat, and the engine quits on the idle check, is it still OK to fly (assuming you run it up for a bit to clear out the excess fuel vapor)? How long is it supposed to be able to idle before it dies?

In my situation I ran it up for 2 minutes after it quit and it ran just fine the whole way home...

 

[Eagle]

[ QUOTE ]
I cannot think of any engine, other than small engines used on lawn mowers that rely only on 'splash oiling'.

[/ QUOTE ]

well you should add piston aircraft engines to the list then.

The upper half of the engine, uses the high pressure oil, but the lower half gets it in the rather low tech (for simple discussion, everything below the cylinder is splash lubricated)

A quote directly from the Mattituck site..

· Keep RPM To A Minimum At Start Up
After engine start up we should run the engine at the slowest RPM that the engine will continue to run at. If it will stay running at 700 RPM then that is the RPM we should be running at not 1400 RPM. Allowing the engine to run at a high RPM right after start with out letting it warm up can cause premature wear to many internal engine surfaces that are splash lubricated. Some of these splash lubricated components, most notably the camshaft and tappets, are especially susceptible to damage right after start with cold oil and high RPM operation. Keeping the RPM to a minimum will limit the amount of interaction of these components before enough oil has splashed around inside the engine to provide sufficient splash lubrication.

Hey but what do they know…

 

[ananoman]

I'll try again. The main problem I had with your original post is:
1.The lower half of the engine is not lubricated by the spinning crankshaft hitting the oil in the oil pan and splashing it around.
2.The lower half is lubed by pressurized oil. (Usually when referring to the 'bottom end' of an engine, this in the engine block and it's contents) I never actually said that no parts of the engine were lubed by oil splashing around.

Quote:
Also, on the lycoming 320-360-540 engines, you should not be sitting on the ramp with the engine running below 1000rpm. The crank shaft splashes through the pool of oil and this is how the lower half of the engine gets lubed.(low pressure side) if you run at 600rpm, it will not splash it around enough, (of course if it is a rental airplane who cares unless you are the guy driving when it gives out.)

This is not correct. The engine is lubed by a pressurized oiling system. Which I described. I also noted that:

Only a very few parts of the engine are lubed by oil splashing around. The cylinder walls and cam lobes are two that I can think of. Oil is not thrown out of the crankcase, but thrown out of the bearings on the spinning crankshaft.

What Matticuck has to say does not refute anything that I have said.

A quote directly from the Mattituck site..

· Keep RPM To A Minimum At Start Up
After engine start up we should run the engine at the slowest RPM that the engine will continue to run at. If it will stay running at 700 RPM then that is the RPM we should be running at not 1400 RPM. Allowing the engine to run at a high RPM right after start with out letting it warm up can cause premature wear to many internal engine surfaces that are splash lubricated. Some of these splash lubricated components, most notably the camshaft and tappets, are especially susceptible to damage right after start with cold oil and high RPM operation. Keeping the RPM to a minimum will limit the amount of interaction of these components before enough oil has splashed around inside the engine to provide sufficient splash lubrication.

You also say that:

The upper half of the engine, uses the high pressure oil, but the lower half gets it in the rather low tech (for simple discussion, everything below the cylinder is splash lubricated)

The last I checked both the crankshaft and the connecting rods are located below the cylinders. They both ride on pressurized plain bearings. This is the source of the 'splashing' oil that lubes the cylinder walls and cam lobes. Not the crankshaft sloshing through a pool of oil. Both the cylinder walls and the camshaft have special features to help them survive. The cylinder walls in most engines are nitrided so they are very hard and are honed with a rather rough stone in a crosshatch pattern to hold oil for the piston rings to ride on.. The camshaft and lifters are both extremely hard, to the point of being brittle.

Mattiruck is correct. The majority of engine wear takes place at engine start. This is when engine parts are not covered by a protective coat of oil. Letting a cold engine roar to a high RPM is one of the worst things that you can do.

I guess just don't know where you are going with this.

 

[Eagle]

I think we are splitting hairs, the main bearings, push rods, tappets ( some accessories) and the like all have oil supplied by the HP oil system, pressure controled by the vernatherm and the spring bearing (next to the top of the number 4 cyl.)

The Pistons, Piston Pins, Cams, everything below the rings... is splash oil, vs HP oil.

as a result, the bearings rarely if ever go bad, it is the rust/wear on the cams and pitting that create problems during overhaul, ( problems.. pilot vernacular for big bucks$)

the Lycoming factory school, recommend not idling below 1000rpms to allow the max splas foe the lower half parts, (again excluding berings)

If yo want I have the book next to me I can scan the schematic and post it later on..

Another tidbit I picked up from the school.. Lycoming recommends oil changes ever 50 hours.. you know why? Because continental (TCM) does. the instructors will tell you point blank... do it ever 25 hours. but if they advertised it, then Continental would be out saying. LOOK LYC NEEDS TWICE THE OIL. ( they claim, but I have no first hand knowledge, that TCM really recommends the 25 hrs as well.)