What is the state of the oil in an automobile engine while it’s running? Sketches describing how lubrication works show a horizontal oil level division between air above and oil below, with a few drops of oil being kicked up so they hit cylinder walls. Typically the oil level is around the crankshaft centerline or somewhat below but within the travels of the connecting rod bearings.
Isn’t the oil and air going to be whipped into a furious froth? There are these big objects occupying a significant fraction of the volume that are traveling around in significantly long paths. In a car engine allowed to go as fast as 6000 RPM, they are completing 100 cycles per second.
My car engine is a horizontally opposed or “flat” engine. The only drawing I found online shows the oil level exactly on the crankshaft centerline, meaning that half the piston and cylinder profile is submerged. How is that not going to turn the oil and air into a frothy mixture?
How does the oil pump suck just liquid oil in such an environment?
From the perspective of a 60’s era hotrodder…Engines I am familiar with had a windage tray. It was bolted to the crankshaft main caps. It’s purpose is to keep the oil in the pan and away from the crankshaft.
Any number of 1930s-1940s engines had “splash” lubrication, where the crank throws were intended to hit the oil, splashing it up into the cylinder bores, etc. Not especially efficient, and superseded by “full-pressure” lubrication in the late 40s.
that image is inaccurate. The Subaru boxer engines have a sump (oil pan) below the main bearing girdle, so that the crankpins and counterweights shouldn’t be dipping into the oil in the pan. They also have some form of windage tray to prevent oil from sloshing up and being picked up by the crankshaft.
here’s a cutaway drawing of a 4.6 liter Mustang V8. notice how the oil in the pan is entirely below the “swing circle” of the crankshaft.
Now, if you over-fill your engine with oil, this can be a problem.
small 4-stroke lawn & garden and industrial engines tend to use splash lubrication; the connecting rod end cap has a dipper on it to dip into the sump and fling oil around the engine.
If you over-fill a typical car engine, the oil will come up to the level of the moving bits and you will end up whipping the oil into a froth. Oil that has been whipped into a froth doesn’t circulate very well through the engine, so this ends up being basically just as bad as running an engine without enough oil in it. Major engine damage can result from it.
I actually knew some small engines and older engines used a splash system. I was thinking about modern automotive engines when I posted, my bad I wasn’t clear.
Some high performance engines have a dry sump system where the oil is purposely suctioned out of the crankcase into a separate storage area then pumped into the bearings from there. This keeps the oil from splashing around during high-G turns.
This is all very interesting. How about airplanes with piston engines? How can they fly inverted, or on their side, or straight up or down, and so forth? Not that they do these things a whole lot, but, if plane engines seized up as soon as you flew in an unusual orientation, air shows would be a lot shorter.
Airplanes are not all the same, you probably can not fly every plane type inverted.
I know you arent flying the planes that use the small little carb’d flat 4’s (Think VW engined mini planes etc) inverted, or at least not a heck of a lot.
I dont think loops and aerobatics are in the average planes forte?
But what i know of the engines in things that fly in extreme manners, they have a dry sump, oil is in an oil tank and the pickup is mobile. possibly the extractor as well
They are also fuel injected as carbs also dont care much for the float bowl being inverted etc.
Gaah. Accidentally clicked [close] on a good post. Here’s the improved (= shorter) version:
Folks just above have nailed it for airplanes. Ordinary airplanes are rigged for fuel and oil feed only under positive Gs. Which doesn’t prevent aerobatics, but does restrict the airplane to purely positive G maneuvers such as inside loops and most rolls while disallowing negative G “pushover” maneuvers lasting more than a few seconds.
For sustained negative G operations you need what’s called an “inverted fuel system” or “inverted oil system”. See http://www.musclebiplane.org/htmlfile/invert.php for several nice pages with pix and diagrams of a typical inverted oil system as installed on one brand of aerobatic sport biplane.
In World War II, Spitfires would stall if they went inverted or if they pulled a high-G turn or did any kind of negative-G maneuver. This was due to the carburetor though, not oil. This was fixed somewhat with a device called Miss Tilly’s Orifice (I’m not making that up) which was basically a restricting diaphragm that prevented too much fuel from entering the carburetor, as excess fuel would be forced above the float during negative Gs, which would first starve the engine, and then when the plane went back to positive Gs, the extra fuel would drop past the float all at once and flood the engine, causing a stall. Going inverted too long would still cause a stall, even with Miss Tilly’s Orifice installed.
German planes were fuel injected and didn’t suffer from this problem. Once the Germans realized that the Spitfires and other Allied planes had this problem, they would try to maneuver in such a way to cause the planes to stall, giving the Germans a big advantage.
Planes that are designed to fly inverted have what’s called a Flop Tube in the fuel tank. When the plane inverts, the tube flops to the top of the tank, along with the fuel. Planes without a flop tube will suck air instead of fuel out of the tank when inverted, causing a stall.
Some plane engines have an external oil tank and inject oil into the engine. If the plane is designed to fly inverted, this oil tank will also have a Flop Tube so that it will pick up oil regardless of the plane’s orientation. For engines that don’t use an external oil tank, a second set of oil pickup tubes can be added to the top of the engine so that it will still circulate the oil when inverted.
If a plane isn’t designed with things like Flop Tubes or top-side oil pickup tubes, then it can stall if it pulls a negative-G maneuver.
Note that you can roll or loop an airplane while maintaining positive Gs throughout the maneuver.
Tex Johnston rolled a Boeing 707 during a demonstration flight and kept positive Gs through the entire roll. The president of Boeing did ask him to never, ever do that again, though.
Oil stays below the crankshaft in a wet sump system. In a splash lubrication setup (like in a horizontal shaft Briggs and Stratton), it usually stays below the crank and is splashed by a arm that sticks off the bottom of the connecting arm.
Smokey Yunick claimed in his book to have put a glass viewing port in an engine and then used a strobe to watch what happened to the oil as windage trays were installed, modified, etc. Some of the oil stayed caught up in the vortex around the crankshaft, and this never really disappeared unless they installed a dry sump system (caveat: it’s been decades since I read the book, and it was Smokey Yunick telling all this stuff.)
Among WWII combat a/c, some had a very tight limit on negative g flight due to the fuel system as has been mentioned: a plain float carburetor engine might cut out immediately on negative g. That was a tactical problem, since less negative g limited opponents might adopt a quick ‘right side up’ pushover into a dive as an escape maneuver against a plain float carbureted opponent in situations where it took too long for the float carbureted a/c to half-roll inverted and enter the dive using positive g. There were various solutions, either retaining a float carburetor with modifications for negative g, a so called pressure carburetor that didn’t rely on gravity, or fuel injection.
But a typical oil system, even ‘dry sump’, will eventually fail to lubricate the engine in non-positive g flight if the scavenging pump is at the bottom of the crankcase. The separate tank provides some seconds of oil supply. For example the P-51D’s pilot manual specifies a 10 second limit on inverted flight because of the oil system. This wasn’t really a tactical issue though since negative g combat maneuvers lasting more than a few seconds weren’t likely. http://www.wwiiaircraftperformance.org/mustang/P-51D-manual-5april44.pdf
see pg 13 for the instruction, the dry sump oil system is illustrated on pg 10.
As mentioned, aerobatic show a/c might want to fly inverted for longer.
Looks like the ultimate solution for an aircraft is a 2 cycle rotary
which of course is not exactly the ultimate engine, unless its WWI.
But the engine itself does not care, since it has no up or down, the entire engine spins when running, so just work out the fuel delivery system.
