How airtight is a cylinder?

Airtight enough that a quick blast of pressure can make it move. Not airtight enough to hold pressure for any significant length of time.

Valve leakage, however minor, and more importantly, ring leakage, which is relatively major when considered over more than a second or two, will bleed off any applied pressure in a few seconds, at most. (Ring leakage is pretty huge when considered over a more than a few seconds - The end gaps are wide open - it's just a matter of the pressure "traveling the maze" to escape into the crankcase)

As far as the force needed to push a piston down, that's going to depend partially on the specific type of engine, and partially on the exact condition of the engine you're working with - different model engines are inherently "tighter" or "looser" than others, and even between two of the same model, the difference in the amount of wear and/or assembly tolerances that were used in putting them together, the ambient temperature, what brand/weight of oil is in them, and probably about a bajillion other variables could have a major impact on how much force is required to move a piston.

I would assume, from the way your question is phrased, that you're thinking of something like zapping a charge of compressed air into a cylinder (perhaps via the spark-plug hole?) to move the piston and thereby turn the engine? Should work, so long as you don't expect the cylinder to hold pressure for very long.

I've forgotten the brand/model, but a few *OLD* farm tractors used a variation on that concept as their starting mechanism - Attach a device containing what amounted to a blank 12 gauge shotgun shell, manually turn the flywheel to a specific point, set choke, magneto, throttle, etc, then snap the firing pin on the device, and it spun the engine over for the start (hopefully...) One running, you detached the device, tucked it into its own little dedicated pocket next to the engine, and went out to do the day's plowing/haying/whatever.

I've also worked with at least one older diesel front-end loader that used compressed air to start in the same manner - run the (battery powered - go figure!) compressor to pump up the tank, while it's pumping up, use a heavily geared-down hand crank to rotate the engine until the "PRIME" mark on the flywheel lines up with a pointer, squirt a shot of ether (not too much, not too little - JUST RIGHT, or it won't fire! there was a definite art to it!) into a dedicated gizmo on the side of the engine, then use the crank to roll the engine more, until the "START" mark on the flywheel lines up with another pointer. Once you had all that done, and the compressor had shut off, you hit a dump-valve that blasted the air into #1 (which, when the "START" mark was aligned with the pointer, was just past TDC) through a dedicated port, which spun the engine over fast enough to fire #3, which you had preloaded with ether, and you were up and running - Usually... Unless it was ungodly cold. Then, you might have to lather/rinse/repeat half a dozen times, all the while with the foreman breathing down your neck and griping about how long it was taking to get that damned loader running, we're burnin' daylight, man...

Farm tractors or WWII aircraft engines? Maybe both??

Forget art--what a g-d PITA!!

I'm told that big diesel locomotives, and probably other large engines, used compressed air to start them. Altho the Q comes up: Is the compressed air always injected into the cylinders, or just powering an air-starter motor? From your description, injecting air into the cylinders is quite the ordeal. Indeed, the injectable pistons have to be at the right position!

Air makes sense, in that it reduces the instantaneous very high draw from a battery. Air starter motor would be best, I would think.

As another poster points out, leakage around rings is substantial. However, the rings are designed in such a way that high pressure in cylinder expands rings against piston wall, and forces ring down on piston land. So the leakage is actually less at combustion pressure than if you just put a few psi in cylinder and measured leakdown.

BTW, look up the term "brake mean effective pressure" on google, and see if you can find some values for a few modern engines. It will give you some idea of the AVERAGE pressure during cycle.

As another poster points out, leakage around rings is substantial. However, the rings are designed in such a way that high pressure in cylinder expands rings against piston wall, and forces ring down on piston land. So the leakage is actually less at combustion pressure than if you just put a few psi in cylinder and measured leakdown.

BTW, look up the term "brake mean effective pressure" on google, and see if you can find some values for a few modern engines. It will give you some idea of the AVERAGE pressure during cycle.

Energy density.

Likely both, but I've never personally encountered the airplane like I have the tractor. However, now that you bring it up, I'd bet the tractor I was thinking of was an old Allis-Chalmers... Wouldn't surprise me in the least to find that they swiped the starting technology from their military engines to use on their civilian engines. Or vice-versa.

I've seen another loader that used an air-powered "turbine" type gizmo that otherwise worked the same as a more conventional electric starter. It was definitely newer (probably at least 20 years newer) than the "injected" kind I had to fight with.

It could be on the loader I had to deal with! When it worked "as planned" it wasn't bad - only took a minute or so to do the presetting, plus the time to wait for the tank to pump up if needed. When it didn't catch the first time, though, it was rather a pain.

That part wasn't really all that big a deal. "crank to here, spray ether, crank to here". The tricky part was getting the ether-spray amount right. Too little, and it didn't fire with enough "oomph" to light the next cylinder, too much, and it didn't fire at all. Either way, "try again"

Well, I'd assume that refinement came later.

(Then there's the fun of dealing with old rice-field Cats - pull some shutters to open the exhaust, prime the carb on the gasoline pony motor, wind up the starter (yes, wind-up, like a watch or a wind-up toy) of the pony, get it running, give its exhaust time to warm up the head of the main engine, make sure everything on the main (diesel) engine is set correctly, engage the pony-to-main clutch, and hope the main catches, close the exhaust shutters, shut down the pony...)

Here's a guess since I have seen air starters on semi's as well as locomotives. For both semi's and trains the braking system relies on compressed air for the braking system. Therefore both applications already have an air compressor and storage vessel on board.

But, how is the compressed air used to spin up the engine? three possibilities: introduce directly into the cylinders that are on their induction or expansion strokes (need to know crank position to know which one is where at startup), blast into intake manifold and have a shutoff valve upstream to force the air through the intake valves, or just have an air motor instead of the electrically driven starter motor. I'm betting it's either going to be options two or three; whichever is cheaper to implement.

On small and medium size marine diesel engines, compressed-air driven starter motors are the norm. The ones I'm familiar with use rotary vane-type air motors powered by ship's service air (90-120psi). Large, low-speed diesels (the ones used for main propulsion, think three stories tall and as much as 100,000 hp or more) use high- pressure air (on the order of 33 bar or almost 500psi) admitted directly to the cylinders controlled either by a starting air distributor (like an ignition distributor for air) or the engine can actually have dedicated cam lobes that will operate air-start valves when made to do so. These systems have very large compressors and tanks, as such engines are normally direct-reversing (to go backwards, you run the engine backwards), so reversing the engine involves stopping it and then starting it in the opposite direction. While maneuvering in port, this might have to be done several times in fairly rapid order, thus necessating large reserves of compressed air.

Harry

Compressed air starters were popular on some early cars before the Kettering style electric starters took off.

???

I've always been under the impression that "Kettering" referred specifically to a coil-and-breaker-points type ignition system, not a starter?

Bendix is the name I associate more closely with electric starters. (probably because most of them use a "bendix" to put the drive gear into contact with the flywheel's ring gear)

Depends on what kind of a cylinder/piston arrangement it is.About a week or two ago, I saw a tv program about a guy who invented a new kind of a pogo stick.It works on air pressure,will jump about thirty or more feet high.Years before that, a guy in Wichita invented a ''spark plug'' pogo stick.His prototype pogo stick used a piston and a spark plug from a lawn mower.

Engines are just glorified air pumps anyway.Ignition, fuel, air and compression. cuhulin

Don Bruder wrote in news:4663278e$0$14097$ snipped-for-privacy@news.sonic.net:

Charles F. Kettering was an inventor who devised the ignition system you describe.

He was also the first man to successfully use a small, light electric motor to start an auto engine. His "temporarily overloaded" small electric motor design is used to this day in all cars.

Vincent Bendix is associated with the starter drive design that bears his name. This is a type of starter drive that brings the starter pinion and the ring gear into contact with each other. Bendix attached that to the existing Kettering-style starter.

The Major was a shotgun start tractor. It was actually a common starting method for quite a range of equipment. From stationary engines to aircraft. I collect engines and tractors and have worked on a lot of the old stuff and I enjoy seeing all the different "break through" technologies that are popping up and claiming to be "new".

Diesel locomotives use electric starters for the most part, although some use compressed air- GE locomotives use the generator that the engine normally drives to generate power for the traction motors as a motor. EMD locmotives use multiple large autmotive type starters on the flywheel.

Diesel engines in ships almost always use compressed-air start, as do a lot of big stationary engines- like generating plants and flood-control pumping stations, etc. This is sorta the 1000-8000 horsepower class engines.

The latter in everything except the HUGE Sulzer-type diesels that are 3 stories tall and drive the prop shaft directly with no reduction gear. A compressed air motor physically smaller than an electric motor of the same power, and it doesn't overheat if the engine is "cranky" and won't fire right off. I've heard a few ship diesels run the air reservoirs completely empty before starting in cold weather, so the engineer had to wait for the compressors to fill the tanks and try again. An electric starter would have been a molten pile of metal after that kind of treatment.

Yes, but that is done for the really, really big diesels. Its the only practical method, and allows for finer control. Those engines actually reverse direction when the ship needs reverse thrust, so the valve gear is already complex to allow the engine to run backward when needed. Plus the transition from "starting" to "running" is kinda gradual and allows finer maneuvering control.

Read up on Charles F. Kettering, you'll be amazed. He invented (among other things) the electric starter, the breaker point ignition, and was largely responsible for the original Oldsmobile "rocket" v8 engine. He also was involved in the founding of what became Dayton Electric Company (better known as Delco). And he was involved in medical research as well, perfecting incubators for infants. He's the "Kettering" in the Sloan-Kettering foundation.

Bendix corp perfected one of the gear engagement mechanisms for electric starters, and "Bendix drive" became a kind of generic term for starter drive gear systems.

Back in the 1970s I read about (or maybe I saw it on tv) some diesel engines used on fishing boats in England.They were old very simple diesel engines with a dome shaped top on the engines.Before starting up the engines with a rope pull, they used hand held blow torches to preheat the cylinder area of the engines. cuhulin

This Kettering guy musta been real bright--or manic. :)

Is this Dayton the same as Grainger's Dayton? I always thought Dayton was Grainger's "house brand".