OT: Why can't a petrol engine start itself?

On Thursday, March 22, 2018 at 4:41:34 PM UTC-4, James Wilkinson Sword wrot e:

in the right position gets an injection of fuel and air mix and a spark and stars the engine rotating, without the need for a starter motor? Especial ly 8 cylinder engines, surely one is always in the right position to get it going?

How are you going to get that injection of air? The air gets pulled in on the intake stroke, then compressed. The compressed air isn't going to stay there forever, the rings and valves don't seal perfectly. Plus one cylinder firing probably isn't going to generate enough momentum in the engine to compress the next cylinder.

You still need compression. They do have airplane engines that start with a cartridge like a shotgun blank tho.

If I blow air and petrol vapour into a tin can then drop in a match, it'll blow up. I don't need to squash it do I?

As seen in the movie "Flight of the Phoenix"

As trader already mentioned, a single cylinder firing most likely wouldn't be enough to get the ball rolling. Maybe if it had already recently run and was still nice and toasty warm and you had a great shot of fresh air/fuel into the combustion chamber. You might (doubtful) get it to start and run from a single initial hit, then.

That's assuming you had some way of keeping the fuel air mixture and compression up on the cylinder you're going to try and start with. For all that effort on a seriously doubtful succesful start, might as well use the trusty electric starter. It'll either start your engine, or, run your battery down trying!

Yes, but don't those engines have valves or something that disables compression build on cylinders so you can spin the prop or cartridge start? Then closes them for compression once it's shaft is upto a certain speed? or vacuum?

You still need compression to get a Otto cycle engine to run. I suppose it is because there is not enough volume at TDC to get in enough fuel/air mix to fire off the bang necessary to turn the motor over but when you fill a cylinder at the bottom of the stroke there is plenty of volume but no place to go. Squeeze it down and apply the spark at the top and off it goes.

"seven days and seven nights" or many other movies with old planes in them.

Koffman IIRC

In the early days of high powered electronic ignition there were reports of fords, being shut off warm and the key then turnrd on, starting from someone slamming the hood. ONE spark and away it went.

Would need a good flywheel.

I often "bumpstart" my ranger - slow walking speed in reverse - just let the clutch out for half a second - definitely less than 1/2 turn of the engine and it comes to like (with 159000km on it)

Nope. you pull the prop through so the designated cyl is at top dead center, then "pull the trigger" - that's how a Field Marshall tractor started.

With a Koffman, the shell didn't give compression - it fired a "rack and pinion" type of device that physically cranked the engine

Not sure what "ignition" means on a diesel engine, but when I were a lad, our school bus would keep running if you turned it off and removed the key. You had to press the "stop" button to stop it running.

On Fri, 23 Mar 2018 01:14:51 +0100, Peeler

Some have glow plugs to start them but compression generates the heat to ignite the fuel, this you no longer need the key. The shut off restricts air flow to stop it.

So what happens on a normal car? Does switching off the ignition automatically shut off the air?

Old farm tractors had a knob attached to a cable. The cable was attached to the injector pump. Pulling on the knob and cable would keep the injector pump from working somehow. I think it's VW making a big deal out of running a combination gasoline/diesel engine. It isn't quite the same thing but some old farm tractors started on gas then ran on diesel. A video of one here: Some old tractors and crawlers used pony motors to start the main engines. The pony motor was just a little gasoline engine.

No. It shuts off the electricity to the ignition system. That eventually stops the power to the spark plugs. Video here: The moderator talks funny.

My vintage Mercedes, they shut off the fuel. I would think on all the new e lectronic fuel injected they would do the same, it's even easier.

it off and removed

I've been around a lot of diesel power units over the years. They all shut off the fuel somehow with the possible exception of one. I think it was a Detroit brand. It cut the air supply to stop the engine.