Reversibility of Two Stroke Engines: Exploring the Impact of Crosshead Bearings

[yudzark]

Are large two stroke engines with cross-head bearings reversible?

 

[Nidum]

Some kinds of large engine can be designed to be reversible but I don't understand the significance of cross head bearings in this question ? What is it that you actually want to know ?.

 

[XZ923]

2-stroke engines can be run backwards if the ignition timing is off (or deliberately by design as Nidum pointed out). I'm not sure how you're factoring a cross-head into the timing mechanism.

 

[Dr.D]

The cross-head has no bearing on the ability of the engine to run backwards. Many large engines, designed for ship propulsion, can and do run in either direction to go forward or astern. Those large ship's engines are often cross head designs, but that is to allow them to be double acting and has nothing to do with reversibility.

 

[Nidum]

Two stroke engines with crosshead bearings - as opposed to having gudgeon pins in the piston - work a little differently anyway . Mainly in the way that the incoming air charge is pre-compressed for the next firing cycle .

Crosshead or piston gudgeon pin design difference does not of itself make the engine reversible or not though .

 

[Nidum]

Just aside - anyone ever tried to kickstart a big 1950's vintage British motorbike ? Many a luckless rider got propelled quite violently into the nearest hedge when the engines on these tried to start the wrong way .

 

[XZ923]

Just aside - anyone ever tried to kickstart a big 1950's vintage British motorbike ? Many a luckless rider got propelled quite violently into the nearest hedge when the engines on these tried to start the wrong way .

I've very rarely seen a British bike that old and on the rare occasion I do it's usually because someone's freaked out by the positive ground system, but I've had it happen on older dirtbikes (70s/80s). A buddy of mine once tried to start it on a hill while pointed uphill; you can probably guess the rest. Fortunately he just had a few bumps and bruises.

 

[Averagesupernova]

Some older outboard boat motors were reversible. I recall seeing one many years ago that had 2 starters and 2 ignition systems. One for each direction. They were known as dock slammers because when coming up to dock, the engine was shut off and started in the opposite direction and at the most inopportune moment would fail to start and slam into the dock.

 

[Ketch22]

As Nidum has said several of the older marine engines were designed this way. It was relatively common in older tug boats where "direct reversing" was the term employed. The gear box was a simple reduction gear, and there was simple make or brake clutch. When the direction of thrust needed to be changed one simply declutched, turned off the engine, restarted in the opposite rotation, then engaged the clutch.
Crosshead bearings had no direct effect, however as a side effect the compression prior to injection made it easier to have the engine run either way.
Side note: this style of propulsion was not particularly efficient in either direction but it was workable.

 

[yudzark]

Some kinds of large engine can be designed to be reversible but I don't understand the significance of cross head bearings in this question ? What is it that you actually want to know ?

I apologise for my lingo error. BTW I wanted to know about the technological developments over the years in reversing mechanism.
Although I used crosshead bearing as an engine specification. And I do know that it plays no part in reversing the engine.

 

[yudzark]

If you look closely at my profile picture, it is the x-head you will be looking at.

 

[Tom.G]

If you look closely at my profile picture, it is the x-head you will be looking at.

Hmmm. Looks like a crankshaft, con rod, and piston here. (VERY big ones! Ocean going vessel?)

 

[Rx7man]

The biggest reasons any two stroke can't be reversed is if they have a supercharger for scavenging, and/or if they have a gear pump to pressure lubricate the bottom end... If the oil pump runs backward you won't get oil (piston style pumps would still work correctly though), and if the supercharger works backward, things will get weird really quickly!

 

[yudzark]

Hmmm. Looks like a crankshaft, con rod, and piston here. (VERY big ones! Ocean going vessel?)

Yes it is an ocean going vessel.

 

[yudzark]

The biggest reasons any two stroke can't be reversed is if they have a supercharger for scavenging, and/or if they have a gear pump to pressure lubricate the bottom end... If the oil pump runs backward you won't get oil (piston style pumps would still work correctly though), and if the supercharger works backward, things will get weird really quickly!

The maritime industry uses turbochargers and not superchargers, we compress the exhaust gas and reuse it in scavenging the engine and we also use exhaust gas to run Economiser gas boiler in a waste heat recovery system.
The bottom end bearing is lubricated from drilled holes in the crankshaft, oil is sent by an external pump(screw pump) outside the engine, the bearing has wedges to sustain elastohydrodynamic lubrication as an oil film.
The crosshead bearing is lubricated by a telescopic pipe(MAN B&W design) or articulating pipe(SULZER/Wartsilla design).
What you said is correct and holds true for 4-stroke engines without a cross-head bearing. The process is totally different in a large two-stroke engine used on a ship.

 

[Nidum]

 

[Rx7man]

The maritime industry uses turbochargers and not superchargers, we compress the exhaust gas and reuse it in scavenging the engine and we also use exhaust gas to run Economiser gas boiler in a waste heat recovery system.
The bottom end bearing is lubricated from drilled holes in the crankshaft, oil is sent by an external pump(screw pump) outside the engine, the bearing has wedges to sustain elastohydrodynamic lubrication as an oil film.
The crosshead bearing is lubricated by a telescopic pipe(MAN B&W design) or articulating pipe(SULZER/Wartsilla design).
What you said is correct and holds true for 4-stroke engines without a cross-head bearing. The process is totally different in a large two-stroke engine used on a ship.

I just cannot figure out how the presence or absence of a crosshead bearing has any relation to the reversibility of an engine. it slides back and forth the same way regardless of engine direction.

I just provided a few examples of engineering designs that would be detrimental to operating in reverse.

I'd like to add that 2 stroke engines with valves (Detroit) will also not run properly in reverse because the gas flow is going to be completely wrong

 

[yudzark]

 

[yudzark]

I just cannot figure out how the presence or absence of a crosshead bearing has any relation to the reversibility of an engine. it slides back and forth the same way regardless of engine direction.

I just provided a few examples of engineering designs that would be detrimental to operating in reverse.

I'd like to add that 2 stroke engines with valves (Detroit) will also not run properly in reverse because the gas flow is going to be completely wrong

Please check
Comment #10 - I have already apologised for incorrectly asking the question and I understand that the x-head bearings are not related to the reversing of an engine.
Also have I expressed my goal behind asking the question in the same comment.
Comment #18 - For purview, I have shared some videos for further insight.