Question about starting a Two Stroke and scavenge air

Peter Short
19th June 2009, 03:14
I have been wondering how a modern two stroke uniflow marine engine gets enough scavenging air to start?

Firstly I was wondering about the later Doxfords (1970's) which I assume use their turbocharger to provide scavenge air? If so, what provides the scavenge air before the turbo gets up to speed?

Then I began to wonder about any of the modern uniflow two strokes, and wondering if they have auxillary electric-driven supplies for starting?

I have read where some of the older marine diesels used electrically-driven scavenge supplies for all their running, i.e. no mechanical pumping of any kind.

Thanks for any help.

Hugh Wilson
19th June 2009, 03:52
At low engine speeeds and whilst maneouvering, electrically driven auxiliary blowers are used, usually two, which automatically start and stop dependent on scavenge air pressure.

Satanic Mechanic
19th June 2009, 06:27
Its not actually the fact they are uniflow that's the problem, its the turbocharging system. Modern engines use a constant pressure system whereby the cylinders exhaust into a receiver rather than straight to the turbocharger(s) which means the initial pressure is dissipated before entering the turbocharger. So before the turbochargers can supply enough air the pressure in the exhaust receiver must be high enough i.e. the engine must be going fast enough. In order to make up the difference two electrically driven fans are used exactly as Hugh described.

The advantage of this system is that it is optimised for NCR operation

Peter Short
20th June 2009, 04:06
Thanks for your replies, thats interesting.

On reflection I shouldn't have said "uniflow", I guess any two stroke design will need scavenging air from some source.

tugtere
4th August 2009, 04:08
Peter, I have sailed with a couple of Norwegian built "Wichman" engines (abt, 1800 hp) which had 5 roots type mechanical blowers for slow-speed air supply and on top 2 turbo-chargers for high speed. Good solid engines too.Mono directional thru C-P props.

cubpilot
4th August 2009, 17:37
If the memory is correct the output of electric fans is such that they provide enough air (ie pressure) for the initial start up and part of the air needed for dead slow speeds. to keep running at low rpm needs more air than the fans can supply and before there is enough exhaust gas flow to spool up the turbos the engines rely on under piston pumping.
i joined one ship to be told that they had terrible problems starting and running at slow speeds. sure enough when we sailed it was touch and go as we manouvred. put the thinking cap on during the voyage and at next port removed as many inspection covers as i could along the middles and found that 40% of the scavenge ports at the back of the engine were blocked ( ie where you could not reach from the scavenge trunking) and many reed valves were stuck open. job and finish and the rest of time in port off for the lads. on sailing them up top thought they had a new engine.

Philthechill
4th August 2009, 17:57
If the memory is correct the output of electric fans is such that they provide enough air (ie pressure) for the initial start up and part of the air needed for dead slow speeds. to keep running at low rpm needs more air than the fans can supply and before there is enough exhaust gas flow to spool up the turbos the engines rely on under piston pumping.
i joined one ship to be told that they had terrible problems starting and running at slow speeds. sure enough when we sailed it was touch and go as we manouvred. put the thinking cap on during the voyage and at next port removed as many inspection covers as i could along the middles and found that 40% of the scavenge ports at the back of the engine were blocked ( ie where you could not reach from the scavenge trunking) and many reed valves were stuck open. job and finish and the rest of time in port off for the lads. on sailing them up top thought they had a new engine.If anyone has any doubts about what makes Marine-Engineers the creme-de-la-creme, of all Engineers, then this snippet about how one of our elite brethren "put-the-thinking-cap-on" and solved a problem should dispel any doubts!! Well done cubpilot!!! Salaams, Phil(Hippy)

tugtere
7th August 2009, 04:48
cubpilot, I had a similar happening on a 9 cyl. Halberstadt MAN don't let anyone tell you there are no valves in a two stroke, this thing had 8 per cylinder=72 and I serviced them all myself. and didn't she run sweet after that. We went that trip from Cape Byron Aus. to Majuro, Marshall Islands on 7 cyls, 2 hanging up on the wall.Broken pistons, continually breaking telescopic pipes (water cooled pistons) e/r natives always turning off snifter valves because they made a mess on the plates. I might write a story about that ship.Crazy punjabi old man and kiwi chief coming to blows at regular intervals Got to Inchon and the old man wanted to go into Seol so we put him on a bus pointed that way, he came back six days later, bus took him right to the DMzone on the way to Pyongyang where he was arrested for not having documents.Not many Koreans speak punjabi. regards ray(Jester)