# Question about starting a Two Stroke and scavenge air



## Peter Short (Apr 13, 2008)

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.


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## Hugh Wilson (Aug 18, 2005)

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.


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## Satanic Mechanic (Feb 23, 2009)

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


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## Peter Short (Apr 13, 2008)

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.


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## tugtere (Dec 6, 2007)

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.


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## cubpilot (Aug 18, 2008)

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.


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## Philthechill (May 8, 2007)

*Say's it all!!!*



cubpilot said:


> 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)


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## tugtere (Dec 6, 2007)

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 do***ents.Not many Koreans speak punjabi. regards ray(Jester)


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## Victor J. Croasdale (Nov 28, 2016)

I did my industrial training at Kincaids building B&W engines, I spent some time in the test dept. Later I went to sea with Denholms.
If the engine had been stopped long enough that the turbos had stopped, then the normal procedure was keep the engine turning on air until the turbos were running at least 1,000 rpm then put fuel on. If the stop was brief then the turbos would be going fast enough to put fuel on as soon as the engine was turning.


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## JClaude60 (8 mo ago)

MAN two stroke engines had two scavenging collectors , the underpart of the pistons were pumping air in a collector while the other was fed by a turbocharger .
Both collectors were connected by the famous " butterfly valve" , the secret of a good operation when manoeuvring was to put those collectors in serie or parallel ( butterfly valve ) to avoid turbochargers to cavitate .
Little is to be found on internet on this glorious piece of german engineering .
At sea speed , Man engines were great, easy to work on, however Sulzer is on the top


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## Waddie (Aug 2, 2014)

It was always thought that you MUST turn the auxiliary fans off (if not automatic) basically as soon as you got out of dead slow.
I was Chief on a VLCC in 2011 when we were asked to carry out fuel consumption tests at all speeds down to 8 knots.
At full speed we burnt around 100t/day (14knots if we were lucky), knocking off a knot and that was down to below 90t, and I think we got it down to 60t.
Obviously a massive saving, but at the lower speeds the auxiliary blowers had to be running, however this was deemed safe by the manufacturers and didn't appear to cause any problems, so looking back it was probably a myth about running with the fans on (though I don't recollect the reason why).
I was told to ensure that we had a spare motor and bearings for the blower, which I duly ordered, but was never around when or if the vessel was ever run at these super low speeds.


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## Peter Hewson (Mar 12, 2019)

I never researched it thoroughly. But always assumed (right or wrong) that the Underpiston volume on a Diaphram Doxford was sufficient to get a start along with a GOOD first blast of start air to get the Turbines producing?. P and J types. I know we never had any real start issue`s. A bit of sooty exhaust perhaps, which soon cleared?. I remember the turbo`s once running, took a hell of a long while to spool down, hence the need on some engines (B&W?) for the Gravity "reserve" Lube tank, up in the "gods".

Pete


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