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- 04/02/2013 at 10:06 am #4537Dave BarkerKeymaster
Flicking through the latest copy of Yachting Monthly I was intrigued by one of the engines in the group test of small outboards. Selva have produced a 4-stroke 2.5hp motor with some characteristics akin to a 2-stroke engine. Lubrication is by mixed petrol and oil, with the oil stored in a separate tank rather than the sump – the magazine reckons that this will only need filling perhaps once a season. This means that the engine can be stored any way up, and doesn’t require tedious and messy pre-mixing of fuel & oil, whilst still achieving EU emissions standards. The fuel tank is a generous 1.35 litres, giving over 3 hours running in the magazine’s cruising speed test (4 knots).
Starting is spring-assisted, which sounds interesting, and (unfortunately) the motor has a centrifugal clutch. The actual engine is air-cooled, and the leg water-cooled, which probably explains the mid-range weight (16.5 kg) and relatively noisy running.
Yachting Monthly makes the Selva 2.5 best all-rounder in the group test, but of course their readers’ requirements are slightly different from ours. Good to see some innovation at this end of the range.20/02/2013 at 1:58 pm #11349AnonymousInactive
Whenever the sailing publications review small outboards, Selva tend to get a good write-up, and they often seem to have features outside the Japanese-dominated norm. What you describe appears to be a dry sump system. It was cutting-edge motor racing technology some decades back.03/03/2013 at 10:16 pm #11365
A dry sump has a scavage pump in the crank case lifting any oil to the seperate resevoir that the feeds the oil pump. This idea intended to stop the drag on a crank paddling oil and mainly to stop the pick up pipe being starved during extreme braking and cornering, i suppose it would save oil seeping through the rings and guides when the engine was led down but oil mixed with fuel seems perplexing! To make this feasable you would have to do away with the oil pump and oil and pass the fuel/oil mixture around the crankcase and into the cylinder, presumably via the vave train……
I think the tappets would struggle for lube and isn’t the point of a 4 stroke to be fuel efficient? These things would smoke like billy o and are probably noisy because the tappet box is digesting itself. I think maybe it’s a dry sump with a seperate oil tank and the fueling system as standard 4 stroke. (‘change’ oil once a year?)
This has got me thinking now, off to check up!04/03/2013 at 8:11 am #11366
Yes, they are a 2 stroke that fires less often. I would steer clear personally. See how they stand up.10/03/2013 at 3:57 pm #11368sailingdaveMember
So then, if they are a two stroke, fuel and oil mix, how can they be sold legally for recreational use in Europe??10/03/2013 at 11:47 pm #11369
The mixture is that of a 2 stroke but it fires every fourth stroke with the aid of a valve train. It will smoke the same as a two stroke but on paper it’s a four stroke. I think it may be called ”taking advantage of a loophole”.
The efficiency will be down to the fact that it will run lean and therefore hot. Modern engines have pressurised coolant so they can run leaner and hotter without boiling. Probaly why the Seva is air cooled. I guess you could save on the weight of a cooker in the boat and fry your bacon and eggs on the Selva’s head!
It’s a good idea to put the right amount of oil in a 2 stroke mix as too much oil will NOT have the advantage of extra lubrication but WILL make the tired old smoker run lean and hot.16/03/2013 at 3:53 pm #11372sailingdaveMember
Very interesting indeed. A loophole in the law does seem to be exploited then. Defeats the object really. I assume therefore that the law must have been written to ban ‘two stroke’ engines but should have been written to ban any engine that runs on a fuel/oil mix instead of a wet sump?
Think I’ll stick to my ‘normal’ engines.17/03/2013 at 9:33 am #11374
I may prove to be a winner but i’ve never been a gambling man. If you want a light and minimalist 2 stroke there are a few 2hp mariner/yamaha outboards still about on ebay. I have had one for 23 years and bought another for spares. I don’t know what they weigh but i can hold them both out at arms length when moving them about.
I am thinking of investing in a Mercury/mariner 3.4 four stroke with a long shaft this year, they are only 17kg’s with forward and neutral. I would use a smaller engine but i am using ‘Wotnot again’ as a rescue boat and means of return for a friend who is kayaking to the Isle of Man from Whitehaven this year and we may encounter some calms as he prefers light winds for the crossing.30/03/2013 at 11:25 am #11383Dave BarkerKeymaster
The latest issue of YM has now been published, including a letter from a reader querying the “4-stroke” description in last month’s magazine. The YM gear expert confirms that the Selva engine is a dry sump 4-stroke, passing EU emissions regs (!), oil being mixed with fuel, drawn through carburettor & into crankcase, lubricating the bearings. From here it’s drawn into the cylinder through a conventional inlet valve. (So, basically as inferred by other posters here).
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