4 Stroke OB design?

One of the constant criticisms of 4 stroke OB's is their bulk, size and weight on a transom compared to 2 cycle OB's.

We see engineering working away at reducing weight all the time with "new" models each year claiming significant weight reductions - yet they are still quite a deal heavier than their 2 cycles counterparts.

Aso there are concerns with the servicing costs occasionally required to adjust valve train clearances, and also the status of rubber timing belts off the flywheel in a marine environment have many folks yet to be fully convinced of the long term reliability of the new 4 stroke OB technology in marine environments.

Today I had occasion to "glance" thru an aeromodelling magazine - and was captivated, bye the unique design of one particular 4 cycle RC Aeroplane model engine.

Obviously weight considerations of model aircraft engines are critical to flight moreso than even boatings OB weight is critical to bouyancy.

This particular model engine drew on VERY old technology used in both Daimler limousine engines and some "radial" aircraft engine designs of the 1930's leading up to ww2.

What caught my eye was the total lack of overhead poppet type valvetrain & camshaft assembly, and no timing belts.

Induction of fuel and exhaust of combustion gases, was achieved thru use of a wet sleeve cylinder which "rotates" thru 360 degrees constantly, with milled ports in the sleeve "mating" with induction and exhaust port holes in the block at critical points of the 4 stroke cycle.

The rotation of the sleeve was geared off the cranckshaft.

This really was a VERY SIMPLE yet efficient design, and for the life of me I couldn't see why it hasn't been adapted for use in marine 4 cycle outboards.

I could envisage several "benefits" like reduced weight of the engine, since there would be no need of all the overhead valve train, reduced manufacturing costs, as the head would simply be a cap over the cyclinder with a spark plug hole in it.

When time came to rebuild a powerhead - a new sleeve inserted would mean standard sized new pistons & rings could be used and so on.

No expensive maintenance of valve train assemblys, or liability of failure of rubber timing belts etc.

It's a no brainer.

Anyone know why this technology isn't being used in marine 4 cycle OB engines yet?

It's not like it's totally new technology - it has been around for quite a while.

Just curious - a MEG it will never be but it still seemed to have a lot of merit for marine use where reliability and light weight are crucial.

Cheers!

[Stephen Ditmore]

You might want to take a look at this recent Harry Schoell project:
http://www.rocketekmarine.com/

I expect this grew out of the 2-stroke direct injection engine Harry was working on when I worked for him almost a decade ago. At that time he was using a rotary configuration (like old piston aircraft) with the cranckcase doubling as a compressed air intake manifold (I think the flywheel doubled as a turbine-style fan, making it effectively a supercharger). I don't recall what the valving was like. It may have been rotary, but more likely the air passed through ports in the pistons like a conventional 2-cycle. I don't know how the engine differed from the direct injection outboards of today, but I recall the impediment at the time was designing a fuel injector that would mist/atomize the fuel properly as it was injected into the compressed air. Since then Interstate Diesel has come out with injectors that purport to do this effectively, and direct injection 2-stroke outboards have become commonplace.

As for a direct answer concerning rotary valves, I'm afraid I don't have one. It's a very good question. If Mike Hodgeson still works for Harry he might know. Maybe it wouldn't be as simple as first imagined with proper bearings and seals.

[yipster]

I’ve got some (model) airplanes and a chopper. Those 2 stroke and diesel engines aren’t exactly “clean” or build on long-term reliability. 4 cycle RC model engines I haven’t seen yet, but I’m not flying everyday either. AMC/Johnson made some 15.000 rotary OB’s I learned but cant find details on. Rotary engine techniques have come a long way. Still thoughts on them vary, most keep mentioning the sealing problems, like probably will be in the rotating sleeve. Than there is the industry, not exactly looking for new R&D designs I think. Only beeing a hobby mechanic I can see the many aperant advantages though.

[eshkabible]

hi, i am working on a 22 ft. wakeboarding boat, the hull is taken from another design, and i am basically changing the layout and adding a wakeboeading bridg. the original planes allow a maximum 145hp engine, and the choice of either outboard or inboard, my question is which one is better from a wakeboarder pint of view, while the biggest isse is the stoke and the wake size.