Fast Fred's OMC Mod 50 secrets Thread #2

The reason to operate all four valves at once is to satisfy port-time-angle-area gas flow requirements. Using only two of four valves cuts port/exhaust area in half, greatly limiting engine power. Either way, the valve train apparatus is complex, heavy, difficult to cool (must cool valves, pistons, etc. in one revolution), and expensive for a two stroke. If there were a great advantage to valved two stroke technology Mercury, OMC/BRP, Yamaha, etc. would have it in production.

Side note: Another lighter, but equally complex and still expensive approach would be a sleeve valve like those used in the Bristol radial arcraft engines of WWII. These eliminated valve springs & poppet valves entirely. Again, if there were promise it would already be on the market.

Keep thinking.........

Agreed on the size, weight, complexity, and that cutting the valve area / cycle in half will make a difference but difficult to know without a cut and try. Hard to guess since the valves and ports in the head were developed for such a different set of dynamics inside the cylinder. Of course if two valves per cycle worked that suggests all four per cycle could work better with less lift and duration. Until they failed.

How about a four valve head from a modern V6 adapted to a two stroke triple? Full sleeves and make a four stroke. With a lot of boost:). Perhaps lube the bottom end like the Stihl 4-Mix saws.

This is lots easier this going out to shop and making stuff.

Barry

Quote:

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Originally Posted by Tim Kurcz

Hi Jeff,

Thanks for asking. Yes, there's been progress:

1) The idle problem has been solved.
2) The front end was removed and reeds inspected - all perfect.
3) Reed stops will be installed as precautionary measure/to improve life.
4) Inter-plenum velocity stacks will be installed to smooth mixture flow.
5) A top entry port will be added to the oil reservoir for turbo oil bypass return.
6) A top loop 1/2" dia oil vent hose will be installed to free up reservoir pressure changes.

All this should be done in time for a Labor Day test run. Adjustments will then be made in preparation for an afternoon's testing Friday Sep 9 at IOA Hillsdale. The plan is to run the boat next to Jimmy Robb and/or Rich Welch on the race course and video the event to be posted on YouTube for all to enjoy. Steady as she goes........

Tim

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All above mods were completed and the turbo was tested briefly Saturday which proved the following:

1) It idles nicely; driveability is excellent.
2) On rough water during hard accelleration, the stern spins to port.
3) Water was too rough and boat too squirrely to attempt high speeds.

The propshaft was dead flat and centerline was even with the bottom; too high. Today it was lowered 1/2" to improve rough water handling. All is ready for Hilldale, and the weather is forecast good. See you there!

Tim

A couple of years ago I gave a lot of thought to building a 3 cylinder axial flow turbo 2 stroke, the idea was to use a commercial 2 stroke block with replacable liners, the head was to have 2 desmodromic inlet valves with 2 plugs & a single overhead camshaft.
the breathing was to be turbo,single carb into the 3 crankcases, out into a common inlet gallery somewhere where it could lube the little ends & cool the pistons, then out & up to the head.
The exhaust would be low & very wide more than 180 degrees wide, giving a longer trapped stroke, exhaust ports either side of cylinder block feeding into a turbo that was offset way over on one side in the hope of damping 6 smaller pulses rather than 3 big ones.

The hopes were that the high scavenging efficiency coupled with the longer trapped power stroke & the higher cylinder pressure when the valves closed (dependant on the boost pressure) would raise the torque enough to cover the slightly lower max revs.
Because of the lower bore/stroke ratio required the piston speed would have to be balanced with the valve speed until they were both civilised.

The problems that I never answered were, where to put the crankcase outlet ports (in a block with 3 cylinders that almost touch each other) so they could do some usefull piston cooling, & how was I to lube the camshaft & everything at that end. I did consider putting the camshaft into the inlet gallery.

Fuel consumption was allways going to be poor when you have to clean & cool the piston with that much of the fresh charge.

I must be getting old when most of my waking thoughts are about 2 stroke motors.

Since we're all using our imaginations, how about a leaf blower supercharged two stroke? There was an article (Mechanics Illustrated?) in which two leaf blowers were used to supercharge a Corvette. Worked very well. All one needs to do is plumb the gas powered leaf blower's output to an airbox that pressurizes the carb(s), with an automatic fuel pressure increasing device.

Jeff

Thanks for that Jeff, but I was rather hoping one of the techies would have torn it apart techniically if there were major flaws in my logic.

Quote:

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Originally Posted by Seagull 170

A couple of years ago I gave a lot of thought to building a 3 cylinder axial flow turbo 2 stroke, the idea was to use a commercial 2 stroke block with replacable liners, the head was to have 2 desmodromic inlet valves with 2 plugs & a single overhead camshaft.
the breathing was to be turbo,single carb into the 3 crankcases, out into a common inlet gallery somewhere where it could lube the little ends & cool the pistons, then out & up to the head.
The exhaust would be low & very wide more than 180 degrees wide, giving a longer trapped stroke, exhaust ports either side of cylinder block feeding into a turbo that was offset way over on one side in the hope of damping 6 smaller pulses rather than 3 big ones.

The hopes were that the high scavenging efficiency coupled with the longer trapped power stroke & the higher cylinder pressure when the valves closed (dependant on the boost pressure) would raise the torque enough to cover the slightly lower max revs.
Because of the lower bore/stroke ratio required the piston speed would have to be balanced with the valve speed until they were both civilised.

The problems that I never answered were, where to put the crankcase outlet ports (in a block with 3 cylinders that almost touch each other) so they could do some usefull piston cooling, & how was I to lube the camshaft & everything at that end. I did consider putting the camshaft into the inlet gallery.

Fuel consumption was allways going to be poor when you have to clean & cool the piston with that much of the fresh charge.

I must be getting old when most of my waking thoughts are about 2 stroke motors.

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The "axial flow" arrangement you discuss is called "uni-flow" in engineering circles. The low row of ports is used in the Detroit Diesel two stroke and my "HKS" two stroke and are actually 360 degrees around the cylinder. Your concept is sound except four valves are needed for area, and can easily be verified by running Gordon Jennings port-time-angle-area equations. Desmodromic valve actuation (ala Ducati) would tolerate high RPM better than conventional spings, but parts (like titanium valves, etc) would be extremely expensive for a one-off. Can't speak to splitting up the exhaust except the plumbing and cooling would be a nightmare. Otherwise, your (global) 1-bbl carb with a plenum intake is the berries. Virgo uses this arrangement and it's been the easiest engine ever to tune.

For all you engine enthusiasts, search for SAE technical papers online. You'll find hundreds of technical papers written about 2 strokes, 4 strokes, rotary (Wankel), Sterling, Rankine cycle engines and more by the leading experts in the industry............

Keep thinking!

Tim

BTW: The twin cams used on my HKS Cosworth-Johnson 45SS twin were converted to roller bearings and lubed with a simple OMC diaphram fuel pump delivering 5W engine oil to all three main bearings. Spray from the bearings lubed the follower buckets and lobes.

Tim the axial-uni flow expression depends of who's work you read last.

Using someone elses cylinder block precluded the use of 360 degree ports & I wanted to get any heat away from the lower portions of the liner as fast as possible, without dropping it's velocity & pressure & 90 to 100 degree wide ports were a lot quicker for me to machine in situ.

With regard to the inlet side of things & the 2 - 4 valve question I felt that Jennings & everyone elses port time areas were made with the assumption that the transfer pressure would drop during the port open time to 1 atmosphere, without exhaust assistance, at a constant boost pressure the flow would greater, how much greater I never calculated as I didn't want to spend a fortune on Blair's 4 stroke book.
Using only 2 valves also gave some uncooled head area to transfer heat into the camshaft gallery where I was hoping to get more fuel vapourisation leaving some lube on the cam gear.
I was also hoping that 2 valves might give less mixing of the fresh charge as the valves could be opened when the boost pressure was slightly greater than the cylinder pressure which would fall much faster with such a large port time area.
Variable valve timing could also be used with a fancy cam chain tensioner.

As Fastjeff said this was just a dream as it would never start without 9 sets of reed valves, even then it would need high speed starting! Which would be more work than the cylinder head.

The valves I planned to obtain from one of the Japanese motor cycle manufacturers as I didn't want to re invent the wheel

Quote:

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Originally Posted by Seagull 170

Tim the axial-uni flow expression depends of who's work you read last.

Using someone elses cylinder block precluded the use of 360 degree ports & I wanted to get any heat away from the lower portions of the liner as fast as possible, without dropping it's velocity & pressure & 90 to 100 degree wide ports were a lot quicker for me to machine in situ.

With regard to the inlet side of things & the 2 - 4 valve question I felt that Jennings & everyone elses port time areas were made with the assumption that the transfer pressure would drop during the port open time to 1 atmosphere, without exhaust assistance, at a constant boost pressure the flow would greater, how much greater I never calculated as I didn't want to spend a fortune on Blair's 4 stroke book.
Using only 2 valves also gave some uncooled head area to transfer heat into the camshaft gallery where I was hoping to get more fuel vapourisation leaving some lube on the cam gear.
I was also hoping that 2 valves might give less mixing of the fresh charge as the valves could be opened when the boost pressure was slightly greater than the cylinder pressure which would fall much faster with such a large port time area.
Variable valve timing could also be used with a fancy cam chain tensioner.

As Fastjeff said this was just a dream as it would never start without 9 sets of reed valves, even then it would need high speed starting! Which would be more work than the cylinder head.

The valves I planned to obtain from one of the Japanese motor cycle manufacturers as I didn't want to re invent the wheel

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The free thinking environment is a great, but we're really getting away from "secrets of the Mod-50". Perhaps it makes sense to start a thread for developmental concepts and engines.

Sam - can you help with this? Thanks!

Tim

What the MOD 50/M31 needs is to have E Tec injectors