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Thread: Announcing the Merc 888

  1. #11
    Team Member daveswaves's Avatar
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    Looks like fun Tim, at one point I laid two inline 6 blocks together like that and considered the challenges. Fun to think about. Agreed, starting will be a challenge, gonna have to spin it up pretty good to get going. Maybe very retarded timing till it kicks. Be nice to roll up a high rpm starter to the prop end and spin from there.

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    Default You're all thinking

    Quote Originally Posted by Fastjeff57 View Post
    It works fine with EMD two strokes, You'll have to be careful backfires, however; they would be very violent.

    Jeff
    Of course, consideration was given to the potential for backfires: Consider that two exhaust ports are open at any given time. Having more cross sectional area then the carb venturi, the path of least resistance is likely through the turbo. With the butterfly is open, the beast would likely belch flame from both ends, regardless! If the butterfly is closed, a crankcase explosion would be split between the turbo and intake. In that case it might blow off the silicone elbow connecting the compressor to the diffuser. As operational behavior cannot be determined until actual testing, I decided the complexity and restriction of reed cages in the prototype could wait.

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    Hay Dave,

    It's great to hear you considered the 12 cylinder opposed. If the 888 works, go for it!

    It is expected the starter should deliver 350 RPM, which should be plenty fast. If not, the circuit will see 24V for some added punch. It's a fun mental and mechanical challenge, all with mostly stock Merc parts (well at least some stock parts).....

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    Team Member R Austin's Avatar
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    Quote Originally Posted by Tim Kurcz View Post
    A SeaDoo blower was considered, but not large enough to provide for 88 cubic inches. Also, there was no reasonable way to install a gear drive to deliver the needed 1:7.25 step up.

    For the purposes of the proof-of-concept prototype, custom .250" thick rods will be made of 954 aluminum bronze or bronze plated 4140 steel running directly on the crank (with 8:1 fuel/oil ratio.

    For starting, two potential methods are envisioned:

    1) Lucky: Crank the engine with the electric starter, inject prime through the starboard side (aft bank) port covers, and hope the exhaust gust is enough to spin-up the (undersized) turbo and provide scavenge flow.

    2) Not so lucky: Crank the engine with the electric starter, apply airflow through the carb with a leaf blower, and prime the aft bank. This will most certainly provide enough flow to spin-up the turbo and provide scavenge flow.
    Tim

    I am afraid that the #1 Lucky option will not work.. No matter how hard you spin with the starter you are not pumping fresh air into the cylinders for combustion. The air is just still moving back and forth between pistons.

    As far as a common roller or any bearing shared by both rods, unless a floating bearing, will be a problem because the roller will have to skid in one rod. Even though the rods are moving in the same direction their angular relationship to the crank pin is opposite of each other.

    Dick

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    Default Pop goes the weasel

    Quote Originally Posted by R Austin View Post
    Tim

    I am afraid that the #1 Lucky option will not work.. No matter how hard you spin with the starter you are not pumping fresh air into the cylinders for combustion. The air is just still moving back and forth between pistons.

    As far as a common roller or any bearing shared by both rods, unless a floating bearing, will be a problem because the roller will have to skid in one rod. Even though the rods are moving in the same direction their angular relationship to the crank pin is opposite of each other.

    Dick
    The chances of the "lucky" solution are slim, but possible. Consider there will be exactly four "pops" from on bank responding to the prime shot. You're correct there is no air motion to recharge the cylinder. However, the turbo was intentionally undersized to allow quicker response to exhaust flow. Won't know until tested.

    You might have missed this, but there will be no rollers for the rods for several reasons, one of which is the obvious conflict in motion. Bronze on steel with oil will have to do; long enough to prove the concept.

  6. #16
    Team Member R Austin's Avatar
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    I did catch the proto rods, I was just noting for those that thought that a shared roller would work and did not understand the conflict of rod motion.

    An interesting concept. There has been numerous time that I have stood 2 blocks face to face and wondered what if. In the mid 80's I was invited to the Merc research building in Oshkosh for a meeting and while there I was asked if I wanted to see a 12 cylinder engine. I though, finally, someone had figured out a way. Only to learn it was 2 1000 or 1250's side by side with a common gear box.

    Good luck with the project. I will have a dyno done shortly, should I put a four cylinder bolt pattern on the mounting plate?

    Dick

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    Team Member daveswaves's Avatar
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    Quote Originally Posted by R Austin View Post
    Tim

    I am afraid that the #1 Lucky option will not work.. No matter how hard you spin with the starter you are not pumping fresh air into the cylinders for combustion. The air is just still moving back and forth between pistons.

    As far as a common roller or any bearing shared by both rods, unless a floating bearing, will be a problem because the roller will have to skid in one rod. Even though the rods are moving in the same direction their angular relationship to the crank pin is opposite of each other.

    Dick
    Dick, I agree that the air in the crankcase will just be shuffling back and forth, however, there will be an intake suction and an exhaust pulse from the non firing piston, not as large as a cylinder that has fired naturally, however , there will be air (and hence the opportunity for fuel) being "pumped" across the top of the piston. If Tim can get it spinning fast enough and light a cyl up the rest should follow. The "air" in question might shuttle in and out of the combustion chamber as well, creating an ever richening mixture until it fires. I will be really lean at the get go unless there is fuel squirted in there to start. The source of the air will be the exhaust side until it fires. Yup lots of opportunity to backfire till it gets going.

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    Team Member R Austin's Avatar
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    I totally agree that the engine will run once started.

    Back in the day, there were many stationary constant RPM power units that ran on just that principle. There were no turbos, blowers or valves of any kind. They were spun and fueled thru intake ports until the depression of the rapid expansion of the escaping gaseous thru the exhaust ports created a depression that allowed the atmospheric pressure to place a new charge in the cylinder. Although very RPM sensitive with just atmospheric pressure.

    Again it goes back to staring and keeping it firing long enough to build the exhaust pressure. Back fire is a real problem because on a dead cylinder there is no expansion to create the low pressure on the intake side. The exhaust port opens first and the low pressure point is at the face of the exhaust ports pulling in air from the exhaust side minus any fresh fuel, just the exhaust of an adjacent cylinder.

    Like staring a turbine engine, spinning to the right pressures to prevent back fire and over temp.

    Again, go for it. Sounds like a fun project to me.

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    Default With a little luck

    It's interesting that the Merc Boxer concept tickled the fancy of others before me. The challenges are many, but with a little luck, the 888 might actually run. If all goes well, it will be fired in February or March. Meanwhile, a few more images for your enjoyment.

    The spacer behind the turbo allows for the rear water bypass, and the diffuser covers are taped in place. Both items will be completed this weekend. Stay tuned and keep those ideas coming!
    Tim

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  10. #20
    Team Member daveswaves's Avatar
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    Nice little turbo Tim, you might have to split that intake manifold.

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