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Halftime Two-Stroke SemiCrower engine

Mix it up, half the time
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I'm smitten with the concept of the Crower six-cycle engine, which has on average one power stroke out of three, compared to the Otto cycle's one out of four.

I'm also taken with the two-stroke, which delivers awesome power from a small package, delivering power on every other stroke.

What if the engine were a two-stroke, but only half the time? In four strokes (two turns of the crankshaft), we have: exhaust / scavenge + intake / supercharge, compression, ignition, and the cycle repeats. The downside of the two-stroke is its very short expansion stroke, and the engines can get pretty hot for their size, since their power output is so much higher than a similarly-sized four-stroke.

But now the cycle doesn't repeat like that. The gasoline cycle does its thing, from intake to exhaust, then waits for two strokes while a water injection / steam expansion cycle goes through two strokes. Then the gasoline two-stroke, then the steam two-stroke. The timing is managed by a camshaft that runs at half crank speed, as in a four-stroke. Unlike a conventional two-stroke, the halftime SemiCrower has its intake and exhaust lobes running with only a little separation between them - careful design of the valve head will be required to minimize unburned gases exiting, and EFI will help to reduce it further.

There are all the benefits attributed to the Crower: engine cooling takes place inside the engine, the expanding steam adds a power stroke, efficiency is increased. Doing it all in a two-stroke package reduces the weight necessary for a required power output, while scavenging and supercharging blowers - powered by whatever is necessary - work hard to keep the exhaust stream from being contaminated with fresh fuel.

The cylinder head needs six holes: spark plug, fuel injector, water injector, and the three valves: fresh air intake for the fuel cycle, fuel exhaust, and steam exhaust. The steam exhaust valve can be operated from the same camshaft as the fuel valves, or can be run on its own shaft.

On the way to being injected into the steam cycle, the water from the condenser runs through heat exchange tubing that cools both the steam exhaust and the fuel exhaust. Having paid for the heat energy, there's no reason not to recapture what might otherwise go out the tailpipe. Strong steel tubing simply wrapped around the exhaust systems would serve for a proof-of-concept prototype. This would permit the injection of water at the highest possible temperature, the better to flash into the driest possible steam upon introduction to the expansion chamber.

When the engine is cold, perhaps a pair of cam lobes can be brought into play to make the engine a full-time gas two stroke until operating temperature is reached, then the starter lobes are selected out (hydraulic lifters with their oil shut off?) and the steam lobe and water injection come on.

Like the Crower, there will probably be plenty of expansion left in the steam to power the scavenger and supercharging blowers. This thing would be a plumber's hell, but I bet it would work a treat.

elhigh, Jul 02 2007

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       I realize that blowby gases and other cross-contamination into the steam/water circulatory path is a certainty. What that circuit needs, then, is the equivalent of a PCV valve - or it may be that an actual one would work admirably - to send those gases to the induction system to be run through the engine.   

       An expansion tank will be a necessity, and it will have to be on the coolest part of the water circuit, to ensure that all the water can be fully condensed and that only combustible volatiles go through the PCV to the induction side.   

       I think someone else has already pointed out elsewhere how the volume of water in such a system could "grow" by absorbing water vapor from the combustion cycle. Fiddle with everything just right, and you could conceivably replace steam that is lost from incomplete scavenging during the steam exhaust stroke with water vapor that cross contaminates from the gasoline exhaust stroke.
elhigh, Jul 02 2007
  

       Looking at this idea again, might you want to make the cam lobes such that the engine is a 4-stroke when cold? Without the steam, the 2-stroke would probably be less efficient than a 4-stroke. Also, 2 gasoline cycles (when the engine is cold) would probably make more power than 1 and then 1 crower (when warm), therefore there might be a drop in power.
acurafan07, Sep 30 2007
  

       Already mentioned in the next-to-last paragraph. It's like a VTEC system, and the 4-stroke gas lobes drop out at operating temperature.
elhigh, Oct 02 2007
  
      
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