The engine looks good for racing, but I see some problems that might limit it's durability. 1. The sliding block in the upper cylinder. A short connecting rod might wear longer. 2. The reed valves. They are potentially exposed to exhaust gases. In a 2 cycle, they are buffered by the crankcase. This engine might have a longer life as a supercharged diesel engine. You could do away with all of the valves and just blow the fresh air into the chamber. Just a thought.
You *can* supercharge petrol engines too. That wouldn't address the durability concerns completely, though. That is the main caveat of normal two strokes. Sarich manages to address a lot of the disadvantages of two-strokes, and gets better performance than four strokes in nearly every category. Just the engines don't last more than about 40-50K Miles. <br>
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I think wear in the slider is the killer, whatever you do, unless you get some materials, which are resistant to friction in a *very* aggressive atmosphere. It is the wear resitance of the air-flow control (valves in particular) which give 4-stroke their main advantage.
Mike Anderson & Shortee,<br>
Thankyou for your thoughts,<br>
I am not sure that you completely understand that this design is not like a two stroke. The underside of the upper piston is just the same as a normal 4stroke sump.There is no fuel mixture there ,Just an oil feed and extraction system. The upper upside down piston has a compression ring or rings ( L shape or Dykes rings) and oil scraping rings witch do not enter the port area but stay above the ports . only the compression rings traverse the ports . The compression rings do arive at a position that the scraper rings traveled to , thus picking up the lubrication.<br>
The stroke of the upper piston is about half that of the main piston and the upper crank revolves at half main crank speed . So the piston speed of the upper piston is about a quarter that of the main piston. This should give a longer wear life than the main piston.<br>
During my first race the back cylinder suffered an oil blockage to the head. The engine did not sieze but it did accellerate the wear markedly. To my surprise it did not weare where I thought it should. The ware between the flat bronze follower or slide plate attached to the piston, and the case hardend steel square block on the crankshaft throw pin was unmeasurable . But it imparted a tipping motion to the block and extensivly wore the aluminium upper slide plate . a part that normally has no load at all.<br>
I think supercharging this engine in petrol form would result in rather spectacular sheets of flame eminating from the exhaust because of the overlap period. this could be stoped if a rotary disk was also used on the intake.<br>
I have had some trouble with plastic reeds but non whatsoever with stainless steel.<br>
The reed valves are protected from exhaust pressures because of the difference of 30 degrees in their openings.<br>
I have tried a con rod . It did work ok but it imparts a less desirable motion to the upper piston and unless it is placed within the piston facing away from the piston crown , the head size is greatly increased. In this position it is always under stretch tension and not compression.<br>
Thankyou for your feedback it is very good to get critical comments <br>
Malbeare<br>
Anyone familiar with FI pneumatic poppet valve springs? I'm looking for a predictive model that includes nonlinear effects. Also, how is the pressure maintained?
Pressure is maintained in pneumatic valves by having a compressor charge up an accumulator. To stop the valves dropping on to the pistons through leakage in storage/transport, the system is connected to a pressurized flask.<br>
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In modelling the system for racing, I suspect (but do not know for certain), that the effects of temperature variation and the inherent compressibility of air are ignored by just 'over pressurizing' the system to respond as required to negate these factors.
is there any info on electronic (solenoid) controlled valving for 4-strokes? What kind of responce speed would be needed and how strong would they have to be to open on the exhaust stroke, and how much power do they need to operate. I read a small article on some development, but no specifics. What is saved by tossing the cam, springs, etc seems like it would be transferred into the electrical generation system to power the valve. Anyone... Bueller?
A solenoid based electronic valve system is unlikely in that the force required (~800 N @ 5000 rpm) is impractical for solenoids at that speed. Solenoids are inductve and as they get bigger (stronger) their time constant also gets slower meaning that high speed and high force are mutually exclusive. Another problem with solenoids is their nonlinearity makes them difficult to manage in control systems.<br>
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There are a number of well done SAE papers on the subject including a compilation "Variable Valve Actuation and Control" SAE #SP-956.<br>
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Storm - thanks for your thoughts.
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The intake valve timing of the sixstroke head is engine demand variable. At low throttle settings the reed valves open late and are so close to the ports that their tips are the virtual port opening area.They open small amounts at low throttle settings and are fully open at full throttle. They open early as well at full throttle depending on how much the extraction efect the exhaust system gives.<br>
The ports are fully exposed at top dead center ( valves fully open) The poppet valve system cannot match this because the valves would hit the piston.<br>
Malbeare
Malbeare,<br>
I'm glad you got some race time on it... However, I don't think slower piston velocity reduces wear... In fact, my experience has been quite the opposite. I have dismantled a couple engines that have been lugged (never revved), and saw some amazing hour-glass shaped wear on the cylinder. This is my best-guess as to why that happened: In a wet lubricating system, there is some minimum velocity between the parts that keeps them from physically touching at a particular force. If you increase the force, you must increase the velocity to keep the parts "flying". There is also some maximum velocity that prevents the lubricant from vaporizing. This is one of the reasons a high torque, low RPM engines need large journals and connecting rod bearings to keep the surface velocity high. Anyway, back to the hour glass thing... The piston moves slower at the ends of its travel, so the rings didn't have sufficient velocity to "fly". This greatly accellerated the wear at these points. In a piston, the rings exert a consistent outward pressure, so the only variable is the velocity between the parts. Your upper cylinder will probably wear quicker than a conventional 2-cycle during idle or low RPM. Just my opinion.
shortee,<br>
I think that the wear factor is unknown.It is not the conventional conrod arangement, but a scotch yoke setup. so the sidethrust is different.I guess if we follow the best practice of nicosil bores and ductile cast iron rings there should not be too much trouble.I havent done enough long term testing to know if your conserns are valid or not. thanks for your input.<br>
Malbeare
