4 into 2 stroke, using NO cams and exhaust port?
4 into 2 stroke, using NO cams and exhaust port?
(OP)
First idea came here:
http://www.eng-tips.com/viewthread.cfm?qid=101322
Then, last night something occured to me. I was reading about pulsejets and it gave me a idea. In a pulsejet, the one way vanes in the front close during combustion, the exhaust gasses escape with high velocity ouit the back, and once pressure has dropped, because air is elastic, it creates a vacuum, pulling open the vanes, and letting fresh air in. The kinetic energy of the hot gasses going out the back is what pulls the air in.
Now, what if we added a exhaust port to the cylinders down near the bottom of the cylinder, then REMOVED the cams entirely, replacing the valve springs with very light springs, with say a 2-5psi opening pressure. Intake with be through all available valves (4 on the motor I hope to use). Also, a forced induction of some sort will be required. I'm thinking the best is a fixed displacement blower, with a belt ratio to give no boost, but just perfectly fill the cylinders without blowing anything extra out the exhaust ports.
Now, imagine this thing in operation. Start with a combustion event. Piston moves down, valves stay closed from the pressure in the cylinder forcing them shut. Piston continues down until it uncovers the exhaust port. At that instant, the pressure and exhaust in the cylinder blows out the port. As the cylinder finishes exhausting, it will suck the valves open at the top, both from a pulsejet like effect of the exhaust creating a vacuum, and the push of the blower. The valves should fly open, and dump a full load of fresh air into the cylinder very quickly because of LOTS of flow area, and top to bottom filling with a blower behind it. Now the cylinder is full. The piston by now is coming back up and covers the exhaust port. Air will now try to reverse out the intake, but can't, as the valves will immediately shut (they are like check valves). The air/fuel is trapped, and gets compressed, then ignited, now repeat the process.
I see several advanatges here over the same motor as a 4-stroke:
2-3x the mass airflow capability. 2x alone from 2x as many induction events, I'd also expect higher VE (from the huge flow area and top to bottom filling)
Slighty improved efficiency. No friction from the cams, no pumping losses from compressing heavy springs. Anyone know the aproximate % of losses from a 4-stroke that go to the valvetrain? Also, per combustion event, there is only half as much frictional losses (2 strokes instead of 4).
Multiply the mass airflow improvement times the efficiency improvement, and we could be talking 2.5-3.5x the horsepower from the same motor.
Why wouldn't it work? Shoot some holes in this, something I'm missing.
http://www.eng-tips.com/viewthread.cfm?qid=101322
Then, last night something occured to me. I was reading about pulsejets and it gave me a idea. In a pulsejet, the one way vanes in the front close during combustion, the exhaust gasses escape with high velocity ouit the back, and once pressure has dropped, because air is elastic, it creates a vacuum, pulling open the vanes, and letting fresh air in. The kinetic energy of the hot gasses going out the back is what pulls the air in.
Now, what if we added a exhaust port to the cylinders down near the bottom of the cylinder, then REMOVED the cams entirely, replacing the valve springs with very light springs, with say a 2-5psi opening pressure. Intake with be through all available valves (4 on the motor I hope to use). Also, a forced induction of some sort will be required. I'm thinking the best is a fixed displacement blower, with a belt ratio to give no boost, but just perfectly fill the cylinders without blowing anything extra out the exhaust ports.
Now, imagine this thing in operation. Start with a combustion event. Piston moves down, valves stay closed from the pressure in the cylinder forcing them shut. Piston continues down until it uncovers the exhaust port. At that instant, the pressure and exhaust in the cylinder blows out the port. As the cylinder finishes exhausting, it will suck the valves open at the top, both from a pulsejet like effect of the exhaust creating a vacuum, and the push of the blower. The valves should fly open, and dump a full load of fresh air into the cylinder very quickly because of LOTS of flow area, and top to bottom filling with a blower behind it. Now the cylinder is full. The piston by now is coming back up and covers the exhaust port. Air will now try to reverse out the intake, but can't, as the valves will immediately shut (they are like check valves). The air/fuel is trapped, and gets compressed, then ignited, now repeat the process.
I see several advanatges here over the same motor as a 4-stroke:
2-3x the mass airflow capability. 2x alone from 2x as many induction events, I'd also expect higher VE (from the huge flow area and top to bottom filling)
Slighty improved efficiency. No friction from the cams, no pumping losses from compressing heavy springs. Anyone know the aproximate % of losses from a 4-stroke that go to the valvetrain? Also, per combustion event, there is only half as much frictional losses (2 strokes instead of 4).
Multiply the mass airflow improvement times the efficiency improvement, and we could be talking 2.5-3.5x the horsepower from the same motor.
Why wouldn't it work? Shoot some holes in this, something I'm missing.





RE: 4 into 2 stroke, using NO cams and exhaust port?
Heat release would be greater, but that can be managed.
All together, I think a very mechanically friendly way to make power.
RE: 4 into 2 stroke, using NO cams and exhaust port?
This all seems too good to be true, but I still don't see a reason why it wouldn't work.
Oh yeah, I plan to use electronic fuel injection, with methanol as the fuel. Will inject toward end of induction cycle, to seperate fuel and exhaust gasses.
RE: 4 into 2 stroke, using NO cams and exhaust port?
The valve would have to be uncovered for a much longer time then just 10 degrees abdc. Also the way it works the engine would only run low rpms I believe unless you were pushing a massive amount of boost to force out the exhaust air. But even thin i think the mixxing of hot and cool gasses would ignite. So mayby an in cylinder mounted fuel injector would be nessasary. Having the boosted air push out the exhaust air in a shorter period of time and then firing an injection of gasoline down into the cylinder. Seperating exhaust from intake. It would be a 4 stroke without four strokes if you set it up correctly.
I think mounting a port midway down the cylinder of a correct size would produce the results desired. Most power is generated in the top of the stroke anyways. By mid stroke you have very little to gain in completing the stroke. I think the boost pressure needed might need to be on the upwards of 20-30 psi. Mounting blowers on both teh intake and the exhaust ports and pushing air in. This would cause an equal pressure gradient on both sides of the chamber more effectively removing all exhaust from the chamber.
The system im describing i believe would require a good amount of octane. I believe the prescribed horsepower figures are hopeful at best.
Sounds like some serious math and tuning would need to be worked out. Then analyzing where the blowby down into teh crankcase was managable while both sides of the piston rings were exposed.
-Travis-
RE: 4 into 2 stroke, using NO cams and exhaust port?
RE: 4 into 2 stroke, using NO cams and exhaust port?
With the cams gone, and valve springs replaced with light weight springs, the intake and exhaust valves will become plain poppet valves.
As long as the fuel isn't in the inital front of air pushing out the exhaust, I don't see igniting being a problem. I can easily set the injectors to fire at the perscribed time. Also note, with methanol as a fuel, the chances are greatly reduced compared to gasoline. Much lower EGT, and a higher auto-ignition temperature.
If I have a port about 1" tall, with a 3.38" stroke, and the bottom of the port is flush with the top edge of the piston ABDC, the port will be uncovered somewhere around 120 degrees (60 before BDC, 60 after).
RE: 4 into 2 stroke, using NO cams and exhaust port?
RE: 4 into 2 stroke, using NO cams and exhaust port?
With modern design and use of light weight materials like titanium, 3000rpm might just be possible with the smallest practical valve size.
Regards
pat pprimmer@acay.com.au
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RE: 4 into 2 stroke, using NO cams and exhaust port?
http://limtechnology.com/
al1
RE: 4 into 2 stroke, using NO cams and exhaust port?
RE: 4 into 2 stroke, using NO cams and exhaust port?
Pancholin
RE: 4 into 2 stroke, using NO cams and exhaust port?
RE: 4 into 2 stroke, using NO cams and exhaust port?
If you study modern (or ancient)two strokes,you will see that the piston skirt effectively seals the exhaust from the crankcase, there will not be any great pressures involved here, and a ring will not be necessary, this is a well proven method.(used daily in millions of two strokes).
I agree that conventional poppet valves would not make good automatic opening valves, they would soon get out of control at higher revs. As for lightweight reed valves, I don't think that they could handle combustion temperatures and pressures.
Other ideas that have been used in large diesels are blower fed inlet ports at the bottom of the swept area and camshaft controlled exhaust valves up top.
TTMOTORS
Modern two stroke design has all the port open timing honed to a fine art now and all the relevant data regarding this is easily accessable,( no need to re-invent it).
OBANION
Ever checked the cost of methanol? and you need to use twice the volume of methanol as you would for petrol (gas), but I'm not sure whether you are talking racing or just normal road use.
Anyway, it doesn't hurt anyone to try all these things sometimes, and you may discover something no one else has, ( a bit like panning for gold in an abandoned site!
There is (or was)a book called "Some unusual Engines" published maybe 20-25 years ago, and if you can find it, it is really worthwhile reading