4 into 2 stroke, using NO cams and exhaust port?

[obanion]

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.

 

[obanion]

One additional postive I forgot to mention. You would get this additional power without increasing the "breaking" point over regular 4-stroke operation. You get your power improvements from having 2x as many combustion events, and less total losses. The maximum cylinder pressure remains near the same. So the pistons, rods, bearings, won't be getting hit any harder, the way they would if we added power by nitrous or turbo. The "pulses" of torque the crank sees won't be greater, just packed together 2x as much.

Heat release would be greater, but that can be managed.

All together, I think a very mechanically friendly way to make power.

 

[obanion]

FYI, I think a Eaton M112 supercharger would be a PERFECT match to the 3L motor I'd do this with. At 1.84L/rev on the supercharger, and a 12000RPM redline, I could set the pulley ratio such that I get 3L/engine rev (thereby filling all the cylinders 100% every cycle), and be able to go to 7360RPM on the motor, where it will hit 780CFM of air. That should be big time horsepower. On a 4-stroke, 780CFM would be over 700HP. On this motor, with added efficiency (hope), could I pray for 800HP?

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.

 

[TTmotors]

Not being a 2 stroke expert or anything but the bottem mounted exhaust valve would cause some signifigant problems in ring wear. Also the flow of air is not instantanious.

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-

 

[SMOKEY44211]

The only drawback I can see is finding a reed valve that would be robust enough to withstand combustion chamber pressure and at the same time be able to respond relatively small pressure differential during the intake event. After you've overcome those hurdles you may find the intake very restrictive. With all its faults the poppet valve would accomplish this task. How about 4 poppet style intake valves, (wouldn't require much lift) direct chamber injection, exhaust through bottom of the cylinder?----Phil

 

[obanion]

SMOKEY44211, I think you need to read my original post again. I was initially suggesting essentially what you said. This is a 4 valve per cylinder 4 stroke motor to begin with, and all of the valves would be used for intake, with a near bottom 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).

 

[SMOKEY44211]

I think the effects of inertia of the valve mass would severely limit the usable rpm. I've seen some antique engines that used this method you are suggesting on the intake valve. (light spring pressure no cam) They were used to power a wood cutting saw. No need for large rpm fluctuations for that application. My thinking about reed valves came from your refference to pulse jet engines. I think the reed valve may have some possibility of success of going camless but I doubt if the rpm capability would equal or exceed the range of a cam opened intake.---------Phil

 

[patprimmer]

The so called atmospheric valve used in antique autos seem to have a top end limit of about 2000rpm.

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
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[al1]

The below web site depicts what I believe this thread subject is all about, including pressure differently valves. I don’t believe that this is new stuff as the Detroit diesels have been doing this for a long time and the pressure differently valves have been tried many times as well.

http://limtechnology.com/

al1

 

[obanion]

Thanks guys, I'll look into that limitation. That was my primary unknown, the limitation of the valves.

 

[Pancholin]

Guys I think ya'll should check out the famous 6-71 series GMC 2 stroke diesels. You have done a pretty good job describing them. except the intake cam is still there. The valves are too heavy to hope the pressure change alone would close them and there wouldn't be combustion if the piston closed them. BTW; Ringed two strokes don't have problems with ring wear but they must be pinned to prevent rotation.

Pancholin

 

[icguy]

Interesting idea. How do you plan to seal the exhaust from the crankcase, and vice-versa? An added bottom ring would seem an easy solution, if there is room. This may open a whole new can of worms though...

 

[dunno]

ICGUY,
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