Designing 2-Stroke & 4-Stroke Engines: Possible to Have 6 or 8-Stroke?

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Discussion Overview

The discussion centers around the feasibility and design considerations of 6-stroke and 8-stroke engines, contrasting them with the more common 2-stroke and 4-stroke engines. Participants explore theoretical possibilities, practical challenges, and historical examples related to engine design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question the purpose and mechanics of additional strokes in engines, asking what the extra strokes would accomplish.
  • One participant argues that higher stroke engines may waste energy due to increased friction from more rotations before achieving a power stroke.
  • Another participant mentions that engines with more than four strokes, such as hit/miss engines, have been historically designed and used, although they operate on a modified 4-stroke principle.
  • A participant proposes a theoretical 6-cycle engine design that would eliminate the blowdown phase, suggesting it could improve fuel efficiency but expressing skepticism about its feasibility.
  • There are suggestions that electronically controlled valves could enable a second ignition sequence in a modified engine design, though concerns about mechanical balance and efficiency are raised.
  • One participant shares a conceptual chart for a 6-stroke engine, indicating a playful exploration of the idea rather than a serious proposal.

Areas of Agreement / Disagreement

Participants express a mix of curiosity and skepticism regarding the practicality of 6-stroke and 8-stroke engines. There is no consensus on the feasibility or design specifics, and multiple competing views remain regarding the advantages and challenges of such engines.

Contextual Notes

Discussions include references to historical engine designs and theoretical concepts that may not have been fully explored or resolved, leaving open questions about their practical implementation and efficiency.

aniket
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Hey friends,
My question is that why only 2-stroke & 4-stroke engines have been
made?Why not 6-stroke or 8-stroke?Is it possible to have such engine? Any
useful suggestions are welcome.
 
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well, the question is, do you have an idea for an 8 stroke engine. What would the other 4 strokes be for?

Regards,

Nenad
 
What would the two other steps be in a 6 stroke? What would differentiate a 4 stroke from an 8 stroke?
 
The main reason higher stroke engines are not used is because energy is consumed by the motor in the form of friction when it rotates. If you wait for the engine to rotate 6 or 8 times before you get a power stroke, you waste energy through frictional losses.
 
More than 4 stroke engines have already been designed and used. Apparently no one here has heard of a hit/miss engine.
 
Averagesupernova said:
More than 4 stroke engines have already been designed and used. Apparently no one here has heard of a hit/miss engine.
An antique engine club exhibits at our county fair each summer, in fact about two weeks from now. They have quite a few of the hit/miss engines running. I’ve only seen them operate as stationary engines hooked up to old time farm equipment, were talking about circa 1900 as far as age. All have enormous flywheels that turn about 100-300rpm and a centrifugal governor controls the spark. It appears the flywheel turns about 20 revolutions before spark is allowed and than two or three exhaust noises are heard followed by coasting until the speed drops sufficiently to repeat the process. All were single cylinder and had a cast iron bowl atop the cylinder head holding water, which evaporated and cooled the engine. Fun stuff! :biggrin: I watch for hours until my wife drags me away.


...
 
A hit and miss engine is a conventional 4 stroke with a governor that prevents it from getting a power stroke when the RPM's go up too high, so its really not a 6 or 8 stroke in the sense it isn't designed to operate on that many strokes, it is designed to operate on 4 strokes. There's a good article that describes the operation of the governor here.

They are pretty cool though, and to think those massive engines were all that was available back then. Not exactly a chain saw motor.
 
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Not only does the governor control spark it holds the exhaust valve open as the link above shows. The intake valve is just a check valve. There really is no carburetor in the conventional sense and it is not uncommon to see them with an open crankshaft and connecting rod. When the exhaust valve is held open no fuel air charge is drawn into the cylinder. When the speed drops to a preset level the exhaust valve is allowed to close and the engine goes through the 4 conventional strokes that we all know. After the RPM comes up the exhaust valve stays open until the RPM drops to the preset level and it starts over again. I have one of these engines and when they are loaded heavy enough they will hit every time running as a conventional 4 stroke.


One other thing I will add is that a lot of them did not actually have spark plugs. They had a set of points in the combustion chamber. The magneto built up enough current going through a coil in series with the internal points and when it was time to fire internal points opened up and there was enough of an arc for ignition.

Maytag also had hit miss engines for washing machines but I believe they operated more as a 2 stroke. I think they wasted the fuel while the exhaust valve was held open. IIRC they used a conventional spark plug and had kick starters on them.
 
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Actually, I can see an advantage in a "6-cycle engine", if someone could figure out how to do it (I don't dee how). Remember, the heated gasses after combustion, need more "expansion' volume than did the original intake that was compressed. The normal Otto Cycle has:
Intake
Compression
(Ignition)
Combustion/Power/Expansion
Blowdown
Exhaust
In the P-V diagram, the vertical line after the "expansion" stroke, represents unburned gasses being blown out of the system (sometimes called "blowdown"). If, somehow someone could figure out how to come up with something like:
Intake
Compression
(Ignition)
Combustion/Power/Expansion - first half
First Exhaust
Combustion/Power/Expansion - Second half
Blowdown (remainder)
Second exhaust
This would eliminate the "Blowdown" phase, but I don't see a way it can be done. If it could it would increase fuel efficiency at the expense of power - much as is done in the Atkinson Cycle.


KM
 
  • #10
Kenneth Mann said:
If, somehow someone could figure out how to come up with something like:
Intake
Compression
(Ignition)
Combustion/Power/Expansion - first half
First Exhaust
Combustion/Power/Expansion - Second half
Blowdown (remainder)
Second exhaust
This would eliminate the "Blowdown" phase, but I don't see a way it can be done.
It should be possible with electronically controlled valves and spark. A gas analyzer just outside the exhaust port could trigger the valve to close as soon as raw fuel is detected, thus trapping it and initiating a second ignition sequence. Of course, that would only work if there's also oxygen left over. I can't help thinking that it would make for one incredibly unbalanced motor, since it would have be done on a per-cylinder basis. It might be fun to tinker with some sketches.

edit: In that system, the second burn would be under almost no compression, so I don't know if enough power could be extracted from it to overcome the mechanical losses of missing a fresh intake charge and burn. :confused:
 
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  • #11
Rats! I got back to late to do another edit. Oh well, consider this a PS instead. Some thoughts occurred to me in the bar tonight, but I'm too tired to post them right now. I'll do up a chart in Illustrator and post it tomorrow.
 
  • #12
Okay, guys... here's what I came up with. The stroke labelled '5' is simply to ensure that there's enough air for the unburned fuel to mix with. '5 1/3' is the actual second power stroke. I think that it's kind of silly, but I had fun playing with it. The thing would still probably have a power curve like a bipolar weasel on a treadmill.

http://img374.imageshack.us/img374/8746/6strokechart4cj.jpg
 
Last edited by a moderator:
  • #13
So the hit miss engine is just a four stroke engine with a rev limiter on it?
 

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