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Variable Stroke Engine

Revetec meets VTEC
  (+8, -4)
(+8, -4)
  [vote for,
against]

Revetec is currently developing an engine design that uses cams instead of a crankshaft to control the linear movement of the pistons. This is rather difficult to explain in words, so refer to the linked website for nice animated pictures.

Honda and other car manufacturers have been making cars with variable valve timing for years now--the premise is simple, a camshaft with two or more sets of cam profiles which can be switched between to provide varying qualities of performance. This technique could also be applied to the "crankshaft" of the Revetec engine, allowing multiple profiles of piston movement.

For instance, there could be a short-stroke and long-stroke set of cams. The long-stroke set, with deep valleys and lower peaks on the cam, is used at low RPMs to produce more torque. The short-stroke set, more circular in shape, is used at high RPMs to allow a higher redline and reduce vibration.

This would be mechanically difficult, given Revetec's use of counter-rotating cams, but Revetec's engine could be made with a simpler single set of cams, at the small cost of re-introducing inertial twisting under changes of engine speed (this is present in all conventional engines, and the point of the conter-rotating cams was to eliminate it--but for high-performance, we can live with a bit of engine twisting). This would vastly simplify the problem of variable stroking, to the point where it would basically just be a re-application of existing variable valve technology.

5th Earth, Jul 19 2006

Revetec http://www.revetec.com/
The Revetec "Controlled Comustion Engine" [5th Earth, Jul 19 2006]

Saab VCE http://www.saabnet....n/press/000318.html
A better way to skin the cat. [jhomrighaus, Jul 24 2006]

Stroking an engine http://en.wikipedia.../wiki/Engine_tuning
specifically maintains compression ratio [5th Earth, Jul 24 2006]

RandCam from RegTec http://www.regtech....otaryprinciple.html
better than pistons, but Revetec has a development contract w/ an actual manufacturer [5th Earth, Jul 24 2006]

Revetec 2.4 X4 http://i84.photobuc...tec/X4manifold1.jpg
Revetec 2.4 X4 Aircraft Engine Prototype [revetec, Feb 13 2007]

Integrated Flywheel http://www.iskra-ae...heel_generators.php
[jhomrighaus, Feb 15 2007]

Revetec X4 engine first day running http://s84.photobuc...X4_1st_day1_001.flv
Running on 50% of the fuel than a conventional engine [revetec, Mar 28 2007]

Revetec Picasa Album http://picasaweb.go...vetec/RevetecImages
Images of Revetec Prototype Engines [revetec, Jun 03 2007]

Atkinson cycle http://en.wikipedia...wiki/Atkinson_cycle
"By adjusting the linkage to allow a power stroke longer than the compression stroke, the engine can achieve greater efficiency than with the Otto cycle engine." [BJS, Jun 04 2007]

Thermodynamic cycles http://en.wikipedia...Thermodynamic_cycle
[BJS, Jun 04 2007]

[link]






       I like it. [+]
david_scothern, Jul 22 2006
  

       I think it is a good idea to incorporate a transmission into the engine, which is basically what I see this idea as, but I don't necessarily like this exact version of the idea.
BJS, Jul 22 2006
  

       [BJS], error noted and corrected. However, this is not an idea to incorporate the transmission into the engine. This idea would only effect the power output of the engine, not how that power is transmitted to the wheels--a normal external transmission would still be necessary.   

       [BrauBeaton], Revetec's engine has two cams per pair of cylinders, which rotate in opposite directions. This is so when the engine changes speed, there will be no torque applied to the engine block from the reaction of accelerating the cams. By removing the counterrotating cam, and using a single cam spinning in a single direction for each pair of cylinders, the engine is mechanically simpler and it is easier to make a multi-profile cam that can be slid back and forth as per my idea. This has the disadvantage of re-introducing the problem of changes in engine speed twisting the engine block, but this is a problem that all engines with conventional crankshafts already suffer. Therefore, it is not a significant impediment to construction of the engine.
5th Earth, Jul 22 2006
  

       I was sort of wrong about the transmission part I mentioned earlier.   

       This idea does not change the stroke of the engine like the name implies, and it does not change the power of the engine like I believe you think it does, it only changes the torque and speed ratio of the output.   

       Would the engine have to be stopped to move the cams?   

       I think this idea is pretty good as far as piston engines are concerned, but I don’t really like piston engines that much.
BJS, Jul 23 2006
  

       Not sure why you want to vary the stroke --- am guessing it is to change the compression.   

       If so it is easier to move the cylinder head and this idea is baked...
madness, Jul 24 2006
  

       Saab Has a variable compression engine that does the same thing and is much simpler. See Link
jhomrighaus, Jul 24 2006
  

       I think I didn't understand this idea correctly before, so my previous annotations sound kind of weird, but I think I understand the idea correctly now, and I don't like it.
BJS, Jul 24 2006
  

       [BJS] fixed for real this time. Thanks for noticing, it really is a stupid mistake.   

       Re: changing the stroke, I'm sorry, but it does change the stroke. a cam with a very non-round profile causes the pistons to move a large distance--a large stroke. Rounder cams cause the pistons to move a short distance--a shorter stroke. This is how cams work, and how "stroke" is defined. By changing the average diameter of the cam, compression ratios can remain constant despite the changing strokes. (if desired, but frankly changing the CE could be good too)   

       At low speeds and constant compression ratios and cylinder diameters, a longer stroke produces more torque than a short stroke because the overall displacement of the engine is increased. This is what "stroking" and engine for performance does--ref. wikipedia article. However, longer strokes limit the redline of the engine, as moving the pistons a long distance is difficult at high speeds. Short-stroke engines have poor torque, but make up for it with a higher maximum redline, which produces more peak power (power=torque*speed). This is partly why Formula 1 engines are so small yet produce so much power--their extremely short strokes allow redlines in the 20,000 RPM range.   

       So the point is to have a long stroke at low speeds for the extra displacment, and therfore torque, and then shorten the stroke at higher speeds to allow the power-producing high redline. Shortening the stroke will cost you torque, but the increased redline will more than make up for it.   

       The engine should not need to be stopped to change cams. I admit ignorance as to how this is done, but Honda VTEC engines perform exactly this feat on their valve-controlling camsshafts--switching between cam profiles while the engine is running. Therefore, I can say with reasonable certainty that it is possible in my application too.   

       As for not liking piston engines, well, wankels have poor fuel economy, quasiturbines are unproven and shrouded in pseudoscience, and gas turbines have hard-to-use power curves and require high-speed gearboxes. I'm looking forward to the RandCam myself, but the Revetec engine is currently likely to be the first to see use in real cars (ignoring the old turbine car concepts)   

       [BrauBeaton], reading the article myself, I can only state that you're also wrong in your interpretation. The counterrotating cams aren't even attached to the drivetrain, so they cannot possibly have a direct effect on the power output of the engine ("The inner cams transfer their force to...a counter-rotating balance shaft"). Besides, I'm at a loss to determine how splitting the force between two cams could possibly double the power transfer Each cam gets 1/2 the force, 1/2+1/2=1=no change in overall force. The Revetec engine produces more power than a conventional engine because it has a piston-movement profile better suited to the expansion dynamics of burning gasoline, and because it has fewer moving parts, which waste energy.   

       To quote the last paragraph: "but there’s still a lot we can do with the cam shape, including profile changes, matching the dwell to the expansion rate of the fuel, etc." This explicitly states that different cam profiles affect the performance of the engine.   

       [jhomrighaus], I grant that Saab's engine changes the compression ratio. However, it has no effect on stroke (granted, I seem to be having difficulty convincing people that stroke is actually changed, and that changing compression ratios are only a secondary consideration).   

       :P
5th Earth, Jul 24 2006
  

       I believe the confusion is that people (like me, but I now understand what you mean) think you are proposing to change the number of stokes per combustion cycle (4-stroke cycle, 2-stroke cycle, etc.), rather than changing the distance of each stroke.
BJS, Jul 24 2006
  

       [revetec] You cannot use a variable type VVT system on our engine drive cams.   

       The reversing cam is geared and is a drive cam. Comments like "The counter-rotating cams aren't even attached to the drivetrain, so they cannot possibly have a direct effect on the output of an engine" is incorrect. They transfer 50% of the power to the drive shaft. It is geared to do so.   

       The counter rotating cams redirect side thrust to rotational drive, so it is not advantageous to use only one drive cam.   

       We do not vary the stroke. We can design different piston acceleration rates by way of cam design to suit other fuels and to change breathing characteristics.   

       As far as transfering force, we reach a better torque lever earlier in the stroke and maintain it for longer.   

       Our piston stroke does not determine the torque lever like a conventional engine.   

       A conventional engine loses 36% mechanical transfer throughout a power stroke, 36% being forced down on the main journal. Because we deflect thrust normally sent to the main journal to the counter-rotating cam, we only lose 11% of force down into the main journal.
revetec, Feb 13 2007
  

       Re: [Revetec] Nice to hear from you. I hadn't considered issues of side-loading when I proposed removing the counter-rotating cams. I see now that there is more than just mechanical stability at issue.   

       I'm also glad to be corrected on the fact that the both cams for each cylinder are attached to the drivetrain. From descriptions on the website, I got the impression that only one cam drove the drivetrain, and the reversing cam existed solely for rotational balance purposes.   

       I accept your statement that stroke length does not affect torque lever in your engines. However, changing the stroke does change the displacement of the engine, and changing the displacement does have a radical effect on an engine's performance. Stroke length also has a very general relationship to the maximum safe RPM of the engine. I believe that the ability to dynamically vary the stroke would still impart some benefits.   

       You also state that variable-cam technology cannot be applied to the Revetec engine's drive cams. I felt confident that such a thing could be achieved given successes in related areas, but I'm not versed in the complexities of the Revetec engine. I'd like more details why this apparently cannot be done, but I can take your word for it.   

       Anyway, It's good to hear from you. The new X4 engine looks very promising.
5th Earth, Feb 14 2007
  

       Sweet deal!
quantum_flux, Feb 14 2007
  

       Thanks for the warm reception. It's Brad the inventor of the Revetec design.   

       In relation to the stroke not determining the torque output, we can produce two identical engines but in one engine we will put bigger piston bearings and change the cams to suit. Now both engines have the same bore and stroke, but...by putting in bigger diameter bearings, we increase the torque lever. The larger the torque lever the lower revving engine is achieved. But with more torque on tap it is not need to rev the engine as high to accelerate the same. What the sweetspot is? We are working on it at the moment.   

       Has everyone seen our new X4 engine? the 2.4 litre billet engine is only 160mm from front to back, and weighing in at under 100kgs. We will drastically reduce this weight soon. The problem is getting all the accessories like alternators and starter motors to fit. :-)
revetec, Feb 15 2007
  

       I'm afraid i'm turning into an ecology freak; sorry but with UK's being on the list of areas that will possibly need to be evacuated due to global warming predictions ill have to give any idea that needs oil a [-]. In my opinion, every post that envisions a oil-burning-engine as anything else than history deserves a [-].
sweet, Feb 15 2007
  

       [sweet], that's rather perverse. This idea would work just as well with biodiesel or alcohol fuel.
5th Earth, Feb 15 2007
  

       //The problem is getting all the accessories like alternators and starter motors to fit. :-)//   

       This sounds like an ideal application for one of the newer integrated flywheels(see Link). Then starter alternator are all in one and the overall weight is lower and efficiency is higher. Strap a PS pump on Top and Ac compressor on side with a simple belt and wham o, your in business.
jhomrighaus, Feb 15 2007
  

       Noticed you posted a link to the X4 engine running--congratulations guys! Looking forward to some dyno curves once you get it tuned up.
5th Earth, Mar 30 2007
  

       It's AWESOME to see actual inventors commenting on their inventions.   

       Just sayin'.
elhigh, Mar 30 2007
  

       You're right 5th Earth.   

       The long term future is not about burning oil. It is about trying to reduce the carbon reserves on the surface of the planet. The carbon reserves are at their highest and we need to start using the resources that exist on the surface rather than pulling it from under the ground. Ethanol or biodiesel is the solution. Even if we now all use electric cars charged from nuclear or hydro generators, the carbon reserves on the earth's surface will remain the same. The only way now to rectify the problem completely is to bury carbon reserves back underground.
revetec, Apr 02 2007
  

       Hey Brad, do you have any half baked ideas that you would like to post on this website?
BJS, Apr 02 2007
  

       I have heaps of them but I don't want to post them as they are all marketable....Hmmm....I'll have a think about it. I have some funny ones but I don't want to wreck my credibility. :-)
revetec, Apr 04 2007
  

       I finally understand this idea properly, and so I decided to vote + for it.   

       Hey while you're at it, why not have a cam that would have a profile which would make the engine run on the Atkinson cycle? Or some other cycle even?
BJS, Jun 04 2007
  

       "don't want to wreck my credibility"...man, you're posting here. That ship has already sailed. ;-)
half, Jun 04 2007
  

       A little late, but I just realized a problem. Vtec switches cam profiles because it locks the rocker arms from the high-rpm cam lobes to the rocker arms of the low-rpm cam lobes (the ones actually connected to the valve stems), and there is no sliding or movement of the camshaft itself. How would this work with a camshaft as the crankshaft, some sort of piston rocker-arms?
acurafan07, Oct 21 2007
  

       Independent certified testing of the X4v2 engine by Orbital Australia   

       The Directors of Revetec are pleased to announce that the X4v2 engine achieved a repeatable Brake Specific Fuel Consumption (BSFC) figure of 212g/kW-h (38.6% engine efficiency) with a best figure of 207g/kW-h (39.5%) at our requested target test of 2,000rpm with a BMEP load of 450kpa (approximately 75% load) and an air/fuel ratio of 15.2:1 using 98 RON petrol and a 10:1 compression ratio. We also achieved a BSFC figure under the same rev and load conditions using an air/fuel ratio of 14.5:1 of 238g/kW-h (34.4%).   

       Cheers Everyone.
revetec, Mar 31 2008
  
      
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