The Revetec Controlled Combustion Engine

Revetec Website ^ | 10/19/05

[Jack of all Trades]

The REVETEC Engine design consists of two counter-rotating “trilobate” (three lobed) cams geared together, so both cams contribute to forward motion. Two bearings run along the profile of both cams (four bearings in all) and stay in contact with the cams at all times. The bearings are mounted on the underside of the two inter-connected pistons, which maintain the desired clearance throughout the stroke.

The two cams rotate and raise the piston with a scissor-like action to the bearings. Once at the top of the stroke the air/fuel mixture is fired. The expanded gas then forces the bearings down the ramps of the cams spreading them apart ending the stroke. The point of maximum mechanical advantage or transfer is around 10deg ATDC (the piston moving approximately 5% of its travel) making the most of the high cylinder pressure.

This compares to a conventional engine that reaches maximum mechanical advantage around 60deg ATDC. (after the piston has moved through 40% of its travel, losing valuable cylinder pressure). The effective cranking distance is determined by the length from the point of bearing contact to the centre of the output shaft (NOT the stroke). A conventional engine's turning distance is half of the piston stroke. The piston acceleration throughout the stroke is controlled by the cam “grind” which can be altered to give acceleration to suit a certain fuel and/or torque application. This also allows different port timing on opposite strokes, increasing efficiency on 2-Stroke engines.

The piston assembly slides rigidly through the block eliminating piston to cylinder-bore contact. This reduces wear and lubrication requirements. This also reduces piston shock to a negligible amount making ceramic technology suitable. One module which comprises of a minimum of five  moving components, produces six power strokes per revolution. Increasing the number of lobes on each cam to five produces ten power strokes without increasing the number of components. The CCE integrates well with existing power plants and can utilise almost all existing engine technology with increased efficiency.

Summaries of CCE advantages are as follows;

• approximately one quarter the size and weight of a conventional engine (for similar applications) combined with improved output substantially increases power/weight and torque/weight ratio.
• fewer moving and total components. As a result of fewer components, more easily manufactured than conventional engines.

• identical cylinder head assembly (“top end”) to conventional engines. Most existing head technology can be either adapted or utilised.
• Flexible design - can be four-stroke, two-stroke, petrol, diesel or gas, natural of forced aspiration.
• Eliminated irregularly reciprocating components such as connecting rods.
• Output shaft can be run in either direction if multilobed cams with symmetrical lobes are employed.
• All rotational forces are counteracted via the counter rotating cam – eliminates the need for a heavy flywheel.
• Torque and power output can be varied using a fixed capacity and piston stroke.
• The CCE can be designed to operate at greatly reduced operating speeds while delivering high torque output.

• Substantial reduction in stroke reduces heat loss through cylinder wall.
• Extended piston dwell is possible because engine design allows a lower than normal compression ratio to be used reducing power loss from compression cycle.
• Maximum mechanical advantage can be applied to output shaft at only 10 degrees ATDC utilising high cylinder pressure early in the stroke, compared to around 60 degrees ATDC for conventional engines.
• Lower emissions can be achieved due to increased control over combustion.
• Extremely low idle speed due to increase in mechanical efficiency at the top of the stroke.
• Little or no bore contact/piston side thrust, which reduces wear on cylinder bore.
• Can have different port timing on compression stroke than power stroke allowing better control two-stroke).
• Lower centre of gravity.
• Due to controlled piston acceleration rates the CCE reduces engine vibration.

• A hollow output shaft can be utilised for specialty applications, such as peristaltic pumps.

[Gandalf_The_Gray]

Also the load on those "lobes" has to be tremendous.

You've spotted the weak link in this design.

There was a very similar design about thirty years ago that used the same double ended piston idea (six pistons, twelve cylinders) and went this design one better. The cylinders were disposed in a circle around the main shaft like a revolver, in the center instead of two counter rotating cams there was a single swash plate similar to an air conditioning compressor. (a swash plate is a disk which intersects the shaft at an angle and wobbles back and forth as the shaft rotates, the angle of intersection being the stroke) The plate used in this engine was trick in that it was designed to provide two strokes per revolution so it was a bit more complex then a flat plate on an angle. The trick allowed the output shaft to run at half speed compared to the pistons. This allowed a simple face cam track at either end to directly power the intake and exhaust valves from the main shaft without timing gears (think double overhead cams WITHOUT the cam shaft!). Ignition could be driven from the same cam (at the time breaker-less ignition was just make a start) No distributer was required to run a standard four cycle engine with twelve power strokes per revolution.

All told you had a twelve cylinder engine, shaped like a thirty gallon drum with about 1/4 the parts count of a V8 with about the same displacement and a torque curve that started down near your socks. The built in two to one reduction made it a natural direct drive of aircraft propellers and as I recall one of the light plane companies was looking at it. (Cessna maybe, I don't remember for sure) I've never seen it since.

Back to the RevTec: Actually, the two cams seam to me to be unnecessary. The mechanism is desmodromic, that is the distance measured across the cam is a constant as it rotates (conjugate surfaces). The result is that piston motion in either direction will cause the cam to rotate. Key point: there is no method to adjust for "lash" or clearance except to dial it into the dimensions. ANY wear on the cams will cause the worst case of "rod knock" you've ever heard. As Nathan has pointed out, this is a highly loaded area in this design and therefore adjustment mechanisms would have to be robust in the extreme yet very stable over the operating temperature range. A formidable task! That is very probably why that "revolver" engine never saw production as it shares the same weakness.

Regards,
GtG

[Gandalf_The_Gray]

PS I forgot to mention that the claims made for the RevTecengine are largely based on the fact that cylinder motion is constrained by a cam with the possibility of other than modified simple harmonic motion as provided by a crankshaft/connecting rod mechanism. As such every claim that they made also applies to the "revolver" engine I described above as it also is a cam mechanism at heart and within reason, cam contours may be modified to provide arbitrary acceleration profiles. As I said previously, the cam may indeed be the thing that keeps either of these engine in the lab because as the heart of the engine it may not be up to the task.

Regards,
GtG

[Gandalf_The_Gray]

a prototype F-1 engine with a notched cylinder running along the top of the head that acted in place of the valve train

Are you sure it was a cylinder and not a series of spherical constructions? That sounds a lot like the Coates engine.

See Here

[Chode]

it was a cylinder with notches running around the side that ran along the top of the head and turned clockwise on one head and counter on the other

it might of been a two stroke as i didn't get a real good look at everything...

the problem seemed to be having the port open long enough to get all the fuel it needed when turning +9000rpm

i can't get your page to open right now but i'll keep trying

[patton]

you need to read this; don't feel bad, I had to look at it for 10 minutes before I understood how it works.

Engine mounts need to be substantial, but with more cylinders, this problem goes away.

Note that it is in a racing car this weekend.

Fascinating.

[Chode]

i got it to load and yes it is the same idea, but i'm sure the one i saw was just a machined cylinder... maybe their prototype they were talking about a spring loaded section that was ok for most of the rev's at lower rpm's but at high rev, the spring would give a little and therefore let the ports stay open longer. anyway that was the theory and i never saw it again.

[edsheppa]

Here's another interesting one: Quasiturbine

[Chode]

how do they attach the drive shaft??? i looked at the site, cool design but i can't figure it out...

[Gandalf_The_Gray]

Just the other night I came across the Stelzer engine while surfing the net...I was wondering how they would get power out of it since there was not a rotation to harness.

The design shown is again nothing new. We had one in the 1960s in the ME Thermo lab where I attended college. That one was a diesel cycle but that really doesn't matter, gas or diesel they are all running as two stroke engines. They are sometimes called "bounce piston" engines, named for the chambers at the extreme end of the piston. The sealed chamber allows air to compress and limit the stroke of the piston, stopping it and returning it in the down-stroke direction rather like a spring. It also how the engine is started, you inject compressed air into the bounce chamber to get the piston moving. After it fires on the down-stroke the cycle continues naturally.

As to how to take off power, there are several methods. The center piston shown in the animation at the website illustrates a double acting pump or compressor which could move either liquid or gasses. An alternative would be to reciprocate a magnet through the center of a solenoid coil, thus providing and alternating current output. Another method would be to use the exhaust gas as a heat source for a central heating plant.

And I saved the best for last, a piston powered rocket (reaction) motor. Way back when I was a pup, I was a regular reader of Science & Mechanics. One particularly memorable issue had a cover feature of a backyard built go cart powered by something called the "screaming demon". It was essentially a bounce piston engine with both the exhaust ports plumbed thru a convergent/divergent DeLaval nozzle. The nozzle was jacketed and a stream of water was passed thru it to keep the thing from melting. The water flashed to steam and was ported out of the throat of the nozzle to augment the thrust of the engine by adding mass to the hot gas flow. The thing was fueled with and acetylene/air mixture. If you remember your chemistry, acetylene does not take well to pressure. Anything much above 15psi or thereabout and it detonates. That normally is annoying but in this case it means you don't need an ignition system, you just valve a bit of compressed air to the bounce chamber and your off to the races. This thing could easily push the cart to 80 mph (like all rockets , the thrust continues to accelerate you 'till you chicken out). If you hit a bump, you can go "airborne". The only way to measure the speed of the engine cycle was to record the impressive exhaust note and try to beat the signal against a signal generator. Estimates ran over 100,000 cycles per minute! Anyway, that how you do it.

Regards,
GtG

[Jack Black]

I haven't looked at what cam system they use, either 1, 2 or 4, but that's a lot of cam to turn. Even if say, that engine maxed out at 3,000 rpm shaft speed, that valve train would run at a punishing 9,000 rpm. If you've ever built a weekend race engine, it takes some doing to keep the valve train together at those speeds, plus expense, and it doesn't last long. The lobe problem may be overcome with some good metallurgy, but I doubt that the valve train issues can be. Not for a commuter car application at least. With 4 overhead cams, it would run pretty good for a while no doubt. It would be a train wreck if something let go through.

What about using it for a two stroke. Yes, not a street engine, but the Jap bike makers still build 2 strokes for racing motors. With a two stroke you get rid of valves, which goes to the point you and others have made that these things have lots of valve openings and closings.

[Paradox]

The second problem related to the surface geometry of the combustion space. For it's displacement any Wankel will have a greater surface area in it's combustion chamber then any equivalent piston engine.

Exactly, I believe this problem is probably more fundamental that the seals problem. You could imagine some technical solution to the seals, and many of those engines lived a fairly long life without problems (my father owned a 1979 RX-7, sweet little car..). But the problem you mentioned above is simply unsolvable and to most, its not obvious.

Most people thought (think?) that the fact that it is a ROTARY engine makes it, by definition, more efficient, being that the output you desire (flywheel rotation) is also rotary, as compared to converting the up-and-down motion of pistons into rotation, but that is simply not true.

I suppose time will tell with this engine, I'll believe it when I see it!

[Gandalf_The_Gray]

What about using it for a two stroke...With a two stroke you get rid of valves, which goes to the point you and others have made that these things have lots of valve openings and closings.

Most two strokes use "crankcase compression" as a means of scavenging exhaust gas and providing a fresh fuel/air charge. Since the pistons in a RevTec design are essentially linked in lockstep as one mechanism, there is no net volume change in the "crankcase" during the complete stroke so no scavenging is possible. That would have been the simplest mechanism but the design prevents it's working. Failing that, you could always use a "scavenging blower" like two cycle diesel engines do. GM has used positive displacement blowers on their truck engines for years and the technology is well developed (just neither simple, compact, nor cheap). These blowers and well known to the drag racing fraternity. Larger marine engines use the same strategy because the two stroke design will run in either direction, you just need a reversing gear for the blower not the output shaft going to the propeller.

The blower needs to be positive displacement to develop compression when starting, if you could figure out how to overcome that requirement a much simpler turbocharger would suffice.

Regards,
GtG

[bmwcyle]

It looks like someone was playing with old brake shoes.

[edsheppa]

Look here and scroll down to the picture of the Carraige Engine Output Mechanism.

Lot's more information here.

[Gandalf_The_Gray]

You could imagine some technical solution to the seals...

Your analysis is spot on. The apex seal problem is a materials problem and is a difficult but not impossible nut to crack. Given the appropriate materials it should be possible to run a Wankel with out oil injection to lube the seals. Hydraulic vane pumps running fireproof water based fluids have similar problems with vane tips being highly loaded and running at very high surface speeds. These problems have been solved with proper metallurgy and/or ceramics. I have confidence that the apex seal problem could be solved.

The chamber geometry problem is inherent in the design and there is nothing that can be done about it. Unlike the piston engine where a variety of chamber designs are possible. Hemi, pent-roof, squish all reflect varying degrees of exposed flame area and thereby are optimal for differing engine parameters. A hemi allows the largest valving area possible, giving better breathing (low end torque) at the expense of quench area (combustion inefficiency). The squish compromises breathing but gives the lowest quench area and highly efficient combustion. A pent-roof is a middle of the road design that works well with multiple intake and exhaust valves at the expense of mechanical complication. A design engineer must weigh all of these considerations when striving for a balanced design. Unfortunately, the Wankel principle does not allow this latitude in tinkering with parameters.

Considering engines in the abstract, almost anything that causes air to move can be made into an engine. A fan can become a turbine engine by adding a second fan and a heat source (turbines have been built that run on powdered coal!). A positive displacement pump of any sort can be made into an engine by mixing fuel into its working fluid and igniting it at the proper point. That being said, consider the piston engine as we know it. Cylinders are easy to machine, arbitrary cam like surfaces are not. Cylinders can make a piston bore and a piston with liberal tolerance allowed and still be sealed with a simple cast iron ring. Crank mechanisms are essentially a bunch of cylindrical surfaces connected by as-cast metal. Insert bearings with forced lubrication are easy and robust, forgiving of large tolerances. Needle, roller, or ball anti-friction bearings are also forgiving of lubrication requirements, but inherently more complicated, bulkier, have finicky tolerances, and are expensive (& noisy!). Poppet valve technology works in everything from lawnmowers to F1 racing at ridiculously high rpm. The rotary valve (Coates engine) is a complex solution looking for a problem that doesn't exist (looks neat though!).

My point is engines are fun! They are fun to dream up, they are fun to build. You might even be able to get somebody to finance your dream engine and go for production. But the Otto cycle piston engine has been in continuous production for over one hundred years. It has been subject to a process of continuing development by market pressure, optimizing virtually every aspect of it's function. We have pushed the laws of thermodynamics and economics about as far as they will go. Todays engines represent the pinnacle of sophisticated simplicity, balancing manufacturing cost, performance, and efficiency. It is not an easy task to replace it. Adding cost and complexity will not suffice, a break with the past is needed not a different way to do the same thing (fuel cell?? maybe).

So endith today's lecture.

Regards,
GtG

PS If it don't go, chrome it!

[Paradox]

Ah, thanks for the flashbacks to ICE 101 and 102 I took back in college. I learned more about engines in those two semesters than probably 99% of "car guys" know today.

[Gandalf_The_Gray]

Here's another interesting one: Quasiturbine

I saw an almost identical engine back in the '60s, invented by an Indian (east) engineer. He built a working prototype with hand tools on the drainboard of his kitchen sink (I can't make this stuff up!). Anyway the combustion space was defined by something like a square box, hinged along the edges, which was alternately squeezed inward along the vertical axis (horizontal expanding outward) and then contra wise by a camming arrangement. Same mechanism as the engine shown here. Intake/exhaust porting was done thru the side plates similar to a Wankel.

Basically, this design carries the same killer problem as the Wankel, a very large combustion chamber area. The exposed area is much larger then an equivalent piston engine. This means that the combustion process is "quenched" by the cooler chamber, runs more inefficiently, and produces more pollutants. There is no way to overcome that fact, it is inherent in the design.

Regards,
GtG

[Lee'sGhost]

Take a look at the schematic, the weak link has got to be the drive belt. Looks like the setup the Wrights had to drive the two props on their original flyer.

[Jack of all Trades]

I don't know, it doesn't strike me as being all too dissimilar from a standard DOHC arrangement. A pretty convincing case has been made for the cam being the weak link. I showed the link to some guys I work with and they just rolled their eyes at the though of grinding such a large profile.

Still pretty cool I think. Glad it generated some interest.

[Gandalf_The_Gray]

flashbacks to ICE 101 and 102 I took back in college. I learned more about engines in those two semesters than probably 99% of "car guys" know today.

I went to Milwaukee School of Engineering in the early '60s. I was in their first class of internal combustion engine AAS (2yr degree) to graduate. I had more thermo, heat transfer, & related physics then most ME's ever get in four years. I know that is true 'cuz I went back and got a BSME too. I had to correct some of the engineering professors on fine points of thermo at times (not necessarily a wise thing to do unless you know you're right and young and foolish!)

Retired now, but I look back fondly on those days, I'd go back and do it again if I could.

Regards,
GtG