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Australian Cam-Drive Gasoline Engine Reaches 39.5% Efficiency in Independent Testing; Potential for Production Engine in China

The 2.4-liter X4V2 prototype was originally designed for an aviation application. Click to enlarge.

Australia-based Revetec is designing what it calls the Controlled Combustion Engine (CCE)—a cam-drive gasoline spark-ignited internal combustion engine that is smaller, lighter, cleaner, less expensive to manufacture and that produces higher torque due to higher mechanical transfer than equivalent conventional engines.

Revetec has prototyped 6 different versions of Revetec engine designs over the last 10 years. The latest version, the X4V2, was designed as a development engine for the aviation industry, and in early 2008 it was independently tested by Orbital Australia.

We modified the X4V2 engine to increase fuel efficiency focusing around the 2,000rpm range where most driving occurs, then sent the engine to Orbital Australia Pty Ltd for independent Certified testing...We tested the engine under the standard air/fuel ratio of 14.5:1 and also at our desired air/fuel ratio of 15.2:1 which maximizes the efficiency of the current configuration.

The Directors are pleased to announce that the X4V2 petrol engine achieved a repeatable Brake Specific Fuel Consumption (BSFC) figure of 212g/kWh (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%).


The engine. The Revetec cam-drive engine uses a pair of counter-rotating trilobate (three-lobed) scissor cams geared together, so both cams contribute to forward motion, rather than a crankshaft. Two bearings run along the profile of both cams (four bearings in all) and stay in contact with the cams at all times.

The trilobate cams.The X4 design.

The bearings are mounted on the underside of the two inter-connected pistons, which maintain the desired bearing-to-trilobe 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 20-30deg ATDC (when the piston moves approximately 10% of its travel) making the most of the high cylinder pressure.

Piston and cam assembly. Click to enlarge.

This compares, says Revetec, to a conventional engine that reaches maximum mechanical advantage around 60-70deg ATDC—after the piston has moved through just over 40% of its travel, losing approximately half of the 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 suit a wide variety of fuels, torque requirements and/or rev ranges. The piston assembly slides rigidly through the block via an oil pressure fed guiding system eliminating piston to cylinder-bore contact, reducing wear and lubrication requirements in the cylinder, and also reducing piston side shock—making ceramic technology suitable.

One engine module can comprise two trilobate cams and either two or four pistons in an “X” configuration. The counter rotation is performed by a reverse gear set at a 1:3 ratio shaft providing two strokes of a piston to 360 degrees of output shaft rotation—the same as a conventional engine.

Revetec calculates that while a crankshaft connecting rod device in a gasoline engine is approximately 65% efficient in matching the mechanical device and cylinder pressure to an output shaft, the Revetec engine bottom end design is approximately 85% efficient.

In addition to improved efficiency, torque performance is strong. In tests using asymmetrical trilobes, Revetec says, it has achieved almost 90% of peak torque from the earliest in rpm it could start the dynamometer tests. The X4V2 aviation engine with asymmetrical Trilobes achieved 180 N·m (133 lb-ft) of torque @1,300rpm with a peak torque of 203 N·m (150 lb-ft) @3,000rpm.

Commercialization prospects. Revetec has signed agreements with a German university and a Chinese group which is funding a testing and development program. Revetec says that it is assured from the Chinese group that upon satisfactory conclusion of the test and development program two of China’s top ten car companies will jointly develop an automotive production engine. Testing was slated to recommence during late May 2009 at the university, with approximately a month of work required.

This test regimen will take pressure readings taken from within the engine’s cylinder head, as well as multiple pressure sensor readings from within the manifolds. This data will help in the modification and optimization of the engine design.



Their system is definitively an improvment compared to conventional crankshaft connecting rod, the problem is that the resulting configuration of the cylinders are not very convenient in particulare to drive the valve where you need heavy rod along the cylinders (like on the Harley Davidson).

The main ineterest is that you get rid of piston slap and manage the torque better than in a conventional architecture, the down side is that you need a gear because the rotation of the shaft is too slow. Using a music or a stratified DI would improve the performances further, but I think that the architecture is too different of conventional engine for the automakers to use it.


If its fairly quiet it might make a good engine for CHP


I have thought of a cam radial modified two stroke for a range extended EV. Efficiency and long life would be a nice design for range extender power. An added bonus is slower speed and more quiet operation with cleaner combustion.



This engine already exist it is being developped by Bertone in italy. I also think that a 2 stroke engines working in very lean mode with not piston slap would be the best approach


Interesting - and no piston side loading is good.
But they are about 40 years too late in eliminating piston slap.

“Smaller, lighter, cleaner, less expensive to manufacture and that produces higher torque”. Since they do not say “higher horsepower”, just what is it smaller and lighter than?

Since they claim higher torque but not horsepower we might assume this is a slow engine. So when they say “The point of maximum mechanical advantage or transfer is around 20-30deg ATDC (when the piston moves approximately 10% of its travel) making the most of the high cylinder pressure.” they imply higher piston speed at 30deg ATDC, maybe matching a normal engine.
Either way piston speed profile and mechanical advantage is a second order effect on power and efficiency, at best.

“ … crankshaft … engine is .. 65% efficient … the Revetec engine bottom end design is 85% efficient.” Only if you redefine what “efficiency” means.

Longevity and noise might be adversely affected by the clearance between the cams and the bearings at opposite ends of the con rods, and mismatch between the two cam profiles on opposite sides of the pistons.


I like the cam race design. There have been several patents files using this design for rotary engines, but not radial. I like the modified two stroke, because it eliminates valves. It is a slower motor, but that would be what I would want for an EV range extender.

Roger Pham

The earlier point of maximum mechanical advantage in the power stroke in this engine simply means that the crankshaft will get a quick jolt of torque early with each power stroke and then this torque quickly diminish as the piston loses its pressure.

In contrast, a crank-slider mechanism in normal piston engine allows the torque to be more gradually distributed throughout the power stroke, lessening the strain on the crankshaft, thereby reducing torsional vibration and stress.

The 39.5% efficiency is too good to be true, unless this engine is running Atkinson cycle. I would like to see this number confirmed by an independent third party. Time will tell.


What's the valving and power-to-weight ratio for this potential aviation engine?



I don't agree with your statement, the torque will be better distributed on this engine than it is on a normal piston-engine, much better. The way it works in a standard engine in this regard is just terrible.

The gain in efficient is probably due to the reduced friction of the zero slap piston, as well as the more sinosoidal motion which avoids triggering the combustion to early like in a standard engine.

Asides Orbital is a serious company with a proven track of record and achievemnt in engine design, I have no reason to think that they measurement are wrong especially when it comes to test the product of a competitor.


This is Revetec, not Orbital. I saw an explanation of the Scotch Yoke compared to the crankshaft. They both had difference curves when compared to sine waves. It sounds like they can contour the cams to produce the desired result, something difficult to do with a crank.

Roger Pham

Right, SJC, the lobed cams can be contoured to give sinusoidal output when the tips are round, or more linear if the tips are more pointy.

Likewise, even the slider-crank mechanism (normal engine) output curve can be tuned by varying the length of the connecting rod with respect to stroke length to adjust the curve of the output. If the connecting rod is long, the output can approximate sinusoidal curve. However, if the connecting rod is short, then, more mechanical advantage can be had earlier in the power stroke and more sudden and less even torque output on the crankshaft.

The notion that more efficiency can be obtained by having early mechanical advantage in the power stroke is difficult to understand. Mechanical energy from the piston will be transmitted to the crankshaft almost entirely minus friction loss, with little difference regarding how quick or how slow this occurs. It has to do with the law of conservation of momentum and energy.


Perhaps there is an efficiency gain from lower frictional losses. I have a hard time seeing a reduction in production costs when every piston needs its own camshaft. I'd also would think there there would be higher emissions from the bottom two cylinders as gravity does not keep the oil in place.

IMO, a two cylinder, two stroke, turbocharged version of this design would make an excellent range extender.


I would make it 90 degree opposing and stand it on its nose. Then every cylinder gets the same treatment. With modified two stroke, you can use the other side of the piston for good scavenging without mixing gasoline with the crankcase. Combine direct injection and you have a good engine.


The press releases for impractical “NEW” ICE engines often share some characteristics.
They read like the editor might not understand physics.
This allows the gullible to interpret it as a BIG breakthrough and the slightly more critical to just assume they meant something else that might make sense.

The “crankshaft 65% - Revetec design 85% efficient.” is typical BS.
As Roger says “It has to do with the law of conservation of momentum and energy.”

Why Orbital? Is this a logical choice for an unbiased test by an impartial lab?
What happened to their tests “in Germany by a prominent University”?

A 90 degree 2-stroke with crankcase scavenging would require sealing the crankcase between the different pistons.
I don’t know why a more sinusoidal motion would significantly help combustion.


"..sealing the crankcase between the different pistons..."

That is not hard to do, now that you have no conventional connecting rods with their odd motions. You have a straight rod now just going back and forth.


ToppaTom:  noise and such could be dealt with using hydraulic lash adjusters between the piston assemblies and the cams.

While this is a very clever engine, I feel that it's a bit late.  We will lose production of petroleum faster than these things can make their way into the vehicle fleet, and our only real solution is electric propulsion.


Electric with range extension is a possibility. People may like the quick re energizing without the large electrical power required for quick recharging. I still like the air-electric hybrid idea. A few minutes refilling the air pressure tanks and you are good to go.

Roger Pham

Compressed air contains only ~5% of the energy of Hydrogen at a given volume and pressure, while being much heavier than Hydrogen. The energy efficiency of Compressed-air car is no better than that of a FCV, such that, if a FCV is capable of 200-mile range, then, the "air car" would only be capable of 10-mile range on the same tank. The mechanical energy of compressed Hydrogen can be harnessed to add more range to the car, albeit only 5% increase in range. If you are willing to put up with the used of a compressed gas as fuel in very thick and heavy tanks, might as well settle for Hydrogen, or CNG, or ANG (Adsorbed Natural GAS). BTW, adsorbed Hydrogen or NG can store a lot more energy than compressed Hydrogen or NG at any given pressure, thus allowing less expensive and more convenient fill up.

As Eng-Poet mentioned, electric propulsion, either BEV for smaller, shorter range vehicles, or FCV's (or Hydrogen-ICE in the interim) will be most promising in the not-so-far future.


Interesting. Its practical application will not only be challenged by its relative "lateness", but it looks like its shape will be extremely unfriendly to packaging in a transverse front-drive setup. That still leaves a lot of range extender applications open, though, and if it's really worth it, the world could always try longitudinal packaging again. It's just that almost all existing transmissions that will accept a longitudinally mounted engine are for rear wheel drive applications.


If 10 cubic feet of air compressed to 4500 psi can run an expander/alternator for 40 miles, it will have propelled a lot of commuters to and from work. If you have enough batteries for 40 miles, you can satisfy the transient high current demands of the electric motor, do regenerative braking and still have a good state of charge when returning to the suburbs for running errands. Once you get back home, 5 minutes of tank to tank air pressure transfer and you are ready to go again. I see it as an urban/suburban/commuter car.


Right. Proponent of the Air Car don't suggest it as a replacement for the highway cruiser. Instead they put it out as a lower tech, cheaper alternative to the BEV.

Roger Pham

That volume of air (.27 m^3, or cubic meter), if expanded isothermally at room temperature, can generate only 12.8 kwh. However, the expansion process is not isothermal, but more like polytropic, emitting very cold air out of the exhaust pipe, so, you can get but ~1/2 of that energy before even considering mechanical losses in the air motor. I don't see how can any USA-street-worthy car run for 40 miles out of 6.5 kwh. (The formula is 100xlnPa/Pb kJ/m^3 of initial gas volume before compression = 100xln300x0.27x300= 100x5.7x0.27x300=46,200 kJ = 46,200kJ/3600= 12.8kwh)

An efficient BEV can do 4 miles/kwh, so 6.5kwh x 4mi/kwh = 26 miles range max. The French "Air Car" can get only 7-10 miles range in real life when demonstrated to reporters.


"With adiabatic storage, the heat that appears during compression is also stored, then returned to the air when the air is expanded..."

" practice round trip efficiency is expected to be 70%. Heat can be stored in ...a fluid such as hot oil (up to 300C).."

If you make your expander/alternator a motor/compressor, you can store the heat of compression on the car to heat the compressed air before it enters the expander.


The only place such trivial amounts of energy storage really fit is in hybrid vehicles, recovering braking energy for re-starting.  If we want to replace liquid fuels, air is not going to do the job; the most efficient thing we have is batteries.

Where's Firefly Energy's traction battery offering?  We can use it about now.


I don't agree, but that just my opinion not THE opinion. If people can quick fill in a few minutes and go 40 plus 40 miles on air and batteries, I think it might have some use as an urban/suburban/commuter car.

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