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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.


Roger Pham

10 cubic feet of air plus the weight of the tank weigh a lot. The air itself weighs 1.3 kg/m^3 at 1 bar pressure, in this case, 0.27 m^3 x 300 bar x 1.3kg =105 kgs, or 231 lbs. (If filled with H2, however, the H2 will weigh but 7 kgs) Then, the weight of the air tank, even if made from carbon fiber, will weigh ~140 kg (308 lbs), based on Quantum's carbon-fiber tank capable of holding 5% of H2 by weight. The air motor capable of at least 3 or more stages of expansion and re-heating or heat exchanging with ambient air won't be light nor small, either.
Combined, 308 lbs + 231 lbs = 539 lbs weight for 6.5 kwh for energy storage (more, perhaps 8-9 kwh can be obtained if the car is driven very slowly at about 15-20 mph whereby the expanded air has the chance to absorb more ambient heat and expands more).

Then, after all this, where do you find the room or spare weight to put in additionally 40mi's worth of battery and electric motor plus motor controller? 10 cubic feet of space will fill up the whole trunk having 15 cubic feet, since the tank must be cylindrical and spherical on both ends, taking up additional space. Batteries can be rectangular in shape and can conform in tight spaces, thus taking up less space.

Then, home charging to 300 bar pressure is somewhat problematic, since a 3-stage compressor plus a large tank for 300-bar compressed air storage will cost thousands of dollars. Plus, the rather inefficient process involved in both compressing the air and the use of compressed air will consume a lot more electricity, perhaps twice as much electricity, in comparison to the charging of a BEV, for a given driving range.

Adiabatic compressed air storage will work for about 10 bars, with the final temperature of the compressed air about 753 degrees K, or 480 C, but working with 300 bars, cooling of the compressed air in 3 or more stages is a must if you don't want your equipment to melt from the heat of thousands of degrees C. Polytropic compression with multiple stage of cooling is the only way feasible, and that introduces significant energy lost to the cooling of the hot compressed air.


Tanks and air weigh up but so does 12 kw of extra batteries. The expander/alternator does not have to weigh much. Quick recharge is the key feature, as well as cost and lifespan. Quick charging batteries takes LOTS of power and reduces life span. A station or home could have compressors and tanks for filling in a few minutes. This is a key feature.

Roger Pham

The weight of Li-ion Iron Phosphate battery is 115 wh/kg, so for 12 kwh, will be 104 kg, or 230 lbs, which is much less, and takes up much less room, leaving you with a full trunk space for grocery shopping.

The most cost-effective and practical option for now is home fill up with CNG or ANG in a tank smaller enough to fit under the rear seat. With ANG, the filling pressure will be much less, 500-750 psi, allowing low-cost home compressor, and light-weight tanks. With use of bio-methane, the fuel is also renewable. With future low-cost water electrolyzer, and adsorb H2 storage requiring low pressures, home refilling with H2 will be optimal. With future lowering of cost of solar PV cells, you could store solar energy from your roof top as H2 and fill up your car once per week.


I agree, if and when fuel cells become cost effective, long lived and readily available, they would be preferable. When they will be available at a reasonable price with long life is still in question.

I don't like combustion and I like quick refill without compromising life span. It remains to be seen how lithium batteries respond to deep cycling and rapid charging over many years in the field.

Buyers may not want to replace $10,000 worth of batteries every 5 years. The 12kwh of batteries in the air electric hybrid would be kept at a high state of charge most of the time. I would use NiMH anyway, because they are proven over many years.


But compressed air is just too inefficient and too heavy.

Adding air tanks and air motors and batteries and electric motors and range extenders and recuperators and heat exchangers only makes it less practical and less affordable.
- - The costs add up but the technologies themselves are not necessarily synergistic.


If there were BEVs with 24 kwh of lithium batteries that sold for $25k and could be recharged in a few minutes, then I would agree. There are lots of claims about lithium batteries, but I do not see them making taxis out of Escape hybrids with lots of lithium batteries and running them 24/7 for years, let alone charging them in minutes and having the batteries last for 10 years.

Every time you charge a battery from deep cycle, it takes a bit of life out of them. This is not the case with compressed air. After 1000 deep cycles, the system pretty much works the way it did on the first day. This is not the case with batteries. If real validated progress is made on lithium and the prices come way down and we can build millions of BEVs with 1000s of lithium cells per vehicle each year and they last for decades, then that is great.

I am advocating an alternate method. I would like to use biofuel in fuel cells in a PHEV, but that probably will not happen soon. The market would indicate where it is heading. If next year, in 2010 if people are buying EVs in great numbers and there is a waiting list like their was for the Prius, I would say that the market is indicating their preference. But I like options and choices that let people decide what they want. Especially when it is good for the environment and the country.

Roger Pham

A123 and AltairNano batteries promise cycle-life durability far exceeding current Li-ion, and can be quickly charged in minutes. Research in Lithium chemistry is on-going at rapid pace. Let's hope that the cost will gradually come down to be cost-competitive with gasoline...or gasoline will go up and up to the level that will promote rapid growth in BEV and PHEV.


Toshiba has a rapid charge, are working with VW and maybe others. Now where people are going to get the 200 kw to do a 5 minute charge, I don't know, but we went over that months ago.

I hope to heck that they make massive progress in batteries. I hope to heck that they free up the making of large format NiMH, but I can hope or I can do. Doing usually makes much more progress for everyone.


If you look at the weight burden of carrying a large compressed air tank, it would not surprise me if a compressed-air "pump up" hybrid would burn more fuel to haul the tankage around than it would save.

Batteries have far better capacity in Wh/kg, and it's more efficient to charge a battery than to compress air anyway.


I might modify the design to use smaller tanks and lower pressure, but use engine heat for expansion. This would be an air/electric PHEV.

Roger Pham

So, your car now has additionally an engine (ICE) as well, beside the air motor, smaller air tank, 12 kwh battery, electric motor, and now, fuel tank for the ICE as well?
Assuming that the engineers can fit everything under the hood with triple expansion and reheating air motor (a lot of plumbing!) AND and the ICE, and use up the entire trunk for the air tank, battery, and fuel tank...which will turn off many buyers since they have no room for groceries nor luggages when carrying some many people would want to mechanically linking high-pressure couplings every night to fill up the air tank, charge up the battery, and then having to fill up the gas tank evey once in a while? And how many people out of that group will want to fork out extra $$$ to buy such a complicated and overweight vehicles?

The Camry Hybrid and Fusion Hybrid turned me off due to their excessive weights and small trunk space.


No one has to attach any air hose. The air is provided by the expander/compressor when you plug it in to charge the batteries. The idea is short, medium and long for energy. Short is the air, medium is the battery and long is the engine. If I can get 4 kwh out of the air and 8 kwh out of the batteries and 300 miles out of the engine and gasoline, it solves most uses.

Roger Pham

Oh, sorry, I had in mind a compress air tank at home for recharging your car, so that you can charge the car's air tank in 5 minutes, since that's what you posted before. This is convenient if you have solar PV panels and want to store the excess solar energy produced in the day time.

You now propose to use the air motor as the air compressor to recharge the air tank using grid electricity. It may work, too, but a little bit difficult if you also wanna use engine's waste heat to reheat the expanding air. Makes for more complicated plumbing.

By using engine's waste heat, it means that the engine must always be running parallel to the air motor in order to generate the heat...kinda diminish the role of a PHEV as a petroleum sparing means, in which, the engine is only turned on once the battery is depleted below a certain level.

IMHO, the best bet for now is to try to make the batteries more affordable and more durable...while trying to make the H2 electrolyzer more affordable and more efficient, and perfecting H2 adsorption matrix to allow high-capacity H2 storage at much lower pressures. H2-ICE, when optimized, can get close to 50% efficiency, in case the FC can't be made cheaper, more durable, or in large quantities. Looking at the likes of the Honda FCX Clarity for the shapes of things to come. We are getting close.


Rational people change their minds to perfect the outcome. You keep the faith that soon hydrogen cars will be running everywhere and I will implement an air hybrid that will perform well until that day, which should be quite a while.

Roger Pham

Success is in the detail. My negativism regarding compress-air energy storage simply reflects the ideas I have accumulated in mind from past practices. You may have better ideas in mind or in paper that may be able to solve past problems associated with compress-air/air hybrid. Please keep working, researching, and calculating all the minute details... and all possible angles...
Best Wishes.

PS: How soon H2 Cars will be roaming everywhere will depend more on how soon Congress will have the gut to tax petroleum out of existence...gradually, of course, so that it won't wreck what ever's left of the fragile economy!


A 1930s Pierce Arrow had an air starter, I understand some race cars have air starters. This is nothing new, it is how you use it that counts. Air can easily provide take off power, so a smaller electric motor is needed with less strain on the batteries. Air can easily absorb regenerative braking so that you do not need super capacitors. There are a lot of advantages and the engineers at UCLA have shown this.

"..the air hybrid engine improved fuel efficiency by 64 percent in city driving.."


I'm strongly negative on both hydrogen and "air cars" (other than air hybrids with storage too small for "pump up" capability), and for many of the same reasons.  What does that make me, SJC?

I note that the examples you cite fit my model:

  • the Pierce Arrow would have had a tank too small for hybrid operation.
  • the UCLA air hybrid does not mention operation on air alone.
Further, the exhaust-heat recovery of the UCLA car requires fuel to be burned while power is being produced.  This may be a way to get a substantial boost in low-speed performance and city fuel economy without a highway penalty, but it's far from a panacea and should not be treated as one.


I think air can be used for regenerative braking and take off. If that means a smaller motor, less batteries with longer lives then a contribution has been made. You can have your opinions and that does not make you anything...which is the point in and of itself.

Roger Pham

But let's consider the gaining success of HEV's like the Prius and new Insight, being top sellers in Japan where the gasoline price is >$5.00/gallon. The cost of producing HEV's is getting lower and lower with increase in production number and experience gained, eventually, perhaps on par with non-hybrids. I don't see how air-hybrid can offer anything extra that would warrant considerable development cost. The air storage tank is bulky, and so is an air motor, in order to fit under the hood...whereas, the electric motors and batteries are much more compact. Ford has been experimenting with Air Hybrid concept since 2003, using the engine itself as both engine and air motor...and also the hydraulic hybrid concept...and yet, we only see now the Escape and Fusion HEV's.


Race cars have been looking at air KERS for years. They would not bother if it were too big nor too heavy.

Not only can air capture braking energy, but it can be used to cool the passenger compartment, electronics, motor and become a supercharger. Think outside the box and you might see.


Compressed air power is cheap.
Compressed air science is easily understood.
Compressed air technology is well developed and simple (because there is no phase change like steam and even steam is well understood).
Compressed air engines are cheap and light and powerful and reliable.
Compressed air power can be combined with an ICE using the same pistons.
Where are they, why do they not fill the streats?
Because they are stunningly inefficient.

If you want to save the compressed heat and also have multi stage reheat expansion, the cost and weight will skyrocket and the efficiency will still suck.

There are no secrets here and no conspiracies.
It’s dirt simple.
Cars powered by static electricity and cars powered by compressed air are not in the market (that is fact – not opinion) because they do not make sense (that should be obvious).
Dreams are good but dreams are not real; dreams are dreams.


Not compressed air powered, but compressed air assisted, electro magnetic assisted and internal combustion it a tribrid.

You can make straw man responses and make statements that are erroneous, but easy to knock down or you can address what is being proposed. I don't care what you choose to do, because you are pretty much irrelevant and most people on here know that....IMO.


"efficiency will still suck."

Air compression and expansion is efficient, but has low energy density. You need to get your facts straight.

If you use air compression to store the braking energy, you can store quite a bit of it quickly. Expanding it can create good power as well as provide cooling and supercharging.

Storing the heat of compression is not complex, you have a heat exchanger in oil, not heavy nor large. It may not be necessary to store the heat of compression but just dissipate it. You have more than enough heat from the engine for expansion.

Roger Pham

Your continual enthusiasm inspite of the negativism in the GCC forum is admirable. We can recall that when the first HEV was released, it was also negatively received by GM and others...most people said it would be too expensive and that the mfg would lose money instead of making any profit. Now, HEV's are important money maker for Toyota and Honda while sales of conventional cars went flat. Engineering is equally art as science. The art part is how to make the science not just work, but to excel at gaining customer's acceptance, market share and profit.

Your engineering challenge now is how to package a "trihybrid" with low-enough cost, simple and reliable enough to avoid expensive warranty expenses, and with enough seats and trunk space competitive with a non-hybrid ICE vehicle or an HEV. Best wishes.

PS: an electric HEV can also capture braking energy, cool the passengers with electric AC when the engine is not running, and also provide boost to the engine during acceleration...anything that an air-hybrid can do...with much larger energy storage capacity.


I contend that air can capture more braking energy with a lower cost system. You do not get "free" cooling and supercharging with electric. I don't care about negative comments, you guys are nothing anyway :)

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