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

Revetec1
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%).

—Revetec

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.

RevaniX4
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.

Revetec2
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.

Comments

Engineer-Poet

More energy than what?  Lower cost than what?  I think you need to be more specific. ;-)

Face it, batteries have a far higher energy/weight ratio than air tanks.  But if the engine can be used as both the compressor and the air motor with minimal added hardware, the air hybrid will be able to add stop/start and small regenerative energy storage at high power levels with fairly low cost.  Long, thin tubular air tanks can double as body stiffeners.  That would appear to be its niche.

Roger Pham

You're right. Most new ideas are negatively received...No one can know the idea as intimately as the inventor...Only thru perseverance and diligence in further research and experimentation can one truly find out.

But, the cooling effect by gas expension is not free. It was paid for by the work done in compressing the gas. The gas does not have the chance to expand completely, hence less work was done by the gas. In fact, Brayton-cycle refrigeration is a lot less efficient than vapor-compression refrigeration, that's why we don't see it in commercial application. (ie. electric scroll compressor using CFC refrigerants is the most efficient form of cooling)

Supercharging is very limited in an engine with high compression optimized for maximum efficiency, limited by detonation limit. To allow for more supercharging,alcohol injection or variable compression scheme will be necessary, increasing cost and complexity. On the other hand, the Prius has a 76 hp electric motor...if you give a stronger Li-ion battery that can deliver 50 kw of burst power, it would have an equivalent boost of 100%, equal to a boost of 15 psi pressure. That's a heck of a boost, with very high instantaneous torque unmatched by any comparable ICE.

ToppaTom

Air compression and expansion are inefficient, and have low energy density besides. You need to get your facts straight.
If you try to use air compression to store braking energy, you must carefully control the rate, particularly if you want to do it quickly - and an electric brake can do it more simply and precisely.
All of compressed air’s apparently simple capabilities appeal to those not familiar with the basic sciences. Those who do not understand that dissipating the heat of compression is a large loss of energy and that storing the heat of compression is expensive and inefficient.
And do not understand that partially overcoming these deficiencies is complex and expensive.
It is not worth adding this complexity to an electric, a hybrid or an ICE car.

That's why it is not done, there is no conspiracy or widespread ignorance here.

Engineer-Poet

Air compression is not necessarily inefficient, so long as the heat of compression is either reclaimed or replaced from waste heat.  That seems to be what the air-hybrid concepts do.  If the amount of energy stored is small, the tanks and regenerators are of reasonable size and weight.  The high specific power of the air cycle gives better performance than small hybrid-electric systems, and reclaimed waste heat can even boost thermal efficiency.

What an air hybrid cannot do efficiently is use off-vehicle power or external compressed air.

Roger Pham

Compress air motor's power drops rapidly with decrease in air pressure, linearly or even more rapidly if one does not lengthen the pressure cutoff in response to dropping pressure. With low energy storage capacity, we can see the power boost from an air motor will be quite unsatisfactory, since the power boost kept sagging when the car tries to accelerate.

By contrast, battery's voltage remains near maximum up to 90% of battery capacity, and battery holds much more energy, meaning that a battery's power boost will "keep on coming" until the car reaches its maximum speed potential.

If one keeps the air tank at low pressure at cruise speed in order to recuperate braking energy, then there will be hardly any power boost available from the air motor when cruising the highway and wanting to pass an 18-wheeler ahead. This means that the engine cannot be downsized much, and highway mpg will suffer. If one keeps high pressures in the air tank, then braking energy recuperation will be very limited. Of course, the vehicle can be provided with a "city mode" and "hwy mode" buttons to decide how much pressure to keep in the air tank, but most people may not be able to remember to push the buttons.

By contrast, an HEV can cruise the hwy at 60% battery capacity to provide a sustainely-powerful boost up to the car's speed limit, and still able to recuperate braking energy by charging the battery to 70-80% of maximum SOC. For a 3000-lb car, the energy recuperable from 60 mph is ~0.11 kwh, and the Prius has a 1.3-kwh battery.
By contrast, the energy available for 60 liters of air at 20 bars when expanded isothermally is only 0.09 kwh. An air tank larger than 60 liters or greater than 20 bars would be impractical.

However, the most important reason to go HEV is to be able to crank out PHEV's quickly on the same platform, and to develop facility to produce future FCV's based on similar electrical platform.

Engineer-Poet

60 liters of storage only requires about 8 meters of 10 cm ID tubing.  There are lots of places in a car where this could double as structural members.  Further, the expansion will NOT be isothermal; there will be either regenerated compression heat or engine exhaust heat.

The issue of highway passing is not so big either.  The compressed air supplies stop-start, regenerative braking and low-speed augmentation (the "intake" stroke can be a power stroke supplied by compressed air, providing 1 power stroke per revolution).  High-speed power can come from turbocharging.

I agree that the ultimate outcome is electric drivetrains (but I think the FCEV is dead; it'll be BEV).  The immediate problem is that the industrial base to build piston machinery is here and base for motors and controllers and batteries isn't.  If we can squeeze more out of the existing base of engine plants we'll be better off.

SJC

"For a 3000-lb car, the energy recuperable from 60 mph is ~0.11 kwh, and the Prius has a 1.3-kwh battery."

I believe that those numbers are close, however you are trying to put 30 kw into a 1.3 kwh pack over the 10-12 seconds that you are decellerating. A 20C charge rate may not get all the energy that braking can offer.

Roger Pham

Right. The power input to the battery will be 33 kw minus efficiency loss in the generator, should be 90+% efficient. This is higher than the rated 21 kw power of the battery.

SJC

I have no idea how much braking energy that a Prius captures on average, I just think an air system can capture a lot without straining the batteries on capture or takeoff.

Roger Pham

Ah, please calculate the displacement (hence the size) of the air compressor/motor and the rpm required to absorb 33kw of power, before you can conclude that "an air system can capture a lot..." A separate air compressor will have a hard time fitting inside the hood together with the engine, if you make it larger than the engine.

An engine doubles as an air compressor will have a hard time absorbing that much power (33kw) even if the valves are configured to run on two-stroke compression cycle. Calculate the engine MEP vs. rpm to calculate hp and do the same for the air compressor. The dead space on the cylinder head will greatly reduce volumetric efficiency when the pressure builds up. For example, if the CR is 10, when your pressure ratio is approach 20 or so, your volumetric efficiency will be reduced to ~1/2 for near-adiabatic compression, or even more, to 1/3 of initial displacement, for polytropic compression. You will need a continously-variable transmission to fine-tune the engine rpm used for braking energy recuperation. The engine's rpm will be much higher during braking recuperation because the cylinder's MEP will be a lot lower (1/3 to 1/2) than during actual combustion. This means a significant level of engine friction that will rob a lot of energy. Don't be surprised if 1/3 of the car's kinetic energy will be lost in engine/compressor friction.

OTOH, in a Prius, just install a 33-kw battery or higher power, if you want to capture braking energy faster. A Lithium battery can provide 2.5 kw/kg, (up to 4 kw/kg for Hitachi Lithium). 33 kw would only require 13.2 kg of battery (29 lbs). An electric motor/generator has negligible friction.

Roger Pham

PS: Engine's friction increases as the square of the rpm. Doubling the rpm will quadruple the friction.

Engineer-Poet

Roger,

It's interesting that you mention power and compression ratios and such.  Compression brakes are very good ways to dissipate power, and an Otto or diesel engine is essentially an air compressor.  Last, the power required for a given braking force is proportional to the vehicle speed; you don't need the same power at 5 MPH that you need at 25, and friction losses in the compressor are certainly acceptable if the alternative is to use friction brakes.

Let's consider an example familiar to me, the 1.9 liter VW TDI engine.  This has a compression ratio of 19.5:1 per Wikipedia.  Using the simplifying assumptions of γ (Cp/Cv) of 1.40 [1], no heat losses and 100% volumetric efficiency [2], the pressure ratios would be 13.7 at 2/3 of the air expelled, 24.2 at 50%, and 36.3 at 1/3.  The air leaving the compressor at the 1/3 fraction would be at 533 PSIA (over 500 PSI gauge pressure).

The net work done per cycle is the work of the upstroke minus the work recovered in expansion of the residual air volume.  The upstroke is divided into the compression phase and the isobaric expulsion phase.  The net work on 1.9 liters of intake air volume is -629 J/rev at 6.5:1 effective compression ratio (1.4 MPa outlet pressure), -597 J/rev at 9.75:1 ECR (2.5 MPa outlet pressure), and -466 J/rev at 13:1 ECR (3.7 MPa outlet pressure).  The net crankshaft work doesn't start falling off a cliff until you get well over 500 PSI at the outlet (it's down to a bit over half at 15:1 ECR).

This outlet air would be hot.  Much of its energy can be stripped out as heat before it goes to the storage tankage, reducing its volume by more than half.  This energy can be stored in a regenerator and put back when the air is taken out again.

Suppose we have a car like my Passat, cruising along fully loaded at 2000 kg at a speed of 45 MPH.  We need to brake it to a stop in 10 seconds, for an average braking power of 40.5 kW and an average braking force of 4.02 kN.  To pull 40.5 kW at 629 J/rev requires 64 rev/sec or 3860 RPM.  This is clearly not going to work at full speed (twice the average power) but at lower braking rates or lower speeds (25 MPH) it is quite feasible to put all the braking power through the engine and store it as compressed air.  Above an effective compression ratio of 12:1, the braking power can be doubled or more by dumping the remaining air charge out the exhaust after the piston reaches TDC ("Jake brake").  This will be at the cost of some noise, but can also drive a turbocharger and increase the intake air pressure and braking power further.

For launching the car from a stop, there are two options:

  1. Use the engine as a 2-cycle expander, running on air alone.
  2. Use stored air to drive the intake stroke as a power stroke, also supplying extra air charge for boost pressure all the way from idle speed.
These options could let the car "creep" silently for a distance, or launch very quickly even with a small engine.  The two options could be selected with a Sport/Economy switch.

[1] Assuming the polytropic gas law for an ideal gas, Pv^γ = C
[2] Volumetric efficiency can exceed 100% if there is sufficient effect from tuned intakes.  I'm assuming 2-cycle compression so there will be no exhaust to operate a turbocharger.

Roger Pham

Thanks, E-P, for your detailed analysis, which addresses the issue quite accurately.

Yes, it is possible to stop the car quite rapidly using engine braking alone, although that will require higher engine rpm's. In the case of your Passat 1.9 liter diesel engine with higher CR, more effective braking can be had at a relatively lower rpm of 3860. However, if the CR is only 10, I'm quite certain that the rpm required will exceed the engine's redline. It's the high rpm's that creates high engine friction loss and hence, reducing the recuperable energy if braking at high rate. This is to address SJC's contention that air-hybrid can recuperate more energy during rapid braking than an HEV. All that's required in an HEV for rapid braking recuperation is stronger battery and stronger power electronics.

"Jake Brake" is great for engine breaking but not for energy recuperation.

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