Turbine cars seemed inevitable in 1963, when 50 Chrysler Turbine demonstrators hit the streets. Low parts count, reduced maintenance, and absolute smoothness were the selling points, but by 1979 efficiency and emissions woes scuppered the automotive turbine.
Now, micro-turbine range extenders like those in Jaguar’s C-X75 and Mitsubishi’s MI-Tech concepts are beginning to hit the road. What’s more, there’s a new patent-pending rotary-engine concept that promises to combine turbine- and piston-engine advantages for even greater efficiency and lower emissions.
In a traditional turbine, burning fuel isn’t contained and forced to act on a crankshaft. Rather, expanding gases blow against fan blades to rotate a shaft, and excess air provides cooling.
However, Astron Aerospace’s new trademarked Omega One rotary engine contains the combustion like in piston engines, improving torque and simplifying emissions control while retaining turbine advantages like air cooling, low parts count, compact size, and smooth operation. The Omega is primarily meant for aviation, but an EV range extender application is being codeveloped. It incorporates various Technologue “greatest hits” ideas, including June 2006’s split-cycle concept of conducting compression and combustion in separate spaces, and homogeneous-charge compression ignition (HCCI).
Imagine a smooth gear featuring a single, wide tooth rotating against another perfectly smooth gear incorporating one notch to fit the first gear’s tooth. These rotate in a housing machined to fit tight (0.001-0.005-inch clearance) to the surface of the notched gear and to the outer edge of that single tooth. Side plates complete each enclosure with similar operating clearance to the rotors. The tooth is not expected to need seals, dramatically reducing friction.
In one gear pair, the tooth separates the intake and compression volumes; in the other, combustion and exhaust. The compression one is about a third larger, which allows it to “supercharge” the combustion chamber via a small air storage tank that sits between these rotor pairs. Blow-off valves regulate intake pressure (180 to 320 psi is the expected range).
Intake air enters the combustion chamber when a port in a rotating plate in the compression tank aligns with a hole in the rotor side plate, positioned where the combustion tooth emerges from its notch to begin another rotation. As compressed air enters this large and unrestricted port, fuel is supplied by port and/or direct injectors that provide stratified charge (rich near the plug) at lower rpm or homogeneous charge at higher rpm. Spark-assisted HCCI is possible up to 10,000 rpm.
Once combusted, the expanding gases push the back side of that tooth most of the way around its circular path, providing loads of time to extract work from the fuel. Then the exhaust leaves through an open port near where the tooth re-enters its notch, pushed by the front side of the tooth on its next pass. This port can be ducted to a turbo and/or a catalyst.
In the computer, this 14 x 15 x 23-inch engine whirrs out 600 hp at 15,000 rpm and 1,000 lb-ft, achieving 80 percent thermal efficiency(!). Cooling air from a turbinelike fan blows through the large, hollow shafts supporting the rotor pairs and past the finned outer housing. Only these shafts’ bearings are lubricated, so the combustion chamber is never exposed to lubricant. And the HCCI flash combustion plus low combustion chamber surface area should lower engine-out NOx sufficiently to eliminate the three-way catalyst, which doesn’t work with excess air. There will be excess air, especially under low-load conditions in “skip-fire” mode, when the engine doesn’t fire every revolution.
These are Astron’s claims. But enough smart people (including some at Los Alamos Laboratory) saw sufficient merit in the concept to fully fund the venture in a matter of days.
Now comes the hard part: building and operating a proof-of-concept engine. I’m concerned about the lack of seals and lubrication of the rotors operating at relatively close tolerances, and I worry that parts exposed to varying amounts of heat will expand at different rates, causing interference. Exotic materials and tight tolerances can be budgeted into a pricey aviation engine, but can this be made to work on an automotive range extender budget? Color me intrigued and optimistic about the aviation engine’s future, a bit more skeptical of its automotive prospects.
More on the Astron EV Range Extender
- The combustion chamber size is about 250cc, though the expansion chamber is vastly larger (akin to a 12-inch piston stroke).
- Like a turbine, it can run on a variety of fuels, and it always runs unthrottled.
- Pumping energy is very low with this design. Because the engine runs unthrottled, there’s no air-pumping work against a vacuum at low loads; the lubrication needs are modest because only the main shaft bearings are lubricated, and air-cooling means there’s no coolant to pump.
- Ignition pulses in this rotary result in minimal vibration, so skip-fire mode (in which at 10,000 to 15,000 rpm it might only fire every 500 revolutions under low load) is not noticeable the way cylinder deactivation is in a piston engine. And routing the surplus air pressure back into the intake during such operation reduces compression work.
- The pressurized air enters the combustion chamber at near sonic speeds, which helps atomize and thoroughly mix port-injected fuel at higher engine speeds.
- The number of parts is roughly equal to that of a one-cylinder lawnmower engine.
- Scalable: The Astron Omega can be shrunk, expanded, or compounded by adding more rotor pairs, and there’s a taller “three rotor” design, wherein a larger main rotor gets two teeth instead of one with smaller upper and lower discs, each having a single notch timed to receive the two teeth, meaning two combustion and exhaust events can happen during each revolution. Astron’s computers suggest a 250-pound three-rotor aviation version like this might produce 4,500 hp.
- Astron officials reckon that such flexibility of size and output mean that this literally revolutionary new engine might conceivably replace nearly any current powertrain. They also suggest it might serve as primary motivation (rather than just electricity generation), and that in such fitments its linear power delivery and broader rev range might even negate the need for a transmission. I’m more skeptical of this assertion given the 18,000-rpm speed range and the fact that all the power is at the top end (the opposite of an electric machine), in light of the fact that the 60,000-rpm Chrysler Turbine still relied on a three-speed TorqueFlite automatic.
- To reduce manufacturing costs of this close-tolerance engine, Astron envisions employing wire-cut electrical-discharge machining (WEDM—also known as spark eroding), wherein an incredibly thin electrically charged brass wire erodes metal electrically. This strikes me as a slow, low-volume manufacturing solution. Cost: Astron CEO Matthew Riley estimates the automotive range extender could be produced for about $1,000.
A Bit More About the Aviation Version
- The rotating masses are made of titanium, versus aluminum in the automotive engine.
- The bearings are ceramic versus conventional Babbitt bearings (and ceramic might coat some other parts, as well).
- Peak speed is 40,000 rpm, versus 18,000 for the automotive one.
- Power and torque are much higher, and the cost is more like $50,000 per engine, which Riley notes is a comparative bargain in the personal aviation engine market (assuming that price estimate holds after the Omega completes the extensive reliability/durability/maintenance-scheduling testing required for aviation certification).
- A big advantage to this design over the traditional turbine is its ability to filter the intake air, which makes the Omega engine vastly more tolerant of sandstorms and less vulnerable to bird strikes.