Supercritical fuel injection and combustion

  • 11-Jan-2010 09:05 EST
TSCi liquid injection.jpg

A comparison of standard direct injection of liquid fuel and Transonic’s novel supercritical injection process (as viewed through an optical engine fitted with a quartz window) shows that the new TSCi fuel-delivery system does not create fuel droplets.

Throughout the history of the internal-combustion engine, engineers have boosted cylinder compression to extract more mechanical energy from a given fuel-air charge. The extra pressure enhances the mixing and vaporization of the injected droplets before burning. Recent work by Mike Cheiky, a physicist and serial inventor/entrepreneur, is focusing on raising not only the fuel mixture’s pressure but also its temperature.

Cheiky's aim, in fact, is to generate a little-known, intermediate state of matter—a so-called supercritical (SC) fluid—which he and his co-workers at Camarillo, CA-based Transonic Combustion believe could markedly increase the fuel efficiency of next-generation power plants while reducing their exhaust emissions.

Transonic’s proprietary TSCi fuel-injection systems do not produce fuel droplets as conventional fuel delivery units do, according to Mike Rocke, Vice President of Marketing and Business Development. The supercritical condition of the fuel injected into a cylinder by a TSCi system means that the fuel mixes rapidly with the intake air which enables better control of the location and timing of the combustion process.

The novel SC injection systems, which Rocke calls “almost drop-in” units, include “a GDI-type,” common-rail system that incorporates a metal-oxide catalyst that breaks fuel molecules down into simpler hydrocarbon chains, and a precision, high-speed (piezoelectric) injector whose resistance-heated pin places the fuel in a supercritical state as it enters the cylinder.

Encouraging test results

Company engineers have doubled the fuel efficiency numbers in dynamometer tests of gas engines fitted with the company’s prototype SC fuel-injection systems, Rocke said. A modified gasoline engine installed in a 3200-lb (1451-kg) test vehicle, for example, is getting 98 mpg (41.6 km/L) when running at a steady 50 mph (80 km/h) in the lab.

The new technology, in addition, is achieving significant reductions in engine-out emissions. Some test engines reportedly generate only 55-58 g/km of CO2, a figure that is less than half the fleet average value established by the European Union for 2012. Two automakers are currently evaluating Transonic test engines, with a third negotiating similar trials.

Cheiky founded several biofuel and battery start-ups such as Zinc Matrix Power before establishing Transonic in 2006 to address inefficiencies in standard internal-combustion (IC) engine cycles. After the successful vetting of his radical SC fuel-injection concept at Cal Tech, he succeeded in obtaining funding from Venrock (the Rockefeller family), Canyon Partners, and Silicon Valley technology guru Vinod Khosla.

The 48-employee firm is finalizing a development engine for a test fleet of from 10 to 100 vehicles, while trying to find a partner with whom to manufacture and market TSCi systems by 2014.

“A supercritical fluid is basically a fourth state of matter that’s part way between a gas and liquid,” said Michael Frick, Vice President for Engineering. A substance goes supercritical when it is heated beyond a certain thermodynamic critical point so that it refuses to liquefy no matter how much pressure is applied.

“People might remember from chemistry class that there’s a triple point on the [temperature vs. pressure] phase diagram of water, for instance, at which water exists simultaneously as ice, water, and vapor, but few know that there’s another critical point at and around which a fluid will exhibit gas-like and liquid-like properties,” he explained.

Ignition timing's the key

SC fluids have unique properties. For a start, their density is midway between those of a liquid and gas, about half to 60% that of the liquid. On the other hand, they also feature the molecular diffusion rates of a gas and so can dissolve substances that are usually tough to place in solution.

Additionally, an SC fluid has a very low surface tension. This enables quicker mixing, and it exhibits catalytic activity that is two to three orders of magnitude faster than the purely liquid form of the substance.

Cheiky formulated his concepts following a fundamental analysis of the IC engine, which he felt “offered a lot of low-hanging fruit for improvement,” Frick reported.

“He made the leap of faith that if you eliminate the time it takes to vaporize fuel and the heat lost with contact with the cylinder walls, you could improve the base efficiency of an engine far beyond what would normally be possible to achieve with, for example, a diesel running out of cycle,” Frick noted.

The TSCi system, he explained, uses supercritical fuel to place most of the combustion in the hot eddy of gas that forms at the center of a standard diesel cylinder chamber. Cheiky figured that by changing the ignition delay so that the fuel ignited in that area, the flame can be kept away from contact with the walls, which take heat out of the engine.

It was also decided to limit combustion to within the first 20 to 30 degrees past top-dead center, to make full use of the mechanical energy created by burning while reducing the heat lost to the exhaust.

Use of SC fuels also “negates the various vaporization rates of any fuels that might be burned,” which makes the process fuel neutral, Frick said. And as long as SC conditions are maintained, little time is needed to vaporize the fuel no matter the fuel.

“And because gasoline has a significantly longer ignition delay than diesel, for example, to run gas and for control of the delay we like to use a catalyst to reduce the fuel octane rating,” he said.

13:1 sweet spot

To minimize friction losses, the Transonic engineers have steadily reduced the compression of their test engines to between 20:1 and 16:1, with the possibility of 13:1 for gasoline engines.

“There may be some advantage to going a little higher, but we’re trying to keep the fuel system within the range that OEMs understand,” Frick concluded. “The combustion process is already radical enough.”

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