Transonic was exhibiting at the Department of Energy’s ARPA-E Energy Innovation Summit in Washington earlier last week with a supercritical fuel injection system that can improve the fuel economy or gas mileage of internal combustion engines by between 50-75%.

That’s almost unbelievable, but stay with me.  The Transonic fuel injection system is based on supercritical fluid principles, that is any substance at a temperature and pressure above its thermodynamic critical point. The TSCi Fuel Injection System (Transonic Super Critical Injection) comprises new fuel injectors that take the fuel charge to a supercritical state just prior to its direct injection at Top Dead Center (TDC) of the cylinder.

TSCi Fuel Injection Operation. Click image for more info.

Michael Frick, Transonic Vice President for Engineering explains, “A supercritical fluid is basically a fourth state of matter that’s part way between a gas and liquid.”  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.

Supercritical fluids have unique properties. To begin, 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.  Also supercritical fluids have 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.

Transonic’s TSCi gasoline fuel charge enters the cylinder at around 400 °C—compared to about 100 °C for a conventional liquid direct injection fuel charge—at precisely Top Dead Center (TDC, 0° crank angle). The supercritical charge facilitates short ignition delay and fast combustion, with the energy released focused just on pushing the piston down. The fast combustion minimizes crevice burn (between the piston and cylinder wall) and partial combustion near the cylinder walls, and prevents droplet diffusion burn. The TSCi system supports more efficient engine operation over the full range of conditions—from stoichiometric air-to-fuel ratios at full power to lean 80:1 air-to-fuel ratios at cruise.

Transonic Technology Injector Demonstrator

So what’s actually happening?  The injector is acting much like current technology except that the fuel is heated to the supercritical level.  At such temperature the injection brings along the heat needed for injection, the fuel(s) are already in a near gas state with almost no liquids to burn, thus the ignition is very fast and the fuel needed can be metered to provide the needed pressure in the chamber to achieve power.  At low power one might suspect the flame front doesn’t get to the cylinder wall before the fuel is consumed.  Thus the making of excess heat, the major share of efficiency losses is minimized if not avoided.

The TSCi system puts the supercritical fuel in place where most of the combustion in the hot eddy of gas forms at the center of a standard diesel cylinder chamber.  By changing the ignition delay so that the fuel is ignited in the close confined area, the flame can be kept away from contact with the walls, which take heat out of the engine. Transonic has 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.

Mike Rocke, Transonic Vice President of Marketing and Business Development says the TSCi systems are “almost drop-in” units, including “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.

Software fuel management and control is key to facilitating the extremely fast combustion and is provided by advanced microprocessing technology. The TSCi injection system can also be supplemented by advanced thermal management, exhaust gas recovery, electronic valves, and advanced combustion chamber geometries. Through the use of its software, the TSCi system can optimize the use of any combustion chamber geometry or piston bowl shape.

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 fueled engines.

There is a wealth of unanswered questions.  Most obvious is that the engine will need built to diesel strength.  Then the more obscure such as will air throttling be needed?  One wonders how well the injectors might last, the fuel pressures involved, and the fuel heating system and its lifespan as well.

Transonic’s own testing using a mid-size vehicle on a chassis dynamometer resulted in EPA highway fuel economy of 64 mpg (3.7 L/100km), with city testing being finalized this year, estimated at 47 mpg (5.0 L/100km). Compare that to the 2010 Prius which delivers 51 mpg US (4.6 L/100km) EPA city and 48 mpg (4.9 L/100km) highway. According to Rocke the TSCi system would add about $1,500 to the cost of the vehicle in substituting TSCi for a current technology injection set, compared to about a $4,000 expense for a current full hybrid solution.

One can expect if Transonic can use a business model that runs to volume over high individual fees, the adoption and further innovation and improvements could come fast and furious.  Lets hope so.


8 Comments so far

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