Given that the traditional four-stroke Otto-cycle engine piston engine only has a thermal efficiency of 25-30 percent, there is clearly still plenty of room for improvement. While most of the green automobile attention in recent years has been focused on electrification, liquid fuels still have about 100 times the energy density of today's best lithium-ion batteries, a difference that probably won't change significantly any time in the near future.
With that in mind, there is still plenty of effort being expended on improving the humble internal combustion engine. These efforts range from completely different architectures like EcoMotors' opposed piston opposed cylinder (OPOC) to new combustion processes such as homogeneous charge compression ignition (HCCI). One of the more interesting combustion-related developments comes from a California-based startup known as Transonic Combustion. In 2007, the company was claiming it could get an ICE vehicle to 100 miles per gallon. A lot has happened since then, and we finally have a better idea what the company's technology is. We sat down with CEO Brian Ahlborn to learn more about what the company is working on, and you can read all about it after the jump.
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The heart of Transonic's technology is a new fuel delivery system conceived by company founder Mike Cheiky. Cheiky's idea was to get the liquid fuel into a supercritical state before injecting it into the combustion chamber. Traditionally, matter has been thought of as having three states, solid, liquid and gas and any given material can exist in one of those at any point in time depending on the temperature and pressure. Fuels like gasoline and diesel generally only burn after they are vaporized.
The supercritical state is essentially a fourth phase of matter that lies between liquid and gas that has properties of each as well as unique properties of its own. Achieving a supercritical state requires raising the temperature above the boiling point of the fluid while also increasing the pressure. According Ahlborn, the supercritical fluid can burn much faster than it can in a "normal" gaseous state, something that provides a number of advantages with respect to efficiency and emissions.
There are two major aspects to Transonic's technology, the fuel preparation and the direct injection system. The fuel delivery system is an evolution of current direction injection systems that use a common high-pressure (200-300 BAR) rail to deliver fuel directly to each each combustion chamber through individually controlled injectors. Before fuel is injected, the preparation system gets it into the supercritical state and this, according to Ahlborn, is where the "secret sauce" lies.
Ahlborn was reluctant to get into too many details of its proprietary system, but did reveal that the fuel temperature is increased from about "100 degrees centigrade to approximately 350-400." The fuel is also catalyzed, and although Ahlborn again declined to be specific about exactly what this means, he did respond to our query with, "I wouldn't necessarily draw the conclusion that we heat it in the presence of a catalyst." Ultimately, the goal is to have the fuel "be better prepared for an optimal combustion" says Ahlborn.
In a traditional piston engine, up to one-third of the energy of combustion is lost to heat transfer through the cylinder walls and into the coolant. One way to reduce some of this energy loss would be to have the actual combustion concentrated closer to the center of the cylinder and away from the walls. The claim from Transonic is that the faster burn rate of the supercritical fuel consumes the fuel before the flame front gets to the cylinder walls, thus reducing the heat transfer. In this way, more of the available chemical energy in the fuel can be transformed into mechanical energy to push the piston down.
With traditional fuel delivery systems, ignition typically occurs while the piston is still rising up in the cylinder, leading to pumping losses as the expanding gases push back against the piston. The faster burn rate of the supercritical fluid would make possible to delay ignition to either top dead center or afterwards, thus reducing those losses. Transonic's current prototype engines use compression ignition, like a diesel, while running on regular 87 octane gasoline. However, unlike homogeneous-charge-compression-ignition (HCCI) engines, they require no spark-plug or cylinder pressure sensor. As with many other details, Ahlborn declined to reveal the compression ratio being used in the prototype Transonic engines, although it's believed to be about 15:1.
The Transonic system also allows the engine to run at air-fuel ratios that are, in Ahlborn's words, "much leaner than conventional," going as high as 80:1. Normally, such lean air-fuel ratios can lead to combustion temperatures that rise above 600 degrees C, which in turn leads to the production of nitrogen-oxides. This is exactly what happened in modern diesel engines as the air-fuel ratios got leaner, in part to reduce particulate emissions. Ahlborn declined to get specific about the combustion temperature but acknowledged that it is below the NOx generation temperature and the engineers are doing some "neat tricks" to keep it there.
Transonic has consistently claimed that its engines are able to meet all current Tier 2 Bin 5 emissions limits without resorting to the expensive and bulky particulate filters and selective catalytic reduction systems required on contemporary diesel engines. The only after-treatment required by a Transonic-equipped engine is a conventional three-way catalytic converter. Supercritical fluid fuel injection is also claimed to be compatible with a range of fuels including gasoline, diesel, ethanol and butanol. While the engines have been tested with multiple fuels, most of the ongoing work is focused on optimizing for gasoline since the retail infrastructure is the most prevalent.
While Transonic's approach will obviously slash the cost of the exhaust after-treatment, it's unclear how much of a price premium the fuel pre-treatment will add. According to Ahlborn, the system is still being optimized for production and the engineers are continually reducing the part counts. As with many other aspects of the design, details were scarce.
Transonic has seven engine dynamometer cells at its Camarillo, CA facility and has purchased a number of engines from various automakers that have been modified with its fuel system. The engineers have been able to push the supercritical fuel system to a 25-30 percent improvement in fuel efficiency over the base gasoline engines. In order to validate its own internal test results, Transonic shipped several stock engines plus two modified engines from automakers to a third-party engineering test lab in Detroit earlier this year. The results from the un-named lab achieved a high degree of correlation with those from Transonic. In fact, Ahlborn says that the emissions results achieved both internally and at the outside lab were better than the initial predictions. Subsequent testing and analysis has allowed the engineers to better understand the properties of the supercritical fluid and why it achieved those results.
In addition to engineers and designers that are working on building and developing prototype hardware and control systems, Transonic has 10 PhDs working on mathematical models of supercritical fluids, the fuel preparation components and the injectors. These highly sophisticated models are needed for up-front analysis of component sizing, flows and calibration before prototype parts are produced. So far, Transonic has built and tested between 500 and 1,000 injectors from which they have collected data for the modeling process. Ultimately, using the simulation models should cut the lead time for new product applications from two or three years down to just six months.
While the bulk of the development work has occurred in its own labs and independent of customers, Transonic is working with three different automakers to test prototypes based on modern current-generation engines that have sufficient real-world data to provide a good baseline. Ahlborn explains that he is trying to keep his team focused on the the R&D required to get a viable, robust product to market as soon as possible. However, keeping some potential customers in the loop will also provide a sanity check on their work to make sure that what they create is commercially suitable from a cost, performance and packaging standpoint for different applications. There is always a risk when sharing too much information too early, but Ahlborn feels that the potential benefits in this case are worth it.
Ahlborn's self-proclaimed "big-hairy-audacious-goal" is to have Transonic go into business as a supplier of fuel systems to the auto industry by the 2014-15 time-frame. Given the three to five year lead times required to bring a product to market in this industry, that doesn't leave a lot of time for an automaker to commit to a program with Transonic. Ahlborn is well aware of the difficulty of meeting his target, but he believes the internal combustion engine, "is a long-term product for many decades still to come" and says, "we believe there is a quantum leap breakthrough in what we're doing" and that, "there will be a lot going on commercially next year (2011)."
Evidently Ahlborn is not alone in that belief. Transonic has been able to attract a substantial amount of venture funding from Vinod Khosla and, in May of this year, the company enticed retired General Motors executives Bob Lutz and Don Runkle to join its board of directors. Runkle's presence is particularly interesting since he also currently serves as the CEO of Ecomotors. There's been no public discussion of combining supercritical fluid injection with the Ecomotors OPOC architecture, but there doesn't seem to be any reason it couldn't be done.
Transonic Combustion still has a long road ahead of it to prove that it can beat the fuel efficiency of a diesel engine with cleaner emissions and a lower cost. Much more detail and public testing will be required to validate the company's claims, but this seems like one to watch.