Researchers at Syracuse University (New York) have developed a method to prepare, inject and combust supercritical (SC) diesel fuel. The central part of the method is a new fuel system including high-temperature fuel injectors and a common rail to deliver SC fuel-diluent mixtures for combustion over ranges of conditions which will significantly improve engine efficiency, reduce PM and NOx and mitigate the environmental thermal impact. Heat required to bring the fuel to SC states is recovered from the exhaust gas of the engine. George Anitescu from Syracuse presented the work in a poster at the DEER 2009 conference in Dearbon, Michigan.
A supercritical fluid is any substance at a temperature and pressure above its thermodynamic critical point. The injection and combustion of supercritical fuels is also the core of Khosla-backed startup Transonic Combustion’s technology. (Earlier post.)
Given the high reactivity of diesel fuels compared to gasoline, the ignition and combustion of pre-mixed fuel-air charges poses a challenging control problem. To address this, the Syracuse team proposes the injection of a SC fuel into SC air followed by a near instant mixing before ignition. At SC conditions, however (T≤450 °C, P ≥600 bar), the fuel cokes before injection.
To attenuate that risk, the researchers use a diluent such as CO2, H2O or exhaust gas.
The mixing of SC fuel-diluent with SC air occurs near instantaneously due to very high molecular diffusion in SC fluids. The SC combustion is homogeneous and nearly complete within the reaction space and is faster and cleaner than traditional droplet combustion because high reaction efficiencies are due to the complete solubilization of the fuels and oxidation products within the bulk oxidant.
To date, experiments were performed to prepare homogeneous fuel-diluent SC mixtures, which were characterized for phase transition, density, solubility, viscosity and mass/energy balance. Diffusivity experiments and design/constructing of a SC fuel injector are in progress.—Anitescu et al. (DEER 2009)
The Syracuse team expects that the implementation of their concept—which is patented—will enable:
- Near complete combustion of diesel fuel and recovery of up to 50% of the exhaust heat;
- elimination of ~80% of criteria pollutants and a significant reduction of the air thermal impact;
- downsizing (smaller combustion chambers for the same torque and power);
- minimizing the parasitic pumping of large excess air; and
- the elimination of after-treatment systems.
Transonic’s TSCi technology. While keeping detail about its technology restricted to signers of non-disclosure agreements, Transonic has provided an overview of its approach on its website.
The TSCi Fuel Injection system comprises new technology fuel injectors, next generation electronic control unit, high efficiency fuel pump, pressure accumulator, and advanced engine control software. The technology is optimized with modern high compression diesel architecture engines, near-term running on gasoline while longer-term utilizing advanced low carbon footprint bio-fuels.
A key feature of our technology is a new type of fuel injector that utilizes supercritical-state fuel. Supercritical-state fuel facilitates short ignition delay and fast combustion, precisely controls the combustion that minimizes crevice burn and partial combustion near the cylinder walls, and prevents droplet diffusion burn. The engine control software facilitates extremely fast combustion, enabled by advanced microprocessing technology. The injection system can also be supplemented by advanced thermal management, exhaust gas recovery, electronic valves, and advanced combustion chamber geometries.
Transonic says that its fuel system supports 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, with engine-out NOx at just 50% of comparable standard engines. Real-time programmable control of combustion heat release results in dramatically increased efficiency.
Along with operating on gasoline, the Transonic technology can utilize fuels based on their chemical heat capacity independent of octane or cetane ratings. This allows the use of economical, highly functional mixtures of renewable plant products which are not practical in either conventional spark or compression ignition engines.
Transonic has successfully run on gasoline, diesel, biodiesel, heptane, ethanol, and vegetable oil in dynamometer testing. Recently, engineers achieved seamless operation alternating between several different fuels on a customer’s engines in its Camarillo test facilities.
George Anitescu, Ronghong Lin, Lawrence Tavlarides (2009) Preparation, Injection and Combustion of Supercritical Fuels (DEER 2009)
George Anitescu, Lawrence L. Tavlarides, and Dan Geana (2009) Phase Transitions and Thermal Behavior of Fuel-Diluent Mixtures. Energy & Fuels 23, 3068–3077 doi: 10.1021/ef900141j