Smart fuels - photochemical ignition control

Smart fuels represent a radically new concept of sustainable energy carriers whose novelty lies in their ability to adapt their combustion properties upon irradiation with light. They promise significant energy savings and lower emissions from aircraft and automotive propulsion by permitting the use of advanced combustion technologies. A combination of photochemistry and combustion kinetics is hereby used to solve the problem of combustion control. The outcome is a smart fuel that can adapt its chemical properties in real time and will serve as a high-value sustainable transportation fuel. The type of smart fuel that was investigated in this study uses ignition control chemistry achieved through photochemical ring-opening of fuel molecules. This means that a fuel contains a component that is sensitive to light (UV light in this case) and will change its structure when irradiated. The reaction is similar to that which permits the human body to form vitamin D through exposure to sunlight. SINTEF Energy Research has been working on fuel production, while SINTEF Industry performed the analysis and NTNU the engine tests. Some people believe that the era of combustion engines is soon over and the transport sector will be completely electrified. However, combustion of liquid fuels will for a long time be needed in shipping, aviation and long-haul transport, where electrification will be difficult to realise. It is important that combustion in the future occur in the most optimal way to minimize stress on the environment. At the same time, it is evident that further technical optimizations of the combustion process in form of an engine control unit will be difficult without introducing new concepts such as a method of optimizing also the fuel in run-time, a smart fuel. Biofuels will be very well suited as the basis to produce smart fuels and by using it in highly efficient engines with low emissions, the end use will be viable as well. For some time now, researchers have worked on developing the so-called Homogeneous Charge Compression Ignition (HCCI) engine which combines (and exceeds) the high efficiency of the diesel engine with the low emissions of NOx and particles of the petrol engine. The main challenge related to HCCI combustion is that ignition timing cannot be controlled in the same way as in the diesel engine (by fuel injection timing) or in the gasoline engine (by spark timing). The combustion can only be controlled kinetically, and one way to obtain this is to vary the reactivity of the fuel according to the operating conditions of the engine as proposed here. The HCCI technology is therefore especially suitable to demonstrate the smart fuel concept. What did we do in the Smart fuels project? First, we needed a proof that the chemical reaction is possible. A small UV lamp and small amounts of chemicals (20 mL altogether) were used for this purpose. We mixed model fuel (isooctane and n-heptane) with an active compound that is UV sensitive (cyclohexadiene). The concentration of active compound was 5%. The analysis confirmed that the we managed to produce the compound we aimed at (hexatriene) and the UV light altered the structure of the active compound. The experiments were then scaled up to 150 mL and 400 mL. Finally, the smart fuel was injected in a research engine and changes in ignition were monitored. The analysis confirmed that 15-40% of the active compound is converted to the structure-altered product, which gave 0.5-1.5% product concentration in the fuel. According to the theory visible changes in ignition should appear when the fuel contains only 1% of the product. Engine tests showed that the addition of the reactant increased the ignition delay and reduced heat release rate compared with the control tests. But the product containing fuel had an even longer ignition delay and heat release rate was significantly reduced compared with both the control fuels. The active compound and product also reduced flame intensity observed by a camera. This study has successfully validated the use of a photo-chemical smart fuel to significantly change the ignition quality of a fuel in HCCI combustion mode and demonstrated the concept of an on-board smart fuel applications for internal combustion engines. Further investigations will address the optimization of the active compound conversion efficiency during UV irradiation in order to obtain a practically feasible irradiation timing for on-board applications.

'Smart fuels' represent a new concept of sustainable energy carriers whose properties may be altered and controlled through irradiation with light. This enables further optimisation of the combustion process beyond what is possible with traditional control units in combustion devices, which may be important with increased use of biofuels in the transport sector. The most apparent demonstration of smart fuels is internal combustion engines. Here, smart fuels can be demonstrated to realise Homogeneous Charge Compression Ignition (HCCI) technology. HCCI engines promise 20% fuel savings over state-of-the-art diesel engines and negligible emissions of NOx and soot. The HCCI mode is entirely controlled by chemical kinetic parameters, which has been one of the obstacles for wider use of this technology. With the smart fuels concept, the ignition property of a single fuel is adapted in real-time on board a vehicle with the help of a photochemical reactor. Components which are altered by reaction with light to form new components with different reactivity, are added to a base fuel, e.g. a sustainable bio-derived fuel. This represents a radically new approach, and if implemented in 80% of the EU's light and heavy duty engines used in road transport by 2030, this could contribute a reduction of 4.6% in total EU CO2 emissions. In future aviation turbines smart fuels provide a means to achieve flame stabilisation by auto-ignition. This is expected to result in improved flame stability and safety, as well as drastically lower NOx and particulate emissions. The project goal is to demonstrate the concept by constructing a simplified photo-reactor and prove that the changes in chemical composition and ignition properties of the reacted fuel is large enough to control the ignition timing of a HCCI engine. This includes chemical analysis, ignition property testing, engine experiments and chemical kinetics modelling.