A Fundamental Study on CH* and OH* Flame Emissions as Indicators of Heat Release and AFR at Engine Relevant Conditions of Temperature and Pressure

Over 95% of the UK energy needs utilize combustion, so advanced combustion monitoring and measurement techniques are essential to the development of clean and efficient engines, boilers and furnaces. Flame chemiluminescence, which is light emitted from intermediate (radical) chemical species formed during combustion, has long been thought to hold detailed information about the chemical processes occurring in the reaction zone, such as the local air-fuel ratio and fuel consumption rate. However, there is conflicting information within the published literature as to the interpretation of chemiluminescence measurements as indicators of local heat release rate and air fuel ratio - parameters that are of tremendous interest and importance to combustion researchers of engines and burners. Accordingly, the overall objective of this work is to determine whether the chemiluminescence intensity emitted from two radical species that commonly occur in hydrocarbon flames (OH* and CH*) can be used as a measure of heat release rate under 'engine relevant' conditions, and also whether their relative intensities can be used to determine the air fuel ratio.Fundamental combustion experiments will be performed in a spherical vessel with central ignition and windows - otherwise known as a combustion bomb. Homogeneous mixtures of air, fuel, and residual combustion gases (representative of the range of conditions typically encountered in combustion engines) will be prepared and burnt in the vessel. The resultant OH* and CH* light emissions will be recorded by photomultiplier tubes fitted with appropriate spectral filters. The combustion bomb method is highly advantageous for this work in many respects: after ignition, a spherical flame propagates radially through the mixture compressing the unburned gas ahead of the flame front. Thus, flame chemiluminescence data is obtained for a sequence of linked temperatures and pressures from a single experiment. The combustion bomb allows data collection across a wide range of pressure and temperature (pressures up to 30 bar and unburned gas temperatures up to 850 K can be investigated). Moreover, by varying the initial temperature and pressure, the effects of pressure and temperature can be decoupled and correlations generated for the effect of temperature and pressure on the OH* and CH* chemiluminescence intensity. The Internal Combustion Engines Group (ICEG) at Oxford University has substantial experience (over 16 years) with combustion bomb experiments. The experience gained from these previous works has led to 3 notable innovations in the field:+ The use of free-fall experiments to eliminate the effect of buoyancy.+ The introduction of a multi-zone combustion model for data analysis, so that the effect of dissociation and the temperature gradient in the burned gas (typically 500 K) is incorporated into the analysis of flame front position and pressure rise.+ The use of 'real residuals' by retaining part of the previous combustion event as residuals, as opposed to the conventional approach of using a fixed composition N2/CO2 mixture to represent the residuals.Compared to engines, the combustion bomb provides a simplified (but not simple) experiment with combustion that can be accurately controlled and analysed - unlike an engine the temperature of the unburned gas can be calculated accurately, there is no moving piston, and no heat transfer to the enclosure before the end of the experiment.No major hardware purchases are required for the work. The existing combustion bomb facility, which uses a comprehensive LabVIEW interface for setting-up the experimental conditions and data logging, is available, as are photomultiplier tubes and a well-validated multi-zone combustion model and previously developed MATLAB routines for data analysis.

Despite the many recent advances in alternative powertrain technologies, the internal combustion (IC) engine fuelled with liquid hydrocarbon fuels (including biofuels), is expected by most industry observers to retain its position as the dominant prime mover for road transportation in the foreseeable future (i.e. up to 2050). CO2, a greenhouse gas that is widely believed to be responsible for recently observed changes in the global climate, is a fundamental combustion product of hydrocarbon fuels. The potential damage to public health and the economic cost of climate change is now well recognized. The 'STERN REVIEW: The Economics of Climate Change' concludes that the scientific evidence is now overwhelming: climate change is a serious global threat, and it demands an urgent global response. The overall costs and risks of climate change will be equivalent to losing at least 5% of global GDP each year, now and forever. In contrast, the costs of action - reducing greenhouse gas emissions to avoid the worst impacts of climate change - can be limited to around 1% of global GDP each year. (http://www.hm treasury.gov.uk/sternreview_index.htm HM Treasury 2006 ISBN number: 0-521-70080-9). Presently, road transport accounts for 21% of the CO2 emissions in the UK. It is clear therefore that there are significant economic and public health benefits to be gained from the development of improved combustion engine technologies that lead to improvements in fuel economy and the reduction of exhaust emissions. The urgency with which these technologies are required is also clear. The proposed research is focused on the development of a diagnostic technique for combustion engine research. It is vital for the automotive industry to maximise the energy efficiency of new engines in order to reduce greenhouse gas emissions. To achieve this goal will require an increasingly detailed knowledge the fundamental combustion properties of hydrocarbon fuels and of the processes (air-fuel mixing, emissions formation, heat release, etc.) that occur within the combustion chamber of an IC engine. Improved combustion diagnostics are prerequisite to attaining the required level of understanding. The potential impact of the proposed research is therefore significant with the potential beneficiaries including: The combustion research community, who as a whole will benefit from experimental data over much wider ranges of temperature and pressure than are presently available (since most existing measurements have been made at close to ambient conditions). These data can be used for: + Validation of computer models of combustion that predict excited state intermediate species. + Determining the effects of combustion variables (pressure, temperature, equivalence ratio) on OH* and CH* flame chemiluminescence. The engines research community who will benefit particularly as, for the first time, chemiluminescence intensity data will be made available under `engine relevant' conditions and will include the potentially important influence of residual gases. In turn, this data will be used to: + Develop new diagnostic tools for combustion research, and + Significantly improve the interpretation of existing data on flame chemiluminescence in engines Society who will benefit when the data is used to develop cleaner and more efficient combustion of fuels used in vehicle engines, furnaces and boilers, and aircraft engines. All data will be archived on a WWW based system, and this will include both the raw experimental data, and the results from data analysis and modelling. The potential impact of the research will be maximized by the publication of the results in the open literature and by the presentation of the work at an international conference.