Low Temperature Combustion with Thermo-Chemical Recuperation to Maximize In-Use Engine Efficiency

Abstract

The key to overcome Low Temperature Combustion (LTC) load range limitations in reciprocating engines is based on proper control over the thermo-chemical properties of the in-cylinder charge. The studied alternative to achieve the required control of LTC is the use of two separate fuel streams to regulate timing and heat release at specific operational points, where the secondary fuel is a reformed product of the primary fuel in the tank. It is proposed in this report that the secondary fuel can be produced using exhaust heat and Thermo-Chemical Recuperation (TCR). TCR for reciprocating engines is a system that employs high efficiency recovery of sensible heat from engine exhaust gas and uses this energy to transform fuel composition. The recuperated sensible heat is returned to the engine as chemical energy. Chemical conversions are accomplished through catalytic and endothermic reactions in a specially designed reforming reactor. An equilibrium model developed by Gas Technology Institute (GTI) for heptane steam reforming was applied to estimate reformed fuel composition at different reforming temperatures. Laboratory results, at a steam/heptane mole ratio less than 2:1, confirm that low temperature reforming reactions, in the range of 550 K to 650 K, can produce 10-30% hydrogen (by volume, wet)more » in the product stream. Also, the effect of trading low mean effective pressure for displacement to achieve power output and energy efficiency has been explored by WVU. A zerodimensional model of LTC using heptane as fuel and a diesel Compression Ignition (CI) combustion model were employed to estimate pressure, temperature and total heat release as inputs for a mechanical and thermal loss model. The model results show that the total cooling burden on an LTC engine with lower power density and higher displacement was 14.3% lower than the diesel engine for the same amount of energy addition in the case of high load (43.57mg fuel/cycle). These preliminary modeling and experimental results suggest that the LTC-TCR combination may offer a high efficiency solution to engine operation. A single zone model using a detailed chemical kinetic mechanism was implemented in CHEMKIN and to study the effects of base fuel and steam-fuel reforming products on the ignition timing and heat release characteristics. The study was performed considering the reformed fuel species composition for total n-heptane conversion (ideal case) and also at the composition corresponding to a specific set of operational reforming temperatures (real case). The computational model confirmed that the reformed products have a strong influence on the low temperature heat release (LTHR) region, affecting the onset of the high temperature heat release (HTHR). The ignition timing was proportionally delayed with respect to the baseline fuel case when higher concentrations of reformed gas were used. For stoichiometric concentration of RG, it was found that by increasing the proportion of reformed fuel to total fuel (RG), from 0% to 30%, the amount of energy released during the LTHR regime, or HR{sub L}, was reduced by 48% and the ignition timing was delayed 10.4 CA degrees with respect to the baseline fuel case. For RG composition corresponding to certain operational reforming temperatures, it was found that the most significant effects on the HCCI combustion, regarding HR{sub L} reduction and CA50 delay, was obtained by RG produced at a reforming temperature range of 675 K-725 K.« less