10Hydrogen offers an attractive alternative to conventional fuels for use in spark ignition engines. It can 11 be burned over a very wide range of equivalence ratios and with considerable exhaust gas recirculation. 12 These help to minimise pumping losses through throttleless operation and oxides of nitrogen (NO x ) pro-13 duction through reduced temperature. Full understanding of hydrogen-fuelled engine operation requires 14 data on the laminar burning rate of hydrogen-air residuals under a wide range of conditions. However, 15 such data are sparse. The present work addresses this need for experimental data. Spherically expanding 16 H 2 -air flames were measured at a range of temperatures, pressures, and equivalence ratios and with vary-17 ing concentrations of residuals of combustion. Unstretched burning velocities, u l , and Markstein lengths, 18 L b , were determined from stable flames. At the higher pressures, hydrodynamic and diffusional-thermal 19 instabilities caused the flames to be cellular from inception and prohibited the derivation of values of u l 20 and L b . The effect of pressure on the burning rate was demonstrated to have opposing trends when com-21 paring stoichiometric and lean mixtures. The present measurements were compared with those available in 22 the literature, and discrepancies were attributed to neglect, in some works, the effects of stretch and insta-23 bilities. From the present measurements, the effects of pressure, temperature, and residual gas concentra-24 tion on burning velocity are quantified for use in a first step towards a general correlation. 25 Ó 2004 by the Combustion Institute. Published by Elsevier Inc. All rights reserved. 29Hydrogen is a very attractive alternative to tra-30 ditional fossil fuels as an energy carrier due to its 31 very clean combustion and the ease of manufac-32 ture. Because of its high flame speed, leading to 33 near constant volume combustion, and wide flam-34 mability limits, a hydrogen-fuelled engine has the 35 potential for high efficiency. The power output of 36 such engines can be varied by changing the equiv-37 alence ratio to use very lean mixtures at low loads. 38 Oxides of nitrogen are minimised, while maintain-39 ing adequate power, by varying the amount of 40 EGR during stoichiometric operation [1]. In both 41 cases, the throttle valve is not used, except maybe 42 at idling, and pumping losses are minimised.43 Thus, hydrogen engines use a large range of equiv-44 alence ratios, and EGR concentrations can be 45 very high with stoichiometric operation.

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