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.

It is well known that adding hydrogen to natural gas extends the lean limit of combustion and that in this way extremely low emission levels can be obtained: even the equivalent zero emission vehicle (EZEV) requirements can be reached. The emissions reduction is especially important at light engine loads. In this paper results are presented for a GM V8 engine. Natural gas, pure hydrogen and different blends of these two fuels have been tested. The fuel supply system used provides natural gas/hydrogen mixtures in variable proportion, regulated independently of the engine operating condition. The influence of the fuel composition on the engine operating characteristics and exhaust emissions has been examined, mainly but not exclusively for 10 and 20 percent hydrogen addition. At least 10 percent hydrogen addition is necessary for a significant improvement in efficiency. Due to the conflicting requirements for low hydrocarbons and low NOx, determining the optimum hythane composition is not straight-forward. For hythane mixtures with a high hydrogen fraction, it is found that a hydrogen content of 80 percent or less guarantees safe engine operation (no backfire nor knock), whatever the air excess factor. It is shown that to obtain maximum engine efficiency for the whole load range while taking low exhaust emissions into account, the mixture composition should be varied with respect to engine load. [S0742-4795(00)02001-9]

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This paper describes an experimental investigation of heat transfer inside a CFR spark ignition engine operated at a constant engine speed of 600 rpm. The heat flux is directly measured under motored and fired conditions with a commercially available thermopile sensor. The heat transfer during hydrogen and methane combustion is compared examining the effects of the compression ratio, ignition timing and mixture richness. Less cyclic and spatial variation in the heat flux traces are observed when burning hydrogen, which can be correlated to the faster burn rate. The peak heat flux increases with the compression ratio, but the total cycle heat loss can decrease due to less heat transfer at the end of the expansion stroke. An advanced spark timing and increased mixture richness cause an increased and advanced peak in the heat flux trace. Hydrogen combustion gives a heat flux peak which is three times as high as the one of methane for the same engine power output.

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