Hydrogen as a Reciprocating Engine Fuel
An analytical and experimental investigation of hydrogen as a reciprocating engine fuel has been carried out The analytical portion involved application of a thermodynamic model to predict trends in power, efficiency and emissions with hydrogen fuel. The experimental investigation was carried out using a single cylinder CFR engine modified to run on hydrogen. Both aspects of the investigation focused particular attention on the unthrottled direct injection hydrogen engine because of its suitability in terms of power, efficiency, and control of flashback, preignition and emissions. Model calculations and experimental results are described and compared.
The analytical model was of the thermodynamic type with the burning rate specified by a semi-empirical turbulent flame speed correlation. The hydrogen was assumed to be injected and fully mixed prior to ignition. The model was adiabatic and the burnt gases were assumed to remain unmixed in order to estimate the temperature gradient across the cylinder. Nitric oxide (NO) emissions were computed by applying the finite rate extended Zeldovich mechanism to the combustion products. Reactions involving N<sub>2</sub>O were found to be of no importance in nitric oxide production even under the leanest operating conditions. Model calculations demonstrated the feasibility of operating a direct injected, hydrogen fueled reciprocating engine unthrottled over a wide range of conditions. The lean region with equivalence ratio less than 0.6 gives a particularly attractive combination of high efficiency and low NO emissions. Due to their relatively high flame temperatures and fast burning rates, hydrogen-air mixtures give rise to pronounced NO decomposition during the expansion stroke for mixtures richer than an equivalence ratio of 0.8. As a result, the specific NO emissions show a maximum near an equivalence ratio of 0.8 and a sharp decrease for richer mixtures. The specific emissions are also relatively insensitive to compression ratio.
The experimental investigation was carried out using both the high swirl prechambered GFR cetane rating head and the standard CFR octane rating Otto cycle head. Tests with fuel injection near top center piston position on the prechambered head demonstrated the feasibility of quality regulated operation on hydrogen. At high compression ratios (greater than 17), spark plug ignition was accompanied by large and rapid pressure fluctuations. On the other hand, glow plug ignition gave rise to very smooth combustion. There was also evidence that late injection did not allow sufficient mixing of fuel and air in the engine, and hence, resulted in incomplete combustion.
Experiments with the Otto cycle head indicated that increasing the duration of fuel injection, and advancing the beginning of injection to an early part of the compression stroke resulted in improved combustion efficiency and increased power and thermal efficiency. However, near stoichiometric mixtures, engine operation under these conditions exhibited preignition tendencies at high compression ratios. The experiments indicate the necessity for careful control of fuel-air mixing in the directly injected engine. Measured nitric oxide levels generally are in satisfactory agreement with the model calculations. Again, lean operation leads to high efficiency with very low emissions.
It is concluded that the unthrottled reciprocating engine, with direct cylinder injection of hydrogen fuel is a feasible and attractive mode for hydrogen usage. Significant fuel economy and emissions benefits, obtained by ultra lean operation, are-possible with hydrogen.
KeywordsBurning Rate Equivalence Ratio Flame Speed High Compression Ratio Combustion Duration
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