Experimental Studies of Hydrogen as a Fuel Additive in Internal Combustion Engines

Abstract

Combustion of hydrocarbons in internal combustion engines results in emissions that can be harmful both to human health and to the environment. Although the engine technology is improving, the emissions of NOx, PM and UHC are still challenging. Besides, the overall consumption of fossil fuel and hence the emissions of CO2 are increasing because of the increasing number of vehicles. This has lead to a focus on finding alternative fuels and alternative technologies that may result in lower emissions of harmful gases and lower CO2 emissions. This thesis treats various topics that are relevant when using blends of fuels in different internal combustion engine technologies, with a particular focus on using hydrogen as a fuel additive. The topics addressed are especially the ones that impact the environment, such as emissions of harmful gases and thermal efficiency (fuel consumption). The thesis is based on experimental work performed at four different test rigs:

1. A dynamic combustion rig with optical access to the combustion chamber where spark ignited premixed combustion could be studied by means of a Schlieren optical setup and a high speed video camera.

2. A spark ignition natural gas engine rig with an optional exhaust gas recycling system.

3. A 1-cylinder diesel engine prepared for homogeneous charge compression ignition combustion.

4. A 6-cylinder standard diesel engine

The engine rigs were equipped with cylinder pressure sensors, engine dynamometers, exhaust gas analyzers etc. to enable analyses of the effects of different fuels. The effect of hydrogen blended with methane and natural gas in spark ignited premixed combustion was investigated in the dynamic combustion rig and in a natural gas engine. In the dynamic combustion rig, the effect of hydrogen added to methane on the flame speed and the flame structure was investigated at elevated pressure and temperature. A considerable increase in the flame speed was observed when adding 30 vol% hydrogen to the methane, but 5 vol% hydrogen also resulted in a noticeable increase. The flame structure was also influenced by the hydrogen addition as the flame front had a higher tendency to become wrinkled or cellular. The effect is believed to mainly be caused by a reduction in the effective Lewis number of the mixture. In the gas engine experiments, the effect of adding 25 vol% hydrogen to natural gas was investigated when the engine was run on lean air/fuel mixtures and on stoichiometric mixtures with exhaust gas recirculation. The hydrogen addition was found to extend the lean limit of stable combustion and hence caused lower NOx emissions. The brake thermal efficiency increased with the hydrogen addition, both for the fuel lean and the stoichiometric mixtures with exhaust gas recirculation. This is mainly because of shorter combustion durations when the hydrogen mixture was used, leading to thermodynamically improved cycles.

Two types of experiments were performed in compression ignition engines. First, homogenous charge compression ignition (HCCI) experiments were performed in a single cylinder engine fueled with natural gas and diesel oil. As HCCI engines have high thermal efficiency and low NOx and PM emissions it may be more favorable to use natural gas in HCCI engines than in spark ignition engines. The mixture of natural gas, diesel oil and air was partly premixed before combustion. The natural gas/diesel ratio was used to control the ignition timing as the fuels have very different ignition properties. The natural gas was also replaced by a 20 vol% hydrogen/natural gas mixture to study the effect of hydrogen on the ignition and combustion process. Also, rape seed methyl ester (RME) was tested instead of the diesel oil. The combustion phasing was found to mainly be controlled by the amount of liquid fuel injected. The presence or absence of hydrogen resulted in only marginal changes on the combustion. Because the diesel oil and RME have much lower autoignition temperatures than both hydrogen and natural gas, the properties of the liquid fuel may overshadow the effect of the hydrogen addition. A large difference however, was found between the RME and the diesel oil with the necessity to inject much more RME than diesel oil to obtain the same combustion phasing.

The last experiments with compression ignition were performed by using a standard Scania diesel engine where the possibilities to reduce particulate matter (PM) and other emissions by introduction of combustible gas to the inlet air (named fumigation) were investigated. Hydrogen, methane and propane were introduced at different rates replacing up to 40% of the total fuel energy. Also, a biodiesel consisting of mainly RME was tested instead of the diesel oil. Because of the low density of hydrogen gas, less of the fuel energy could be replaced by hydrogen than by the two other gases. Higher rates of hydrogen would sacrifice the safety by exceeding the lower flammability limit in the inlet air. Only moderate reductions in PM were achieved at high gas rates, and because of the limitation in the practical achievable hydrogen rate it was not possible to obtain considerable reductions in PM emission by hydrogen addition. The NOx emissions were found to be little influenced by the fumigation, but the THC emissions strongly increased with increased methane and propane rates, especially at a low engine load. Propane fumigation resulted in considerably less THC emissions than methane fumigation. The biodiesel resulted in higher PM emissions than the diesel fuel at low load, but was considerably lower at the higher loads. This is believed to be because of the low volatility of the biodiesel which may lead to emissions of un-burned fuel at low load when the temperature is low. At higher loads this is believed to be less of a problem because the temperature is higher, and the oxygen content of biodiesel is believed to increase the PM oxidation and/or reduce the formation of PM.