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Although HCCI engines promise low NOx emissions with high efficiency, they suffer from a narrow operating range between knock and misfire because they lack a direct means of controlling combustion timing. A series of previous studies showed that reformer gas, (RG, defined as a mixture of light gases dominated by hydrogen and carbon monoxide), can be used to control combustion timing without changing mixture dilution, (λ or EGR) which control engine load. The effect of RG blending on combustion timing was found to be mainly related to the difference in auto-ignition characteristics between the RG and base fuel. The practical effectiveness of RG depends on local production using a fuel processor that consumes the same base fuel as the engine and efficiently produces high-hydrogen RG as a blending additive.

Homogeneous Charge Compression Ignition (HCCI) engines offer high fuel efficiency and some emissions benefits. However, it is difficult to control and stabilize combustion over a sufficient operating range because the critical compression ratio and intake temperature at which HCCI combustion can be achieved varies with operating conditions such as speed and load as well as with fuel octane number. Replacing part of the base fuel with reformer gas, (which can be produced from the base hydrocarbon fuel), alters HCCI combustion characteristics in varying ways depending on the replacement fraction and the base fuel auto-ignition characteristics. Injecting a blend of reformer gas and base fuel offers a potential HCCI combustion control mechanism because fuel injection quantities and ratios can be altered on a cycle-by-cycle basis.

Homogeneous Charge Compression Ignition (HCCI) combustion offers high fuel efficiency and some emissions benefits. However, it is difficult to control and stabilize combustion over a significant operating range because the critical compression ratio and intake temperature at which HCCI combustion can be achieved vary with operating conditions such as speed and load as well as with fuel octane number. Replacing part of the base fuel with reformer gas, (which can be produced from the base hydrocarbon fuel), alters HCCI combustion characteristics in varying ways depending on the replacement fraction and the base fuel auto-ignition characteristics. Because fuel injection quantities and ratios can be altered on a cycle-by-cycle basis during operation, injecting a variable blend of reformer gas and base fuel offers a potential HCCI combustion control mechanism.

As a first step toward better understanding of the effects of flame stretch on combustion rate in SI engines, the burning velocity of a premixed, spherical, laminar methane-air flame propagating freely at standard temperature and pressure was investigated. The underlying un-stretched burning velocity was computed using CHEMKIN 3.7 with GRI mechanism, while the Lewis number and subsequently the Markstein length were deduced theoretically. The burning velocity of the freely growing flame ball was calculated from the un-stretched burning velocity with curvature and stretch effects accounted via the theoretically deduced Markstein length. For the positive Markstein length methane-air flame, flame stretching reduces the burning velocity. Therefore, the burning velocity of a spark-ignited flame starts with a value lower than, and increases asymptotically to, the underlying un-stretched burning velocity as the flame grows.

The effects of turbulence on 60% stoichiometric, premixed methane-air flame propagation were investigated using high speed schlieren video and pressure trace analyses. The mixtures were centrally spark-ignited at 300 K and 101 kPa in a 125 mm cubical chamber. Turbulence was up to 2 m/s intensity with 2 to 8 mm integral scale. With quiescent mixtures, buoyancy convected the slow-burning flame upward onto the upper wall, resulting in dramatic heat loss. With turbulence, the burning rate was enhanced profoundly, though partial flame quenching resulted in cyclic variability at higher turbulence levels. Despite this partial quenching, these ultra-lean flames generally resisted total extinguishment over the conditions tested.

The different roles played by small and large eddies in engine combustion were studied. Experiments compared natural gas combustion in a converted, single cylinder Volvo TD 102 engine and in a 125 mm cubical cell. Turbulence is used to enhance flame growth, ideally giving better efficiency and reduced cyclic variation. Both engine and test cell results showed that flame growth rate correlated best with the level of high frequency, small eddy turbulence. The more effective, small eddy turbulence also tended to lower cyclic variations. Large scales and bulk flows convected the flame relative to cool surfaces and were most important to the initial flame kernel.

Diesel engined buses are the major means of transportation in many urban and suburban areas. Compared with other transportation systems, bus fleets are flexible, effective and low in capital cost. However, existing buses contribute to a serious air pollution problem in many cities. They also consume large amounts of diesel fuel, which is a concern for national economies where locally available natural gas could displace the more expensive petroleum-based fuel. New engine designs significantly reduce pollutants and some use alternative fuels. However, there is a huge infrastructure of existing diesel buses. Expensive new buses or bus engines will only gradually displace them, particularly in countries with weaker economies. The urgently required fuel replacement and pollution reduction benefits must be deferred into the future. These factors lead to the requirement for an economically viable, clean-burning conversion system to convert existing diesel engines to natural gas fuel.

High speed Schlieren video and pressure trace analyses were used to study the turbulence effects on burning velocities in a constant volume combustion chamber. Propane-air and methane-air mixtures of equivalence ratios between 0.75 and 0.96 were ignited at 101 kPa and 296 K. Schlieren images of flame growth were recorded on video at 2000 frames per second while combustion chamber pressure was simultaneously recorded. Turbulence at ignition was up to 7 m/s intensity with 2 mm or 8 mm integral scale, produced by pulling a perforated plate across the chamber prior to ignition. In the analysis, the turbulence parameters during combustion were adjusted for the effect of decay and rapid distortion in a closed chamber. Results of both video and pressure trace analyses show a linear relationship between turbulent burning velocity and turbulence intensity as expected.