Abstract
This paper presents an experimental analysis of dual-fuel combustion based on the performance, emissions, and in-cylinder combustion measurements with gaseous propane or gaseous methane as the primary fuel and diesel as the pilot fuel. Two different sets of experiments were performed on a 1.9 litre four-cylinder engine at a constant engine speed of 1800 r/min: first, constant-pilot-quantity experiments, allowing the primary fuel concentration and the brake mean effective pressure to vary; second, constant-brake-mean-effective-pressure experiments, allowing the percentage energy substitution of the primary fuel and the pilot quantity to vary. In the constant-pilot-quantity experiments, the apparent heat release rate profiles showed the influence of the preignition chemistry and gaseous fuel burn rates on the dual-fuel combustion phasing and duration, the fuel conversion efficiency, and the engine-out emissions. With a fixed pilot quantity, the nitrogen oxide emissions were either reduced or unaffected while the smoke levels were increased or unaffected with increasing primary fuel concentration. The carbon monoxide and total unburned hydrocarbon emissions decreased and the fuel conversion efficiency increased as the pilot quantity or the primary fuel concentration was increased. Overall, diesel–propane combustion yielded higher carbon monoxide emissions, lower total unburned hydrocarbon emissions, and slightly higher fuel conversion efficiencies than diesel–methane combustion did. In the constant-brake-mean-effective-pressure experiments, at a brake mean effective pressure of 2.5 bar, diesel–propane and diesel–methane combustion behaved very similarly, the primary differences being in the preignition chemistry and the ignition delay trends. At a brake mean effective pressure of 2.5 bar, the nitrogen oxide and smoke emissions were simultaneously reduced while the carbon monoxide and total unburned hydrocarbon emissions were increased. At a brake mean effective pressure of 10 bar (a baseline diesel fuel conversion efficiency of 38%), diesel–propane fueling was prone to rapid earlier combustion while diesel–methane combustion was slower. For diesel–methane combustion at a brake mean effective pressure of 10 bar, the fuel conversion efficiency decreased to 37.1% as the percentage energy substitution was increased to 51%. For diesel–propane combustion at a brake mean effective pressure of 10 bar, the fuel conversion efficiency increased to 39% as the percentage energy substitution was increased to 46%. At high-brake-mean-effective-pressure–high-percentage-energy-substitution and large-pilot-quantity–high-equivalence-ratio conditions, diesel–propane combustion showed an apparent departure from the classical three-phase dual-fuel combustion to a distributed volumetric combustion process that resembled a “diesel-regulated homogenous-charge-compression-ignition-like” combustion process.
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