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For high-load applications, natural gas represents a clean burning, readily available, and relatively inexpensive alternative to number 2 Diesel fuel. However, the fuel’s poor ignitability has previously limited implementation to spark ignited and dual-fueled engines. These approaches suffer from reduced peak load and high engine-out particulate emissions, respectively, requiring lean operation and expensive aftertreatment to meet regulatory standards. A high-temperature combustion strategy can overcome the difficult ignitibility, allowing for true Diesel-style combustion of pure methane-the least ignitable and least sooting component of natural gas. In order to achieve this result, a compression system was designed to supply fuel at pressures suitably high to achieve good mixing and short injection durations, and a solenoid-actuated Diesel fuel injector was modified to function at these pressures with a gaseous fuel.

The use of neat alcohols, namely methanol and ethanol, in direct-injection, compression-ignited engines is difficult, most notably due to their poor ignitability. By employing a high-temperature combustion strategy, this challenge may be overcome, thus creating the opportunity for using these oxygenated and inherently low-sooting fuels for heavy-load applications. Experimental data are provided from a single-cylinder research engine that shows particulate matter (PM) emissions for Diesel-style combustion of both methanol and ethanol that are below the current US Government regulation limit. The level of particulates remained low up to stoichiometric ratios of fuel and air. A complete emissions analysis indicates a high combustion efficiency of ∼ 96% at stoichiometric conditions. In order to achieve reliable combustion, some form of intake-air preheating was required.

There is significant motivation to extend the operating range of naturally aspirated HCCI combustion to high load (8-12 bar IMEP) to attain a combustion strategy with the efficiency benefits of HCCI but without the lost power density of a lean or highly diluted charge. Currently, the high-load limit of HCCI combustion is imposed by a phenomenon commonly known as ringing. Ringing results when the kinetically-driven autoignited combustion process proceeds in such a way as to form strong pressure waves which reverberate in the engine. Inhomogeneities and gradients in mixture reactivity lead certain regions to react ahead of others, and as a result, coupling can occur between a pressure wave and the reaction front. This paper seeks first to sort several related but distinct issues that impose the high load limit: ringing, engine damage, peak in-cylinder pressure, peak rate of pressure rise, and engine noise.

Heat transfer is one of the largest causes of exergy destruction in modern engines. In this paper, exergy distribution modeling was used to determine the potential of reduced engine heat transfer to provide significant gains in engine efficiency. As known from prior work, of itself, reducing heat transfer creates only a small increase in efficiency-most of the exergy is redirected into the exhaust stream-requiring both mechanical and thermal recovery of the exhaust exergy. Mechanical regeneration, through turbocharging and over-expansion, can lead to efficiencies exceeding 50%. Adding thermal regeneration, through high enthalpy steam injection or a bottoming cycle, can increase the efficiency potential to approximately 60%. With implementation of both mechanical and thermal regeneration, the only remaining cause of substantial exergy destruction is the combustion process.