by Rishi Venugopal
Senior Staff Engineer, Achates Power, Inc.
Last month, I had the opportunity to organize and co-chair a technical session at SAE World Congress on efficiency and emissions in compression-ignition combustion. In my session as well as several others, there were a number of papers focused on advanced combustion concepts aimed at simultaneously controlling emissions and improving engine efficiency. This is not surprising as we head towards CAFE regulations for on-highway applications and, likewise, strict emissions standards for industrial power generation and marine applications. At this year’s World Congress as well as over the past few years, there has been a surge in the R&D activity of universities, research institutes and OEMs to accelerate the evaluation and development of advanced combustion regimes.These regimes unanimously focus on aggressive in-cylinder control of NOx and soot emissions, while trying to overcome the historical drawbacks of sacrificing fuel consumption improvements. Some of the concepts—such as partially-premixed combustion (PPC), reactivity-controlled compression ignition (RCCI) and gasoline compression ignition (GCI)—have received significant attention and primarily condition the in-cylinder charge so as to achieve relatively long ignition delays and reduced NOx formation rates while still improving fuel efficiency. In RCCI, for instance, by appropriately conditioning the charge with a low reactivity fuel (e.g. gasoline) and a high reactivity fuel (e.g. diesel) in combination with multiple injections and an overall lean background equivalence ratio, experimental and computational studies on research engines have shown very good potential to simultaneously control NOx and soot emissions. They have also demonstrated the ability to achieve good fuel efficiency due to reduced heat transfer—albeit with higher unburned hydrocarbon and CO emissions. Though these advanced combustion regimes have been shown to be effective primarily on select single-cylinder research engine operating points, they present significant operational challenges on conventional four-stroke engine architectures. Low load operating points, which are heavily weighted on a light-duty application for instance, tend to suffer from poor ignitability and high coefficients of variation, thereby deteriorating engine performance. These challenges stem from the fact that the four-stroke cycle typically fully scavenges the cylinder leaving only a small amount of exhaust gas residuals from the previous cycle. Essentially, this near fully scavenged cylinder diminishes the reactivity of the background mixture into which the lower reactivity fuel is injected during RCCI operation. Apart from this significant drawback, there is the additional challenge of packaging both gasoline and diesel injectors on the cylinder head of a four-stroke engine. A majority of the studies have opted for port injection of gasoline to overcome this limitation. In doing this, however, you lose the inherent advantages of direct injection, which helps tune the background mixture stratification through multiple injections before the high reactivity fuel (diesel) is injected to ignite the mixture. More recently, four-stroke research has invoked concepts—such as variable valve timing (VVT) and negative valve overlap (NVO)—to increase exhaust gas residuals and improve low-load performance. However, these technologies are expensive and complex given the marginal improvement that they potentially offer. At Achates Power, we have developed and optimized an opposed-piston, compression ignition architecture over a decade of rigorous engine research. This engine synergistically offers numerous advantages for advanced combustion concepts, such as RCCI and GCI. As shown in the schematic below, Achates Power’s uniflow-scavenged opposed-piston architecture is essentially a flow-through device when the port windows are open, and can be designed not to fully scavenge the cylinder and, thereby, retain significant amounts of exhaust gas residuals. This “hot EGR” mixture in the cylinder is much more reactive than the trapped charge in a conventional four-stroke engine. As a result, it’s ideally suited for the ignition and combustion of higher octane and lower reactivity fuels, such as gasoline and natural gas. Furthermore, partially scavenging the cylinder reduces pumping losses, which tend to be significant at lower loads.
With regard to the packaging dilemma, the Achates Power opposed-piston architecture incorporates injectors mounted on the cylinder wall and, therefore, benefits from a wealth of real estate to package the injectors all around the bore. As a result, direct injection of multiple fuels can be incorporated with relative ease. Injection patterns can also be extensively optimized to tune performance and emissions for the fuels and applications of interest. These advantages—combined with our proprietary combustion chamber constructions that provide excellent mixing and high rates of turbulent kinetic energy—make the Achates Power engine an ideal platform for advanced combustion regimes (such as RCCI and GCI) that are production-feasible. Achates Power’s opposed-piston architecture is well poised to radically improve today’s engine technologies as well as future engine concepts that incorporate advanced combustion regimes. It can also be operated on a suite of alternative fuels.