The U.S. Army is working with civilian industrial companies through Small Business Innovation Research grants to reduce fuel consumption and enhance the power density of diesel engines used in field generators.
Since the Army uses a range of generators from 2 to 100 kW (3 to 134 hp), the solution must work across all sizes, without adding much weight or complexity to the diesel engines themselves. The first obvious choice is to turbocharge the engines. However, that would require reinforced engine blocks because smaller diesel engines cannot withstand the high peak pressure from turbocharging.
An alternative is to provide engine boost by increasing the amount of oxygen in the air, somewhat the same as turbocharging but without the drawbacks. It is a simple concept: to get more power more oxygen should be put into the engine to burn fuel more efficiently. The benefit is that oxygen enrichment results in much lower peak pressures than turbocharging for the same increase in power.
Mainstream Engineering Corp. focuses on transitioning advanced thermal control and energy conversion technology into cost-effective commercial products. Dr. Paul Yelvington, Senior Chemical Engineer and technology lead for energy conversion projects at Mainstream, says that oxygen-enriched combustion (OEC) has already been used for industrial combustion processes such as process heating, melting, metal cutting, and incineration.
“In fact, Army researchers had already experimented with the oxygen enrichment of flames to produce electricity using thermoelectric devices, which brought about the interest in trying to apply the technology to an engine,” he said.
The issue with developing this technology into a viable process has always been that the oxygen-enrichment system for use in engines has never really been perfected. “People have tried putting these two together but they really never optimized the tuning of the engine or the operation of the air separation membrane,” said Yelvington.
He explained that OEC offers several advantages for diesel engines aside from boosting power. Products of incomplete combustion (CO, hydrocarbons, soot, etc.) are reduced by increasing the oxygen concentration at a constant fueling rate. Also, burn duration and ignition delay are reduced, which allows faster cold starts and the ability to use lower quality fuels.
Through experimentation, Mainstream found that certain polymeric membranes are preferred for separating air to generate an oxygen-enriched air stream.
“These membranes have the best balance of size, weight, power consumption, and durability for this application when compared to competing methods, i.e., pressure swing adsorption, cryogenic distillation, mixed conducting ceramic membranes,” said Yelvington. “Polymeric membrane modules consist of a bundle of hollow membrane fibers encased in a cylindrical enclosure. When a pressure differential is applied across the membrane, the difference in partial pressures creates a flow through the membrane, generating an O2-rich permeate stream and a N2-rich retentate stream.”
Mainstream built computational models for the smallest and largest engines used by the Army, the theory being that all models in between would be easily accommodated. Pressure transducer and thermocouple assemblies were applied to the engines to measure key parameters such as cylinder pressure, intake pressure, exhaust temperature, and oil temperature. Fuel consumption was measured as were CO, NO, and NOx emissions.
Yelvington noted that the membrane technology selected allowed them to make the device as compact as possible. “For the 2-kW system, the OEC unit is only about a foot long and under five inches in diameter. It fits right into the frame of the small generator and only adds about 10 pounds to the unit.”
The result is a compact OEC unit that gives diesel engines a boost in power; reduced CO, hydrocarbons, and soot emissions; faster cold starts; and minimal stress on small engine blocks. Preliminary designs have shown that increases in power of 15% are easily attainable, and increases of 30% or more are expected after further tweaking of the engine controller and air separation equipment.
The obvious next step is a transition to diesel-powered vehicles.
“With air separation membranes, you can produce nitrogen-enriched air as easily as oxygen-enriched air. Producing nitrogen-enriched air is perfect for a vehicle because it greatly reduces the amount of NOx in the exhaust,” said Yelvington.
One way NOx reduction is currently done in diesel engines is using exhaust gas recirculation, where some of the exhaust is recycled back into the engine.
“However, you have to live with recycling unburned hydrocarbons and soot back into the engine, resulting in fouling of components,” he said. “Using nitrogen-enriched air, you get the same thing or better, reducing the NOx without fouling the engine. Actually Mack Truck and Caterpillar are currently looking at nitrogen enrichment to control NOx.”
The real niche for OEC is the small engine because large engine blocks can handle turbocharging. OEC will allow smaller engines to be used in applications now using larger, costlier generators.
“The nice thing about oxygen enrichment is that you get more power out of the engine without increasing the cylinder pressure and stressing the block,” said Yelvington. “You can get away with using standard, naturally aspirated diesel blocks without increasing the mechanical strength of the block.”