The News of the Internal Combustion Engine’s Demise May Be Exaggerated
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At the Low net-GHG Fuel and Engine Technologies Symposium at the University of Wisconsin in June, researchers noted that the life of familiar internal combustion engines may be extended. This will happen via the use of fuels created from renewable crops, agricultural waste, and even solar energy.
Charles Mueller and a team of researchers at Sandia National Laboratories developed ducted fuel injection (DFI) (described in Q3 2020’s Equipment & Maintenance Update). To refresh your memory, the system provides a small duct or tube a short distance from each orifice in a diesel injector’s tip. The fuel “entrains” or grabs on to some air prior to entering the duct, much as it does in any diesel spray, but it then passes through the duct prior to ignition. The tube is just a little bigger (in diameter) than the spray, and passing through it generates turbulence that substantially improves fuel/air mixing. The spray of fuel entering the combustion chamber is normally stratified, with both rich and lean areas. But, passing through the duct has the effect of, as Mueller says, “making the rich areas leaner, and the lean areas richer.”
With standard diesel combustion, there is a NOx/particulate trade-off that causes a sharp rise in particulate with increased EGR. The more consistent mixing with DFI allows the use of more EGR to kill NOx, bringing the engine-out NOx to levels that will require only minimum aftertreatment.
For example, with about 33% exhaust mixed with intake air, standard diesels will see particulate rise nearly 10 times. With DFI, particulate actually decreases slightly. At this EGR percentage, NOx drops to about one-fortieth of normal levels. Even higher levels of EGR with lower NOx may prove practical.
Research running DFI with renewable fuels like one made of 50% sewage sludge and 50% ethanol, both of which had been processed to create a diesel-like liquid fuel, showed even better results because these fuels produce very little soot. Modifying the design of the duct so the resulting spray would be just slightly rich in fuel (just slightly less oxygen than needed for complete combustion) cut the NOx to engine-out levels typical of today’s engines, but without EGR. The combination of modern, high-pressure injection and the duct enables a precise and uniform tailoring of the mix of fuel and air in the combustion chamber.
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DFI will be explored by a consortium with the goals of providing fuels and engines capable of an 80% reduction in net-carbon emissions, together with an 80% reduction in engine-out NOx and soot, within three years. In many respects, these potential results represent the near-perfection of the diesel engine that advanced researchers have dreamed of for years, and this simple system can likely be produced in retrofitable form. If the in-cylinder duct system proves durable and doesn’t develop problems with deposits, we’ll see both new and existing engines with simpler aftertreatment in the near future.
Another promising fuel is dimethyl ether, in some respects an ideal diesel fuel. DME was invented in an oil industry laboratory, and closely resembles propane in that it needs to be stored under moderate pressure (75 psi) to keep it in liquid form. But its chemical structure prevents particulate, and its cetane rating is at least two points above that of diesel. Like diesel burned with ducted injection, the fuel combusts soot-free with high levels of NOx-killing EGR. The only rub emissions-wise is that it can form carbon monoxide if not properly mixed with air. It also does not penetrate the chamber and mix with air as well as diesel fuel. But, research done by Volvo and Chalmers University of Technology in Sweden showed that with enough injection pressure and a special combustion chamber, CO2 could be controlled.
It can be made from many sustainable forms of waste or agricultural products, including ethanol. The net effect is an 85% reduction in CO2 emissions. The fuel lacks lubricity and compresses when pumped, so there are major challenges in designing a durable injection system. But a durable pump of unusual design can produce enough pressure (500 bar or 7,350 psi), and an injector with exotic parts like a ceramic plunger and some of silicon nitride has shown improved injector life. Blending with other fuels can help, too. A pilot program in Europe saw 10 heavy-duty trucks drive 1.5 million kilometers using 1,000 tons of DME.
DME can carry large quantities of hydrogen in low-pressure tanks, and the DME can then be readily converted back to pure hydrogen.
Cummins was not represented at the symposium this year, but the company has been vocal about its hydrogen engine development. With hydrogen also likely to be produced via solar and wind farms, but fleets shy about adopting fuel cell-electric tractors, Cummins is working on a 15-liter hydrogen-fueled engine.
Jim Nebergall, general manager of Cummins Hydrogen Engine Business, described the hydrogen engine as “a hybrid of our natural gas and diesel engines. It will be spark-ignited yet direct injected.” The spark plug itself will closely resemble those used with natural gas.
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Nebergall explained that hydrogen creates such rapid flame travel that supplying a mixture of fuel and air to the cylinders via the intake manifold as natural gas engines do can result in the flame traveling back into the manifold at the point when the intake valve opens at the top of the exhaust stroke. A low-pressure direct injection system will keep the fuel out of the engine until there’s no possibility of it coming in contact with hot exhaust. Forcing the fuel into the cylinder using direct injection shortly before it will be burned helps to prevent pre-ignition, too. Hydrogen disperses extremely rapidly, so the charge ends up being homogeneous, or uniform and fully mixed, similar to what happens in natural gas engines.
However, the hydrogen engine will also employ the diesel characteristic of lean running. He says, “We are also looking at lean burn versus stoichiometric in order to achieve greater power density.” Stoichiometric refers to supplying just enough air to completely combust all the fuel as gasoline and the Cummins natural gas engines do, while the hydrogen engine will ingest significantly more air than a stoichiometric engine, which should allow the engine to produce about 20% more power.
The combustion chamber will be similar to what is seen with a gasoline auto engine, in the shape of a pent roof, with the valve stems at an angle either side of vertical. This allows slightly larger valves and places most of the combustion chamber in the head, though there will be a small bowl down in the piston. The result will be an engine with a compression ratio similar to what is seen with natural gas — in the range of 12-13:1 versus the diesel ratio of nearly 20:1, as well as SCR (selective catalytic reduction) to control NOx. Cummins is exploring EGR to determine whether it will be best to include it in the combustion recipe for the lowest emissions and highest efficiency.
The design goal is a 15-liter engine with power and torque characteristics similar to a diesel. The hydrogen engine will be included in the company’s new platform that adapts to various fuels with parts that are nearly identical below the head gasket. A DPF may be fitted just to catch the minuscule amounts of soot created by the oil the engine consumes, but it’s likely to be maintenance-free.
The hydrogen engine will be about 5% less efficient than today’s diesels at about 42%, but it will use the same cooling package and actually run slightly cooler (with less heat rejection).
Carbon-fiber fuel tanks will handle higher pressures than those needed with natural gas and may be rated at up to 700 bar or 10,200 psi. Equipment capable of allowing quick refueling is under development and a 500-mile range is planned.