News of the demise of the internal combustion engine may be overstated
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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 could be extended. This will be done through 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 the Q3 2020 Equipment and Maintenance Update). To jog your memory, the system provides a small conduit or tube a short distance from each orifice in the tip of a diesel injector. The fuel “entrains” or grabs air before entering the line, much like it does in any diesel spray, but then passes through the line before ignition. The tube is just a little larger (in diameter) than the spray, and its passage generates turbulence which considerably improves the air/fuel mixture. The fuel spray entering the combustion chamber is normally stratified, with rich and lean areas. But going through the conduit has the effect, as Mueller puts it, “of making rich areas leaner and poor areas richer.”
With standard diesel combustion, there is a NOx/particulate trade-off which causes particulate matter to increase sharply with an increase in EGR. The more consistent mixing with DFI allows the use of more EGR to kill NOx, bringing non-engine NOx to levels that will require minimal after-treatment.
For example, with approximately 33% exhaust mixed with intake air, standard diesel engines will see particulates increase nearly 10 times. With DFI, the particles actually decrease slightly. At this EGR percentage, NOx drops to about one-fortieth of normal levels. Even higher levels of EGR with less NOx may come in handy.
Research looking at DFI with renewable fuels like one made from 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 duct design so that the resulting spray is just slightly fuel-rich (just slightly less oxygen than needed for complete combustion) reduces NOx to levels typical of today’s engines, but without EGR. The combination of modern high-pressure injection and ducting enables precise and even matching of the fuel and air mixture in the combustion chamber.
DFI will be explored by a consortium with the aim of delivering fuels and engines capable of reducing net carbon emissions by 80%, as well as an 80% reduction in NOx and soot emissions, within three years. In many ways, these potential results represent the near-perfection of the diesel engine that advanced researchers have dreamed of for years, and this simple system can probably be produced in retrofittable form. If the in-cylinder ducting system proves durable and does not develop deposit issues, we will see new and existing engines with easier aftertreatment in the near future.
Another promising fuel is dimethyl ether, in some ways an ideal diesel fuel. DME was invented in a laboratory in the petroleum industry and closely resembles propane in that it must be stored under moderate pressure (75 psi) to keep it in liquid form. But its chemical structure prevents particles and its cetane number is at least two points higher than that of diesel. Like diesel burned with ducted injection, the fuel burns soot-free with high levels of NOx-killing EGR. The only drag in terms of emissions is that it can form carbon monoxide if not properly mixed with air. It also does not enter the chamber or mix with air or diesel fuel. But research by Volvo and Chalmers University of Technology in Sweden has shown that with sufficient injection pressure and a special combustion chamber, CO2 can 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. Fuel lacks lubrication and compresses when pumped, so there are major challenges in designing a durable injection system. But a durable pump of unusual design can produce sufficient pressure (500 bar or 7,350 psi), and an injector with exotic parts like a ceramic plunger and silicon nitride has shown improved injector life. Mixing with other fuels can also help. A pilot program in Europe saw 10 heavy goods vehicles cover 1.5 million kilometers using 1,000 tonnes of DME.
DME can transport large amounts of hydrogen in low pressure tanks, and the DME can then be easily converted back to pure hydrogen.
Cummins was not represented at the symposium this year, but the company spoke about the development of its hydrogen engine. With hydrogen also likely to be produced via solar and wind farms, but fleets are reluctant to adopt fuel cell electric tractors, Cummins is working on a 15-litre hydrogen 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 ignition but direct injection. The spark plug itself will closely resemble those used with natural gas.
Nebergall explained that hydrogen creates such rapid flame travel that supplying a mixture of fuel and air to the cylinders through the intake manifold, as natural gas engines do, can cause the flame to flash back into the manifold as the intake valve opens at the top. of the exhaust stroke. A low-pressure direct injection system will keep fuel out of the engine until there is no possibility of it coming into contact with hot exhaust gases. Forcing fuel into the cylinder using direct injection shortly before it is burned also helps prevent pre-ignition. Hydrogen disperses extremely quickly, so the charge ends up being homogeneous, or uniform and fully mixed, like what happens in natural gas engines.
However, the hydrogen engine will also use the diesel characteristic of lean running. He says, “We also look at lean burn versus stoichiometry to achieve greater power density.” Stoichiometric refers to delivering just enough air to completely burn all the fuel as Cummins gasoline and natural gas engines do, while the hydrogen engine ingests significantly more air than a stoichiometric engine, which should allow the motor to produce approximately 20% more power.
The combustion chamber will be similar to what is seen with a gasoline automobile engine, in the form of a sloping roof, with the valve stems at an angle on either side of the vertical. This allows for slightly larger valves and places most of the combustion chamber in the head, although there is a small bowl 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 compared to the diesel ratio of nearly 20:1, as well as SCR (catalytic reduction selective) to control NOx . Cummins is exploring EGR to determine if it would be best included in the combustion recipe for the lowest emissions and highest efficiency.
The design goal is a 15-litre engine with diesel-like power and torque characteristics. The hydrogen engine will be included in the company’s new platform that adapts to various fuels with nearly identical parts under the head gasket. A DPF may be installed just to catch the tiny amounts of soot created by the oil consumed by the engine, but it is likely to require no maintenance.
The hydrogen engine will be around 5% less efficient than current diesels at around 42%, but it will use the same cooling package and will actually run slightly cooler (with less heat rejection).
Carbon fiber fuel tanks will withstand higher pressures than needed with natural gas and can be rated up to 700 bar or 10,200 psi. Equipment capable of rapid refueling is being developed and a range of 500 miles is planned.