|Emissions profiles for HCNG and CNG under part load condition. Click to enlarge.|
The use of a 30% hydrogen blend with natural gas in a direct injection natural gas engine results in a significant extension of the lean limit; an increase in engine thermal efficiency; a reduction in CO2 and HC emissions; and a reduction in NOx under certain conditions, although with a slight increase in others according to a paper presented by engineers from Orbital at the ANGVA Natural Gas Vehicle conference in Bangkok.
A variety of other studies have shown that the use of a blend of hydrogen and compressed natural gas (HCNG) in a conventional spark injected gaseous engine extends the lean limit, improves thermal efficiency and reduces emissions. Direct injection of gaseous fuels already has been shown to extend the load range of engines. The Orbital study builds on both by examining the effect of HCNG in a direct injection engine.
The Orbital engineers used a 4-stroke 454cc single-cylinder research engine running with a compression ratio of 12.4:1 to compare the performance of the 30% HCNG blend with CNG. The engine has a centrally located direct injector and close proximity spark plug. Testing was conducted at 2000 rpm and at two load points; 300 kPa IMEP (part load) and wide-open throttle (full load).
At the part load condition, earlier direct injection timing returned higher efficiency and lower emissions for both HCNG and CNG. The Orbital engineers found that at part load the HCNG mixture:
Extends the lean operating limit to lambda 1.8 from 1.4 for CNG alone;
Increases maximum indicated thermal efficiency from 30.6% to 32.8%;
Lowers exhaust temperatures by extension of lean limit;
Returns NOx formation of 2 g/kWh when operating leaner than lambda 1.4;
Reduces hydrocarbon emissions by 40%; and
reduces CO2 emissions by 21%.
At full load, the use of HCNG increases the engine output when injecting the fuel after intake valve closure. For earlier injection timings however, more air is displaced by HCNG resulting in lower output. At the full load condition:
Indicated thermal efficiency is higher for HCNG at all injection timings;
Formation of NOx is higher for HCNG at all the injection timings tested;
Hydrocarbon emissions are reduced by approximately 40%; and
CO2 emissions are reduced by 16%.
|The FlexDI system offers fuel flexibility. Click to enlarge.|
Orbital recently introduced its new FlexDI modular direct injection fuel system technology at the 5th International Clean Vehicle Exhibition and Forum in Beijing. Orbital’s core technology—the Orbital Combustion Process (OCP)—uses air-assisted, low-pressure direct fuel injection rather than high-pressure injection to atomize the fuel charge. With OCP, fuel is first metered into an injector pre-chamber via a conventional automotive port injector (MPI), and then delivered into the combustion chamber with the assistance of air at pressure.
The FlexDI system enables engine manufactures to develop one spark-ignited engine family capable of handling different fuel types through changes to the fuel system. This opens up the potential for manufacturers to offer customers gasoline, ethanol, CNG, hydrogen and SI heavy-fuelled engine variants utilizing the same base engine design. FlexDI can also be configured for Bi-Fuel or Tri-Fuel operation. (Earlier post.)
C. Bleechmore, S. Brewster, H. Hochstadt (2007) Hydrogen Addition Strategy for Lean Limit Extension of a Natural Gas Engine