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Direct Injection of HCNG Extends Lean Limit, Increases Thermal Efficiency, and Can Reduce NOx

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



I assume that the increase in thermal efficiency is because the combustion velocity increases when there is hydrogen mixed with the natural gas.
I suppose that when the engine is using the 30% hydrogen blend, the spark timing can be delayed a little bit.


"The Orbital engineers used a 4-stroke 454cc single-cylinder research engine to compare the performance of the 30% HCNG blend with CNG.
- Reduces hydrocarbon emissions by 40%; and
- reduces CO2 emissions by 21%."

Can anyone explain why 30% HCNG does not lead to a proportional decrease in hydrocarbons or CO2?
Reducing unburned hydrocarbons suggests improved combustion, but reducing CO2 by less than 30% sounds odd.

At present market prices, burning 30% hydrogen in an ICE sounds expensive.

Hydrocarbon emissions from CNG are lower than fossil diesel which is attractive for reducing smoke in cities.
However, fuel consumption in Spark Ignition engines fueled by CNG is higher than Compression Ignition engines.

It would be interesting to see what results they get from a low % of hydrogen in a Compression Ignition engine.


One of these engines might be nice for a series hybrid.

Rafael Seidl

Blending hydrogen with NG means you still have most of the logistics hassles associated with hydrogen, specifically hydrogen embrittling. Besides, you still have to produce the hydrogen.

The most interesting tidbit here is that the fuel was directly injected. Port injection of gaseous fuels displaces air in the intake manifold, reducing power. Direct injection eliminates the need to add a boost device just to compensate.

If you want to reduce throttling losses in a gasoline engine by leaning down the fresh charge, a cheaper way is to use 20-30% of externally cooled EGR coupled with 10-15% greater displacement. The asymmetry is due to the fact that exhaust gas features a higher specific heat index than air. However, it might be difficult to apply this concept to an NG engine because that fuel is already hard to ignite (120-130 octane) and flame propagation is slow, unless an accelerant like hydrogen or the less effective propane is added. Slow combustion limits engine speed and EGR would exacerbate this problem. Perhaps the compression ratio could be raised even higher to compensate, improving fuel economy at the expense of a heavier engine and additional NOx formation.

At very low loads, a variable compression ratio mechanism or else charge stratification may be needed to secure complete combustion at the high EGR rates needed to minimize throttling losses.

@ Polly -

perhaps the 30% blend refers to volume rather than mass fraction. It is true that stoichiometrically operated NG engines deliver lower fuel economy than CI engines in part load, because they are still throttled. Using externally cooled EGR should eliminate that disavantage, at least on the basis of fuel mass. In volumetric terms, a gaseous fuel - even if stored at high pressure - will always deliver less than a dense liquid.

Btw, while NG engines feature very low non-methane HC emissions, non-toxic methane is a potent greenhouse gas. You still want complete combustion and not just to save on fuel cost.



I would guess (without trying to do the chemistry equations) that if you reduce unburnt hydrocarbons that would lend itself automatically to more CO2 produced (more fully combusting the CH4 of the CNG to release more of the Carbon). I would think the increase in production of H2O and CO2 would balance out with the reduction in unburnt Hydrocarbons out the tailpipe.


Orbital used to use compressed air with their injection. I do not know if this is the case here.


Notice how the benefits of HCNG aren't that much better than just CNG.

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