Because of its ready availability, low cost and potential for reducing emissions, gaseous fuels have long been a promising substitute for liquid fuels for fuelling internal combustion engines. Natural gas is one example of such a gaseous fuel. Other examples include hydrogen, methane, ethane, propane, LPG, butane and mixtures of such gaseous fuels as well as gaseous fuel mixtures containing one or more of hydrogen, carbon monoxide and methane each of which can be produced synthetically.
In particular, replacing diesel fuel with gaseous fuels provides the potential for reducing emissions and lowering operating costs as diesel fuel burns with higher concentrations of pollutants and is generally more expensive than many gaseous fuels. The challenge, however, has been to substitute diesel fuel with gaseous fuels while maintaining the performance of diesel-fuelled engines including the efficiency of a diesel engine.
One way of maintaining the performance found in a diesel engine while using gaseous fuel is by directly injecting the fuel into a combustion chamber when the piston is near top dead center causing the fuel to burn in a diffusion combustion mode or in a stratified combustion mode where fuel and air are partially mixed.
Any direct injection engine benefits from techniques to ensure complete and efficient combustion of the fuel. Inefficient combustion results in higher emissions and reduced engine performance. Combustion in a diffusion combustion mode, in general, occurs at the fuel/air interface generally defined by the fuel jet. That is, the fuel has limited opportunity to mix with the intake air charge prior to combustion. Therefore, increasing the surface area of fuel exposed to the air charge helps promote combustion as more fuel is allowed to burn when desired during the early part of the power stroke. Diesel engines attempt to increase surface area between air and fuel to promote mixing and combustion by atomizing the diesel fuel and introducing mixing into the combustion chamber. For a gaseous fuel, atomization is not relevant, however; mixing is important for promoting combustion. Diesel-fuelled engine mixing is predominantly generated by the jet injection and the environment created by the geometry of the combustion chamber interacting with an intake charge drawn into the combustion chamber. This mixing can be important as well in promoting mixing of a gaseous-fuelled direct injection engine. The applicant, however, has found that, unlike diesel fuel, the properties of the gaseous fuel itself are useful for promoting mixing and, therefore, combustion.
As well as generally promoting combustion, variations in the fuel quality and charge properties influence combustion. Fuel quality can vary considerably for many gaseous fuels such as natural gas. One of the prior art methods of compensating for lower grade fuels (lower heating value) tends to encourage longer injection duration for direct injection engines and this technique is appropriate for gaseous fuels as well. However, long injection durations can negatively impact the efficiency of the engine. The same technique can be used in high exhaust gas recirculation (EGR) engines resulting in the same drawbacks.
One method of managing gaseous fuel mixing generally and a method for promoting combustion to adjust for the variations in the properties of the fuel (low quality fuel), intake charge (EGR levels) is to take advantage of the compressibility of the gaseous fuel. The compressibility of a gaseous fuel can be used to enhance combustion by enhancing mixing. As an additional benefit, the compressibility of gaseous fuels has also been found to extend the power range of gaseous-fuelled engines. While pressurizing gaseous fuels has been used previously-generally to force gaseous fuel into a combustion chamber (thereby utilizing the compressibility of the gaseous fuel), prior art has taught away from using a range of pressures take advantage of the compressibility of gases in order to promote combustion. By way of example, see Miyake M., et al., “The development of high output, highly efficient gas burning diesel engines”, CIMAC, 1983. Generally, injection pressure (and consequently compressibility of the gaseous fuel) has been driven by the need to force gas into the combustion chamber at full load (see U.S. Pat. No. 5,771,857, column 4, line 39).
U.S. Pat. No. 6,708,905 recognizes the benefits of gaseous fuel compressibility under limited circumstance. The disclosure provides for an injector nozzle design for taking advantage of gaseous fuel compressibility at low pressures to deliver a supersonic gaseous fuel flow at the exit of the injector nozzle into a combustion chamber to promote shock wave turbulence of the gaseous fuel in the combustion chamber. The disclosed injector nozzle design provides for such fuel flow at relatively low pressures. The drawback of the design is, in general, the sought supersonic flow within the combustion chamber is dependent on the cylinder pressure which varies throughout the engine map. The injector design can only take advantage of shock wave turbulence of a supersonic flow over a discrete range of the engine map.
Similarly, Tice J. K., et al., “Field Test and Development of a Low-Cost Mechanically Actuated, Enhanced Mixing System for Emissions Reduction”, Gas Machinery Conference (October, 2003: Salt Lake City), discusses supersonic gaseous fuel flow delivered by low pressure injector nozzle designs similar to those discussed in U.S. Pat. No. 6,708,905. As noted above, however, such an injector has limited application. Moreover, this publication teaches away from a high-pressure injection of gaseous fuel as undesirable for the reason that this art is directed at providing for a method of creating a homogeneous mixture of fuel and air for a spark ignited engine. There is no discussion of appropriate techniques for high pressure directly injected gaseous-fuelled engine designs that take advantage of the efficiencies found in diesel/compression ignition engines.
In the present technique, a gaseous fuel is introduced that utilizes the high pressure direct injection of gaseous fuel and the compressibility of the gaseous fuel to promote combustion over the range of the engine map resulting in enhanced power and compensating for poor fuel quality in gaseous-fuelled direct injection internal combustion engines.