There are emissions benefits of operating diesel engines using natural gas. Natural gas, by way of example, is a clean burning fuel (relative to diesel). Specifically, natural gas, in general, allows engines to operate with reduced emission levels of particulate matter (PM), hydrocarbons, greenhouse gases and nitrogen oxides (NOx).
Additionally, replacing conventional fuels with natural gas, which is a relatively abundant and broadly available fuel, helps to reduce dependence on oil.
In this disclosure the invention will be described in relation to natural gas fuelled engines. However, other gaseous fuels may be substituted with similar benefits. By way of example, gaseous fuels such as methane, ethane, propane and lighter flammable hydrocarbon derivatives, as well as hydrogen and hythane™ (a mix of natural gas and hydrogen) and other such gaseous fuels may also be used with the invention.
Recent developments in natural gas fuelled engine technology has shown that natural gas injected into a combustion chamber at high pressure can result in engine performance that is similar to diesel engine performance. Such a high pressure direct injection technology being developed by the Applicant is known as HPDI™ technology, which provides a solution to emissions levels associated with diesel-fuelled engines while incurring little or no penalty in regards to diesel fuelled engine performance.
Natural gas using current diesel engine technology, requires an ignition assist strategy. Unlike diesel fuel, natural gas does not auto-ignite quickly when injected into the combustion chamber environment established in a typical operating diesel engine. Therefore, in order to ensure timely ignition and combustion of natural gas in such engines, an ignition assist is provided. For the purposes of this disclosure, the auto-ignition temperature of a fuel is the temperature of the intake charge within the combustion chamber that will cause auto-ignition and combustion of that fuel. Auto-ignition should occur within a time period after directly injecting the fuel that is suitable to provide the required energy to the pistons. For clarity, ignition assist strategies are needed for the other gaseous fuels noted above as well as some liquid fuels such as methanol and other fuels that have relatively high auto-ignition temperatures.
One such ignition assist strategy employs a glow plug or other hot surface projected into the combustion chamber. Natural gas impacting on such a hot surface will ignite and burn.
A second ignition assist strategy is to use a small quantity of diesel as a pilot fuel. Here diesel is injected into the combustion chamber shortly before or at approximately the same time as the natural gas is injected, when the piston is near top dead center. As diesel will generally auto-ignite under the conditions established within the chamber when the piston is near top dead center, the combustion of this diesel will trigger the ignition of the main gaseous fuel, such as natural gas.
In this disclosure, “near top dead center” means within 30 degrees of top dead center.
There are technical challenges associated with both approaches. With hot surface ignition, if combustion is initiated at one point within the combustion chamber, incomplete combustion can occur resulting in increased hydrocarbon emissions and loss of combustion energy that could otherwise be utilized to drive the piston. In order to promote complete combustion, the flame generated at the hot surface needs to be propagated throughout the combustion chamber so that all of the injected natural gas is ignited. However, since flame propagation can be imperfect, gas directed into parts of the chamber furthest removed from the glow plug may fail to ignite or it may burn incompletely. See by way of example Mueller, C. J. and Musculus, M. P., “Glow Plug Assisted Ignition and Combustion of Methanol in an Optical DI Diesel Engine”, SAE paper 2001-01-2004. If flame propagation is not rapid enough to initiate combustion of gas within the combustion chamber, the directly injected quantities of natural gas may become too lean for ignition.
When using pilot fuel for the ignition assist strategy, two fuels must be accommodated. That is, a separate fuel supply system is needed, adding complexity in the engine. Also, depending on the amount and type of pilot fuel, there may be undesirable emissions generated by combustion of the pilot fuel.
Further, a pilot fuel system adds cost to a natural gas engine. There are the capital costs associated with the pilot fuel system as well as the maintenance time and costs for such maintenance.
Other means may be used to help ignite HPDI™ natural gas engines. By way of example, natural gas can be spark-ignited after the gas is directly injected. However, spark ignition of a directly injected natural gas has many of the same challenges as glow plug ignition.
The present invention addresses the problems noted above.