The Ultra Lean Burn Partially Stratified Charge Natural Gas Engine

It is well known that lean operation of homogeneous- charge spark-ignited engines is effective in increasing thermal efficiency and reducing exhaust emissions. In particular, the lower combustion temperatures provided by a lean air-fuel mixture result in a significant reduction in NOx emissions. Lean operation is normally restricted, however, by the “lean-limit” of combustion, as measured by the air-fuel ratio above which ignition is impossible, or combustion is incomplete. In order to extend the lean limit of operation a new “partially-stratified charge” combustion concept has been developed. This technique relies on the fact that a stronger initial flame kernel produced following the spark event should also be effective in igniting a very lean mixture which may not otherwise ignite, or which may result in incomplete combustion. An innovative spark-plug insert design, in which a small portion of “pilot fuel” is injected directly near the spark plug electrodes, provides a small pocket of relatively rich mixture that ignites more readily than the main, very lean, combustion charge. This has then been used to extend the lean-limit of operation of natural-gas fuelled spark-ignition engines, resulting in reduced brake specific fuel consumption and significantly lower levels of NOx emissions. This process has also been shown to be effective in increasing the stability of combustion, thereby reducing cyclic variations in cylinder pressure. Recent data obtained using this concept in a single-cylinder research engine are reported in this paper. In addition, some initial studies of the partially stratified-charge mixing process are also described.