The advantages of applying Compressed Natural Gas (CNG) as a fuel for internal combustion engines are well known. In addition to a significant operating cost savings due to a lower fuel price relative to diesel, there is an opportunity to reduce the engine's emissions. With CNG combustion, some emissions, such as Particulate Matter (PM) and Carbon Dioxide (CO2), are inherently reduced relative to diesel fueled engines due to the nature of the combustion and the molecular makeup of the fuel. However, it is important to consider the impact on all emissions, including Total Hydrocarbons (THC) and Carbon Monoxide (CO), which can increase with the use of CNG. Nitrogen Oxides (NOx) emission is often reported to decrease with the use of CNG, but the ability to realize this benefit is significantly impacted by the control strategy and calibration applied.
FEV has investigated the emissions and performance impact of operating a heavy-duty diesel engine with CNG in a dual fuel mode. The CNG was introduced via injectors mounted to an inlet pipe located upstream of the intake manifold. The fumigation approach included a mixer to improve the distribution of gas prior to delivery to the cylinder. The initial investigations sought to determine how the performance of a heavy-duty diesel engine would be affected by the introduction of CNG. For this effort there was no change to the base engine calibration, and the ability to maximize substitution of diesel with CNG was investigated. It was observed that the ability to maximize substitution of diesel with CNG across the operating map was limited by extremely high THC levels, combustion instability and limitations in peak cylinder pressure and exhaust gas temperature.
With the application of a simplified engine calibration with a single diesel injection and Exhaust Gas Recirculation (EGR), timing adjustments allowed higher CNG substitution levels in several areas of the operating map. A further increase in gas substitution along with higher fuel conversion efficiency, improved combustion stability and even lower emissions could be achieved through Reactivity Controlled Compression Ignition (RCCI) combustion. This approach required a unique injection strategy along with a careful balance of EGR rates and boost pressure. Under this combustion regime it was possible to observe a simultaneous reduction of NOx and PM emissions, approaching engine-out emission levels that could avoid, or significantly minimize, aftertreatment of these species.
With the desire to quickly apply CNG systems to existing diesel engine architecture in an effort to reap the benefit in fuel cost savings, manufacturers and system developers must be careful to understand the full impact on the engine's performance and emissions. Tests conducted as part of this investigation have revealed that an un-optimized approach to CNG introduction can lead to extreme THC emissions that mostly consist of Methane (CH4). In addition, the maximum gas substitution level is significantly limited in most regions of the engine operating map. Thus, the ability to specifically tune the calibration for operation with CNG is essential to achieving the maximum benefit in fuel cost savings and emission control.