Extending Substitution Limits of a Diesel–Natural Gas Dual Fuel Engine

Abstract

New drilling techniques have increased availability and decreased costs of oil and gas. The decreased costs have caused an increase in drilling activity. The well sites have a large power demand that is typically met by diesel engines for the drilling derrick, fracking pumps, and electrical power. Dual fuel retrofit kits are being increasingly used at well sites to reduce operating costs and the amount of fuel trucked in to the site. Natural gas (NG) is cheaper compared to diesel and can be delivered to the site by the pipeline limiting the disturbance to surrounding communities due to diesel truck loads. The purpose of this work is to examine the performance of a typical dual fuel retrofit kit commissioned for field operation on a 6.8 L tier II diesel engine. After the baseline commissioning, the mechanisms limiting further substitution were clearly identified as engine knock similar to end gas auto-ignition in spark-ignited engines and governor instability. Two methods are examined for their ability to increase substitution limits by adjusting the start of injection timing (SOI) and the intake air manifold temperature. Retarding the SOI is able to delay the onset of knock at high loads and therefore increase the substitution level by around 4% at full load. At high loads, lowering the air manifold temperature is able to increase the substitution levels by around 10%. Preheating the intake air was able to increase low load substitution levels by 10% as well.