Design considerations for an engine-integral reciprocating natural gas compressor

Abstract

Abstract Conventionally, compressed natural gas (CNG) vehicles are refueled using a high-cost, centralized, and sparse network of CNG fueling stations that has primarily been developed for the use of fleet customers. An engine-integral reciprocating natural gas (NG) compressor has the capability to disrupt the incumbent CNG market by enabling the use of NG for personal transportation, fueled at home, from the preexisting low-pressure NG infrastructure, at low parts count, using conventional components, and therefore at low incremental costs. The principal objective of this paper is therefore to describe and analyze the dynamic and thermal design considerations for an automotive engine-integral reciprocating NG compressor. The purpose of this compressor is to pressurize storage tanks in NG vehicles from a low-pressure NG source by using one of the engine cylinders as a multi-stage reciprocating compressor. The engine-integral compressor is developed by making changes to a 5.9 l displacement diesel-cycle automotive engine. In this novel design and implementation, a small tank and its requisite valving are added to the engine as an intermediate gas storage system to enable a single compressor cylinder to perform two-stage compression. The resulting maximum pressure in the storage tank is 250 MPa, equivalent to the storage and delivery pressure of conventional CNG delivery systems. Dynamic simulation results show that the high cylinder pressures required for the compression process create reaction torques on the crankshaft, but do not generate abnormal rotational speed oscillations. Thermal simulation results show that the temperature of the storage tank and engine increases over the safety temperature of the natural gas storage system unless an active thermal management system is developed to cool the NG before it is admitted to the storage tanks.