Preventing Fuel Tank Oxygen Ingress for a Bimodal CNG Internal Combustion Engine
Zachary Taie and Christopher Hagen

Abstract: We have created a bimodal compressed natural gas (CNG) internal combustion engine which can compress residential natural gas from approximately atmospheric pressure at the home to US CNG on-vehicle storage pressures (250 bar).  This engine acts both as a refueling compressor and as a power producing device for vehicle locomotion.  This is to say, one cylinder of this engine acts as a compressor at times and at other times combusts as normal.  
One of the challenges of this engine is determining the sequence of events for switching from combustion mode to compression mode.  As one might imagine, it is undesirable to compress atmospheric oxygen into a CNG tank (even though it would be unlikely for the mixture equivalence ratio to go under the rich limit).  The work here describes approaches to mitigate undesirable in-tank fuel air mixing during mode switching.
Processes will be model numerically to determine expected results and experiments will be conducted using a fast oxygen sensor in the fuel line.