Cummins is working with a scheme for waste heat recovery to boost performance of its heavy-duty diesel engines to 50% Brake Thermal Efficiency while meeting the upcoming 2010 EPA emissions specifications.
Cummins has already demonstrated heavy-duty diesel engine performance of 45% Brake Thermal Efficiency with 2007 EPA emissions (earlier post). On the emissions front, meeting 2010 requirements entails a significant decrease of NOx emissions from 1.2 g/bhp-hr to 0.2 g/bhp-hr.
Brake Thermal Efficiency represents in percentage terms the amount of energy converted from fuel into useful mechanical work by the engine. A engine with a higher BTE level is more efficient, offering the potential for increased fuel efficiency and associated reductions in CO2 emissions. Percentage point improvements in this are hard-earned.
|The majority of the heat energy of the fuel is unused|
With average heavy-duty diesel BTE of around 42%, most of the heat energy released through combustion is lost. Cummins—and many others—reason that minimizing heat rejection while improving BTE would be ideal. Low Heat Rejection Engines (LHRE) using a variety of mechanisms have been a subject of study for years.
Cummins is dividing its work on this project into two components: continued optimization of the base engine, and development of the waste heat recovery system.
Work on the base engine platform involves further optimization of combustion, improvements to the high pressure common rail injection system, more low-temperature cooled EGR, and reduction of mechanical parasitic losses.
For the energy recovery system, Cummins has opted to go with a Rankine Bottoming Cycle system for the generation of electricity from the waste heat.
The basic Rankine cycle has four stages:
A working fluid is pumped from low to high pressure by a pump. Cummins is using a mix of Flourinal and water.
The pressurized liquid is heated at constant pressure by an external heat source (in this case, the exhaust gas) to become a superheated vapor.
The superheated vapor expands through a turbine to generate power output.
The vapor then enters a condenser where it is cooled to become a saturated liquid. This liquid then re-enters the pump and the cycle repeats.
|Concept Layout for Max BTE Demo Using Rankine Bottoming Cycle system.||Demonstration schematic.|
Cummins considered and rejected other approaches, including variants and combinations of Rankine Bottoming, turbocompounding and thermoelectrics, because of complexity or constraints on power delivery.
Current thermoelectric waste heat recovery schemes still do not provide enough power recovery for Cummins’ needs. The Rankine Bottoming Cycle, according to Cummins’ analysis, is proven in other industries, capable of recovering and delivering adequate power, could work for the out-of-vehicle high-BTE demonstration and is suitable for future on-board integration.
The fixed-nozzle, axial inflow turbine generator projected for the demonstration has a 50,000 rpm design point speed, with 400–450 VAC output. Target power is approximately 45kWe, with 77% turbine efficiency at the design point. Flourinol cools the bearings and the generator. This unit is not designed for future in-vehicle applications.