Illustrative split cycle engine layout in a V configuration (cross section view). Based on a modified MTU Series 396. Gurr (2016). Click to enlarge.

The new cycle combines both Diesel and Ericsson Cycles into an integrated process to recover waste heat within a single thermodynamic cycle. The Ericsson cycle comprises two isothermal and two constant pressure (isobaric) processes. (An Ericsson engine based on the Ericsson cycle is an external combustion engine, because it is externally heated.) To improve efficiency, the engine has a regenerator or recuperator between the compressor and the expander.

Key changes to implement the integrated cycle are isothermal compression and heat addition from the exhaust gas to the charge air after the end of compression.

To transfer exhaust gas heat at the end of compression requires the use of a “split cycle” in which the standard four-stroke sequence is split into two separate elements. Intake and compression are carried out in one cylinder; combustion plus expansion and exhaust in another. Gas is transferred from the compression cylinder to the expansion cylinder via a heat exchanger that transfers heat from the exhaust to this high pressure compressed gas.

Using isothermal compression enhances the recovery of exhaust gas waste heat and significantly improves cycle efficiency.

Ricardo has explored a number of isothermal compression technologies, including use of a water spray during compression and also direct injection of cryogenic nitrogen or air. The use of a cryogen for isothermal or sub-cooled compression results in a more efficient compression process, and also realizes a reduction in pumping work, while increasing the thermal differential across the heat exchanger, driving increased capture and reinvestment of waste heat.

The benefits of a split cycle have been demonstrated numerous times by different bodies. Split cycle with waste heat recuperation but with water fulfilling the role of compression cooling fluid was demonstrated by National Power and presented here at CIMAC in (2004 and 2007) by Coney. Employing a cryogenic fluid to cool the charge air during the compression stroke is the inventive step taken more recently by Ricardo. However, should that additional fluid run out, the engine can run on in a conventional recuperated split cycle mode, utilizing some of the split cycle benefits to a lesser extent, such as waste heat recuperation. Industry progress in the key technologies and Ricardo’s innovations are enabling the split cycle technology to now be implemented. This new activity coupled with fuel saving and CO₂ drivers has brought about a new interest in split cycle.

—“The 60% Efficiency Reciprocating Engine: A Modular Alternative to Large Scale Combined Cycle Power”

For the CIMAC paper, Ricardo focused on a total cost of energy assessment of a recuperated split cycle reciprocating internal combustion engine with liquid nitrogen as the cryogenic fluid introduced for compression cooling. The basis of this evaluation was for a power generation application, replacing single large Combined Cycle Gas Turbines (CCGT) units with modular and flexible split cycle genset installations.

CCGT units of > 250 MW are achieving 55-60% brake thermal efficiency (BTE) at full load. Ricardo says that its split cycle technology also has the potential to achieve circa. 60% while also enabling a more distributed power solution, allowing for further efficiency increases through reduced grid losses.

For the CIMAC analysis, Ricardo used three unit power capacities: 4MW, 9MW and 18MW. Total cost of energy was calculated for the split cycle solution at these nodes; the total cost of energy was also evaluated for baseline diesel powertrains and diesel powertrains with Organic Ranking Cycle (ORC) waste heat recovery fitted.

The analysis found that the split cycle engine could achieve class leading efficiency levels in the 1-4 MW range, comparable with very high efficiency large scale CCGT technology. Overall, other than solid oxide fuel cells (SOFCs), which are not anticipated to be available for some time yet, a <15MW split cycle can achieve a class leading BTE.

The Innovate UK-funded split cycle research program to install, commission and test a single cylinder combustor was completed earlier this year. The EPSRC-funded ULTRA project now takes over test bench testing of the engine.

Ricardo is also proposing a three-year collaborative program to design, build and test a multi-cylinder split cycle engine, most likely based on a heavy-duty diesel (HDD) platform to accelerate development with two compression cylinders and four combustion/expansion cylinders. Running in 2-stroke operation, this unit would match conventional specific power and torque profiles.

Resources

• Adam Gurr (2016) “The 60% Efficiency Reciprocating Engine: A Modular Alternative to Large Scale Combined Cycle Power” CIMAC 2016 paper Nº 267