Ricardo and Univ. of Brighton working on advanced combustion system for heavy-duty vehicles; CoolR features recuperated split-cycle with isothermal compression using cryogenic injection
|The concept of the Ricardo Split-Cycle engine. The recuperated engine uses isothermal compression via cryogenic injection to enable significant exhaust to compressed gas heat transfer. Source: Neville Jackson. Click to enlarge.|
Ricardo and the University of Brighton will model and evaluate an advanced split-cycle combustion system aimed at substantially reducing the carbon emissions of heavy-duty vehicles. The feasibility study is part-funded by the UK Technology Strategy Board as one of the winning submissions for the recent “Disruptive technologies in low carbon vehicles” competition.
Unlike many previous research projects that have focused on refining existing four-stroke engine technology to reduce fuel consumption and emissions from heavy-duty vehicles, the CoolR project will examine a fundamentally new split cycle combustion concept based on a recuperated split-cycle with isothermal compression using cryogenic injection. (In an isothermal process, the temperature is constant.) Professor Neville Jackson, Ricardo’s Chief Technology & Innovation Officer, had presented an overview of the basics of such an approach at the SAE 2011 High Efficiency IC Engines Symposium in April.
The Ricardo Split-Cycle engine concept incorporates the following:
- Liquid Nitrogen (LN2) injection during compression to control temperature rise and increase mass.
- LN2 produced using the engine (efficiency of generation is a key parameter).
- Recuperator to transfer heat from exhaust gas to compressed air.
Turbine (Brayton cycle) efficiency can be substantially improved through recuperation, Jackson noted. Recuperated Brayton cycle efficiency improvement is a function of exhaust and end of compression temperatures, as well as of compression ratio. However, reciprocating engines with high compression ratios offer less opportunity for recuperation.
Lower compression ratios would provide more opportunities for recuperation, he said, but with lower overall efficiencies. High compression ratios reduce this temperature difference with less scope for recuperation. The key challenge is to maintain high pressure ratios for good simple cycle efficiency and to increase the capacity for recuperation.
|Example of a recuperated diesel cycle with high compression ratio and isothermal compression. Source: Neville Jackson. Click to enlarge.|
The key to successful recuperation with high pressure ratios is isothermal compression, Jackson said. Further, isothermal compression reduces compressor work; test show about a 17% reduction in energy requirement. However, implementation of practical isothermal compression is challenging.
A recuperated combustion engine transfers exhaust heat to the working gas at the end of compression and at constant volume. It requires a separate compressor and expander—e.g. cross-linked reciprocating piston and cylinders.
|The IsoEngine concept. Click to enlarge.|
Ricardo has already successfully demonstrated a spilt-cycle isothermal compression engine in static form for power generation purposes. The “IsoEngine” prototype demonstrated by the company for energy utility Innogy in the 1990s used water injection to achieve a thermal efficiency in excess of 60% in comparison with around 43% for a current state-of-the-art on-highway heavy duty diesel engine.
The IsoEngine separated compression and combustion/expansion processes in different cylinders. The charge air was compressed isothermally by spraying a large volume of water into the cylinder during compression to 100 bar. The charge air was heated to 750 °C by the low pressure exhaust gases in the recuperator.
Combustion occurred in the separate cylinder at constant pressure by staged injection of the fuel; work was extracted through expansion of the combustion products and recovered through the crank.
|Ricardo has patented an approach to a split-cycle isothermal compression engine with cryogenic injection. Click to enlarge.|
While water injection would not be practical for a vehicular implementation, the CoolR concept aims to achieve the same thermodynamic benefits using liquid cryogen injection. Allowing for the energy costs of cryogen production, this would result in a thermal efficiency improvement of around 40%. This is significantly better than that of other promising technologies also currently being researched such as exhaust heat recovery concepts based on thermo-electric generation or the Organic Rankine Cycle which offer improvements of around 10-15%.
Key requirements for such a mobile application would be the ability to handle transient loads and speeds, compact packaging, and fast start and load acceptance, Jackson said in his talk. There might also be consideration of the capability for kinetic energy storage and release.
In his talk, Jackson said that a 2-liter split-cycle engine operating at 50 kW and 40 bar Pmax could utilize more than 18 kW of exhaust heat and deliver indicated efficiency of 60%.
In this one year feasibility project, the partners will carry out a concept study aimed at addressing the fundamental questions that industry will face if such a radically new technology were to be adopted. In doing so, it is intended that a road map be developed to identify the necessary work required to bring the CoolR concept from feasibility to systems prototype and beyond.
The global imperative to reduce the carbon footprint of road transportation is now almost universally accepted. While electrification, hybridization and improvements of the existing internal combustion engine offer a pathway to sustainability for light vehicles, a major problem remains in the heavy duty sector. By fundamentally reviewing the underlying thermodynamics of the internal combustion engine in a manner unseen for many decades, we believe that the CoolR spilt cycle cryogenic injection combustion concept offers the prospect of very significant improvements in thermal efficiency and hence reduced carbon dioxide emissions in the economically crucial heavy vehicle sector.—Nick Owen, project director for research and collaboration at Ricardo UK
Neville Jackson, “Future Reciprocating Combustion Engine Efficiency – How much further can we go?” (SAE 2011 High Efficiency IC Engines Symposium)