Developing an Alternative Engine Concept
As the transportation industry begins to enter the next stage of power sources, a wave of new options is becoming available to consumers. What was once an industry dominated by gasoline spark-ignited and diesel compression-ignition engines is now diversifying into electrified, natural gas, hydrogen fuel cell and bio-sourced powertrains. However, not all of these alternatives present viable options for the long-haul heavy-duty trucking market due to its extensive distance and payload requirements.
Ricardo, a global engineering and environmental consultancy, has developed a new engine concept called CryoPower that the company believes can offer significant emissions and operating fuel cost reductions over current diesel powertrains. The engine, which has been in development for 10 years, is now ready for a full-system demonstration and subsequent pilot applications. To achieve this next stage, Ricardo has spun the CryoPower technologies and assets off into their own company, Dolphin N2 Ltd.
The CryoPower engine leverages two unique combustion techniques for reduced emissions and fuel consumption—liquid nitrogen and split combustion. Combining these two methods has allowed Ricardo to claim a 30% reduction in CO2 output as well as a 20% improvement in operating fuel costs. The company believes that the long-haul trucking and stationary power generation applications would be the first beneficiaries of these new technologies.
“We’re aiming at the heavy-engines market,” Neville Jackson, Chief Technology and Innovation Officer for Ricardo told Truck & Off-Highway Engineering. “The key element in choosing a large engine is total through-life cost. Because we can deliver the reduction in fuel cost where that cost is a significant portion of the through-life cost, then what we are offering becomes extremely appealing.”
Future applications like permanent power and train engines are also viable options for CryoPower technologies. However, the company also realizes this is not the alternative option for every market’s needs. “We recognize that for mobile applications of lighter duty and short-haul that electrification offers some strong benefits,” Jackson said. “But when you get to the heavier end, the energy density of batteries becomes less practical over 500 or 1000 miles.”
Liquid nitrogen and split combustion process
The CryoPower combustion process begins like most engines, but then quickly deviates from the standard course. Air enters the cylinder during the intake stroke. During the combustion stroke and after the intake valve has closed, liquid nitrogen is injected into the cylinder. The spray evaporates and cools the intake air as the pressure increases. Near TDC (top dead center), the exhaust valve opens and the high-pressure intake charge is released from the compression cylinder into a heat exchanger.
Once in the heat exchanger, the intake charge is heated by the exhaust gases. The intake mixture is then released into a second cylinder over a short crank angle near TDC. Fuel is then injected into this high-pressure and high-temperature condition, which allows for ignition and expansion without the typical mixing tumble and swirl requirements. After the power stroke, the combustion gas is released from the expansion cylinder through an exhaust valve and back through the heat exchanger. This process is designed to work with or without EGR (exhaust gas recirculation) assistance.
“There are three specific areas that deliver high efficiency, not because the engine is split cycle but what the split cycle enables,” Jackson explained. “The first is that as we compress the gas in the compression cylinder and as we cool the gas down, we are doing less work than a conventional engine with the same compression pressure—between 15 and 20% less compression work. The second thing we are doing is recovering exhaust gas heat to the hot part of the cycle like a recuperating gas turbine. The third thing is having the two cylinders have different swept volumes. We have a lower volume in the compression cylinder than we do in the expansion cylinder, so we have a true Miller cycle.”
The only additional components required for the system include the liquid nitrogen storage tanks and injectors as well as the heat exchanger between the compression and expansion cylinders, which Ricardo calls a recuperator. An image of a possible CryoPower engine application shows a six-cylinder configuration. In this instance, two cylinders are used for compression and four cylinders are used for expansion. This allows for four power strokes per crankshaft revolution since the expansion cylinders function on a two-stroke cycle.
A decade in the making
Ricardo has been developing the CryoPower technologies for the past 10 years. The research has been focused on proving individual improvements to the powertrain before bringing it together in a single system. The liquid nitrogen was tested to determine the process of injection and evaporation. Other gases were tested, but nitrogen was determined to be the best option due to it being a stable, inert gas and its evaporative properties. The valvetrain was also designed to handle the split combustion process.
However, one aspect proved to be more difficult than the others. “The one critical area we struggled with was the combustion system,” Jackson told TOHE. “More recently, we have had a major breakthrough in getting the combustion system to work and to work exceptionally well. That’s been the breakthrough in getting a split-cycle, recuperating engine to work.”
The team at Ricardo has partnered with local U.K. universities to aid with the development research. The University of Brighton and the University of Southampton have helped with providing additional simulations to prove out these new combustion techniques. While Ricardo has been the leader in the development process, the universities have allowed for more experimental research that has led to some of the combustion advancements.
Next steps: demonstration and further refinement
As those developments have come to realization, the time has come to apply them in demonstration and pilot units. All CryoPower property and assets are being spun into their own company called Dolphin N2. While Ricardo will remain an active member of Dolphin N2, the company believes now is the time to include other partners in this research.
“We have reached the stage of maturity in all of the subsystems now,” Simon Brewster, CEO of Dolphin N2 told TOHE. “Now is the time to piece all of those together and take them into a multi-cylinder demonstrator that will look like a normal engine so you can simply install it into a vehicle or a gen-set.”
The cost of this next phase to bring the CryoPower technologies into a production atmosphere is beyond the standard investment level of a single company. Dolphin N2 now fits into a venture capital investment space that will include multiple backers. While Ricardo did evaluate the opportunity to bring the engine to market through a single customer, the team felt the technology still requires some continued development that goes beyond the abilities of a single company over a shorter timeframe.
Dolphin N2 realizes that the long-haul trucking and power generation industries are in need of fuel-efficiency improvements along with emissions reductions to meet both future regulations and consumer demands. While the CryoPower system has been in development for a decade, the market is now ready to accept these alternative power source technologies. The liquid nitrogen and split combustion processes allow for these improvements while minimizing the changes required to current infrastructure.
Another powertrain option appears poised to come to market in the not-too-distant future.