Many vehicles now have CO2 outputs of under 100 g/km, but the majority of them are small cars or those that use a hybrid powertrain.
For the Syner-D program – a project led by Ricardo and part-funded by the UK’s innovation agency, the Technology Strategy Board – the aim was to deliver a premium diesel demonstrator vehicle displaying a CO2 reduction of 30 percent, achieving Euro 6 tailpipe emissions, and suffering no compromise in transient driveability and overall performance.
At the completion of the program, the results exceeded the target, with a measured test-cycle CO2 reduction of 32.5 percent.
No Hybrid Technology
The goal was to achieve these targets without costly hybridization.
“Ricardo is a firm believer in hybrid technology, but there is an argument that it currently represents a relatively high cost for the CO2 benefits delivered,” explains Andy Ward, Ricardo’s head of light duty diesel engines.
“While hybrid cost benefits continue to improve, with Syner-D, we wanted to target what was possible with a conventional powertrain.”
The collaborative three-year effort included input from Jaguar Land Rover, Valeo, SKF, Shell and Lontra – each company supplying specific components and expertise in its specialized area.
An important factor for the development team was to make use of technologies that were available off the shelf, the challenge being to demonstrate they could all perform on a vehicle together to amplify the overall benefit.
“These components are already available, so instead of developing new technologies, a lot of effort went into tailoring them to work with the rest of the system,” explains Raoul Day, chief engineer, performance and calibration, at Ricardo.
There were a number of key areas that Ricardo and its partners wanted to focus their efforts on in order to achieve their desired CO2 target. These included:
The base vehicle for the Syner-D project was a Jaguar XF, powered by a 3.0-litre Euro 5 diesel engine. The first step of the project would be to downsize this to the 2.2-litre DW12 inline four-cylinder unit calibrated for Euro 6 emissions levels.
“The objective was to achieve comparable transient performance, and going from a 3-litre to a 2.2-litre goes against that, so we then have to add some technology to increase the performance beyond the baseline 2.2 litre,” explains Ward.
To achieve the targeted torque, analysis was carried out using Ricardo’s WAVE program, identifying the optimal systems configuration.
“We selected an advanced boost system comprising a low-pressure turbocharger and high-pressure mechanical supercharger, which is quite a novel approach,” said Ward.
The low-pressure turbo was retained from the larger engine, while a Roots-type belt-driven supercharger was added. The supercharger is declutched to minimize losses when not required. In this situation, the air path is bypassed around the supercharger.
In addition to the advanced boosting, a low-pressure EGR circuit is used in combination with SCR (selective catalytic reduction) aftertreatment, giving more capability than is required to meet the Euro 6 emissions targets.
“We could have selected a two-stage [turbo] boosting setup, as that technology is already in the market, but our approach presented a more interesting research and development proposition,” says Ward, adding that the target of Euro 6 emissions could not be ignored.
“If we had used two-stage turbo boosting, adding NOx aftertreatment would have meant temperature issues from the two turbines because of the temperature drop along longer pipe lengths; this in turn would mean the NOx aftertreatment would be less effective.
“Having a supercharger at the high-pressure stage means the exhaust system layout is unaffected, and you’re left with the same high temperature as you would get on a standard, single stage turbo variant. Basically, while there are some synergies between supercharging and NOx aftertreatment, you don’t find the same synergy between two-stage boosting and NOx aftertreatment.”
At the same time as changing the base engine, the Syner-D team also swapped the original six-speed transmission for an eight-speed unit, which can operate at more efficient speeds and load points. Additional calibration work was carried out to minimize up-shift speeds, and with the new unit, it is now possible for the enhanced torque delivery of the engine to be used when operating off the [emissions test] cycle, limiting the perceived changes in performance in the process.
System temperature control
“The coolant heat storage tank was a key technology in helping to achieve our CO2 reduction goals,” explains Ward.
“It is similar to the device housed in the Toyota Prius, which releases heat retained in coolant to boost engine temperature at the start of a test. Valeo provided a lot of the elements of the thermal system, and these were integrated by Ricardo into a functioning system, making them work in the most efficient way possible.”
In simulations using Ricardo’s VTherm program, an improvement in fuel economy of 4.7 percent was predicted with a combination of zero coolant flow during warm-up, elevated coolant temperature when hot, and heat storage tank release.
Roller bearing crank
To reduce friction, Ricardo used a number of measures including a roller bearing crankshaft from SKF and low-viscosity lubricants courtesy of Shell.
The challenge was to develop the world’s first high-speed diesel with a roller bearing crankshaft, a technology typically applied to specialized gasoline engines. The main benefit to the Syner-D development team of using this crankshaft configuration was the reduction in frictional losses within the engine – especially in the warm-up phase, thus helping to cut CO2 emissions.
In the case of Syner-D, the inner bearing race is the actual crank face machined to suit the barrelled roller elements. The oil feed to the connecting rods is maintained through the crankshaft.
Together with SKF, Ricardo embarked on a rigorous test bench-based evaluation program.
“We ran an intensive, high-load 100 hour durability test on the system, because one of the key concerns around a development such as a roller bearing crank is that it is durable,” says Ward.
“During testing we also encountered a material hardness issue that was rectified, so it was a doubly good result as we learned something about the engine but also got the result that we needed.”
The engine oil was specified using test bed mapping data at a range of operating conditions to identify the overall optimum oil for warm up and fully hot fuel consumption benefits. Data compared the standard 5W/30 to 0W/20 and 0W/30 formulations. The testing resulted in 0W/30 being chosen for its lower temperature friction improvements. The 0W/30 oil supplied by Shell was also successfully used for the durability test of the roller bearings.
An important step forward on Syner-D programme was to consider the engine and aftertreatment system as a whole within the Ricardo Efficient Calibration toolset, and to employ the optimizer to select engine calibration to get the best overall result from the system.
“The traditional approach is to divide the problem into aftertreatment and engine, and calibrate them individually against targets set respectively for each,” explains Ward.
“However when you combine them you don’t get the optimum result since the performance of the aftertreatment depends on exhaust temperature and engine calibration modifies the temperature. With a combined model of the engine and aftertreatment system, we can achieve the right tailpipe NOx at minimum cycle CO2, allowing the optimizer to take into account all the complex trade-offs to balance the calibration at any given time in the drive cycle.”
Through the Syner-D project, Ricardo believes it has identified areas that will become common on passenger car vehicles in the future.
“We have demonstrated the feasibility of systems that might not be achievable in today’s cars, but which are certainly technically possible,” reasons Ward.