Despite being in existence and undergoing continuous development for 140 years, the internal combustion engine can still be improved, and Volvo recently highlighted one of its recent improvements that resulted in more fuel-efficient heavy-duty engines in the U.S.
“Over the past years, focus has mainly been on using computers to more efficiently control the combustion process,” says Jan Eismark, technical specialist in combustion systems. “It’s therefore, particularly pleasing that the change we have just made is exclusive to the hardware and the design of the piston crown.”
The more efficient combustion halves the quantity of soot particles and has reduced fuel consumption by an average of 2%.
However, it did require cutting-edge technology to prove that the initial ideas were correct and to identify the exact design of the piston crown. This involved using computational fluid dynamics (CFD) and high-speed filming of the combustion process inside the cylinder.
When the idea arose, the combustion development team was trying to reduce the amount of soot particles in exhaust gases in a project partly funded by the Swedish Energy Agency. They were using a single-cylinder engine to test different pistons and fuel injectors. The results of both engine test results and the CFD calculations showed substantial differences in soot quantities.
“The design of the combustion chamber seemed to be significant,” says Jan Eismark, who started to sketch possible changes to the design.
In a standard piston, the injector is located in the piston top and the fuel is sprayed toward the cylinder sides through a number of orifices in the injector. The combination of heat and pressure causes the fuel to ignite before hitting the wall. The flame hits the wall of the combustion chamber at a speed of up to 50 meters per second, spreads along the piston bowl wall and then collides with adjacent flames at an angle of 180 degrees, still at a high speed.
When the flames collide, they compete for the oxygen available there. At the same time, the oxygen in the center of the combustion chamber is never fully utilized.
“Therefore, we wanted to identify a method of leading the flames into the center of the combustion chamber and thereby better utilize the oxygen there,” explains Eismark.
They found they could do this using the ridges or waves in the piston crown. The piston has six such ridges and the injector, which is located in the center of the piston top, has six orifices to ensure the fuel is sprayed between the ridges. Eismark owns the basic patent for the technology together with his colleague Michael Balthasar.
But a clever idea will remain only an idea unless it can be industrialized. The new piston crown must be of at least the same quality as a standard piston crown and it should not cost any more to manufacture.
“Together with our suppliers, we have invested considerable effort into identifying a manufacturing method for the new piston crown,” says Frank Löfskog, responsible for the piston system for heavy-duty engines at the Volvo Group.
Standard pistons are forged and then finished by machining on a precision lathe, but this method will not work with the new design. Instead the piston must be precision forged to eliminate the need for further processing.
“The piston development team, the supplier and the combustion development team have had many discussions and conducted numerous tests to identify exactly how the piston crown should be designed to deliver the desired advantages and, at the same time, be possible to manufacture in a cost efficient manner,” says Löfskog.
The new piston crowns are ready and you'll see the new technology in engines in trucks from Volvo and Mack starting in January 2017.