Truck
​In 2017 the new piston arrived in the first trucks in the United States​​.
​Photo: Volvo Truck Corporation

Wave Piston Design Lowers Fuel-consumption

​Volvo’s new Heavy-Duty diesel-engines are more fuel efficient due to a new, smart, wave-shaped, piston design. The new design reduces fuel-consumption by two percent and halves the quantity of particulates. The idea of the piston shape came from Volvo AB. In collaboration with Chalmers, the idea could be refined and realized.
​Ten years ago, diesel-engineer Jan Eismark was struggling with a problem of reducing emissions from Volvo's engines. The permitted limit values for soot particles and nitrogen oxide emissions were constantly lowered. One big challenge is that the particle and soot emission formation in the combustion chamber is just like a rocking board. The methods limiting soot particles increase nitrogen oxides and the methods that lower nitrogen oxides increase soot particles. The challenge was to lower both. 

Jan Eismark conducted a variety of engine experiments with different pistons and fuel injectors and saw that the soot emissions were very different. The conclusion was that the shape of the combustion chamber, which is completely shaped by the piston top, ought to be very important. 

In the case of a standard piston, the injector is located in the centre of the piston bowl (combustion chamber) and the fuel is sprayed towards the sides of the bowl through a number of orifices in the injector. The combination of heat and pressure causes the fuel to ignite before it reaches the combustion-chamber walls. The flame hits the wall at a speed of up to 50 meters per second, it then spreads along the piston bowl wall at an angle of 180 degrees where-after it collides with the adjacent flames. When the flames collide, they compete for the available oxygen. At the same time, the oxygen in the centre of the combustion chamber is never fully used. 

"We wanted to find a way to lead the flames more inwardly into the combustion chamber to better utilise the available oxygen there", says Jan Eismark. 

Jan Eismark became an industrial PhD student at Chalmers, to develop the idea together with Chalmers’ researchers through studying fundamental mixing and spray phenomena and combustion mechanisms. 

"The research work in the project has been very extensive and includes, in addition to Volvo's engine experiments, advanced computerised combustion calculation and high-speed recording of the combustion inside the cylinder", says Ingemar Denbratt, director of the Combustion Engine Research Centre, where the research at Chalmers was conducted. 

The research was used to improve the combustion system and resulted in the unique wave design in the piston bowl. The injector position in the centre of the piston bowl has six holes allowing the fuel to be injected in between the waves helping the flames to be directed towards the centre of the piston bowl. The available oxygen could therefore be consumed more efficiently. 

After that, industrialisation has been taken over by AB Volvo and in 2017 the new piston “arrived” in the first
trucks in the United States. Fuel consumption has been reduced by two percent and particulate emissions have been halved. According to AB Volvo, the concept give big fuel savings on Volvo's products and reduction in carbon-dioxide emissions of 5 megaton per year. 

(For comparison, a return flight between Sweden and Thailand corresponds to approximately 2.5 tonnes of carbon-dioxide emissions per person, hence 5 megaton of carbon-dioxide emissions corresponds to approximately two million Thailand voyages.)

Published: Thu 28 Jun 2018.