Diesel: Crystal clear
Piezo injectors are the most precise means of getting the right quantity of fuel into the combustion chamber. But the technology must evolve
- Published in Focus.
Piezo injector: Development experts want to exploit the improvements
Dr Klaus Wenzlawski
Technical expert for diesel engine combustion, Continental
Euro 6 will lead to greater market penetration of piezo injectors because of the demand for better minimal fuel mass accuracy and controllability.
Unlike other direct-acting piezo injectors our models have no hydraulic coupling device and therefore give greater freedom in injection rate shaping.
We’re working on quicker opening and closing times to improve HC and CO emissions for reduced compression ratios effective in low load areas with cold engine temperatures.
Looking to Euro 6 and beyond, our experience is that you cannot improve the injection system alone and hope everything will be well. If we consider that a 1% dispersion of intake oxygen results in a NOx dispersion of about 10%, it can easily be understood that most work has to be done on cylinder charge and its control. The fuel injection equipment plays the smaller part.
The main challenge for these advanced combustion modes, such as premixed charge compression ignition (PCCI), is a stable air: fuel ratio in a narrow tolerance band. The main contributor here is cylinder charge and its control via exhaust gas recirculation (EGR), not the injection system, for the reasons given above. But, because of the smaller injector-to-injector and shot-to-shot dispersion of piezo injectors, it’s easier to stay within the tolerance band.
There will be different injection modes such as multiple injection and rate shaping in different engine map areas to find the best compromise between fuel consumption and emissions. We are also seeking to replace some sensors with virtual ones.
Along with the start of series production for Euro 6, by then we will also be ready for the introduction of continuous injection rate shaping.
Because of CO2 reduction, downsizing, and especially downspeeding, will be used more and more. But no OEM will accept any increase in NVH. We’re sure that with a more sophisticated control of heat release we can find the right compromise between fuel consumption, pollutant reduction and NVH, so that combustion noise will not be an issue.
We’re integrating cylinder pressure sensing functionality into our software portfolio. It will contribute to the reduction of engine-out emission dispersion and EGR stability.
We’re improving simulation of the whole system, including injector behaviour. Most useful are improvements in nozzle design through CFD modelling of cavitation, spray plume shape and penetration, droplet size and distribution and spray break-up. It helps us to understand mixture formation and combustion.
The greatest challenge we have to face for piezo development is finding the correct balance between functional improvements and cost.
