In this feature we're looking at some interesting developments in fuel system technology for common rail diesels. In previous posts, we've looked the pressure wave phenomena in common rail diesels and how this can significantly affect the accuracy with respect to the quantity of injected fuel per stroke.
Denso has addressed pressure wave phenomena and taken the intelligence in diesel engine fuel systems to the next level with the introduction of their Intelligent Accuracy Refinement Technology (i-ART). The technology features a fuel-pressure sensor with an integrated microcomputer which monitors injection pressure, based on various input data. The whole assembly is integrated into the top of each fuel injector. The closed-loop system precisely manages injections of fuel to match specific drive cycle conditions. It replaces the single pressure sensor typically positioned in the fuel rail. Denso engineers have stated that i-ART can improve fuel efficiency by 2%, compared with open-loop systems. It was developed to enable diesel engines to meet Euro 6 regulations with a reduced after-treatment burden. Toyota also is using i-ART systems in upcoming 3.0-L commercial diesel engines.
Fig 1 - The new range of Volvo power units includes Denso i-ART technology (source: Volvo)
A conventional injection system could only detect an injection quantity based on indirect methods such as combustion or an engine rotation fluctuation. The i-ART system enables a direct detection of the injection quantity, each injector is equipped with a built-in fuel pressure sensor to measure injection pressure inside the injector itself. Based on the information from the built-in pressure sensor, the Engine Control Unit (ECU) reads fuel pressure values for each injection rapidly, and calculates an actual injection quantity and timing for each cycle, based on this information, using a rapid waveform processing technique. The learning value for the injection quantity and timing calculated with the i-ART system are applied to subsequent injections and adapted throughout its lifetime.
Fig 2 - System overview - i-ART intelligent injectors and feedback data flow (source: Denso)
The actual pressure wave form generated by the i-ART pressure sensor is shown in Figure 3. The system performs a pre-processing by compensation to the non-injection pressure waveform in order to estimate the injection quantity and timing correctly. It then calculates the injection rate based on the processed pressure waveform which is optimised by filtering. The injection rate can be expressed by five parameters of a trapezoid shape. Calculating the area of the trapezoid, the injection quantity is obtained. (Figure 3 lower diagram)
The i-ART system learns the injection quantity and timing constantly while the engine is in operation - there are two advantages to using this characteristic. The first is the possibility to use a triple pilot injection strategy - which allows a lower a compression ratio to be used, as less heat is needed to be able to ignite the fuel under all operating conditions. This is due to the improved mixture formation which promotes efficiency in the early stages of fuel injection/initial burning. In addition, this allows a sufficient preheating effect for the fuel with a reduced overall cylinder temperature, such that NOx and PM can be reduced. As a second advantage, in conjunction with cetane number detection, a stable combustion with minimised combustion noise can be achieved irrespective of the variation of cetane number with fuels in certain markets.
Figure 3 - Fuel pressure waveforms at the i-ART injector (source: Denso)
This technology is a big leap for common rail diesels, but also a significant step forward for measurement technology that can now be employed in production. There are significant advantages to being able to establish the fuel pressure directly at each injector, at the point of injection, as this helps considerably in being able to model the injection rate and fuel mass per stroke. The ultimate goal is to develop an injector where the rate and quantity of injection can be varied without a step and within a cycle. This would then facilitate the ability to truly control the combustion and energy release in a diesel engine, with high precision, on a cycle-by-cycle basis. I wonder who will get there first - Bosch, Denso, Continental, or someone else....assuming they haven't already!