Fuel injection systems
turbo engine running at 2,200 bar injection pressure, while Denso, which supplies the complete 2,500 bar common rail system for the new Volvo engines, is promising to reach 3,000 bar in 2015. In a research paper published in 2011, Denso engineers discuss EGR rates of over 40% to achieve NOx emission values 20% lower than today’s rates, with increased boost helping reduce smoke emissions, and post-injection events reducing soot output and easing the load on the after-treatment system.
PIEZO-ELECTRIC OR SOLENOID OPERATED? As well as upsetting big-hitters such as Bosch, the pioneering of piezo- electric injectors by Siemens VDO (now part of Continental AG) in 2000 provided a huge stimulus to the diesel market. Much faster and with greater switching accuracy than the traditional solenoid or unit injectors, piezo injectors allowed engine designers far greater freedom in creating complex multiple injection patterns that improved efficiency, reduced emissions, and made diesels smoother and more civilised. Continental is now working on
piezo-electric injectors capable of handling up to 2,500 bar, operating in conjunction with closed loop control similar to that of Denso. The new injectors have miniaturised piezo stacks, enabling greater hydraulic
efficiency and much reduced leakage; an important factor when vehicles have stop-start enabled. Denso engineers agree that it is possible to meet upcoming requirements using piezo-electric activation, which, according to Bosch, is 10 times more powerful than solenoid operation. However, in electing to go for solenoid injectors for its G4S fourth-generation common rail system, as fitted to the new Volvo engines, Denso cites the higher robustness and longer lifetimes of solenoids – clearly an important consideration when the self- compensating feedback loop allows the injector to continue working indefinitely at peak performance. Bosch, too, is working on 2,500
bar solutions using piezo injectors. For precisely metering the tiniest amounts for advance and post- injection, as well as for consistent quality over its service life, the inline piezo injector satisfies the highest standards, says the company, and, thanks to its modular design, this system can be adapted to the requirements of engines with between 4 and 12 cylinders.
GASOLINE DIRECT INJECTION Fuel pressures in petrol engines are much lower, generally running at between 100 and 200 bar, though Marelli is testing systems at 500 bar. The current attention in GDI is on
Gasoline direct injection system from Bosch
particulate matter (PM), something of a surprise discovery on early engines and now thrown into sharper focus by Euro 6 regulations that stipulate particle numbers. The injector spray pattern must
thus avoid wetting the cylinder walls and other components at start-up – a cause of raised PM emissions – yet it must also generate a concentrated zone of air-fuel mixture that can burn stably at part loads, allowing the engine to operate in its economical lean-burn mode for as much of its duty cycle as possible. Again, multiple injections, often
with fuel quantities as minute as 1 milligramme, are key to stable combustion. Ricardo has demonstrated this in its HyBoost project engine, based on the three- cylinder Ford Fox GDI: injections during the intake and compression strokes allow EGR to be increased, helping cut raw NOx emissions – and giving 160Bhp (119kW) from one litre.
WITHIN REACH The message coming from the automakers and the top suppliers is clear: with the aid of key technologies such as the latest fuel injection systems, the 95 g CO2 threshold is well within reach for 2020; certainly for city and compact-sized cars, even those with petrol engines. VW’s engines are already among the best, but CEO Martin Winterkorn expects further efficiency increases in the group’s TDI and TSI engines of 15% by 2020. Bosch, likewise, signals the potential for a 30% gain between 2011 and 2020, and even a seemingly small development such as Denso’s new injector is alone worth a 1% improvement. Small wonder, then, that diesel is
set to go on to even greater things – but do not dismiss petrol. Volkswagen insiders say that, however much its engineers improve diesel performance, parallel improvements are in the pipeline for petrol, too.
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www.automotivedesign.eu.com May/June 2013
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