Continental recently unveiled a new systems solution that dramatically increases the precision of solenoid-type fuel injectors for gasoline direct-injection (GDI) engines. The system, called COSI (COntrolled Solenoid Injection), will help powertrain engineers better control cold-start emissions. It is slated for 2013 production.
“To date solenoid injectors cannot be controlled with optimum precision during what is known as the ballistic operating mode. The tolerance band tends to be too wide,” said Andreas Greff, Vice President Engine Systems at Continental, which presented the COSI details at the 2011 Vienna Engine Symposium.
“COSI makes it possible to reduce the minimum injected fuel per shot and to do that on a stable injector-to-injector level which ensures proper and clean combustion,” Greff explained.
Once a modern GDI engine and its exhaust gas aftertreatment have reached operating temperature, emissions levels will be extremely low. The most critical phase is the cold start period that extends until the catalytic converter lights off. Most of the particulate emission, for instance, is produced during this phase. To speed up the converter light-off, the fuel dosage can be increased under cold-start conditions.
However, powertrain engineers know there is a trade-off between injecting more fuel and the impact of this strategy on potential in-cylinder wall-wetting and the level of soot formation. Also fuel consumption as such has to be kept in mind in order to meet CO2 emissions targets.
An effective way to reduce wall-wetting at low temperature levels is to split up the total fuel injection into several smaller shots. By doing this, the single spray cone’s liquid length is reduced, which brings down the risk of liquid fuel colliding with the cylinder wall. With solenoid injectors, multiple injections can be challenging, though, as the injection behavior between ballistic and linear injector operation is quite different.
Resulting needle movement variation can either mean that too little fuel is injected, which can cause misfiring, or too much fuel is injected, leading to soot formation. Both are unacceptable. Thus Conti's development of the COSI system.
Measuring needle opening time
COSI consists of an injector, an ASIC (Application Specific Integrated Circuit) chip that receives and computes a current signal from the injector, and the engine control software that integrates into the injection realization module, which in turn translates the signal from the injector into the duration of the needle lift.
This is compared to an absolute set value, and the needle closing time is adjusted accordingly. The underlying electrical principle is a correlation between the needle position and the amount of current that flows through the actuator coil while the voltage is applied during the injection.
Unfortunately, this correlation is not very linear during ballistic injector operation, such as when the needle directly transits from the opening ramp to the closing ramp.
“A lot of know-how has gone into the algorithm that calculates the needle closing time, based on the current signal from the magnetic coil,” said Greff.
3-mg shots of fuel
The information about the duration of the needle lift can be used to adjust the individual amount of fuel per cylinder.
“We can control the minimum injection quantity per shot down to 3 milligrams of fuel for injectors with a high maximum flow,” Greff noted. This level of precision can be used to balance the combustion in the individual cylinders, and it ensures proper ignition despite multiple minimum injection.
In order to inject enough fuel per cylinder, the total quantity can be split up in multiple shots during one injection cycle.
“Our tests have shown that the control is effective. During test runs of a four-cylinder DI gasoline engine with and without COSI, the test run without COSI showed misfiring, while this did not occur with COSI active,” Greff explained.
In principle, controlled solenoid injection would work with any solenoid injector. In reality, the results depend on the quality of the current signal coming from the injector coil. Said Greff: “We have optimized our solenoid injectors for their signal behavior to ensure that the signal can be better used for a control function.”
The COSI software module will be part of Continental’s third generation of engine management system (called EMS 3) from 2013 onward. Nevertheless, it will remain the OEM’s decision whether or not to make use of the function. In order to exploit COSI’s benefits, the ASIC needs to be integrated into the electronic engine control unit and the solenoid injector needs to be suitable.
“If COSI is used in combination with our new high-pressure gasoline fuel pump, this means the pump’s rapid pressure buildup can be utilized during cold-start practically from the first shot in multiple-injection mode,” Greff added.
A piezo alternative?
The bottom line seems to be that COSI is designed to better exploit the electrical limits of solenoid injectors and thus push the limits of precise minimum-quantity fuel injection. By adding this control function, solenoid injectors can be used for gasoline-injection strategies that would have required piezo injectors.
Yet not even COSI will bring solenoid injection to the precision level and speed of piezo technology, Greff believes.
“The principle limitations of solenoids still apply," he asserted. "However, we squeeze the best out of this technology within the limits of physics. Our tests have shown that this can bring down the soot formation after cold start by several percent.”