GM’s HCCI Demonstrator Combines a Set of Enabling Technologies and Strategies for Extending Operating Range
|GM approaches to extending the operating range of classic HCCI. Adapted from Yun et al. (SAE 2009-01-0499) Click to enlarge.|
General Motors last week again highlighted the progress it is making with advanced HCCI (homogeneous charge compression ignition) gasoline engines by showcasing a demonstrator unit. GM has also begun to publish SAE papers describing the techniques and strategies it is using to extend the fuel-efficient, low-emissions HCCI operating range down to lower load regions.
To extend HCCI to idle (shown last year, earlier post), GM is using a Multiple Injection and Multiple Ignition (MIMI) strategy, combined with the use of in-cylinder fuel reforming during recompression as a bridge technique up to the classic HCCI operating range, said Paul Najt, Lab Group Manager, GM Powertrain Systems Research. (Najt, then at the University of Wisconsin, and David Foster published the first study of a gasoline-fueled four-stroke HCCI engine in 1983.)
In fall 2007, when we first introduced the full functional driving [HCCI] car to the press [earlier post], it operated in the classic HCCI mode—sort of the middle speed load range. Then in the Spring of 2008...we demonstrated to the press idle to 55 mph. The big thing there was to incorporate these technologies [in the SAE papers] to push the HCCI down to very light loads. Since that time, we have been working on extending the HCCI domain to higher speeds and loads.
One of the obvious issues was waiting until we had good IP protection before we had it on the road.—Paul Najt
GM has not yet published a paper on the techniques it is using to extend HCCI to those higher speeds and loads, with speeds up to 60 mph (what GM showcased last week). Najt said that GM is using its available injection and valve management technologies with thermal and fuel stratification to slow down the burn rate.
The real barrier is the combustion-induced noise. To extend the HCCI load range to higher speeds, we need to control the noise. We are doing different things with injection timing and the cylinder.
HCCI. Operating an engine with high air-fuel ratios (i.e., lean burn) has become of increasing interest as a means to provide a significant improvement in fuel consumption. A number of approaches to this have emerged, including gasoline direct injection in stratified mode, which can provide overall fuel consumption improvement of up to 25% compared to a conventional engine. Downsides to this approach include combustion stability and NOx emissions.
HCCI offers the benefit of reduced fuel consumption while emitting only low levels of NOx, therefore avoiding the expensive exhaust aftertreatment systems which are necessary for most lean combustion engines. In the HCCI combustion process, air and fuel are premixed and then ignited by compression in the cylinder. The auto-ignition occurs in multiple points throughout the charge, providing a parallel energy release without flame propagation.
HCCI combustion has a number of challenges, however, including cold start, a limited operating range, and controlling combustion over the entire load/speed range. A practical gasoline engine at this point would combine HCCI combustion with more convention spark ignition (SI) combustion. Two areas of significant research focus are thus (a) extending the HCCI operating range and (b) improving mode transition between HCCI and SI.
Multiple Injection Multiple Ignition and in-cylinder reforming. The main obstacle for light-load HCCI is that the combustion cycle is too cold to support auto ignition. To achieve the conducive conditions, fuel is injected during recompression; i.e., when the gas temperature and pressure are high. The injected fuel experiences partial reforming to produce extra heat required for auto ignition. At very light loads, however, in-cylinder reforming is not enough.
To tackle that, GM adopted a stratified combustion concept combining the reforming strategy. The stratified part of the fuel is ignited by a spark and the resulting propagating flame compresses further the remaining fuel-air mixture in addition to piston compression, allowing the in-cylinder conditions to reach the temperature and pressure required for auto-ignition.
...fuel stratification late in the main compression is very important to develop robust HCCI idle combustion.—Yun et al. (SAE 2009-01-0499)
GM uses six control parameters—three injection timings, two spark timings and negative valve overlap (NVO)—to develop HCCI idle combustion. Applied in a 2.2-liter four-cylinder engine, GM demonstrated a 25% fuel consumption benefit at idle operation with lower NOx emissions and a similar level of combustion stability compared to a SIDI (spark ignition direct injection) engine. Other findings reported in GM’s SAE paper describing MIMI include:
Very low NOx emissions (<1.0 g/kg) and good stability (<10 kPa stdv of IMPE, gross indicated mean effective pressure) can be achieved at 800 rpm, 85 kPa NMEP (net indicated mean effective pressure).
The injection timing and the separation between injection timing and ignition timing in the main compression are critical to achieve strong flame propagation.
Strong flame propagation is a key factor for robust HCCI idle operation.
The injection timing and ignition timing during the recompression play a crucial role in determining the amount of reforming.
NOx emissions can be controlled by adjusting the amount of reforming.
The injection timing of the second pulse can be varied to obtain desired combustion phasing.
NVO and fuel split ratio are good tools to achieve optimal trade-off between NOx emissions and combustion stability.
GM also uses a combination of in-cylinder reforming and stratified combustion to concept to support the extension of low-load HCCI operation above idle conditions, but below the classic HCCI range.
The amount of heat released during the recompression varies with the amount of fuel injected, with the injection timing, and with the temperature and composition of the gas left over from the previous cycle.
For engine control purposes the fuel mass burned during recompression should be closely monitored to achieve the desired trade-off between combustion stability and NOx emissions. PMEP [pumping mean effective pressure], a pressure ratio or a pressure difference over the recompression period show a linear correlation with the fuel mass burned during recompression and can be used as surrogate parameters for engine control.
Over a wide range there is a linear correlation between the fuel mass burned during recompression and the injection timing that can be used to control fuel reforming. A new injection and ignition strategy allows for improved low-load HCCI operation but more work is need to make good use of the additional degrees of freedom.—Wermuth et al. (SAE 2009-01-0923)
Future work. “You have to remember,” said Najt, “that HCCI is a combustion process. We are trying to integrate this combustion process into a lot of different technologies.”
Although the current demonstrator engine is naturally aspirated, GM is looking at boosted HCCI applications as well, in addition to applications with hybrid transmissions and different fuels. GM already has a strong diesel HCCI effort underway. If a dedicated E85 option was feasible (from an infrastructure point of view), the high octane in ethanol could support some interesting features from an auto-ignition chemistry, Najt said.
On the diesel side, the HCCI technology is evolving from traditional diesel compression combustion to a partial homogeneous mixture (PCCI) , and then into purely HCCI. “With the diesel, its more an evolutionary process rather than more of a step process,” aid Najt.
The next major chunk of work, Najt said, is to try to use all of the tools available to continue to push up to higher speeds and loads. GM is collaborating with the national labs and universities, and has seen successful HCCI all the way up to 16 bar BMEP—a very high load.
Nicole Wermuth, Hanho Yun and Paul Najt. Enhancing Light Load HCCI Combustion in a Direct Injection Gasoline Engine by Fuel Reforming During Recompression (SAE 2009-01-0923)
Hanho Yun, Nicole Wermuth and Paul Najt. Development of Robust HCCI Idle Operation Using Multiple Injection and Multiple Ignition (MIMI) Strategy (SAE 2009-01-0499)
“Pressure to Perform: The Internal Combustion Engine in a Sustainable Future”, Paul Najt, FastLane Blog 20 May 2009