GM exploring different valving strategies to extend HCCI operation for high loads; benefits of a Positive Valve Overlap approach
|Representation of the limited operating range of robust HCCI operation in a conventional 4-stroke SI engine. Click to enlarge.|
Although Homogeneous Charge Compression Ignition (HCCI) combustion offers significant efficiency improvements compared to conventional spark ignition gasoline engines, traditional HCCI combustion can be realized only in a limited operating range.
GM has been steadily working on extending the operating range of HCCI. In 2009, it showcased a demonstrator unit, and has been publishing on techniques and strategies it is using to extend the HCCI operating range. To extend HCCI to idle (shown in 2008, 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. In a new SAE paper, GM now describes two different valving configurations to maximize the extension of high load limit HCCI.
|HCCI combustion results from the spontaneous auto-ignition at multiple points of thoroughly mixed fuel-air charge throughout the volume of the charge gas. HCCI combustion typically occurs in two stages: a low-temperature heat release (LTHR) occurs first, followed by a high-temperature heat release (HTHR).|
|Broadening LTHR and HTHR, and reducing the maximum rate of pressure increase during LTHR and HTHR, increases the operating range of a HCCI engine. Despite that being well known, it is still difficult to operate in HCCI mode over a wide range of loads.|
|Combustion phasing (the timing of auto-ignition) is inherently difficult to control; the rapid rate of heat release by a HCCI engine as its load increases can lead to mechanical and noise problems; with rapid combustion, the maximum rate of pressure rise limits the ability of HCCI engines to achieve medium and high loads; HCCI is sensitive to fuel composition; and fuels often do not auto-ignite at low loads. (Earlier post.)|
|HCCI operation is known for very low NOx emissions. However, as the amount of fuel mass injected increases, the NOx emissions increase due to local hot spots generated by incremented inhomogeneity. These will reach unacceptably high levels without after-treatment, GM notes.|
HCCI operation at high load range is limited by a trade-off between combustion noise and combustion stability. In prior work, the group from GM R&D’s Propulsion Systems Research Labs, led by Paul Najt, concluded that the HCCI load operation is limited by the air availability due to a low lift cam when spark assisted HCCI combustion was applied.
(Najt, then at the University of Wisconsin, and David Foster published the first study of a gasoline-fueled four-stroke HCCI engine in 1983.)
The new study compares the ability of two different valving strategies—negative valve overlap (NVO) and positive valve overlap (PVO)—to improve the efficiency of high load HCCI by controlling the amount of internal residual fraction and pumping loss.
The team explored the configurations in a single-cylinder four-stroke engine. Prototype cylinder heads were equipped with four valves per cylinder and direct fuel injections. Compression ration was 12.0:1. The configuration of the piston, spark plug position and injector position was designed to enable stratified charge combustion.
The engine featured a fully flexible valve actuation (FFVA) system. Peak valve lift of intake and exhaust valves was varied based on the strategy.
In NVO, peak valve lift of intake and exhaust valves was 5.0 mm and 6.0 mm, respectively. The opening duration of intake and exhaust valve was 145 and 160 crank angle degrees (CAD), respectively. Low lift valve profile was designed considering the compatibility with a two-step mechanism.
In NVO, exhaust valve closing (EVC) timing determines the internal residual (hot residual gas) mass fraction. The decreasing residual gas fraction means that more air is inducted in the cylinder, changing the air/fuel ration. Since EGR rate give additional dilution to the charge, in order to maintain stoichiometric operation, EGR needs to be adjusted.
In PVO with high lift valve, the same valve profiles as conventional SI operation were used: 10 mm of peak valve lift and 240 CAD of valve duration for both intake and exhaust valves. Here, the amount of PVO determines the trapped residual mass. As engine load varies (injected fuel mass changes), the amount of PVO or EGR needs to be adjusted in order to maintain stoichiometric operation.
The experiments were performed at stoichiometric HCCI operation to enable NOx reduction with conventional three-way catalysts (TWC).
When the NVO strategy was employed, as engine load increases, combustion phasing must be retarded to reduce combustion noise due to high amount of hot internal residuals. Retarded combustion phasing at high load HCCI operation is a key solution for the NVO strategy.
On the other hand, when the PVO strategy was applied, optimal residual fraction can be achieved resulting in efficiency improvement due to optimal combustion phasing, lower pumping loss and less heat transfer loss. The high load limit was successfully extended to 10 bar IMEPg (Gross Indicated Mean Effective Pressure) while maintaining good efficiency and complying with emissions requirements.
...The main characteristics of PVO strategy can be summarized as follows: more favorable in-cylinder charge condition in terms of temperature due to more EGR and less hot internal residuals enables to obtain low ringing even with advanced combustion phasing condition. In addition, since more fraction of the total fuel injected was burned by flame, less fraction of total fuel injected was involved in auto-ignition.
Another benefit of PVO strategy compared with NVO strategy is efficiency improvement. Lower pumping loss was achieved through PVO strategy regardless of engine operating conditions. In addition, more advanced combustion phasing helps to increase IMEP near TDC region. Net specific fuel consumption was improved by 3 to 7%.—Yun et al.
Hanho Yun, Nicole Wermuth and Paul Najt (2011) High Load HCCI Operation Using Different valving Strategies in a Naturally-Aspirated Gasoline HCCI Engine (SAE 2011-01-0899)
Hanho Yun, Nicole Wermuth and Paul Najt (2010) Extending the High Load Operating Limit of a Naturally-Aspirated Gasoline HCCI Combustion Engine. (SAE 2010-01-0847)