This content is not included in
your SAE MOBILUS subscription, or you are not logged in.
High Load HCCI Operation Using Different Valving Strategies in a Naturally-Aspirated Gasoline HCCI Engine
Abstract:
Homogenous Charge Compression Ignition (HCCI) combustion offers
significant efficiency improvements compared to conventional spark
ignition engines. However, due to the nature of HCCI combustion,
traditional HCCI combustion can be realized only in a limited
operating range. In order to maximize fuel economy benefits the
HCCI operating range needs to be extended to higher loads. One
immediate benefit is to maximize the portion of the standard
driving cycles (NEDC, FTP, etc.) that can be run with HCCI
combustion, so that transitions between SI operation and HCCI
operation can be avoided.
The HCCI operation at high load range is typically limited by a
trade-off between combustion noise and combustion stability. In a
previous research, we showed how to improve this trade-off using
spark-assisted HCCI combustion strategy, and concluded that the
HCCI high load operation is limited by the air availability due to
a low lift cam when spark-assisted HCCI combustion was applied.
In this research different valving configurations were
investigated to maximize the extension of high load limit of HCCI
operation. Then two different valving strategies, such as Negative
Valve Overlap (NVO) and Positive Valve Overlap (PVO), were compared
to improve the efficiency of high load HCCI operation by
controlling the amount of internal residual fraction and pumping
loss.
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.