Low temperature combustion (LTC) refers to a broad range of in-cylinder combustion strategies for the reduction of NOx emissions from diesel combustion; NOx is formed primarily by a thermal mechanism, which production rates increasing exponentially with temperature. LTC strategies reduce combustion temperatures by the dilution of the in-cylinder combustible mixtures, either with excess charge gas to create more fuel-lean mixtures, or with moderate to high levels of EGR.
However, challenges remain in diesel low temperature combustion implementation due to combustion inconsistency or instability. To address this, a team from Tsinghua University has devised an in-cycle combustion feedback control method based on the detection of the Start of Combustion (SOC) in diesel LTC. A paper describing their method is published in the journal Applied Energy.
Currently, the main approaches for achieving diesel LTC are late fuel injection and multiple injection. The late injection method comes from the MK [modulated kinetics] system. The idea is to extend the ignition delay and allow more time for premixing. In this manner, the combustion can be controlled somewhat independently from the auto ignition properties of the fuel. One common technique is to place the main injection near TDC. Large amount of EGR (Exhaust Gas Recirculation) is usually used in combination to further prolong the ignition delay and lower combustion temperature. Advanced injection strategy is another important aspect in LTC realization. One example is the Toyota UNIBUS system, which integrates both early and late fuel injection. The heat release process could be manipulated more precisely with multi-injection strategies.
However, several challenges remain in implementing diesel LTC concepts. In particular, combustion consistency and stability control are some of the most challenging obstacles. Diesel LTC combustion is similar to HCCI combustion, which is, to a large extent, still dominated by chemical kinetics. This makes it very sensitive to the temperature and components of the intake charge. A slight change in intake charge temperature or component can result in quite different results … Typically a large amount of EGR is adopted to achieve controllable and stable LTC combustion. The EGR amount demanded varies largely under different working conditions and especially at the LTC and CI [compression ignition] transition range, which is typically accompanied by a sharp change in EGR rate.—Yang et al.
The Tsinghua team set out to determine a new feedback parameter for in-cycle combustion control. After proposing SOC as a combustion feedback control parameter, they developed a new on-line SOC detection algorithm using a 0.2 MPa pressure difference between motored cylinder pressure and actual in-cylinder pressure as the threshold for combustion start. They then designed their in-cycle combustion feedback control method based on SOC.
They then ran a series of experiments to find out how EGR influences the SOC and CA50 of two-stage diesel LTC.
The research was performed using a 4.75L four-cylinder, high-pressure common-rail diesel. A commercial Pressure Sensor Glow-Plug (PSG) was installed for each cylinder. They replaced the original Bosch ECU with a Tsinghua-developed prototype ECU, comprising two systems: the traditional engine control core for conventional engine management tasks and an integrated iCAT (in-cylinder combustion analysis tool).
The SOC detection algorithm was implemented in the iCAT. Sampling the cylinder pressure for each cylinder every 0.2 degrees of crank angle, the iCAT core processes cylinder pressure sampling and combustion state parameters estimation tasks. iCAT sends the SOC results to the engine management core through a dedicated serial peripheral interface (SPI) bus.
The control method as applied in this work adjusts the second injection timing according to SOC detection. Top-level findings were:
The SOC detection method can detect SOC in the cycle very soon after start of combustion and the result is very reliable in the -50ATDC to 40ATDC detection range.
Experimentation showed that the SOC of the pilot injection advances when the EGR rate increases while the combustion phase of the main injection will be postponed, resulting in a postponed combustion center (CA50) under hot EGR condition with a split injection strategy. The SOC method adjusts the main injection timing based on the SOC of the pilot injection. Results from the experiments showed successful reduction in CA50 fluctuations.
The SOC detection method should apply for both single and multiple injection strategies in the -50ATDC to 40ATDC range, regardless of combustion mode.
Normally more than one injection is needed if the in-cycle control method is to be applied.
Fuyuan Yang, Jinli Wang, Guojing Gao, Minggao Ouyang (2014) “In-cycle diesel low temperature combustion control based on SOC detection,” Applied Energy, Volume 136, Pages 77-88 doi: 10.1016/j.apenergy.2014.09.014