Use of Early Exhaust Valve Opening to Improve Combustion Efficiency and Catalyst Effectiveness in a Multi-Cylinder RCCI Engine System: A Simulation Study

Abstract

Low Temperature Combustion (LTC) strategies such as Reactivity Controlled Compression Ignition (RCCI) can result in significant improvements of fuel economy and emissions reduction. However, they can produce significant carbon monoxide and unburnt hydrocarbon emissions at low load operating conditions due to poor combustion efficiencies at these operating points, which is a consequence of the low combustion temperatures that cause the oxidation rates of these species to slow down. The exhaust gas temperature is also not high enough at low loads for effective performance of turbocharger systems and diesel oxidation catalysts (DOC). The DOC is extremely sensitive to exhaust gas temperature changes and lights off only when a certain temperature is reached, depending on the catalyst specifications. Uncooled EGR can increase combustion temperatures, thereby improving combustion efficiency, but high EGR concentrations of 50% or more are required, thereby increasing pumping work and reducing volumetric efficiency. However, with early exhaust valve opening, the exhaust gas temperature can be much higher, allowing lower EGR flow rates, and enabling activation of the DOC for more effective oxidization of unburnt hydrocarbons and CO in the exhaust.In this paper, a multi-cylinder engine system simulation of RCCI at low load operation with early exhaust valve opening is presented, along with consideration of the exhaust aftertreatment system. The combustion process is modeled using the 3D CFD code, KIVA, and the heat release rates obtained from this combustion are used in a GT-Power model of a turbocharged, multi-cylinder light-duty RCCI engine for a full system simulation. The post-turbine exhaust gas is fed into GT-Power's aftertreatment model of the engine's DOC to determine the catalyst response. It is confirmed that opening the exhaust valve earlier increases the exhaust gas temperature, and hence lower EGR flow rates are needed to improve combustion efficiency. It was also found that exhaust temperatures of around 457 K are required to light off the catalyst and oxidize the unburnt hydrocarbons and CO effectively. Performance of the DOC was drastically improved and higher amounts of unburnt hydrocarbons were oxidized by increasing the exhaust gas temperature.

Effect of Valve Timing and Excess Air Ratio on Torque in Hydrogen-Fueled Internal Combustion Engine for UAV

“…On the other way, when the opening time of the exhaust valve advanced excessively, the effective work resulting from the blowdown and scavenging effect decreased due to the exhaust gas leaving the cylinder. The pressure drop during the earlier part of the expansion stroke significantly influences the decrease in expansion work [30]. As a result, the torque and efficiency decreased, as shown in the figure.…”

Improving Exhaust Temperature Management At Low-loaded Diesel Engine Operations Via Internal Exhaust Gas Recirculation

“…Therefore, brake thermal efficiency is reduced up to 1.2 %. However, the trade-off engine has to suffer to improve exhaust temperatures is relatively low in comparison to conventional fuel-consuming methods [9][10][11][12].…”

“…On previous studies, early exhaust valve opening (EEVO) was examined by many researchers as a solution for low exhaust temperatures at low loads [9][10][11]. In EEVO mode, exhaust discharge starts while piston is relatively close to top dead center (TDC) where in-cylinder pressure and temperature are still high.…”

Modelling and Control of Combustion Phasing of an Rcci Engine

“…in which the ignition delay ( ) is estimated using the previously parameterized correlation developed to predict SOC, i.e., the denominator of the integral on the left side of Equation (11). A corrected temperature and pressure for the duration of combustion is assumed in estimating the ignition delay of the mixture, calculated using: The parameter accounts for the heat losses during the combustion as well as any compression heating or expansion cooling.…”

“…BD model (BDM), previously shown in Equation (2-12), was parameterized using the KIVA simulation data. An approach similar to that of the MKIM is used to parameterize Using the values derived for , , , , and (Table 2-7) in Equations (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17) and (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19), it could be noted that as SOC is advanced, the value of and decrease. As mentioned before (due to lower average temperatures and pressures) as SOC is advanced, the BD increases.…”