Active Control Turbocharging Boosts RCCI Engine Efficiency
Low-temperature combustion (LTC) strategies such as reactivity controlled compression ignition (RCCI) offer notable efficiency gains and ultra-low NOx and soot emissions, but their inherently low exhaust enthalpy poses challenges for turbocharging. Conventional variable geometry turbochargers (VGTs) can regulate boost and backpressure, yet a fixed vane position cannot fully exploit the pulsating exhaust flow, limiting turbine efficiency and increasing pumping losses.
Active control turbocharging (ACT) addresses this by continuously oscillating the VGT rack position with a sinusoidal signal whose frequency scales with engine speed. By synchronizing turbine inlet area changes to exhaust pulse timing, ACT can extract more energy from the flow. Earlier studies by Rajoo, Pesiridis, and Martinez-Botas demonstrated torque and power gains in conventional diesel engines, but its potential in LTC concepts had not been explored.
This investigation used coupled GT-POWER and KIVA-3V simulations of a GM Z19 DTH 1.9 L light-duty diesel converted to RCCI, operating at 8 bar BMEP and 3,000 rpm—representative of highway cruise. The Garrett M53 turbocharger’s NS111 radial turbine was modeled with supplier maps, and in-cylinder combustion was resolved with a reduced PRF mechanism for iso-octane and n-heptane. Model calibration matched experimental pressure traces, heat release rates, torque, and turbo speed within acceptable accuracy.
The ACT control replaced the baseline VGT logic with a sinusoidal rack position equation. A design of experiments varied amplitude ratio (20–100% of maximum allowable) and phase angle (0°, 30°, 60°, 90°, 240°), constrained by peak cylinder pressure (180 bar) and peak pressure rise rate (14 bar/°CA) limits. Air-fuel ratio and EGR fraction were held constant to isolate ACT effects.
Results showed amplitude ratio had a stronger influence on combustion and turbocharger performance than phase angle. Increasing amplitude raised trapped mass at intake valve closure, boosting peak cylinder pressure and advancing CA50 timing. At 40% amplitude and 0° phase, brake specific fuel consumption (BSFC) improved by 2.8% over VGT, with PCP and pressure rise rate within limits. Above 40%, gains in gross indicated efficiency were offset by higher pumping losses from elevated backpressure, and mechanical limits were exceeded at high amplitudes.
Turbocharger shaft speed and turbine isentropic efficiency rose with amplitude up to about 60%, then efficiency declined as excessive vane closure at minimum rack positions restricted flow. Phase angle effects were modest, with efficiency peaking near 90° as more of the blowdown pulse was utilized, then dropping at 240° when only low-energy evacuation flow remained. Compressor efficiency improved with ACT, moving operating points closer to the map’s peak, but gains plateaued beyond 60% amplitude.
Torque increased across all ACT cases due to higher mass flow and fueling, with maximum values at 90° phase for each amplitude. BSFC trends indicated optimal efficiency at moderate amplitudes; within a given amplitude, BSFC varied slightly with phase due to changes in pumping work.
Emission analysis revealed NOx rising sharply with amplitude, reflecting higher in-cylinder temperatures from rapid heat release. Soot remained very low and decreased further with amplitude. Unburnt hydrocarbons and CO showed no consistent trends, with combustion efficiency near 97% in all cases.
Compared with prior ACT studies on conventional diesels, this RCCI application delivered fuel economy benefits at medium load, whereas earlier work found negligible or negative effects at low and high loads. Differences in turbine rotor type (radial vs. mixed flow), tip clearance, and vane mechanism (pivoting vs. sliding wall) likely contributed to divergent trends, as did the additional constraint on pressure rise rate in RCCI operation.
The map-based turbine model in GT-POWER may underrepresent unsteady flow effects of sinusoidal vane motion, reducing apparent phase sensitivity. Higher-fidelity one-dimensional or CFD-based turbine models could better capture these dynamics.
Overall, ACT demonstrated clear potential to enhance turbocharger efficiency and reduce BSFC in RCCI engines at cruise, but benefits are bounded by combustion controllability and mechanical limits. Further optimization of fuel reactivity, EGR, and valve timing could enable higher amplitude operation without exceeding pressure constraints, unlocking greater efficiency gains.
