Application of Variable Geometry Turbine on Gasoline Engines and the Optimisation of Transient Behaviours

Abstract

Downsizing of the internal combustion engine (ICE) through turbocharging is one of the primary approaches to reduce fuel consumption (FC) and emissions. The challenges for turbocharged engines include meeting low-end torque and transient torque response. While the Variable Geometry Turbine (VGT) is a proven measure for Diesel engines, quantitative understanding is still required to optimise the use of VGT on gasoline engines.

The accuracy of engine simulation, which is crucial in the design process, is largely affected by turbocharger modelling. Therefore, a novel turbocharger on-engine mapping facility was developed to improve the understanding and simulation accuracy of turbocharging system operating under variable pulsating and thermal conditions.

Experimental results demonstrated a 48Nm (17.7%) improvement in maximum torque and 4% FC improvement in the boosted region at 2000rpm through replacing the Fixed Geometry Turbine (FGT) with VGT on a 2.0 litter direct injection gasoline engine. Simulation was carried out using GT-Power to analyse the improvements. A 2.3% FC improvement was predicted with 0.6% from reduction in pumping work, 1.5% from combustion thermal efficiency and 0.2% from variation of compressor efficiency.

To investigate the effects of turbine size, three VGT sizes were selected and simulated. Depending on the size of VGT, the FC at engine speeds 3500rpm and below was reduced by 0.5 - 1% from the reduction of pumping work. The engine torque knee-point was advanced by 100 - 250rpm, and the transient response was improved by up to 4 seconds (73.4%). Therefore, further downsizing can be enabled by using VGT. To achieve the full potential especially the FC benefits at high speeds, a variable nozzle type VGT assisted by an additional waste-gate was proposed.

It is challenging to manage VGT during transients efficiently primarily because of the non-monotonic characteristics of the turbine response to VGT position. A methodology was established to investigate and optimise the transient response. To overcome the challenges and to reduce calibration effort, a semi-physical control strategy was developed. The optimum VGT trajectories were captured by this control strategy, while PID controller could not reach target engine torque in four of six cases simulated. Compared with PID controller, the turbocharger response time was improved by 15-19% in the two cases where PID controller can reach the target. This novel structure is also applicable to other complex systems such as two-stage turbocharging system and Diesel air-path system.

Date of Award	22 Jun 2016
Original language	English
Awarding Institution	University of Bath
Supervisor	Chris Brace (Supervisor) & Sam Akehurst (Supervisor)