Abstract
This paper presents the experimental results of a new look at throttle body injection (TBI) for LPG, targeting the same objectives as fourth generation LPG systems. The TBI system is configured from CFD studies to minimise cylinder-to-cylinder air-fuel variations, enabling the lean limit to be extended as far as lambda 1.6. Operating variables were optimised using CFD simulations and by experimental investigation.
Special effort has been made with the TBI system to encourage good mixing. The injection angles and location of the two-injector outlet tubes was studied in detail. The authors used Fluent® simulation, validated with a manifold test rig with injected propane gas to optimise the gas jet orientation and velocity at critical engine operating points.
Additionally, an in-line, six-cylinder engine with a compression ratio of 11.7:1 (up from the standard 9.65:1) over a range of speed/torque conditions was tested, with the air/fuel ratio varied from lambda 1.0 to 1.6. Cylinder-to-cylinder air-fuel variations were measured over a range of speeds and throttle positions representing most of the steady state parts in the Euro drive-cycle for light-duty vehicles. The spread between the leanest and richest cylinders was expressed by NVar, the sum of the unsigned variance from the mean over the mean.
It is shown that mixing characteristics have a considerable effect on emissions and thermal efficiency. Comparisons of thermal efficiency and specific emissions of HC, NOx, CO2 and CO against liquid and gaseous phase LPG multipoint port injection (MPI) systems tested on the same engine show superior mixing of the throttle-body system.
The paper demonstrates that fuel injection pressure, injector nozzle location and angles are important in achieving good mixing as demonstrated by the low NVar parameter and high efficiency. Additionally, results of NVar are presented as multidimensional maps to enable the development of control strategies.
Notable improvements in emissions and thermal efficiencies were achieved when compared with original gasoline sequential port injection system. At lambda 1.6, the NOx is low enough to meet Euro 4 standards without the need of a reducing catalyst. Best emissions observed were reductions of 88% for HC, 98% for NOx, 92% for CO, 36% for CO2 and a rise of 26% in thermal efficiency.
Keywords:LPG, lean burn, throttle-body injection, mixture preparation, emissions