Michael Howard Shelby

Effect of Exhaust Gas Temperature Limits on the Peak Power Performance of a Turbocharged Gasoline Engine

Peak power of an engine is typically constrained by the maximum obtainable airflow. This constraint could arise directly from the airflow limitation imposed by the throttle restriction (typical for a naturally aspirated engine), or indirectly from other factors, such as various temperature limits for component protection. In this work, we evaluate the airflow limit for a turbocharged gasoline engine as dictated by the constraints on the turbine inlet temperature. Increasing the limit on the turbine inlet temperature requires the exhaust manifolds and turbine to be made out of more expensive materials that withstand higher temperatures. This expense is justifiable if operating with higher turbine inlet temperature allows noticeably higher power output, and not merely increases the allowable airflow. Experimental data show that under some conditions the increase in airflow does not increase the peak power. The effects of increasing airflow on the peak power and turbine inlet temperatures are systematically analyzed through individual accounting for the different losses affecting the engine torque. The breakdown analysis presented in this work indicates combustion phasing as a major contributing factor to whether increasing the flange temperature limit would increase the peak power.

A New Analysis Method for Accurate Accounting of IC Engine Pumping Work and Indicated Work

In order to improve fuel economy, engine manufacturers are investigating various technologies that reduce pumping work in spark ignition engines. Current cylinder pressure analysis methods do not allow valid comparison of pumping work reduction strategies. Existing methods neglect valve timing effects which occur during the expansion and compression strokes, but are actually part of the gas exchange process. These additional pumping work contributions become more significant when evaluating non-standard valve timing concepts. This paper outlines a new analysis method for calculating the pumping work and indicated work of a 4-stroke internal combustion engine. Corrections to PMEP and IMEP are introduced which allow the valid comparison of pumping work and indicated efficiency between engines with different pumping work reduction strategies. Several example data sets are presented which illustrate the method and the necessity for the corrections when analyzing engines with non-standard valve timings. The upper limit potential improvement in BSFC available from PMEP reduction is presented and compared to the actual benefit obtained with three pumping work reduction strategies: 1) variable valve timing, 2) intake charge dilution (stratified lean operation) and 3) cylinder deactivation.

Scavenging in a turbocharged gasoline engine

The phenomenon of air escaping the engine intake directly to the exhaust during valve overlap is commonly known as scavenging. This phenomenon is primarily observed at low-speed, high-load in engines with significant overlap between intake valve opening and exhaust valve closing. Evaluation on a turbocharged-gasoline engine shows increased low-speed torque when operating under scavenging conditions. This paper investigates the occurrence of scavenging and analyzes its consequences. A methodology is presented to infer the amount of scavenging using airflow and in-cylinder pressure measurements. Scavenging increases catalyst exotherm when operating with stoichiometric exhaust. A model is proposed to predict the additional exotherm.

Improving Transient Torque Response for Boosted Engines with VCT and EGR

Modern gasoline engines have increased part-load fuel economy and specific power output through technologies such as downsizing, turbocharging, direct injection, and exhaust gas recirculation. These engines tend to have higher sensitivity to driving behavior because of the steady-state efficiency versus output characteristics (e.g., sweet spot at lower output) and the dynamic response characteristics (e.g., turbo lag). It has been observed that the technologies aimed at increased engine efficiency may improve fuel economy for less aggressive cycles and drivers while hurting fuel economy for more aggressive cycles and drivers. The higher degrees of freedom in these engines and the increased sensitivity make controls and calibration more complex and more important at the same time. With the interactions between the dynamic response characteristics of the powertrain and the driver in mind, a dynamic control strategy for variable cam timing (VCT) and exhaust gas recirculation (EGR) is developed. The strategy allows actuator positions at steady-state optimal values when possible yet a fast response proportional to the driver request in transients. The aim is to strike a balance, which is tunable, between steady state efficiency and transient response. Most of the calibration process is algorithmic and based on standard engine mapping data. Experimental results for fuel economy on drive cycles and performance testing from powertrain and chassis dynamometers for two powertrain configurations are reported. Analysis shows improvements in terms of fuel economy and driver demand tracking on drive cycles as well as improved performance metrics. In particular, it is demonstrated that it is possible to simultaneously improve transient performance and fuel economy.