Heat Transfer in the SI/HCCI Engine

Heat Transfer in the SI/HCCI Engine

Abstract:
Experimental studies are carried out to provide qualitative and quantitative insight into gas to wall heat transfer in gasoline fueled Spark Ignition (SI) and Homogeneous Charge Compression Ignition (HCCI) engines. Fast response thermocouples are embedded in the piston top and cylinder head surface to measure instantaneous wall temperature and heat flux. Heat flux measurements are then used to assess several heat transfer correlations, in order to refine and validate mathematical models.

Background:
The Homogeneous Charge Compression Ignition (HCCI) is currently under widespread investigation, due to its potential to increase thermal efficiency while greatly decreasing harmful exhaust pollutants.

Auto-ignition and combustion rates in a Homogeneous Charge Compression Ignition (HCCI) engine are very closely coupled to the heat transfer phenomena. Hence, a thorough understanding of the heat transfer process is critical for extending the load range and managing thermal transients. An improved heat transfer model, valid for a range of HCCI conditions, is an essential element in the development of predictive thermo-kinetic simulations.

The fuel, air and residual gas are generally expected to be well mixed in an HCCI engine and its combustion is often described as controlled auto-ignition. The combustion process starts at multiple locations and is governed by chemical kinetic rather than turbulent flame front propagation, as in Spark Ignition (SI) engines, or a stratified diffusion flame, as in conventional highly stratified Compression Ignition (CI) engines. Hence, turbulence and mixing rates have a much smaller effect on HCCI combustion, while in-cylinder thermal conditions have a critical impact on HCCI ignition timing and burning rate. The thermal condition of the combustion chamber is closely tied to heat transfer from the hot gas to the walls. Thus, good understanding of the heat transfer process in the combustion chamber is prerequisite for developing HCCI engine control strategies and thermal management schemes.

Crank/Piston Instrumentation

Researchers:
Dennis Assanis
Zoran Filipi
Mark Hoffman

Sponsors:
General Motors

Goals:
The goal of this work is to experimentally quantify the effect of various operating parameters on in-cylinder heat transfer and HCCI combustion. It is important to understand the sensitivity of chamber thermal conditions on HCCI performance and emissions and the details behind HCCI specific heat transfer phenomena. This will help to allow more effective means to more precisely control HCCI combustion and to explore the limits of its range of operability. Additionally, experimental results can be used to improve heat transfer model for HCCI engine simulation and support control strategy. The main technique behind accomplishing this is through the use of fast response piston and head surface thermocouples which are used to accurately quantify wall temperatures and heat flux.

Achievements:

Development of a telemetry system that allows reliable in-cylinder temperature and heat flux measurements
Full thermal and performance characterization of the GM-UM single cylinder HCCI engine
Development of a new global heat transfer correlation for HCCI combustion