Dynamic Diesel Engine Timing with Microwaves—The General Motors Approach

Conventional production methods for setting initial pump timing in indirect injection diesel engines are subject to tolerance stack-up errors which contribute to vehicle-to-vehicle variations in tailpipe emissions and smoke levels. This problem has been addressed through the development of a new dynamic timing apparatus in which injection pump timing adjustments are made under hot-test conditions. The new timing apparatus employs microwave sensing of the combustion chamber to determine the dynamic piston top dead center (TDC) of a cylinder to within ±0.1° crankangle. Furthermore, the onset of prechamber luminosity, rather than injector needle motion, is used as the signal to be timed. This signal is abrupt, readily detectable and more directly related to engine performance than is start of injection. Both the microwave and luminosity sensing functions are integrated into a single probe which is used in place of a glow plug.

Experimental results which illustrate the accuracy of the location of TDC by microwaves and the dependence of prechamber luminosity on engine operating conditions are given. In addition, data are presented which indicate that the dynamic timing method can significantly reduce the initial timing related vehicle-to-vehicle variation of exhaust emissions. The production implementations of this timing system by Chevrolet and Oldsmobile are also described.