The Effect of Valve Overlap on Idle Operation: Comparison of Model and Experiment 932751
Validation of the Ford General Engine SIMulation program (GESIM) with measured firing data from a modified single cylinder Ricardo HYDRA research engine is described. GESIM predictions for peak cylinder pressure and burn duration are compared to test results at idle operating conditions over a wide range of valve overlap. The calibration of GESIM was determined using data from only one representative world-wide operating point and left unchanged for the remainder of the study. Valve overlap was varied by as much as 36° from its base setting. In most cases, agreement between model and data was within the accuracy of the measurements.
A cycle simulation computer model provides the researcher with an invaluable tool for acquiring insight into the thermodynamic and fluid mechanical processes occurring in the cylinder of an internal combustion engine. By applying such a model in conjunction with experimentally obtained data, one can obtain accurate calculations of quantities not amenable to routine measurement, including the residual mass fraction (RMF) and time-resolved values of in-cylinder gas temperatures and rates of mass flow past the intake and exhaust valves.
If its physically based submodels have been extensively validated with experimental data, then a model can also be used in the engine design process. A wide range of values for a particular design parameter (e.g., valve overlap) can be examined with the model, which can accurately assess the trade-offs generally inherent in such design changes, and select the most promising cases for testing in hardware. As a result, the overall design process is not only more comprehensive, it is also accelerated by the elimination of a significant amount of testing.
The development and use of the Ford General Engine Simulation program (GESIM) as a diagnostic research tool has been well documented over the last decade or so[1, 2, 3, 4, 5, 6]. It now has submodels to account for the most important physical processes affecting engine performance; principal among these are submodels for turbulence, swirl, combustion, heat transfer, and gas exchange with the manifold. It is necessary only to validate the accuracy of GESIM's predictions in order to employ it as a design tool.
This paper presents the results of a study undertaken to validate GESIM's predictions of the effects of valve overlap on engine operation under idle conditions. This particular test was chosen both because of the importance of valve timing as, a design parameter and because the requirement of accurate predictions at idle is a fairly severe test of all the important submodels.
The relationship between valve timing and idle quality is of particular interest to the designer if the engine is to be equipped with the traditional fixed valve events. In this case the events are necessarily a compromise among the competing goals of high-end power, low-end torque, and idle quality. It is often desireable to attempt to improve an engine's top-end performance simply (and relatively inexpensively) by changing the cam; however, such a change generally entails increasing the overlap of the intake and exhaust events. As a result, engine operation (as measured by fuel economy and combustion stability) at part load and lower speeds suffers.
Valve overlap extends from intake valve opening (IVO) until exhaust valve closing (EVC). During this period, under throttled conditions, there is a pressure drop from the exhaust manifold to the intake manifold, and combustion products flow back into the combustion chamber through the exhaust valve. These products combine with those which never left the chamber to constitute the residual, or internal EGR. The more throttled the condition, the larger the pressure drop, and the smaller the amount of charge ingested; hence, the RMF increases. Idle, the most throttled condition, thus has the highest RMF in the engine operating range for a given valve timing.
The low mean piston speed at idle produces very little in-cylinder turbulence, which, along with the high residual, results in slow burn rates and requires significant spark retardation away from MBT in order to achieve acceptable combustion stability. All of these effects have a negative impact on fuel economy. Without a model to help quantify these trade-offs, a considerable amount of hardware testing is required to determine an acceptable set of valve events.
As variable cam timing (VCT) systems become more widely used in production engines, it becomes reasonable to consider designing a set of valve events specifically for the idle condition without consideration of their effects on operation at other points in the engine speed-load map. The determination of the “best” combination of intake and exhaust timings would be significantly assisted by a well validated model.
The idle condition will expose weaknesses in any of the principal submodels. The condition is highly throttled; combustion rate is very sensitive to the large RMF which results, hence the gas exchange process must be well modeled to obtain sufficiently accurate RMF values. In addition, the combustion submodel must exhibit the proper trends for burn rate with RMF dilution. Finally, the speed is low, which allows heat transfer to become very significant; inaccuracies in the heat transfer submodel will show up as erroneous predictions for ISFC or fuel flow.
The remainder of this paper separates the validation of GESIM with valve overlap into three parts. First, valve overlap is systematically varied and data acquired for a single-cylinder research engine at the idle condition. Next, GESIM is applied to model the same set of conditions. Finally, the comparison of predicted and measured results is discussed.