Research Projects

Project at a Glance

Project Status: complete

Report Published February 1973:

Title: Study of vacuum spark advance disconnect as an NOx control measure

Contractor: Automotive Environmental Systems, Inc.

Contract Number: R-772-73-04 & 2-299-10

Research Program Area: Emissions Monitoring & Control

Topic Areas: Mobile Sources & Fuels


A comprehensive study of the value of vacuum spark advance disconnect (VSAD) as a means for reducing oxides of nitrogen emissions from vehicles in the State of California was executed by AESi under the sponsorship of the California Air Resources Board. A sample of 100 typical 1966 through 1970 model year consumer-owned vehicles was selected on a basis proportional to the presence of these vehicles in the California vehicle population. Exhaust emission tests were performed to measure carbon monoxide, hydrocarbons, carbon dioxide, and oxides of nitrogen under four different operating conditions before and after the disconnection of the vacuum spark advance. These four operating conditions included simulated stop and go traffic driving, 50 mph level road cruise, 50 mph 8% grade and the Federal CVS driving schedule. In addition, measurements of exhaust, engine coolant, and inlet air temperatures were made before and after vacuum spark advance disconnect, under operating conditions likely to induce engine overheating. Driveability tests were performed on all vehicles before and after the disconnection of the vacuum spark advance. Vehicle owners also completed a driveability questionnaire after a full week of operating their vehicles with the vacuum spark advance disconnected.

Under all four testing conditions, VSAD resulted in a substantial average reduction in NOx emissions. An average NO, reduction of 38% was measured using the 1972 Federal test procedure. The average corresponding reduction in hydrocarbons was 13% while carbon monoxide and carbon dioxide changed only slightly. When tested under stop and go traffic conditions, NOx decreased 29% after VSAD. Hydrocarbons, carbon dioxide and carbon monoxide were not significantly affected under this condition. A sizeable reduction in NOx of 51% after VSAD was observed at 50 mph cruise, with a decrease in HC and CO of 23% and 7% respectively. The reduction in NOx at this driving condition was pronounced because the greatest differential in spark timing after VSAD occurs at high speed, part throttle load. Changes in emission levels at 50 mph S% grade were nominal reflecting the similarity of spark timing before and after VSAD at this operating condition.

Engine coolant inlet air and exhaust temperatures increased for over half the test vehicles after VSAD. Exhaust temperatures for 50 mph 8% grade and stop and go showed substantial increases. The trend toward increased exhaust temperature and coolant temperature was not consistent, and a few vehicles experienced reductions in these and coolant temperatures. The driveability tests performed by AESi's technicians, as well as the response offered by the vehicle owners regarding driveability suggested that, on the average, a slight penalty in driveability might be anticipated. Fuel mileage as calculated from emissions data on the 1972 Federal test was penalized an average of 3.5% by VSAD. Statistical analyses of the emissions data and other quantitative results of the program are presented in this final report.


For questions regarding this research project, including available data and progress status, contact: Heather Choi at (916) 322-3893

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