Effect of Intake Valve Deposits and Gasoline Composition on S.I. Engine Performance 922263
Valve deposits in gasoline engines increase with time, absorbing fuel during acceleration and releasing fuel during deceleration. Valve deposits insulate the heat release from the cylinder and this phenomenon is the cause of bad fuel vaporization. In this way, the deposits greatly affect the driveability and exhaust emissions.
Using a 3.OL MPI(Multipoint Injection) engine, we measured the quantity of fuel that deposits at the intake port, and the throttle response (using a wall-flow meter made by Nissan Motor Co.1), 2) to study the deposits effect on driveability and exhaust emissions at a low temperature. The deposits were formed on the intake valve surface (about 8.0 on the CRC deposit rating scale) through 200 hours of laboratory engine stand operation.
At low temperature, C9 and C10 hydrocarbons tend to stick to the intake port surface and intake valve as “wall-flow”; this is one cause of bad driveability. We sampled and analyzed pre-ignition gas from the cylinder using a solenoid valve, finding that light gasoline vaporises easily and heavy gasoline tends to become wall-flow during acceleration. The A/F momentarily becomes lean, which is not good for emissions. In a worst case, abnormal burning such as misfire can occur. If deposits cling to the intake valve, they absorb fuel during acceleration, irrespective of the cooling water temperature, and the absorbed fuel enters the cylinder later. By using a Ispecial valve-with thermosensors buried on the combustion chamber side of the valve and manifold side of the valve-we observed a correlation between the insulating effect of the heat release from the cylinder by the deposits and the A/F response delay. The emissions from a car with a dirty intake valve become worse during the LA4 emissions cycle, especially NOx and hydrocarbon emissions.