Basic Engine, 4-CYLINDER Petrol: NVH Saab 9-3 I

Engine vibration

Great efforts are continually being made to improve comfort for car drivers and passengers. Two areas that have been addressed are engine vibration and noise level, both of which are a result of the basic design of the engine. Through combustion, during which chemical energy is converted into mechanical energy, gas forces are generated that act on the piston crown. The reciprocating motion of the pistons and connecting rods, combined with the rotation of the crankshaft, generate inertial forces that act on the engine block and cause it to vibrate in various ways. At low engine speeds, these gas forces are greater than the inertial forces but at high engine speeds, the inertial forces are greater than the gas forces. The most significant forces arise periodically once or twice per crankshaft revolution. They are known as first-order and second-order forces respectively. The first-order inertial forces are completely cancelled out since the crankshaft is balanced and the two piston pairs, 1-4 and 2-3, reverse their direction simultaneously when they reach top dead center and bottom dead center.

Identifying Second-Order Forces Acting Vertically. Scheme 190

Inertial forces are generated because both of the descending pistons in a 4-cylinder engine at a certain crankshaft angle move a longer distance than the ascending pistons (the lateral movement of the crankshaft accelerates the descending but delays the ascending piston movement). Therefore, the common center of gravity of the ascending and descending masses varies, which leads to ascending and descending forces that vary periodically twice per crankshaft revolution and make the engine vibrate vertically.

Valve noise

1. Valve chatter after long-term parking (longer than 48 hours). During prolonged parking, the oil drains out of the tappets' high-pressure chambers and air is forced in. Once the engine is started, it will take approx. 15 minutes for the air to be purged from the tappets. During this time, valve noise will be heard (chatter). The engine speed should not be allowed to exceed 3000 rpm until the valve chatter has ceased. See step 4 .
2. Valve chatter on starting the engine after an oil change. If valve chatter did not occur before the oil change, it will disappear of its own accord when the engine oil pressure has reached normal level.
3. Temporary noise on starting a cold engine. This noise is perfectly normal and will disappear of its own accord when the engine oil pressure has reached normal level.
4. Valve chatter if the engine is started after performing work on the engine that has involved turning over the engine manually or with the starter motor. The noise is due to one or more tappets being drained of oil. The noise will cease after being driven for a while. In severe cases, it may take up to 15 minutes at 2000-3000 rpm. At lower engine speeds, it will take much longer to expel the air and at higher speeds, there is a danger of damaging the valve tappets.
5. Noise after changing valve tappets. Cause and remedy, see 4 .
6. Noise occurring after a short period at idling speed with warm engine. If the engine is kept at 1500 rpm or above, the noise will soon cease. The noise is due to low oil pressure in the hydraulic tappets at idling speed.
7. Noise occurring at high engine speeds, which disappears after a period at idling speed. This noise is due to a high concentration of air in the oil at high engine speeds. The air originates from leakage on the suction side of the oil pump, e.g. a leak in the suction pipe.
8. Noise from individual tappets independent of how the car is driven. The probable cause is an impurity trapped in the tappet's non-return valve. The defective tappet (tappets) can be most easily detected once the engine has been turned off and the camshaft cover removed by pressing all the unloaded tappets. The tappet that feels soft (spongy) is the defective one. Change the defective tappet (tappets).