"Blowby" denotes the part of the total amount of engine ventilation gas that escapes from the combustion chamber, past the piston ring system and into the crankcase [22, 23]. Furthermore, gas escaping past valves and shaft seals or the turbocharger causes changes in the absolute amount of ventilation gases (see Fig. 12). As the amount of blowby cannot be measured directly, measurement of the total ventilation gas is often referred to as blowby measurement. However, this can only give an indirect indication of the sealing performance of the piston ring system.
Since the piston ring system is a dynamic sealing system, the quality of sealing is governed both by the velocity of the system and the differential pressure at the point of sealing. When stationary, such a system is therefore more or less ineffective.
A typical diesel engine map such as shown in Fig. 13 indicates, as may be expected, that the amount of ventilation gas increases with increasing mean effective pressure (equals differential pressure above the piston ring system). However, if this map is not plotted per unit of time - i.e. R/min - as in Fig. 13, but the quantities are related to the engine speed (R/rev) as in Fig. 14, then it will be apparent that the sealing quality of the system improves with increasing engine speed. These maps show no anomalies.
In gasoline engines, on the other hand, there are more frequent anomalies as can be seen from a representative gasoline engine ventilation map (Fig. 15). Up to a typical engine speed (here about 4500 rpm) a gasoline engine often exhibits a similar ventilation behaviour to the diesel engine described previously. Upwards of this speed, however, the gas quantity rises significantly at low mean effective pressures and at the highest revolutions even surpasses the full load values. This anomaly is generally referred to as "flutter" (for details see "Ring Flutter").
Fig. 12: Ventilation Gas Paths
Fig. 13: Ventilation Map (Diesel)
Fig. 14: Ventilation Map (Diesel)
Fig. 15: Ventilation Map (Gasoline)