15.2.
Starter Motor Capacity
The starter motor must fulfil all the criteria discussed earlier. From the comparison of the engine cranking torque with the minimum cranking speed (Fig. 15.2), the torque required from the starter can be determined.
Manufacturers provide characteristic curves of starter motor, which are discussed in more detail in the next section. The curve represents torque, speed, power and current consumption of the starter at 293 K and 253 K. The power rating of the motor is provided as the maximum output at 253 K using the recommended battery. Figure 15.5 shows the comparison of the power output for the starter with the engine size. A greater torque is required for engines with a lower number of cylinders due to the greater piston displacement per cylinder. As a very general guide the stalled starter torque required per litre of engine capacity at the starting limit temperature is;
(i) 12.5 Nm/1 for twin cylinder engines.
(ii) 8.0 Nm/1 for four cylinder engines.
(Hi) 6.5 Nm/1 for six cylinder engines.
(iv) 6.0 Nm/1 for eight cylinder engines.
(v) 5.5 Nm/1 for twelve cylinder engines.
To understand the relation between torque and
power consider the following example. Assume that under the worst conditions (253 K) a four cylinder 2 litre engine requires 240 Nm to overcome static friction and 80 Nm to maintain the minimum cranking speed of 100 rpm. With a starter pinion to ring gear ratio of 10:1 the motor must be able to produce a maximum stalled torque of 24 Nm and a driving torque of 8 Nm. It is assumed that stalled torque is generally three to four times cranking torque.
Torque (T) is related to power (P) as;
Power = torque x angular velocity (P = To), where, co = 2 n ra/60 and n is rpm. Therefore, the output power developed at 1000 rpm with a torque of 8 Nm (at the starter) is about 840 W. In Fig. 15.5 the ideal choice appears to be the starter marked ‘c\ The maximum permissible recommended battery would be 55 Ah for normal condition and 255 Ah for cold start performance.
Fig. 15.5. Power output of the starter compared to engine size.
