There’s no need to switch off the air-conditioner before cranking the engine

Does it make any difference whether I switch off the air-conditioner before I start up or shut down the engine?

In the past, all automotive air-conditioning systems relied on belt-driven compressors.

The pulley at the front of the compressor freewheels until the air-con is switched on. An electromagnetic clutch then engages the pulley to the compressor shaft to drive an internal piston or rotor.

The earliest air-con compressors used reciprocating pistons, before being superseded by the more efficient and compact rotary compressors.

Typically, an automotive air-conditioning compressor saps between 4kW and 7kW of power, while the electromagnetic clutch draws at least 4 amperes of electric current to maintain grip.

With the air-conditioner switched on when cranking the engine, the starter motor receives less battery power because of the extra load of the compressor.

In modern cars, the electronic control unit (ECU) automatically disables the compressor during start-up. The ECU may even delay the activation of the blower fan until the engine settles into idle.

Hence, there is no longer any need, and no real advantage, to switching off the air-con manually.

While today’s air-conditioning compressors are conceptually similar in that they are essentially rotary devices, not all of them are belt-driven.

Electric vehicles use electric compressors that do without a clutch.

Instead, they are electronically controlled for variable compression of the refrigerant, making them far more efficient than the old on-off control clutch systems.