Question: It has often been mentioned in CAR that a turbocharged engine needs to idle a few seconds after starting before being driven to allow the oil to lubricate the turbo’s bearings. You have also stated that a turbocharged engine should be allowed to idle for a few minutes after driving to allow the turbo to stop spinning before having its oil supply cut when the engine is shut down. Now, it has been reported that the latest Audi A3 and Mercedes-Benz A-Class will introduce turbocharged petrol and diesel engines with the option of stop/start systems. What has changed on these engines’ turbos to allow such a feature?
GERRIT GRUNDLING, By email
Answer: A turbo-development engineer in Europe supplied CAR with the following answer:
Turbos on new-generation, highly boosted engines have watercooled centre housings. The coolant (taken from the engine cooling) acts as a heat sink after engine shutdown to prevent the oil from coking. The layout of the cooling system and turbo should be designed to promote natural circulation or ow (a thermosiphon effect) after the engine is switched off. However, this “natural cooling” often isn’t enough and an electric water pump (in addition to the mechanical water pump) is used to provide forced cooling for a few minutes after the engine is switched off. During development, testing is done to check if an additional electric water pump is really needed.
Software can also prevent the stop/start system from kicking in if conditions are unfavourable. We would still recommend allowing for idling time before switching off a turbocharged engine.