August 30, 2023—Why it's not possible to jump start a car by hooking a vacuum to its exhaust pipe
In theory it would not be possible to jump start a car by hooking a vacuum to its exhaust pipe, no matter how strong the vacuum is.
(Assuming a normal four-stroke engine that would otherwise idle fine.)
The intuition is that in order to make the engine complete a full four-stroke cycle, you’re going to have to overcome the compression stroke of at least one piston. Yet the compression ratio of the engine is always greater than 1:1 (the motor in my car has a compression ratio of 10.5:1). Therefore it will always take more force on the piston to complete the first compression stroke than the force that you’ll get from 1 atmosphere of pressure. Yet a vacuum can only give you the force of up to 1 atmosphere of pressure.
Remember, it’s the starting conditions that you have to overcome in order to start the engine. So while the intake pressure of an already idling engine will be negative (relative to ambient) and thus the peak pressure of its compression stroke will be much lower, a stopped engine that needs to be started doesn’t have this luxury. And even if it did, that negative intake pressure would be working against your vacuum on the exhaust.
I’m not sure there is one. Unless you’re thinking of designing one such vacuum powered jump starter—in which case, don’t.