I am struggling to understand why it is recommended to push start a car in 2nd gear rather than 1st - in terms of the physics involved, I fail to see a reason why this would be the case. I have compiled where I am at currently with a for/against list for each option.
Reasons why 1st gear would be better
- The engine has a greater mechanical advantage, so an engine that is not firing correctly, is sputtering and not producing as much torque as a normally running engine is less likely to be forced into a stall than in 2nd gear.
- For a push speed of X, the engine will be spinning at a higher RPM than in 2nd gear and I assume a higher RPM gives the engine a better chance of starting?
Reasons why 2nd gear would be better
It would give the people pushing the car a greater mechanical advantage than 1st gear, which may make it easier to keep pushing the car while attempting to start it. However, I'm not sure how important this is, as the aim is surely to get the engine running on its own, so continuing to rely on the force provided by the people pushing the car, even after the clutch has been engaged, already suggests the push start has not been successful?
If the push start is unsuccessful and the car comes to a halt, it should do so over a greater distance/time in 2nd gear than 1st, thereby reducing the risk of injury to the people pushing the car, which could happen if the car stops suddenly.
Reasons why gear selection should be irrelevant
- Assuming the retarding effect of each engine stroke is a constant and the push speed (and therefore momentum) is the same in both cases, then surely the number of strokes before the car comes to a halt should be the same in both cases. As mentioned above, I assume the likelihood of a successful start is probably related to RPM. However, if it is not and is simply a fixed chance that the engine will start during each stroke, then the likelihood of the engine starting should be directly related to the number of strokes, thereby suggesting there is no difference between the two options? - While the car may stop sooner in 1st than 2nd, the number of rotations of the engine should be the same. Admittedly, higher RPM is likely to increase engine braking and therefore starting in 1st gear may mean greater retardation per stroke, possibly favouring 2nd gear in terms of increasing the number of strokes before the car stops. Having said that, given the engine does a lower RPM for each MPH in 2nd than 1st, then the car has to slow down more in 1st than 2nd to reach the speed where the RPM will be too low for the engine to run without stalling, which could in fact go in favour of suggesting that 1st gear will in fact have more potentially successful strokes than 2nd. Overall, it seems as though these benefits would approximately counteract the negatives for each gear in this case.
In terms of the physics involved, I fail to see an obvious reason why being in 2nd gear will be beneficial. Have I missed anything? Or is this more of an old wives' tale than actually being something that will impact the likelihood of a successful push start?