Powering a car with both a reciprocating engine and a jet engine (at the same time) I’m not sure if this is the best stack exchange to ask this, but what kind of behaviour can we expect from a car at launch that is powered by both a conventional reciprocating engine (connected to the wheels via a gearbox) and a jet engine at the same time? My intuition is that the maximum acceleration of a car at standstill is limited by the available traction from the tyres. A jet-powered car does not have this limitation, but it has many practical problems to overcome: poor response, high fuel consumption, the danger of the hot exhaust gases etc.
If the car were being driven by the jet engine at the same time the wheels are driven, would the two motor forces work in harmony, or would there be undesired behaviour like wheel slip, gear slip, etc? Would love to hear your thoughts as I have no idea.
 A: I imagine it would be a total fail. If jet engine produced linear acceleration is greater than internal combustion engine linear acceleration,- then there will be unavoidable tire slipping, due to the fact that car's engine is not on par with jet engine, hence in this scenario car's engine would act as passive breaks.
Second point - maneuverability issues. While car's torque in case of driver tilted wheels nicely rotates car in a turn,- jet engine would have just pushed car simply forward in old direction, resulting again in tires slipping, because jet force is applied to car COM. Have you seen an aircrafts rushing in turns at airports at $50~km/h$ ? They are very slow at this. This could be a bit fixed by rotating jet engine in a turn,- for producing acceleration in a direction which driver wants to be. But this have to be strictly synchronized with turning of wheels. Which is again a big issue.
Lastly as you mentioned already, jet engine has poor response,- it's good for long-lasting acceleration, but not for sudden and frequent acceleration changes to which internal combustion engine is optimized for. If jet engine again will be not synchronized with a car engine by responsiveness,- it will result in tire slipping, maneuverability issues, car engine fuel consumption ineffectiveness  and/or other problems.
Well... unless you want to beat Guinness World Record on fastest car in a world,- then remove car engine at all, attach jet engine to it, and accelerate only in a straight line, then you will beat them all. But your mixed solution for maneuvering in a cities where we spend at least $20\%$ of time just standing in traffic jams,- is a no-go.
A: At the moment of launch, the car would have a higher forward acceleration because the force acting on it would be greater than it would be with only an internal combustion engine.  This of course assumes that the force applied by the jet engine more than compensates for the increase in mass.
At some point, depending on the throttle position among other things, the car would reach a velocity where the internal combustion engine can no longer accelerate it.  Assuming the jet engine is still accelerating the car forward, the velocity would increase beyond this point, and the internal combustion engine would then be responsible for some drag.  Whether this causes the tires to slip or has some effect on the transmission depends on the type of transmission, the condition of the roads and tires, and, of course, the magnitude of the forces.
