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Why it is easy to start the vehicle on hot day than on cold days? Since on winter days it is diffcult to start than on hot days I thought it is due to the low temperature which in turn affects the chemical reaction in the cell.Since energy is produced in the cell due to reaction.But I don't think it is the correct one..

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up vote 4 down vote accepted

There are two main reasons

As temperature is decreased the voltage of a car battery decreases and it's internal resistance increases. This means the battery can supply less current.

As temperature is decreased the viscosity of oil in the engine increases so the engine is harder to turn over.

In the days before fuel injection there was a third factor because carburettors are less good at vaporising fuel at low temperatures, but this won't be a problem for most modern cars.

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The viscosity of the fuel could slow down the injector rates, unless the design specifically compensates for that. – Skaperen Mar 2 '13 at 16:34

Well in winter day's the cylinder's in which combustion takes place and the fuel as well as surrounding air are at very low temperature so when you start the engine the cold fuel and air mixture enters the cold combustion chamber and at that time the temperature of the spark produced by the spark plug for a very little instance of time is not sufficient to rise the temperature of the fuel gas mixture to the level at which it can burn and start the engine. Well after sometime you see that engine just start's and shut's down. I think this is because when the fuel burn and start expanding the heat is instantly absorbed by the cylinder walls and it stop's the expansion of the gas due to which piston is not able to move and engine don't start.

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