Inside the cylinders of a diesel engine, the mixture of fuel and air is compressed to about 1/20 of its original volume. although diesel engines have no spark plugs to ignite this mixture, it ignites anyway because_.
a) increases in temperature decrease molecular motion
b) rapid decrease in volume raise the temperature of the mixture
c) rapid increases in volume raises the temperature of the mixture
d) change in temperature is controlled by the cooling system
e) diesel fuel and air ignite at room temperature
Look at b. Doing work on the system (increasing pressure and decreasing volume) raises the temperature.
The correct answer is (b) rapid decrease in volume raises the temperature of the mixture.
To understand why, let's break it down step by step:
1. Inside the cylinders of a diesel engine, a mixture of fuel (diesel) and air is compressed. This compression process reduces the volume of the mixture.
2. According to the ideal gas law, when the volume of a gas decreases, its pressure and temperature increase, given that the amount of gas and the number of gas molecules remains constant. This happens because compressing the mixture forces the molecules closer together, increasing their collisions and overall kinetic energy.
3. In the case of a diesel engine, as the fuel-air mixture is rapidly compressed to about 1/20 of its original volume, the pressure and temperature of the mixture rise significantly. This increase in temperature occurs due to the rapid decrease in volume, causing the molecules to collide more frequently and with higher energy.
4. The rise in temperature caused by the compression process is sufficient to ignite the diesel fuel without the need for a spark plug. Diesel fuel has a higher autoignition temperature than gasoline, which means it ignites under high temperature and pressure conditions.
Therefore, the correct answer is (b) rapid decrease in volume raises the temperature of the mixture.