CNG Vehicle Fueling Animation

When fueling a Compressed Natural Gas (CNG) vehicle tank, the amount of fuel in a “full” tank will vary depending on ambient temperature and fill rate. The dispenser automatically stops filling when it reaches a pressure that will allow the gas inside to safely expand or contract based on typical daily temperature changes.

Try it and choose different fill speeds and temperatures and watch what happens.

Fast-Fill at 0°F

0 minutes – Gas rapidly cools
CNG moves rapidly from the dispenser (high pressure) into the fuel tank (low pressure). The gas cools upon expansion in the tank – a phenomenon known as the Joule-Thomson effect.

1 minute – Pressure rises, temperature rises
As more gas enters the tank, its pressure increases, thereby elevating the tank’s internal temperature. The heat generated during this process is called the heat of recompression. Although the gas collecting in the tank is warming, the nozzle may still feel cold because of the Joule-Thomson effect — where the gas just entering the tank rapidly cools upon expansion.

3 minutes – Gas heats up
Because the tank is filling so quickly, the extra heat created doesn’t have time to dissipate from the walls of the tank. As the gas becomes warmer, the molecules move a bit faster and the pressure increases. The heated molecules collide against one another and the tank wall with greater force and frequency. This causes the gas to expand, taking up a bit more room inside the tank. But, because it is a cold day, the molecules are still moving relatively slowly and more of them can fit in the tank than on a hot day.

4 minutes – Tank continues to fill
Unlike a liquid fuel dispenser that shuts off when the liquid reaches the top of the tank, a CNG dispenser stops when it reaches a certain pressure. The pressure at which the dispenser stops filling is based on multiple factors, most notably, the temperature and, therefore, how much pressure the tank can safely handle if the ambient temperature were to rise.

5 minutes – Dispenser shuts off, operator disconnects
The dispenser stops filling when it reaches 3,000 psi. At that moment, the amount of fuel in the tank is only 16 gasoline gallon equivalents (GGEs). Because the gas heated up during the fill process, it took up more room and could not fill to its 20-GGE capacity.

1 hour – Gas cools to ambient temperature
The inside of the tank gradually cools to the ambient temperature (0°F) and the gas molecules begin to move more slowly. The tank’s internal pressure drops to around 2,500 psi. If the temperature outside were to rise or if the vehicle were to enter a 70°F garage and stay parked, the gas would move faster and expand, taking up more room in the tank, and the pressure would safely rise to 3,600 psi, the tank’s rated pressure.

Fast-Fill at 70°F

0 minutes – Gas rapidly cools
CNG moves rapidly from the dispenser (high pressure) into the fuel tank (low pressure). The gas cools rapidly upon expansion in the tank – a phenomenon known as the Joule-Thomson effect.

1 minute – Pressure rises, temperature rises
As more gas enters the tank, its pressure increases, thereby elevating the tank’s internal temperature. The heat generated during this process is called the heat of recompression. Although the gas collecting in the tank is warming, the nozzle may still feel cold because of the Joule-Thomson effect — where the gas just entering the tank rapidly cools upon expansion.

3 minutes – Gas heats up
Because the tank is filling so quickly, the extra heat created doesn’t have time to dissipate from the walls of the tank. As the gas becomes warmer, the molecules move faster and the pressure increases. The heated molecules collide against one another and the tank wall with greater force and frequency. This causes the gas to expand, taking up a bit more room inside the tank. Because it is a warm day, the molecules are moving quickly and fewer of them can fit in the tank than if it was a cold day.

4 minutes – Tank continues to fill
Unlike liquid fuel, where the dispenser shuts off when the liquid reaches the top of the tank, a CNG dispenser stops when it reaches a certain pressure. The pressure at which it stops filling is based on multiple factors, most notably, the temperature and, therefore, how much pressure the tank can safely handle if the ambient temperature were to rise.

5 minutes – Dispenser shuts off, operator disconnects
The dispenser stops filling when it reaches 4,100 psi. At that moment, the amount of fuel in the tank is 15.5 gasoline gallon equivalents (GGEs). Because the gas was already warm and also heated up during the fill process, it took up more room in the tank and could not fill to its 20-GGE capacity.

1 hour – Gas cools to ambient temperature
The inside of the tank gradually cools to the ambient temperature (70°F) and the gas molecules move more slowly. The gas contracts and the pressure drops to about 3,600 psi, the tank’s rated pressure. If the ambient temperature rises or falls from this baseline, pressure will safely rise or fall accordingly. A tank is designed to handle 125% of its rated pressure, so if the temperature outside were to rise, the pressure would remain within a safe range.

Fast-Fill at 100°F

0 minutes – Gas rapidly cools
CNG moves rapidly from the dispenser (high pressure) into the fuel tank (low pressure). The gas cools rapidly upon expansion in the tank – a phenomenon known as the Joule-Thomson effect.

1 minutes – Pressure rises, temperature rises
As more gas enters the tank, its pressure increases, thereby elevating the tank’s internal temperature. The heat generated during this process is called the heat of recompression. Although the gas collecting in the tank is warming, the nozzle may still feel cold because of the Joule-Thomson effect — where the gas just entering the tank rapidly cools upon expansion.

3 minutes – Gas heats up
Because the tank is filling so quickly, the extra heat created doesn’t have time to dissipate from the walls of the tank. As the gas becomes hotter, the molecules move faster and the pressure increases. The heated molecules collide against one another and the tank wall with greater force and frequency. This causes the gas to expand, taking up more room inside the tank. Because it is a hot day, the molecules are moving very quickly and fewer of them can fit in the tank then if it was a cold day.

4 minutes – Tank continues to fill
Unlike liquid fuel, where the dispenser shuts off when the liquid reaches the top of the tank, a CNG dispenser stops when it reaches a certain pressure. The pressure at which the dispenser stops filling is based on multiple factors, most notably, the temperature and, therefore, how much pressure the tank can safely handle if the ambient temperature were to rise.

5 minutes – Dispenser shuts off, operator disconnects
The dispenser stops filling the tank when it reaches 4,100 psi. The tank is designed to handle 125% of its rated pressure, so stopping at 4,500 psi would have maximized the number of gas molecules let into the tank on this hot day. However, most stations cannot fill past 4,100 psi. So, when the flow shuts off at 4,100 psi, the tank only contains 14 gasoline gallon equivalents (GGEs) of fuel. Because it was already hot outside and the gas also heated up during the fill process, the tank could not fill to its 20-GGE capacity.

1 hour – Gas cools to ambient temperature
After disconnecting from the dispenser, the inside of the tank gradually cools to the ambient temperature (100°F). The gas molecules move more slowly. The gas contracts, and pressure slowly drops to about 3,600 psi, the tank’s rated pressure. If the vehicle were to enter a 70°F garage and remain parked, the pressure would gradually fall to 3,300 psi.

Time-Fill at 0°F

0 hours – Gas enters tank
CNG slowly moves from the time-fill post (moderate pressure) into the fuel tank (low pressure). Initially, the gas cools very slightly upon expansion in the tank – a phenomenon known as the Joule-Thomson effect.

1 hour – Pressure and temperature increase slightly
More gas enters the tank over several hours, and its pressure gradually increases, thereby slightly elevating the tank’s internal temperature. The heat generated during this process is called the heat of recompression.

2 hours – Gas heats up slightly
As the gas molecules become warmer, they move faster, colliding against one another and the tank wall with slightly greater force and frequency. This causes the gas to expand, taking up a bit more room inside the tank.

3 hours – Heat has time to dissipate
Gas continues to slowly enter the tank, and pressure continues to rise, but the heat inside the tank created from the fill process has time to dissipate into the surrounding environment. The tank remains slightly warmer than the ambient temperature of 0°F.

4 hours – Tank continues to fill slowly
Unlike liquid fuel, where the dispenser shuts off when the liquid reaches the top of the tank, a CNG time-fill system stops when it reaches a certain pressure. The pressure at which it stops filling is based on multiple factors, most importantly, the temperature and, therefore, how much pressure the tank can safely handle if the ambient temperature were to rise.

6 hours -Flow of fuel stops
The flow of fuel from the hose shuts off when the tank reaches a pressure of 2,700 psi. Because the tank filled so slowly and the heat from recompression had time to dissipate from the tank walls, the gas stayed cool, didn’t move too fast, and therefore did not take up a lot of room in the tank. When fueling stops, the tank contains 18.6 gasoline gallon equivalents (GGEs) of fuel. But, because the gas heated up during the fill process, it took up extra room in the tank and could not fill to its 20-GGE capacity.

7 hours – Gas cools to ambient temperature
The inside of the tank gradually cools to the ambient temperature (0°F) and the gas molecules begin to move even more slowly. The gas contracts and the tank’s internal pressure drops to about 2,500 psi. If the temperature outside were to rise or if the vehicle were to enter a 70°F garage and stay parked, the gas would move faster and expand, take up more room in the tank, and the pressure would safely rise to 3,600 psi, the tank’s rated pressure.

Time-Fill at 70°F

0 hours – Gas enters tank
CNG slowly moves from the time-fill post (moderate pressure) into the fuel tank (low pressure). Initially, the gas cools very slightly upon expansion in the tank – a phenomenon known as the Joule-Thomson effect.

1 hour – Pressure and temperature increase slightly
More gas enters the tank over several hours, and its pressure gradually increases, thereby slightly elevating the tank’s internal temperature. The heat generated during this process is called the heat of recompression.

2 hours – Gas heats up slightly
As the gas molecules become warmer, they move faster, colliding against one another and the tank wall with slightly greater force and frequency. This causes the gas to expand, taking up more room inside the tank.

3 hours – Heat has time to dissipate
Gas continues to slowly enter the tank, and pressure gradually rises, but the heat inside the tank has time to dissipate into the surrounding environment. The tank remains slightly warmer than the ambient temperature of 70°F.

4 hours – Tank continues to fill slowly
Unlike liquid fuel, where the dispenser shuts off when the liquid reaches the top of the tank, a CNG time-fill system stops when it reaches a certain pressure. The pressure at which it stops filling is based on multiple factors, most importantly, the temperature and, therefore, how much pressure the tank can safely handle if the ambient temperature were to rise.

6 hours – Flow of fuel stops
The flow of fuel from the hose shuts off when the tank reaches a pressure of 3,800 psi. Because the tank filled slowly, the heat from recompression had time to dissipate from the tank walls. This allowed the temperature to stay as low as possible and ensure that as many gas molecules as possible could enter the tank. When fueling stops, the tank contains 18 gasoline gallon equivalents (GGEs) of fuel. Because the gas heated up during the fill process, it took up more room in the tank and could not fill to its rated 20-GGE capacity.

7 hours – Gas cools to ambient temperature
The inside of the tank gradually cools to the ambient temperature of 70°F, the gas contracts, and pressure drops to about 3,600 psi, the tank’s rated pressure. A tank is designed to handle 125% of its rated pressure, so if the temperature outside were to rise, the pressure would remain within a safe range.

Time-Fill at 100°F

0 hours – Gas enters tank
CNG slowly moves from the time-fill post (moderate pressure) into the fuel tank (low pressure). Initially, the gas cools very slightly upon expansion in the tank – a phenomenon known as the Joule-Thomson effect.

1 hour – Pressure and temperature increase slightly
More gas enters the tank over several hours, and its pressure gradually increases, thereby slightly elevating the tank’s internal temperature. The heat generated during this process is called the heat of recompression.

2 hours – Gas heats up slightly
The hot ambient temperature is also warming the gas. The gas molecules move faster than they would at lower temperatures, colliding against one another and the tank wall with greater force and frequency. This causes the gas to expand, taking up more room inside the tank.

3 hours – Heat has time to dissipate
Gas continues to slowly enter the tank, and pressure gradually rises, but some heat inside the tank has time to dissipate into the surrounding environment. The tank remains slightly warmer than the ambient temperature of 100°F.

4 hours – Tank continues to fill slowly
Unlike liquid fuel, where the dispenser shuts off when the liquid reaches the top of the tank, a CNG time-fill system stops when it reaches a certain pressure. The pressure at which it stops filling is based on multiple factors, most importantly, the temperature and, therefore, how much pressure the tank can safely handle if the ambient temperature were to rise.

6 hours – Flow of fuel stops
The tank reaches a pressure of 4,100 psi, and flow from the hose shuts off. Because the tank filled slowly, the heat from recompression had time to dissipate from the tank walls. This allowed the temperature to stay as low as possible and ensure that as many gas molecules as possible could enter the tank. When fueling stops, the tank contains 17 gasoline gallon equivalents (GGEs) of fuel. Because the gas was already hot and also heated up during the fill process, it took up more room in the tank and could not fill to its rated 20-GGE capacity.

7 hours – Gas cools to ambient temperature
The inside of the tank cools to the ambient temperature of 100°F, the gas contracts slightly, and pressure drops to about 3,900 psi. A tank is designed to handle 125% of its rated pressure, so even though the pressure is above the rated pressure of 3,600 psi, it is still within a safe range. If the vehicle were to enter a 70°F garage and remain parked, the pressure would gradually drop to 3,600 psi, the tank’s rated pressure.