The pressure and temperature of a gas are linked. As pressure goes up and down, so does temperature.
Take a container that has a fixed amount of gas. A plunger seals one end of the container.
A pressure gauge is attached, and a thermometer.
Pushing in the plunger increases gas pressure. It forces the gas molecules into a smaller space.
As a result, they move faster and make more impacts.
This causes the gas to heat up.
And the overall temperature of the gas rises too. An increase in pressure produces a rise in temperature.
Pulling out the plunger reduces gas pressure. It gives the molecules more room to move. They make fewer impacts.
The energy level of the molecules falls.
And so does the temperature of the gas. A fall in pressure produces a fall in temperature.
Now let’s see what happens when it is temperature that changes first.
When a gas is heated up, its molecules gain energy and start to move more quickly. They make more impacts.
This increases the pressure the gas is exerting. Increasing the gas temperature produces increased pressure.
Cooling has an opposite effect. The particles lose energy and slow down. They make fewer impacts.
Pressure falls.
So, for a fixed amount of gas, higher temperatures produce higher pressures. And lower temperatures produce lower pressures.
This is put to use in compression-ignition or diesel engines. Air in the combustion chamber is highly compressed by the piston. This causes such a large rise in air temperature that when fuel is sprayed into it, the mixture ignites.
Pressure & volume
The cylinder of a bicycle pump contains air. Its exit is sealed with a pressure gauge.
As the plunger is pushed in, the air is forced into a smaller volume. At the same time, the pressure gauge shows an increase in pressure.
It is this increase in pressure that allows the pump to do its work.
When the plunger is pulled out, the volume occupied by the gas grows larger, and the pressure drops.
Larger volume, smaller pressure
Smaller volume, higher pressure.
Pressure and volume are in what is called an inverse relation. As one rises, the other falls.
Temperature & energy
The temperature of a gas is a measure of how much energy it has. And the more energy it has, the more work it can do.
This is because heating gas particles makes them move faster, and they exert larger and larger pressure on the surfaces of their container
Higher pressure means the particles exert increased force. Until eventually, they are able to do work.
This is especially true for the very high temperatures generated in combustion. The more energy the air-fuel mixture has, the more force it exerts on the piston, and the more work the piston can do. That’s what happens during the power stroke.
Understanding power and torque
When an piston is forced down the cylinder during the power stroke it applies the force to the connecting rod. The connecting rod then causes the crankshaft to turn. The force that makes the crankshaft turn is called torque. The metric unit for the measurement of torque is newton meters, the imperial measurement is pounds feet.
If we assume that a shaft has a lever attached perpendicular to its axis, and that lever is 1 meter long. If a force of 100 newton were applied to the end of the lever, the torque applied to the shaft is 100 newton per meter or 100 newton meters. Similarly if a force of 100 pounds were applied to the end of the lever that was 1 foot long, the torque applied to the shaft is 100 pounds er foot or 100 pounds feet.
Power is a term used to describe how much work is done in a period of time. An engine produces POWER by applying TORQUE to a ROTATING shaft. So the measurement of engine power is calculated from the amount of torque applied to the crankshaft and the speed at which it is turning. When expressing engine power it is necessary to express not only the power value, but to include the engine speed at which it occurs.
The metric measurement of power is the Kilowatt. The imperial measurement is the Horsepower.
The watt is the metric system measurement of power, however engine power is expressed in kilowatts, as the watt has such a small value. A kilowatt is equivalent to 1000 newton per meter per second.
The term horsepower was expressed by James Watt, who determined that a horse could lift 330 pounds, 100 feet in one minute. Therefore 1 horsepower is equal to 33,000 pounds per foot per minute. 1 horsepower is more than a person can produce.
There are different standards of power measurement. These are ECE, SAE and DIN
The ECE standard is European. Engine power is measured at 99 kPa of dry air and 25°C (77 F). Friction torque is not taken into consideration at all.
The SAE standard American. Engine power is measured at 99 kPa of dry air and 25°C (77 F) and applies a friction correction and uses a default Mechanical Efficiency (ME) value of 85%. This is approximately correct at peak torque but not at other engine operating speeds.
The DIN standard is determined by the German automotive industry. engine power is calculated at 101.3 kPa of dry air and 20°C (68 F). With the advent of ECE standards, the DIN is rarely used.
