Post by metalbeast on Sept 6, 2005 12:44:37 GMT -5
Volumetric Efficiency
An engine's volumetric efficiency it defined as the amount of air and fuel present in an engine related to atmospheric pressure. Lets look for example at a hypothetical cylinder that has a volume of 100 ci of displacement. Lets consider for this example that the volumetric efficiency of this engine is 100%. During the intake stroke for a cylinder, once the piston reaches BDC, 100 ci of fuel and air is pulled into the cylinder. Now let us consider and engine with a volumetric efficiency of only 80%. During the intake stroke for a cylinder, once the piston reaches BDC, only 80 ci of fuel and air are pulled into the cylinder and 20 inches of vacuum. It is easy to ascertain that the higher the volumetric efficiency an engine has, the more power it can produce.
Volumetric Efficiency is calculated as follows:
VE = actual engine CFM/theoretical CFM *100%
Typically, naturally aspirated engines have volumetric efficiencies between 80-90%. The volumetric efficiency of an engine is affected by a number of components
Forced induction engines can achieve volumetric efficiencies well above 100%. Forced induction increases the pressure well above atmospheric pressure inside the cylinders. This increase pressure is the result of more air and fuel being forced into the cylinders. This is the reason such phenomenal horsepower numbers can be seen with turbocharged and supercharged engines
Thermal Efficiencies
Its no surprise that the internal combustion engine is not the most efficient machines on the planet. One gallon of gas contains about 22,000 BTU of energy. Only a small portion of this energy is converted into power. The rest is lost to friction, heat and noise. The typical reciprocating gasoline engine has a thermal efficiency of 0.2-0.25. This means that for every gallon of gasoline that is consumed by a vehicle, only 5500 BTUs is converted into mechanical work. Rotary engines are slightly more efficient that reciprocating gasoline engines by about 2-4%. Diesel engines are even more efficient and have thermal efficiencies of over 0.3. The thermal efficiency of an engine is affected by the following:
Even a slight increase in thermal efficiency can significantly increase the power output of an engine. For example: Lets say an engine for a stock street car has a thermal efficiency of 0.23. Now look at a race engine that has a thermal efficiency of 0.28. On the face, it seems that an increase of 0.05 is insignificant. However if we look at the numbers more closely, we see that this is an increase of 22% in thermal efficiency. More efficiency means more power in the end ;D
An engine's volumetric efficiency it defined as the amount of air and fuel present in an engine related to atmospheric pressure. Lets look for example at a hypothetical cylinder that has a volume of 100 ci of displacement. Lets consider for this example that the volumetric efficiency of this engine is 100%. During the intake stroke for a cylinder, once the piston reaches BDC, 100 ci of fuel and air is pulled into the cylinder. Now let us consider and engine with a volumetric efficiency of only 80%. During the intake stroke for a cylinder, once the piston reaches BDC, only 80 ci of fuel and air are pulled into the cylinder and 20 inches of vacuum. It is easy to ascertain that the higher the volumetric efficiency an engine has, the more power it can produce.
Volumetric Efficiency is calculated as follows:
VE = actual engine CFM/theoretical CFM *100%
Typically, naturally aspirated engines have volumetric efficiencies between 80-90%. The volumetric efficiency of an engine is affected by a number of components
- Head design
- Intake design
- Headers/exhaust
- Cam design
- Valve train design
- Piston ring design
Forced induction engines can achieve volumetric efficiencies well above 100%. Forced induction increases the pressure well above atmospheric pressure inside the cylinders. This increase pressure is the result of more air and fuel being forced into the cylinders. This is the reason such phenomenal horsepower numbers can be seen with turbocharged and supercharged engines
Thermal Efficiencies
Its no surprise that the internal combustion engine is not the most efficient machines on the planet. One gallon of gas contains about 22,000 BTU of energy. Only a small portion of this energy is converted into power. The rest is lost to friction, heat and noise. The typical reciprocating gasoline engine has a thermal efficiency of 0.2-0.25. This means that for every gallon of gasoline that is consumed by a vehicle, only 5500 BTUs is converted into mechanical work. Rotary engines are slightly more efficient that reciprocating gasoline engines by about 2-4%. Diesel engines are even more efficient and have thermal efficiencies of over 0.3. The thermal efficiency of an engine is affected by the following:
- Head design
- Cam design
- Ignition design & timing
- Exhaust design
Even a slight increase in thermal efficiency can significantly increase the power output of an engine. For example: Lets say an engine for a stock street car has a thermal efficiency of 0.23. Now look at a race engine that has a thermal efficiency of 0.28. On the face, it seems that an increase of 0.05 is insignificant. However if we look at the numbers more closely, we see that this is an increase of 22% in thermal efficiency. More efficiency means more power in the end ;D
