This was emailed to me recently -- I guess it's going around as a technical "resource" in certain circles.
Enjoy:
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Now this is some great stuff for any Engineer!
Interesting article. Unfortunately, it starts with the bad physics - "Engines don't make horsepower; they convert fuel into torque." This is simply not true.
The correct statement is "Engines make power. They convert fuel into
power."
I am taking the time to make this correction because if you want a fast
car, you need to work with the correct physics. Power makes cars quick and
fast. And airflow makes power.
Power, not torque, is what moves a car. For example, if you were modifying
an engine and doubled the peak torque while holding the peak power the
same, the acceleration would basically be the same. However, if you doubled the peak power while holding the peak torque the same, the acceleration would double.
The physics are simple and quite clear.
Force = mass x acceleration (1)
Hence
Acceleration = force / mass (2)
And
Power = force x velocity (3)
Hence
Force = power / velocity (4)
Combining (2) and (4)
Acceleration = power / (velocity x mass) (5)
Acceleration is directly proportional to power, not torque.
You could start with (2) and work in torque for force, but you would have
to know tire radius and gearing to get force from torque and it would work
its way around to being torque x rpm, which is power so you would get the same result.
Torque is force without motion. No motion, no acceleration. Torque in
motion is power.
Power comes from converting the chemical energy in the fuel/air mixture to
heat energy and then converting the heat energy to mechanical energy.
Power is the rate of energy conversion. Since fuel/air mixture is essentially
constant, power is directly related to airflow. A piston engine is
essentially an air pump. The more air it pumps, the more power it makes.
Airflow is the product of cubic inches (air pumped per revolution),
revolutions per minute, and volumetric efficiency).
Airflow = cubic inches / 2 x rpm x volumetric efficiency
The " / 2" is because a 4-stroke only draws air every other revolution.
This is the classic approach to power. Make it bigger. Turn it faster.
Improve the volumetric efficiency by helping it breath with fewer losses -
bigger carb, bigger cam, bigger manifold, bigger heads, bigger valves -
improving the design and/or size of everything that restricts flow.
Enjoy:
----------------------------------------------------------------
Now this is some great stuff for any Engineer!
Interesting article. Unfortunately, it starts with the bad physics - "Engines don't make horsepower; they convert fuel into torque." This is simply not true.
The correct statement is "Engines make power. They convert fuel into
power."
I am taking the time to make this correction because if you want a fast
car, you need to work with the correct physics. Power makes cars quick and
fast. And airflow makes power.
Power, not torque, is what moves a car. For example, if you were modifying
an engine and doubled the peak torque while holding the peak power the
same, the acceleration would basically be the same. However, if you doubled the peak power while holding the peak torque the same, the acceleration would double.
The physics are simple and quite clear.
Force = mass x acceleration (1)
Hence
Acceleration = force / mass (2)
And
Power = force x velocity (3)
Hence
Force = power / velocity (4)
Combining (2) and (4)
Acceleration = power / (velocity x mass) (5)
Acceleration is directly proportional to power, not torque.
You could start with (2) and work in torque for force, but you would have
to know tire radius and gearing to get force from torque and it would work
its way around to being torque x rpm, which is power so you would get the same result.
Torque is force without motion. No motion, no acceleration. Torque in
motion is power.
Power comes from converting the chemical energy in the fuel/air mixture to
heat energy and then converting the heat energy to mechanical energy.
Power is the rate of energy conversion. Since fuel/air mixture is essentially
constant, power is directly related to airflow. A piston engine is
essentially an air pump. The more air it pumps, the more power it makes.
Airflow is the product of cubic inches (air pumped per revolution),
revolutions per minute, and volumetric efficiency).
Airflow = cubic inches / 2 x rpm x volumetric efficiency
The " / 2" is because a 4-stroke only draws air every other revolution.
This is the classic approach to power. Make it bigger. Turn it faster.
Improve the volumetric efficiency by helping it breath with fewer losses -
bigger carb, bigger cam, bigger manifold, bigger heads, bigger valves -
improving the design and/or size of everything that restricts flow.
