Wednesday, August 13, 2008

exhaust flow in an automobile-relation between performance and air flow

Components that influence airflow into the engine are the:

  1. air filter
  2. intake air piping
  3. mass air sensor (if applicable)
  4. throttle body or carburetor
  5. intake manifold
  6. camshaft
  7. intake port and valve of cylinder heads
  8. turbo's compression, section, and supercharger (if applicable)

Components that influence airflow out of the engine are the:

  1. exhaust valve and exhaust ports of the cylinder heads
  2. camshafts
  3. exhaust manifolds
  4. turbo's turbine (if applicable)
  5. exhaust tubing
  6. catalytic converters
  7. muffler

When these components are modified to increase flow out of the engine, pumping losses are reduced. Pumping losses refer to the amount of horsepower (HP) used to push the exhaust gases out of the cylinders on the engine's exhaust stroke. Since less HP is used to get the exhaust out of the engine, more horsepower is available at the flywheel. An added benefit of reducing pumping losses is that fuel mileage will also increase.

No matter how much additional air is forced into the engine, no additional HP will be made unless additional fuel is also added. The energy that makes HP in an engine comes from the combustion of the fuel, not only the air. In general, every two HP produced requires one pound of fuel per hour. When modifications are performed that increase airflow into the engine, more air is available for the combustion of fuel. The combustion of the additional fuel is what translates into additional HP.

Air flow is not just influenced by the size (area) of the paths it takes into and out of the engine. It is also influenced by the speed at which it moves.

Specific Port Flow (cubic meter/sec) = Flow Velocity (m/s) x Average Path Area (m2)

Whenever an engine modification increases the average area of the airflow paths into and out of an engine, there is a chance the velocity of the flow will decrease. Most of the time the factor of velocity decrease is very small compared to the area increased, so flow is generally increased. If modifications are taken too far, the velocity will decrease more than the area increases, so flow is adversely affected (example - four inch exhaust system on a 1.6 liter engine).

In the following section, we will analyze the components of an exhaust system in a car and how air flows from the engine to the outside environment.

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