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Forced Induction

Forced Induction

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Forced Induction

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  1. Forced Induction Ram-air, Superchargers, Turbochargers and Nitrous Oxide

  2. Forced Induction Theory • An average normally aspirated engine will achieve approximately 85% volumetric efficiency • With forced induction, a volumetric efficiency well in excess of 100% can be achieved • By increasing the amount of air entering the engine, more fuel can be burned which creates more cylinder pressure and more power

  3. Ram-air Induction • May use forward facing hood scoop, cowl induction, or a forward facing air inlet tube exposed to a higher than ambient pressure area • Uses the speed of the vehicle to pressurize the intake system

  4. Superchargers • Engine driven air compressor capable of pressurizing the intake of an engine • Types • Roots • Screw • Centrifugal • Axial flow

  5. Roots Supercharger • Positive displacement type • Acts as an air pump • Constant boost pressure across the full RPM range • Numbering System (example 6-71) • 6 cylinder • 71 CID per cylinder

  6. Screw Type • Internal compression type • Similar to roots style • Lobes interlock and compress the air along the way • More efficient than roots style

  7. Centrifugal • Similar to a turbocharger except it is driven by the engine • Uses an impeller to compress air • May be connected to engine by hoses • May be mounted in more convenient positions

  8. Axial Supercharger • Not widely used • Similar in operation to a jet engine • Multiple compressor rotors progressively compress air

  9. Supercharger Efficiency • Volumetric efficiency • Ratio of theoretical airflow and actual airflow • Example: one revolution of the supercharger should displace 5 CFM but in operation only 4 CFM are displaced per revolution - 80% VE • Adbiatic efficiency • Ratio of how much engine power is used to run the supercharger to how much air is compressed (air heating has a negative impact on adbiatic efficiency) • Roots – 50% • Screw – 65% • Centrifugal – 75%

  10. Turbochargers • Exhaust gas spins the turbine wheel which then drives the compressor wheel • Disadvantages • Increased backpressure • Turbo lag • A wastegate causes exhaust gas to bypass turbine wheel when boost pressure reaches a pre-determined level – it is operated by an actuator which senses induction system pressure

  11. Turbochargers • Rotational speeds may exceed 10,000 RPM • Turbo charger bearings are oiled with engine oil (oil starvation a major cause of turbo failure) • Manufacturer’s suggest allowing engine to idle 30 – 90 seconds before shutting engine off

  12. Nitrous Oxide • Nitrous oxide increases engine horsepower by supplying more oxygen to the combustion process which allows more fuel to be added • With nitrous the engine is supplied with more oxygen than it could draw through the induction system normally

  13. Nitrous Oxide Chemistry • N20 = nitrous oxide (2 nitrogen atoms and 1 oxygen atom) • The nitrogen acts as a buffer and slows the combustion process • This prevents major cylinder pressure spikes which would cause part failure • Nitrous is 36% oxygen by weight • Atmosphere is 21% oxygen • Boiling point of nitrous at sea level is 128 degrees • When N20 reaches 572 degrees Fahrenheit it separates into one oxygen atom and two nitrogen atoms • At this point the oxygen is available to the combustion process

  14. Nitrous Oxide Use • The number one concern when using nitrous oxide is “going lean” • Nitrous oxide is stored in a bottle at approx 900 PSI • Engine components must be selected to withstand the increased cylinder pressures

  15. Nitrous Oxide Systems Arming Switch - + Throttle Switch NOS Nitrous Solenoid Carburetor Fuel Pump Fuel Solenoid Nitrous Plate

  16. Nitrous Use • 100 –150 horsepower systems are generally safe for a stock V-8 engine in good mechanical condition • Limiting factor is cylinder pressure. • More nitrous, more cylinder pressure • More cylinder pressure, more power • More cylinder pressure, more likely that stuff breaks