Basic Engine Design, Part 2

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Basic Engine Design, Part 2

• Rob Shanahan
• Apr. 2, 2006

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How to increase engine output

• Increase volumetric efficiency
• Better gas flow
• Forced induction
• Altered valve timing
• Improve engine efficiency
• Better combustion efficiency
• Lower internal friction
• Reduce heat loss
• Better fuel

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Brake Mean Effective Pressure

• Not actually a measure of pressure, just
  expressed in pressure units (N/m2 or psi)
• A measure of work output from an engine
• BMEP Horsepower__________
• (Swept volume Vs) ( rpm/120)
  

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Compression ratio

• Higher compression gives greater combustion
  efficiency, up to a point
• Efficiency goes down after a certain point due to
  need to retard ignition timing to avoid knock

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Ignition timing

• For every combination of engine rpm and load,
  there is an optimum ignition setting
• A dynomometer is required to map these points
• Steve Dose has been helping the SAE projects

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Internal Friction

• Low tension piston rings
• Thinner piston rings
• Low tension valve springs
• Lighter valves allow lower tension valve springs
• Low-friction coatings
• Main and rod bearings
• Piston skirts
• Cam lobes and followers

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Reduce heat loss

• In general, less heat rejected to the cooling
  system or atmosphere results in more heat energy
  to be converted to mechanical energy
• Exhaust coatings and wraps
• Coated piston crowns

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Better fuel

• Alcohol has both greater knock resistance and
  greater cooling effect due to higher latent heat
  of vaporization
• Toluene has greater energy density
• Nitromethane adds both energy and oxygen to the
  combustion reaction

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Increased airflow

• Greatest power increases are in the cylinder head
• Start by balancing the intake and exhaust flow
• Test manifolds as well to make sure they are not
  a restriction to flow
• Testing flow vs. valve lift helps to choose the
  proper cam profile

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Forced induction

• Intake manifold pressure can be increased above
  atmospheric with a supercharger or turbocharger
• Either device requires power from the engine in
  order to drive it, either from mechanical drag or
  increased exhaust manifold backpressure
• Output increase is greater than the power
  required to drive it, so it is a net gain

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Supercharger

• Mechanically driven, positive displacement pump
• Instant throttle response
• New designs are approaching the efficiency of
  turbos
• Can be clutch driven

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Turbocharger

• Good efficiency in cruise
• Needs time to spin up, so there is a lag when
  throttle is opened
• Lots of hot, whirling bits

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Altered valve timing

• Performance cam opens valve earlier, lifts it
  higher, and closes it later
• Area under the curve is a good measure of cam
  performance
• Duration is degrees that valve is open
• Lift is the greatest distance valve is raised off
  the seat

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Altered cam timing, cont.

• When duration increases the overlap period also
  tends to increase
• Increased overlap is bad for low rpm drivability
• Increased overlap tends to shift output to higher
  rpm range

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Altered cam timing, cont.

• As rpm increases, the optimum valve timing shifts
• Fixed cam timing is therefore a compromise
• Variable valve timing (VTEC) is an attempt to
  optimize valve timing across the operating range

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My approach

• Cylinder head is key to power increase
• For cams, lift is better than duration
• Torque is much more important than peak
  horsepowerarea under the curve
• A heavy crank extends bearing life
• Extreme compression is not worth it
• Dyno tuning is a must