Basic Engine Theory

1
Basic Engine Theory

• Rob Shanahan
• 3-8-06

2
What is an Engine?

• Requires external fuel source (motor does not)
• Converts chemical energy to mechanical energy
• Only about 40 effecient
• Generates a lot of waste heat and sound
• Easiest to think of it as an air pump

3
Two-Stroke Engine Operation

• Simplest design with few moving parts
• Highest power/weight ratio
• Ineffecient, dirty and noisy
• Needs lubricant in fuel
• Narrow power band
• Banned in FSAE

4
Diesel Engine Operation

• No spark, ignition occurs when fuel is introduced
  to hot gasses
• Run very high compression ratios
• Emit a lot of particulates due to high fuel/air
  mixtures
• Emit a lot of NOX due to high combustion
  temperatures
• Parts must be heavy

5
Rotary (Wankel) Engine

• Smooth due to lack of reciprocating parts
• High power to weight ratio
• No valves
• Low maintenance
• Hot, noisy exhaust
• Banned in FSAE

6
Four-Stroke Engine Operation

• This is the engine design in most cars on the
  road
• Makes up the great majority of competition
  engines
• Also the engine design used in all SAE
  competitions

7
Four-Stroke Engine, cont.

• 4 strokes are intake, compression, power,
  exhaust

8
Compression Ratio

• Ratio of swept volume V1 divided by combustion
  chamber volume V2
• Ranges between 81 and 101 for most street cars
• Diesels run around 221
• Competition engines are usually between 101 and
  141

9
Rotating vs. Reciprocating Mass

• Crankshaft, flywheel, and lower portion of
  connecting rod are considered rotating mass
• Piston and upper portion of connecting rod are
  considered reciprocating mass
• Cams and valves also rotate and reciprocate, but
  mass is low

10
Two-valve vs. Four-valve

• Most engines are 2-valve, one intake and one
  exhaust
• 4-valve engines use two intake and two exhaust
• Two small valves have greater annular area than
  one large one
• Better flow at low lift

11
Exhaust Tuning

• When exhaust pulse hits low pressure area, a low
  pressure wave is reflected back up the tube
• By tuning exhaust primary tube length, low
  pressure pulse can arrive just as exhaust valve
  opens

12
Exhaust Tuning, cont.

• Commonly called headers
• Note equal length primary tubes
• Collector is the part where the primaries merge
  and the point from where the wave is reflected
• Tuned for a particular RPM

13
Intake Tuning

• The intake tract can also be tuned for a
  particular engine RPM
• In this case, intake was detuned to smooth out
  the torque peak
• Ultimate peak HP is not always the goal

14
Valve Timing

• Camshafts are the part that synchronizes all the
  motions
• Cams control the operation of the valves and
  therefore the operation of the engine
• Competition cams will typically have both greater
  lift and duration than stock