2 Stroke Engines 51001

1
2 Stroke Engines (5/10/01)

• No Separate induction and exhaust
• Occur Simultaniously
• Efficiency depends on scavenging
• Gas Exchange Process determines mixture
• Possible to Short-Circuit fuel direct to
  exhaust
• Must be forced induction

2
2 Stroke Engines

• Issues
• High emissions due to short circuiting
• Oil in fuel due to open sump, I.e. sump used to
  transfer intake charge
• Dynamic Waves flow important
• Can be either spark or compression ignition
  engines
• Intake port in side of cylinder in this simple
  model.

3
Why bother?

• Inexpensive to produce
• Low compression
• Can be high RPM, but do not need close tolerance
  because of high bypass
• Applications
• Off-road vehicles (motorcycles, ATV, snowmobiles)
• Lawn implements
• Outboard engines
• Small generators
• Anything where light weight is important and
  efficiency is not

• Low mass
• Few moving parts
• No cams, valves, etc.
• Only crank, piston, rod need move
• Higher specific power (power to weight)
• Low efficiency due to lost fuel, oil consumption
• High emissions, dirty

4
The 2 Stroke Cycle
5
Two stroke cycle

• 60 BBDC Exhaust opens (120 ATDC) Exhaust begins
  exiting, and contines to do so, ambient pressure
  (Pinf) reached.
• 55-50 BBDC, ATDC Intake opens, stays open until
  100 BTDC, charge is forced from crankcase or
  induction source, incoming charge displaces
  exhaust and forces out exhaust port, also mixes
• Scavenge process ends and intake port closes (55
  ABDC), ambient pressure, exhaust continues to
  exit
• 60 ABDC, exhaust port closed, compression begins
• 60 BTDC Crankcase pressure lt Pinf, crankcase
  port opens and charge fills crankcase
• Ignition occurs 10-40 BTDC, crank/flywheel
  momentum carries piston upward, Powerstroke
  begins at TDC
• Crankcase port closed at 60 ATDC, before flow out
  of crankcase. Crankcase pressure at Tinf at
  approx TDC

6
More Notes 2 Stroke

• Crankcase port may be replaced by reed valve
• Simplifies timing in that crankcase intake is
  pressure driven only
• Eliminates backflow from crank port
• May also use disk valve on crank (demo) so timing
  can be adjusted and need not be symmetric about
  TDC.

7
2 Stroke Performance Parameters

• How can you define cylinder volume?
• Delivery Ratio
• Mass delivered
• /Vsdensity
• Scavenging Efficiency,
• mass delivered retained
• /mass trapped
• Trapped Efficiency
• Mass deliverd retained/
• mass delivered
• Charging Efficiency
• Mass delivered
• /Vsdensity

8
Efficiency Ranges

• Delivery ratio 0.6 to 0.95
• Scavenging 0.7 to 0.9
• Trapping 0.6 to 0.8
• Charging 0.5 to 0.7

9
Scavenging Systems
10
Cross Scavenging

• Most popular
• Simple
• Inexpensive
• Deflector is troublesome, but reduces amount of
  intake charge that flows into exhaust

11
Loop Scavenging

• Eliminates need for deflector on pistom
• Many ports needed
• Exhaust ports directly above intakes
• Takes advantage of gas momentum
• Unported wall acts as deflector for incoming
  intake jet

12
Schurnle Loop Scavenging

• Modified loop scavenging (1920)
• Allows all ports on same level
• Note flow un U and also in plan view
• More efficient than loop

13
Uniflow Poppet Scavenging

• Allows for high degree of axial swirl by directed
  vanes in ports
• Exhaust exits at top of cylenders
• Requires use of poppet valves, I.e. complicated
  matters.
• Good for deisel apps which require high level of
  swirl. Injector is central

14
Uniflow opposed piston Scavenging

• Uses a second piston as to block off exhaust
  ports
• Less expensive than poppet valves? Looks like a
  mess.

15
Pumping arrangements

• Under Piston
• Crankcase used as intake pump
• Lubrication done by adding oil to fuel mixture
• Difficult to impossible to run pressure lube
  system
• Typical of small implement/inexpensive designs

16
Pumping arrangements

• May be Positive displacement pump
• Can also use exhaust valve for better in cylinder
  flow
• Blower is considered low pressure, near
  atmospheric

17
Pumping arrangements

• Supercharging refers to higher than atmospheric
  intake charge
• May be positive displacement or dynamic
  cpmpressor
• Turbos work well and can take advantage of
  unburnt fuel in exhaust to drive turbine (far
  fetched)
• Opposed layout shown makes little sense

18
Mixing Models - Benson

• Skip details in book, know concepts

19
Engine Performance and Technology

• 2 Stroke
• Max BMEP is about 6.4 bar _at_ 4000RPM
• SFC is 400g kg/kWh at 3000 RPM
• 4 stroke
• high as 11 bar
• SFC (4) 275 g/kWh
• 2 Stroke makes power every stroke, so 2-stroke is
  about 15-30 higher power

20
2 Stroke Disadvantages

• Short Circuit
• Unburnt A/F mixture flows directly from intake
  port to exhaust port
• High fuel consumption and high hydrocarbon
  emissions
• Especially bad at low speeds because intake
  charge has more time to flow.
• Net compression usually lower because of trapping
  efficiency and exhaust residuals in charge
• Effective mixture is well below Stochiometric
  because effectively you have EGR in the mix, so
  you may have as much as 30-50 lower available
  charge per cycle

21
Efficiency and Power

• Note relationship of charging efficiency and
  power (BMEP), and Trapping efficicncy vs. RPM

22
Direct Injection Orbital 2 Stroke

• Injects pressure (5 bar) fuel after exhaust port
  closes (but before combustion begins)
• Simultaneous Blast of 5 bar air shoots in with
  fuel to atomize fuel and allow for rapid burning
• Eliminates fuel short circuit since fuel not
  introduced until after exhaust closed
• Weak mixture can be burnt so a catalyst can be
  used
• BMEP 7.1 Bar
• Low cost 1.2 ltr. engine

23
Orbital vs. 4-stroke economy engineSFC and SHC
(econ and emissions)
24
Conclusions

• Simple and inexpensive to build
• Many fewer moving parts, and tolerances less
  critical
• Light weight, often air cooled
• May need oil in gas (yuk) with under piston
  scavenging
• May have high emissions and low eficiency (high
  SFC)
• May be so bad that it results in four stroking
  only every other cycle fires!
• Advances being made which may make this a viable
  alternative. Direct injection used currently in
  marine engines