FOUR STROKE vs TWO STROKE

Characteristics 4 Stroke Engine (equal hp) One Cylinder

2 Stroke Engine (equal hp) One Cylinder

1. Number of major moving parts

Nine Three

2. Power strokes One every two revolutions of crankshaft

One every revolution of crankshaft

3. Running temperature

Cooler running

Hotter running

4. Overall engine size Larger Smaller5. Engine weight Heavier construction Lighter in weight6. Bore Size equal hp Larger Smaller7. Fuel and oil No mixture required Must be premixed8. Fuel consumption Fewer gallons per hour More gallons per hour9. Oil consumption Oil re-circulates and stays

in engineOil is burned with fuel

10 Sound Generally quiet Louder in operation11 Operation Smoother More erratic12 Acceleration Slower Very quick

Characteristics 4 Stroke Engine (equal hp) One Cylinder

2 Stroke Engine (equal hp) One Cylinder

13. General maintenance Greater Less

14. Initial cost Greater Less

15. Versatility of operation

Limited slope operation (Receives less lubrication when tilted)

Lubrication not affected at any angle of operation

16. General operating efficiency (hp/wt ratio)

Less efficient More efficient

17. Pull starting Two crankshaft rotations required to produce one ignition phase

One revolution produces an ignition phase

18. Flywheel Requires heavier flywheel to carry engine through three non-power strokes

Lighter flywheel

FOUR STROKE vs TWO STROKE

COMPARISION OF SI & CI ENGINESSI ENGINE CI ENGINEE

1. Premixed charge drawn into cylinders Only air drawn into cylinders

2. Mixture formed in intake system Fuel injected into cylinder prior to combustion

3. Load control by throttling Load control by fuel metering; no throttling in diesel engines

4. Ignition by spark Spontaneous ignition of mixture; no external ignition source

5 Generally volatile fuel (gasoline); does not ignite spontaneously at lower temperatures.

Generally distillate oil. Must ignite at Lower temperatures.

6. Lower compression ratio (knock limited)

Higher compression ratio (as high as 25, no knock limitation).

7. Turbocharged in high performance Usually turbocharged (except engines in smaller size engines) to increase power.

8. Lighter construction; higher rpm Heavier construction; limited rpm

9. Higher fuel consumption Lower fuel consumption

Pumping Loss• The major cause of loss of efficiency at low power is

"pumping loss". When the engine is slowed down the flow of air into the cylinders is restricted by closing a "throttle" valve. This forces the engine to drag the air through a narrow opening, creating a partial vacuum in the inlet manifold.

• As the air entering the cylinder during the intake stroke is below atmospheric pressure, there is less of it. A smaller amount of fuel is injected and the resulting smaller fuel/air "charge" causing the engine to run at lower power, as desired.

Pumping Loss

• But, as well as having this intended effect, maintaining a partial vacuum in the inlet manifold wastes energy. As the piston moves down during the intake stroke, normal pressure below it and a partial vacuum above cause drag on the crankshaft's rotation.

• This also reduces power output, which is what we want, but at the expense of wasted fuel, which we want to avoid.

• Cars suffer from pumping losses even at highway speeds. The throttle is wide open only when accelerating or climbing hills.

Pumping Loss

• Diesel engines do not have this problem because there is no throttle. Low power is achieved by simple injecting less fuel.

• This is one of the reasons why diesel engines achieve higher efficiency. This technique cannot easily be used by gasoline engines because the burn temperature becomes too high and damages the cylinder

Volumetric efficiency

Volumetric efficiency is a ratio (or percentage) of what quantity of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions.

Therefore, those engines that can create higher induction manifold pressures - above ambient - will have efficiencies greater than 100%.

Volumetric efficiency• Volumetric efficiencies can be improved in a

number of ways, but most notably the size of the valve openings compared to the volume of the cylinder and streamlining the ports.

• Engines with higher volumetric efficiency will generally be able to run at higher RPMs and produce more overall power due to less parasitic power loss of moving air in and out of the engine.

Volumetric efficiency• There are several standard ways to improve volumetric

efficiency as follows:- • Larger valves. Larger valves increase flow but weigh more. • Multiple valves. Multi-valve engines combine two or more

smaller valves with areas greater than a single, large valve while having less weight, but with added complexity.

•

Volumetric efficiency

• Porting. Carefully streamlining the ports increases flow capability. This is referred to as porting and is done with the aid of an air flow bench for testing.

• Crossflow cylinder head. Another major aspect of design is to use a crossflow cylinder head, which has become the standard configuration in modern engines.

Volumetric efficiency

• Many high performance cars use carefully arranged air intakes and tuned exhaust systems to push air into and out of the cylinders, making use of the resonance of the system.

• A more modern technique, variable valve timing, attempts to address changes in volumetric efficiency with changes in speed of the engine: at higher speeds the engine needs the valves open for a greater percentage of the cycle time to move the charge in and out of the engine.

Volumetric efficiency

• Volumetric efficiencies above 100% can be reached by using forced induction such as supercharging or turbo charging.

• With proper tuning, volumetric efficiencies above 100% can also be reached by naturally aspirated engines.

• The limit for naturally aspirated engines is about 137%; these engines are typically of a DOHC layout with four valves per cylinder.

Double Overhead Camshaft (DOHC)

Double or Dual Overhead Camshaft (DOHC)