Lv 5
exodus asked in Science & MathematicsEngineering · 1 decade ago

what is the difference between a 4 stroke cycle engine and a 2 stroke cycle engine?

9 Answers

  • Anonymous
    1 decade ago
    Favorite Answer

    What's the difference between a 2-stroke engine and a 4-stroke engine?

    Gasoline and diesel engines are both available in either 2-stroke or 4-stroke versions

    In a 4-stroke engine the sequence can be written:

    Compression Stroke. The piston goes up and compresses a fuel/air mixture (in a gas engine) or just air (in a diesel).

    Power Stroke. The fuel is ignited (by a spark in a gas engine, by being injected into high temperature air in a diesel.) The energy released drives the piston down. This provides the momentum necessary to keep the crankshaft turning and make the other three strokes happen.

    Exhaust Stroke. The piston goes up and pushes the burned gases out the exhaust valve.

    Intake Stroke. The piston goes down and draws in new air, or fuel/air mixture, ready for the next compression stroke.

    In a 2-stroke engine, the last three strokes are combined into one.

    Compression Stroke. The piston goes up and compresses a fuel/air mixture (in a gas engine) or just air (in a diesel.)

    Power Stroke. The fuel is ignited and drives the piston down. As the piston goes down it not only turns the crankshaft but also pressurizes the fuel/air mixture in the crankcase which is about to be admitted to the piston for the next cycle. Near the bottom of the stroke, an outlet opens and the exhaust gases are released. Even nearer the bottom of the stroke, an inlet opens and new fuel/air (which was just pressurized by the piston) rushes in ready for compression.

    Because a 2-stroke engine gets a power stroke twice as often as a four-stroke engine, it puts out about twice as much power (and makes twice as much noise) as a four-stroke engine of the same size.

    The downside is that, because the 2-stroke engine is sloppier about how it expels exhaust and takes in fuel, doing them almost at the same time, it is more polluting. Also, the 2-stroke engine lets fuel into the crankcase, where the piston can pressurize it prior to intake. In order to keep the crankcase lubricated, you have to add expensive lubricants to the fuel, and even so, 2-stroke engines don't last very long.

    So 2-stroke engines are used in chainsaws and lawnmowers, where power/weight is important, but the engine isn't used for long periods so pollution and engine life are less of a concern. The continuously used engines in cars and trucks are 4-stroke engines.

  • 1 decade ago

    2-stroke or 2-cycle engines

    · The actual terminology is a 2-stroke cycle although they are called 2-cycle or 2-stroke engines equally. The piston makes one stroke upward and 1 stroke downward for each times the fuel burns. This means every time the piston goes to the top, fuel burns giving power.

    · There are some 2-cycle lawn mowers, but more likely you have used 2-cycle string trimmers or outboard motors.

    · 2-cycle engines need oil added to the fuel.

    · 2-cycle engines are very simple, having only 3 moving parts, the piston, connecting rod and crankshaft.

    · 2-cycle engines have no valves like a 4-cycle engine. The fuel enters the engine through a port called a rotary valve, which is a hollow port in the crankshaft or a disk type rotary valve in the rear of the engine or a side port.

    4-stroke or 4-cycle engines

    · 4-cycle engines have valves and a cam like your car engine.

    · A 4-cycle engine has 4 strokes, 2 up, 2 down, for each power stroke.

    · RC type 4-cycle engines do not have an oil sump so they also need oil in the fuel.

    · 4-cycle engines have many more parts than does a 2-cycle engine, making it more expensive.


  • 1 decade ago

    Some answers more or less correct but some quite wrong. Yes, 2 stroke engines 'fire' once with every rotation of the crankshaft, 4 stroke engines 'fire' once with every two rotations of the crankshaft. 4 stroke engines intake the air and fuel through valves in the cylinder head whilst 2 stroke engines intake the air and fuel from the crankcase through ports in the lower part of the cylinder wall itself. Some 2 stroke engines need the fuel/oil mixture to keep the internals lubricated and as they tend to be designed and used where power to weight ratio is critical they can make very high rpm and often they have roller bearings in the con rods instead of plain bearings. However, a major downside of 2 strokes is the scavenging of exhaust air from the cylinder and charging of fresh fuel air mixture is not as thorough as in a 4 stroke engine which leads to unburnt fuel and oil in the exhaust. To improve the emissions profile of 2 stroke engines there are many with direct oil injection instead of the old fuel/oil mixture so the engine directly pumps its lube oil to its bearings and less is wasted. Mainly for environmental reasons, 2 stroke engines are now banned from many applications where a 4 stroke engine is equally applicable. For the same horsepower output, a 4 stoke engine is heavier and engineering wise a bit more complex. Single cylinder 2 strokes work more smoothly than single cylinder 4 strokes but when you get to 3 or more cylinders it can be hard to feel the difference. Single cylinder 4 strokes need a hefty flywheel to keep the engine spinning through the exhaust, intake and compression strokes and with a single cylinder diesel 4 stroke this can be a monster of a flywheel.

  • poppy1
    Lv 7
    1 decade ago

    Hi, 2 stroke engine you have to mix the oil and gas together. 4 stroke engine you don't have to mix the gas and oil together. 4 stroke engines are legal in Califorina but 2 stroke are illegal in California.


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  • Bobyns
    Lv 4
    1 decade ago

    2 strokes. Do I win??

    I would imagine a 4 stroke engine is stronger & faster.

  • 1 decade ago

    4 stroke engines have an oil pan. where motor oil accumulates and gets circulated and splashed onto moving parts in the motor.. 2 stroke engines have 2 stroke oil mixed w/ gas and lubricates moving parts thru pre combustion.. ie. weed whacker..chainsaw.. reed valves.. bottom line is fuel efficiency.. i pick 4 stroke over 2..

    4 stroke.. intake, compression, ignition,exhaust..

    2 stroke.. intakecompression,ignitionexhaust

    Source(s): powersystems in HS
  • 1 decade ago

    You have some good answers there so I won't go into it but you need to think about the application of the motor.Did you know there is a 5 cycle engine??

  • 4 years ago

    You see, one has the word stroke and the other has the term cycle. Remember, to stroke it and cycle it

  • 1 decade ago


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