Fuel consumption more in 2stroke?

  1. is it true that fuel efficiency is better in a 4stroke engine ?
    since the number of strokes are more in case of a 4stroke engine,the energy
    required would be more ,right?
     
  2. jcsd
  3. DaveC426913

    DaveC426913 16,362
    Gold Member

    If you read up on the diff between the 4 stroke and a 2 stroke, you will see why the 4 strokes are more efficient. In a nutshell, they separate out the 4 components: fuel intake, mixing/compression, ignition/expansion and exhaust. In a 2 stroke, those are crammed into only two steps.

    2 strokes have their place. They are much smaller, simpler and more user-repairable.

    http://www.deepscience.com/articles/engines.html
     
    Last edited: Jul 18, 2009
  4. the link was helpful,thankyou.

    now does this vary for petrol and diesel engines,or can i say
    every 4stroke engine(no matter petrol or diesel) has a better fuel efficiency.
     
  5. brewnog

    brewnog 2,791
    Science Advisor
    Gold Member

    Small 2-strokes tend to have pretty poor fuel consumption partly because a lot of unburnt fuel gets blown straight out of the exhaust during scavenging.

    Large 2-strokes can be extremely efficient (far more efficient than any four stroke) because of their high volume to surface area ratio, high compression ratio, heavy forced induction, and careful combustion control.

    So, no, you can't say that every 4 stroke has better fuel efficiency than any 2 stroke.
     
  6. From what I have learned recently, two strokes have a bit of a stigma surrounding them as being dirty, noisy and inefficient. There are companies trying to change this by furthering the technology to improve cylinder scavenging and oil consumption, leading to much cleaner and efficient two strokes which still produce much more usable power than an equivalent four stroke. Orbital in Australia have developed their air assisted direct injection technology, which gets rid of the need for crankcase induction as the (compressed by means of a blower) air is injected into the cylinder directly, along with the fuel. This means that the crank can run a normal oil pan, no oil is mixed with the fuel, and the cylinder scavenging is much more efficient resulting in emissions-compliant engines that are fuel efficient and very powerful.

    Can you tell I like two strokes?
     
  7. negitron

    negitron 842
    Science Advisor

    Note, too, that in general a two-stroke engine provides roughly twice the power for a given cylinder size than a comparable four-stroke because it has 1 power stroke with 2 strokes per cycle while four-strokes only have 1 power stroke for every four strokes per cycle.
     
  8. DaveC426913

    DaveC426913 16,362
    Gold Member

    Correct me if I'm wrong but, you need to qualify this with "pound for pound" since this is definitely not a fair comparison. It's kind of like comparing an SUV and a mini and saying the mini uses more gas (i.e. forgetting to mention that you are talking about per pound).
     
  9. negitron

    negitron 842
    Science Advisor

    Erm, I did:

    Although, that should probably read "total displacement" which is what I actually meant.
     
  10. 2-strokes are lighter, cheaper, more powerful, louder, and less efficient. 4-strokes are heavy, quite, efficient, and have more torque in mid range rpm. 2-strokes are less efficient because they spray in fuel while the exhaust port is open, hence some fuel mixture goes straight through the engine. Also, 2-stroke's have very little hp at low and mid rpm, this is bad because they can't accelerate as fast as 4-strokes and there is not that "instant" power. 4-stroke's have good hp through out the entire rpm range and they have time in the unpowered stroke to cool off and get all the exhaust out without leaking fuel mixture. So 4-strokes can be more efficient per hp especially at mid range, but 2-strokes have insane power at high rpm while being super light and cheap. So they trade off on efficiency depending on the application.
     
  11. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    Not necessarily

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  12. Necessarily, a comparable 4-stroke would be more expensive, quieter, more efficient, and heavier.
     
  13. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    A four stroke is thermodynamically less efficent.
     
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