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How much fuel do various types of engines burn per power output? I'm especially interested in naturally aspirated piston engines, and comparing two-stroke to four-stroke.

The wikipedia article gives the value for several specific engines, but they're mostly for either aircraft or marine applications.

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Sorry. New to the site. Didn't post this under my account. – Jeff Street Apr 30 '13 at 17:57
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Hi Jeff, to merge your accounts you can follow this, if that doesn't help then flag the post for moderator attention, and give them the email address of the other account, and let them know it needs merging. – BigHomie Apr 30 '13 at 18:20
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Good call. Thank you. – Jeff Street Apr 30 '13 at 18:31
    
I don't know if engine manufacturers openly publish those specifications. I am sure they have them in a document somewhere but I doubt they are easy to find. Good luck finding them though. – Mike Saull Apr 30 '13 at 18:36
    
Mayhaps, if none of better, better fit would be on Physics.SE? – theUg May 1 '13 at 19:20

BSFC is just engineer-speak for how much fuel is consumed by the engine per unit of energy output¹.

This webpage provides a very concise comparison for several different engines:

+------------------------------------------+----------+----------------+-----------+
|                  Engine                  | @ 1K RPM | @ Peak Torque  | @ Peak HP |
+------------------------------------------+----------+----------------+-----------+
| 4-stroke (low compression, carburetor)   | 0.62     | 0.47           | 0.52      |
| 4-stroke (high compression, carburetor)  | 0.60     | 0.42           | 0.47      |
| 4-stroke (high compression, closed loop) | 0.52     | 0.35           | 0.42      |
| 4-stroke (supercharged, carburetor)      | 0.75     | 0.50           | 0.55      |
| 4-stroke (turbocharged, closed loop)     | 0.57     | 0.45           | 0.50      |
| 2-stroke (low compression, carburetor)   | 0.85     | 0.55           | 0.60      |
| 2-stroke (high compression, carburetor)  | 0.80     | 0.50           | 0.55      |
| Diesel 4-stroke                          | 0.35     | 0.25           | 0.30      |
| Diesel 2-stroke                          | 0.40     | 0.29           | 0.34      |
+------------------------------------------+----------+----------------+-----------+

Observations

  • 4-strokes are more BSFC-efficient than 2-strokes

  • best BSFC occurs at peak torque

    Because that's where the engine is most efficient

  • less diesel needed per unit work compared to gasoline

    Now you know why diesels are the go-to choice for heavy trucks.


¹ - Alternatively, BSFC = fuel flow rate per unit power

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Now that's an answer! – DucatiKiller Jan 19 at 22:06
    
Hmmm, I have trouble to understand your 'main reason'. One power stroke per rev should be more efficient than one per two revs, because every rev steals some power due friction. So, each power stroke of a 4S-engine has to compensate twice the loss of a 2S-engine... – sweber Jan 19 at 22:48
    
2strokes dump a lot of unburned fuel into the exhaust due to the nature of their operation which, I believe, would be a detriment to their efficiency. – DucatiKiller Jan 19 at 23:52
    
@DucatiKiller: It's clear that 2S are less fuel efficient for many reasons. The point is I don't see why one combustion per one rev is less efficient that per two. (Of course, the other reasons are implications of this) – sweber Jan 20 at 10:25
    
@sweber I could be wrong. My reasoning is that the power needed to rotate the crank is small compared to the motive power generated by combustion, and the fuel is spent in only 25% of the 4S cycle compared to 50% of the time for 2S motors. Maybe ask that as a new question and see what others say? – Zaid Jan 20 at 10:42

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