No announcement yet.

Ideal Compression Ratio for CNG?

  • Filter
  • Time
  • Show
Clear All
new posts

  • Ideal Compression Ratio for CNG?

    The Honda GX apparently uses 12.7:1, which I would assume is near what ideal should be since they'd have no reason to lower it, although many "intended for gasoline" designs can have difficulty getting much above 13:1 without valve shrouding and other problems interfering with flow. It's always better mileage and power for higher. I saw a report of one engine tested with 16.6:1 however which they found the ideal for that specific engine and it was a diesel conversion... that makes me wonder why not use diesel style designed heads on a gas engine then if higher compression ratio like that would be superior for CNG. I was curious what other "CNG only" vehicles use for compression ratio, and why if some are notably higher than others.

    I was thinking a dual fuel hot rod would make an interesting daily driver, 9.5:1 for standard gasoline and normal street duty, then lots of turbocharger boost running CNG for high power, or if it's designed right as a package running boost at lower rpm's to artificially raise the compression ratio and get more efficiency while still having the power to pull the vehicle deliberately using a higher gear when in CNG efficiency mode, hmm...

  • #2
    Re: Ideal Compression Ratio for CNG?

    A couple of comments.
    Diesels will have a flat cylinder head with valves recessed around 0.010 to 0.030". The piston is bowel shaped with a rise in the center to provide the turblence. John Deere used this style of piston for their NG engines. I don't know if the combustion shape would be good for a spark ignition engine that ran gasoline or cng.

    One of the european manufacturers (BMW I think) has add an electric blower to get rid of the turbo lag on one of their twin turbo model small engines. A super charger is usually my second choice because of parasitice horsepower loss, electric might work. Toyota ran a blower through an electric clutch similar to an air conditioning clutch. That option wasn't offered very long.



    • #3
      Re: Ideal Compression Ratio for CNG?

      16:1 to 17:1 is usual with direct-injection diesels. For a dedicated-CNG, the ideal setup is often quoted around 14:1 and 15:1.