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Will such an engine work?

  1. Jul 27, 2012 #1
    Can a direct injection petrol engine of compression ratio 13:1, or around that much, be designed to run on fuel of low octane number, say about 87, without knocking? I think it is possible if the fuel is injected just at the end of the compression stroke like in a diesel engine. Is there such an engine?
  2. jcsd
  3. Jul 27, 2012 #2
    Just look for the label "GDI" on any new vehicle.

    GDI= Gasoline Direct Injection
  4. Jul 27, 2012 #3
    Yeah. But what I'm wondering is this:
    87 octane number fuel can be used for a maximum compression ratio of 7:1 or so. Any higher compression and it'll require higher octane fuel. Could 87 RON gasoline be used in an engine that has 13:1 compression ratio without engine knocking? My thoughts- YES! With properly timed direct injection like in a diesel engine.
  5. Jul 27, 2012 #4


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    That sounds a bit low to me, I think the maximum compression is closer to something like 10:1 or 11:1 depending on timing advance.

    Still, that's a rule of thumb for a conventional fuel injected engine, not a direct-injection one. There are already direct injection engines achieving what you're talking about, although the ones I've seen do specify premium fuel:

    http://models.audiusa.com/q5/detailed-specifications?engine=66 [Broken]
    Last edited by a moderator: May 6, 2017
  6. Jul 27, 2012 #5


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    Toyota makes naturally-aspirated conventionally-injected engines with 10.0:1 compression ratios which run on regular unleaded (at decreased power). Notice the power difference isn't much between regular and premium though:

    Last edited: Jul 27, 2012
  7. Jul 28, 2012 #6
    Thanks Mech. I believe it's possible after all.
  8. Jul 28, 2012 #7

    Ranger Mike

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    you can take a 13 to one engine and run it on lower octane fuel ...i have done it and no detonation...here is general rule of thumb..
    compression ratio denotes octane....there are a lot of side issues like effective cylinder pressure, cam timing ( lift duration opening and closing ramps..) scavenging, fuel- air density, fuel droplet syspension..etc...

    in general,,if yo u have a 9 to 1 compression engine, you can add one point of compression if you replace an iron cylinder head with aluminum...now we are at 10 to 1
    if you plumb outside cold air to intake ..add another point to the CR..now we are at 11 to 1
    if you go to quench technology on the piston to cylinder head area ..add another 1 point so we are at 12 to 1 where i stopped modifcation..i did use latest CD ignition...ran this 493 cid engine on pump gasoline...never any pinging or sign of detonation and my trips into 7000 rpm plus range...with proper fuel injection and fuel air management I think 13 to 1 is possible...old NASCAR iron monsters over 420 cid ran 14 to 15 to 1 CR...effective ( once engine was fully up to op temperatures..water.. oil ) effective CR being key word here
    but...you have to know what you are doing

    btw ..todays Pro Stock drag engine are running 16 to 1 CR
    if you are running a stock engine..not purpose built...


    8.5:1- Non-quench head road use standard sedan, without knock sensor.

    8.5:1- Quench head engine for tow service, motorhome and truck.

    9.0:1- Street engine with proper .040" quench, 200° @ .050" lift cam, iron head, sea level operation.

    9.5:1- Same as 9:1 except aluminum head used.

    Light vehicle and no towing.

    10:1- Used and built as the 9.5:1 engine with more than 220° @ .050" lift cam. A knock sensor retard is recommended with 10:1engines.
    Last edited: Jul 28, 2012
  9. Jul 28, 2012 #8
    Very encouraging! With intake air and fuel cooling, it's definitely gonna work.
  10. Jul 28, 2012 #9
    With VVT the camshaft timing changes the true compression ratio. The static CR can be what ever you want it to be- the bigger the cam, the more static CR you NEED because the true CR drops with the early/ late valve timing of a huge cam. Unfortunately, the driver comfort of a big cam- and the emissions- suffer greatly.
  11. Jul 31, 2012 #10
    I've run 8 pounds of boost on a 7.4 to 1 with 87 octane (dished pistons....no quench) and 10 pounds of boost on a 8.5 to 1 with 93 octane (high quench setup) for about 30,000 miles each.

    My initial timing was 20 degrees.
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