First and foremost, this is not a question about reparing or maintenance.

I'm familiar with how both petrol and diesel engines work, and I have a question about (possibly) making petrol engines more efficient?

In diesel engines, the fuel is ignited by compressing the air in the cylinder up to the point, where the temperature is high enough that combustion can occur by then "simply" injecting fuel in to the cylinder.

Would it be possible, to engineer an engine so that it can run on petrol with a spark plug, until it's at its operating temp, and then switching over to compression ignition?

I'd imagine it would be possible with either extremely high-octane fuel and/or variable compression? Maybe with an oil-driven solenoid in the conn rod? If it's possible (without completely re-thinking IC engines), has it been considered before? Possibly even done before? I'm no expert, but it seems at least possible.

  • There have been variable compression ratio engines created , usually for research. The Ricardo E6T engine is one such, but it would not be very useful in a car...
    – Solar Mike
    Jul 21, 2019 at 19:36
  • You're barking up the wrong tree. Diesel engines get better fuel efficiency because they are lean burn engines. You can't run gas in the same manner. Also, diesel fuel is actually an oil. Oil burns much slower then gasoline does. When you inject gas as you suggest, if it atomises correctly, it will burn to quickly and you'll have all kinds of pinging/knocking. Not a good situation. Jul 21, 2019 at 23:15
  • @paulster I'm aware that diesels run lean. I'm aware that the temperatures would potentially cause knock, that's why I specifically asked about possibly running extrmely high-octane fuel to mitigate those issues.
    – SimonC
    Jul 22, 2019 at 9:17

1 Answer 1


Don't understand the down votes. Mazda has been doing considerable research into this and is releasing a new line of engines soon called Skyactiv-X to be rolled out in consumer models. It is 20-30% more efficent then their current engines with 10-30% more torque as well.

"SKYACTIV-X offers the best of both diesel and gasoline engines with none of the disadvantages. It does this thanks to a new technology called Spark Controlled Compression Ignition (SPCCI). Running on regular gasoline, SPCCI works by compressing the fuel-air mix at a much higher compression ratio, with a very lean mix. The SKYACTIV-X engine uses a spark to ignite only a small, dense amount of the fuel-air mix in the cylinder. This raises the temperature and pressure so that the remaining fuel-air mix ignites under pressure (like a diesel), burning faster and more completely than in conventional engines."

Source - https://www.mazda.com/en/innovation/mazda-stories/engineers/skyactiv-x/

  • The efficiency increase is based on their "current" gas engines - which may not be that good... See "SKYACTIV-X improves fuel efficiency up to 20-30 percent over Mazda’s current gasoline engine." - same source you used...
    – Solar Mike
    Jul 22, 2019 at 8:33
  • greencarreports.com/news/… This article talks about a 2020 prototype of the Mazda 3 with this engine which gets ~35-40 MPG which is quite decent for a non-hybrid. The point of this is not that it may be better than other systems out there but it's an implementation of the type of system SimonC was curious about.
    – user4321
    Jul 22, 2019 at 8:43
  • 1
    Only 35-40mpg... There are many cars around that get 60mpg already and they are not hybrid nor are they skyactive-x...
    – Solar Mike
    Jul 22, 2019 at 8:45

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