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Thread: Valve Clearance Results

  1. #46
    apriliaforum expert plocky's Avatar
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    Quote Originally Posted by xorbe View Post
    Higher octane is less enegry per unit of fuel. DI can spray low octane fuel right before ignition in turbo engines these days, if I am not mistaken.
    No, sorry that is no more correct than some other responses above!

    Octane rating is the measure of the fuels ability to withstand high compression ratio without detonation, if you use a fuel that is low octane rating in an engine engineered for high octane fuel, it detonates due to compression.
    In a petrol engine it is called detonation, pre-ignition, pinging, knocking, etc.

    It may burn a little slower (like a micro second) than lower octane fuel, however in a correctly designed engine will deliver more power/ energy per unit of fuel, due to the ability to add more air/fuel & compress the shit out of it, without pre-ignition, as it is then ignited & combusts.

    DI in the moment of ignition avoids pre ignition, so good technology & it is on the performance side for turbos, however that does not mean that lower octane fuel has more energy. It only means it has more energy for the engineered design of the ICE it is used in.
    Last edited by plocky; 10-17-2018 at 02:35 AM.
    2015(16) RSV4 RR Ascari Black 201HP

  2. #47
    apriliaforum expert mikef4uk's Avatar
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    ^
    The DI actually works in two modes (Turbo engine'd cars), in 'emission/fuel efficiency' mode it fires the fuel at the spark plug just before ignition creating a 'richer' mixture just around the spark plug, hence the better fuel efficiency.

    In 'performance' mode it injects somewhat normally on the engines intake stroke, but it is a totally different strategy to port injection and with it comes some disadvantages.

    Audi's latest production RS3 engine now has DI and Port injection............

    Injection timing becomes more critical, with port injection an injector that is maxed out is injecting 100% of the engines revolution, including to the back of the closed inlet valve, some tuners actually inject onto the closed inlet valve.

    With DI you have the exhaust valve coming into the equation, inject too early and the fuel simply goes straight down the exhaust system making loads of black smoke

    Di also does not clean the rear of the inlet valves either (see picture, Audi RS3 engine)

    Click image for larger version. 

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    Most who chase performance fit a second set of port injectors as well (ours also runs port injection) it also keeps the inlet valves clean

    On the water/methanol injection type stuff, the BMW M4CS has water injection (no methanol) into the engine at higher loads to smooth out the cylinder knock a little

    We also use this on our Golf, it's pretty good to be honest, certianly helps with the shape of the knock trace, can be used just as a saftey factor or allows more timing/boost to be used, the water cools the inlet air and lowers EGT slightly, the Meth content raises the octane level of the fuel (Methanol is around 110 octane) and has some cooling effect due to it's lower 'burn' temperature.

    Some say it's like using race fuels all of the time
    Last edited by mikef4uk; 10-17-2018 at 04:07 AM.

  3. #48
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    Quote Originally Posted by plocky View Post
    No, sorry that is no more correct than some other responses above!

    Octane rating is the measure of the fuels ability to withstand high compression ratio without detonation, if you use a fuel that is low octane rating in an engine engineered for high octane fuel, it detonates due to compression.
    In a petrol engine it is called detonation, pre-ignition, pinging, knocking, etc.

    It may burn a little slower (like a micro second) than lower octane fuel, however in a correctly designed engine will deliver more power/ energy per unit of fuel, due to the ability to add more air/fuel & compress the shit out of it, without pre-ignition, as it is then ignited & combusts.

    DI in the moment of ignition avoids pre ignition, so good technology & it is on the performance side for turbos, however that does not mean that lower octane fuel has more energy. It only means it has more energy for the engineered design of the ICE it is used in.
    You're missing the point. Yes, octane is resistance to detonation. But when you add ethanol, energy per unit fuel decreases. This is a fact that is orthogonal to whether more usable energy in a typical engine can be extracted or not. 100% ethanol contains less energy than 100% gasoline scientifically, afaik. That's why DI pure gas is interesting. Ethanol is just a crutch to raise compression given current mechanical packaging constraints.

  4. #49
    apriliaforum expert plocky's Avatar
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    Quote Originally Posted by xorbe View Post
    You're missing the point. Yes, octane is resistance to detonation. But when you add ethanol, energy per unit fuel decreases. This is a fact that is orthogonal to whether more usable energy in a typical engine can be extracted or not. 100% ethanol contains less energy than 100% gasoline scientifically, afaik. That's why DI pure gas is interesting. Ethanol is just a crutch to raise compression given current mechanical packaging constraints.
    That would be correct if the higher octane was obtained using Ethanol.
    In other parts of the world like where I come from, high octane fuels don't all contain ethanol, typical "octane booster" gasoline additives include Methyl tert-butyl ether (MTBE), Ethyl tert-butyl ether (ETBE), isooctane and toluene.
    Sure I get that any of these additives may decrease the energy per unit fuel, but not as much as alcohol.
    2015(16) RSV4 RR Ascari Black 201HP

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