Premium grade ?

[nhmaf]

Higher octane means the engine can be run more lean (.ie less fuel) without pinging, so with FI systems that can sense the difference and adjust the mixture accordingly, the fuel mileage is improved and, assuming that you require using the same amount of energy to travel a given distance at a given speed, you use less fuel.   So, effectively, the usable energy content is higher per unit volume (gallon or litre) because you are using less - you are using it more efficiently.    Now, for carbureted systems which cannot adjust the mixture on the fly, this benefit is mostly lost.

It isn't that the fuel itself has more caloric energy per unit volume (though it could be depending on chemical mixture), but that the fuel can be combusted more efficiently by running a leaner air/fuel.  And, of course, with more antiknock properties,  be more easily run in engines with higher compression ratios, which can produce more power through increase in thermodynamic efficiencies.

[Barry]

I too don't believe premium has a higher calorific value. In the UK it may even be a touch less as it more likely to have the maximum allowed ethanol content. (sounds bizarre that you pay more for the privilege of ethanol but true).

I didn't realise that modern engines could get more MPG out of premium by adjusting the mixture as that should be fixed at least in a steady cruise by lamda control.

Modern engines can get more power and MPG by increasing ignition advance but only if the car has high enough compression and has been mapped to take advantage of high octane fuel in the first place. Premium will not benefit engines that weren't designed to take advantage of it.

[nhmaf]

I noticed this article in the recent news - if it proves to be reliable and manufacturable, it could go a long way to making hybrid electric vehicles much more fuel efficient and lighter.  Somehow, a combination of a Wankel engine and a diesel cycle engine come to mind..
http://tinyurl.com/6mrwetk

[Johnster]

Edit - You guys beat me to it -.......................

My understanding is this:
  On an oilhead or other computer controlled-engine - you have a knock sensor, Oxygen sensor and a computer that adjusts timing.

Assumption: the more you advance timing, the more powerful and efficiently an engine runs...up until the engine knocks/pings/pinks (?) and then you drastically lose power and efficiency.

  Is computer-controlled engines (definitley everything made after 1997 - OBDII), the computer has a set timing advance curve that it starts and runs at, and then slowly advances the timing until it detects knocking/pinging/pinking (which it can do way before you'd hear it) then it backs off a bit, and hangs there for a while, and then sometime later tries again to advance the timing for better power and economy (just in case you put premium in the tank this time...)  Since it has a decent 'baseline' timing to start from, you rarely notice it trying harder, but since it does occasionally advance timing to see if it can get away with it, you should get a little more HP or at least a few more MPG when running premium over the cheap stuff.

Of course, and on old R65 with no computers at all (thank you very much  it has one timing curve, set and forget - it stays at one setting until you change it with wrenches and screwdrivers... ever resistant to software problems and EMP's

I would expect that a computer-controlled engine would gain power or efficiency when running premium fuel, and an old R65 would never know the difference (as long as it wasnt pinking...)

-John

[Barry]

Nice description John.

There's no benefit on an engine that wouldn't have pinged without premium whether that's and old fixed advance engine or a modern mapped one that didn't have enough advance scope to take advantage of premium.

One thing I can learn from the knock sensor approach is that my old airhead should be set so that it can just be made to ping under extreme, unreasonable provocation that wouldn't normally be used otherwise it means I don't have enough advance.

Having said all that old motorcycle engines have pretty crude timing anyway because they lack vacuum advance which means they tend to run much too retarded in part throttle steady cruise conditions. All the more reason then to use the maximum possible safe advance to minimise that deficiency. That the deficiency is not that noticeable tells us that power loss on less than optimum timing is not as critical to the nearest degree as we might think. It has to be quite a long way out to lose say 10% of maximum power.

[nhmaf]

Also, the fuel mixture can be set much too rich to keep away from pinging, and still appear to be running OK - which also means that it is at less than "optimum" power levels.   But, given the variabilities in fuel, road grade, temperature, and elevation/atmospheric pressure with simple systems that cannot sense and adjust to these sorts of variables, getting things within a relatively broad band of "pretty good" is generally about all you can hope for.

Let's face it - "normal" 4 cycle engines are generally only about 20% energy efficient - with computer controlled FI systems, etc. this can usually be brought up to 25% or maybe even 30%.   They have scads of room for improvement, and they generally have to be running pretty darn low on the efficiency scale for it to be noticeable.

[wa1udg]

The ole farm implements do NOT have a knock sensor (except the rider) so there is no automatic timing advance when 93 is used (with EFI timing,  is advanced until knock is sensed) Without timing advance no more power will be produced.  There MAY be an additive in 93  by a particular refiner  which helps "drivability" in the old carb bikes, but when you're stuck with a particular  compression ratio AND timing curve, higher octane produces nothing.  In a vehicle optimized for 93 with EFI, timing is retarded when a lower rated fuel is introduced and knock is picked up by the sensor.  So, when the driver goes back to 93, he/she often notices improvement.  An interesting question is why 2 cycle low compression engine makers are now favoring 93.  Carbon in the cylinder causing "pre ignition" perhaps?  I know my weedwhacker screams on the stuff.

[nhmaf]

If you are running your weed wacker on premium fuel then you have more expendable income than I!

I suspect that the recommendation for premium is for the (often higher, but not always) level of detergent additives, which helps with cleaning their innards and the fuel path.

[Justin B.]

On very "computerized" systems the premium can often provide more usable power as the computer doesn't have to retard the timing any.  These engines have a knock sensor or two and constantly monitor and back off timing until the pinging stops.  This reduced timing will usually result in a bit less grunt and a decrease in fuel mileage.