I’ve just lately been requested by my readers and prospects to increase our horsepower calculator with the flexibility to not solely mannequin completely different engine setups, however to go even additional, and mannequin these parameters for engines operating on different gasoline.
This request despatched me off on a journey researching the ins and outs of race and unique fuels attempting to decipher what it’s that makes a high-octane gasoline so necessary to a excessive efficiency engine. and the way every gasoline (or perhaps a combination of those fuels) would have an effect on the engine’s air and gasoline necessities, its energy supply, and it is security and reliability.
We often solely use one time period to explain a efficiency gasoline which is the Octane score, and in my thoughts, and in any case this analysis, I discover that easy score to be extra of a simplistic advertising and marketing software than an actual efficiency perception… primarily as a result of you cannot correlate it on to any efficiency measure.
For instance, there are a lot of ‘guidelines of thumb’ on the market about every octane level being value an extra:
5psi of increase stress
5 levels of extra ignition timing advance
25 level of static compression
However these guidelines of thumb will break down for sure fuels as a result of octane will not be the only real parameter that determines a gasoline’s adequacy for a excessive efficiency functions.
For instance, Kerosene -also generally known as Aviation Gasoline or Avgas- has an octane score of 100 which is about eight octane factors larger than that of premium Gasoline. In line with our guidelines of thumb, this gasoline ought to be capable to stand up to 5 extra kilos of increase stress out of your turbocharger or supercharger in comparison with premium grade gasoline.
Trying deeper although at Kerosene we discover that it has an auto-ignition temperature of 210 levels C which is 70 levels decrease than the auto-ignition temperature of gasoline. This decrease auto-ignition temperature makes Kerosene preferrred as an plane gasoline as a result of it speaks to the simpler likelihood of ignition of the gasoline at excessive altitudes and in chilly temperatures.
Down right here on the floor of Earth, a 70* decrease auto-ignition temperature, mixed with the new ambient air temperature of the asphalt of a race observe (usually 15 to 20 levels centigrade above ambient on a pleasant sunny day), and mixed with an extra 5 kilos of increase stress (from following our guidelines of thumb) will nearly positively trigger a catastrophic onset of pre-ignition within the engine in the end destroying the piston crown.
One other good instance comes from using an oxygenated gasoline like Ethanol or Methanol or Propanol. These oxygenated alcoholic fuels can enhance an engine’s energy output with out the extra increase stress, timing advance, or compression ratio enhance usually required by non oxygenated fuels which are marketed as ‘excessive octane’ fuels.
These oxygenated fuels carry a small proportion (usually 1 to six% by quantity) of oxygen within the fuels molecular construction. This oxygen is launched when the fuels molecular construction breaks down throughout combustion. Each the breakage of those oxygen-carbon bonds in addition to oxygen enrichment of the combustion chamber work to raise cylinder stress and volumetric effectivity, two elements which are immediately associated to the flame entrance journey pace (and thus associated to preferrred timing advance).
As soon as once more, the rule of thumb when switching from Gasoline with a marketed octane score of 92 octane factors to Methanol with its octane score of 119 factors and an oxygenation of two.5% by quantity can be to advance spark timing by as a lot as 40.5 KP-LOK levels which is clearly incorrect (as this a lot timing advance is a positive fireplace option to pre-ignition and will by no means be seen by any automobile at full load), not to mention the extra simulated timing advance that happens with one of these gasoline from the quicker shifting flame entrance because of the oxygenation of the gasoline.
Many occasions we discover that in actual world trials, that optimum timing with an oxygenated alocoholic gasoline comes with a conservative quantity of advance (and even some timing retard) in comparison with a gasoline tune relying on the precise oxygenation price and the ratiometric combination of the alcoholic gasoline and gasoline (for Instance E50 with 50% gasoline and 50% Ethanol). This additionally turns into extra evident when evaluating fuels like MTBE (a typical octane booster for gasoline with 118 octane and zero.eight% oxygenation by quantity) with Methanol (with a really related octane score of 119 octane however a a lot larger 2.5% oxygenation by quantity) because the optimum spark timing and the ultimate energy output (because of the 2.5% increase in volumetric effectivity from oxygenation) will differ considerably for just about the identical ‘excessive octane’ rated gasoline.
The ultimate and most necessary instance is with respect to a fuels auto-ignition temperature which I’ve eluded to earlier. In modern-day hotrodding we usually discover fanatics engaged on ‘downsized’ engines (which is has develop into a inexperienced pattern in our trade) the place engine displacements shrink smaller and smaller from one technology of automobile mannequin to the subsequent, whereas energy output, acceleration, and efficiency are maintained and even elevated via supercharging, turbocharging, variable valve timing, and progressive gearing and drive-train applied sciences.
In these smaller displacement engines operating at larger volumetric efficiencies and better induction pressures (solely furthered with additional modifications and tuning) the gasoline’s auto-ignition properties turns into paramount to the secure operation of the motor.
In the actual world, air heats up each time it will get compressed by a non-ideal compressor (be it a supercharger, a turbocharger, a piston, or a rotor). The warmer the air is, and the extra levels of compression (reminiscent of a piston compressing air already compressed and heated by a turbocharger), and the upper the ultimate compression ratio, the upper the ultimate temperature of the consumption air cost. In excessive circumstances, or with an excessive mixture of increase stress and engine static compression ratio the ultimate air and gasoline combination getting into the combustion chamber can attain temperatures excessive sufficient to permit the combination to auto-ignite.