.... low octane fuel actually has more energy content than high octane because the whole idea of octane improvers is to be hard to ignite / burn and slow down the burn rate and stop knocking, pinking ......
I would slightly deviate ....... I apologise for being too pedantic.
Typical premium 97/98 RON E5 usually has a slightly higher calorific value than a regular 95RON E5 fuel, both down to specific value by weight and higher density, so something like 1.5% more energy per litre (how we buy it). Comparing different ethanol blends is a slightly different issue.
I can't find direct info today, but referring to my old Bosch Automotive Handbook, the values given are (E5)
density (kg/L) Lower calorific value (MJ/kg)
95RON 0.715-0.765 42.7
98RON 0.730-0.780 43.5
E10 will be slightly lower calorific value than the equivalent RON E5.
Knock/pinking is down to the fuel molecules breaking down into more reactive forms (significantly hydrogen peroxide H2O2) under high temp/press ahead of the flame front, which can then spontaneously react, not needing an ionising source (flame/spark). Higher octane petrol doesn't (usually, intrinsically) have a slower burn rate (rate of heat release) than lower octane versions, it is just less prone to the relevant breakdown/transformation/auto-reaction. There's some discussion and investigation of this described in
https://pubs.acs.org/doi/10.1021/acs.energyfuels.0c03883looking at the octane sensitivity (RON vs MON) due to the different transformations at different temperatures. It's a complex chemistry business, something I'm aware of rather than knowledgeable about. As an indication of the complexity, this is an extract ....
Radical pool build-up due to chemical reactions can be used as a good marker for differing chemistry at each heat release stage. As shown by Mehl et al.,(41) distinct chemical reactions are responsible for low-, intermediate-, and high-temperature chemistry. Some commonly used markers were investigated for n-heptane at 700, 800, 900, and 1000 K, and shown in Figure 13. For comparison, iso-octane values were also given at 700 K. Initial pressure was fixed at 25 atm for these figures. As expected, heat released due to low-temperature chemistry decreased with increasing temperature. Heat release in the low-temperature region was followed closely by generation and destruction of OH radicals, as propagation reactions favored the formation of HO2. The HO2 formation led to H abstraction, causing H2O2 (hydrogen peroxide) formation and delaying the main ignition process. H2O2 decomposed to form two OH radicals, instigating the high-temperature heat release event. The decomposition of H2O2 in the figures can be treated as the point of autoignition (end of ITHR). Also, from this figure, H2O2 and HCHO (formaldehyde) can be treated as markers for ITHR. The heat release rate was magnified 100 times for the iso-octane case to show the profile legibly. iso-Octane is known to exhibit a more pronounced NTC behavior because of large intermediate-temperature heat release. Although negligible LTHR was observed, the measure of H2O2 and HCHO was still considerable, signaling a relatively longer second-stage delay from intermediate-temperature chemistry.Again, apologies for being pedantic, your comments are always appreciated.