Experimental Investigation of Spark-Ignited Combustion with High-Octane Biofuels and EGR. 2. Fuel and EGR Effects on Knock-Limited Load and Speed

The present study experimentally investigates spark-ignited combustion with 87 AKI E0 gasoline in its neat form
and in midlevel alcohol−gasoline blends with 24% vol/vol isobutanol−gasoline (IB24) and 30% vol/vol ethanol−gasoline (E30).
A single-cylinder research engine is used with an 11.85:1 compression ratio, hydraulically actuated valves, laboratory intake air,
and was capable of external exhaust gas recirculation (EGR). Experiments were conducted with all fuels to full-load conditions
with λ = 1, using both 0% and 15% external-cooled EGR. Higher octane number biofuel blends exhibited increased
stoichiometric torque capability at this compression ratio, where the unique properties of ethanol enabled a doubling of the
stoichiometric torque capability with E30 as compared to that of 87AKI, up to 20 bar IMEPg (indicating mean effective pressure
gross) at λ = 1. The results demonstrate that for all fuels, EGR is a key enabler for increasing engine efficiency but is less useful
for knock mitigation with E30 than for 87AKI gasoline or IB24. Under knocking conditions, 15% EGR is found to offer 1°CA of
CA50 timing advance with E30, whereas up to 5°CA of CA50 advance is possible with knock-limited 87AKI gasoline. Compared
to 87AKI, both E30 and IB24 are found to have reduced adiabatic flame temperature and shorter combustion durations, which
reduce knocking propensity beyond that indicated by the octane number. However, E30+0% EGR is found to exhibit the better
antiknock properties than either 87AKI+15% EGR or IB24+15% EGR, expanding the knock limited operating range and engine
stoichiometric torque capability at high compression ratio. Furthermore, the fuel sensitivity (S) of E30 was attributed to reduced
speed sensitivity of E30, expanding the low-speed stoichiometric torque capability at high compression ratio. The results illustrate
that intermediate alcohol−gasoline blends exhibit exceptional antiknock properties and performance beyond that indicated by
the octane number tests, particularly E30.