Search Results

Net heat release rates and knock characteristics were derived from in-cylinder pressures for different fuels in a single-cylinder engine; the effect of an ashless antiknock, N-methyl aniline (NMA) was also studied. The maximum net heat release rate (MHRR) resulting from the final high-temperature chemistry determines the knock intensity. Paraffinic fuels have similar knock intensities at comparable knock occurrence frequencies. Aromatic fuels have significantly lower MHRRs and give much lower mean knock intensities for a given knock occurrence frequency compared to paraffinic fuels. Adding NMA to a paraffinic fuel increases the spark advance required to get a chosen frequency of knock occurrence as it increases the octane number of the fuel but has little effect on MHRR and hence knock intensity.

A four-cylinder engine with a slice between the head and the block carrying instrumented plugs has been used to study the growth of combustion chamber deposits and knock. Deposit thicknesses vary substantially at different locations, the thickness generally being greatest at the coolest surfaces. If a dirty engine is run on a low-boiling-point fuel such as a primary reference fuel, deposits are removed and octane requirement is reduced rapidly. Of the head deposits, those in the cooler squish region where the end gas is likely to be situated affect knock more than the deposits in the hotter regions. Different fuel additives have different effects on deposits in different areas. For instance, an additive might cause a substantial increase in deposit thickness in the hotter areas and a slight increase in total deposit weight but can control deposits in the cooler squish regions and so reduce octane requirement increase (ORI) compared to the base fuel alone.

The auto-ignition or anti-knock quality of a practical fuel is defined by the Octane Index, OI = (1-K)RON + KMON where RON and MON are the Research and Motor Octane numbers and K is a constant depending only on the pressure and temperature variation in the engine. K decreases as the compression temperature in the unburnt gas at a given pressure in the engine decreases and can be negative if this temperature is lower than in the RON test. As spark ignition (SI) engine designers seek higher efficiency knock becomes more likely. Moreover such initiatives - direct injection, higher compression ratios, downsizing and turbocharging - will reduce the unburnt gas temperature for a given pressure and push the value of K downwards. In Europe there is evidence of a monotonic decrease in the average K value from 1987 to 1992. In 37 different Japanese and European cars (34 models) equipped with knock sensors that have been tested K has been found to be negative in most cases.

The anti-knock or octane quality of a fuel depends on the fuel composition as well as on the engine design and operating conditions. The true octane quality of practical fuels is defined by the Octane Index, OI = (1-K)RON + KMON where K is a constant for a given operating condition and depends only on the pressure and temperature variation in the engine (it is not a property of the fuel). RON and MON are the Research and Motor Octane numbers respectively, of the fuel. OI is the octane number of the primary reference fuel (PRF) with the same knocking behaviour at the given condition. In this work a wide range of fuels of different RON and MON were tested in prototype direct injection spark ignition (DISI) engines with compression ratios of 11 and 12.5 at different speeds up to 6000 RPM. Knock Limited Spark Advance (KLSA) was used to characterize the anti-knock quality of the fuel. Experiments were also done using two cars with DISI engines equipped with knock sensor systems.