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Technical Paper

Fuel Anti-Knock Quality- Part II. Vehicle Studies - How Relevant is Motor Octane Number (MON) in Modern Engines?

2001-09-24
2001-01-3585
The effect of fuel anti-knock quality on power and acceleration performance is studied in twenty-three European and Japanese cars equipped with knock sensors. The anti-knock quality of a fuel in a given car and operating condition is defined by its octane index OI = RON - KS where K is a constant for that condition and S is the sensitivity, (RON-MON), and RON and MON are the Research and Motor Octane numbers respectively. The higher the octane index, the better the antiknock quality of the fuel and the better the power and acceleration performance. K is often assumed to be 0.5 so that OI =(RON+MON)/2. However, it is found that in most cases considered here, K is negative so that for a given RON, a fuel with higher sensitivity (lower MON) has better anti-knock quality and better performance. Even when K is not negative it has a small (< 0.2) positive value so that MON contributes much less to fuel anti-knock quality than generally assumed.
Technical Paper

Studies of Knock in a Spark Ignition Engine with “CARS” Temperature Measurements and Using Different Fuels

1995-02-01
950690
A “CARS” System using a modeless dye laser has been extensively calibrated and shown to give average temperatures of acceptably good accuracy. It has been used to measure temperatures in the end-gas of a single-cylinder E6 engine under knocking conditions using propane, commercial butane, iso-octane and a primary reference fuel made up of 90% iso-octane and 10% n-heptane by volume. These measurements show that there is significant heating of the end-gas because of pre-flame chemical reactions for all the fuels except propane. Propane has to be compressed to a much higher pressure compared to the other fuels studied in order to make it knock. At a given engine operating condition, there is significant cycle-by-cycle variation in both combustion and knock.
Technical Paper

Fuel Blend and Mixture Strength Effects on Autoignition Heat Release Rates and Knock Intensity in S.I. Engines

1996-10-01
962105
In general, the rate of heat release during combustion in a spark ignition engine, can have two components: one due to normal burning in a propagating flame, and another due to autoignition in the end gas. It has been possible to separate these two components by analysing the pressure trace of a single cylinder engine. From this, the volumetric autoignition heat release rate can be inferred and studied in some detail. To approximate this rate in an Arrhenius form presents difficulties, in so far as it is not possible to measure the temperature at the instant of maximum heat release rate, at the onset of knock. However, it was possible to measure end gas temperatures by the CARS technique prior to autoignition and then to estimate the temperature at the onset of autoignition by extrapolation. Estimation of the temperature at the instant of maximum heat release rate has enabled kinetic parameters to be assigned in an Arrhenius expression for this rate over a range of temperatures.
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