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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.

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.

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.

A 1.8 litre 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 some of their effects on engine operation. Different techniques for measuring deposit thickness, knock onset and deposit effects on the thermal characteristics of the cylinder have been developed. Deposit growth as measured by deposit weight on the plugs is reasonably repeatable from run to run and cylinder to cylinder. The presence of deposits already in the cylinder does not affect deposit growth on clean plugs introduced into the combustion chamber. Deposit thickness and morphology vary substantially at different locations, the thickness being greatest at the coolest surfaces. Deposits increase the flame speed and reduce the metal temperatures just below the surface. They also reduce the mean heat flux away from the cylinder.