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A “thermal pulse” technique has been used to measure thermal diffusivity and thickness of combustion chamber deposits continuously during engine operation. The technique uses a fast-response thermocouple junction at the combustion chamber wall surface and a simplified model which describes the effect of the deposit on the measured temperature cycle. Results from 13 tests using four different fuels and three different commercial additive packages are discussed in the paper. Thermal diffusivity values in the range of 0.85 - 4.2 x 10-7 m2s-1 were measured. Deposit growth is normally a continuous process. However, occasionally deposit flaking events characterised by a sudden significant decrease in deposit thickness were observed.

A field problem associated with flakes of combustion chamber deposits getting trapped on the exhaust valve seat and causing starting problems has appeared recently. Four fuels have been tested in three different car models using a deposit flaking road test procedure. For each piston top, flaking can be characterised using T1 and T2, the mean deposit thickness on the piston crown before and after flaking respectively. A new measure of deposit flaking, ΔT, the mean of (T1-T2) averaged over all cylinders has been introduced and its variance established for the standard test using one of the models. ΔT quantifies the actual amount of deposits that have flaked and is likely to be a more relevant indicator of flaking for startability problems than Rw, the mean of the ratio of T2 to T1, used in previous work. Deposit flaking is directly related to an increase in valve leakage rates and startability problems.

A quick, convenient, objective and sensitive engine test to monitor the activity of a gasoline detergent additive package in the inlet system has been developed. The test is based on an optical technique which detects, by monitoring atomic emission, the increase in the movement, caused by the additive package, of an appropriate marker from the inlet manifold/port into the combustion chamber. The test is not sensitive to deposit formation tendencies of the fuel and additive package components and hence cannot be used to assess the cleanliness performance of unknown gasolines or of additive packages whose components themselves produce deposits. Since it is developed in a carburetter engine, the results are relevant to carburetter engines. The results from the test are repeatable and correlate very well with results from a variety of more expensive engine and road tests. They are also in line with existing knowledge about detergent packages reported in the literature.

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 high resolution study of the early stages of flame development in a spark ignition engine has been performed. Full flame velocity distributions are presented and are discussed in terms of the nature and causes of fluctuations in the flame propagation. The simultaneous sampling of a number of engine variables such as gas velocity, flame velocity, pressure etc. is necessary for a more complete description of the controlling variables. Techniques are described and data presented for such a multiple sampling experiment.

Deposits derived primarily from the fuel but with some contribution from the oil are formed on all internal surfaces of a spark-ignition engine. Interest in combustion chamber deposits (CCD) has increased as fuel and additives technology to control deposits in other parts of the engine has matured. This paper reviews the literature on CCD. It discusses the mechanisms involved in the growth of CCD, their properties and structure, the engine and fuel-related factors that influence their growth and finally the effects of CCD on engine performance and emissions.

There is increasing concern that small flakes of combustion chamber deposits (CCD) can break lose and get trapped between the exhaust valve and the seat resulting in difficulties in starting, rough running and increase in hydrocarbon emissions. In this paper we describe experimental observations which might explain how this flaking of CCD occurs and the factors that might be important in the phenomenon. The experiments include thirty one engine tests as well as tests done in a laboratory rig and show that some CCD flake when they are exposed to water; indeed water is far more effective in bringing this about than gasoline or other organic solvents. The hydrophilicity of the deposit surface which determines the penetration of water and the inherent susceptibility of the relevant deposit layer to inter-act with water are both important. Consequently there are large differences between deposits produced by different fuels and additives in terms of their susceptibility to flake.

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.