Search Results

A technique, based on the unique relationship that exists between the dielectric constant of a fully oxidized exhaust gas sample and the fuel:air ratio of the unburnt mixture, is presented. To measure the dielectric constant a special cell has been constructed through which the fully oxidized gases are passed. The capacitance of the cell is then directly proportional to the dielectric constant of the gases so that changes in mixture strength are detected as changes in cell capacitance. These changes in cell capacitance are measured electronically and are continuously recorded as an analogue voltage on a chart recorder. The cell design is such that the residence time of the gases is 1/10 s at a flow rate of 2 1/min-1.

The formation of combustion chamber deposits in modern SI engines is predominantly derived from hydrocarbon fuels and occurs as a consequence of the quenching action of the combustion chamber walls on the flame. A laboratory experiment has been designed which enables rapid generation of deposit material in the form of viscous brown liquids. Heating these deposits produces material that is consistent in composition and physical appearance with mature engine deposits. The deposit-forming tendency of a number of individual hydrocarbon species has been determined. The amount of deposit increases with i) the amount of unsaturation present in the molecular structure and ii) the boiling point of the hydrocarbon fuel being burned. A structurally derived parameter for each hydrocarbon molecule is found to correlate well with deposition rate, allowing a unified treatment of the different generic forms of hydrocarbons in which deposit-forming tendency is linked to molecular structure.

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.

In order to estimate the influence of the fuel composition on speciated hydrocarbon emissions from gasoline engines a model has been developed for the processes undergone by the fuel which escapes the main combustion event. One of the most important ways that this occurs is by trapping in crevices followed by mixing and partial oxidation with the hot burnt gas during the power and exhaust strokes. This complex process has been modelled by recognising some important characteristics. It is observed that the fraction of a fuel species emitted is well correlated with its rate constant for reaction with OH radicals and that this is independent of the rest of the fuel composition. This means that (a) the chemistry is significant (not just mixing) and (b) the radicals carrying out the oxidation originate from the burnt gases. The decoupling of radical concentrations from the fuel composition considerably simplifies the modelling.

Significant differences have been observed in the behaviour of elastomeric seals exposed to various automotive diesel fuels. This behaviour is governed not only by the chemistry of the elastomer but also by the aromatic content of the fuel and is typical of elastomer/fluid interactions occurring under diffusion control. Although no significant differences were observed in the response of nitrile elastomers exposed to peroxides, the use of antioxidant additives in “low” aromatic diesel fuel needs to be considered. The normal seal housing design criterion is such that seal integrity should not be compromised by the use of “low” aromatic fuels in normal operating circumstances. Some three years' experience in the Swedish market supports this view.

As a first attempt to assess the scope for reducing the emissions from gasoline fuelled vehicles in the European market via fuel changes, an experimental scouting programme has been conducted to examine the effects of fuel composition and properties on both regulated emissions and detailed exhaust gas composition. This 4 fuels/4 vehicles programme involved exhaust emission tests on a chassis dynamometer, and was carried out mainly on non-catalyst vehicles, using two commercial gasolines and two (“reformulated”) test gasolines representing “intermediate” and “extreme” changes of the base gasoline design. The fuel characterization includes analysis by hydrocarbon type and carbon number; the exhaust analysis comprises both regulated emissions and hydrocarbon speciation measurements. In the case of regulated emissions, the fuel effects are, as expected, relatively small in comparison with the effect of installing exhaust catalysts.

Departures from optimum stoichiometry during transients (acceleration and deceleration) and cold start can lead to significant degradation in driveability and emissions control. Such departures occur as a result of a complex interplay between fuel transport mechanisms and the fuelling strategy. The relative contributions of several of these mechanisms are affected by fuel composition. To help understand these effects an open-path differential infra-red absorption technique has been used to monitor the transient evolution of the fuel vapour phase directly within the combustion chamber. The sensor projected a narrow infra-red beam which traversed the cylinder of an optical access engine along an open path under the head, and measured the path-integrated attenuation caused by absorption of the infra-red radiation by the fuel vapour. It operated in the near infrared (NIR) spectral region around 2.3 μm, an absorption band in hydrocarbon species containing methyl groups.

Combustion chamber deposit (CCD) formation in SI engines is a complex phenomenon which is dependent on a number of fuel and engine parameters. A mathematical model has been developed, based upon a previously proposed mechanism of CCD formation, which describes the physical and chemical processes controlling the growth of deposits in SI combustion chambers. The model allows deposit thickness to be predicted as a function of time, taking into account gasoline composition and factors influenced by engine operating conditions. Piston top deposit thicknesses predicted by the model for 38 unadditivated fuels show a strong correlation with data from three different bench engine tests. The model offers the possibility of predicting the amount of CCD produced by unadditivated gasolines for a range of engine designs, operating conditions and test durations.

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.

The transport of fuel droplets into the combustion chamber of an SI engine and their subsequent evaporation has been studied, using a new optical diagnostic technique, Interferometric Laser Imaging for Droplet Sizing (ILIDS), which allows temporally and spatially resolved measurements of droplet size distributions. The measurement technique and its application to in-cylinder engine measurements are described. Measurements were made under warmed-up conditions, with open valve injection timing, in a Ricardo Hydra single cylinder engine. The results showed differences in the evolution of the droplet size distribution in cylinder with variations in load and speed. At 1200 rpm under full load, droplets arrived quickly into the cylinder, and were small, the Sauter Mean Diameter (SMD) being in the region 10-12 μm on arrival, so that mixture preparation was good.