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A set of model fuels has been designed, using the Major-Component Fuel approach, to represent a range of gasoline mid-range and back-end volatilities. The thermo-physical properties of the model fuels have been used, together with a simple model of inlet system, to calculate liquid-vapour mass fractions in the inlet system, and the composition of the inlet port wall film. This has enabled the effects of gasoline volatility, speed, load and inlet port wall temperature to be studied systematically. The results indicate that, in cold start, only some 20-30% of the injected fuel is vapourised in the inlet port, leading to an accumulation of liquid fuel in the inlet port wall film reservoir. As the engine warms up, the mass of fuel in the reservoir decreases, and its composition changes, becoming progressively richer in heavy end species. Mid-range volatility affects the cold start behaviour, whilst back-end volatility affects the approach to fully-warmed up operation.

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.

An experimental programme has been carried out to quantify the influence of Combustion Chamber Deposit (CCD) removal on vehicle acceleration performance, fuel consumption and tailpipe emissions in several modern European car models. Vehicles were performance and emissions tested dirty', following accumulation of 16,000 kilometres (10,000 miles) with a light duty cycle, then ‘clean’, following removal of CCDs. This scheme was repeated for one model using a heavy duty driving cycle. Additional tests were carried out on three vehicle models equipped with knock-sensors for which ignition timing was monitored. CCDs reduced fuel consumption relative to the clean engine, in amounts dependent on vehicle model. CCDs had only small, detrimental effects on acceleration performance and power. They generally (but not always) increased NOx emissions and had variable and usually small effects on HC and CO emissions.

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.

Vehicle-based studies have shown that a reduction in the aromatic content of gasoline fuels can result in increased NOx emissions from catalyst-equipped vehicles. A study with simulated exhaust gas has shown that light paraffins, especially methane, are unreactive and cause substantial breakthrough of unreacted NO over the catalyst. However, unsaturated exhaust components including aromatics are effective reactants and play a large part in converting NO over the catalyst. Engine tests have shown that methane is predominantly produced by fuel paraffins and olefins, but hardly at all by aromatics. Thus it appears that methane generated during combustion of low aromatics fuels may be the cause, wholly or in part, of the poor NO conversion efficiency observed when catalyst-equipped cars are operated on such fuels. However, it cannot be excluded that low aromatics fuels are associated with increased air-to-fuel ratio which will also contribute to poor NO performance.

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 “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.

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.

Environmentally adapted diesel fuels defined by the Swedish Government contain extremely low levels of sulphur and have limited aromatics contents. Road trials and pump durability tests of these fuels revealed unacceptable wear in injection pumps due to low lubricity. Additive solutions were identified using bench tests and then proven in field trials. Market experience has substantiated the findings that fuels using the chosen additive give fully satisfactory performance. This paper illustrates how practical solutions to lubricity questions can be found, and is applicable wherever specifications demand fuels requiring a high degree of hydroprocessing.

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.

The benefits of diesel fuel additives have been demonstrated in a broad range of performance and operational areas, from the refinery, through storage and distribution, to fuel dispensing and vehicle operation. The customer is certainly aware of their effects on fuel performance in many of these respects, such as cold-weather operation, ease of starting, foaming, odour, etc. An area of particular interest in customer perception, however, is fuel economy. Excluding the use of after-market fuel-treatment devices, it is claimed that additives of different types can improve fuel economy, for example by improving combustion, by maintaining injection equipment in optimum condition, or by reducing engine frictional losses.

As a result of increasing concerns over air quality, environmental legislation has led to more stringent emissions limits for diesel engines and vehicles. This has affected both engine manufactures and fuel suppliers. Whereas in the US, only the fuel requirements for heavy-duty diesel engines are of key interest, in Europe light-duty diesel applications are also important since diesel-powered passenger vehicles are accepted by customers and their market penetration has increased rapidly. This paper gives an update of Shell's ongoing research on correlations between diesel fuel quality and particulate emissions in both heavy- and light-duty applications. In heavy-duty testing (both steady-state and transient), sulphur is the dominant fuel property affecting particulate emissions. After sulphur correction, fuel effects are small and can best be described by a combination of cetane number and density.

Six full range gasolines were tested in two engines (one with a catalyst) operated at 4 steady states. Engine-out regulated emissions responded to equivalence ratio, Φ, in the accepted manner. For both CO and NOx, there was a characteristic, single emissions response to changes in Φ. Changing fuel composition will primarily alter the production of these emissions by modifying the stoichiometric air/fuel ratio, projecting engine operation onto another part of the Φ response curve. These Φ effects, which are independent of engine design, also determine how operating conditions affect engine-out CO and NOx. Speciated hydrocarbon measurements at engine-out and tail-pipe confirm results seen in previous test-cycle based programmes.

Following a small scouting programme to examine the scale of emissions benefits achievable by different degrees of gasoline base fuel redesign (SAE 930372), a larger programme has been initiated to investigate more systematically the influence of individual fuel parameters on tailpipe emissions. This coordinated study has been spread across five participating Shell Group laboratories, using a set of common fuels specifically designed and centrally blended for this purpose. Additionally, subsets of these fuels have been used for detailed systematic examination of selected topics within the overall programme scope. This paper summarises the plan for the integrated study. It describes the composition and properties of the fuels and their blending. The results covered here are those of chassis dynamometer-based regulated emissions studies conducted on a composite fleet designed to represent a range of vehicle technologies, using a variety of regulatory driving cycles.

Total aromatics have no influence on particulate emissions. This is an unexpected finding from a comprehensive programme to determine the influence of diesel fuel properties on heavy-duty particulate emissions. 30 fuels and five engines representing a variety of manufacture/technology were tested. To reveal causative influences, key fuel properties were intentionally uncorrelated and had a wide range of values. Engine sensitivities to fuel quality were found to differ considerably. Properties that most influenced emissions were sulphur content, density and cetane number. Some engines were totally insensitive to polyaromatics but in others, small influences are possible. The emissions benefits of specific fuel property changes are quantified.

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.

A method is proposed to quantify the activity of a detergent at a selected engine operating condition by measuring the amount of additive required to reduce nozzle fouling to zero in a well-defined engine test procedure. To determine this critical detergent concentration, a series of engine tests is performed using increasing levels of additive, until a zero level of fouling is achieved. These results are then used to construct a detergent concentration response map, defining unambiguously the additive activity and allowing quantitative comparisons of different detergents or additive packages. Concentration response plots are presented from both bench engine test series and vehicle road trials which demonstrate the wide variability in the amount of different detergents required to achieve comparable deposit control. From these maps the performance of the detergents may be compared in terms of keep-clean and clean-up activity for the range of engines and operating conditions.

Engine Sludge has reappeared in the last decade as a source of operation problems and in manufacturers warranty claims in Europe and the USA, due to engine malfunction and in some cases engine failure through oil starvation. This sludge has become known as ‘Black sludge’ or ‘Hot sludge’ in Europe. As a result of the problem, bench engine tests have been developed in Europe (CEC-L-41-T-88), and the USA (ASTM Sequence VE). Both these tests have been shown to be particularly sensitive to changes in the fuel composition, even between batches of the same gasoline. A field trial has been conducted by Shell Research Ltd at Thornton Research Centre, to study the effect of fuel composition and fuel-lubricant interactions on the propensity to form sludge, using a mileage accumulation cycle designed to be severe with respect to sludge formation.

The role and required function of a diesel fuel detergent are discussed and related to the process of nozzle deposit formation. A bench engine test method is described which uses an air-flow technique to measure the fouling produced in IDI nozzles and hence quantify the effectiveness of detergents in the engine test. Data are presented demonstrating the discrimination in detergent performance which may be obtained by applying this method at different operating conditions. Results of ECE 15.04 emissions tests are presented from vehicles having nozzle fouling levels ranging between 1-70%. These emission levels are plotted versus nozzle fouling. From these data, it is concluded that reducing fouling produces a systematic reduction in unburnt hydrocarbon emissions. An optimum fouling level of 15-40% is identified to minimise particulate emissions from this engine.

Vehicle driveability is a function of gasoline volatility, ambient conditions and engine design. The ability to predict driveability performance from a knowledge of fuel/air mixture temperatures and gasoline properties would greatly assist both fuel and engine development. Accordingly, a model to predict engine hesitation under full-throttle accelerations (a major driveabilty malfunction) has been developed. Hesitation occurs when the fuel/air mixture reaching the combustion chambers is too lean to burn. Thus the model is based on the calculation of heat flow and air/fuel vapour ratios in the engine inlet manifold. Chassis dynamometer tests for two different cars using a range of fuels and a range of test temperatures have shown that the model gives an accurate prediction of mixture temperatures and engine hesitation under full-throttle conditions.