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Evaluating performance differences between fuels in vehicles on the road suffers from two problems. Measurements can be made in pairs of vehicles driven in parallel. Each vehicle is subjected to identical conditions but the vehicles and drivers may not be perfectly matched. Alternatively, tests can be conducted consecutively in a single vehicle. Variation between different drivers and engine builds is eliminated, but weather and road conditions may differ between runs. To address these problems, Shell have constructed a “dual-fuel truck” in which the fuel injection system of a V6 diesel engine has been divided into two halves, supplied from two fuel tanks. The whole engine can operate from either fuel tank, switching between them in a matter of minutes. Alternatively, each half of the engine can be supplied with a different fuel. In this way, two fuels can be compared in a single vehicle - in the same engine build, with the same driver, under identical driving conditions.

A road test has been conducted to quantify the benefits provided by a high-performance gasoline additive package in a fleet of cars representative of Europe, SE Asia, and South America. The emissions, fuel consumption, and engine cleanliness benefits of additised versus untreated gasoline were compared in 15 pairs of cars. A further 6 cars were operated on a mixture of fuels to show the benefits of additised fuel versus mixed fuelling. The design of the experiment was based on a similar road test conducted in 1991. Through careful test design and execution, it has been possible to assess the performance of the package at a high statistical confidence level. The package provides a high level of inlet system cleanliness, a significant reduction in fuel consumption and reduced HC emissions.

It is desirable to predict the cold weather driveability performance of fuels by means of an index based on simple measurements such as the ASTM distillation curve. In the past, several such indices have been proposed from the analysis of vehicle test results. In contrast, this paper describes how a driveability index can be derived from first principles - namely, the physics of fuel vapour formation. A number of present and proposed driveability indices were evaluated by comparing the way they correlate with the calculated enthalpy requirement of fuels. It is concluded that E100+E150 best meets the need for a simple index and is robust across a range of air/fuel ratios.

Cold weather driveability has mainly been assessed in Europe using the CEC M-08-T-83 procedure. This was developed in the 1970's for a vehicle population mainly equipped with carburetted engines. However, modern cars equipped with fuel injected engines and electronic engine management systems show very few driveability problems in the CEC test, making it difficult to show significant differences in driveability performance between cars or fuels. Shell Research have developed a new cold weather driveability test cycle for use on climate-controlled chassis dynamometers. Tests in a number of fuel-injected vehicles have shown more demerits than in the CEC test under the same conditions. The new cycle is therefore more useful for demonstrating differences in cold weather driveability where the CEC cycle produces demerit levels close to zero.