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Direct Injection Spark Ignition (DISI) engine technology is becoming increasingly common in the South African and global vehicle parcs. South Africa is in a unique position because a significant portion of all liquid fuels consumed are synthetically produced from coal and gas. These fuels are mainly supplied into the inland regions, particularly the Gauteng province, the economic heartland of South Africa and the most densely populated area in the country. It is important to understand the performance of synthetic fuels in the latest generation engines, in order to ensure that these fuels are fit for use in these new applications. The latest generation DISI gasoline engines (also known as Gasoline Direct Injection™ and Fuel Stratified Injection™) differ significantly in operation to older Port-Fuel-Injected (PFI) engines.

The paper describes the application of an enhanced ignition delay model, developed by Yates and co-workers, to the operation of a single cylinder HCCI engine. The ignition delay description was further expanded in this paper to incorporate the effects of air-fuel ratio changes (to cater for lean operation) and residual exhaust gas. Variable combustion (heat release) duration was added, yielding improvement on the common assumption of a fixed duration. The model was compared to measurement performed in a variable compression ratio, single cylinder Ricardo E6 engine and the paper details the engine preparation, test procedure and results. A variety of fuels were tested and modelled and results for two fuels (n-heptane and methanol) were presented in the paper. These two fuels represent extremes of autoignition resistance (with octane numbers of 0 and 106, respectively).

Diesel engines operate with an open-loop fuel control system and the engine torque is therefore affected by variations in the fuel density. Five vehicles, representing a range of different injection technologies, were tested on six fuels having densities ranging from 819.5 to 840.1 kg/m3 @ 15°C. The results indicated that rotary distributor-pump systems were considerably more sensitive to fuel density variations than the common-rail systems or unit-injectors. The fuel density variations caused acceleration performance deviations ranging over 7%. Various external factors (vehicle loading, air conditioner, under-inflated tires, open windows, headwinds, road gradients and different road surfaces) caused deviations in the acceleration performance of comparable magnitudes.