Alrayyes, Taleb
(2011)
The effect of ethanol-gasoline blends on SI engine energy balance and heat transfer characteristics.
PhD thesis, University of Nottingham.
Abstract
Ethanol is one of a group of hydrocarbon fuels produced from bio-mass which is attracting interest as an alternative fuel for spark ignition engines. Major producers of ethanol include Brazil, from sugar cane, and the USA, from com. Reasons for the growing interest in ethanol include economic development, security of fuel supply and the reduction of net emissions of carbon dioxide relative to levels associated with the use of fossil fuels. Unlike gasoline, which is a mixture of hydrocarbon compounds suited to meet a range of start and operating requirements, ethanol is a single component fuel with characteristics which make engine cold starting difficult, for example. Hence, ethanol is generally used in a blend with gasoline, accounting for <5% in EU pump-grade gasoline to 85% by volume for so called flex-fuel vehicles.
Although ethanol is already available in the marketplace, there are aspects of its effects on engine behaviour that are unresolved, including its effects on engine thermal behaviour and heat transfer. These have been investigated in the experimental study presented in this thesis. The aims of this work included determining the effect of ethanol content in blends on combustion characteristics, energy balance, gas-side heat transfer rate and cylinder instantaneous heat transfer.
This study covers a range of loads, speeds, spark timings, equivalence ratios and EGR levels representative of every day vehicle use, and has been restricted to fully warm operating conditions. The investigations have been carried out on a modern design of direct injection, spark ignition engine. The performance of different ethanol-gasoline blends has been compared at conditions of matched brake power output.
The emissions data for NO, HC, CO and C02, which was used to calculate combustion efficiency, show a decrease in their levels proportional to the increase in ethanol content in the fuel blend. This is owing to an increase in combustion efficiency and change in chemical structure and physiochemical properties.
Compared to gasoline, running on 85% ethanol produces slightly faster rates of burning in rapid burn stages of combustion. Typically, the reductions in rapid burn angle are 4%. Results show that the effects do not vary in proportion to the ethanol content in the fuel blend. This is attributable to the fact that, at low and medium ethanol content, the enhancement in combustion gained by oxygen availability is offset by its higher enthalpy of vaporisation and lower heat content.
Energy balance data show an improvement in thermal efficiency proportional to the increase in ethanol ratio. This is due to improvement in combustion efficiency and a reduction in coolant and exhaust losses.
Results for gas-side heat rejection show that a correlation developed for engines run on gasoline can be used without any modification. The heat rejection rate has been inferred from measurements of heat rejection to coolant adjusted to allow for the contribution of engine rubbing friction. The apparent insensitivity to ethanol content is attributed to a combination of factors. These include the increase in fuel flow rate for a given energy supply being offset in its effect on charge flowrate by a reduction in stoichiometric air/fuel ratio.
Gas-side heat transfer results from both the exhaust port and the cylinder show a clear decrease when running on 85% ethanol compare to gasoline. This reduction was also observed in the total measured heat loss to coolant. The magnitude and phasing of instantaneous heat loss is not sensitive to the use of ethanol during combustion. However, as the combustion starts to terminate, lower heat loss for medium and high ethanol content was observed due to the reduction in the combustion product temperature. The results from the C 1 C2 correlation and instantaneous heat transfer are comparable.
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