Search Results

The benefits of blending ethanol into gasoline fuel are well established. Ethanol’s high latent heat of vaporisation and chemical auto-ignition resistance combine in producing significant knock resistance, enabling higher compression ratio and/or higher charge boosting. Its high flame speed characteristics result in shorter burn durations. Its high knock resistance and rapid burning enable ignition phasing optimisation. These factors all improve the efficiency of spark ignition (SI) engines. Current “flex-fuel” vehicles are designed to operate on both conventional gasoline as well as blends containing higher volumes of ethanol and/or methanol, the former being commonly known as E85. The American Society for Testing and Materials ASTM D5798 specification for ethanol fuel blends was adapted in 2011 to prescribe a minimum ethanol content of 51 % with the remainder able to consist of low octane blending streams.

A study was conducted to investigate the relative influence of charge cooling and autoignition chemistry on the greater knock resistance seen by alcohol fuels compared to petrols when operating under “Beyond RON” conditions in a Port Fuel Injection (PFI) engine. The methodology employed was that of a modelling study calibrated and validated using experimental data, with ethanol and iso-octane used as representatives of the alcohol fuels and petrols respectively. A two zone combustion model combined with an empirical knock model formed the centre of the modelling work, with the experimental investigation conducted on a boosted PFI engine. The comparison of knock resistance between ethanol and iso-octane showed that autoignition chemistry plays the largest role in the knock resistance advantage of ethanol. This dominance by autoignition chemistry is partly aided by PFI's poor use of the charge cooling capacity of ethanol.

Knock in Spark Ignited (SI) engines has received significant research attention historically since this phenomenon effectively restricts the compression ratio and hence the thermal efficiency of the engine. The latent heat of vaporization (LHV) of a fuel affects its knock resistance in production engines as well as affecting its Research Octane Number (RON) rating. The reason for this is that evaporative cooling of the fuel lowers in-cylinder gas temperatures resulting in reduced tendency for end-gas auto-ignition. Controlling of the fuel-air mixture temperature to 422 K at the inlet port as per the Motor Octane Number (MON) test method ensures full evaporation of the liquid fuel, and hence LHV is assumed to have little effect during this procedure. LHV therefore has a strong influence on a fuel's Octane Sensitivity (OS) - the difference between its RON and MON values.

The equation found on page 994 contains errors and is corrected in the erratum.

This paper describes a quasi-dimensional multi-zone model of the CFR engine. The engine cylinder was divided into multiple zones containing the unburned air-fuel mixture, which experienced different temperature-pressure histories during the compression stroke and flame propagation phases of the engine cycle. This allowed for the simulation of a temperature gradient within the cylinder, which is postulated to be the cause of the Cascading Autoignition characteristic of the CFR engine. A Wiebe function description of the flame front propagation was used to describe the normal combustion process; mass and energy were transferred proportionally from the unburned zones to a single burned zone. A Functional Global Autoignition Model (FGAM) was used to describe the autoignition chemistry in each of the unburned zones and an equilibrium approach was used to determine the composition of the burned zone.

Homogeneous Charge Compression Ignition (HCCI) engine technology has been an area of rapidly increasing research interest for the past 15 years and appears poised for commercialisation through the efforts of international research institutions and manufacturers alike. In spite of significant worldwide research efforts on numerous aspects of this technology, the need still exists for accurate and computationally efficient fuel auto-ignition models capable of predicting the heat release dynamics of two-stage auto-ignition, especially for full boiling range fuels, sensitive to the effects of pressure, temperature, fuel equivalence ratio and inert dilution.

Homogeneous Charge Compression Ignition (HCCI) engine technology has been an area of rapidly increasing research interest for the past 15 years and appears poised for commercialisation through the efforts of international research institutions and manufacturers alike. In spite of significant worldwide research efforts on numerous aspects of this technology, the need still exists for accurate and computationally efficient fuel auto-ignition models capable of predicting the heat release dynamics of two-stage auto-ignition, especially for full boiling range fuels, sensitive to the effects of pressure, temperature, fuel equivalence ratio and inert dilution.

This paper describes the set-up and testing of a single cylinder 25cc, air cooled, 4-stroke Spark Ignition (SI) engine converted to run in Homogeneous Charge Compression Ignition (HCCI) mode with the aid of various combustion control systems. The combustion control systems were investigated regarding their effects on combustion stability and heat release phasing. Engine operation was compared with unique findings from previous work done on a very small 2-stroke HCCI engine. HCCI engine operation was possible between 1000 - 4000 rpm when using Diethyl Ether (DEE) as the test fuel. Maximum operational fuel-air equivalence ratio (Φ) was 0.75 when operating without Exhaust Gas Recirculation (EGR). This relatively high equivalence ratio was attainable due to thermal gradients induced by the high surface area to volume ratio of the small engine combustion chamber, resulting in high chamber heat transfer.

In three recent engine testing research projects involving comparisons of Low Temperature Fischer Tropsch (LTFT) synthetic diesel with conventional crude derived diesel, findings have included indications of significantly lower engine cylinder wear rates in engines running on Fischer Tropsch (FT) diesel. Close examination of the engine oil analysis from the second comparative study has strongly indicated that the differences in cylinder wear rate can be ascribed to the choice of fuel. None of the three studies were originally formulated for this aspect of comparison and only the second study is able to prove that this is in fact a fuel specific advantage attributed to FT diesel fuel. This paper reports on the details of the three projects in respect to this issue, presents analysis of the experimental data and preliminary investigations attempted in an effort to understand this phenomenon.

This paper examines the successful use of HCCI combustion in a standard issue model-aero “diesel” engine. This two-stroke engine, unlike the more common glow-plug versions, operates without any form of combustion initiator. The fuel and air are premixed using a simple carburettor and ignited by piston compression only. The engine therefore operates in HCCI mode even though it is referred to as a “model diesel engine”. Of particular interest is the fact that the engine is easily started from cold, warm and hot conditions. It runs stably from idle to over 11000rpm and is shown to run at high load points across the speed range with extremely conservative pressure rise rates. Furthermore, this engine is shown not to exhibit any knocking (high pressure oscillations) within its normal range of operation. The speed-load operational envelope of the engine is mapped out using a range of propellers and a propeller speed-load calibration rig.

A recently improved Arrhenius fuel auto-ignition model was combined with a single zone explicit discrete Homogeneous Charge Compression Ignition (HCCI) engine model in order to investigate a wide range of combinations of fuel type with engine setup and operational configurations. The engine model was validated using experimental data from a single cylinder variable compression ratio engine running in HCCI mode. The model was used to identify promising and problematic areas for the combination of fuel properties, engine configurations and operational ranges. Insights regarding the interaction between trapped gas pressure and temperature histories and auto-ignition reaction rate surfaces in the pressure and temperature domain are presented.