Alcohol Fueled Engines

Hannu Jääskeläinen

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• Introduction
• Compression Ignition
• Pilot Ignition
• Spark Ignition
• Emissions

Introduction

While the application of alcohol fuels to internal combustion engines can make use of some unique properties of alcohols, there are significant challenges that must also be addressed. Some of the properties of alcohol fuels that need to be considered include a high octane rating, a low cetane rating, a high latent heat of vaporization, a low calorific value relative to hydrocarbon fuels, poor lubricity, a high affinity for water, corrosivity and low soot production.

The high octane number and low cetane number can be an advantage or disadvantage depending on the engine type it is being used in. For positive ignition engines using a premixed charge, the high octane rating allows knock to be avoided and allows higher engine power and torque levels to be produced compared to gasoline fueled engines. For compression ignition engines, the low cetane rating means ignition is difficult and requires the use of high charge temperatures or large quantities of ignition improver additives.

Work at Sandia in the 1980s determined that ethanol autoignition occurs only at temperatures above 950 K (677°C) but in-cylinder temperatures above 1100 K (827°C) are necessary for achieving ethanol and methanol ignition delays comparable to diesel, and for limiting in-cylinder pressure rise rates [6101]. Others suggest that maintaining a charge temperature at the beginning of fuel injection of approximately 943 K (670°C) was sufficient to provide stable operation [6056]. Despite the large differences in ignition delay between methanol, ethanol, isooctane, and cetane at lower temperatures, their ignition delays converge and reach a limiting value for temperatures above 1100 K [6101].

The use of cetane improver additives is another way to achieve compression ignition of alcohol. However, under diesel-like conditions, the quantity of improver additive can be one or two orders of magnitude higher than for diesel fuel. For example, a common diesel fuel cetane improver, 2-ethylhexyl nitrate (EHN), was reported in 2007 to be used at concentrations of 0.05% to 0.4% mass in diesel fuel [2110]. In ethanol and methanol, to achieve a cetane number of 40 under diesel-like conditions requires concentrations of EHN in the range of 10% volume, Figure 4 [5856]. To keep additive concentrations lower, higher in-cylinder temperatures can also be combined with the use of ignition improvers [6077].

Unlike diesel fuels, methanol and ethanol have single-stage ignition characteristics with no low or intermediate temperature heat release. This makes their combustion phasing sensitive to temperature when used in premixed combustion concepts and can make control, especially during transients, challenging [6102][6103]. This has led some to conclude that ethanol and methanol are more suitable as fuel for compression ignition engines with diesel-like operation [4880].

The high latent heat of vaporization of alcohols has several possible effects. It can lead to a reduction in charge temperature which can be exploited to improve cylinder filling, reduce compression work and avoid knock. It can also make it more challenging to achieve compression ignition and increases the in-cylinder temperature requirements. Low temperature performance can also be affected if insufficient heat is available to vaporize the fuel.

Ethanol and methanol also have low soot production, Figure 1 [6067]. Ethanol has a single C-C bond and can produce soot while methanol has no C-C bond, only a C-O bond, and produces little measurable soot even under fuel-rich conditions. This also avoids the soot-NOx trade-off and allows high EGR rates to control NOx emissions [5859]

Methanol’s ability to burn fuel rich and not produce soot potentially makes it a good choice for combined cycles based on an internal combustion engine where the unburned fuel in the engine’s exhaust can be utilized in an auxiliary power producing cycle that utilizes the engine’s exhaust.

Alcohols can cause corrosion of some metals and alloys. Aluminum and zinc are susceptible to corrosion by ethanol and methanol. Corrosion inhibitors have been developed for gasoline/ethanol blends. The low lubricity of alcohols may require lubricity additives to avoid wear problems with high pressure fuel injection equipment. They can also form acids during combustion which could lead to higher demands on lubricant additives and possibly more frequent oil changes [6073][6069].

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