There are a number of energy alternatives to gasoline and diesel engines.  For those who are not familiar with the difference between the engines and the fuels that they require, let's have a quick overview.  For those of you who need more information, you can easily get a more comprehensive explanation of engines and fuels elsewhere on the internet .

Gasoline Engines

Gasoline engines are also known as spark-ignition (SI) engines.  These engines can only burn a homogeneous fuel mixture, that is to say one that is premixed before it enters the combustion chamber.  The combustion chamber is simply the volume between the top of the piston and the cylinder head.  SI fuels must have a high resistance to self ignition so that only the spark within the combustion chamber initiates the combustion process.  Remember that the fuel mixture will already have a high temperature just prior to the spark near the top of the stroke of the piston.

The relationship between temperature and pressure is a well known engineering principle (well known to engineers) described by the Ideal Gas Law.  As you reduce the volume of air in a cylinder, you will increase its pressure and its temperature.    You can easily appreciate this relationship when you inflate a bicycle tire and you notice that the pump becomes hotter as the tire increases in pressure.  An important factor in the efficiency and performance of any engine is its compression ratio which is simply the ratio of the volume of a cylinder when the piston is at the bottom of its stroke compared with the volume of the cylinder when it is at the top of its stroke.  Generally, the higher the compression ratio, the higher the thermal efficiency of the engine.

Once the piston is near the top of its stroke in the cylinder and the fuel mixture is highly compressed, the spark plug initiates the combustion process, which is simply a flame that progresses across the combustion chamber.  As the flame propagates, the temperature obviously increases in the combustion chamber  If you increase temperature of air in a cylinder, you will increase its pressure.  If you keep increasing the temperature of any fuel mixture, eventually you will reach the temperature at which the fuel mixture will spontaneously ignite.  This spontaneous ignition is known as knock and is very destructive to engines when it is severe.

SI engines are designed to have the flame progress from the spark plug to all regions of the combustion chamber.  Engine designers will introduce swirl into the fuel mixture movement in the combustion chamber through optimized placement of the intake valve(s) and shape of the combustion chamber to maximize the turbulence and speed of the flame.  Designers will also design a compression ratio into the engine to limit the compression pressure as well. 

The relative resistance to self-ignition can be expressed with the Octane number (O.N.).  In a special test SI knock testing engine, a fuel mixture of Iso-Octane and Normal Heptane consisting of 100% Iso-Octane has an octane number of 100.  In the same engine, a fuel mixture consisting of 100% Normal Heptane has an octane number of 0.  Higher compression engines require fuels of higher octane number. There are two techniques for computing the octane number: research and motor.  The antiknock index for gasoline found on the pump is generally the average of the research and motor numbers.  Octane numbers over 100 are extrapolated.

An important advantage for commonly available alternative fuels is their higher relative octane numbers.  Propane or LPG has a O.N. of about 104 whereas natural gas is around 120, and 100 for Methanol and for Ethanol.  These numbers will vary somewhat depending upon the test method.  Compare this with 87 for regular grade gasoline and 93 for premium.  The point is that engines optimized for alternative fuels can potentially have higher outputs than for gasoline engines.

Diesel Engines

Diesel engines are also known as compression-ignition (CI) engines.  Diesel engines are designed to burn a non-homogeneous fuel mixture, where the fuel is introduced in the combustion chamber.  The very high compression ratios of the diesel engine produce extremely high pressures and temperatures in the combustion chamber.  The temperature of the air at the top of the piston's stroke is high enough to spontaneously ignite the fuel as it is sprayed into the combustion chamber.

CI fuels must have a low resistance to self ignition because only the heat of compression ignites the fuel as it is sprayed into the combustion chamber.  The diesel fuel is rated according to its ignition quality, which is expressed with the Cetane number (C.N.).  The fuel mixture in this case is a blend of Normal Cetane and Heptamethylnonane.  In a special CI test engine, a fuel with a ignition characteristics of 100% Cetane has a Cetane number of 100.  In that same engine, a fuel with the ignition characteristics of 100% Heptamethylnonane has a Cetane number of 15.  Fuels with higher Cetane numbers burn more smoothly and cold start more easily than those with lower Cetane numbers.

Diesel engines can also benefit from alternative fuels.  Supplying a premixed high O.N. gaseous fuel to the engine can help its performance significantly.  CI engines are different from gasoline engines also in the way power output is controlled.  In an SI engine, power is controlled by the throttle valve.  The more air, the more power.  There is no throttle valve in a CI engine and power is controlled by the amount of fuel sprayed into the combustion chamber.  The more fuel, the more power.  Even under full power, a CI engine still has more air available for combustion than the amount of fuel being supplied.  A little gaseous fuel introduced into the engine makes better use of the air in the combustion chamber.

Energy Concepts

Understanding the characteristics of the various alternative fuels requires the understanding of some important energy concepts.

Energy Content

The different fuels can be compared with their energy content, or more precisely, their specific energy.  Specific energy is energy per unit mass:  BTU/lb or J/kg.  A BTU (British Thermal Unit) is defined to be the amount of energy required to raise one pound of water one Fahrenheit degree.  Similarly, a Calorie is the amount of energy required to raise one gram of water on Celsius degree.  A Joule is a metric unit derived from the Newton - Metre.  Specific energy is also known as the calorific or heating value.

All of the fuels we are considering here are hydrocarbons, that is to say composed of both hydrogen and carbon atoms.  When the carbon atoms are completely reacted with oxygen (or burned), their product of combustion is carbon dioxide.  When the hydrogen atoms are completely reacted with oxygen, their product of combustion is water.  Since the exhaust temperature of any real engine cycle is higher than the boiling temperature of water, the energy available when steam is condensed to liquid water is not available.  Therefore, the energy released from burning a hydrocarbon fuel whose products of combustion are at a higher temperature than the boiling point of water is known as the lower heating value (LHV).  Similarly, when the products of combustion from liquid water, the amount of energy released is the higher heating value (HHV).  The difference between the two is the latent heat of evaporation (or condensation) of water.  The latent heat of evaporation is dependant upon the pressure at which the evaporation takes place but the variation will be small at the the range of exhaust pressures considered in internal combustion engines.

When we compare the energy available from various fuels for internal combustion engines, we always use the lower heating value.  When do we use the higher heating value?  One example is the modern high efficiency furnace, which is an external combustion process.  These furnaces are known as condensing furnaces that reason and they do transfer additional energy to the air that heats your house as the steam condenses to water in the secondary heat exchanger.

Fuel Volatility

This concept is more applicable to SI engines and describes the fuels' ability to evaporate.  Even though gasoline is stored as a liquid, it must become a gas before it can react with air.  Gasoline is also not a pure element but is composed of several components.

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Last modified: November 12, 2004