The Heat Engine

The heat engine is a thermodynamic principle that has been engineered and produced for decades. The heat engine is currently one of the top means of transportation and is also one of the leading causes of an increased amount of greenhouse gases in the atmosphere. The thermodynamic principles behind the engine are the kelvin statement and the carnot cycle. These two and environmental impacts will be covered in later posts.

The Kelvin Statement

Below is the Kelvin statement drawn out. The Kelvin statement states that heat energy from a source of heat, will do work within a system. The amount of work done will not be 100% of the heat energy. The remaining heat energy is lost to the surroundings or to a cold source. This statement is behind the basic setup of automobile engines. The heat source is the combustion of gasoline, while the cold source is water/antifreeze mixture.

The Carnot Cycle

The Carnot Cycle, illustrated below, was made in Nicolas Carnot which allows a series of expansions and compressions in combination with the Kelvin Statement to produce work. An isothermal expansion and compression wouldn’t create any work but in combination with an adiabatic expansion and compression it allows for work to be produced. The efficiency of any engine is the total work done divided by the change in heat energy of the isothermal expansion. The work done is the area created by the isothermal expansion and compression and the adiabatic expansion and compression. The efficiency of this work will always be less than 1. The efficiency is found from the following equation: (Th-Tc)/Th.
So using an approximate temperature of the combustion of around 2200K for Th and a warm day in summer of about 300K this will give you an efficiency of 86%. Efficiency can also be measured by the total work done divided by the change in heat from the isothermal expansion.

Sources of Heat and Cooling

Following the Kelvin statement, the engine requires a heat energy source. The source of heat for the engines in our cars comes from the combustion of fossil fuels which provide a large amount of heat energy. This heat energy forces the expansions to take place. In order for the engine to come full circle, the engine must be cooled or the engine would fail. One of the best methods to cool anything is to douse it with water. Since the water could simply evaporate due to the extreme heat or freeze while left outside in winter and damage the engine, an antifreeze/coolant agent was created to mix with the water. This agent decreases the freezing point and increases the boiling point to avoid these engine issues.

Benefits of the Engine

The heat engine has provided us with a means of transportation by boat, car, train, or plane. Although these engines vary, the result is the same. A simple and faster means of travel from here to there. These engines, along with the ability to carry larger objects, have reduced the amount of human labor needed to move items from one place to another.

Damages to the Environment

Although the engine has made our lives easier, the widespread use of cars, trucks, and planes have been taking their toll on the environment. The combustion of the fossil fuels with air have created an increase in a number of greenhouse gases, the primary one being CO2 and the more subtle ones being NO and NO2. In any combustion CO2 is a product, and the more carbons on your combusting molecule the more CO2 created. Since air is comprised of 78% Nitrogen, that percentage is sent into the engine along with O2 and at high temperatures and pressure a reaction can take place that yields NO2 and NO. These 3 gases when in the atmosphere capture energy from the sun and that energy radiated from the earth (radiated energy from natural processes and reflected light). This increase in energy absorption heats up the atmosphere and thus the rest of the earth as well, causing increased flooding, more powerful storms, and an earlier and increased melting of the poles. NO and NO2 also cause ozone depletion which allows more radiation to bombard the earth, causing an increase in cancer in humans and a loss in biodiversity.

Improvements to Help Decrease the Impact

Over recent years we’ve started to notice the growing problems floating into the atmosphere. With these problems accumulating and becoming a larger threat to the Earth’s natural order, we’ve started looking at various ways to cut greenhouse gas emissions. One way to do this is to change the fuel for the fire that drives the engine. Ethanol mixtures and biodiesels have been developed to continue the use of our transportation. Originally ethanol and vegetable oil were used as fuels but since the discovery of cheap petroleum came about, we switched over to this new and inexpensive fuel. Since these new fuels still produce CO2 emissions other strategies involve phasing out the engine and moving towards electric and hydrogen cars which emit water or nothing at all.

Biodiesel

Diesel engines are used as the power source for many machines including generators, trains, cars and trucks,and many others. We will be looking at what biodiesel is, how it affects the environment compared to methane, and how realistic it is as a source of energy.

What is a diesel engine?

            The engine was developed by a German inventor by the name of Rudolf Diesel in 1893. A diesel engine differs from the traditional gasoline burning internal combustion engine in that it the heat from the compression of the piston to ignite and burn the fuel instead of a ignition caused by a spark igniting a mixture of air and gasoline. Diesel engines have much greater efficiency compared to a gasoline engine due to the much higher compression ratio. Diesel engines in automobiles can very easily be anywhere from 30 to 50% more efficient than a gasoline engine of similar power.

Sources of Diesel

            Originally diesel was made from vegetable oils, but because diesel refined from petroleum became cheaper, that became the main source of diesel fuel. In recent years especially the last decade biodiesel made from vegetable and animal fats has become a much more sought after commodity due to how much cheaper it is to make. Eight to ten years ago many restaurants would give away used cooking oil because it was a convenient way to get rid of it. Today however many restaurants realize the value of the used oil and sell it usually for around $1 per gallon. The majority of biodiesel being made today is made by hobbyists who have homemade apparatuses to carry out the chemical reactions to convert the oil into a usable fuel. The cost of making the biodiesel is typically around $1 per gallon also. So with roughly $2 per gallon spent in making the diesel it comes out to be much cheaper than the nearly $4 per gallon typically being paid at the pump.

How is biodiesel different from diesel made from petroleum?

Petroleum diesel

 
Biodiesel

            The cooking oil is reacted with an alcohol to create a long chain ester, which is biodiesel. Diesel from petroleum are just long alkanes.

Effects on the environment

            From a strictly monetary standpoint synthesizing your own biodiesel makes sense. From a standpoint of reducing your carbon footprint however it becomes more complicated. Looking at the amount of CO2 released in the combustion of biodiesel in grams of CO2 per kJ of energy, the combustion of biodiesel releases more CO2 than the combustion of methane. With methane producing .06 grams of CO2 per kJ of energy released and methyl acetate producing .1 grams of CO2 per kJ of energy released.

            Clearly from calculations alone it can be shown that the combustion of methane does not contribute to CO2 pollution more than the combustion of biodiesel, in fact it is the other way around. The only area where there could be an argument about the contribution one fuel versus the other makes to global warming and the amount of CO2 released into the atmosphere would be in the amount of energy required to covert each of these fuels into usable forms. Some might say that biodiesel has the upper hand in that it is already being made from waste products from restaurants. But making biodiesel in this manner is really only feasible on a small scale. Also even though the combustion of methane produces less CO2 than biodiesel methane itself is much worse for the environment than CO2 and biodiesel. If methane is not combusted full, methane will be released into the environment causing more polution.

Combustion reactions for different fuels

Methyl acetate CH3COOCH3 + 7O2 → CO2 +6CO2

Ethyl-methyl ester C2H5COOCH3 +H2O → 8H2O +4CO2

 

Methane CH4 +2 O2 →CO2 + 2 H2O

 

Thoughts:

Kody: I think that we should be phasing out the engine and moving to cleaner and better forms of energy rather than using the carbon based compounds from thousands of years ago. Given the damages and continuing of vehicular usage I think it’s going to be a while before it gets better.

As far as biodiesels go, I think it’s just a means to an end. It’s nice to reuse items but it’s still a combustion reaction with CO2 produced. Given the massive amount of cars combusting carbon compounds at any point during the day the amount released is staggering. I think it’s an engineering last attempt to keep the engines we have while new ones are being developed.

Matt: I think that since petroleum based fuels are not going away over night, more needs to be done to improve the efficiancy of the engine while at the same time decreasing the amount of emmisions given off by engines. I think more research needs to put into clean diesel engines in small cars.

For biodiesel i do not think it is fiesable on a large scale. i think the amount of energy that would be required to produce diesel from soybean oil would make it counterproductive and would probably just drive costs too high. I think it is a decent way for people to make their own on a small scale for a cheaper price than what would be paid at the pump. But biodiesel in no way really has a better impact on the environment than do petroleum based fuels.