Internal Combustion Engines And Fuel Cells

Aim

I will decide whether it would be viable for fuel cells to be a replacement for the internal combustion engine and analyse the three different types of fuel cells to decide which the best design is.

Summary

This report will discuss in detail how a fuel cell works and if it could be an appropriate replacement for the common internal combustion engine. From my research I found out that fuel cells aren't as futuristic and complicated as I first conceived and I will also analyse the three different forms of fuel cells in order to determine a suitable replacement. Though I believe that the fuel cell is a viable development and is far more environmentally friendly I will also put forward the argument for why we chose to use and continue to use the internal combustion engine.

Introduction

'There is only so far you can go with the internal combustion engine, and it is very important that we explore different directions' .

Every year this statement become truer as experts say that at most we have a 50 year supply of fossil fuels left and for the millions of people that use cars it is a very serious problem. Not only will we run out of petrol in the near future which gives Ј1.2 trillion to the market each year but our habitat is suffering heavily from air pollution from compounds such as carbon dioxide, nitrogen oxide and sulphur dioxide. This results in global warming and acid rain and with its continuation it could result in a very inhospitable place for future generations. This is why huge Trans National Companies have spent billions of pounds on research into alternatives to petrol as a fuel. This is why I am also going to examine one of the main contenders that could possibly remove the internal combustion engine.

The internal combustion engine

The petrol engine must be first examined to show why it is not a feasible option for the future. The combustion engine has two main problems:

* Its powered by fossil fuels which are non renewable

* It is highly inefficient

With regards to the first point there is the option of converting to a renewable fuel such as diesel oil made from plants or used chip pan oil however, this is very expensive and the combustion engine would be made even more inefficient by these fuels.

The combustion engine must be studied to show how it is inefficient. To do this a Carnot Engine must first be studied. This type of engine is the most efficient type and like a combustion engine it operates in a thermodynamic cycle called a Carnot cycle. This cycle, like an internal combustion engine, takes place between a hot and cold reservoir (tank). The Carnot cycle is simply:

The steps are as following:

1. A-B: The petrol is ignited by the spark plug causing the gas to expand,

causing the piston to do work. The gas expansion is propelled by the

transference of heat from the high temperature reservoir.

2. B-C: The gas continues to expand, causing the piston to do work

and so causing the gas to cool.

3. C-D: The surroundings now do work on the gas causing heat

to transfer from it to the low temperature reservoir.

4. D-A: The piston does work by compressing the gas causing its temperature to rise, bringing it back to the same state.

This cycle demonstrates how a perfect heat engine should work, but even this perfect theory of an engine is not 100% efficient and shows how only a limited amount of heat energy can be used. The Second Law of Thermodynamics (Carnot's Law) describes this. It states that with the use of two heat reservoirs a heat engine is unable to convert all of the heat supplied into mechanical energy.

This illustrates that even under ideal conditions, a heat engine can be severely limited. Its efficiency is given by Carnot's equation:

((T1 - T2) / T1) x 100

T1 - this is the temperature of the high temperature reservoir that uses the heat energy

T2 - this is the temperature of the low temperature reservoir (sink tank) where the rejected

heat energy is transferred.

However, as guessed, a real internal combustion engine does not work under ideal conditions, further reducing its efficiency. Its cycle demonstrates this:

This cycle is very different to a Carnot cycle. This is due to friction in the engine, resulting in less heat being transferred and therefore preventing ideal conditions at each step. In addition the entropy of a real material changes with temperature, which the Carnot cycle does not take into account and so the efficiency is further reduced.

From this equation the maximum efficiency of a combustion engine can be calculated:

The combustion temperature of petrol is 2300K (T1) and the average temperature of the sink tank is 925K (T2):

((2300 - 925) / 2300) x 100 = 59.78%

This demonstrates that the internal combustion engine only has a maximum efficiency of around 60%. However, this value is purely theoretical as due to many other energy loss factors the efficiency value is much less:

* Just in the engine around 38% of the power is lost through exhaust heat, 36% is lost through water heating and 6% is lost through motor friction. This leaves 20% of the original total being converted into mechanical energy .

* Conventional cars also lose much energy to friction when braking which just dissipates into the atmosphere.

* In addition aero-dynamic drag is also a large cause for energy loss as many cars on the road today are not streamlined.

All in all the overall efficiency of a car containing an internal