environmentalism and car safety

It is taken for granted that use of hydrocarbon fuels are harmful to the climate and this has to be reduced.  However, it is very difficult to make a car which is not powered by hydrocarbons and still gives acceptable performance.  Tesla is a noticeable exception to this.  Recent crash tests have shown the incredibly good performance of the Tesla sedan.  Lacking the technology of Tesla, and with a desire to reduce the price point of the car, manufacturers have taken to reducing the weight of the car.  This would help the underpowered motor handle the everyday tasks expected of the car.  However, the weight reduction might come at the expense of structural safety.  Add to this the classification of quadricycles, which have no safety requirements whatsoever, and the well meaning environmentalist is in danger of going extinct.

“air” engines

Every once in a while, one hears about a cool new air engine, which will solve all our transportation problems.  Consider an engine which is supplied by a 200 bar cylinder.  Suppose this cylinder is about the size of a small bike tank, say 10 L.  It is fairly easy to show that this amount of air is equivalent to about 5 g of gasoline.  That being said, a bike would consume only around 0.1 g/s, so that this tank of gas would be enough to run the machine for an hour or so.  Here I have assumed that the air can be expanded adiabatically to 1 bar (atmospheric pressure).  However, note that this will mean that the final temperature of 300 / 200 ^ (0.4/2.4) = 66 K.  This is much below the atmospheric pressure boiling point of nitrogen and air.  In actual practice, the power will be generated by a positive displacement device like a vaned rotor or a piston engine.  The energy input to the air can be increased by a solar concentrator heating the compressed air cylinder.  This would reduce the work required to fill the compressed air cylinder, and at the same time provide more margins on the outlet temperature.  The upper limit on the heating is the material structural margins.  If super alloys like inconel are used, then temperatures can be fairly high.  However, if we are restricted to mild steel due to cost considerations, then the temperature limit will be much lower.  Also, mild steel will have to be replaced after repeated thermal cycling due to property degradation.

Stirling Engine Applications

Introduction

Stirling engine is something completely different from the internal combustion engines which are very common today. Like the electric motor at the beginning of the twentieth century, the Stirling engine fared quite well compared to the steam engine in the early 18th century. However, due to the refinements in the steam technology, and the later invention of the internal combustion engine, the sales suffered.

Just a Toy?

There are two main types of Stirling engines which are used today—the toy low temperature differential engine, and the more conventional external combustion or solar powered engine. Some of the toy engines can even be powered by the heat produced by the human body. However, there is usually no path from this kind of engine to one that can be put to some practical use. I have found many stores selling such model engines, but none that produce any useful power. Typically, these model engines drive a flywheel, where the only torque that they have to overcome is the frictional resistance.

Advantages of the Old Style engines

One feature of the older Stirling engines compared to the internal combustion engines was the very low power to weight ratio. This meant that the engine, even though it had a large displacement, produced very little power. An engine putting out less than 30 W could easily weigh over 100 kg, and overcoming friction with such a low power output is a great challenge. However, an advantage of operating an engine of this nature was the relatively low stresses that acted on it. This meant that material of lower quality, and also cheaper, more primitive manufacturing methods could be used. The absence of an explosive power stroke also meant that the engine was quieter and ran trouble free. The same cannot be said for the newer engines, which have higher power to weight ratio, use exotic materials and work with gases like hydrogen and helium instead of air. As you may expect, the engineering challenges in such a design poses are tremendous.

The Stirling Engine was invented by Robert Stirling (1790-1878) , a minister in the Church of Scotland, in the early 19th century. It is a heat engine working on the principle of a closed cycle, i.e., the cyclic expansion and compression of a working fluid (air in many cases). The working fluid undergoes the Stirling cycle, which consists of two constant volume processes and two isothermal processes. The efficiency of a Stirling engine is the same as that of a Carnot engine working between the same temperatures. The closely related Ericsson cycle replaces the two constant volume processes with constant pressure processes.

Theory of Stirling Engines

The key feature in a Stirling engine is the regenerator, which is usually a metal matrix, made of wires. Regeneration is used in many engines—for instance, in steam engines it is used to increase the temperature of the input water using the energy of the exhaust steam. This makes the steam easier to condense, and lesser amount of heat has to be supplied to heat the water. A regenerator is also used in open cycle gas turbines based on the Brayton cycle. In the Stirling engine, the regenerator connects the high temperature expansion space to the low temperature compression space. Consider the case where the compression space piston is at the BDC, and the expansion space piston is at the TDC. In this case, all the fluid is in the compression space, and is at the lowest temperature. The volume is maximum. During compression, the compression piston moves towards the TDC, while the expansion piston remains at the TDC. Heat is rejected from the compression space to the surroundings. In the transfer process, the compression piston keeps moving towards the TDC, while the expansion piston moves away from the TDC, so that the volume remains constant. The fluid moves past the regenerator, gaining heat, and enters the expansion space with temperature T_max. In the expansion process, the expansion piston moves towards the BDC, while the compression piston remains stationary. The temperature remains constant as heat is added from an external source. In the final stage, the transfer of fluid back to the compression space takes place at constant volume. During this process, the regenerator cools the fluid by absorbing heat, which is later gives back to the fluid when it is transferred from the compression space to the expansion space. Thus, there is not net gain or loss of heat by the regenerator. Clearly, for a given swept volume and pressure ratio, the work done by a Stirling engine is much larger. However, the efficiency of a Stirling engine is the same as that of a Carnot engine, namely η = 1 – T₂/T₁.

Practical Issues

The power output for a Stirling engine is quite low as the pressure ratio increase beyond 2 is very difficult without the use of valves. The primary difficulty is reducing the dead space in the regenerator. A very low value of this dead space leads to inadequate heat transfer and fluid friction losses.

Net Energy for a Stirling Engine

Stirling engines can use any form of heat to produce mechanical work. The use of a Stirling engine does not heat a room compared to an electric fan. If you have an electric fan circulating air throughout the room, then electrical energy coming in through the wires is converted to mechanical energy. This can only increase the energy content in the room, ultimately increasing the temperature. If you cannot use a heat pump, it would be best to use such an engine (in the form of a fan), it will not cool the room, since the work that is produced by the engine is eventually converted to heat energy inside the room (first law). A Stirling cooler was used by MSI in one of their systems in CeBIT recently.