Section 9 – A Steam Engine for the Twenty First Century

Such an engine, if it is to attract the interest of manufacturers of prime-movers in the small to medium power band, and particularly if it is to gain acceptance by the automobile industry, must possess the following features:

1. Exhaust emissions which can meet the increasingly rigorous limits demanded in the USA, EU and Japan.
2. Ability to achieve virtually zero emissions by using hydrogen as a fuel.
3. Power densities approaching those of internal combustion engines.
4. Noise output no worse than that emitted by internal combustion engines.
5. Mass production manufacturing costs similar to internal combustion engines.
6. Fuel consumption levels at least as good as gasoline engines and preferably better than diesel engines.

Based on the evidence of research work on steam engines for automobiles between 1970 and 2003, it is reasonably safe to assume that currently planned emissions levels can already be met by employing known combustion technologies. Using hydrogen as a fuel also does not present a problem in the steam engine.

Power densities for most of the prototypes of the 1970s were significantly lower than those achieved by internal combustion engines. One notable exception (Carter), however, demonstrated that a light weight engine running at speeds similar to those encountered in modern internal combustion engines is possible. It would be reasonable to assume, therefore, that acceptable power densities could be achieved today.

Internal combustion engines create noise as a result of the cyclic explosions of the combustion mixture. Since classical steam engines did not depend on explosions, they were remarkably quiet power plants. Indeed, Bessler’s steam powered aircraft was so quiet that the pilot was able to converse with people on the ground whilst flying past. Unfortunately, it is more difficult to suppress noise in a modern steam-engine concept. The California steam bus project showed that noise from some of the prototype demonstrator vehicles was no better than from contemporary diesel powered buses. The source of the noise was from the combustion air blower, condenser fans and from the combustor itself. If high-pressure water pumps are employed, these too can be a source of noise. Nevertheless, by close attention to design details of the auxiliary systems, there is no reason why a modern steam engine should be any noisier than its internal combustion rival.

No modern steam engine has ever been mass produced and the prototypes of the 1970s could not have matched the costs of manufacture of contemporary internal combustion engines. Their valve and control systems were much more complicated and they required more mass of materials because they were that much bigger. Since then, however, the cost of the internal combustion engine has doubled by the addition of emission reduction systems, as well as electronic control and injection systems. Bearing this in mind and also acknowledging the fact that at least one prototype high-speed small automobile engine was made, it is now certain that a steam engine, if it were to be put into mass production, would find it easier to rival the mass production costs of the internal combustion engine.
This leaves the most difficult problem of them all – fuel consumption. The Basic Rankine cycle of the steam engine is less efficient in extracting thermal energy than is the Carnot or Diesel cycle of the internal combustion engine. To argue otherwise would be to argue against the laws of thermodynamics. Quite simply, the expansion process in the cylinder of the internal combustion engine extracts more thermal energy than does the expansion process in the steam engine. Of course, both the steam engine and the internal combustion engine waste most of the energy of the fuel they burn by discharging it as heat to atmosphere. The internal combustion engine loses heat to the exhaust gases and the radiator while the steam engine loses heat to the exhaust gases and to the condenser.

The problem facing the mobile piston steam-engine until now is that it wastes two to four times the heat energy that is wasted by the internal combustion engine. Due to the fact, however, that it relies on external combustion, the steam engine has the potential to recover waste heat from the steam and from the combustion gases and add it back as energy on the inlet side of the engine. This is something that the gasoline or diesel engine cannot do because of the substantial loss of volumetric efficiency which this would entail. The heat produced by the internal combustion engine is therefore irrecoverably wasted whilst that in the steam engine is substantially recoverable.

It is this characteristic of the steam engine which redeems it. Heat recovery from the condenser as well as from an exhaust combustion gas heat exchanger is used to pre-heat feed water and combustion air. As the steam engine uses ‘external combustion’, volumetric efficiency is not an issue. The only limit to heat recovery is the effectiveness of the heat- exchange systems. In classical steam engines such systems were bulky and added significant cost. They were altogether uneconomic and impractical is such applications as the steam locomotive. Whilst the Rankine cycle itself limits the extraction of heat energy in the working cycle of the steam engine, there is practically no limit to re-using waste heat in this type of machine.

Fig. 3 Energy Flow Comparison between Internal Combustion Engine and Steam Engine with Efficient Heat-Recovery System

This can be seen by comparing energy flows in the steam and internal combustion engines. In this example, the assumption is that both engines consume the same amount of fuel and both give the same power output and that energy losses due to friction, noise etc are ignored.
Referring to Fig. 3:

E = Fuel energy
W = Work extracted by expander
Qe = Heat energy to exhaust
Qr = Heat extracted from condenser and combustion gases

In the case of the internal combustion engine, the fuel energy (E) results in work extracted by the expander (W) and waste heat (Qe) dumped by the exhaust and cooling system. So that quite simply:
E = W + Qe

In the case of the steam engine, the same external values and therefore the same equation applies but with the superimposition of a heat recovery flow. Recovered heat (Qr) is added to the same fuel energy (E’) and produces the same work (W) as in the internal combustion engine in this example. Qr is then extracted from the exhaust steam and combustion gases for re-circulation, giving the same waste heat value (Qs). This is expressed as:
E = W + Qe where Qe = Qs +Qr
Qs – real heat loss
Qr – heat re-covered from Qs

E = W +Qs + Qr
E – Qr = W + Qs
E’ = W + Qs where E’ < E and Qs < Qe

The Rankine steam cycle extracts less energy from the fuel in the expander than the internal combustion cycle does for the same work. In the above example, however, this deficiency is compensated for by adding a system which recovers an amount of heat from the steam and combustion exhaust gas which is equal in value to the difference in the energy extraction capacity of the two cycles.

To achieve this, an efficient condenser and/or combustion gas heat-recovery system must be employed and it is the execution of such a system, of an acceptable size and at an acceptable cost, that is now in sight. Heat-exchanger technologies have advanced considerably since the 1970s and offer the possibility of a compact system for the steam engine.

Employment of such a condenser together with a flash-steam generator and a modern combustion system in an engine monitored and controlled by an ECU (Electronic Control Unit), which emulates latest internal combustion engine control practice, allows complete flexibility to optimize the operation of a steam engine.

The technological limitations of the internal combustion engine which have been exposed by environmental concerns and legislation have galvanized the engineering world to re-think its approach to prime-movers. The application of twenty first century technologies to the steam engine now make it a serious candidate in the race to replace the internal combustion engine in this role.

>> Additional Information

   Copyright  © 2004 Huzar Power Ltd. All rights reserved.