Section 4 – Emissions and the Steam Engine

So the steam engine attracted the attention of the automobile industry again when environmental concerns over air quality in cities became a significant issue. The reason for this renewed interest is simple. The combustion process in a steam engine is external and continuous, making it easier to achieve close to ideal burning at a temperature of 1700ºK, which results in much smaller emissions of NOx, HC and particulates. By contrast, in the internal combustion engine, the burning occurs within a cylinder and is cyclical. Here, an explosive mixture of fuel and air is repeatedly ignited, quenched and exhausted in a time of fifty milliseconds or less. The gases initially burn in the cylinder at around 3000ºK, a temperature which encourages the formation of oxides of nitrogen. After the initial explosion, the hot combustion gases are cooled by the cylinder walls to around 500ºK and this, in turn, induces the formation of carbon monoxide and unburned hydrocarbons. Finally, the short time available in the whole internal combustion cycle makes it difficult to complete the combustion process fully, and soot or particulate material (PM) is emitted – a problem which particularly affects diesel engines.

It is for the purpose of reducing emissions that makers of internal combustion engines have been obliged to modify their products and install systems to meet legally enforced environmental regulations. These have progressively involved the fitting of catalytic converters, fuel injection systems and electronic ignition. However, the challenge to the internal combustion engine, particularly as a power source for automobiles, is becoming more difficult to face year by year as the legislation-driven emissions regulations become more stringent. The following tables demonstrate the scale of the problem. Emissions from new automobiles sold in 2005 will have to be around half those of vehicles sold since 2000 to meet the new rules.

EU emissions limits for spark-ignition engines

Standards	Introduction	CO g/km	HC g/km	Nox g/km	HC+Nox g/km
EU Stage 1	1992	2.72	-	-	0.97
EU Stage II	0996	2.2	-	-	0.5
EU Stage III	2000	2.3	0.2	0.15	-
EU Stage IV	2005	1.0	0.1	0.08	-

EU emissions limits for diesel engined passenger and light commercial vehicles

Standards	Introduction	CO g/km	Nox g/km	PM g/km	HC+Nox g/km
Euro 1	1992	2.72	-	0.2	1.75
Euro II	1995	1.0	-	0.15	1.00
Euro III	2000	0.5	0.5	0.05	0.5
Euro IV	2005	0.4	0.25	0.025	0.3

Limits will be lowered further from 2007

When electronics, injection systems, catalytic converters and filters are added together, the cost of emissions reduction systems is (in 2003) already of the order of fifty percent of the cost of a new engine. As automobile industry researchers push combustion technology and exhaust-gas conditioning systems to their limit, it is becoming increasingly difficult to see where further improvements to satisfy environmental concerns can be made. These costly systems are in effect ‘crutches’ to support a form of propulsion which has probably reached its technological limits. For this reason there is growing interest in more radical approaches. Electric vehicles powered by diesel-electric engines such as the Honda Insight offer one approach. The diesel engine used in this hybrid operates at constant speed which permits optimization of the combustion process and makes it easier to achieve tighter emission limits. The complexity, size and first-cost of a diesel engine, alternator and electric motor combination, however, have invited another look at alternatives such as steam engines which were rejected for these very same reasons several decades ago.

Much lower emissions from the steam engine are the starting point to the argument for taking another look at this form of power. The other relevant factor is cost. As the cost of the internal combustion engine has climbed to accommodate emissions reduction systems, so the cost gap between it and a potential volume-produced steam engine which does not require such systems has closed. These are two very important points leading the argument for re-considering the steam engine as a candidate for small to medium prime movers. When these points are added to the fact that new materials, technologies and innovations now make it possible to build a high-speed steam engine with similar power densities to an internal combustion engine, the idea becomes even more interesting. Finally, when cost reductions through mass production techniques are applied, the steam engine becomes a very strong candidate to compete in the prime-mover market place in the twenty first century.

>> Steam automobile engine research since the 1930s (Section 5)