Last week in
Alternative Cars, Part 4
we looked at the possibility of cars powered by humans. This week, we look at
the potential for steam-powered cars.
In the first few decades of last century,
steam-powered cars were not uncommon. Like battery-electric cars, at times it
appeared in fact as if they would triumph over cars powered by internal
combustion engines. If battery-electric cars were seen as quiet and
sophisticated, steam cars were strong and effective – and unlike the new-fangled
and temperamental internal combustion engine cars, used technology already
largely familiar to the public from railway steam locomotives.
Old steam cars tend to be the butt of jokes made
much later in the history of automotive endeavour – the Stanley Steamer being
often cited as the archetypical snail. But with a 1906 outright world record
speed of 127.66 mph (205.5 km/h), not all of Mr Stanley’s steamers were slow...
So is it worthwhile revisiting steam cars? Let’s
take a look.
Function
Steam cars use external combustion engines, that
is, combustion of the fuel occurs outside of the cylinders. Fuel is burned to
raise the temperature of water that subsequently changes state to steam. As more
steam is produced, it rises in pressure. The steam is then fed to a
reciprocating engine where the pressure of the steam forces the piston to move.
A connecting rod joins the moving piston to a crankshaft that in turn drives the
wheels.
However, this description, while not wrong, is
likely to give rise to quite incorrect notions. For example, one tends to
picture a kettle boiling on a gas stove – that is, a relatively large quantity
of water heated by a flame. In fact, even at the time of the steam car’s
greatest popularity nearly 100 years ago, flash steam boilers (where a
relatively small quantity of water passes through long, heated tubes, so
changing to steam as it progresses) were being used. This design reduces the
time for a cold start-up to well under 60 seconds.
A flash steam boiler has additional advantages –
the required strength to withstand high pressures is more easily achieved in a
small diameter tube (and not a large diameter boiler), and with much less water
volume being heated at any one time, a dangerous boiler explosion is much less
likely. Additional heat can also be added to the steam through the use of
superheaters – a heat exchanger that uses the burning fuel to further increase
the temperature of the steam – so reducing the amount of condensation that
occurs as the steam moves to the cylinder.
Multi-piston engines can be used, which smooths
the torque pulsations and allows more heat energy to be extracted from the steam
through multi-staging. In addition, the valve timing of piston steam engines
(even railway locomotives) has always been variable. Usually termed “cut-off”
(as in “10 per cent cut-off”), the variability in valve timing allows for an
early closing of the intake valve, so reducing the mass of steam admitted to the
cylinder and allowing it to do work through expansion. This reduces torque but
improves efficiency.
In automotive uses, a condenser was added to
massively decrease the consumption of water. Rather than the exhaust steam going
up the chimney to provide a draft for the fire (as occurs in steam locomotives),
the draft for the burner was provided by a mechanical or electric fan and the
steam was condensed back into water for re-use.
The greatest efficiencies are gained for steam
engines when the working pressure is highest, superheating is greatest and
condensing pressures are lowest – normally well below atmospheric pressure. (The
latter is achieved by careful design of the steam engine, such that the exhaust
steam pressure is as low as possible. If air is excluded from the system, the
pressure in the condenser will automatically be much lower than ambient.)
Advantages
A major advantage of reciprocating steam engines
is that, for a given engine, as the cut-off of steam is made earlier, efficiency
improves. For example, a 4 cylinder, 4.3 litre steam engine with a feed pressure
of 1000 psi might have a thermal efficiency of near 30 per cent when working
with a cut-off of 5 per cent (and develop 85 bhp), grading to an efficiency of 6
per cent at 70 per cent cut-off (and develop nearly 1200hp!)* So, completely
unlike a conventional throttled internal combustion engine, efficiency actually
improves at the small loads most commonly encountered in car use.
Secondly, this variability in power output, and
the ability to be able to temporarily overload a steam engine (that is, for a
short period, use up steam more quickly than it is being produced) allows even
small but strong engines (say under 1 litre in capacity) to produce a very short
tem output of several hundred horsepower.
Reciprocating steam engines develop maximum torque
at a standstill. One steam car of 1920 could accelerate without a gearchange
from zero to 150 km/h. Given that current transmissions have a volume and mass
perhaps half that of the engine, packaging improvements could be made.
Another advantage is that the combustion can be
very tightly controlled. Compared with an internal combustion engines, where the
volume, pressure and flame behaviour are always varying, an external combustion
engine varies only in the mass of fuel being burned. This implies that emissions
could be reduced.
Finally, a wide variety of fuels can be used.
These include gaseous fuels like LPG and CNG, and liquids like kerosene and
petrol.
In terms of pure efficiency, it is unlikely in the
short-term that reciprocating steam engines would better conventional internal
combustion engines. But the lack of a gearbox, ability to burn a variety of
fuels (including low octane fuels) and the achieving of highest efficiencies at
lowest loads may well change the ‘real world’ economy results.
Disadvantages
However, there are also distinct
disadvantages to automotive steam power.
As steam cut-off is reduced, the torque variation
per power stroke increases. This is because when steam is first admitted, full
pressure is felt within the cylinder; as the steam expands, the pressure drops.
Therefore, a smooth engine running short cut-off would need multiple cylinders,
so increasing frictional losses and cost.
The higher the pressure of steam that is used, the
stronger the boiler, feed tubes and engine need to be made. In addition, higher
boiler pressures mean the condensed steam takes up a greater volume, requiring a
larger condenser. Very high temperature steam creates problems in lubricating
the cylinders; if the lubricant is mixed with the steam, it needs to be removed
before the water is reheated.
Even a flash-steam boiler will take a finite time
before steam pressure is available.
Finally, while the engine doesn’t need to idle at
traffic lights, steam pressure needs to be maintained – the longer the pauses in
stop-go city traffic, the lower the efficiency.
Solution
When reading about the 1920s steam powered cars,
one is struck by how current progress in metallurgy and electronic control
systems lend themselves very well to facilitating the development of modern-day
steam cars. The control systems for fuel flow, water flow, steam flow and
cut-off were all then mechanical. Aspects such as predicting steam consumption
were difficult.
These days, electronic control of variable flow
solenoid valves could easily control fluid flows, and the mapping techniques and
self-learning developed for internal combustion engine management units would
lend themselves to these control systems. Even exhaust oxygen sensor feedback is
directly applicable to the combustion of the fuel.
Common rail diesel fuel injection systems now use
pressures that are enormous. Certainly, diesel fuel systems do not have to
withstand high temperatures as well, but the metallurgical develops in these
systems would have application to high pressure steam.
It would seem that the following steam engine may
prove successful:
-
A small capacity (eg 1 litre) multi-cylinder
engine (eg six cylinder) engine working at a high steam pressure (eg 1500+
psi)
-
Flash steam boiler
-
Full electronic control of water, fuel, and steam
flow
-
Electronically controlled variable cut-off from
0-70 per cent
-
Direct drive to the wheels
Conclusion
Of all the automotive engine developments that are
currently mentioned, the reciprocating steam engine attracts the least notice.
But with its combination of high torque at low speed, increasing efficiency as
the power output of a given engine decreases, full control over combustion, and
versatility of fuels, perhaps it’s time for another look.
*Ayres, R & McKenna, R, Alternatives to the
Internal Combustion Engine, John Hopkins University Press, 1972. ISBN 0 8018
1369 7
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