A series of lessons by David C

Dec 2010

Steam Engines

Part 10

The little engine

Now perhaps I’ve given you the impression that Newcomen’s steam

pumps created a revolution in the way things were done in England

in the 1700s.

If that’s the case, I’d better make

it very clear that the steam pump

was very limited in what it could

do, and was adopted only very

slowly

by the businessmen of the time.

It costs a lot of money to build a

steam pump, and if you’ve been

successfully running a business that

draws power from a water wheel for

all of your working life, why should

you want to change to steam?

It was generally the new startup

companies that bought Newcomen’s

pump, and these generally were

textile factories that made easy use

of the pumps’ up-and-down

thumping action.

Newcomen’s pumps were more like hammers than

engines. They’re good for hitting things, or pulling water

out of the ground, but they’re not very good for turning

things.

There were some attempts to get the

rising and falling piston to operate a

crank that would turn a wheel, but

these were always unsatisfactory

because of the pump’s jerky action.

Joseph Cugnot in France no doubt experienced this

problem when trying to drive his steam tractor in 1769.

The tractor would have lurched forwards staccato fashion

with each pulse from the two pistons; turning the vehicle

would have required disengaging one of the pistons

while the other continued to thump away on the other

side of the vehicle.

No wonder he crashed the tractor into a wall.

The strictly up-and-down motion of the

pump

was a major limitation to what it could

do.

Compare that with water wheels,

which had been in operation for

centuries. Whole industries had evolved

around the smooth, circular motion that

these wheels created.

So who really needed a steam pump,

other than the mine owners for whom

the machine had been invented?

Well, as you’ve seen in the last

lesson, there were a number of

people who saw possibilities for the

steam pump if only it could be made

a good deal smaller than it presently

was.

It would have been good for example

to have a small steam pump in a boat

to operate a set of paddles.

Still others saw the possibility of

driving a carriage across the ground,

as long as the pump was small

enough to sit on the wheels.

But Newcomen’s steam pumps were big for a reason.

First, they had to be strong enough to lift water out of the ground.

But just as important was their efficiency.

Large piston chambers don’t lose heat

to the outside world as quickly

as small chambers do.

Let’s suppose you had a small steam pump,

consisting of a small boiler and a small cylinder and a small

piston.

Let’s suppose the cylinder is only 10cm in diameter.

That means the steam in the cylinder can never be

more than 5cm from the wall of the cylinder.

Now steam is a pretty good insulator,

but it still means that about 20 percent of the steam

is within a centimeter of the cylinder wall

and can be affected by the outside temperature.

Compare that with a cylinder that is a metre in diameter.

Now only 2 percent of the steam is in contact with the cylinder

wall,

and that means only 2 percent of the steam is going to be

affected.

That’s just the beginning. Now consider what happens when

you inject a spray of cold water into the cylinder.

Of course, you want to cool the steam down so that it will

condense, but you don’t want to cool the cylinder wall down. If

the cylinder is a metre across, you have 98 percent of the

steam condensing without touching the cylinder wall.

But if the cylinder is just 10cm across, only 80 percent of the

steam can do that. The rest is either trickling down the

cylinder wall or picking up heat from the cylinder wall and not

condensing.

But if the cylinder is just 10cm across, only 80 percent of the

steam can do that. The rest is either trickling down the

cylinder wall or picking up heat from the cylinder wall and not

condensing.

Whichever is the case, you have a

machine that is going to start off well

but within a few beats of the piston is

going to reduce to a pathetic little

wobble that has no power in it at all.

Consequently there was a drive to

make steam pumps ever bigger, not

smaller, and this was becoming

increasingly easy to do with each

passing decade as iron-casting and

iron-cutting techniques improved.

To make steam pumps smaller didn’t

make an awful lot of economic sense.

This fact would have been well known

to the university students in Glasgow in the

1760s.

By this stage, 50 years after

the invention of the steam pump

it was possible to go to university

and learn how to build one.

The University of Glasgow had a

scale model of a Newcomen pump

that they would show to the

students, and anyone observing

this little thing in action got a very

clear lesson that little steam

engines simply didn’t work

because … well ... it didn’t work.

It was a nuisance to the lecturing

staff, however, because they wanted

this little model to demonstrate how a

steam engine ‘should’ work.

Rather than build a bigger model, they

asked the university’s lab technician if

he could tinker with it and make the

thing work a little better.

The lab technician was a young

fellow named James Watt, and by the

time he’d fixed that damned thing,

he had an idea that was going to

make him fabulously rich and make

him perhaps the best known

engineer who ever lived.

Young James’s solution was to fit a

second tank into the design; one kept

perpetually cold so that any steam

directed into it would condense very

quickly.

Naturally, this tank was to be called the

condenser.

Well, when you introduce steam to the

piston chamber, it of course pushes the

piston up as you’d want it to do.

But when you want to condense the

steam to bring the piston down again,

you no longer spray water into the

piston.

Now you open a valve that lets the

steam escape to the condenser, where

of course it condenses.

The advantage of this design is that the

piston chamber can be kept hot at all

times; you no longer lose heat by

spraying water into it.

If the piston is to be kept hot at all

times,

it makes sense now to put a blanket

around it so that no heat can

radiate off into the open air.

These and other minor

modifications

made the steam pump some 5

times more fuel-efficient than it had

ever been,

which inspired Watt very quickly to

visit a lawyer and slap a patent on

his modifications before anyone else

discovered them.

He couldn’t see it at the time

– nobody could –

but young James’s modifications made

all steam engines efficient, no matter

what size they were. That meant (in

theory) it was possible to build a steam

engine small enough to fit onto a set of

wheels and drive it down a road.

There would have to be a few more

modifications, however, before

‘locomotion’ was truly possible.

Interestingly, Mr Watt would be

responsible for all of those essential

modifications.

And most interesting of all, he

wouldn’t even know what those

modifications were leading to.

End

dtcoulson@gmail.com