Engines may steal the limelight. But it’s the humble piston that really handles all the pressure and drives cars, bikes, boats, trains, planes, cranes, and big industrial mach-anes. Today, we’re going to look into the history of the engine piston and how it ultimately ignited the development of the modern world.
But before we take the proverbial rings off and strip the piston back to its early ancestors, let’s take a moment to revel in its reciprocating simplicity.
Introducing the piston (again)
A crown. A skirt. Two or three rings. Maybe a little bit of engine oil. And a broken con rod in hand. My partner was right. This year’s Halloween costume does sound a lot like a piston. Because in essence, these are the mechanical components that help convert energy into motion. And as most of us know, they play an important role in internal combustion engines and hydraulic and pneumatic systems.
Whilst on paper, their job might sound pretty linear, it’s actually full of ups and downs. In fact, inside an internal combustion engine, there are four types of ups and downs that turn chemical energy into rotary motion.
The rise and fall of the engine piston
There are four distinct stages (or strokes) of piston movements. And in a four-cylinder engine, for example, each piston does a different stroke at the same time, so power stays smooth.
- Intake stroke. The piston moves down, sucking in air and fuel through the intake valve.
- Compression stroke. The piston moves up and compresses the air-fuel mixture ready for ignition.
- Power stroke. The spark plug does its job, and the mixture combusts, forcing the piston down and generating POWERRR.
- Exhaust stroke. Now, the piston moves up again, letting the exhaust gases run free via the exhaust valve.
A two-stroke engine follows the same stages but combines the compression and power strokes and the exhaust and intake strokes. So, as the piston moves up, the air-fuel mix compresses. Near the top, the spark ignites, and an explosion forces it down, generating the POWERRR and letting out the exhaust gases whilst drawing in new air and fuel.
And, of course, diesel engines have no spark. Instead, they rely on more compression (hence the higher compression ratios vs petrol engines) to combust the air-fuel mix, meaning the piston is exposed to more loads.
However, internal combustion engines differ in size and application. So, as you’d probably expect, the shape and size of the pistons must differ too. Let’s explore more.
Piston design: Rising to the occasion
So yes, as engines differ, pistons differ. And the shape and size of the piston can dramatically alter performance. I briefly touched on it with my Halloween costume, but there are four main parts to the piston.
We have the crown (or head). This is where the combustion occurs. The skirt is like a handleless mug that guides the piston inside the cylinder. Next, the piston rings, of which there are two types: compression and oil. These seal the piston to the cylinder wall and stop excess oil and general combustion gunk from leaking into other parts of the engine. And then the piston pin. This connects the piston to the connecting rod so the reciprocating motion transfers to the crankshaft.
Types of engine pistons
Flat-top pistons
These are flat on top (shock) and create efficient combustion distribution, allowing higher compression ratios. They’re pretty common and used in many petrol engines.
Dome (convex) pistons
These have a raised centre, increasing the surface area of the piston head, which decreases the combustion chamber volume. This then raises the compression ratio (think more energy from air-fuel mixture). Racing engines often have them because they produce more POWERRR.
Dish (concave) pistons
These ‘curl’ upwards and increase the combustion chamber volume, lowering the compression ratio. Turbo or supercharged engines can use them because they create more ‘boost pressure potential’. (I don’t think that’s the technical term, though)
Trunk pistons
These are often used in large engines with lower compression ratios. The longer skirt helps stabilise the piston so it can travel longer distances more efficiently. It also helps distribute some loadings, helping them last longer.
Slipper pistons
On the flip side, or should I say slip side, we have slipper pistons. These have a smaller skirt and focus more on weight and friction reduction, allowing higher engine speeds and better performance in high-revving engines.
Here’s a side by side guide on how they look.
Just how big (and small) do pistons get?
So one of the biggest (if not the biggest) internal combustion engines is the Wärtsilä RT-Flex96C engine. Standing at a monstrous 13.5m tall and producing 109,000bhp and 7,600,000Nm of torque across 14 cylinders. So, the pistons are pretty big too… Each one weighs over 5 tonnes, is ~6 metres long and has a bore diameter of 960mm. Here’s a video sharing more.
Now, at the other end of the scale, the alleged smallest V12 engine has a bore diameter of 11.3mm and a stroke of 10mm. Or smaller still, the George Luhrs single-cylinder engine has a bore diameter of ~3.2mm and stroke of ~4mm.
So, shapes and sizes vary!
But they didn’t just come out of nowhere. There is a path to their development. And if there’s one thing we know about pistons, it’s that they always (well, almost always) rise to the occasion.
The engine piston’s rise to POWER
The pistons' early ups and downs date back to 150 BC when it was supposedly used in air-pumping systems for metal working (furnaces). From then, piston developments seemed a little slow because the next recorded development arrived in the form of a steam engine - in the 18th century.
Nicolas-Joseph Cugnot, a French military engineer, produced a steam-powered vehicle in 1769, becoming one of the first to use piston-driven POWERRR. Although, it was allegedly a little unpredictable in its direction and speed.
However, like a big locomotive, once developments started to roll, they became hard to slow down.
Streets ahead
In 1794, Robert Street designed a single-cylinder engine that had a coal-fired furnace at the bottom and a cooling jacket at the top. This was allegedly the first liquid fuel (petrol) engine.
Fuel was fed into the cylinder, and air was pumped manually so the piston would rise. The coal-fired furnace would evaporate the fuel, igniting it and then pushing the piston up to drive a rocking beam connected to dewatering pumps in coal mines. As the piston cooled, gravity lowered it, pulling the beam and powering the pump’s plunger on the opposite end.
Slow, yes. But important nonetheless.
Running on fumes
Until now, engines ran on coal burned outside of the cylinder (in a furnace), making them external combustion engines.
Combustion took its first steps inside the engine block when Samuel Morley invented his “gas engine” in 1826. Despite being very inefficient, his design ignited fuel (a coal gas and air mixture), resulting in a cylinder pressure boost. A first for the time.
Although, I think we need a special pat on the back to Robert Boyle, who discovered the underlying gas principles here in 1662.
Boyle’s Law states that at a constant temperature, the volume of gas is inversely proportional to the pressure exerted by the gas. Or, put differently, a gas will fit inside an enclosed space, but as it shrinks, pressure increases.
Replace enclosed space with 'cylinder', and you almost have Morley’s discovery.
Keep this in mind, because not long after Samuel Morley’s engine design, Benoît Paul Émile Clapeyron entered the scene. Here’s what he discovered…
The Ideal situation
In 1834, 8 years after Morley’s engine design, Benoît Paul Émile Clapeyron, a French engineer, linked Boyle’s Law with other pre-existing gas laws. He proposed a new one that helped us better understand the behaviour of gases and the impact of gas temperature in enclosed spaces. What he proposed was the Ideal Gas Law. (See here for its derivation)
If we rearrange for P, pressure, we can see that as compression occurs and volume decreases, pressure will increase. If you then consider that as the fuel ignites, temperature also increases then pressure will rise even faster, meaning a heck of a lot more POWERRR.
Whose shoulders does the compression fall on? That’s right. Our humble piston.
A seal of approval
So far, most of our timeline has followed engine developments. But in 1852, John Ramsbottom invented the piston ring, replacing mostly hemp and leather equivalents.
A step in the right direction, sure, but it led to uneven wear. So he improved it.
And in 1855, Ramsbottom’s split piston ring hit the market, becoming a standard in reciprocating engines.
No compress. More impress.
Étienne Lenoir raised the piston game in 1860 and invented the first two stroke engine. For its time, the design was pretty special with the piston playing an important, double-acting role.
Unlike modern engines, Lenoir’s didn’t compress the fuel mixture before ignition and it also had two power strokes for each up-and-down cycle. In other words, a power stroke at either end. A doubly efficient way. These designs allegedly followed those of Philippe Lebon’s (from 1801).
Pat, pat, pat, pat (4 strokes…)
In 1862, Alphonse Beau de Rochas theorised the ideal conditions for an efficient internal combustion engine. Within it was a four-stroke cycle sequence: intake, compression, power and exhaust. Unlike Lenoir’s two-stroke engine in 1860, de Rochas suggested four for optimal efficiency.
Nikolaus Otto caught drift and, in 1876, developed the first four-stroke engine, completely changing the potential of an engine and piston partnership. (It's maybe worth noting that Otto also built a 2 stroke gas engine in 1861 that did well, winning Gold at the 1867 Paris Exposition!)
Around the Benz
Next, Karl Benz. In 1879, he developed his first two-stroke engine and was granted a patent in 1880 for it. He commercialised it, developed it, and ultimately then parked it.
Because in 1886, he took what he knew and patented the first ‘practical’ petrol-powered car, using a single-cylinder four-stroke engine and producing a staggering ~0.7hp (yes, there is a decimal point in there!).
From wheels… to wings and wheels
With an engine that works and successfully moves people without exploding, it’s only human nature to see if you could put it in the air, right? The Wright brothers thought so, and their plane, the Wright Flyer, used a similar four-stroke engine in 1903 - it also used an aluminium engine block!
You see, up until now, pistons were mostly made from heavy ol’ cast iron. But as the demand for engines grew, so did the demand for performance improvements. And reducing the weight of components seemed an intuitive area to explore.
In 1920, Karl Schmidt developed the first pistons from an aluminium alloy (an Al-Cu alloy that’s used in aviation). However, folks soon discovered (when their engines broke) that the Al-Cu alloy they were using was rather brittle, thanks to the high iron content. So iron left and nickel and cobalt entered, improving the elasticity.
The power curve
It’s difficult to pinpoint when exactly, but around this time, piston heads started to change shape. For example, car racing was becoming more popular, and dome pistons started to crop up in engines more regularly (likely for their POWERRR potential?)
And like it did with aluminium adoption, WW2 also provoked rapid developments in IC engines. Aerospace engineers made them more efficient and more powerful - only to drop them completely and let jet engines take over (in bigger, faster planes, at least).
More pistons in more pies
So, now four-stroke engines were well established. They were spreading into more areas of life, and more engineers could get their hands on them. And like we always do, we change stuff.
Material science was still catching up, and there was a transition away from aluminium pistons. Because of the higher forces and temperatures with diesel (and some high-performance) engines, pistons were made from two materials.
The head was a steel alloy, and the skirt was aluminium. Steel offered better strength, wear resistance and thermal stability, whereas aluminium helped keep the weight down. The same market also saw pistons with longer skirts to help stabilise the side loads and better cooling systems.
Standing strong under pressure
Fast forward to today, and piston designs are still changing. We’ve since had the introduction of short skirt (slipper) pistons, reducing weight and friction and, hence, improving engine efficiency and performance.
Ovality and barrel contouring have also become more popular. This further helps to control the effects of thermal expansion and friction.
Heck, we’re even using more of them at once, too. V8s have come (and gone?...), 9-cylinder radials in aircraft, W12s and even the monstrous RT-Flex96C engine. Pistons have come a long way, and I’m sure they’ll continue to be tweaked, improved and tailored to more niche applications.
But what’s interesting, for me, at least, is that alongside the development of pistons and their engines, a completely different means for mechanical motion was occurring…
From bangs to hisses
Whilst internal combustion engines rely on lots of mini explosions to move pistons, pneumatic systems use compressed air to move them. And their hiss-tory is just as interesting. So, it’s time for a crash course in pneumatic systems.
Blown away
Pneumatic systems look as though they began with our rather hairy, primitive hunter-gatherer cousins. They used them as blowguns to help them hunt! (Apparently delivering a pressure of 1-3 psi…).
At the time, I don’t think they were identified as pneumatic tools because the word pneumatic derives from the word “Pneuma”, which in ancient Greek means ‘wind’.
And that’s rather fitting because the Greek mathematician, Hero of Alexandria, invented some of the first ever truly pneumatic devices. One of which was the ‘aeolophile’, often regarded as one of the first ever steam engines. (Although I don’t think it looked much like they did in the 1900s…). Nevertheless, air pressure created rotational motion, and this opened the door to how thermal energy could become mechanical.
Take my breath away
Moving forward into the 17th century, Otto von Guericke removed the air and created a revolutionary vacuum pump in 1650. It created a partial vacuum, which is cool in itself but even cooler because it let others study air pressure in more detail, in particular its impact on respiration and combustion.
Through the Industrial Revolution, pneumatics progressed further. And a lot of theories became reality, changing how we would later use (or could use) pneumatics. For example, Alfred Beach, the American inventor, built a pneumatic subway train in New York in 1867.
Shortly after, in 1871, Samuel Ingersoll invented the pneumatic drill (great for mining) and Charles Brady King the pneumatic hammer in 1890.
The bright PSIde of life
Of course, the developments have continued through the 20th and 21st centuries, where many pneumatic systems find themselves slyly hidden in lots of manufacturing and everyday uses.
So, whereas you might find an internal combustion engine piston in the engine of a car, bus or train, you might find pneumatic pistons in brake and suspension systems or even the doors! Of course, pneumatics have expanded beyond just the automotive - you’ll find them in material handling, food processing and fancy gaming setups and maybe even some really impressive tool collections.
Maybe that now begs the question…
What’s the differences between pneumatic pistons and internal combustion engine pistons?
It’s probably easier to start with the similarities between pneumatic pistons and IC pistons. They’re both pistons. So they both turn pressure into repetitive up and down motions - usually as a means of moving something else (e.g. wheels, actuators, tools). They also both require sealants, either piston rings or O-rings. But beyond that, they differ.
Generally speaking, you’ll use pneumatic systems for quick, repetitive tasks (e.g. assembly lines, material handling, food production and tooling – in fact, air's natural ‘compressibility’ makes it particularly great for shock absorption). On the other hand, you’ll use internal combustion engines for applications that need continuous power (e.g. vehicles and heavy machinery).
Of course, compressed air is also safer than fuels. I’d rather smoke my food myself, I think…
So, whilst they work in a similar way, the pistons serve very different reciprocating roles.
A Pneu Type of Piston
Now, as we know pistons and pneumatic systems are used all over the place. But the team here at MetMo have spotted a gap (or a vacuum, if you will) in the market. Because there’s one more area where pistons and pneumatics could take over… together… an area that sits very close to our heart. And one that’s come from a lot of dangerous and dare we say hiss-terical situations…
For now though, watch this space and be prepared to be blown away. Speak to you soon.