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Breakthrough: The overhead camshaft – Part two
2 years ago
Today's engines contain a carryover from the steam age
We’ll concentrate on the poppet valve here, but before forging on, we should at least tip our hat to other ways of letting the ‘suck’ cycle get air in and the ‘blow’ get the exhaust gases out. Of course, two-stroke motors have no valves at all, instead having sneakily machined ports in the cylinder walls that are unveiled at just the right moment to let in the mixture of air, fuel and oil that these wonderful things thrive on, then at just the right time to let the exhaust gases ring-a-ding their smoky way into the atmosphere. But let’s tarry no more on two-strokes – that way lies madness and witchcraft. Very early engines used simple spring-loaded ‘pop’ valves – simple, but inefficient, as the gases needed to overcome the spring force to shoulder their way into or out of the cylinder.
The only serious competitor to the poppet valve was the sleeve valve, invented by one Charles Knight around 1904. This system involves a cylindrical sleeve concentric with but outside the cylinder bore. Inlet and outlet ports were machined into this sleeve, a little like the ports in a two-stroke. By rotating this sleeve, the valves could be opened and closed at the appropriate times.
The system was quickly adopted by Willys – the same company that would go on to be evermore associated with the GPW or Jeep. The big advantage of this Willlys-Knight setup was silence. Valve systems of the day, with their looser tolerances than modern engines, were tickety-tappety affairs, whereas Knight’s sleeve was almost silent in operation, hence leading to its adoption by various luxury makes including Daimler. It was also used in various radial aircraft engines, including those powering the Bristol Beaufighter and the Hawker Typhoon. [Phew – we finally shoehorned in the mandatory military aircraft reference: it’s virtually a contractual obligation in this series.]
"Two valves in the cylinder head could only ever open up 50 per cent of the available combustion chamber. Whereas if you draw four concentric circles inside the other circle, you now cover 68.56 per cent of that area – an improvement of 37 per cent"
But enough diversions, and back to the poppet valve. It has dominated the car engine since Car Number One – yes, the OG Benz Motorwagen of 1886 already used a poppet valve for the exhaust. And it’s still there, poppetting away under the bonnet of your car to this very day.
Now, the natural setup is to have one entrance to and one exit from the cylinder – one inlet valve, one exhaust valve. Hence two-valves-per cylinder heads were the natural and first solution.
But as we know, no sooner had the first cars puttered uncertainly down a cobbled street than some people were already thinking of racing them. Within a very few years of Bertha Benz taking her husband’s tricycle for the world’s first proper drive in 1888, racing was – as it arguably remains – the front-line of automotive technology. So, very quickly, the early engineers started to figure out how to get more air into the cylinders, and how to get the exhaust gases out faster.
And some clever chap who had paid attention to ‘pi’s and r-squared in his basic geometry class figured out that two valves in the cylinder head could only ever open up 50 per cent of the available combustion chamber. Whereas if you draw four circles inside the other circle, you now cover 68.56 per cent of that area – an improvement of 37 per cent. In the marginal-gains world of motor racing, that’s massive.
“It had a desmodromic valvetrain (meaning it had a rocker arm to close the valve as well as opening it – a system still associated with Ducati) and, believe it or not, four spark plugs for its single cylinder as well as those four valves. In 1908”
Sadly, we don’t know the name of the chap who did the maths and figured out that cramming four slightly smaller valves into the cylinder would give better gas flow. But we do know which car he applied it to. They don’t get much more obscure than the 1908 Ariès VT – a race car that competed in the small car or voiturette class in just a few races that season: to no great success. But under its hood was a single-cylinder 1.4-litre engine, with the cutting-edge overhead cam pioneered by the Maudslay Motor Company just six years before. And this time, that SOHC drove four valves instead of the usual two. This engine – whose significance only came to light as recently as 2014, by the way – was a remarkable thing. It also had a desmodromic valvetrain (meaning it had a rocker arm to close the valve as well as opening it – a system still associated with fancy-pants Ducati motorcycles) and, believe it or not, four spark plugs for its single cylinder as well as those four valves. In 1908.
Just four years later, we saw the twin overhead cam and four valves per cylinder come together for the first time in the slightly-less-obscure Peugeot L76 race car that we already mentioned in part one. A far more formidable beast than the spindly and forgotten Ariès, this monster was powered by a 7.6-litre inline four that produced a (then) tarmac-ripping 148bhp. And to prove that race drivers have always had balls of steel, one Jules Goux drove a streamliner L76 to 103mph on the Brooklands banking. He took one across the Atlantic and became the first non-American winner of the Indy 500, making sure to keep his hydration levels up by downing three bottles of champagne during the race, of course. Good ol’ days.
So there we have it – by 1912, engineers had already figured out the ultimate form of valve-gear: twin overhead cams driving four valves per cylinder.
Twohig would welcome the return of 16v badges
Hold on, hold on. The ultimate? What about five-valve heads? If cramming four small valves into the head gave more geometric ‘coverage’ and hence better breathing, surely cramming in more valves would be even better? Peugeot again led the way with this theory – as early as 1921 it had followed up its Indy-winning four-valver with a five-valve, triple-overhead cam engine.
But the 1980s and 1990s were the heyday of the five-valve head, with the likes of Audi, Yamaha, Ferrari and Toyota developing and marketing five-valvers. But here’s the rub – by a quirk of geometry, cramming five circles into a bigger circle actually gives you less surface area than you can cover with just four: 68.45 per cent, to be precise. And actuating that fifth valve that our extra little circle represents gives you a whole lot more mechanical complexity in the head, as your valves no longer line up neatly in two parallel rows. So five valves quite literally gave you no more bang for your buck than four. Which is why they have tended to fall from favour in recent decades. So I stand my ground – the four-valve head really is the ultimate.
Just a pity that those ‘16v’ badges that proudly adorned many a bootlid in the ’80s and ’90s have fallen out of fashion.
