Fast Five In-Line For 2017 Audi TT RS

Standing in one of the pit garages at Jarama Circuit, near Madrid —location for the launch of the newTT RS — there is a display of various engine parts for the latest Audi five-cylinder engine, demonstrating just how the company has cut 26kg from the unit’s weight (it saved 18kg with a new, aluminium crankcase).

“I had one of those as my first car,” says a voice behind me. It’s Stephan Reil, the amiable head of Quattro GmbH and therefore the man in charge of Audi’s RS models. He’s pointing at the Audi 100 way over on the left-hand side of the chronologically ordered photos. He tells me that although the engine was good, the three-speed auto gearbox it was attached to was terrible. Nonetheless, I can imagine how much the 18-year-old Reil would have appreciated having this impressive car to run around in, the quintet of pots doing just what Audi intended and lifting it above all the four-cylinders in status.

Mercedes actually put a five-cylinder engine into a production car two years before Audi, and Lancia had a five-cylinder engine in a truck back in the 1930s, but those were both diesels; Audi was the first to bring a petrol five-cylinder engine to market. The main reason that this configuration hadn’t been used before this was the difficulty of getting the fuel into the cylinders with carburettors. Use one carb and the outer cylinders would be much farther away than the middle cylinders, meaning uneven fuelling. Use two carbs and things were equally tricky because one carb would be supplying three cylinders and the other only two. Diesel engines obviously don’t suffer from this issue as they have always used fuel injection, but it wasn’t until fuel injection in petrol cars became more widespread in the 1970s that the petrol five-cylinder became a more viable option.

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Audi was looking at a five- or six-cylinder engine to take its cars upmarket and a six was deemed too bulky (plus the competition in Munich had rather made the straight-six its own). The new 2.1-litre naturally aspirated five-cylinder unit was based on the EA 827 four-cylinder (that had already appeared in the Audi 80 and Audi 100) and arrived in the Audi 100 SE in 1977, putting out 100kW. A naturally aspirated diesel followed in 1978, but things really got interesting a year later when the first turbocharged five-cylinder petrol arrived.

With 125kW and 265Nm, the Audi 200 ST was initially the top model, but then came the Ur-Quattro.

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Audi Ur-Quattro

From here we have the line of legendary rally cars that everyone is familiar with, from the 225kW car that Hannu Mikkola piloted to a win in Sweden in ’81 to the mighty Sport Quattro S1 (E2) that Walter Rohr’ took up Pikes Peak in 1987. There were the racing cars, too, most notably the incredible IMSA GTO, which produced 530kW from not much more than two litres.

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Audi Sport Quattro S1’s engine

On the road there was the wonderful RS2, but after that the five-cylinder largely faded from Audi’s line-up until 2009. In the interim there was, of course, the rather lovely Volvo/Ford in-line five, which kept the fire burning. When a five-cylinder re-emerged in an Audi, it did so under the bonnets of the TT RS, RS3 and RS Q3 and the engine has since gone on to win seven consecutive Engine of the Year awards.

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Audi IMSA GTO

But what makes a five-cylinder attractive to a manufacturer? Well, if there was a marketing department for fives, it would tell you that they are smoother than fours but not as bulky as sixes. Which is largely true. The reason a five is smoother than a four is all to do with secondary-order vibrations (feel free to skip to a couple of paragraphs at this point). A four-stroke engine fires all the cylinders once in every 720 degrees of rotation of the crank. In a four-cylinder engine that means four powerstrokes, and 720 divided by four equals 180, which means there is a powerstroke within every 180-degree rotation of the crank. A powerstroke can last no longer than 180 degrees, so if there is an even firing order, there can be no overlap between powerstrokes. Intuitively this sounds like something that should be balanced, but because the acceleration of a piston is greater at top-dead-centre than bottom-dead-centre, they don’t cancel each other out in the way you might expect.

Add an extra cylinder, though, and there is now a powerstroke every 144 degrees, meaning there’s overlap between the power phases. Basically the crank is always under load, so the engine runs more smoothly. There are still primary-order imbalances — those caused by forces that occur once per rotation of the crankshaft — but these are countered with weights on either end of the crankshaft. A firing order of 1-2-4-5-3 also produces the least primary imbalance, so is the one preferred in road cars.

Of course, the most common reason for people falling in love with the five-cylinder engine over the last 40 years is because of the sound. Whether it’s a Quattro ripping through a gravel stage or an RS2 starting up in the street, a five produces as distinctive a war cry as any engine, and Audi plays it to perfection. Apparently the sound can be represented by the musical interval 5:2, but that really is beyond my understanding.

Perhaps with all the downsizing of engines that is going on, we will see more five-cylinders appearing in performance cars. I certainly hope so, but I suspect whatever they appear in, the first instinct on hearing a five-cylinder burst into life will be to think it is an Audi. Anyway, now to find out what that latest, lightest five-cylinder turbo is like…

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