Inline fives - you’ll find them in service today on the Ford Ranger and Mazda BT-50. Various manufacturers including Merc, Audi, Ford, Volvo and Jeep have had their dalliances with inline fives in both petrol and diesel.
But the reason inline five-cylinder petrol engines are late bloomers on the automotive landscape is simple: Carburettors suck. If you try to fuel an inline five with a single carb, you run into unavoidably inconsistent fuelling problems between the cylinders. It’s just not symmetrical.
If you fit multiple carbs - two or three - one of the carbs always fuels more (or fewer) cylinders, making it inherently difficult, bordering on impossible, or at the very least practically unworkable, to balance up the fuelling.
Hypothetically, you could fit five carburettors - one for each port - but that’s not really a viable suggestion if you want to save cost over a comparable six-cylinder engine…
Multi-point fuel injection solved that problem and made five-cylinder petrol engines viable because one central fuel pump feeds five injectors, one in each inlet port, making even fuelling easy to achieve.
This also explains why five-cylinder diesels were more historically popular - decades earlier - because diesels are all compression-ignition fuel injected engines, obviously.
The other thing about five cylinders is that in a four stroke engine, one complete cycle is two revs - 720 degrees or crank rotation.
720 degrees, divided by five cylinders is 144 degrees between firing strokes.
And 360 degrees divided by 144 doesn’t give you a whole number - so a five-cylinder engine can’t share crank phases, unlike a six-cylinder or an eight-cylinder engine.
Inline fives need five separate crank pins at 72 degrees to deliver even firing pulses. And the main advantage of an inline five over a four is that the firing pulses are a lot closer together.
There’s a few inherent balance deficits in inline fives: They have planar imbalance on the reciprocating mass, and on the rotating mass, and on compression. So you need heavy counterweights to mask all that, and this results in an engine that doesn’t rev up all that fast.
They’re sort of OK for secondary balance (depending on which firing order they use) almost comparable to an inline six in the best case. And that means you can design them a bit undersquare (with a relatively long stroke relative to the bore) without compromising high-rpm smoothness.
With an under-square five, the small bore/long stroke design means you can reduce the overall engine length a bit and put them in an engine bay about the same size as you’d need for a four-cylinder … provided the beancounters will wear the cost of the additional the parts, the extra machining, and screwing them all together.
Another way of looking at this, perhaps, is you can have a three-point-something inline five for a lot less cost and less physical space than an inline six, or slightly more space (but still less cost) than a V6. It’ll be slightly rougher, but, hey, we’re saving the big bucks here…
Volkswagen uses a 3.0-litre inline six in the Amarok. Ford uses a 3.2 inline five in the Ranger.
You can say that the Amarok is more powerful - it certainly is - but it’s spinning 50 per cent faster and only making 12 per cent more power … so the torque production is falling in a hole.
(Power equals torque times revs, right? Do you really want to rev a diesel like the Amarok to 4500 revs for maximum acceleration at any particular road speed? Or would you rather be at 3000 revs, like in the Ford/Mazda engine? I’ll take 3000.)
If you want to be the full nerd on inline fives, here’s the pub conversation-stopper: There are 24 different potential firing order permutations that support a 144-degree phasing of the crank. (For a four-stroke.) But they only commonly use two of those permutations in production.
1-2-4-5-3 is the combination that minimises the crank-speed ‘rocking couple vibration’ - and it’s the most common firing order for inline fives on road cars.
But 1-5-2-3-4 minimises secondary imbalance and that’s the one they use commonly in V10s. Which are essentially two inline fives, Siamesed and sharing a crank.
The Audi R8, the BMW E60 and E61 M5s and the E63 and E64 M6s use that second firing order. I think the Dodge Viper was like that as well.
The 1-5-2-3-4 order in the V10s has a high primary rocking couple and is not such a good choice for inline fives. But it suits the V10s - and it allows higher revs. Statistically, nobody knows that about the firing orders.
