GM was developing (will release soon?) a new line of straight-6 and 8 truck engines, I believe. And the Volvo 5 cylinder is an inline, right?
I guess there are more oddball engines than I realized. Volkswagen used to have (and maybe still does) a V-6 with only a 15 degree angle between the cylinder banks. I found one site that said they did it because a typical 60 or 90 degree V-6 is so wide that it becomes hard to work on. In the 1930s, Cadillac had a V-16. When Formula 1 went from 1.5 to 3 litre in 1967, BRM took two of their flat-8s and attached them to make an H-16. I think Volkswagen (again) has a W-12 (two of the narrow-angle V-6s, or something like that) they’re tinkering with. Subaru and Porsche like horizontally opposed engines. And Saabs are so fundamentally weird there must be something funky about the engine. People have tried damn near everything, and I hope they keep at it. But for the most part it’s inline-4s (small), V-6s (medium) and V-8s (large).
A couple of things here. On a four cylinder engineThere are only two crank throws. When #1 piston is at the top of its stroke (called top dead center) #4 is also at the top of its stroke. Assuming that #1 has both valves closed mixture compressed, and spark has been introduced, then #4 is closing the exhaust valve and the intake valve has just opened. As #s 1 & 4 head down #1 is producing power and number 4 is drawing in a fresh mixture. When #1 & 4 were at the top of their stroke, #s 2 & 3 were at the bottom. As #1 moves down producing power #3 is moving up compressing the mixture so that when it gets to the top of its stroke (actually a few degrees befre) the spark plug will ignite it and it (#3) will produce power.
EACH PISTON ONLY FIRES ONCE EVERY OTHER REVOLUTION.
The reason for the size limit on 4 cylinder engines is that at the top and bottom of each stroke you have 4 pistons and four rods all changing direction at the same time. If the displacement goes over about 2.5L this is so much mass that the engine viabrates like a mutha.
For smothness, the more cylinders the better.
On a four cylinder piston throws are 180 degrees apart. (pistons travel in pairs)
On a five cylinder piston throws are 72 degrees apart (Pistons travel individualy)
On a six cyliner piston throws are 60 degrees apart. (Pistons travel in pairs.
On an eight cylinder piston throws are 45 degrees apart (pairs)
Ten cylinder piston throws are 36 degrees apart (pairs)
and on a V-12 for you Ferrari, Jag and BMW guys pistons are 30 degrees apart and the pistons travel in pairs.
Oh yeah on your list of straight six engines add Volvo. It is a modular design and shares design and parts with the 4 and the five.
You are right about SAAB they made a V-4 used in the Sonnet. When those boys advertised that they don’t build cars they build SAAB’s they weren’t kidding. 
Oh one more thing aobut how many cylinders are powering the car. On an engine turning 6,000 RPM one complete revolution of the crankshaft takes 10 milliseconds. So it doesn’t power it for long.
The Honda S2000 has a 240 hp four cylinder, so you’re not too off the wall, Mole.
:sigh: The commercial went, “We don’t make compromises, we make Saabs.”
I’l throw in a few oddities into the mix, these are all bike engines,
Morbidelli put a V8 850cc engine into a bike,
http://www.italia.co.uk/morbidelli/
I looked horrible to my eyes but the killer was that it was hugely expensive, on top of which I reckon that keeping it maintained in tune would be mucho expensive too.
The other killer for that bike is that tyre technology currnetly allows bikes only up to around 190mph on the road and those kinds of speeds can be achieved readily enough with 4 cylinders at much less cost, and a good deal less weight.
There are a couple of differant configurations for in-line triples, most manufacturers use 240 degree firing intervals, however this may change as Benelli has brought out its 900cc Tornado which has one revolution of 120 degree firing follwed by one revolution of nothing.
http://www.tornadobenelli.com/
Triples usually have a very flat torque curve, you can use almost any gear at any revs, but they also do not rev as high as 4cylinder machines, and revs*torque = power.
The ‘big bang’ engines such as on Hondas 500cc 2stroke 180bhp+ Grand Prix machines have all 4 cylinder fures within a very short period, less than 45degrees, this allows the tyre to deal more easily with the power pulses, the tyre gets a ‘rest’ period.
I would not think such a firing order would work on the road, since road tyres do not get anything like as hot enough to provide the levels of grip that racers have
Bikes also use massive v-twins, the current largest is the Honda VTX1800, such things are not to be found on cars, possibly the cost of them would make them uncompetitive, but bikers don’t seem to worry too much about mundane matters such as cash.
http://www.motorcycle.com/mo/mchonda/01vtx.motml
back to Whack-a-moles question, one thing you are missing is the role of the flywheel, each piston puts energy into this, which the flywheel stores as momentum and produces torque as its output.
At any given time then the vehicle moves as a result of that energy stored in the flywheel, not the pistons themselves.