Given a four cylinder car engine, one assumes there will be four equally- spaced ignitions every 90' of crankshaft rotation. Does the same apply for *all* engines in terms of the number of their cylinders? For example, a 5 cylinder engine firing every 72', a V8 every 45' and a 12 cylinder every 30'? Do I have this right?
Dunno about that, but I had a twin cylinder Honda motorbike where the ignitions were spaced strangely. The pistons operated 180 degrees apart, but being a four stroke, this meant two ignition events followed by two intake events. The contact breakers (two, no distributor) were spaced 90 degrees apart on the camshaft.
A number of Hondas did that at the time (mine was the CB72). They made a distinctive 'double beat' noise. I have no idea if twin cylinder engines still do this.
I remember reading an article of an experiment where a BSA engine was re- engineered to do the same (modified crankshaft, camshaft etc.) Apparently it then revved faster. How true that was, I don't know.
For 4 cylinders: 180 degrees - there are 2 rotations of the crank per full firing cycle.
Each cylinder only fires once per 720 degrees, so (in e.g. a straight 4) they're 180 degrees apart, not 90.
Other than the factor of 2.
Traditional parallel twins (by which I mean pre-Honda) have the crank-pins on the same axis (not 180 degrees apart), so this makes the firing interval symmetrical (once per rev, that's for a four stroke of course). V-twin motorbikes normally have the same arrangement, and this makes the firing interval uneven.
When you get to multi-cylinder Vees it all gets complicated, and depends on the angle between the cylinders and the location of the crankpins.
AFAIK most flat twin motorbikes have crank-pins at 180 degrees, and this gives them an even firing interval. That's one of the reasons that traditional BMW flat twins are relatively smooth, since the pistons are always moving in opposite directions. On a traditional British type parallel twin, there is a counterbalance weight on the flywheel to help balance out the effect of the pistons, but of course these only go up and down, while the counterbalance weight goes round and round. If you see what I mean.
Nope, still not getting it. Let's take a 5 cylinder in-line car engine as a template for this argument. How many degrees of crankshaft rotation between firing pulses?
One assumes wrong. In a 4-stroke.
One every 180 degrees.
Apart from forgetting the 4-stroke and the fact that any other engine than an inline one of a single bank, is likely to have irregular firing intervals. yes.
Listen to an old Veedubs flat four, or a light aircraft..chacha-pause cha cha
Or a V8,
In fact there is so much wrong I have to say 'no'.
With a V configuration, it depends on the angle of the V.
No.
It depends on the position of the crankpin[1] with regard to TDC of any particular cylinder; which (for example) is the reason for the differenece between a Ford V8 and a Ferarri.
[1] which can, of course be at any angle relative to any other crankpin.
No. The crankshaft is "flat" in four cylinder engines (and v8). There are two ignitions at 180deg per revolution. The firing order is (usually 1-4-3-2.
If there is a distributer, it runs at the same speed as the cam shaft, ie half engine speed.
No, a four cyclinder usually fires every 180 degrees. At 90 deg, all the pistons would be in mid stroke, halfway up/halfway down.
That article is one of the worst articles on wiki I have ever read.
"A V8 engine gets a power stroke every 90 degrees: 720° ÷ 8 = 90°, which is four power strokes for each revolution of the crankshaft."
Utter total bollocks. The last phrase is true - four power strokes per crankshaft revolution, but the angle between power strokes is not a constant 90 degrees.
It all depends on the crank angles chosen, and that will be a dynamic balancing choice largely, plus the angle between the banks.
Its quite possible to build and engine with for cylinders all of which fire simultaneously, and have the phases 0,0,0,[720]...
It would be an off balance nightmare, but it could be done.
The distinctive sounds of e.g. V6s V8s and flat fours and flat sixes shows that the firing angles are not equal divisions of a single rotation.
There is a compromise to be struck between dynamic balance and uneven torque delivery.
That depends...
Why did nobody develop the rotary engine for land transport, one assumes it was due to the size of the engine as it would need to be two cylinders in all directions, but still, they do seem more elegant. Brian
You are assuming a crank with one throw per cylinder?
Exactly so.
Form factor was vile for rotaries, gyro effect was vile, light weight was not so important, and there was no problem in having water cooling. So the rotary had no real advantage and some disadvantages.
Form factor is crucial in car design - thats what made the Mini such a breakthrough.
850cc of decent engine twisted sideways with the drive train folded to put the gearbox underneath, and then drive shafts to the front wheels obviating a transmission tunnel and allowing a smaller passenger box. Tack a little boot on behind the rear seats and that's a mini. Other issues like rubber cone suspension made for a very compact layout too.Mini was a triumph of packaging.
VW beetle is good too, the flat four makes a shorter engine, and that feeds local driveshafts too. Allowing a bit more space for passengers in a compact layout. And that has torsion bar suspension, too. Again compact.
Yes, it's amazing how such a tiny car can feel so roomy inside. Contrast with the more modern BMW Mini which is a bloated eyesore with no redeeming qualities.
Are we reading the same article? Nothing about V8s in the link I followed.
For a 5 cylinder in-line, the usual design seems to have crank pins at
72 degrees, giving an even firing interval, but I was interested to see that the five cylinder Honda 125 ridden by Ralph Bryans in the 1960s had 60 degree pins, but a missing cylinder. It's all about optimising balance.HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.