Define the cycle of a 4 cylinder engine

[Zak33]

OK....we had inlet, we had fueling and we had the timings of an engine.

The big BANG has occured......one of our mighty pistons is FLYING down the bore......its pushing the crankshaft around and making our car move......

So on the premise that we have a four cylinder engine (forget howmany valves, its just got inlet and exhaust valves) we have a fuel system, we have an Ignition system just waiting to spark up our fuel.....

How does the engine turn, and when does the fuel/air mix explode and then how does it get out of the engine?

I'll start.......

Each of 4 pistons are connected to a CRANKSHAFT...this is a metal shaft in the bottom of our engine, looks like this



It has a central axis.....the bit that doesnt flap about when the engine is turning.......and off from that axis are 4 points for the pistons to connect too....so that when they go UP AND DOWN.....this thing goes ROUND AND ROUND......

[Zak33]

Please not that picture is the most horrible crankshaft I have ever seen......but I used it BECAUSE it is a perfect example with nothing in the way....you can see in your mind the central axis that is spins around and the 4 place for the pistons to join onto....

in a modern car, to keep it smooth, there are weights cast into the crankshaft and they keep the pistons inertia counteracted...ie they stop your engine shaking itself apart at high revs....

They come in all shapes and sizes.....





Now as with all good subjects its not that easy......

first: How far the piston goes up and down (the STROKE) effects howlong each of those connections has to be from the central axis.
second: How high its gonna rev determines if it needs to be lighter (and weaker)
third: WHEN the pistons go up and down is determined by how its mad and in what order the engine is gonna fire....
fourth: How big is the rotation...ie HOW MUCH ROOM DOES THE LOWER part of the engine NEED for this thing to turn.....cos if the piston moves a long way up and down.....the crankshaft needs to be turning in a wider arc....cos the piston comes down further and ends up pushing further and further OUTWARDS, not just dowwards....dont forget ithe crankshaft goes ROUND and ROUND but pistons just got straight up and down

[jon bda]

SUCK
SQUEEZE
BANG
BLOW

[Zak33]

That big picture....note the holes drilled into the balancer weights....3 on the left one, 2 on each of the visible right ones....

to make a race engine, you need to put this Crankshaft on balancing machine and spin it FAST.....then get the machine to tell you just where the weight needs removing to make it better balanced....

And to think in Grand Turismo its a mod you have all paid for, and you might not haveeven known WHY

Smother means it increases revs easier and then goes higher as well.......but its an art form...its what motor sport is about.

Stunning huh.....and this is in the bottom of all your engines spinning at up to 6000 revs per minute.......thats 100 times per second....

and it weighs lots and lots.....its steel....

mindboggling really.....

right....who else wants a go?

Who wants to try to define how the pistons are timed..ie which ones go up and down while connected to this crankshaft.

[Zak33]

Originally posted by jon bda
SUCK
SQUEEZE
BANG
BLOW

Itsa beautifull description of the way the fuel is detoanted, but we mean the entire engines cycle...go on...I know you know....but wording it is HARD

[Skii]

bloody hell you lot might as well be talking Chinese

[Zak33]

Ironically.......it might be worth knowing in the Me109.......summit makes it fragile

[sdp]

I'll have a go;

So you turn the key and the starter motor engages with the flywheel and turns it and as the fw is bolted onto the crank that turns too.

SUCK
The inlet valve opens, the piston is cranked down the bore and pulls in the mixture.

SQUEEZE
The inlet closes and the piston goes up the bore, compressing the mixture to around 8 - 11:1.

BANG
The spark plug sparks and the mixture burns. The piston is slammed back down the bore and turns the crank wot its connected to by a con rod.

BLOW
Boom - the exhaust valve opens and the piston is shoved back up the bore by the action of another piston BANGing down n turning the crankshaft.

In a four cylinder engine the power (BANG) strokes of each cylinder come one after another, unlike say a v8 where they overlap to give the v8 its reknowned smoothness of torque. The 1-3-4-2 (n the other variation which I forget) firing order of the 4c is important in reducing vibration.

The flywheel helps to smooth out the power impulses. During one complete cycle, each piston goes up or down the bore 4 times; on the firing stroke it is exerting force down through the con rod and so turning the crankshaft, on the other 3 strokes it's being flung up n down by the continual rotation of the crankshaft. 4 strokes hence '4-stroke' engine.


/Care should be taken when messing with lightened components, cos for example a set of top quality ultra light forged pistons might be great for zooming round a track but they're not so good on road cars where long term reliablity normally comes before speed - basically don't go too mad with lightened components; the distinction between road and race components is there for a reason. Balancing the con rods and the rest of the crankshaft assembly is always good though.

[sdp]

I think valve and cams need mentioing here too; if you mod a car so it revs higher you may well need stronger valves springs to prevent bounce and perhaps a hotter cam to increase the valve lift wouldn't go amiss too but again a cam that is happy up to 9,000rpm but doesn't kick in til 3,000rpm is not much good on a road car.


(sdp hopes he sounds like he knows what he's on about )

[RocketmanX]

Good point about the valve springs sdp. The lift of the cam and ramp angle and duration at the top of the cam have more to do with the valve spring rates.

A hydraulic lifter cam has lobes that angle up to max lift pretty gradually because of the inablility of the hydraulic tappets to function under more extreme lift and lobe ramp angle. A solid tappet can take more angle and as well take more revs since it's solid. A roller tappet is even better and able to take very severe lobe ramp angles due to the roller end on the tappet. Some roller cams, such as they use in my oval racing V8, have ramp angles approaching straight up and down
The roller tappet is kept from "flying" off the top of the lobe by a rev-kit, at least in a V8. The rev kit is a barrel and spring that goes between the head and the top of the roller tappet and keeps the tappet firmly seatedon the camshaft. Top end racing V8's over here can now be ran a wjole 500 mile race in excess of 9000 rpm's The 9:1 I use (comp. ratio- a tterm used here to define a specific type of racing V8, 9:1's turn produce all there power at high revs and are lighter internally and run higher revs than a high comp. V8) is turned about 8700-8900 on a weekly basis and will run a whole season between rebuilds. Average about 20 races a year.

A good solid tappet V8 is capable of running around 7500 rpms's regularly and a hydraulic tappet around 6000.

Of course F1 cars have the extreme in lifter tech with their hydraulic valve trains and 19000+ rpm's
Don't know much about those sytems though.

If you want to get funky with piston firing timing then check out a Harley-Davidson, don't know much about bikes but I know they get their unique sound from the firing order. I believe some bikes like Ducati have some wierd firing orders as well.

[Zak33]

Wooah....boys. calm down

We aint done camshafts in basic yet ....so while I know you too know all about them...we gotta explain it better to the public

Right.

At the bottom of the engine is the crankshaft.......its connected to the pistons, by a Con Rod. We know that now.

These old boys go up and down in the cylinder block.....but what is above it all?

That all looks like this (invisible egine block!)



____________________________________________________

But something needs to "be on top"

Its a cylinder head. THIS ONE IS UPSIDE DOWN TO SEE THE VALVES



Now....there are valves in it......they look like this



these are held SHUT with springs and kept sealed and straight with valve guides


___________________________________________________
........above them, is a CAMSHAFT

it looks like this

[Zak33]

the plan is this:

The camshaft has LOBES on it......when it SPINS, the lobes PRESS DOWN onto the top of the VALVE.....that forcesit open, even though a spring is trying to hold it shut.

When it turns further, the LOBE releases pressure on the valve....and it shuts again.