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evilleppy87 t1_iwdrhoe wrote

Is it just me, or is the exhaust timing wrong?

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Inle-rah t1_iwehpwm wrote

First thing I thought - I totally wanna lobe that cam. It’s gonna “pop” when the intake opens. Love the animation and rendering though.

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ClassBShareHolder t1_iwfgub4 wrote

Not animation. It’s a polycarbonate model. I too thought it was animation until I looked closer at the cylinders and connecting rods.

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Inle-rah t1_iwfhaeo wrote

Holy guacamole! That’s awesome!

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bentsea t1_iwfim5p wrote

It would be if the timing were right. As it is, it's just a physical trigger for deep psychological trauma.

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guerroconpollo t1_iwdvhhq wrote

Definitely not wrong. It's like it's missing a beat in a dance.

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yodazer t1_iwgbtv8 wrote

It is very wrong. It looks like they are sucking in the exhaust instead of pushing it out.

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MarcoMontana t1_iwgjusc wrote

The cam is not timed at all, this engine if it started would rumble an pop all over the place.

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cbxsix t1_iwg5had wrote

It's off because of the huge amount of lash. The valves open too late and close too early because only the ~top half of the cam is actually hitting the valve stem. Otherwise it's a cool model!

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Wayed96 t1_iwgc821 wrote

All the timing is wrong. But it gives the general idea I guess

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shifty_coder t1_iwgob0g wrote

Isn’t the ignition order wrong, too? Would a three cylinder fire sequentially, or wouldn’t it fire 1-3-2 or something

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Elkazan t1_iwh07to wrote

1-3-2 is the same as 3-2-1 which is also sequential, just in the other way. Would it change anything? Seems pretty equivalent to me.

Disclaimer: I don't know about engines.

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shifty_coder t1_iwh0owc wrote

That makes sense. It doesn’t matter what the crank order is, the ignition order will be sequential.

1-3-2-1-3-2

3-1-2-3-1-2

2-3-1-2-3-1

2-1-3-2-1-3

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ct_rugen t1_iwfdnrn wrote

You're telling me you see that but you haven't been 180 off on a cam or distributor timing. I know Ive missed a timing belt or chain a few tooths off a lot of times

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allwaysnice t1_iwfs3bi wrote

I know nothing other than 20 years ago when I watched this on Magic School Bus and it still felt wrong.

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Aggrador t1_iwghsgt wrote

Doesn’t look wrong to me. Red exhaust valve opens as the piston rises and blue opens when the piston drops on each of the cylinders. I will say though, that this does look like an awfully unbalanced engine between cylinders 1 and 3 having so much time between the firing orders, as oppose to 1 to 2 or 2 to 3.

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johnnySix t1_iwgj3j7 wrote

It also looks like the middle gear is going the opposite direction of the arrow and handle.

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drsbd t1_iwdsbdp wrote

I think it's just you but don't quote me on that

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evilleppy87 t1_iwdssjt wrote

Shouldn't it be open til the piston reaches TDC? If so that means with those cams it should open later in the exhaust stroke. It seems like they start opening just before the piston reaches BDC, and close half way up the exhaust stroke.

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drsbd t1_iwdur3t wrote

Now that I look again you're right they are only open for roughly half the exhaust stroke. Good eye

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Waldron1943 t1_iwerj2m wrote

They do open before BDC. Imagine a graph with two curves plotted. One curve is the "benefit" you get from leaving the valve closed and using every last bit of pressure. The other curve is the "benefit" you get from opening the valve early and starting exhaust flow. Those two graphs cross before BDC; there's more "benefit" to opening the valve than there is to keeping it closed.

Also, it's actually open past TDC. Right at TDC you've invested energy in establishing flow; if you slam the exhaust valve right then that flow just "piles up" against the valve, which doesn't help you. As a matter of fact, the intake valve opens (and stays open a surprisingly long time) before the exhaust valve closes...that's called "valve overlap". Less overlap = low RPM power, longer overlap = high RPM power. Overlap uses that flow through the exhaust valve to pull more fuel & air mixture into the cylinder.

Valve Overlap

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myawesomeself t1_iwgmva8 wrote

I think this is what everyone expects, that the exhaust should be open near TDC so it might overlap with intake a little, however in the model it clearly closes completely somewhere around halfway up and there is a noticeable gap between the exhaust valve closing and intake opening which is the unusual timing people are talking about.

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Waldron1943 t1_iwhkl0j wrote

Oh yeah, I mean in real engines. The model is all fuxored up; check out the valve clearances!

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Snufflepuffster t1_iwe3wrq wrote

often valves close or open early to take advantage of acoustics (shock waves) to ram air into the cylinder or throw it out. So timing is very nuanced and can be adjusted drastically from engine to engine. In general the exhaust valve isn’t going to match the stroke, it might be early or late, extended or short due to design.

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evilleppy87 t1_iwe8c32 wrote

I'm aware that there's going to be some variability, it just seemed to me that closing 90° before TDC was a little extreme, especially considering in most cases the exhaust valve closes a little after TDC

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Drunken-samurai t1_iwge1ej wrote

Bringing up variability, there is VVT (variable valve timing) which takes advantage of this in real time to use different valve timings at different RPM's to provide more efficiency or power.

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evilleppy87 t1_iwgrf7o wrote

Like Honda VTEC, for example?

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Drunken-samurai t1_iwkt2fq wrote

Yes your right, this is part of what the VTEC acronym is (Variable Valve Timing & Lift Electronic Control).

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Wayed96 t1_iwgmkik wrote

And even a little longer to improve intake speed

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