Engine technology thread.

[old greg]

Originally Posted by cossey

so you wont be saying that wour shiny new FR-S weighs 83 slugs when asked?

Hah! 83 slugs isn't a measure of weight, smartass. :happy0180:

I'd tell them it weighs 12000 Newtons.

[arghx7]

You have to be careful reading a lot of those books. A lot of them are old and are not too relevant to the way modern manifolds are designed in the sense that everything is done with computational fluid dynamics models.

[Dimman]

Originally Posted by arghx7

You have to be careful reading a lot of those books. A lot of them are old and are not too relevant to the way modern manifolds are designed in the sense that everything is done with computational fluid dynamics models.

CFD would be dealing with the inertial stuff, and this is mostly focused on acoustics.

Cosworth managed 170+ hp/L in 1982 with no CFD, so it's not the end of the world not having access to it.

But you are correct on some of the atrocious old manifolds in some of the road car examples. The 'siamesed' 3 port on a 4 cylinder makes me shake my head. Plus non-crossflow heads... But some of the carb stuff applies to ITBs. And some of the old factory designs are superior to current ones thanks to not having to fight with catalyst placement and emissions.

[cossey]

Originally Posted by old greg

Hah! 83 slugs isn't a measure of weight, smartass. :happy0180:

I'd tell them it weighs 12000 Newtons.

I know but given every one uses kilograms or pounds rather than Newtons or pounds force it is in keeping.

[Dimman]

Well here’s what I’m getting out of that book so far: Lots of practical testing is in order. Yay…

Seriously though, there is something to this. The authors detail the importance of ‘mapping’ the wave-forms at different rpm, lengths and possibly throttle positions. They even describe how to build a multi-manometer array and how to plumb it to multiple points in the exhaust with a rotary valve physically timed off the engine (yes it‘s about as complicated as it sounds…). Or their more ‘advanced’ electronic way using an oscilloscope and photographing the resultant ‘oscillogram‘. Oh how times have changed. Something could probably be rigged up digitally with a data-logger with MAP sensors and timed off the crank or cam sensors these days. I would also probably gather some EGT data at a few points along the exhaust as well. Maybe IAT at different points in the intake manifold, too. Oh yeah, and a dyno is necessary. D’oh!

The big thing that their graphing shows, and that the formulas don’t address, is the change of shape/length of the waves as well as their amplitude. This becomes even more interesting as we delve into our modern technology.

The formulas can be the starting point and put us in the initial ballpark for what we want. The whole shorter=top end and longer=bottom end generalizations. As well as 4-1 top end short power band vs. 4-2-1 wider low to mid range power band. Now with the ability to ‘see’ the wave shape and amplitude we can make tube length or cross-section adjustments to match the low-pressure return to the overlap period of the cams at the desired rpm or reduce inertia-caused high pressure zones. We can then also see where the corresponding high-pressure return will occur at (flat spot). We can also get an idea of the strength of the scavenging effect from the amplitude of the wave. Then verify everything with power pulls. This would be the limits of the ‘standard’ tuning abilities.

Now for the fun part. Since depending on rpm, and even EGT’s, the shapes change as well as the timing of the returns. Tough luck in the old days, just pick your compromise. But we now have the ability to phase the cams in real-time. So with phasing we can increase or reduce overlap to maximize a longer scavenging return and minimize or eliminate a pressure return. If the wave is arriving a bit early or late we can advance or retard the cams to meet it. Again, the dyno will tell if any benefits of moving the cams around to ‘catch’ negative exhaust waves pay dividends.

Naturally this can then be extended to the intake side and repeated.

So short version: Even if we put a well-designed header and intake manifold designed for max top-end power together, we will probably see even better results in terms of widening the power band with re-mapping the AVCS.

This will be something to be excited about if there is access to the AVCS in a ‘cracked’ ECU.





And in FT86club forgot topic fashion here’s something for the turbo guys:

I got a bit side-tracked by something in the book’s ‘current’ (pre-catalytic converter, still have leaded gas era) emissions control chapter. Toyota used to use an accessory compressor and air injection system for emissions control. And all of this hardware sounds exactly like what is used for a hard-core anti-lag system (the air is injected directly into the exhaust manifold). On a turbo car that has a MAF system venting a BOV to the one-way valve of the air-injection system would be all it takes (well that and plumbing in the air injectors). Since we’ve metered the air, but don’t use it, the fuel injection will be on the rich side, so there will be extra un-burnt fuel in the exhaust, and it meets the air vented by the BOV and now injected into the hot exhaust manifold to create an ‘afterburn’ effect inside the manifold that will keep the turbine spinning while off-throttle. Basically what Mitsubishi’s Supplemental Air System does. So run out and find a late-70s Toyota with air injection (hopefully not disintegrated to rust) to make your own rally-style anti-lag system for your WRX! Just don’t expect your turbo to last very long…

[Dimman]

Question regarding some language in that book.

In the context of exhaust gas velocity and this formula:

Gas Speed = Piston Speed X (D^2 / d^2)

D= dia of bore
d= dia of port


This is under the condition of "When the coefficient of flow is unity."

What does that mean "coefficient of flow is unity"?

Is it something to do with pressure differentials or something?

[bambbrose]

Originally Posted by Dimman

Question regarding some language in that book.

In the context of exhaust gas velocity and this formula:

Gas Speed = Piston Speed X (D^2 / d^2)

D= dia of bore
d= dia of port


This is under the condition of "When the coefficient of flow is unity."

What does that mean "coefficient of flow is unity"?

Is it something to do with pressure differentials or something?

I believe "unity" in this case means constant. Unity can be defined as "unvaried", and therefore can be said to be constant.

I'm sure for the formula to work, you have to hold the coefficient constant, otherwise there are too many unknowns.

Coefficent of flow = fluid friction, or "the resistance to flow", in layman terms.

[Dimman]

Originally Posted by bambbrose

I believe "unity" in this case means constant. Unity can be defined as "unvaried", and therefore can be said to be constant.

I'm sure for the formula to work, you have to hold the coefficient constant, otherwise there are too many unknowns.

Coefficent of flow = fluid friction, or "the resistance to flow", in layman terms.

Would the pressure difference between the hot exhaust gases and whatever pressure is left in the exhaust affect the speed of the gas as well?

[serialk11r]

I think that gets a bit complicated, the exhaust gas expanding into the exhaust pipe would produce...a wave? And then as the piston rises what happens is affected by how that wave behaves.

[Dimman]

Originally Posted by serialk11r

I think that gets a bit complicated, the exhaust gas expanding into the exhaust pipe would produce...a wave? And then as the piston rises what happens is affected by how that wave behaves.

Well we have two things happening, the pressure wave exiting at whatever the speed of sound in the exhaust pipe is (~1500 ft/sec) but that's moving through whatever gas/atmosphere is already in the pipe. There is also the left-over exhaust gas from the combustion that is exiting at whatever speed it does (that formula, I've heard 300 ft/sec estimates).

The formula I posted seems to be only for the piston 'pushing' the gas out. But in a situation where we open the exhaust early (before BDC, modern performance engine) there is left-over pressure that isn't really doing anything to push the piston anymore. And the idea is that the pressure of the gas pushes itself out (well moves to the lower pressure pipe) so there is less work for the piston to do on the up stroke. But does that pressure difference affect the speed of the exiting gas?

(remember physics is my big weakness...)

[bambbrose]

Are you familiar with free body diagrams? This is a simple piston pointing left with N force, existing gas pressure pointing right with X force, scavenging effect from last cycle pointing left with Y force, etc. The forces can all be said to be in one of two directions for this simple calculation.

However, real life is much more dynamic. The formula varies with the following: rate of valve opening, maximum valve opening, time that valve opens fully, heat of exhaust pulse, any 2nd or 3rd order harmonic or standing wave in the exhaust pulse, etc.

There is a reason dynos exist, the best tune is the one that works in real life. Eistein said turbulence was the one thing he never understood why god created

[old greg]

Originally Posted by Dimman

This is under the condition of "When the coefficient of flow is unity."

What does that mean "coefficient of flow is unity"?

Is it something to do with pressure differentials or something?

'Unity' is obscure math-speak for 1, and flow coefficient refers to the efficiency of the valves/head. So yeah, it's referring to pressure drop across the valves. What it means is that the equation is only true for constant density (zero pressure drop, isentropic efficiency of 1, etc).

The equation comes from the fact that the mass flow rate of the gasses exiting the cylinder must be equal to the mass flow rate entering the exhaust manifold (area*velocity*density must be constant). If the density within the cylinder is higher, the velocity in the runner will be proportionally higher as a result.

[Homemade WRX]

how did this turn into a gas dynamics thread? and I see some people poking around resonance theory

An exhaust manifold (header) will difinitely be one of the first things I start working on...onec I can profile the cam and see what range of cam phasing the exhaust AVCS gear will have.

[Dimman]

Originally Posted by Homemade WRX

how did this turn into a gas dynamics thread? and I see some people poking around resonance theory

An exhaust manifold (header) will difinitely be one of the first things I start working on...onec I can profile the cam and see what range of cam phasing the exhaust AVCS gear will have.

Does an AccessPort allow you to control the AVCS?